Tube plugging of HP carbamate condenser in urea plant

Abstract

The disclosure pertains to tube plugging of submerged carbamate condensation apparatuses of urea plants, in particular in the synthesis section of the plant. A plug is used with an annular part and a frustoconical plug body.

Description

P138004PC00Title: TUBE PLUGGING OF HP CARBAMATE CONDENSER IN UREA PLANTField

[0001] The invention pertains to tube plugging of a submerged high pressure carbamate condensation apparatus, provided as a shell- and- tube heat exchanger, in a urea production plant.Introduction

[0002] In urea production plants, CO2and NH3are reacted, at high pressure (HP), above 100 bara (bar absolute), in the urea synthesis section to form carbamate (ammonium carbamate) that is further reacted to a urea synthesis solution comprising urea and water, and unconverted carbamate. In urea plants of the stripping type, a part of the carbamate is expelled from the urea synthesis solution at a pressure in the HP range, in a HP stripper comprised in the urea synthesis section, with heating and counter-current contacting of the urea solution with a strip gas in the HP stripper. The stripped urea solution is further purified by removal of remaining carbamate, e.g. in one or more steps including a carbamate decomposition step in a low pressure (LP) decomposer (operated at 1 -10 bar abs., preferably 2 -8 bar abs.).

[0003] The gas from the stripper, containing CO2and NH3, is condensed in a HP carbamate condensation apparatus, comprised in the urea synthesis section, to form carbamate in an exothermic reaction. The resulting carbamate-containing liquid is supplied to the reaction zone of the HP synthesis section, which zone can be a separate reactor (typically a vertical urea reactor with trays), or can be a part of the same vessel as used for the HP carbamate condensation.

[0004] The HP carbamate condensation apparatus, as used in the present invention, is typically a shell-and-tube heat exchanger, comprising a shell (vessel), a tube bundle, a tubesheet, and a shell-side compartment. The tube ends are attached to the tubesheet. The tube bundle is arranged at the backside of the tubesheet. At the opposite front side of the tubesheet, a front side chamber (header) is provided, acting as a plenum between a plurality of tubes and a fluid inlet / outlet channel. The shell side compartment refers to the space between the tubes and between the tube bundle and the inner surface of the shell. The shell is usually an essentially cylindrical vessel, possibly provided with a hemispherical cap in thecase of a U-shaped tube bundle. The tubesheet is accessible for inspection and maintenance at the front side (the header is e.g. provided with a manhole or is openable). The direction from the front side to the back side and vice versa is the transversal direction.

[0005] The terms ‘distal’ and ‘proximal’ are used herein as seen from the front side of the tubesheet, i.e. ‘distal’ indicates a transversal position close to the backside, the term ‘ proximal’ a transversal position close to the front side of the tubesheet.

[0006] In the HP carbamate condensation apparatus used in the present invention, the carbamate condensation is carried out in the shell side compartment. In this context, it is noted that cooling fluid (e.g. water, in particular boiler feed water) is hence provided in the tubes; and that the pressure of the cooling fluid is lower than that of the process fluid; i.e. the pressure in the shell side compartment is higher than in the tube bundle. Furthermore, in the apparatus HP carbamate condensation apparatus used in the present invention, the carbamate condensation apparatus is a submerged condensation apparatus, meaning that carbamate-containing liquid is the continuous phase in the shell side compartment and the tube bundle is submerged by said liquid in operation. An example of this type of carbamate condensation apparatus is the so-called pool condenser, or a so-called pool reactor wherein a pool condenser and a reaction zone are combined in a single vessel. The pool condenser and pool reactor can also be referred to as a horizontal submerged condensation apparatus with a U-shaped tube bundle.

[0007] Pool condensers are often, but not exclusively, used in combination with a HP CO2stripper that uses a part or all of the CO2feed of the HP synthesis section as strip gas.

[0008] A background reference for urea production processes is Ullmann’s Encyclopedia of Industrial Chemistry, Chapter Urea, 2010. Fig. 18 of that document shows a process scheme of a synthesis section with a pool condenser. Fig.19 of that document includes a process scheme of a synthesis section with a pool reactor. The pool condenser is operated with raising steam (low pressure, 3 -6 bar abs.) in the tubes, from steam condensate / boiler feed water that is used for cooling. The present invention is useful for plugging such a pool condenser.

[0009] Patent application US 2024 / 0092730A1 proposes using a submerged carbamate condensation apparatus, namely a pool condenser, in a thermal stripping (self-stripping) urea plant.

[0010] A further type of submerged carbamate condensation apparatus of a urea plant comprises a vertical U-shaped tube bundle and a horizontal tubesheet, preferably a bottom tubesheet, with condensation of gas in the shell-side space and cooling fluid in the U-shaped tube bundle. An example is the process scheme in Fig. 29 of Ullmann’s Urea 2010. The present invention is also useful for plugging such a submerged carbamate condensation apparatus.

[0011] In some urea plants, the submerged carbamate condensation apparatus, e.g. the pool condenser, comprises two tube bundles, with different cooling fluids, usually a first tube bundle used for raising steam and a second tube bundle wherein medium pressure (MP) urea solution is heated (at 10 - 70 bar abs, preferably 15 – 35 bar absolute) to decompose carbamate comprised in said urea solution. The MP urea solution typically originates from the HP synthesis section and the carbamate decomposition provides for purification of the urea solution. The present invention is useful for plugging such a pool condenser, for either tube bundle. A background reference for this type of pool condenser is the paper M. Gori, “Stamicarbon’s Ultra Low Energy Design”, September 2018.

[0012] In the pool condenser, the tubes are exposed at the outer surface to the highly corrosive liquid phase in operation, which liquid phase includes carbamate at high pressure (above 100 bar, typically at 120 - 160 bar) and high temperatures (at least 120°C).

[0013] Preventing tube rupture is essential for the safe operation of the pool condenser. Tube rupture (leakage) can be caused by, e.g., corrosion, erosion, or mechanical damage. Accordingly, pool condensers are regularly inspected, usually with a combination of visual inspection and testing techniques such as eddy current and / or ultrasonic techniques, which can recognise that a tube has a reduced tube wall thickness. Leak monitoring (observing increasing conductivity of the steam raised in the tubes) can be used to detect small leaks well before tube rupture. If it turns out that a tube is at risk of becoming leaky or at risk of tube rupture, the tube needs to be ‘plugged’, i.e. the individual tube is to be taken out of operation. After inspection and any required plugging of any tubes, the poolcondenser can be operated further, e.g. until the end of the lifetime of the pool condenser or a major plant maintenance shutdown.

[0014] It is observed that repairing an individual tube in a tube bundle in place in the pool condenser is practically impossible, nor can an individual tube be removed from the closely spaced tube bundle practically for replacement, at least in the case of pool condensers.

[0015] Hence, it is desirable that tube plugging can be done quickly, so that identifying a need for tube plugging during an inspection shutdown does not lead to a long downtime. A disadvantage of welded plugs is the typical need for pretreatment heat and post-welding heat treatment, which can be time-consuming, as well as the risk of cracking in the heat-affected zones.

[0016] One known technique for tube plugging is by using plugs that are welded. For high pressure strippers that are operated with the high pressure corrosive process fluid in the tube bundle and with steam in the shell side compartment and where the tubes protrude through the tubesheet and the tube ends are accessible at the front side of the tubesheet, a plug can be welded into the tube end if the tube end is of good quality. An alternative approach is hammering a solid conical lug in the tube end, with the narrow end of the conical plug first (distal) flush with the front side of the tubesheet, to provide a friction fit, and welding several layers of weld material on top. In HP strippers, the tubesheet front side is provided with a weld overlay of a corrosion-resistant steel on the carbon steel tubesheet plate. A background reference for tube plugging of HP strippers of urea plants is the paper Roes, From ‘potentially’ unsafe to unsafe by inadequate tube plugging, 2015.

[0017] It is customary practice to pierce a tube before plugging it, typically using an internal tube cutter, to provide for venting and draining.

[0018] The carbamate in the high pressure synthesis section of a urea plant is very corrosive (especially at the relatively high temperature in that section). It is well known in the art to prevent severe corrosion by using passivation with oxygen, to provide the surfaces of the stainless steel parts that are exposed to the carbamate with a protective oxide layer. To this end, it is ensured that the liquids in the HP synthesis section, which contains the carbamate, also comprises (dissolved) oxygen, e.g. by adding air to the CO2 feed. The relevant carbamate-containing liquids include carbamate solutions in water and urea synthesis solution comprising urea, water and carbamate. For this reason, stagnant carbamate-containing liquid in the HP equipment is to be avoided.

[0019] Accordingly, crevices should be avoided in this equipment, since the stagnant carbamate-containing liquid in a crevice may cause a specific type of corrosion, even of stainless steel grades that are able to withstand carbamate corrosion with suitable oxygen passivation.

[0020] In submerged carbamate condensation apparatus, the corrosive high pressure fluid is in the shell side compartment, and the tubesheet is provided by a carbon steel tube plate with cylindrical holes from between the front side and the backside (this plate bears the mechanical load), and a corrosion-resistant weld overlay at the backside of the tube. The tube ends are typically flush with the tube backside. Generally, the tubes are not inserted into the holes in the tubesheet. The tube-to-tubesheet connection involves an internal bore weld. For example, the tube ends are typically welded to the backside of the tubesheet, i.e. to the welded overlay, by an internal bore weld. This provides for a crevice-free joint between the tubes and the tubesheet. In an embodiment of the invention wherein the pool condenser has a sleeve extending through the hole in the tubesheet, as inUS 2015 / 0086440A1, the internal bore weld of the tube-to-tubesheet connection is at the backside of the tubesheet between an end of the sleeve, protruding slightly from the tubesheet backside, and an end of the weld.

[0021] The paper “Importance of proper BFW quality in Pool Condenser plants”, Stamicarbon Symposium 2022, shows the detailed construction of the tube-to tubesheet joint of an example pool condenser, with the tubesheet of 425 mm thick carbon steel, 22 mm corrosion resistant weld overlay, inner diameter of the holes in the carbon steel tube of 19 mm, and internal bore weld connecting the tubes to the weld overlay.

[0022] Hence, in pool condensers and other types of submerged carbamate condensation apparatuses, the tubes are frequently not inserted into the boreholes, but are joined to the tubesheet in a crevice-free manner using, e.g. internal bore welding.

[0023] It is desired to provide an improved method of tube plugging of submerged carbamate condensation apparatuses of urea production plants.Summary

[0024] The invention provides a tube plugging method for a submerged carbamate condensation apparatus, and plugs suitable for such methods.

[0025] The invention provides, in an embodiment, a method of tube plugging of a submerged carbamate condensation apparatus of a urea plant, the submerged carbamate condensation apparatus comprising a tubesheet and a tube bundle. Preferably, the tubesheet has a front side and a back side. The front side defines the proximal side and the back side defines the distal side, in the transversal direction. The tubesheet has holes extending between the front side and the back side, said holes providing a concave exposed surface of the tubesheet, the tubesheet comprising a carbon steel plate, and wherein tubesheet comprises a welded overlay at the back side. The tube bundle is arranged at the backside of the tubesheet, wherein the tube ends of the tubes of the tube bundle are connected to the backside of the tubesheet with internal bore welds aligned with said holes. The method comprises providing a plug. The plug comprises: an annular part with a distal side and a proximal side, wherein the annular part comprises a tapered part at the distal side of the annular part, wherein the inner diameter of the annular part decreases over the tapered part in the direction from the distal side to the proximal side, wherein the annular part optionally comprises a sleeve extension part at a proximal side; a frustoconical plug body having a narrow end and a wide end and an outer surface mating with inner surface of the annular part at the tapered part of the annular part wherein the frustoconical plug body comprises a part with an outer diameter that is larger than the inner diameter of the annular part at the proximal end of the tapered part of the annular part; and a rod for pulling the plug body in the proximal direction, thereby moving the plug body into the annular part. The method comprises: inserting the plug in one of said holes in the tubesheet at a transversal position distal from the carbon steel plate, with the wide end of the plug body in the distal direction and with the tapered part of the annular part in the distal direction, and pulling the plug body, with the rod, in the direction of the front side of the tubesheet, thereby compressing the annular part at least in the tapered part thereof against said concave exposed surface of the tubesheet, thereby forming a pressure-resistant seal between the outer surface of the annular part and the tubesheet, wherein after the pulling, the wide end of the plug body and / orthe distal end of the annular part is at a transversal position between the internal bore weld and the carbon steel plate.

[0026] As noted above, the terms ‘distal’ and ‘proximal’ are used herein as seen from the front side of the tubesheet, i.e., ‘distal’ indicates a transversal position close to the backside, the term ‘ proximal’ a transversal position close to the front side of the tubesheet. Preferably, in the event that an operator is carrying out a method of installing a plug as defined in this disclosure, the proximal side is at the front side (close to the operator installing the plug), and the distal side D is at the back side (away from the operator installing the plug from the front side).

[0027] The invention also provides: a submerged carbamate condensation apparatus that comprises a tubesheet having a front side and a back side, the front side defining the proximal side and the backside defining the distal side of the tubesheet in a transversal direction, wherein the tubesheet has holes extending between the front side and the back side, said holes providing a concave exposed surface of the tubesheet, the tubesheet comprising a carbon steel plate, and wherein tubesheet comprises a welded overlay at the back side; a tube bundle arranged at the backside of the tubesheet, wherein the tube ends are connected to the backside of the tubesheet with internal bore welds aligned with said holes. At least one tube of the tube bundle is plugged with a plug, wherein the plug comprises: an annular part with a distal side and a proximal side, wherein the annular part comprises a tapered part at the distal side of the annular part, wherein the inner diameter of the annular part decreases over the tapered part in the direction from the distal side to the proximal side, wherein the annular part optionally comprises a sleeve extension part at a proximal side, a frustoconical plug body having a narrow end and a wide end and an outer surface mating with inner surface of the annular part at the tapered part of the annular part wherein the frustoconical plug body comprises a part with an outer diameter that is larger than the inner diameter of the annular part at the proximal end of the tapered part of the annular part, a rod for pulling the plug body in the proximal direction, thereby moving the plug body into the annular part; wherein the plug is provided in one of said holes in the tubesheet at a transversal position distal from the carbon steel plate, with the wide end of the plug body in the distal direction and with the tapered part of the annular part in the distal direction, wherein the annular part preferably is compressed at least in the tapered part thereof against said concaveexposed surface of the tubesheet, wherein the annular part provides a pressureresistant seal between the outer surface of the annular part and the tubesheet, and wherein preferably the wide end of the plug body and / or the distal end of the annular part is provided at a transversal position between the internal bore weld and the carbon steel plate.

[0028] The invention also provides a carbamate condensation process carried out in that carbamate condensation apparatus.

[0029] The invention also provides a stripping-type urea production plant comprising a urea synthesis section comprising a reaction zone, a stripper, and a submerged carbamate condensation apparatus according to the invention; and a urea production process carried out in such a plant.

[0030] The disclosure pertains to tube plugging of submerged carbamate condensation apparatuses of urea plants, in particular in the synthesis section of the plant. A plug is used which comprises an annular part and a frustoconical plug body.Brief description of the drawings

[0031] Figure 1 schematically illustrates an example submerged carbamate condensation apparatus that is plugged with a mechanical plug, according to the invention.

[0032] Fig. 2 schematically illustrates an example plug useful in the invention.

[0033] Figure 3 schematically shows an example urea production plant comprising a submerged carbamate condensation apparatus, useful for the invention.

[0034] Figure 4 schematically illustrates the transversal position of the plug, after the pulling, in the inventive method.

[0035] Figure 5 schematically shows an example sealing element preferably used in the invention.

[0036] Figure 6 illustrates an example preferred embodiment wherein the annular part is provided with a circumferential groove at the outer surface.

[0037] Figure 7 illustrates a perspective view of an example annular part used in the plug used in the invention.

[0038] Fig. 8 shows an example of a urea production plant wherein the submerged carbamate condensation apparatus receives MP urea solution in a tube bundle, which submerged carbamate condensation apparatus can be plugged according to the invention.

[0039] Fig. 9 schematically shows the tapering angle of the inner surface of the tapered part of the annular part in an example plug useful in an embodiment of the invention.

[0040] Fig. 10 schematically illustrates an example tensioning means useful in an embodiment of the invention.

[0041] Fig. 11 schematically illustrates an example of a plug with a serrated outer surface of the annular part, useful in embodiments of the invention.

[0042] Fig. 12 schematically shows the tapered part located at a transversal position in the tubesheet borehole.

[0043] Fig. 13 schematically illustrates a further example of a plug with a serrated outer surface of the annular part.

[0044] Any embodiments illustrated in the figures are examples only and do not limit the invention.Detailed description

[0045] The invention provides a plug for tube plugging of a submerged carbamate condensation apparatus of a urea plant. The invention also provides a urea plant of the stripping type comprising the submerged carbamate condensation apparatus comprising the tube bundle wherein at least one tube of the tube bundle is plugged with the inventive plug. The invention also provides a method of tube plugging of a submerged carbamate condensation apparatus of a urea plant of the stripping type. The invention also provides a urea production process in such a urea production plant with the submerged carbamate condensation apparatus with the inventive plug.

[0046] The urea plant comprises a high pressure (HP) urea synthesis section, configured for operating at high pressure above 100 bar, the section comprising a reaction zone, a condensation apparatus, and a HP stripper. The reaction zone and condensation apparatus may be provided in a single combined vessel, such as in the case of a pool reactor, or may be provided as a separate urea reactor, or acombination thereof. The condensation apparatus is a submerged carbamate condensation apparatus. Preferably, the synthesis section comprises the urea reactor, the submerged carbamate condensation apparatus, and the stripper.

[0047] The urea reactor is usually a vertical urea reactor with trays, having an inlet or inlets at the bottom and having an outlet for withdrawing urea synthesis solution from an upper part of the reactor, e.g. with a downcomer. The urea synthesis solution, comprising urea, water, and unconverted carbamate, is supplied to a HP stripper. The urea reactor usually has a separate gas outlet at the top.

[0048] In embodiments with a pool reactor as the submerged carbamate condensation apparatus, the pool reactor is a horizontal vessel with a U-shaped tube bundle, with horizontal legs of the tubes. The U-shaped tube bundle extends over a part of the length of the vessel. The part of the vessel with the tube bundle provides the condensation apparatus, where carbamate is condensed. The part, in the horizontal direction, between the bend of the U-shaped tube bundle and the wall of the vessel, provides a reaction zone. The tube bundle and the liquid outlet of the vessel are arranged at opposed sides of the reaction zone in the horizontal direction. The pool reactor comprises baffles. The baffle near the liquid outlet functions as an overflow weir. A liquid level is maintained in the reactor in operation. The U-shaped tube bundle is submerged in the liquid in operation.

[0049] The HP stripper is configured to heat the urea synthesis solution in counter-current with a strip gas that has a relatively low concentration of NH3and / or CO2so as to strip the liquid. The strip gas is e.g. a part or all of the CO2feed. In a further embodiment, the strip gas is an ammonia-rich vapor obtained by reboiling the urea solution. The latter is known as thermal stripping or selfstripping. The stripper is usually a shell-and-tube heat exchanger operated with a falling film of urea solution to be stripped in the tubes, and with a liquid inlet and a gas outlet at the top and a liquid outlet at the bottom, and e.g. an inlet for CO2 feed used as strip gas at the bottom. Steam is typically used as a heating fluid in the shell. The gas from the stripper, comprising NH3and CO2, is supplied, at least in part, to the submerged carbamate condensation apparatus, where it is condensed into carbamate. In an embodiment, the NH3feed is also supplied to the HP carbamate condensation apparatus, i.e. in a CO2 stripping-type plant. The liquid comprising carbamate from the submerged carbamate condensation apparatus is supplied to the reactor.

[0050] In the invention, the submerged carbamate condensation apparatus is a shell-and-tube heat exchanger that is configured for operating at high pressure (HP), i.e. above 100 bar, and with condensation of gas from the HP stripper in the shell-side compartment (also referred to as the shell side space). Furthermore, the submerged carbamate condensation apparatus is configured for operating with liquid as the continuous phase in the shell side compartment, such that the tube bundle is submerged in the liquid in the shell side compartment in operation. This is known as a ‘submerged’ condenser.

[0051] The term ‘condensation apparatus’, as used herein, includes, as an embodiment, a vessel that combines a condensation zone (with a tube bundle) and a reaction zone (without a tube bundle), e.g. in the case of a pool reactor, or in the case of a vertical combination reactor. The reaction zone provides residence time for the urea formation reaction, by dehydration of the carbamate, to take place. Furthermore, also in the condensation zone, some urea may already be formed.

[0052] The submerged carbamate condensation apparatus is e.g. essentially filled with liquid in operation, e.g. has only a fluid outlet at an upper part, or is e.g. operated with a liquid layer of sufficient depth to submerge the tube bundle, e.g. with an overflow weir. In operation, gas to be condensed is introduced as bubbles into the liquid phase. The submerged carbamate condensation apparatus is typically provided with a lining of a corrosion-resistant steel on the inner surface of the vessel that is exposed to the shell side space.

[0053] The liquid withdrawn from the submerged carbamate condensation apparatus comprises, typically, carbamate, urea, and water.

[0054] The submerged carbamate condensation apparatus may have a straight tube bundle or a U-shaped tube bundle. The tube bundle can be vertical or horizontal. In the case of a U-shaped tube bundle, this refers to the direction of the legs of the tubes.

[0055] An example of a horizontal submerged carbamate condensation apparatus with a U-shaped tube bundle, operated with the condensation in the shell-side compartment, is a pool condenser, including a pool reactor. This type of condenser is particularly preferred.

[0056] An example of a vertical submerged carbamate condensation apparatus with a U-shaped ube bundle, operated with the condensation in the shell-sidecompartment, is illustrated in Fig. 28 of Ullmann’s, Chapter Urea, 2010, and is used in the ACES21 urea production process.

[0057] US 2020 / 0306663A1 proposes a HP carbamate condenser, operated with condensation in the shell side compartment, with in one embodiment, a straight tube bundle with tube ends at opposed sides of the tube bundle in the length direction. In principle, such a HP carbamate condenser can be plugged with the present invention.

[0058] The preferably used horizontal submerged condensation apparatus comprises a sparger or (gas distributor) for introducing the gas from the stripper in the shell side compartment, in particular the sparger extends in the horizontal plane below the tube bundle for distributing the gas into the liquid. The sparger is arranged at the bottom of the shell side compartment. In the case of a pool reactor, the gas sparger also extends in the reaction zone.

[0059] The shell side compartment preferably comprises an inlet from an MP carbamate solution, optionally provided as a mixture with NH₃ feed liquid.

[0060] The shell side compartment may comprise an additional sparger for introducing NH₃ feed liquid, which additional sparger extends horizontally below the tube bundle.

[0061] The HP synthesis section optionally further comprises a HP scrubber for gas from the HP reactor or from the pool reactor, in case of a CO₂ stripping type plant.

[0062] The stripped urea solution is for example supplied to a decomposer operated at a lower pressure, e.g. at MP or LP, for further removal of carbamate from the solution, or, for example, first to an MP (adiabatic) flash vessel, and subsequently to one or more decomposers, which are as such known in the art.

[0063] The preferred plug comprises an annular part (which may be a sleeve). The annular part is a cylindrical tubular part, having a length in axial / length direction and an inner and outer diameter, and an inner surface that is concave (in radial cross section perpendicular to the axial direction) and an outer surface that is convex (in radial cross section). The annular part is straight in the length direction, i.e. is not curved in the length direction. The annular part has, in the length direction, a distal end and a proximal end. The plug is installed with the distal end of the annular part facing the tubesheet backside. The outer diameter ofthe annular part closely matches the inner diameter of the fluid passageway (hole in the tubesheet). The hole in the tubesheet is suitably drilled to remove any fouling and oxide layers and to have the correct inner diameter before installing the plug.

[0064] The outer diameter of the annular part is e.g. between 15 mm and 25 mm.

[0065] The annular part is a part of the plug and is inserted in the hole when installing the tube, and remains in place in the tube-plugged submerged carbamate condensation apparatus.

[0066] The annular part comprises, at the distal end, a tapered part. In this tapered part, the inner (concave) surface of the annular part is tapered, with a decreasing inner diameter of the annular part in the direction from the distal end inwards (in the transversal direction). As said, the distal end is at the side of the tubesheet backside when the plug is installed. Hence, the tapered part has an opening that is converging (decreasing in diameter) from the distal side edge inwards.

[0067] The tapered part of the annular part extends, in the length direction, preferably over a distance that is 80 % to 120% of the thickness of the welded overlay, preferably over 80 to 100 % of said thickness.

[0068] The plug comprises a frustoconical plug body (or part), also referred to as a conical plug body. The plug body may be a part of a unitary piece, for instance, the plug body may be joined to a cylindrical part at the wide end or narrow end of the plug body. In other words, the plug body may comprise a frustoconical part, or consist of a frustoconical part. The frustoconical part refers to the outer, convex surface of the plug body. The plug body outer surface typically has a cylindrical cross-section in the radial plane, perpendicular to the length axis of the plug boxy.

[0069] The plug body is tapered, at the outer surface, in a length direction, from a wide end to a narrow end of the plug body, with a diameter at the wide end that is larger than at the narrow end. The plug body is installed with the wide end in the distal direction. The tapering angle is preferably the same for the tapered part as for the annular part. The tapering angle is preferably constant over the length of the plug body. The tapering is e.g. a slope in the range of 1% to 10%. Thetapering is e.g. an angle in the range of 0.5° to 5°. The plug body hence comprises a tapered surface that is oblique in the transversal cross-section.

[0070] The frustoconical plug body comprises a part with an outer diameter that is larger than the inner diameter of the annular part at the proximal side of the tapered part of the annular part. The frustoconical plug body also comprises a part with an outer diameter that is smaller than the inner diameter of the annular part at the distal end of the tapered part of the annular part. The frustoconical plug body also preferably comprises a part with an outer diameter that is larger than the inner diameter of the tapered part of the annular part at a position in the middle of the tapered part in the length direction of the sleeve. The frustoconical plug body also preferably comprises a part with an outer diameter that is smaller than the inner diameter of the tapered part of the annular part at a position in the middle of the tapered part in the length direction of the annular part. These relative dimensions ensure that the frustoconical plug body can be sufficiently pulled into the tapered part of the annular part, to compress the annular part, at least the tapered part thereof, against the concave surface of the fluid passageway.

[0071] The plug body typically comprises, at a proximal side, attachment means, such as a threaded hole (receiving portion) for receiving a (tie) rod that is used, at least, for pulling the plug body. The plug body may also be integral with a rod that is used for pulling the plug body, and possibly also for pushing the plug body.

[0072] The plug also comprises the rod, having a distal end attached to, or joined with, the proximal (narrow) end of the plug body. The diameter of the rod is e.g. less than 30% or less than 20% of the diameter of the plug. In particular, the rod is not used for bearing the thrust of HP fluid when operating the tube-plugged submerged carbamate condensation apparatus, and hence does not need to be very strong and thick.

[0073] The rod can be hollow. The rod is used for pushing the plug body into the hole in the tubesheet, and for subsequently pulling the plug body in the direction of the front side of the tubesheet.

[0074] The plug body, the annular part, and the rod are independently preferably made of a corrosion-resistant type of steel, e.g., austenite steel, more preferably of duplex stainless steel, and preferably are all made of these steels.Preferably, these components are made of duplex stainless steel with 25 wt.% Cr or higher, and at least 7 wt.% Ni.

[0075] For example, these components are made of steel having the elemental composition of, in wt.%: C < 0.03; Cr 21.0 - 23.0; Ni 4.50 - 6.50; Mo 2.50 - 3.50; N -0.12 - 0.20, the balance being Fe and unavoidable impurities, or having the composition of wt.%: C < 0.03; Cr 24.0 - 27.0; Ni 4.50 - 7.50, Mo 2.00 - 4.50; N 0.12 - 0.28; W < 0.50, the balance being Fe and unavoidable impurities. These types of steel are known as UNS S32205 or UNS S32750, respectively.

[0076] In a preferred embodiment, the plug body and the annular part, and more preferably also the rod, are made of a type of duplex stainless steel as described in US 5,582,656A or in US 7,347,903. Accordingly, in an embodiment, the duplex stainless steel has the composition in wt.%: C 0 - 0.05; Si 0 - 0.8; Mn 0 - 4.0; Cr 26 - 35; Ni 3.0 - 10; Mo 0 - 4.0; N 0.30 - 0.55; Cu 0 - 1.0; W 0 - 3.0; S 0 - 0.03; Ce 0 - 0.2; the balance being Fe and unavoidable impurities. However, other types of corrosion-resistant steels, preferably duplex stainless steels, are also possible, such as, e.g., UNS S32906 and S32808

[0077] The plug, highly preferably, comprises a sealing element. Preferably, the sealing element is made of a different material than the plug body. Preferably, the sealing element is made of a polymeric material.

[0078] The sealing element has a circular outer edge, for example is a sheet element with a circular outer edge, e.g. a circular sheet provided with one or more holes, or a ring-shaped sheet. The sealing element could also be an O-ring, for example.

[0079] The sealing element is arranged in a distal part of the plug. In particular, the sealing element is arranged more distal than the proximal end of the tapered part of the annular part. Preferably, the sealing element is arranged, in the transversal direction, more distal than the middle of the tapered part, i.e. the middle in the transversal direction. Optionally, the sealing element is arranged adjacent to the distal end of the annular part.

[0080] In an embodiment, the sealing element is placed adjacent to, and in contact with, the distal planar surface of the plug body and the distal edge of the annular part, and covers the crevice between the outer surface of the annular part and the concave exposed surface of the tubesheet in the hole in the tubesheet, atthe distal end of the annular part, and, in this embodiment, preferably also covers the crevice between the plug body and the inner surface of the annular part.Preferably, the sealing element is a sheet with one or more holes, in this embodiment. By covering either or both crevices with the sealing sheet, crevice corrosion due to oxygen-depleted carbamate solution in the crevices is avoided. In particular in embodiments wherein the exposed surface of the tubesheet is made of austenite steel, covering the crevice between the tubesheet and the annular part is advantageous. This embodiment is preferably combined with a sleeve extension part.

[0081] In another embodiment, the outer surface of the annular part comprises a groove and the sealing element is a ring arranged in said groove, e.g. an O-ring.

[0082] The sealing element preferably has an outer diameter that is at least equal to the outer diameter of the annular part, more preferably is larger than the outer diameter of the annular part. In a preferred embodiment, the outer diameter of the sealing element is equal to or, more preferably, larger than, the inner diameter of the hole in the tubesheet (borehole), preferably larger by at least 0.10 mm, e.g. 0.10 mm to 0.20 mm larger. The outer diameter of the sealing element is preferably larger than the outer diameter of the wide end of the plug body, preferably at least 0.20 mm larger, e.g. 0.20 mm to 0.30 mm larger. The outer diameter of the sealing element is preferably larger than the outer diameter of the annular part, preferably at least 0.10 mm larger, e.g. 0.10 mm to 0.20 mm larger. These preferences in particular apply to the embodiment wherein the sealing element is located adjacent to the plug body.

[0083] In embodiments wherein the sealing element is a ring located in a groove of the annular part, the outer diameter of the ring can be smaller than, equal to, or larger than the outer diameter of the annular part, and smaller than, equal to, or larger than the inner diameter of the bore hole. In particular, the compressing of the annular part against the concave surface may expand the sealing element ring to provide a sealing effect.

[0084] Hence, when installing the plug, the sealing element is compressed. The sealing element material is preferably resilient / elastic, such as to close off the crevice between the concave surface of the hole in the tubesheet and the annular part. This mitigates the risk of crevice corrosion by carbamate, in particular of an austenite welded overlay.

[0085] In embodiments wherein the sealing element is a sheet (with a circular circumference) arranged adjacent to the plug body, the sheet is preferably supported by the plug body over the entire proximal side of the sheet.

[0086] Preferably, the plug body is made of steel and the sealing element is made of a non-steel material. Preferably, the sealing element is made of a non-metal or a non-ferrous metal or alloy. A non-ferrous metal is a metal or alloy that does not contain iron as the main component. Preferably, the sealing element comprises less than 10 wt.% Fe based on the element.

[0087] Preferably, the material of the plug body has a higher Young’s modulus (in GPa) than the material of the sealing element. For example, the material of the plug body has a Young’s modulus of at least 100 GPa and the material of the sealing element has a Young’s modulus below 80 GPa, or even below 20 GPa. For example, steel typically has a Young’s modulus of 200 GPa and PTFE has a Young’s modulus of 0.56 GPa. In a preferred embodiment, the plug body is made of steel and the sealing element is made of a material with a Young’s modulus lower than 80 GPa or lower than 20 GPa. The relatively low Young’s modulus is e.g. useful when the outer diameter of the sealing element is larger than the inner diameter of the convex surface of the tubesheet.

[0088] These preferences for the materials apply not only to the plug body, but also to the annular part and the rod.

[0089] Suitable materials of which the sealing element is for example made, include engineering plastics resistant to ammonium carbamate (at least 10 wt.% in the liquid) and at least 130°C, preferably at least 150°C, up to 190°C (temperatures typical of a urea synthesis section).

[0090] In an example embodiment, the sealing element is made of a material selected from the group consisting of polytetrafluorethylene (PTFE), polyether ether ketone (PEEK), polypropylene (PP), polyvinyl chloride (PVC), polyvinylidene difluoride (PVDF), polyamide (such as Nylon 6-6), and combinations thereof.

[0091] Other materials suitable for the sealing element include, e.g., graphite and aluminium. These materials are used e.g. for the sealing element provided as a sheet adjacent to the plug body. In embodiments wherein the sealing element is an O-ring arranged in a groove of the annular part, the O-ring is more preferably made of a polymeric material.

[0092] Preferably, the sealing element is made of polytetrafluoroethylene, polyether ether ketone, or a combination thereof.

[0093] An advantage of using PTFE for making the sealing element is the low friction coefficient of PTFE, which is advantageous when the plug is pushed into the hole in the tubesheet, especially since the sealing element is in direct contact with the concave surface of the tubesheet during said pushing.

[0094] Figure 1 schematically illustrates an example submerged carbamate condensation apparatus (carbamate condenser) that is plugged with a mechanical plug, according to the invention. The submerged carbamate condensation apparatus (1) comprises a tubesheet (2). The tubesheet is provided with holes (3) (channels, boreholes) which extend from the front side (2a) to the back side (2b) of the tubesheet, in the transversal direction (T). The tubesheet comprises a carbon steel plate (4) and a welded overlay (5) on the backside, the welded overlay is of a corrosion-resistant steel grade. The carbamate condenser further comprises a tube bundle (6) that is arranged in the vessel (9) of the carbamate condenser at the backside of the tubesheet. As illustrated, the tube bundle is a horizontal U-shaped tube bundle, i.e. with horizontal legs, and with the length of the tube legs extending in the transversal direction. Other configurations are also possible. The carbamate condenser comprises a shell side space (10) between the tube bundle and the shell (11) of the vessel. The carbamate condenser has in the shell an inlet (12) to receive gas comprising NH₃ and CO₂ to be condensed in the shell-side compartment to form carbamate, and has an outlet (13) for carbamate solution. The submerged carbamate condensation apparatus comprises a gas distributor (15), e.g. sparger, to distribute the gas from the inlet (12) in the shell side space (10), in particular in the transversal direction.

[0095] As illustrated in Fig. 1, one of the tubes is plugged with a plug (8). In practice, both ends of the tube will be plugged. In practice, the two ends of the tube are in communication with different chambers (14) on the tubesheet frontside (proximal side), namely an inlet chamber and an outlet chamber, for cooling fluid supplied to and withdrawn from the tube bundle. The tube ends (6a) are attached to the tubesheet, in particular to the welded overlay (5), by means of an internal bore weld (7). The transversal direction (T) is shown, as well as the distal direction (D) and the proximal direction (P) in the transversal direction. As illustrated, the proximal side (P) is at the front side (close to the operator installingthe plug) and the distal side D is at the back side (away from the operator installing the plug from the front side). An optional intermediate layer (not shown) can be arranged between the carbon steel plate (4) and the welded overlay (5).

[0096] The hole (3) terminates in a frontside chamber (14) of the carbamate condenser, which chamber acts as an inlet or outlet manifold for cooling fluid to or from the tube bundle, i.e. as a header. The two ends of an individual tube terminate in different frontside chambers. Hence, the submerged carbamate condenser comprises, for one tube bundle, two frontside chambers. For the illustrated U-shaped tube bundle, the two frontside chambers (headers) are both at the proximal side of the tubesheet.

[0097] Fig. 2 schematically illustrates an example plug (8, 200) useful in the invention, in a cross-section in the transversal direction and in a cross-section A-A in the radial direction. The plug comprises an annular part (201) which has a distal side (202) and a proximal side (203). The annular part comprises a tapered part (204) (or tapered section) at the distal side of the annular part. By the tapering, the inner diameter of the annular part decreases over the tapered part in the direction from the distal side to the proximal side, i.e. in the proximal direction (P). The tapering in an example embodiment extends over the entire length of the annular part (length in the transversal direction. As illustrated, the annular part comprises, in this example embodiment, a sleeve extension part (211) at the proximal side. In practice, the sleeve extension part can be longer in the transversal direction, when used, e.g. so as to extend in the proximal direction up to the front side of the tubesheet.

[0098] The tapered part of the annular part has a proximal end (209) where the tapering stops, which can be at the proximal side of the plug or in a position in the middle annular part in the transversal direction. The annular part has a convex outer surface, typically a cylindrical outer surface, typically with an outer diameter closely matching the inner diameter of the hole in the tubesheet. The tapered part can also be referred to as a tapered section of the annular part. The tapered part comprises a part of the inner surface of the annular part that is oblique in the transversal direction. For example, the thickness of the annular part increases over the tapered part, in the direction from distal to proximal.

[0099] The plug also comprises a frustoconical plug body (205) having a narrow end (206) and a wide end (207). The plug body has an outer surface (208) that ismating with inner surface of the annular part at the tapered part of the annular part. This outer surface is oblique in the transversal cross-section. The outer surface of the plug body (205) is convex, and the inner surface of the annular part (201) is concave, in a cross-section in radial direction, perpendicular to the transversal direction (as illustrated, cross-section A-A). The frustoconical plug body comprises a part, e.g. the wide end (207), with an outer diameter that is larger than the inner diameter of the annular part at the proximal end (209) of the tapered part of the annular part.

[0100] The plug also comprises a rod (210) for pulling the plug body in the proximal direction, thereby moving the plug body into the annular part, in particular for sliding the plug body in the opening of the annular part.

[0101] As illustrated, the plug body has been inserted into the annular part until the moment of contact (between the plug body and the annular part). Further pulling of the plug body in the proximal direction (P) (to the right in the figure) results in the plug body pushing against the inner surface of the tapered part and compression of the outer surface of the annular part against the concave exposed surface of the tubesheet.

[0102] Figure 3 schematically shows an example urea production plant comprising a submerged carbamate condensation apparatus, useful for the invention. The urea plant (301) comprises a HP urea synthesis section (302). This section comprises a vertical urea reactor (303), the submerged carbamate condensation apparatus (304) (illustrated as a pool condenser) and a HP stripper (305). Urea solution (306) from the reactor (withdrawn at the top) is supplied to the top liquid inlet of the stripper, which also receives CO₂ feed in this example with CO₂ stripping. The stripped urea solution (307) is expanded to a lower pressure and supplied to a urea purification section (not shown). The gas (308) expelled in the HP stripper is supplied to the shell side space of the submerged carbamate condensation apparatus. Boiler feed water (BFW) is supplied to the tube bundle of the submerged carbamate condensation apparatus. The gas condenses in the shell side space and the resulting carbamate-containing liquid (309) is supplied to the reactor. The shell side space of the submerged carbamate condensation apparatus also receives NH₃ feed.

[0103] Figure 4 schematically illustrates the transversal position of the plug after the pulling. References are the same as in Fig. 2 unless specified otherwise;also in the following figures the same reference signs identify the same or corresponding elements in the different figures. The plug (200) is located in the hole (3) in the tubesheet at such a position that the wide end (207) of the plug body (205) and / or the distal end of the annular part (201), in particular of the tapered part of the annular part, is at a transversal position between the internal bore weld (7) and the carbon steel plate (4). Preferably, at least the part of the annular part (201) that is compressed against the concave surface of the tubesheet is located, in the transversal direction, between the internal bore weld (7) and the carbon steel plate (4), i.e., at the transversal position of the welded overlay (5). Preferably, the tapered part (204) of the annular part is entirely located between the internal bore weld (7) and the carbon steel plate (4), as illustrated. The proximal end (209) of the tapered part is located distal to the carbon steel plate (4). Figure 4 also shows the radial cross-section A-A (perpendicular to the transversal direction T) and shows that the convex outer surface of the annular part (201) is in contact with the convex exposed surface of the welded overlay (5).

[0104] Figure 5 schematically shows an example sealing element preferably used in the invention, based on Fig. 2. The sealing element (501) is attached to the wide end of the plug body (205) using a bolt and nut used as a clamping element (502). The sealing element is, in this embodiment, a circular sheet with a hole for the bolt. The sealing element covers the crevice between the annular part (201) and the tubesheet. The sealing element has an outer diameter that is slightly larger than the outer diameter of the annular part. The sealing element is made, in this example, of a polymeric compressible material. When installed, the circumferential edge, being a circular edge, is compressed against the concave surface of the hole in the tubesheet to provide a tight seal.

[0105] The sealing element is provided at the distal side of the plug, i.e. at the side of the wide end of the plug body (205). The sealing element thereby protects a proximal part of the plug against corrosive fluid entering into the hole in the tubesheet at the distal side. The sealing element thereby protects the annular part (201). In the illustrated embodiment, the sealing element is provided distal to, and adjacent to, the annular part (201), i.e. the entire annular part is proximal to the sealing element.

[0106] In this embodiment, the sealing element (501) is a sheet with at least one opening (503), and the clamping element (502) has a stem (504) and a radiallyextending part (505). The radially extending element extends radially from the stem and provides a recess (506) for the sealing element between the plug body and the radially extending element. The radially extending part (505), e.g. the nut, prevents undesired movement of the sealing element in the distal direction.

[0107] Figure 6 illustrates an example preferred embodiment wherein the annular part (201) is provided with a circumferential groove (602) at the outer surface. The diameter of the groove is in the radial plane, i.e. perpendicular to the transversal direction. The sealing element is provided as a sealing ring (601) in the groove, in particular as an O-ring. The O-ring has the shape of a torus and has a circular cross-section. The cross-section of the ring is preferably larger than the depth of the groove; the minor radius of the torus is preferably more than 50% of the depth of the groove. Hence, before installing the plug, the O-ring protrudes from the groove, as illustrated. The O-ring is made, in this example, of a polymeric compressible material. When the plug is installed, the O-ring is compressed against the concave exposed surface of the tubesheet. This provides a seal. The sealing ring (601) thereby protects a proximal part (603) of the annular part against corrosive fluid possibly entering from the distal side through the annular crevice between the tubesheet and the annular part. Such a crevice may gradually increase in size (thickness) due to crevice corrosion of, e.g., the tubesheet at the concave surface that is exposed to the crevice. The sealing ring is accordingly made of a suitable material that stops crevice corrosion, e.g. a polymeric material that is corrosion resistant. The tapered part (204) is provided in part or entirely proximal to the sealing ring. The sealing ring is provided distal to the proximal end (209) of the tapered part. In the illustrated example embodiment, the outer surface (604) of the annular part (201) is smooth and non-tapered in the transversal direction. The outer surface of the annular part may also be corrugated or provided with serrations, as in Fig. 11, also in combination with a sealing ring (see Fig. 13). The serrations are provided by ridges and circumferential grooves.

[0108] Figure 7 illustrates a perspective view of an example annular part (201) used in the plug used in the invention, the annular part having the tapered part (204) at the inner surface at the distal side (202). The opening (701) of the annular part is also visible, as well as the convex outer surface (702) of the annular part and the concave inner surface (703), and the oblique inner surface part (704) at the position of the tapered part (with convex and concave referring tothe radial direction, and oblique referring to the transversal direction). When installing the plug, the narrow end of the plug body is inserted into the opening at the distal side of the annular part, the plug is introduced into the hole, and thereafter the plug body is pulled in the proximal direction.

[0109] The invention provides a carbamate condensation process carried out in a submerged carbamate condensation apparatus according to the invention, i.e. the submerged carbamate condensation apparatus with a tube plugged by the plug.

[0110] The process comprises: supplying gas comprising CO₂ and NH₃ to the shell-side compartment of the submerged carbamate condensation apparatus, the submerged carbamate condensation apparatus comprising one or more tube bundles, condensing the gas into carbamate in said shell-side compartment, and supplying one or more cooling liquids to said one or more tube bundles of the submerged condensation apparatus.

[0111] In this process, the plug in submerged carbamate condensation apparatus, which plugs a tube of that carbamate condensation apparatus, prevents any high pressure corrosive fluid from the shell side compartment to enter into the front side chamber to which the tube is connected (i.e., the front side chamber adjacent to and in direct connection with the tubesheet hole in which the plug is installed), and thereby prevents contamination of the cooling fluid and avoids a risk of loss of containment.

[0112] In particular, in the process, the plugged tube receives carbamate-containing liquid (e.g. by the tube puncture), and the process involves exposing the distal end of the plug (e.g., the sealing element adjacent the wide end of the plug body, or the wide end of the plug body) to this carbamate-containing liquid, wherein the carbamate-containing liquid preferably has a pressure of at least 100 bar, a temperature of at least 140°C, and contains at least 10 wt.% carbamate or at least 20 wt.% carbamate.

[0113] Preferably, the pressure difference between the process fluid in the shell side compartment and the pressure of the one or more cooling liquids is at least 50 bar, with the higher pressure in the shell side compartment. The shell side compartment contains a liquid comprising carbamate. This tube bundle of the carbamate condensation apparatus is submerged in this liquid in the process.Further preferences and details are as for the carbamate condensation process.

[0114] The inventive urea production process carried out in the urea production plant of the invention, comprises supplying NH₃ feed to the urea synthesis section, e.g. to the shell side compartment of the submerged carbamate condensation apparatus, and CO₂ feed to the urea synthesis section, e.g. at least in part to the stripper, and operating the synthesis section at a high pressure (HP) of at least 100 bar and a temperature of at least 160°C, typically 180 – 220°C to form a urea synthesis solution in the reaction zone of the said synthesis section, and subjecting the urea synthesis solution to stripping in the stripper. The process further comprises supplying gas from the HP stripper to the shell-side compartment of the submerged carbamate condensation apparatus, where the gas is condensed into carbamate in that shell-side compartment (shell side space), and supplying one or more cooling liquids to the one or more tube bundles of the submerged condensation apparatus.

[0115] The hole of the tubesheet in which the plug is inserted, is, at the front side of the tubesheet, connected to (terminates in) a front side chamber. This chamber is located directly adjacent to the hole in the tubesheet. This chamber is a chamber for fluid. A plurality of tubes are connected to this chamber through respective holes. In a preferred embodiment of the process, the front side chamber receives a medium pressure (MP) ( 10- 70 bara, preferably 15 - 35 bara) urea solution also comprising carbamate, e.g. from the tube (outlet chamber) or from a liquid supply line (inlet chamber). The MP urea solution originates from the urea synthesis section of the urea plant. In the process, the heat of condensation released in the shell side compartment is transferred through the walls of the nonplugged tubes of the tube bundle to the MP urea solution, thereby decomposing at least part of the carbamate in said MP urea solution, resulting in a desirable increase of the urea concentration.

[0116] In an embodiment of the urea production plant, the plant comprises a liquid flow line for urea solution from the urea synthesis section to the front side chamber where the hole in which the plug is installed terminates, the liquid flow line preferably including a control valve for expanding the urea solution from HP to MP. Preferably, the liquid flow line includes a gas / liquid separation vessel (e.g. flash vessel) between the valve and the front side chamber of the submerged carbamate condensation apparatus. A background reference for heating MP urea solution comprising carbamate in the tube bundle of a HP submerged carbamatecondensation apparatus is M. Gori, “Stamicarbon’s Ultra Low Energy Design”, September 2018.

[0117] Preferably, in this embodiment, the submerged carbamate condensation apparatus comprises two tube bundles, a first tube bundle for raising steam and a second tube bundle for heating MP urea solution. Preferably, both tube bundles are U-shaped tube bundles, more preferably horizontal U-shaped tube bundles.

[0118] Preferably, at least in this embodiment, the rod is made of duplex stainless steel.

[0119] Preferably, in this embodiment, the submerged carbamate condensation apparatus has a horizontal U-shaped tube bundle, and is e.g. a pool condenser or a pool reactor.

[0120] For example, in this embodiment, the plug has a serrated outer surface of the annular part, and preferably the sealing element is a ring located in a groove in the outer surface of the annular part. For example, plug B is used in embodiments wherein the front side chamber receives MP urea solution also comprising carbamate.

[0121] Fig. 8 shows an example of such a urea production plant, i.e. of a urea production plant wherein the submerged carbamate condensation apparatus receives MP urea solution in a tube bundle. The submerged carbamate condensation apparatus (304) (illustrated as a pool condenser) comprises two tube bundles, a first tube bundle (801) wherein steam is raised and a second tube bundle (802) wherein an MP urea solution comprising carbamate is heated, thereby effecting a desirable carbamate decomposition and achieving a desired increase of the urea concentration. The plant further comprises a supply line (803) for stripped urea solution from the stripper (305), that is expanded to MP by a valve (804) and thereafter subjected to gas / liquid separation, e.g. in a flash vessel (805). The liquid is then supplied to the second tube bundle (802) of the submerged carbamate condensation apparatus (304). The arrangement of the two tube bundles in the vessel is merely an example. In practice, the first tube bundle for raising steam can be arranged below the second tube bundle for raising steam, or can be nested in each other (see, e.g., US 12,186,725).

[0122] The submerged carbamate condensation apparatus comprises a tubesheet having a front side and a back side, and, as said, a plurality of holes(channels, also referred to as boreholes) extending between the front side and the back side. The tubesheet comprises a carbon steel plate to withstand the pressure of the fluid in the shell side compartment. The tubesheet may provide a part of the pressure-containing shell of the vessel of the pool condenser, but may also be provided by a distribution chamber arranged inside the shell side compartment, e.g. as described in US 2020 / 0306663A1. This is especially useful if one of the tube bundles carries MP urea solution. However, alternative configurations are also possible in embodiments wherein a tube bundle carries MP urea solution, e.g. the design with installing a stainless steel sleeve in the tubesheet hole, and having corrosion-resistant weld overlays at both the front side and the back side of the tubesheet.

[0123] The tubesheet has a plurality of holes (channels) extending between the front side and the back side. These holes / channels are typically straight and cylindrical, and are typically boreholes. The carbamate condensation apparatus comprises a header, where fluid flows between a plurality of front side openings (and corresponding tubes of the tube bundle) and an inlet or outlet of the carbamate condensation apparatus. For each functional tube bundle (for one specific cooling fluid), the carbamate condensation apparatus comprises an inlet header and an outlet header.

[0124] The ends of the tubes are welded at the backside of the tubesheet with an internal bore weld, in particular are welded to the welded overlay or, e.g., to a distal end of a sleeve as described in US 2015 / 0086440A1. The tubes, hence, do not protrude through the tubesheet so as to have the tube ends at the front side of the tubesheet. In the projection of each tube end, a hole is provided through the tubesheet. Hence, the holes are cylindrical, perpendicular to the front and back surface of the tubesheet, e.g. horizontal in a pool condenser, and have essentially constant diameter, are straight, and connect the front side and the backside of the tubesheet. The holes thereby provide a fluid passageway for each tube end through the tubesheet.

[0125] In an embodiment, the carbon steel plate comprises, for a fluid passageway, a concave surface that is directly exposed to the fluid in the fluid passageway in operation. For a tube bundle carrying steam I steam condensate (boiler feed water), corrosion at this exposed part will be no problem. In case of a sleeve installed in the hole, e.g. a described in US 2015 / 0086440A1, this sleeve isremoved before installing the plug. Hence, the plug is generally inserted into a hole with an exposed concave surface of the tubesheet.

[0126] The tubesheet comprises a welded overlay of a corrosion-resistant steel on the backside. In case the welded overlay is a duplex stainless steel, an intermediate layer of austenitic steel is, for example, applied between the carbon steel plate and the welded overlay to ensure proper welding.

[0127] In the present invention, the plug is used to provide a sealing of the liquid passageway between the internal bore weld and the carbon steel plate, i.e. at the transversal position of the welded overlay and / or intermediate layer. Thereby, advantageously, any corrosive fluid (in particular, carbamate-containing liquid) entering the tube from the shell side compartment (through a puncture of the tube) does not cause corrosion of the exposed concave surface of the carbon steel plate. In other words, the plug is located, at least with the part of the plug that provides for the liquid seal, at a transversal position between the internal bore weld and the carbon steel plate, i.e. at the part of the tubesheet that is constituted by the welded overlay and, optionally, the intermediate layer, and hence the plug, or at least the part of the plug providing the liquid seal, is arranged particularly at the transversal position of the welded overlay and / or the intermediate layer. The liquid seal is the barrier for the high pressure liquid, which liquid, in case of a tube leakage, enters from the shell side space into the tubes and that approaches the plug from the distal side.

[0128] The thickness of the carbon steel plate is e.g. between 30 and 60 cm. The thickness of the welded overlay is e.g. 10 to 30 mm, for instance 15 mm to 25 mm, or for instance 22 to 25 mm. It was found that this thickness of the welded overlay (including the possible intermediate layer) provides for sufficient thickness to position the plug, or at least the part of the plug (i.e. the tapered part of the annular pat) that is compressed against the concave surface of the hole in the tubesheet to provide for a seal in order to protect the carbon steel plate against corrosive fluid entering the tube from the shell side compartment through a puncture in the tube.

[0129] Accordingly, the plug is to be placed in the hole in the tubesheet at a precise depth (transversal position). For example, the rod is provided, at a proximal end, and at a precisely determined position in the length of the rod, with a radially extending protrusion, which is larger than the hole diameter. Thereby, the tubecan not be pushed further into the hole than the protrusion permits. The protrusion can be provided, e.g., by a nut that is mounted on a proximal threaded portion of the rod.

[0130] In an embodiment wherein the tube end is welded to the welded overlay with an internal bore weld, preferably, an annular groove is provided at the tube backside around the welded overlay position. The annular groove provides for stress relief. This is known as a castellation groove. A background reference in this respect is the paper “Importance of proper BFW quality in Pool Condenser Plants”, Stamicarbon Symposium 2022, in particular Fig. 17 thereof. The inner diameter of the annular groove is preferably equal to the outer diameter of the tube end, such that the welded overlay comprises an annular shoulder with an inner diameter equal to the tubesheet hole diameter, having a distal surface at which the internal bore weld is provided, and an outer diameter that is the same as the inner diameter of the annular groove. The annular groove has a depth in the transversal direction, e.g. a depth of 0.3 x to 1.0 x the tube hole (borehole) radius. The annular groove has a proximal end (the bottom of the groove, when viewed from the tube bundle).

[0131] In some embodiments, the tubesheet is provided with sleeves in the holes, e.g. in case of a design of a pool condenser with a sleeve as described in US 2015 / 0086440A1. Preferably, in this embodiment, the sleeve is removed before installing the plug, e.g. by drilling. Hence, when the plug is installed, the concave surface of the tubesheet is directly exposed to the hole. The concave surface of the hole is for example also thoroughly cleaned, e.g. brushed, to ensure the proper inner diameter.

[0132] The method comprises inserting the plug in one of said holes in the tubesheet, at a transversal position distal from the carbon steel plate. In other words, the plug is inserted in the hole beyond the carbon steel plate, e.g. at the position of the welded overlay or even somewhat in the tube. For example, the plug is inserted in a hole of the tubesheet, at the transversal position of the welded overlay and / or intermediate layer, more precisely such that part of the plug providing the liquid seal is in contact with the welded overlay and / or the intermediate layer.

[0133] The plug is inserted with the wide end first, i.e. with the wide end in the distal direction, facing the tubesheet backside. Generally, before inserting the plug,the annular part is mounted on the plug body with the distal side of the annular part in the direction of the wide end of the plug body.

[0134] The method typically involves bringing the outer surface of the frustoconical plug body in contact with the mating inner surface of the tapered part of the annular part, by moving the plug body relative to the annular part in the proximal direction, and subsequently exerting a force on the plug body in the proximal direction, while maintaining said contact between the mating surfaces (i.e., contacting surfaces) to compress the annular part against the concave surface of the tubesheet hole. At this stage, the plug body in contact with the annular part at the mating, oblique surfaces, acts as a wedge, converting the force in the proximal direction into a radially outward force applied on the annular part.

[0135] In the tube plugging method, the plug body is forcibly moved into the annular part by the pulling of the plug body, wherein the pulling continues after the plug body and the annular part contact each other at the mating tapered parts of both. This causes the deformation of the annular part to lock the plug in place.

[0136] By reference to Fig. 2, the method comprises thereafter pulling the plug body (205), with the rod, in the proximal direction, i.e. towards the front side (2a) of the tubesheet, thereby compressing the annular part (201) at least in the tapered part (204) thereof against said concave exposed surface (2c) of the tubesheet, thereby forming a pressure-resistant seal between the outer surface of the annular part and the tubesheet. Hence, the method involves bringing the mating outer surface of the plug body and inner surface of the annular part into contact with each other, i.e. in mating contact with each other, and thereafter further pulling of the plug body into the annular part. After the pulling, the wide end (207) of the plug body (205) and / or the distal end of the annular part (201) is at a transversal position between the internal bore weld (7) and the carbon steel plate (4). And hence, by reference to Fig. 1, at the transversal position of the welded overlay (5).

[0137] Advantageously, there is no need for welding the plug in the tubesheet hole using welding internally in the hole at a significant depth, i.e. in a hardly accessible place.

[0138] The tube to be plugged can be plugged e.g. because wall thickness loss has been observed, or because it already exhibits a leak. Generally, the tube to plugged is punctured to provide a positive leak before installing the plug.

[0139] In a preferred embodiment, a preferred plug, is used that is referred to as “Plug A” hereinafter.

[0140] The annular part optionally comprises a sleeve extension part, especially for the embodiment of plug A. The length of the sleeve extension part is e.g. between 30 cm and 60 cm. The length of the sleeve is preferably at least 90% of the thickness of the carbon steel plate of the tubesheet. Preferably, for the installed plug, the sleeve is located with the proximal end at the front side of the tubesheet. The sleeve extension part is also a tubular part, typically with the same outer diameter as the annular part, and extends the annular part in the proximal direction.

[0141] For the embodiment of plug A, the wall thickness of the non-tapered part of the annular part is e.g. 1 mm to 3 mm. The tapering of the annular part is e.g. a slope in the range of from 1% to 10%. The tapering angle is e.g. in the range of 0.5° to 5°.

[0142] The annular part has a maximum inner diameter at the distal end, where the tapering is maximum. Preferably, the annular part has a constant outer diameter of the tapered part, and the wall thickness is at a minimum at the distal end.

[0143] The tapered part of the annular part extends, in the length (transversal) direction of the annular part, preferably over 5 to 20 mm.

[0144] The oblique tapered inner surface of the tapered part of the annular part preferably extends, in the radial direction, over at least 50% or at least 70% or at least 90% of the radial thickness of the annular part at the proximal side of the tapered part of the annular part. Preferably, the plug body outer surface is oblique and extends in the radial direction over a radial distance (i.e., has a radial size) that is at least 50%, or at least 70%, or at least 90% of the radial size of the oblique tapered inner surface of the tapered part of the annular part, or even over a radial distance larger than the radial size of the oblique tapered inner surface of the tapered part of the annular part.

[0145] Preferably, when the plug is installed, the tapered part of the annular part and the plug body have contacting surfaces, that are oblique in the transversal cross section, and that have a radial size (extend in radial direction over a length) that is at least 50%, or at least 70%, or at least 90% of the radial thickness of theannular part at the proximal side of the tapered part of the annular part.Preferably, the outer surface of the plug body is in contact with the oblique inner surface of the tapered part of the annular part over a radial distance that is at least at least 50% or at least 70% or at least 90% of the radial thickness of the annular part at the proximal side of the tapered part of the annular part.Furthermore, preferably in these embodiments, the tapering angle is in the range of 0.5° to 5°.

[0146] The preferred relatively large radial size of the contacting surfaces, especially relative to the thickness of the annular part, with furthermore preferably this tapering angle or slope, contributes to the good clamping of the plug, so as to withstand transversal forces by HP fluid in the tube when the tube-plugged submerged carbamate condensation apparatus is operated again.

[0147] Fig. 9 schematically shows the tapering angle (901) of the inner surface (703) of the tapered part (204) of the annular part (201) in an example embodiment of the invention. The oblique inner surface (704) extends (902) in the radial direction (R) over about 50% of the thickness (903) of the annular part, said thickness of the annular part being the thickness at the proximal side of the tapered part.

[0148] This steepness and length of the tapering ensure that the tapered part of the plug and the tapered part of the annular part act as a wedge, converting the pulling force into a force radially pushing outwards to expand the annular part at the tapered part thereof, thereby pushing the outer surface of the annular part against the concave surface of the hole in the tubesheet, which locks the plug in place and ensures the plug withstands the pressure of the HP carbamate when the tube-plugged submerged carbamate condensation apparatus is operating.

[0149] The annular part for example has a smooth outer concave surface.

[0150] Optionally, the concave outer surface of the annular part is provided with circumferential serrations or grooves.

[0151] The inner surface of the annular part, at the tapered part, is optionally provided with circumferential serrations or grooves, or is e.g. smooth.

[0152] The annular part is preferably made of a duplex stainless steel.

[0153] The dimensions of the sleeve are given for the annular part prior to being installed, i.e. prior to being deformed by the pulling of the plug.

[0154] The rod of the embodiment with “plug A” is preferably of such length to extend (protrude) from the tubesheet front side after installing the plug. The rod is a part of the plug and stays in the tube-plugged submerged carbamate. The diameter of the rod is usually 10 – 60 % of the inner diameter of the annular part and of the inner diameter of the sleeve extension part, e.g. 10 to 30 % thereof. The rod extends through the sleeve extension part. The rod, for example, has threaded parts at either end or, preferably, both ends.

[0155] The plug A preferably furthermore comprises tensioning means for pulling the plug body to the tubesheet front side. The pulling involves pulling the plug body through the hole in the tubesheet, with relative motion between the moving plug body and the tubesheet, which does not move. For instance, a ring and a nut(s) are used, which engage the proximal end of the rod and contact the tubesheet front side, such that rotating the rod causes a transversal motion of the rod in the proximal direction, and hence a force in the proximal direction on the plug body.

[0156] Fig. 10 schematically illustrates an example tensioning means (1001) useful in an embodiment of the invention. These tensioning means (1001) are located at the front side (2a) of the tubesheet (2), comprising a nut (1002) received by a threaded portion (1003) of a proximal end (1004) of the rod (210) and further comprising a ring (1005). The ring has an outer diameter larger than the diameter of the hole (3). When installing the plug, the rod is pushed in the proximal direction until the ring contacts the tubesheet frontside and stops the pushing at a desired transversal position of the plug. Subsequently, the nut can be rotated to pull the plug in the proximal direction (to the front side). Optionally, the ring and nut are welded to the tubesheet front side, but this weld does not need to withstand the thrust of the HP carbamate-containing liquid when operating the tube-plugged submerged condensation apparatus by virtue of the plug mechanically locking in place.

[0157] Advantageously, when the tube-plugged submerged carbamate condensation apparatus is operated, the thrust of the high pressure fluid at the distal side of the plug is discharged on the tapered portion of the annular part and therefrom on the welded overlay and any intermediate layer of the tubesheet. Accordingly, the tensioning means do not need to be able to withstand thesethrusts. Although the tensioning means could be welded to the front side of the tubesheet, this weld (if any) does not need to withstand these forces.

[0158] Preferably, the ring essentially closes the opening of the hole in the tubesheet at the front side. Preferably, the ring comprises a small hole for leak detection. In this embodiment, in case of fluid (gas or liquid) passing in the proximal direction through the plug, the fluid flows through the preferably used sleeve extension part and through the leak detection hole in the ring, and the fluid is received in the front side chamber to which the tube is connected. This can be monitored, e.g. by a conductivity increase of steam condensate if that front side chamber is used for boiler feed water I steam.

[0159] In an embodiment wherein the sealing element is a sheet with at least one opening, the plug preferably comprises a clamping element having a stem and a radially extending part. This embodiment is particularly preferred for the embodiment with “plug A”. The stem protrudes through said opening in the sealing element, fixing the transversal position of the radially extending part relative to the plug body. The radially extending element extends radially from the stem, and provides a recess for the sealing element between the plug body and the radially extending element, and prevents undesired movement of the sealing element in the distal direction. The radially extending element is hence configured for clamping the sealing element against the flat surface of the wide end of the plug body. In this embodiment, the sealing element is arranged adjacent to the plug body. An example is illustrated in Fig. 5.

[0160] The clamping element is provided e.g. by a bolt and a nut (providing the stem respectively the radially extending part), with the bolt received by a threaded hole at the distal end (wide end) of the plug body and extending through the opening in the sealing element, and further received by a threaded hole of the nut. In a further embodiment, the stem of the clamping element is provided by a (second) rod, preferably a tie rod, which extends through a channel through the plug body in the length direction of the plug body, and the radially extending part is e.g. a nut provided at the distal end of the second rode, or is unitary with the distal end of the second rod. At the proximal side, the second rod may extend coaxially through a channel through the (first) rod, beyond the front side opening of the tubesheet, and be provided with a nut and bolt. In this embodiment, the nut and the bolt of the second rod are accessible at the front side of the tubesheet andcan be used for tensioning the second rod, to clamp the sealing element more strongly against the wide end of the plug body.

[0161] In a preferred embodiment, a plug called “plug B” is used, and the annular part comprises serrations at the outer surface, provided by circumferential grooves on the outer surface, at the tapered part of the annular part, and preferably the tapered part of the annular part extends over at least 90% of the length of the annular part. Preferably, no sleeve extension part is used in this embodiment. Preferably, the serrations extend over at least 50% of the length of the tapered part (in the transversal direction), or over at least 90 % of that length, preferably the entire outer surface of the annular part is serrated.

[0162] Preferably, in this embodiment (plug B), the serrations are provided by ridges between the grooves, with the ridges typically having a thickness in the transversal direction, at the basis (radially inward) of less than 2 mm, and at the tip (radially outward) of less than 0.5 mm. The ridges are typically spaced apart from each other, in the transversal direction, by 0.2 – 5.0 mm, by grooves. Hence, alternative ridges and grooves are provided. In other words, the ridges separate the grooves. The ridges preferably all have the same outer diameter. The grooves typically have a width in the transversal direction of 0.5 – 5.0 mm, and in the case of at least three grooves, i.e. including a proximal groove, a distal groove, and one or more inner grooves, the inner grooves preferably have a width of less than 2.0 mm.

[0163] The ridges preferably have an angle (in transversal cross-section) in the range of 20° to 80°, preferably in the range of 30° to 60°.

[0164] The dimensions are specified for the plug before installation.

[0165] The number of grooves is e.g. at least 3, or at least 4, or at least 5, generally up to 8 or up to 6 grooves, such as 3 - 6 grooves, wherein preferably one groove of these grooves is occupied by a sealing ring.

[0166] Optionally, the most distal groove has a different width, and correspondingly, the most distal ridge has a larger spacing, namely to accommodate a sealing O-ring. Preferably, the distal ridge of the distal groove has a smaller outer diameter than the proximal ride of the distal groove.

[0167] The grooves on the outer surface contribute to good locking in place of the plug, in the transversal position, so that the installed plug is able to withstandhigh pressures, in particular at the distal side (and high pressure differences between the distal and the proximal side). A background reference for such grooves is US 4,425,943 A.

[0168] Preferably, the annular part on the outer surface comprises deformable ridges that extend circumferentially about the outer surface (periphery) of the annular part, and the annular part also comprises a plurality of grooves disposed between said ridges. Preferably, said ridges are defined by surfaces which converge radially outward, more preferably to form a substantially V-shaped cross-section having a slender outermost tip portion capable of being deformed relative to the tubesheet concave surface without substantially penetrating the tubesheet concave surface when the annular part is forced radially outward against the concave surface of the tubesheet. Preferably, said ridges cooperate with the tubesheet concave surface when the plug body, as a wedge, is forced into said annular part, preferably wherein the ridges deform for providing a fluid-tight seal between the outer surface (periphery) of the annular part and the concave surface of the tubesheet, whereby a leak-resistant joint is provided between the tubesheet and the plug.

[0169] Preferably, the length of the annular part is 20 mm to 30 mm.

[0170] Preferably, the thickness of the annular part is 1.5 mm to 2.5 mm, i.e. the difference between the inner diameter and the outer diameter of the annular part.

[0171] Preferably, the tapering of the inner surface of the annular part is 5 % to 10 % slope.

[0172] Preferably, the tapered part of the plug body has the same slope as the inner surface of the annular part.

[0173] Preferably, the sealing element is an O-ring that is arranged in one of those grooves on the outer surface of the annular part, more preferably in the most distal groove. Thereby, the O-ring protects a plurality of ridges and grooves, located on the annular part outer surface proximal to the O-ring, against the risk of crevice corrosion.

[0174] Fig. 11 schematically illustrates an example of a plug with a serrated outer surface of the annular part, useful in embodiments of the invention.References correspond to those in Fig. 2. The outer surface (702) of the annularpart (201) is provided with serrations (1101). These serrations are provided by ridges (1102) and circumferential grooves (1103).

[0175] Optionally, the inner surface of the annular part also comprises circumferential grooves. A background reference is US5437310A.

[0176] A background reference for the design of the plug body and the annular part is De Rijk (2012), 57th AIChE Safety in Ammonia Plants and Related Facilities Symposium, pp. 295-300.

[0177] The tapering angle of the annular part optionally varies over the length of the annular part, i.e. in the transversal direction, particularly becomes steeper from the distal end to the proximal end. A background reference in this respect is US 2022 / 0282819A.

[0178] Preferably, the rod comprises a breakaway element, especially for plug B. This element breaks when sufficient pulling force is applied. This ensures that the operator does not stop pulling without applying sufficient force. It also protects the tubesheet from damage caused by excessive pulling. A background reference for the breakaway element is US 5,437,310 A and US 4,425,943 A.

[0179] In a preferred embodiment wherein the tubesheet comprises at the backside a castellation annular groove, as described hereinbefore, the annular part of the plug is preferably located proximal to the proximal end of the groove, i.e. proximal to the “bottom’ of the annular groove. In particular, preferably, the distal end of the annular part is located proximal to the proximal end of the annular groove, when the plug is installed, such that the compressing of the annular part against the concave surface of the tubesheet hole does not distort the shoulder of the castellation.

[0180] Figure 12 schematically shows the tapered part located at a transversal position in the tubesheet borehole. The tapered part (204) of the annular part is located with a distal end (202) proximal from (more to the frontside of the tubesheet than) the proximal end (1202) of the annular groove (1201) around the internal bore weld (7), i.e. proximal of the ‘bottom’ of that annular groove, and is also located proximal of the proximal end (1204) of the shoulder (1203), which shoulder is in the radial direction provided between the groove (1201) and the borehole (3). The internal bore weld (7) is provided between the tube end (6a) and the distal end (1205) of that shoulder (1203). The tapered part (204) is also locateddistal to the carbon steel plate (4), i.e. entirely between the proximal end (1202) of the groove (1201) and the carbon steel plate, at the position of the welded overlay (5) and the intermediate layer (1206). Thereby, tapered part (204) is arranged at a part of the tubesheet (2) that is sufficiently strong to withstand the forces when the annular part is compressed against the concave exposed surface (2c) of the tubesheet.

[0181] Figure 13 schematically shows an annular part (201) that is provided with serrations (1101). These serrations are provided by ridges (1102) and circumferential grooves (1103). The most distal serration (1301) is used to hold a circumferential sealing ring (601). Hence, the sealing ring (601) is located at the side of the wide end (207) of the plug body (205). The distal ridge (1302) of the most distal serration (1301) has a smaller outer diameter than the more proximal ridges (1102). Hence, the ridge has a lower elevation (in the radial direction) than the next proximal ridge. In this way, there is a gap (in the radial direction) between the distal ridge (1302) and the tubesheet, in particular the welded overlay (5) once the plug is installed. Hence, the distal ridge (1302) does not contact the concave exposed surface of the tubesheet. The liquid seal of the plug is provided by the sealing ring (601). The smaller diameter of the distal serration (1301) allows for easier mounting of the sealing ring (601).

[0182] Also disclosed is a plug for tube plugging of a shell- and-tube heat exchanger, operated with process fluid in the tube bundle, the plug comprising a frustoconical plug body and a tapered annular part, wherein the annular part has a decreasing inner diameter in the proximal direction, the plug further comprising a sealing element, e.g. sealing ring, at a proximal side of the tapered annular part. This plug is installed at a proximal side of the tubesheet. The tubes of the tube bundles extend through the holes in the tube sheet and terminate at the proximal side of the tube sheet. The tube ends are typically welded to the proximal side of the tube sheet. The proximal side of the tube sheet is typically provided with a corrosion-resistant layer, such as a welded overlay. Generally, further details are the same as for the inventive plug. The heat exchanger is, e.g., a HP stripper, a HP scrubber, or a falling-film, vertical tube bundle, HP carbamate condenser of a urea plant, which units are each operated with a HP carbamate-containing urea liquid stream (for the HP stripper) or HP carbamate solution (for the HP scrubber and falling-film HP carbamate condenser) in the tube bundle and the frontsidechamber(s), and with water / steam in the shell side space at lower pressure than the fluid in the tubes.

[0183] As used herein, ‘carbamate’ refers to ammonium carbamate, as that term is used in the field of urea production.

[0184] High pressure (HP) indicates above 100 bara (bar absolute), medium pressure (MP) indicates 10 – 70 bar absolute, preferably 15 – 35 bar absolute, low pressure (LP) is 1 – 10 bar absolute, preferably 2 – 8 bar absolute.

Claims

Claims1. A method of tube plugging of a submerged carbamate condensation apparatus (1) of a urea plant, the submerged carbamate condensation apparatus comprising:a tubesheet (2) having a front side (2a) and a back side (2b), the front side defining the proximal side and the back side defining the distal side of the tubesheet in a transversal direction, wherein the tubesheet has holes (3) extending between the front side and the back side, said holes (3) providing a concave exposed surface (2c) of the tubesheet (2), the tubesheet comprising a carbon steel plate (4), and wherein tubesheet comprises a welded overlay (5) at the back side,a tube bundle (6) arranged at the backside of the tubesheet, wherein the tube ends (6a) are connected to the backside of the tubesheet with internal bore welds (7) aligned with said holes,the method comprising:providing a plug (8, 200), the plug comprising:o an annular part (201) with a distal side (202) and a proximal side (203), wherein the annular part comprises a tapered part (204) at the distal side of the annular part, wherein the inner diameter of the annular part decreases over the tapered part in the direction from the distal side to the proximal side, wherein the annular part optionally comprises a sleeve extension part (211) at a proximal side,o a frustoconical plug body (205) having a narrow end (206) and a wide end (207) and an outer surface (208) mating with inner surface of the annular part at the tapered part of the annular part wherein the frustoconical plug body comprises a part (208) with an outer diameter that is larger than the inner diameter of the annular part at the proximal end (209) of the tapered part of the annular part,o a rod (210) for pulling the plug body in the proximal direction, thereby moving the plug body into the annular part;inserting the plug (8) in one of said holes (3) in the tubesheet at a transversal position distal from the carbon steel plate (4), with the wide end (207) of the plug body in the distal direction and with the tapered part (204) of the annular part in the distal direction,pulling the plug body (205), with the rod, in the direction of the front side (2a) of the tubesheet, thereby compressing the annular part (201) at least in the tapered part (204) thereof against said concave exposed surface (2c) of the tubesheet, thereby forming a pressure-resistant seal between the outer surface of the annular part and the tubesheet, wherein after the pulling, the wide end (207) of the plug body (205) and / or the distal end of the annular part (201) is at a transversal position between the internal bore weld (7) and the carbon steel plate (4).

2. The method of claim 1, wherein the submerged carbamate condensation apparatus has a U-shaped tube bundle.

3. The method of claim 1 or 2, wherein the submerged carbamate condensation apparatus has a horizontal tube bundle, preferably a U-shaped tube bundle with horizontal leg parts of the tubes.

4. The method according to any of the preceding claims, wherein the submerged carbamate condensation apparatus is a high pressure carbamate condensation apparatus comprised in a synthesis section of the urea plant, the urea plant being of the stripping-type, and the synthesis section further comprising a high pressure stripper and a reaction zone.

5. The method of claim 4, wherein the submerged carbamate condensation apparatus is a pool condenser or a pool reactor and has a horizontal U-shaped tube bundle.

6. The method of any of the preceding claims, wherein the submerged carbamate condensation apparatus comprises at least two tube bundles for different cooling fluids.

7. The method of claim 5, wherein the submerged carbamate condensation apparatus comprises at least two tube bundles for different cooling fluids, wherein the tube bundle comprising the tube to be plugged is connected to receive a medium pressure urea solution comprising carbamate from a urea synthesis section of the urea plant.

8. The method of any of the preceding claims, wherein the plug comprises a sealing element (501), wherein the sealing element:has an outer diameter that is equal to or larger than the outer diameter of the annular part,is made of a non-steel material, preferably of a polymeric material, and is arranged in a distal part of the plug.

9. The method of any of the preceding claims, wherein the plug comprises the sleeve extension part, and wherein preferably the outer surface of the annular part is smooth, more preferably wherein said outer surface is smooth over the tapered part and over the sleeve extension part of the annular part.

10. The method according to any claims 1-8, wherein the annular part of the plug is serrated on the outer surface and comprises circumferential grooves and ridges on the outer surface.

11. The method of claim 10, wherein the plug comprises a sealing element as defined in claim 8, wherein the sealing element is provided as an O-ring in a most distal circumferential groove on the outer surface of the annular part.

12. The method of claim 10 or 11, wherein the tapered part of the annular part extends over the entire length of the annular part.

13. A submerged carbamate condensation apparatus comprising:a tubesheet (2) having a front side (2a) and a back side (2b), the front side defining the proximal side and the backside defining the distal side of the tubesheet in a transversal direction, wherein the tubesheet has holes (3) extending between the front side and the back side, said holes (3) providing a concave exposed surface (2c) of the tubesheet (2), the tubesheet comprising a carbon steel plate (4), and wherein tubesheet comprises a welded overlay (5) at the back side,a tube bundle (6) arranged at the backside of the tubesheet, wherein the tube ends (6a) are connected to the backside of the tubesheet with internal bore welds (7) aligned with said holes,wherein at least one tube of the tube bundle is plugged with a plug, wherein the plug comprises:o an annular part (201) with a distal side (202) and a proximal side (203), wherein the annular part comprises a tapered part (204) at the distal side of the annular part, wherein the inner diameter of the annular part decreases over the tapered part in the direction from the distal side to the proximal side, wherein the annular part optionally comprises a sleeve extension part (211) at a proximal side,o a frustoconical plug body (205) having a narrow end (206) and a wide end (207) and an outer surface (208) mating with inner surface of the annular part at the tapered part of the annular part wherein the frustoconical plug body comprises a part (208) with an outer diameter that is larger than the inner diameter of the annular part at the proximal end (209) of the tapered part of the annular part,o a rod (210) for pulling the plug body in the proximal direction, thereby moving the plug body into the annular part;wherein the plug (8) is provided in one of said holes (3) in the tubesheet at a transversal position distal from the carbon steel plate (4), with the wide end (207) of the plug body in the distal direction and with the tapered part (204) of the annular part in the distal direction,wherein the annular part (201) is compressed at least in the tapered part (204) thereof against said concave exposed surface (2c) of the tubesheet, thereby forming a pressure-resistant seal between the outer surface of the annular part and the tubesheet,and wherein the wide end (207) of the plug body (205) and / or the distal end of the annular part (201) is provided at a transversal position between the internal bore weld (7) and the carbon steel plate (4);and preferably:wherein the submerged carbamate condensation apparatus is as defined in any of claims 2-7, and / orwherein the plug comprises the sealing element as defined in claim 8, and / or wherein the plug is as defined in any of claims 9 – 12.

14. A carbamate condensation process carried out in a submerged carbamate condensation apparatus according to claim 13, the process comprising:supplying gas comprising CO₂ and NH₃ to the shell-side compartment of the submerged carbamate condensation apparatus, condensing the gas into carbamate in said shell-side compartment, andsupplying one or more cooling liquids to one or more tube bundles of the submerged condensation apparatus.

15. A urea production plant of the stripping-type, comprising a urea synthesis section comprising a reaction zone, a stripper, and a submerged carbamate condensation apparatus according to claim 13.

16. A urea production process carried out in a urea production plant according to claim 15, the process comprising:supplying NH3feed to the urea synthesis section, e.g. to the shell side compartment of the submerged carbamate condensation apparatus, supplying CO2feed to the urea synthesis section, e.g. at least in part to the stripper,operating the synthesis section at a high pressure (HP) of at least 100 bar and a temperature of at least 160°C, typically 180 – 220°C, to form a urea synthesis solution in the reaction zone of the said synthesis section,subjecting the urea synthesis solution to stripping in the stripper, supplying gas from the stripper to the shell-side compartment of the submerged carbamate condensation apparatus, condensing the gas from the stripper into carbamate in said shell-side compartment, andsupplying one or more cooling liquids to one or more tube bundles of the submerged condensation apparatus.

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