Electrical isolation flange

Abstract

This invention provides an electrical isolation flange (20) for a directly heated pipe (16) of process fluid. The flange (20) comprises: an upper flange member (22); a lower flange member (24); a non-conductive spacer (26) disposed between the upper and lower flange members (22, 24); a first sealing gasket (28) positioned between the spacer (26) and the upper flange member (22); and a second sealing gasket (28) positioned between the spacer (26) and the lower flange member (24). The spacer (26) comprises topographical features, such as ridges, fins or corrugations, along an outer surface thereof to increase the creepage distance between the upper and lower flange members (22, 24).

Description

[0001] SP3201

[0002] 1

[0003] ELECTRICAL ISOLATION FLANGE

[0004] Field of the Invention

[0005] This invention relates to an electrical isolation flange for a directly electrically heated pipe of process fluid, and to a direct electrical heating system for process fluids .

[0006] Background of the invention

[0007] Fluids in chemical processes often require heating in order for the desired reaction or process to take place . One type of an electrical process heater involves placing heater cables arranged in bundles directly immersed into the process fluid, like in an electric kettle . This is referred to as convective heating . This method has many disadvantages including the development of hotspots due to nonhomogeneous flow and contact with the heating element within the process heating vessel resulting in overheating the process fluid . Contact with the heating element in particular can lead to issues including coking, element damage, element burnout, vessel overpressure, and other failure mechanisms .

[0008] Another method is heating the process fluids in a pipe or coil externally using flames from a gas burner or electrical heating coils referred to as radiant heating . This improved method of heating eliminates the need for a process vessel and heats the process fluid directly inside a tube, coil, or pipe containing the process material to be heated .

[0009] Using a gas burner is inefficient and results in non-uniform heating of the process fluid . Utilising the impedance heating or direct electrical heating method by passing an electrical current through the process tube walls more uniformly heats the tube to the targettemperature, eliminating the need for high temperature radiant heating from burners or process vessels . Using this direct electrical heating, the maximum temperature required for the process is lower due to the improved heat transfer efficiency, resulting in substantially reduced material failure risks . Direct electrical heating of the tubes also can be powered by renewable electricity, substantially reducing carbon emissions compared to combustion fired furnaces and process vessels .

[0010] The use of direct electrical heating for the coils of process fluids requires the coils to be electrically isolated from the rest of the fluid circuit due to the voltage that is passed through the walls of the coils in order to generate the heating . Providing electrical isolation in the context of the heating of process fluids , where the temperatures of the fluids can be in excess of 900 °C and the voltages through the coils can reach 15 kV, presents challenges for electrical isolation of the heated coils due to the high voltages and high temperatures present in the system.

[0011] Electrical isolation methods exist in other fields of technology. For example, cathodic protection systems on structures or equipment such as tanks , process vessels, pipelines and offshore structures employ a sacrificial anode which is preferentially corroded to protect the structure which acts as a cathode . Where passive cathodic protection is insufficient, an external DC power source biases the electrochemical cell to provide sufficient current .

[0012] Where multiple such cathodic protection systems are in place on the same piece of equipment, they must be electrically isolated from each other . However, the temperature requirements in such systems are limited to ambient conditions up to approximately 150 °C, while thebiasing voltage is typically low, at a maximum of 50 V. The isolation flanges used therefore will not provide the necessary electrical isolation for process heating applications .

[0013] In another example, direct electrical heating is also used for flow assurance in subsea pipelines , where low temperatures can lead to the formation of deposits that block the pipeline . However, in this scenario, the temperature and voltage requirements are significantly lower than for heating process fluids and so the requirements for electrical isolation are also significantly less intensive .

[0014] For example, US 6315697 describes apparatus for electrically isolating a direct electrical heating system for a subsea hydrocarbon production pipeline . The structure is dependent on the use of O-rings as sealing structures and so is not sufficient for high temperature applications over 300 °C .

[0015] It is an aim of the current invention to provide an electrical isolation system that enables more efficient heating of process fluids .

[0016] Summary of the Invention

[0017] According to an aspect of the invention, the invention provides an electrical isolation flange for a directly heated pipe of process fluid . The flange comprises : a first, upper flange member; a second, lower flange member; a non-conductive spacer disposed between the upper and lower flange members , wherein the spacer comprises topographical features along an outer surface thereof to increase the creepage distance between the upper and lower flange members along the outer surface of the spacer; a first sealing gasket positioned between the spacer and the upper flange member; and a second sealinggasket positioned between the spacer and the lower flange member .

[0018] The electrical isolation flange of the invention enables direct electrical heating to be utilised for heating process fluids up to 900 °C using voltages of up to 15 kV.

[0019] The topographical features along the outer surface of the spacer may increase the creepage distance between the upper and lower flange members by at least 50% , preferably by at least 100% .

[0020] The electrical isolation flange may further comprise a fixation mechanism configured to generate a first seal between the first sealing gasket, the upper flange member and the spacer, and a second seal between the second sealing gasket, the lower flange member and the spacer .

[0021] The fixation mechanism may comprise at least one opposed pair of bolts , wherein a first bolt of the opposed pair of bolts is received in a bore running through the upper flange member and a second bolt of the opposed pair of bolts is received in a bore running through the lower flange member .

[0022] The first and second bolts may be additionally received in respective first and second blind bores in a bushing made of a non-conductive material . In this case , the

[0023] the first and second bolts may be made of steel .

[0024] Like the spacer, the bushing may also comprise topographical features along an outer surface thereof to increase the creepage distance between the upper and lower flange members along the outer surface of the bushing .

[0025] The topographical features along the outer surface of the bushing may intertwine with the topographical features along the outer surface of the spacer . In this way, topographical protrusions associated with the spacermay be received in topographical recesses associated with the bushing and vice versa .

[0026] The bushing may be made of a ceramic material . The spacer may also be made of a ceramic material .

[0027] The first sealing gasket and / or the second sealing gasket may be a double layer compression sealing ring .

[0028] According to a second aspect of the invention, the invention provides a direct electrical heating system for process fluids . The system comprises : an inlet header; an outlet header; a plurality of directly electrically heated pipes extending between the inlet header and the outlet header; and an electrical isolation flange as described above at each end of each of the plurality of directly electrically heated pipes .

[0029] The direct electrical heating system may further comprise at least one linking cable between the lower flange members of at least one pair of neighbouring electrical isolation flanges .

[0030] Brief Description of the Drawings

[0031] Figure 1 schematically illustrates a process fluid heating system; and

[0032] Figure 2 schematically illustrates an electrical isolation flange .

[0033] These drawings depict one or more implementations in accordance with the present teachings , by way of example only, not by way of limitation . In the figures, like reference numerals refer to the same or similar elements .

[0034] Detailed Description of the Drawings

[0035] Figure 1 schematically illustrates a heating system 10 for process fluids . As can be seen, the heating system 10 comprises an inlet header 12 and an outlet header 14 . The two headers 12 , 14 are connected by a plurality of heating pipes , or coils 16. The coils are made of aconductive material and are connected to an electrical power supply (not shown) .

[0036] In operation, process fluid inside the inlet header 12 flows into the coils 16. The electrical power supply heats the coils 16 by virtue of the resistance of the walls of the coil 16, as is conventional for such systems . The voltage across each coil 16 can be altered in order to adj ust the amount of heat generated, and so adj ust the temperature to which the process fluid is heated .

[0037] Process fluids may require heating to high temperatures , for example up to 900 °C, and so the voltage across each coil 16 may be as high as 15 kV. At such high voltages, there is a significant risk of short circuits or even fire without isolating the coils 16.

[0038] As such, each coil 16 comprises two electrical isolation flanges 20 , one at each end of the coil 16. An electrical isolation flange is shown in exploded cross-sectional view in Figure 2 .

[0039] Each flange 20 comprises an upper flange member 22 and a lower flange member 24 . The lower flange member 24 is in electrical contact with the coil 16 and so is at the same potential as the coil 16, while the upper flange member 22 is at a potential of 0 V, being electrically connected to either the inlet header 12 or the outlet header 14 , depending on which end of the coil 16 the flange 20 is positioned at .

[0040] A cylindrically annular spacer 26 is sandwiched between the two flange members 22 , 24 . The spacer 26 is made from a non-conductive material in order to prevent any direct conduction between the flange members 22 , 24 . The upper and lower flange members 22 , 24 and the spacer 26 have bores 30 through their respective centres that form part of the flow path into the coils 16 for the process fluid . In order to prevent any leakage of theprocess fluid out of the isolation flange 20 , a pair of sealing gaskets 28 are positioned in between the spacer 26 and the upper and lower flange members 22 , 24 , respectively .

[0041] On or both of the gaskets 28 may take the form of a double layer compression sealing ring, as shown in top, cross-sectional , and bottom views in Figures 3a , 3b and 3c, respectively. In this configuration, the gasket 28 comprises an upper layer 28a and a lower layer 28b . As seen in Figure 3b, each of the upper and lower layers 28a, 28b have a triangular cross-sections which combine to form the rectangular cross-section of the gasket 28 as a whole . Viewed another way, the division between the upper and lower layers 28a, 28b is at the top of the gasket 28 on a radially inner side of the gasket 28 and is at the bottom of the gasket 28 on a radially outer side thereof . The slope defined by the diagonal division between the two layers 28a , 28b may be reversed . Configuring the gasket 28 in this way allows for a more even distribution of stresses over the spacer 26, helping to prevent cracks in the spacer 26.

[0042] To generate the seal, the isolation flange 20 comprises a fixation mechanism. In the example of Figure 2 , the fixation mechanism takes the form of opposed pairs of bolts 32 , which pass through bores 34 in the upper and lower flange members 22 , 24 . In each opposed pair, one bolt 32 passes through a bore 34 in the upper flange member 22 and another bolt 32 passes through a bore 34 in the lower flange member 24 .

[0043] While the bolts 32 may be received in the spacer 26, it is preferable for the isolation flange 20 to comprise a pair of insulation bushings 36, which comprise their own blind bores 38 , which receive the bolts 32 . Inclusion of the bushings 36 alleviates some of the mechanical strainon the spacer 26. Like the spacer 26, the bushings 36 are also made of a non-conductive material . Use of blind bores 38 to receive the bolts 32 also means that it is possible to manufacture the bolts 32 from metals or alloys , such as steel , which has beneficial impacts on the maximum compressive strength available to hold the isolation flange 20 together, which in turn increases the pressure rating of the heating system 10 as a whole .

[0044] While the description and figures here discuss and show an isolation flange 20 comprising two opposed pairs of bolts 32 , the skilled person will appreciate that more or fewer pairs of bolts 32 may be present depending on the needs of different applications , or that alternative fixation mechanisms may be utilised to generate the seal between the flange members 22 , 24 , the gaskets 28 and the spacer 26.

[0045] As alluded to above, the spacer 26 electrically separates the conductive flange members 22 , 24 from each other so that the upper flange member 22 remains at ground potential . However, the separation between the upper and lower flange members 22 , 24 means that the isolation flange 20 effectively acts as a capacitor .

[0046] In general with any capacitor, account must be taken of the dielectric breakdown voltage above which the dielectric material between the capacitor plates breaks down and allows current to flow through . In the case of the isolation flange, the dielectric is the spacer 26 (and the bushings 36 if present) . Dielectric breakdown would therefore result in failure of the isolation flange 20 as current would flow through the spacer 26 into the upper flange member 22 and the wider heating system 10.The breakdown voltage, Vb, of a capacitor can be expressed as :

[0047] Vb= Udd,

[0048] where Ud represents the dielectric strength of the capacitor' s dielectric material and d represents the separation between the plates of the capacitor .

[0049] Therefore , in cases where the voltage across a capacitor remains substantially constant, as is the case here with the isolation flange 20 , one way to ensure that dielectric breakdown does not occur is to ensure that the separation between the upper and lower flange members 22 , 24 is large enough that the breakdown voltage exceeds the voltage across the isolation flange 20. To achieve this , the height of the spacer 26 can be varied depending on the voltage requirements of the heating system 10 .

[0050] Another route to increase the breakdown voltage of the capacitor formed by the isolation flange 20 is to make the spacer 26 from a material with a high dielectric strength . There are a number of materials with high dielectric strengths , including glass and polymers such as polystyrene, polyvinyl chloride ( PVC) and Teflon®.

[0051] However, due to the high temperatures experienced by the coil 16 of the heating system 10 , a number of these materials are unsuitable .

[0052] Instead, because of the high-temperature properties required, ceramic materials are an attractive choice for the spacer 26 and the bushings 36, when present . Ceramic materials are renowned for their high melting points and also have very high dielectric strengths .

[0053] In addition to bulk dielectric breakdown of the spacer 26, one other way in which the isolation flange 20 can fail is through conduction along the surface of the insulating spacer 26 or the bushings 36. The creepage distance is defined as the shortest distance between twoconductors along an insulating surfaces therebetween . To increase the creepage distance, the side surfaces of the spacer 26 and the bushings 36 in between the upper and lower flange members 22 , 24 include topographical features, such as ridges , fins or corrugations . This can be seen in Figure 2 , where the spacer 26 and the bushings 36 have corrugated outer surfaces , which increase the creepage distance between the upper and lower flange members 22 , 24 .

[0054] The topographical features present on the side surfaces of the spacer 26 increase the creepage distance by at least 50% compared to the height of the spacer 26 , and preferably by at least 100% compared to the height of the spacer .

[0055] In a similar way, the topographical features present on the side surfaces of the bushings 36 also increase the creepage distance by at least 50% compared to the height of the bushings 36, and preferably by at least 100% compared to the height of the bushings 36. Alternatively, the topographical features present on the side surfaces of the bushings 36 may increase the creepage distance along the side surfaces so that the creepage distance is at least equal to that along the side surface of the spacer 26.

[0056] Conveniently, the topographical features on the outer surface of the spacer 26 may intertwine or interengage with the topographical features on the bushings 36, with the fins , ridges or other topographical protrusions of one component being received in topographical recesses of the other . This provides the isolation flange 20 with a more compact configuration .

[0057] The design of the fixation mechanism seen in Figure 2 , utilising the opposed pairs of bolts 32 received in blind bores 38 also helps to maintain an increasedcreepage distance . For example, even if the bolts 32 were made of a non-conductive material, if the fixation mechanism comprised bolts 32 that ran straight through the upper flange member 22 , the bushings 36 (or the spacer 26) and the lower flange member 24 , creepage paths would exist along the outer surface of the bolts 32 , as well as the inner surface of the bore through the bushing 36 (or the spacer 26) .

[0058] In a heating system 10 that comprises multiple coils 16, linking cables 18 may be included to electrically connect two or more of the lower flange members 24 , as shown in Figure 1. Since the lower flange members 24 are in electrical connection with a respective coil 16, this enables multiple coils 16 to be arranged electrically in series with each other . Such coils 16 can therefore be heated using a single electrical current, despite the coils 16 offering parallel flow paths for the process fluid through the heating system 10 from the inlet header 12 to the outlet header 14 .

[0059] The provision of the isolation flange 20 has a number of benefits for the heating system 10. Being able to successfully electrically isolate the coils 16 from the rest of the heating system enables direct electrical heating to be used safely for process heating purposes at high voltage and high temperature .

[0060] Since currents in parallel circuits are additive, the ability to connect each coil 16 in series enables the use of lower current in the heating system. This decreases electrical losses and improves the efficiency of the heating system 10 , reducing operating costs . The design of the isolation flange 20 is also relatively straightforward, without any sacrificial parts, which also contributes to minimise operational costs .The isolation flange 20 can also be scaled depending on the exact requirements of different heating systems 10 , simply by changing the height of the spacer 26 and the bushings 36.

[0061] While many possible variations of the heating system 10 have been described above , it will be clear to the skilled person that additional variations and modifications can be made without departing from the scope of the invention as claimed in the appended claims .

Claims

C L A I M S1 . An electrical isolation flange ( 20 ) for a directly heated pipe ( 16 ) of proces s fluid, the flange ( 20 ) compris ing :an upper flange member ( 22 ) ;a lower f lange member ( 24 ) ;a non-conductive spacer ( 26 ) disposed between the upper and lower flange members ( 22 , 24 ) , wherein the spacer ( 26 ) comprises topographical features along an outer surface thereof to increase the creepage distance between the upper and lower flange members ( 22 , 24 ) along the outer surface of the spacer ( 26 ) ;a first sealing gas ket ( 28 ) positioned between the spacer ( 26 ) and the upper flange member ( 22 ) ; anda second sealing gas ket ( 28 ) positioned between the spacer ( 26 ) and the lower flange member ( 24 ) .2 . The electrical i solation flange ( 20 ) of Claim 1 , wherein the topographical features along the outer surface of the spacer ( 26 ) increase the creepage distance between the upper and lower f lange members ( 22 , 24 ) by at least 50% .3 . The electrical i solation flange of Claim 1 or Claim 2 , wherein the topographical features along the outer surface of the spacer ( 26 ) increase the creepage distance between the upper and lower flange members ( 22 , 24 ) by at least 100% .4 . The electrical i solation flange ( 20 ) of any preceding claim, further comprising a fixation mechanism configured to generate a first seal between the firstsealing ga s ket ( 28 ) , the upper flange member ( 22 ) and the spacer ( 26 ) , and a second seal between the second sealing gas ket ( 28 ) , the lower flange member ( 24 ) and the spacer ( 26 ) .5 . The electrical i solation flange ( 20 ) of Claim 4 , wherein the fixation mechani sm comprises at lea st one opposed pair of bolts ( 32 ) , wherein a first bolt ( 32 ) of the opposed pair of bolts ( 32 ) is received in a bore ( 34 ) running through the upper flange member ( 22 ) and a second bolt ( 32 ) of the opposed pair of bolts ( 32 ) i s received in a bore ( 34 ) running through the lower flange member ( 24 ) .6 . The electrical i solation flange ( 20 ) of Claim 5 , wherein the first and second bolts ( 32 ) are additionally received in respective first and second blind bores ( 38 ) in a bushing ( 36 ) made of a non-conductive material .7 . The electrical i solation flange ( 20 ) of Claim 6 , wherein the first and second bolts ( 32 ) are made of steel .8 . The electrical i solation flange ( 20 ) of Claim 6 or Claim 7 , wherein the bushing ( 36 ) comprises topographical feature s along an outer surface thereof to increase the creepage distance between the upper and lower f lange members ( 22 , 24 ) along the outer surface of the bushing ( 36 ) .9 . The electrical i solation flange ( 20 ) of Claim 8 , wherein the topographical features along the outer surface of the bushing ( 36 ) intertwine with the topographical feature s along the outer surface of the spacer ( 26 ) so that topographical protrusions as sociated with the spacer ( 26 ) are received in topographical reces ses a ssociated15with the bushing ( 36 ) and topographical protrus ions as sociated with the bushing ( 36 ) are as sociated with topographical reces se s as sociated with the spacer ( 2 6 ) .10 . The electrical i solation flange ( 20 ) of any of Claims 6 to 9 , wherein the bushing ( 36 ) i s made of a ceramic material .11 . The electrical i solation flange ( 20 ) of any preceding claim, wherein the spacer ( 26 ) is made of a ceramic material .12 . The electrical i solation flange of any preceding claim, wherein the first sealing ga s ket ( 28 ) and / or the second sealing gas ket ( 28 ) i s a double layer compres sion sealing ring .13 . A direct electrical heating system ( 10 ) for proces s fluids comprising :an inlet header ( 12 ) ;an outlet header ( 14 ) ;a plurality of directly electrically heated pipes ( 16 ) extending between the inlet header ( 12 ) and the outlet header ( 16 ) ; andan electrical isolation flange ( 20 ) as claimed in any preceding claim at each end of each of the plurality of directly electrically heated pipes ( 16 ) .14 . The direct electrical heating system ( 10 ) of Claim 13 , further comprising at least one linking cable ( 18 ) between the lower flange members ( 24 ) of at lea st one pair of neighbouring electrical i solation flanges ( 20 ) .

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