Method for recovering flammable gas from a pipe section to be taken out of service, in particular without emissions into the atmosphere

Abstract

A method for recovering flammable gas from a pipe section (10) to be taken out of service and intercepted between two line shut-off valves (11, 12), comprising the steps of: - injecting a flow of nitrogen into the pipe section (10) at the first valve (11) so that the injected flow pushes the gas towards the second valve (12) and transfers it into the second operational pipe section (30), said injection of a flow of nitrogen resulting in the formation of a gas / nitrogen interface volume that moves and grows progressively along the pipe section (10); - detecting the gas or nitrogen concentration in a detection zone (40) situated in proximity to or in the bypass channel of the second valve (12); - detecting the arrival of the gas / nitrogen interface volume in the detection zone (40), represented by the detection of a decrease in the gas concentration to a first value lower than 100%; - upon detecting a second gas concentration value lower than the first, shutting off communication between the pipe section (10) and the second operational pipe section.

Description

[0001] DESCRIPTION

[0002] METHOD FOR RECOVERING FLAMMABLE GAS FROM A PIPE SECTION TO BE TAKEN OUT OF SERVICE, IN PARTICULAR WITHOUT EMISSIONS INTO THE ATMOSPHERE

[0003] Technical field

[0004] The present invention relates to a method for recovering flammable gas from a pipe section to be taken out of service, in particular without emissions into the atmosphere. The pipe to which reference is made here is part of a gas pipeline, i.e. a network section containing flammable gas at high or medium pressure. By way of non-exhaustive example, the flammable gas can be biogas, biomethane, hydrogen, propane, ethane, hexane and LPG gas mixtures.

[0005] Prior art

[0006] As is well known, methane gas (CH4) is a highly climate-altering element. Direct emission of methane gas into the atmosphere is 28 times more harmful than an equal weight of carbon dioxide (CO2).

[0007] The network of pipes for the transport and primary distribution of gas to catchment areas is equipped with line valves adapted to shut off pipe sections with a maximum length of 10 km. This design measure is dictated by safety reasons and regulated by ministerial decree DM 18.04.2008 (Ministry of Economic Development).

[0008] It often happens that a network of gas pipelines is subject to the need to take a section out of service: in order to create new connections for the supply or upgrading of new industrial and residential catchment areas, special maintenance, decommissioning in the case of construction of new replacement sections, lowering the installation covering following landslide movements and, much more rarely, for any necessary repairs.

[0009] Taking out of service a network section containing gas at high or medium pressure consists in shutting it off (isolation of the section) by closing line valves which are always provided with by-passes and thus can be used in evacuation for the recovery of the gas contained in the section that has been shut off.

[0010] The taking out of service is usually planned and the network operator undertakes in due time to lower the pressure of the gas to the limits allowed by the contracts for the supply to the affected users and, in the most critical cases, in which the supply must be absolutely assured, it must arrange for an emergency supply with gas cylinder trailers. In any case, taking a network section out of service is an operation to be carried out in the shortest time possible.

[0011] As regards the recovery of gas from high-pressure large diameter pipe sections, there are already mobile units present on the market which are provided with compressors that transfer the gas contained in the section to be evacuated, intercepted upstream and downstream of the aforesaid shut-off valves. However, these units do not manage to transfer all the gas, but succeed at most in depressurising the gas to a pressure of about 2 bar absolute, thus leaving in the pipe a considerable amount of gas, which is then freely vented into the atmosphere until reaching atmospheric pressure (1 bar absolute), then expelling the residual gas corresponding to the geometric volume, also freeing this into atmosphere by flushing with an ejector and a simultaneous free intake of air from the opposite terminal after the injection of a nitrogen buffer to prevent the incoming air from coming into contact with the gas.

[0012] In the case of pipe sections of smaller diameters, recompression with mobile units is not carried out because of the cost and complexity of preparing the site for accommodating such units, as the connection points (e.g. the chambers of shut-off valves) are often positioned in rural settings with very limited access. Therefore, at present, the gas, even in fairly considerable volumes, is depressurised and directly introduced into the atmosphere and the pipe section purged with an ejector as mentioned above.

[0013] Object of the invention

[0014] In this context, the technical task at the basis of the present invention is to propose a method for recovering flammable gas from a pipe section to be taken out of service which overcomes the drawbacks of the abovementioned prior art.

[0015] In particular, it is an object of the present invention to provide a method for recovering flammable gas from a pipe section to be taken out of service, wherein the emission of the gas into the atmosphere is avoided.

[0016] Another object of the present invention is to propose a method for recovering flammable gas from a pipe section to be taken out of service which is capable of totally recovering the gas present in the pipe section.

[0017] Brief description of the drawings

[0018] Additional features and advantages of the present invention will emerge more clearly from the approximate and thus non-limiting description of a preferred but not exclusive embodiment of a method for recovering flammable gas from a pipe section to be taken out of service, as illustrated in the appended drawings, in which:

[0019] - figures 1 and 2 respectively illustrate a first embodiment and a second embodiment of a method for recovering flammable gas from a pipe section to be taken out of service, according to the present invention, in a schematic view;

[0020] - figure 3 illustrates a graphic representation of a gas / nitrogen interface volume in a detection zone.

[0021] Detailed description of preferred embodiments of the invention

[0022] A description of a method for recovering flammable gas from a pipe section 10 to be taken out of service is provided below. In particular, the flammable gas is under pressure. For example, the flammable gas is preferably methane gas. However, the present invention can be applied without any particular modifications to other flammable gases, such as, for example biogas, biomethane, hydrogen, propane, ethane, hexane and gas mixtures like LPG, etc.

[0023] For the sake of clarity of illustration, the flammable gas will hereinafter be briefly referred to as “gas”. The pipe section 10 is interposed between a first operational pipe section 20 and a second operational pipe section 30. The pipe section 10 is intercepted between a first line shut-off valve 11 and a second line shut-off valve 12. Each valve 11 , 12 comprises a bypass channel through which the pipe section 10 can selectively communicate with the respective adjacent operational pipe section 20, 30. In particular, the pipe section 10 communicates with the first operational pipe section 20 via the bypass of the first valve 11. In particular, the pipe section 10 communicates with the second operational pipe section 30 via the bypass of the second valve 12. In other words, the valve 11 , 12 is closed and communication between adjacent pipe sections is possible exclusively through the bypass channel.

[0024] The pipe sections typically have a maximum length of 10 km. The diameters of the sections go from 48” (1200 mm, typical of trunk lines) to 4” (100 mm, typical of supply lines to users) and the pressure of the gas contained can range from 55 to 3 bar absolute depending on the position they are in within the national network and thus the transport service they must fulfil.

[0025] The method comprises a step of injecting a flow of nitrogen into the pipe section 10 at the first valve 11. In particular, the volume of nitrogen injected is at least equal to 3 times the geometric volume of the pipe section 10.

[0026] The injected flow pushes the gas towards the second valve 12 and transfers it, via the bypass, into the second operational pipe section 30. In other words, the gas is transferred into the operational pipe section that is located at the end opposite the one into which the flow of nitrogen is injected. In particular, the operation of moving gas by means of another gas is known as displacement.

[0027] The flow of nitrogen progressively replaces the gas in the pipe section 10 to be taken out of service.

[0028] Preferably, the step of injecting a flow of nitrogen takes place with a flow rate such that the flow of nitrogen is turbulent or in a range of values which are no lower than one order of magnitude compared to the critical Reynolds number.

[0029] The method comprises a step of detecting the gas or nitrogen concentration in a detection zone 40 situated in proximity to or in the bypass channel of the second valve 12.

[0030] The detection of the concentration preferably takes place in a percentage volume fraction. Preferably, the detection takes place by means of an online analyser of the passing fluid.

[0031] In particular, the injection of the flow of nitrogen into the pipe section 10 results in the gradual formation of a gas / nitrogen interface volume. In particular, the interface volume is intermediate between the volume of nitrogen and the volume of gas to be displaced and contains percentage volume fractions of both gases (flammable gas and nitrogen).

[0032] The interface volume moves and grows progressively along the pipe section 10 as the gas is transferred into the second pipe section 30. In fact, given that the volume of nitrogen pushes the volume of gas and replaces it, the interface volume moves accordingly.

[0033] The method comprises a step of detecting the arrival of the gas / nitrogen interface volume in the detection zone 40. This takes place with the detection of a decrease in the gas concentration in the detection zone 40 to below 100%. In other words, this step takes place with the detection of a first gas concentration value other than and below 100%. This corresponds to the detection of a nitrogen concentration other than zero in the detection zone 40.

[0034] In particular, the first value can be arbitrarily chosen. For example, the first value is 98%.

[0035] Upon the detection of a second gas concentration value in the detection zone 40, the method comprises a step of shutting off communication between the pipe section 10 and the second operational pipe section 30. In particular, the route of communication via the bypass between the two pipe sections 10, 30 is closed. The second value is chosen in agreement with the network operator already in a preliminary phase, so that a part of the interface volume (containing both a volume of gas and a volume of nitrogen) will continue to be injected into the second pipe section 30 until the shutting off. The amount injected depends on the calorific value (Wobbe index) of the gas in the pipe and thus on the available margin of tolerance in order to remain within the limits of the network code.

[0036] The detection area located in the bypass channel of the second valve 12 makes it possible to know the parameters of the gas, as well as the gas / nitrogen ratio, prior to its introduction into the adjacent operational pipe section 30. In this manner, the operator can intervene promptly by shutting off communication between the two pipe sections 10, 30 once the desired conditions are reached.

[0037] The method comprises a step of evacuating the flow of nitrogen from the pipe section 10. Preferably, this step comprises a step of depressurising the residual volume of nitrogen in the pipe.

[0038] Preferably, this step takes place at least in part simultaneously with the shutting off of communication between the pipe section 10 and the second operational pipe section 30, or following it. Preferably, the flow of nitrogen is evacuated through the bypass of the second valve 12. Preferably, the flow of nitrogen is emitted into the atmosphere by means of a vent 16. In particular, the vent 16 comprises a depressurisation port 17 or silencer.

[0039] Preferably, the method comprises a step of monitoring the formation of the gas / nitrogen interface volume in the detection zone 40. In fact, the passage of the gas / nitrogen interface volume in the detection zone 40 takes place in a time interval in which there is a progressive change in the percentage volume fractions of the two gases.

[0040] In other words, the interface volume does not evolve solely in space; in fact, it shows a variation in the gas / nitrogen concentration in space which is manifested over time with the passage through the detection zone 40.

[0041] From an experimental test carried out on a pipe section about 16.5 km long, it was possible to observe that the length of the interface in the detection zone 40 was about 176 m.

[0042] As already specified above, the shutting off of communication between the pipe section 10 to be taken out of service and the second pipe section 30 takes place upon the detection of the second gas concentration value in the detection zone 40. This second value is decided on the basis of how large a volume of nitrogen can be introduced into the adjacent section together with the gas. A series of factors influence the evaluation of the nitrogen volume, including above all the calorific value (Wobbe index) of the gas in the pipe.

[0043] This means that it is fundamental to abide by a maximum value of the volume of nitrogen to be injected into the second pipe section 30. Therefore, the step of shutting off communication between the pipe section 10 and the second operational pipe section 30 comprises a step of cutting or separating the gas / nitrogen interface volume into two parts. One part is injected into the second pipe section 30, the other remains temporarily in the pipe section 10 for subsequent evacuation and recovery of the gas remaining in the interface.

[0044] Figure 3 shows a graphic representation of the interface volume in the detection zone 40, connected to an experimental test carried out on site. The volume of nitrogen is represented by the part of geometric space on the left side of the curve, whilst the volume of gas is on the right side of the latter. The curve represents the actual interface between the gas and the nitrogen in terms of relative concentration. The dashed vertical line graphically represents the cutting of the interface at 50% of its length, thus dividing the interface volume into a front part and a rear part. In this case, the two parts are nearly symmetrical. The profile of the concentration of the two fluids along the interface makes it possible, in fact, to note that the nitrogen content in the front half of the interface volume (area highlighted in grey in figure 3) is equal to 4.2% of the entire interface volume and consequently only 8.4% of half the interface volume. Therefore, by making a cut at half the length of the interface volume one transfers a very low volume of nitrogen to the adjacent pipe section. Likewise, the half of the interface volume intended for the recovery of nitrogen in cylinder bundles or trailers has a very exiguous gas content, equal to 4.1% of the entire interface volume and consequently only 8.2% of the half.

[0045] Measuring the concentration in the detection zone 40 is fundamental, therefore, in order for the operator to be sufficiently prepared and cut the interface volume at the pre-established moment in accordance with the network operator. Under the operating conditions of the experimental test, the time of passage of the entire interface volume was about 11 minutes with a constant displacement flow rate.

[0046] Preferably, the bypass of the second valve 12 is selectively communicating with the vent 16 or with an interface gas recovery route 18. For example, the route 18 is connected to gas cylinder trailers or the like.

[0047] In accordance with one embodiment, the method comprises a step of simulating the formation of the gas / nitrogen interface volume. The simulating step comprises a step of calculating a first time for the arrival of the gas / nitrogen interface volume in the detection zone 40. In particular, the first time is the time necessary for the nitrogen to reach the detection zone 40, i.e. the time necessary to detect a decrease in the gas concentration in the detection zone 40 to less than 100%.

[0048] The simulating step also comprises a step of calculating a second time for reaching the second gas concentration value in the detection zone 40.

[0049] Simulating the formation of the interface is decisive for carrying out the above-described operations; in particular, it makes it possible to know a priori, with high precision, the time of arrival of the interface in the detection zone 40 and the time of reaching a percentage volume fraction of gas that is sufficient in order to consider the displacement and recovery operation terminated. As described above, knowing in advance the time, i.e. when the communication between the two pipe sections should be shut off, is of fundamental importance. Preferably, the simulation takes place by means of a commercial two- dimensional finite-element (FEM) calculation model, with axial symmetry, which is populated with values characteristic of the gas at stake and data on mixing kinetics. Such information, entered into the program to carry out the modelling, is within the reach of and available to the person skilled in the art, by virtue of their knowledge or experimentation.

[0050] In particular, a continuous refinement of information on the basis of the collaboration between the simulation and experimental field tests is particularly effective.

[0051] Preferably, the method comprises a step of detecting one or more physical parameters of the flow of nitrogen at the bypass of the first valve 11 . In particular, the detected physical parameters are pressure, temperature, flow rate and concentration of nitrogen. Preferably, this step takes place online by means of a first measuring block 13. Preferably, this step takes place online in the bypass channel of the first valve 11 .

[0052] Preferably, the detection of the nitrogen concentration takes place by means of an oxygen / nitrogen interface analyser. Preferably, the analyser is included in the measuring block 13.

[0053] In fact, according to the present invention it is possible to inject a flow of non-pure nitrogen, for example 95% nitrogen, into the pipe section 10. It is thus fundamental to monitor the concentration thereof prior to its injection into the pipe section 10.

[0054] Preferably, a nitrogen generator 15 communicates with the pipe section 10 via the bypass of the first valve 11. The first measuring block 13 is for example located online in the communication route.

[0055] In particular, the nitrogen to be injected can be in a liquid state and regasified on site (in the case of large volumes or high pressures of use), but it can also be produced directly on site from atmospheric air by means of special membrane generators.

[0056] Preferably, the method comprises a step of detecting one or more physical parameters of the gas at the second valve 12. In particular, the detected physical parameters are pressure, temperature and flow rate, as well as the previously mentioned gas concentration. Preferably, this step takes place online by means of a second measuring block 14. Preferably, this step takes place online in the bypass channel of the second valve 12.

[0057] Preferably, the detection of the gas concentration takes place by means of a gas / nitrogen interface analyser. Preferably, the analyser is included in the measuring block 14.

[0058] The steps of the method described thus far correspond to a first embodiment, illustrated in figure 1 , which is particularly recommended for pipes with a small diameter (for example up to 16-20”) and medium / high pressures (for example up to 25 bar).

[0059] In accordance with a second embodiment, illustrated in figure 2, the method comprises a step of reducing the volume of gas exiting from the pipe section 10. This step is known in the art as recompression. Preferably, this step takes place by bringing the volume of gas from a first value to a second value lower than the first. This means that the operating pressure in the pipe section 10 passes from a first value to a second value lower than the first. Preferably, this step takes place by means of one or more volumetric compressors 19 connected to the bypass of the second valve 12.

[0060] The step of injecting a flow of nitrogen into the pipe section 10 takes place during the step of reducing the volume of gas. Preferably, the step of injecting a flow of nitrogen begins upon the reaching of a third intermediate pressure value between the first and the second.

[0061] In this second embodiment, the injection of the flow of nitrogen supports the recompression. In particular, the nitrogen fulfils the task of displacing the volume of gas that the recompression would leave in the pipe due to operating limits of compressors with low inlet pressures. In this manner, it is possible to work until the gas is completely evacuated, it being possible to rely on an inlet pressure that is always greater than the lower limit.

[0062] The second embodiment is particularly suitable for pipes with large diameters and high pressures.

[0063] Preferably, the method comprises a step of detecting one or more physical parameters of the flow of recompressed gas at the bypass of the second valve 12. In particular, the detected physical parameters are pressure, temperature, and flow rate as well as the previously mentioned gas concentration. Preferably, this step takes place online by means of a third measuring block 14a. Preferably, this step takes place online in the bypass channel of the second valve 12.

[0064] Preferably, the detection of the gas concentration takes place by means of a gas / nitrogen interface analyser. Preferably, the analyser is included in the measuring block 14a.

[0065] The Applicant highlights that two different experimental tests were carried out on a same pipe, applying the known method previously described (combination of recompression and ejector) and the method of the present invention. It was possible to note that in the first case an amount of residual gas of 16,700 Sm3was left in the pipe, it not being possible to go below an inlet pressure of 1.8 bar absolute. Under real conditions, this volume of gas would be subsequently emitted into the atmosphere for a CO2 equivalent value of 177.5 t. Moreover, the recompression operation lasted 48 hours. It is also highlighted that these values strongly depend on the pipe size; in particular, they progressively increase with the pipe diameter.

[0066] In the second case, to begin with the flow of nitrogen was injected at a pressure of 8 bar absolute (the starting line pressure stood at over 35 bar absolute) and it was observed that it was possible to evacuate the entire volume of gas, being able to rely on an inlet pressure that was never less than 5 bar absolute. In this case, the operation required a total duration of 36 hours, with the additional advantage that the pipe section was already purged.

[0067] The savings in terms of pollution and duration of the removal from service of the pipe section is thus evident. The features of the method for evacuating flammable gas from a pipe section to be taken out of service, according to the present invention, are clear, as are the advantages.

[0068] In particular, the proposed method enables a total recovery of the gas, unlike the known methods in which the residual gas is emitted directly into the atmosphere.

[0069] Moreover, once the evacuation has been terminated, the pipe section remains full of nitrogen (inert gas) and is thus completely purged. It is not necessary to proceed with further purging of the internal volume, so cutting and welding operations can proceed without danger. The auxiliary purging effect thus reduces the intervention times and considerably simplifies the operations. With the known solutions, in fact, taking a section of a gas pipeline out of service often gives rise to the need to supply gas to users, above all industrial users which cannot shut down, by means of gas cylinder trailers until operation is restored.

[0070] Furthermore, the proposed method makes it possible to obtain an evacuated, purged pipe section in less time compared to the prior art.

[0071] Moreover, the possibility of intercepting the arrival and passage of the interface in the detection zone increases the reliability of the operation.

[0072] Furthermore, in the second embodiment, a recompression of the gas takes place, which reduces its volume. The simultaneous injection of a flow of nitrogen during the operation makes it possible to avoid reaching the lower inlet pressure limit of compressors and thus to be able to displace all the gas in the pipe section. This was not possible in the known solutions.

[0073] The combined use of recompression and injection of nitrogen enables compressors to still work with a high mass flow rate, since the injection of nitrogen reduces the inlet pressure drop and upon the arrival of the interface in the detection zone (preferably, the bypass for transferring the gas to the adjacent section) makes an already purged pipe section available to the network operator.

[0074] Moreover, with the proposed method the time for taking the pipe section out of service is reduced by 40-45% compared to the time that recompression alone would take. Recompression on its own leaves the pipe with gas at least at an effective pressure of no less than 1 bar and yet to be purged; it should be said that for the complete removal from service of a pipe section one must further take into account that at least a volume of gas equal to twice the geometric volume of the pipe section will have to be emitted into the atmosphere and that the purging operation, usually performed by sucking out the content of residual gases by means of an ejector with a primary flow of compressed air, is a lengthy operation that entails the use of an air compressor with a diesel motor and the injection, in the opposite terminal, of a buffer volume of nitrogen such as to assure the separation of the residual gas from the incoming air drawn in by the ejector.

[0075] Moreover, the possibility of predicting the entity, in volume and length, of the interface between the gas and the displacement nitrogen by means of a two-dimensional calculation model (with axial symmetry) facilitates the steps of preparing the intervention and monitoring the intervention itself in the operational phase. The simulation demonstrated the predictability of the displacement conditions with great precision. In fact, it allows for tracing the expected formation of the interface along the entire length of the pipe and predicting the arrival thereof at the point of transfer, while monitoring the evolution of the percentage volume fractions of nitrogen and natural gas over time and space, as well as reducing human error as much as possible.

Claims

CLAIMS1. A method for recovering flammable gas from a pipe section (10) to be taken out of service, said pipe section (10) being interposed between a first operational pipe section (20) and a second operational pipe section (30), said pipe section (10) being intercepted between a first line shut-off valve (11 ) and a second line shut-off valve (12), each valve (11 , 12) comprising a bypass channel through which the pipe section (10) communicates with the respective adjacent operational pipe section (20, 30), said method comprising the steps of:- injecting a flow of nitrogen into the pipe section (10) at the first valve (11 ) so that the injected flow pushes the gas towards the second valve (12) and transfers it into the second operational pipe section (30), said injection of a flow of nitrogen resulting in the formation of a gas / nitrogen interface volume that moves and grows progressively along the pipe section (10);- detecting the gas or nitrogen concentration in a detection zone (40) situated in proximity to or in the bypass channel of the second valve (12);- detecting the arrival of the gas / nitrogen interface volume in the detection zone (40), represented by the detection of a decrease in the gas concentration to a first value lower than 100%;- upon detecting a second gas concentration value lower than the first, shutting off communication between the pipe section (10) and the second operational pipe section (30).

2. The method according to claim 1 , comprising a step of monitoring the formation of the gas / nitrogen interface volume in the detection zone (40).

3. The method according to claim 1 or 2, wherein the step of shutting off communication between the pipe section (10) and the second operational pipe section (30) comprises a step of cutting the gas / nitrogen interface volume into two parts.

4. The method according to any one of the preceding claims, comprising a step of evacuating the flow of nitrogen from the pipe section (10), said evacuating step comprising a step of depressurising the volume ofnitrogen.

5. The method according to any one of the preceding claims, comprising a step of simulating the formation of the gas / nitrogen interface, said simulating step comprising a step of calculating a first time for the arrival of the gas / nitrogen interface volume in the detection zone (40).

6. The method according to claim 5, wherein the simulating step comprises a step of calculating a second time for reaching said second gas concentration value in the detection zone (40).

7. The method according to any one of the preceding claims, comprising a step of reducing the volume of flammable gas exiting from the pipe section (10), said reducing step resulting in the passage of the operating pressure in the pipe section (10) from a first value to a second value lower than the first.

8. The method according to claim 7, wherein the step of injecting the flow of nitrogen takes place during said step of reducing the volume of flammable gas.

9. The method according to claim 7 or 8, wherein the step of injecting the flow of nitrogen begins upon the reaching of a third intermediate pressure value between the first and second pressure values.

10. The method according to any one of the preceding claims, wherein the step of injecting the flow of nitrogen takes place with a flow rate such that the flow of nitrogen is turbulent or in a range of values which are no lower than one order of magnitude compared to the critical Reynolds number.

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