A method

Abstract

Method of commissioning a new gas pipe 14 into an existing gas pipe network (12), the method comprising isolating ends of the existing gas pipe network 26a, 26b, fluidically connecting ends of the new

Description

TECHNICAL FIELD The present invention relates to a method for commissioning a new gas pipe into an existing gas pipe network. The existing gas pipe network may carry, for example, methane or hydrogen. BACKGROUND Commissioning a new gas pipe into an existing gas pipe network typically occurs by purging. Purging requires fluidically connecting one end of a new gas pipe to an existing gas pipe network and flushing the air out of the new gas pipe with gas from the existing gas pipe network until a desired concentration of gas in the new gas pipe has been achieved, at which point the other end of the new gas pipe can be connected to the existing gas pipe network. Such direct purging therefore discharges valuable gas to the atmosphere. It is an aim of the present invention to address disadvantages associated with the prior art. SUMMARY OF THE INVENTION Embodiments of the invention may be understood with reference to the appended claims. Aspects of the present invention provide a method of commissioning a new gas pipe into an existing gas pipe network. In an aspect of the invention for which protection is sought there is provided a method of commissioning a new gas pipe into an existing gas pipe network, the method comprising: isolating ends of the existing gas pipe network; fluidically connecting ends of the new gas pipe, which defines a new internal volume for containing a gas, to the isolated ends of the existing pipe network to create an isolated internal volume, including at least a part of the new internal volume, wherein the isolated internal volume is isolated from the existing gas pipe network; applying a vacuum to the isolated internal volume below at least -61 kPa gauge; fluidically connecting the isolated internal volume to the existing gas pipe network, through a bypass line including a restrictor, so that the isolated internal volume is filled with gas from the existing gas pipe network; disconnecting the isolated internal volume from the bypass line and de-isolating the isolated ends of the existing gas pipe network so that the isolated internal volume is in direct fluid communication with the existing gas pipe network. It may be that the method further comprises cutting an existing pipe out of the existing gas pipe network, the new gas pipe being configured to take the place of the removed existing Pipe. It may be that the end of the existing gas pipe network comprises a plastic pipe, and that the method comprises isolating the ends of the existing gas pipe network with a squeeze off. It may be that the end of the existing gas pipe network comprises a plastic pipe, and that the method comprises isolating the ends of the existing pipe network with a double squeeze off. It may be that the end of the existing pipe network comprises a metal pipe, and that the method comprises applying a bag off or closing a valve to isolate the existing pipe network. It may be that the method comprising further isolating an end of the new gas pipe where it meets the metal pipe of the existing gas network to create the isolated internal volume and air pockets between the isolated ends of the existing gas pipe network and adjacent respective isolated ends of the new gas pipe and wherein the method further comprises: de-isolating any isolated ends of the new gas pipe, after filling the isolated internal volume with gas through the bypass. It may be that isolating the ends of the new gas pipe comprises applying a squeeze off at the ends of the new gas pipe. It may be that isolating the ends of the new gas pipe comprises applying a double squeeze off at the ends of the new gas pipe. It may be that the method further comprises: calculating a volume of the air pockets between a squeeze off in the new gas pipe and the isolated end of the existing gas pipe; calculating the vacuum pressure required in the isolated internal volume, based on the volume of air pockets, to ensure that the total air concentration introduced into the existing gas pipe network will be no more than 5% of the volume of the new internal volume, when the squeeze off on the end of the new gas pipe is removed. It may be that the method further comprises: calculating a volume of air pockets between a respective double squeeze off in the new gas pipe; calculating the vacuum pressure required in the isolated internal volume, based on the total volume of air pockets, to ensure that the total air concentration, introduced into the existing gas pipe network, will be no more than 5% of the volume of the new internal volume, when the squeeze off on the end of new gas pipe is removed. It may be that the gas is methane. It may be that the gas is hydrogen. It may be that the method further comprises: adding nitrogen to the isolated internal volume, after applying the vacuum to the isolated internal volume, until the proportion of oxygen within the isolated internal volume is below a respective limiting oxygen concentration, before fluidically connecting the isolated internal volume to the existing gas pipe network to fill the isolated internal volume with methane or hydrogen. The limiting oxygen concentration may be the proportion of oxygen below which hydrogen or methane will not ignite at any proportion. Within the scope of this application, it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and / or in the following description and drawings, may be taken independently or in any combination. For example, features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. For the avoidance of doubt, it is to be understood that features described with respect to one aspect of the invention may be included within any other aspect of the invention, alone or in appropriate combination with one or more other features. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which: FIGURE 1 schematically shows a fluid diagram of an existing pipe network and a new gas pipe before being commissioned; FIGURE 2 is a flow chart showing steps of a first example method of commissioning a new gas pipe in an existing gas pipe network according to an embodiment of the invention; FIGURE 3 schematically shows a fluid diagram of the existing pipe network and the new gas pipe of FIG. 1, during commissioning of the new gas pipe by the first example method according to an embodiment of the invention; FIGURE 4 is a flow chart showing steps of a second example method of commissioning a new gas pipe in an existing gas pipe network according to an embodiment of the invention; and FIGURE 5 schematically shows a fluid diagram of the existing pipe network and the new gas pipe of FIG. 1, during commissioning of the new gas pipe by the second example method according to an embodiment of the invention. DETAILED DESCRIPTION FIG. 1 shows a diagram 100 with an existing gas pipe network 12 with a gap 11, and a new gas pipe 14 before it has been commissioned into the existing gas pipe network 12. The new gas pipe 14 is configured to be installed between ends of the existing gas pipe network 12. The new gas pipe 14, defines a new internal volume 50 for containing a gas, and the existing gas pipe network 12 defines an existing internal volume for containing a gas 60. In this example, ends of the existing gas pipe network 12 comprise existing flanges 16 and ends of the new gas pipe 14 comprise new flanges 18, and the new flanges 18 and the existing flanges 16 are configured to meet to connect the ends of the existing gas pipe network 12 to the new gas pipe 14. In other examples, the new gas pipe 14 and the existing gas pipe network 12 may be configured to be connected together by any suitable means, with or without end flanges. Before installing the new gas pipe 14 into the existing gas pipe network 12, an existing gas pipe may be cut out of the existing gas pipe network 12 to create the gap 11, and the new gas pipe 14 may be configured to be installed in place of the cut-out pipe in the gap 11. In other examples, the new gas pipe may simply be a new branch of the existing gas pipe network. Before cutting out an existing gas pipe and / or installing the new gas pipe 14 between the ends of the existing gas pipe network 12, the ends of the existing gas pipe network 12 are isolated to ensure that there is no free flow of gas out of the existing gas pipe network 12. The ends of the existing gas pipe network 12 may be isolated with valves already in place. In other examples, the ends of the existing gas pipe network 12 may be isolated by bagging off, in which a small hole is drilled into the pipe through which an inflatable bag is introduced and inflated to block the path of the gas in the existing gas pipe network 12. In further examples, the ends of the existing gas pipe network 12 may be isolated by a squeeze off in which the pipe is squeezed shut by external clamps. FIG. 2 is a flow chart showing steps of a first example method 200 of commissioning the new gas pipe 14 into the existing gas pipe network 12 according to an embodiment of the invention. FIG. 3 is a diagram showing an example apparatus 300 which is used during commissioning of the new gas pipe 14 into the existing gas pipe network 12 according to the method described with reference to FIG. 2. In this example, the gas in the existing gas pipe network 12 may be methane. In block 202, the method 200 comprises isolating ends of the existing gas pipe network 12, such as described above. When the end of the existing gas pipe network 12 comprises a metal or rigid pipe, suitable methods of isolating the ends include with a valve or by bagging-off. When the end of the existing gas pipe network 12 comprises a plastic or flexible pipe, suitable methods of isolating the ends include with a valve, by bagging-off, or by squeezing-off. For example, in FIG. 3 a first end 20 of the existing gas pipe network 12 and a second end 22 of the existing gas pipe network 12 are shown. In this example, both the first end 20 and the second end 22 of the existing gas pipe network 12 comprise a plastic pipe, and both have been isolated with a double squeeze-off 26a, 26b (i.e., two adjacent squeeze-offs). Having a double squeeze-off 26a, 26b fluidically isolates the first end 20 and the second end 22 more reliably than a single squeeze-off. In block 204, the method 200 comprises fluidically connecting ends of the new gas pipe 14 to the isolated ends 20, 22 of the existing gas pipe network 12. Since the ends 20, 22 of the existing gas pipe network 12 are isolated, this does not fluidically connect the new gas pipe 14 to the gas in the existing gas pipe network 12. In FIG. 3, the new gas pipe 14 is positioned between the first end 20 of the existing gas pipe network 12 and the second end 22 of the existing gas pipe network 12 where existing flanges 16 meet the new flanges 18 so that the new gas pipe 14 is fluidically connected to the ends 20, 22 of the existing gas pipe network 12. Connecting the ends of the new gas pipe 14 to the ends 20, 22 of the existing gas pipe network 12 creates an isolated internal volume 70 which is isolated from the existing gas pipe network 12. The isolated internal volume 70 includes at least a part of the new internal volume 50. In this example, the isolated internal volume 70 includes the entire new internal volume 50, and parts 80 of the existing internal volume 60 up to the first squeeze off 26a adjacent to the new gas pipe 14 on each end 20, 22 of the existing gas pipe network 12. In other examples, it will be appreciated that, if the isolation of the existing gas pipe network 12 is closer to the ends, then the isolated internal volume 70 may not include any parts 80 of the existing gas pipe network 12. In block 206, the method 200 comprises applying a vacuum to the isolated internal volume 70. In FIG. 3, a vacuum pump 40 is disposed on a bypass line 30 which is fluidically connected to the isolated internal volume 70 through a vacuum bypass valve 34. By opening the vacuum bypass valve 34, the vacuum pump 40 can be operated to apply a vacuum to the isolated internal volume 70 down to a maximum required vacuum pressure. In this example, the vacuum pump 40 comprises a vacuum air vent 42 to enable the evacuated air from the isolated internal volume 70 to be discharged to atmosphere. A vacuum is applied to the isolated internal volume 70 so that the isolated internal volume 70 can be filled with gas to a required minimum gas concentration, of typically 95%, without wasting gas through direct purging of the isolated internal volume 70. It will be appreciated that the required minimum gas concentration may be any suitable percentage. The maximum required vacuum pressure that should be applied to the isolated internal volume 70 is dependent on the pressure of gas in the existing gas pipe network 12. For example, an allowable air content which can be introduced into the existing gas pipe network 12 from the new gas pipe 14 may be 5% of the total gas in the isolated internal volume 70 (based on a minimum gas concentration of 95%), when it is at the same pressure as the existing gas pipe network 12. Table 1 below shows the maximum gauge vacuum pressure which is required in the isolated internal volume 70 for existing gas pipe networks 12 at different working pressures. Table 1 assumes that the isolated internal volume 70 is the same as the new internal volume 50 (i.e., a unitary volume, V, of 1), and assumes that the temperature, T, is 293K and the ideal gas constant, R, is 8.31 J / K.mol. Calculation Low pressure pipeline Medium Pressure pipeline Intermediate pressure pipeline A Pressure of methane in existing gas pipe network (kPa Gauge) 3.5 200 700 P3 Pressure of methane in existing gas pipe network (kPa absolute) A + atmospheric pressure (assumed to be 101.3) 104.8 301.3 801.3 Mf Maximum allowable Molar fraction of air (5%) 0.05 0.05 0.05 D Total moles at pressure of existing gas pipe network (unitary volume) T*R 43.03 123.7 328.95 A max Maximum moles of air allowable in the isolated internal volume D * Mp 2.15 6.18 16.45 Pl Maximum pressure to achieve maximum moles of air (kPa absolute) ^■max * 7 * R V 5.2 150.7 400.7 G Maximum pressure to achieve maximum moles of air (kPa gauge) P1-atmospheric pressure -96 -86 -61 Table 1 Table 1 can be distilled into Equations 1 and 2, which show the derivation of the maximum absolute vacuum pressure, Pi, which is required in the isolated internal volume 70 to achieve 5 the maximum allowable air content. Equation 1 below shows the calculation for determining the maximum moles of air allowable in the isolated internal volume 70, nmax, using the ideal gas law: 10 Equation 1 where Mf is the maximum allowable air content (or the maximum allowable molar fraction of air within the isolated internal volume 70), P3 is the pressure of the gas in the existing gas pipe network 12, V is 1, R is the ideal gas constant and T is the temperature. Equation 2 is the 15 calculation for the maximum absolute allowable vacuum pressure, Pi, which is required in the isolated internal volume 70 to achieve the maximum moles of air allowable, nmax: , nmaxRT P3*V RT 1 V RT V Equation 2 In block 208, the method 200 comprises filling the isolated internal volume 70 with gas from the existing gas pipe network 12. In this example, this includes fluidically connecting the isolated internal volume 70 to the existing gas pipe network 12 through the bypass line 30 so that the isolated internal volume 70 is filled with gas from the existing gas pipe network 12. This includes keeping the vacuum bypass valve 34 open and opening a gas bypass valve 35 which connects the gas in the existing gas pipe network 12 to the bypass line 30. In this example, the bypass line 30 comprises a restrictor 32, such as an orifice plate, so that filling of the isolated internal volume 70 proceeds at a rate that does not lower the pressure in the upstream existing gas pipe network 12. In this example, the vacuum pump 40 is on the same bypass line 30 as the bypass line 30 which connects the existing gas pipe network 12 to the isolated internal volume 70 through the restrictor 32. Having them on the same line makes a more space efficient arrangement. In other examples, the vacuum pump 40 may be on a separate line to the line which connects the existing gas pipe network 12 to the isolated internal volume 70 through the restrictor 32. In block 210, the method 200 comprises disconnecting the isolated internal volume 70 from the bypass line 30, which can be done in this example by closing the gas bypass valve 35 and / or the vacuum bypass valve 34, and de-isolating the ends 20, 22 of the existing gas pipe network 12 so that the isolated internal volume 70 is in direct fluid communication with the existing gas pipe network 12, such that the new gas pipe 14 has been commissioned into the existing gas pipe network 12. In this example shown in FIG. 3, the apparatus includes a pressure meter 44 connected to the existing gas pipe network 12 and a pressure meter 46 connected to the new gas pipe 14 at the isolated internal volume 70. The method 200 may comprise, in block 206, applying a vacuum until the pressure meter 46 in the new gas pipe 14 registers the maximum required vacuum pressure. In block 208, the method 200 may comprise filling the isolated internal volume 70 with gas until the pressure meter 46 at the new gas pipe 14 reads the same pressure as the pressure meter 44 at the existing gas pipe network 12. FIG. 4 is a flow chart showing steps of a second example method 400 of commissioning the new gas pipe 14 into the existing gas pipe network 12 according to an embodiment of the invention. FIG. 5 is a diagram showing an example apparatus 500 which is used during commissioning of the new gas pipe 14 into the existing gas pipe network 12 according to the second example method described with reference to FIG. 4. The example apparatus 500 is similar to the example apparatus 300 in that the new gas pipe 14 is located between ends 20, 22 of the existing gas pipe network 12 with new flanges 16 abutting existing flanges 18, but differs in that the second end 22 of the existing gas pipe network 12 comprises a metal pipe, rather than a plastic pipe. In this example, the gas in the existing gas pipe network 12 may be hydrogen. In block 402, the method 400 comprises isolating ends of the existing gas pipe network 12, such as described above, in a similar manner to block 202. In this example, the first end 20 of the existing gas pipe network 12 comprises a plastic pipe, and the second end 22 of the existing gas pipe network 12 comprises a metal pipe. In FIG. 5, the first end 20 of the existing gas pipe network 12 has been isolated with a double squeeze-off 26a, 26b (i.e., two adjacent squeeze-offs). The second end 22 of the existing gas pipe network 12 has been isolated with a double bag-off 28a, 28b (i.e., two adjacent bag-offs). Having a double bag-off 28a, 28b isolates the second end 22 more reliably than a single bag-off. In other examples, the second end 22 of the existing gas pipe network 12 may be isolated by closing a valve. A squeeze-off or double squeeze-off is a preferred isolation method because it can more reliably withstand a higher vacuum against it without leaking. In block 404, the method 400 comprises fluidically connecting ends of the new gas pipe 14 to the isolated ends 20, 22 of the existing gas pipe network 12 in a similar manner as in block 204, as shown in FIG. 5. In block 406, the method 400 comprises isolating an end of the new gas pipe 14 facing the second end 22 of the existing gas pipe network 12 (i.e., the metal end). In this example, the end of the new gas pipe 14 is isolated with a double squeeze-off 56a, 56b, thereby creating an isolated internal volume 570 between the squeeze off 26a on the existing gas pipe network 12 adjacent to the new gas pipe 14 and the nearest squeeze-off 56a on the new gas pipe 14. The double squeeze-off 56a, 56b on the new gas pipe 14 also creates an air pocket 58 between the two squeeze offs 56a, 56b, and between the squeeze-off 56b closest to the bag-offs 28 and an air pocket 62 between the first bag-off 28a adjacent to the squeeze-offs 56 on the new pipe 14. It will be appreciated that in an example in which only a single squeeze-off 56 is applied to the new ga pipe 14, only the air pocket 62 between the squeeze-off 56 and the bag-off 28 will be created. In some examples, there may be a vent point at the air pocket 58. This may allow for monitoring of vacuum pressure loss, which could detect unintentional communication of the isolated internal volume 570 and the air pocket 62 (which would be unintentional direct fluid communication between the isolated internal volume 570 and the existing gas pipe network 12). Block 406 in the second example method 400 has been added compared to the first example method 200 because the metal end cannot be isolated by a squeeze-off, and applying a vacuum to the isolated volume against the bag-off 28 in the second end 22 of the existing gas pipe network 12 may result in leakage of the gas from the existing gas pipe network 12 into the isolated internal volume as the vacuum is being applied, thereby resulting in unnecessary loss of gas. By contrast, the squeeze-off or double squeeze-off 26a, 26b, 56a, 56b can withstand a vacuum more reliably without leakage. In block 408, the method 400 comprises calculating the maximum required vacuum pressure in the isolated internal volume 570, to ensure that the total air introduced into the system will be no more than 5% of the new internal volume 50 at the pressure of the gas in the existing gas pipe network 12. The maximum required vacuum pressure in this second example method 400 may be different to the vacuum pressure required in the first example method 200 since the air pockets cannot be evacuated and will therefore retain 100% air concentration. Block 408 includes calculating a volume of the air pockets 58, 62, as a fraction of the total volume of the isolated internal volume 570 and air pockets 58, 62. The vacuum pressure required in the isolated internal volume 570, is based on this fraction, and the pressure of the gas in the existing gas pipe network 12, to ensure that the total air concentration introduced into the existing gas pipe network 12 will be no more than 5% of the volume of the new internal volume, when the double squeeze-off 56a, 56b on the end of the new gas pipe 14 is removed. Assuming that the total volume, Vt, of the isolated internal volume 570 and the air pockets 58. 62 is unitary (i.e., VT= 1), then the isolated internal volume 570, Vi, and the volume of the air pockets 58, 62, V2, together make 1 (i.e., V1 + V2 = 1). Therefore, the volume of the air pockets V2 is a fraction of the total volume. The air pockets 58, 62, will remain at atmospheric pressure while the isolated internal volume 570 is evacuated. When the new gas pipe 14 has been commissioned into the existing gas pipe network 12 so that it is filled with gas at a pressure equal to the pressure in the existing gas pipe network 12, the total moles of gas, ntotai, can be calculated (using the ideal gas law) with Equation 3, below: _ where P3 is the pressure of the gas in the existing gas pipe network 12, Vt is 1, R is the ideal gas constant and T is the temperature. The moles of air, nmax, which can be allowed in the total volume, Vt, to achieve a maximum allowable air content (or the maximum allowable molar fraction of air, Mf, which is allowed to be introduced into the existing gas pipe network 12 when the new gas pipe 14 has been commissioned and is fluidically connected to, and at the same pressure as, the existing gas pipe network 12), can be calculated with Equation 4 below: nmax = ^F^total Equation 4 The total moles of air, n2, trapped in the air pockets 58, 62 can be calculated with Equation 5 below: P2*V2 n2 RT Equation 5 where P2 is atmospheric pressure, or the assumed pressure within the air pockets, if they are not likely to be at atmospheric pressure, and V2 is the volume fraction of the air pockets 58, 62 compared to the total volume, Vt, of the air pockets 58, 62 and isolated internal volume 570 combined. The maximum allowable moles of air in the isolated internal volume 570, m, to achieve the maximum molar fraction of air, Mf, can be calculated with Equation 6 below: MFP3VT P2y2 nl — nmax n2 ~RT~ Equation 6 Therefore, the required vacuum pressure, Pi, in the isolated internal volume 570 can be calculated with Equation 7 below: ntRT MFP3VT - P2V2 ri = ----= ------------ Fl Equation 7 In block 410, the method 400 comprises applying a vacuum to the isolated internal volume 570 in a similar manner to block 206 of the first example method 200, up to or beyond the calculated required vacuum pressure, Pi, from block 408. If it is found that it is not possible to achieve the required maximum air concentration even if an entire vacuum is applied to the isolated internal volume 570 (i.e., Pi is found to be below OPa absolute with Equation 7 above) because the air pockets 58, 62 are too large, direct purging could be applied to the air pockets 58, 62, and required vacuums pressures from the first example method 200 may be used to ensure the required maximum air concentration is not exceeded. In block 412, the method 400 comprises introducing nitrogen into the isolated internal volume 570. This is particularly useful if the gas in the existing gas pipe network 12, which will be introduced into the new gas pipe 14, is hydrogen, but can also be used when the gas is methane. The nitrogen is introduced into the isolated internal volume 570 until the proportion of oxygen within the isolated internal volume 570 (from the air remaining therein) is below a limiting oxygen concentration. The limiting oxygen concentration may be the proportion of oxygen below which hydrogen or methane (whichever gas is in the existing gas pipe network 12) will not ignite at any proportion. In other examples, block 412 may be omitted from the second example method 400, or may be included in the first example method 200 after block 206 and before block 208. In block 414, the method 400 comprises filling the isolated internal volume 570 with gas from the existing gas pipe network 12 through the bypass line 30 in a similar manner to block 208 of the first example method 200. In block 416, the method 400 comprises disconnecting the isolated internal volume 570 from the bypass line 30, which can be done in this example by closing the gas bypass valve 35 and / or the vacuum bypass valve 34, in a similar manner to block 210 in the first example method 200, and de-isolating the ends 20, 22 of the existing gas pipe network 12 and the ends of the new gas pipe 14 which have been isolated, so that the isolated internal volume 570 is in direct fluid communication with the existing gas pipe network 12, such that the new gas pipe 14 has been commissioned into the existing gas pipe network 12. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the 5 plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Features, integers, characteristics, compounds, chemical moieties or groups described in 10 conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

1. A method of commissioning a new gas pipe into an existing gas pipe network, the method comprising:isolating ends of the existing gas pipe network;fluidically connecting ends of the new gas pipe, which defines a new internal volume for containing a gas, to the isolated ends of the existing pipe network to create an isolated internal volume, including at least a part of the new internal volume, wherein the isolated internal volume is isolated from the existing gas pipe network;applying a vacuum to the isolated internal volume below at least -61 kPa gauge;fluidically connecting the isolated internal volume to the existing gas pipe network, through a bypass line including a restrictor, so that the isolated internal volume is filled with gas from the existing gas pipe network;disconnecting the isolated internal volume from the bypass line and de-isolating the isolated ends of the existing gas pipe network so that the isolated internal volume is in direct fluid communication with the existing gas pipe network.

2. A method according to claim 1, further comprising cutting an existing pipe out of the existing gas pipe network, the new gas pipe being configured to take the place of the removed existing pipe.

3. A method according to any preceding claim, where the end of the existing gas pipe network comprises a plastic pipe, isolating the ends of the existing gas pipe network with a squeeze off.

4. A method according to claim 3, where the end of the existing gas pipe network comprises a plastic pipe, isolating the ends of the existing pipe network with a double squeeze off.

5. A method according to claim 1 or 2, where the end of the existing pipe network comprises a metal pipe, applying a bag off or closing a valve to isolate the existing pipe network.

6. A method according to claim 5, comprising further isolating an end of the new gas pipe where it meets the metal pipe of the existing gas network to create the isolated internal volume and air pockets between the isolated ends of the existing gas pipe network andadjacent respective isolated ends of the new gas pipe and wherein the method further comprises:de-isolating any isolated ends of the new gas pipe, after filling the isolated internal volume with gas through the bypass.

7. A method according to claim 6, wherein isolating the ends of the new gas pipe comprises applying a squeeze off at the ends of the new gas pipe.

8. A method according to claim 7, wherein isolating the ends of the new gas pipe comprises applying a double squeeze off at the ends of the new gas pipe.

9. A method according to claim 7 or 8, further comprising:calculating a volume of the air pockets between a squeeze off in the new gas pipe and the isolated end of the existing gas pipe;calculating the vacuum pressure required in the isolated internal volume, based on the volume of air pockets, to ensure that the total air concentration introduced into the existing gas pipe network will be no more than 5% of the volume of the new internal volume, when the squeeze off on the end of the new gas pipe is removed.

10. A method according to claims 8 and 9, further comprising:calculating a volume of air pockets between a respective double squeeze off in the new gas pipe;calculating the vacuum pressure required in the isolated internal volume, based on the total volume of air pockets, to ensure that the total air concentration, introduced into the existing gas pipe network, will be no more than 5% of the volume of the new internal volume, when the squeeze off on the end of new gas pipe is removed.

11. A method according to any preceding claim, wherein the gas is methane.

12. A method according to any preceding claim, wherein the gas is hydrogen.

13. A method according to claim 11 or 12, further comprising:adding nitrogen to the isolated internal volume, after applying the vacuum to the isolated internal volume, until the proportion of oxygen within the isolated internal volume is below a respective limiting oxygen concentration, before fluidically connecting the isolated internal volume to the existing gas pipe network to fill the isolated internal volume with methane or hydrogen.

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