GB2059548A

GB2059548A - Pressure Surge Relief System

Info

Publication number: GB2059548A
Application number: GB8005189A
Authority: GB
Original assignee: Grove Valve and Regulator Co
Current assignee: Grove Valve and Regulator Co
Priority date: 1979-09-26
Filing date: 1980-02-15
Publication date: 1981-04-23
Also published as: IT1131688B; FR2465937A1; NL8004132A; GB2059548B; FR2465937B1; CA1138745A; DE3026422A1; IT8023372A0

Abstract

A pressure surge relief system for a liquid transporting pipeline (12) comprises a surge relief line (14) adapted to be tapped at one end from the pipeline and connected at the other end to a receptacle (26). A valve (16) in the relief line is conditioned to open quickly to substantial flow when the pressure in the line exceeds a predetermined safe limit. Opening of the valve (16) allows that quantity of liquid which is necessary to prevent the pressure from exceeding said predetermined limit to flow to the receptacle (26). The receptacle (26) is pre-charged with a gas to cushion the impact of the surge and also includes a tube (44) with slots (50) to dissipate energy of the liquid surge and to bring the liquid to a state of quiescence as quickly as possible. A return duct (54) allows flow from the receptacle (26) back to the pipeline (12) when the surge subsides. <IMAGE>

Description

SPECIFICATION Pressure Surge Relief System This invention relates to a pressure surge relief system for a liquid transporting pipeline.

Severe damage can result from pressure surges in a liquid transporting pipeline; such surges can occur when there is any sudden change in velocity of the liquid, as when starting or stopping a pump. Such surges have been relieved in some instances by operation of valves which open whenever they occur. Since many pipeline surges never reach dangerous levels, the diversion of liquid at every occurrence is a needless waste and may require a reservoir of enormous capacity. Particularly where a closed tank is required, this could be very costly.

It is an object of this invention to provide a system which can be used to operate to relieve only those pressure surges which are beyond a limit to be safely accommodated by the pipeline.

According to the invention there is provided a pressure surge relief system for a liquidtransporting pipeline, the system comprising a surge relief line adapted for connection at one end to a pipeline and connected at the other end to a closed receptacle; a surge relief flow-blocking member in said relief line, said surge relief member being conditioned to open quickly to large capacity flow when the pressure of fluid in said relief line reaches a predetermined safe limit above normal pipeline pressure; means for modulating fluid flow through said surge relief member; and means in said receptacle for smoothly stopping flow.

The flow-blocking member may be provided as a surge relief valve of the type having a flexible tube which is constricted around a circular barrier by hoop tension and by pressure of a surrounding pilot gas. When the surge relief valve is opened, liquid in the pipeline is diverted through the relief line to the closed receptable which preferably is pre-charged by a gas at a pressure level sufficient ta cushion the impact and, as is desirable in some cases, to maintain the liquid in its liquid state.

In operation of the system, the pilot gas is set at a predetermined pressure level which is well in excess of normal pipeline pressure, but within the range that can safely be handled by the pipeline.

Hence, liquid is diverted from the pipeline to the closed receptacle only in the event of dangerous surges, and only in a volume to prevent the surge from generating the pressure peak which exceeds the safe design level. In a preferred embodiment, the relief line is connected to a pipe within a closed receptable which pipe has a plurality of apertures along its length through which the liquid flows as it moves along. When the surge has passed, and pipeline pressure subsides, a return line returns the liquid so diverted to the pipeline.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a generally schematic view of a pressure surge relief system embodying the invention; Figure 2 is an end view of the system assembly; Figures 3 to 10 illustrate various modified embodiments of the invention.

Referring now to the drawings, in Figures 1 and 2 there is shown a surge relief system 10 connected for operation to a liquid transporting pipeline 12, and having a branch or surge relief line 14 in open communication therewith. Surges frequently occur when the flowing liquid is subjected to sudden changes in flow conditions, as when a pump (not shown) is activated or deactivated. The system 10 is conditioned to relieve such surges when the exceed a predetermined designed pressure level of safe pipeline operation.

A relief valve 1 6 is provided in the relief line 14, and, in use, excess pressure is relieved through the valve 16, which is biased towards and open position by pressure of liquid in the pipeline. The valve 16 may be of the expansible tube type, such as is that shown in United States Patent Specification No. 3,272,470. Specifically the surge relief valve 16 comprises a cylindrical body shell 18, which is clamped between two closure plates 20, by means of studs 22. The closure plates are, in turn, secured by conventional means to complementary flanges 24 in the branch pipeline 14 which has an output end connected to a storage reservoir 26.

A slotted core 28 is carried within the valve body shell 1 8 and includes spaced, circumferential rows of inlet and outlet slots 30 and 32 on opposite axial sides of an intermediate barrier 34. A flexible, expansible tube 36 is stretched around a cylindrical sealing outer surface 38 on the barrier, normally to prevent flow from the inlet slots 30 to the outlet slots 32.

A control gas, such as Nitrogen from a source 39, is regulated by a regulator 40 to a predetermined pressure Pj as it is supplied to an annular space or jacket 41 surrounding the expansible tube 36 and within the body shell. The jacket pressure P1 is well above normal pipeline pressure Pl, but within the safe design limits of the pipeline, and will enable the valve 16 to open and relieve a surge when that pressure is exceeded.A restraining sleeve 42 with perforations therein limits the expansion of the rubber tube 36, while a spacious chamber 43 around the restraining sleeve 42 and bounded by the shell 18 provides plenty of volume for compression of the control gas, whereby the surge relief valve 1 6 will open quickly in response to pipeline pressure exerted on the interior of the flexible tube 36 when the pipeline pressure reaches a level which is in excess of the jacket pressure Pj.

Extending into the reservoir 26 is an energyattenuating sparge tube 44, which is connected directly to the branch line 14, by a suitable flange connection 46. The attenuating tube 44 is centred within the reservoir 26 by means of arms 48, and is provided with several rows of apertures, which may,Zas shown, be in the form of longitudinally extending slots 50, spaced arcuately around the cylindrical surface thereof.

The surge relief pipe 14 has a U-bend between the relieving valve 16 and the closed receptacle 26 and, for greater accessibility, the axis of the reservoir 26 is, as shown in Figure 2, displaced from the vertical plane of the axis of the valve 1 6.

The U-bend dissipates some energy of the diverted liquid by imposing friction losses on it; the rapid reversal tends to equate thrust forces; and the bend enables the provision of a more compact structure. In addition, the centrifugal force tends to throw the liquid to the outside wall of the pipe. This, together with the laying over of the relief pipe 14 at an angle tends to generate a rotational movement of the fluid in the stream and cause elongation of the rapidly moving front, converting the "piston slug" to a wedge.

It is highly desirable to have more slot area in the sparge tube 44 than is needed to accommodate maximum liquid flow, so that gas is drawn into the sparge tube 44 to reduce the pressure of the fluid, creating a greater pressure differential to increase flow from the valve 16.

In any event, when the surge relief valve 16 opens, the diverted liquid flowing into the sparge tube will flow, drop and be projected, through the apertures 50 whereby energy thereof is attenuated until it reaches a state of quiescence in the reservoir 26.

The reservoir 26 is precharged with a gas, such as air or nitrogen, from the source 39, and regulated by a regulator 51 to a predetermined pressure level Pz to cushion the impact of the surge waves and, in the case of some liquids such as a volatile liquid, for example Butane, to maintain a pressure sufficient to prevent it form becoming gaseous. The closure cap 52 at the end of the attenuator tube 44 entraps gas in the imperforate end portion to provide a further cushion.

The surge relief system 10 is designed to relieve only dangerous surges; most surges will be accommodated in the pipeline itself. Even with dangerous surges, only that amount of pipeline fluid is diverted to the reservoir 26 which is attributable to that portion of the surge beyond the safe limits of the pipeline, taking into consideration a margin of safety. That is, if if is determined that the pipeline can safely handle pressure at a given level, which may be well above normal pipeline pressures, the surge relieving valve 1 6 is set so that it will not open until that level is reached.Then, only that volume of liquid necessary to contain the peak pressure within a safe design level needs to be accommodated in the attenuation tube 44 and cushioned by the precharge pressure P , which is determined on the basis of pipeline design parameters, and may be atmospheric pressure or higher.

Particularly with the loop 1 4a in the branch line 14 disposing the reservoir 26 adjacent to the surge relief valve 16, the entire system 10 is extremely compact and may be assembled and shipped on a single platform, pallet or skid (not shown).

After a surge is attenuated and subsides in the pipeline 12, it is desirable to restore the capability of the system 10 by the transfer of diverted liquid from the reservoir back to the pipeline 12 in order to return the reservoir to full, or near-full; surgerelieving capacity. For this purpose, a small return line 54 which by-passes the valve 16 is provided with an on-off valve 56 in case it may be desired to control the return manually. In addition, or in the alternative, a simple check valve 58 may be installed in the line for automatic liquid return when pressure in the reservoir and branch line 14 downstream of the surge relief valve 16 exceeds that in the pipeline 12 after the surge subsides.In either event, since the liquid in the reservoir 26 and branch line 14 augmented by the compressed gas cushion, is usually well above pipeline pressure, it will return to the pipeline 12 without need of a pump when pressure in the pipeline 10 subsides, to normal or near normal level. With automatic liquid return enabled by the check valve 58, a float valve 60 is also provided so that, when liquid ceases to flow, the float valve will close to prevent loss of the precharged gas in the chamber 26.

For complete drainage of the reservoir 26, a liquid drain line 62 opens from the bottom of the reservoir 26 and has a check valve 64 provided to permit flow.

Figure 3 shows an embodiment in which the surge wave is relieved by directing a quantity of liquid from the surge relief valve 16, through a relief pipe 66, and the energy of the liquid so diverted is attenuated by impacting it in the form of a jet against the end of a cylindrical tank 68. An obvious disadvantage of this embodiment resides in the unidirectional thrust imposed on the system by the impacting jet.

Figure 4 shows an embodiment in which unidirectional thrust is avoided by directing the jet tangentially of the cylindrical tank 70. This generates a swirling mass around the tank 70, dissipating energy while avoiding jet impact. If the relief pipe 72 is located near the top of the tank 70, the liquid will follow a generally spiral path until it reaches the bottom.

In Figure 5, the gas or buffer G contained in the closed end 74 of the energy dissipating chamber 76 is separated from the liquid in the surge by means of a piston 78 or the like. This avoids gas entrainment in the liquid and resultant loss of the cushioning gas when the liquid is returned to the pipeline. The energy of the diverted flow is dissipated in compressing the gas in the closed end 74 of the chamber 76 behind the piston 78. If desired, the gas cushion may be augmented or replaced by a spring 80.

Figure 6 shows how energy is again attenuated by compressing a gas G. The gas is separated from the pipeline liquid by a diaphragm 82 sealed across the closed chamber 84. In addition, the diaphragm may be elastic to dissipate further energy in stretching.

Figure 7 shows an embodiment having a series of accumulators 86 open to the closed chamber 88, each with a bladder 90 charged with a gas under pressure. Hence, the surging liquid flows into the accumulators 86 to compress the gas in the bladders 90, thereby dissipating energy. In addition, the chamber 88 may be pre-charged with a cushioning gas.

Figure 8 shows an embodiment in which the energy is attenuated by providing the closed chamber 92 with a series of bends 94 which impose friction losses on the diverted liquid. In addition the chamber may be of progressively increasing cross-section to dissipate further energy.

Also as shown in Figure 8, the system of this invention is not limited to an expansible tube valve as illustrated in Figure 1. Any embodiment shown will operate with any flow blocking device that opens at a predetermined pressure level. For example, a device 96 with a rupture disc 98 which ruptures at a given pressure to allow full flow will answer the needs of the system.

However, without the expansible tube of Figure 1 to modulate the flow, some other means, such as a bundle of tubes 100 to impose friction losses may by required to lessen the impact against the closed chamber 92.

In the embodiment of Figure 9, the closed chamber 102 is angled upward so that impact of the surging liquid is with a sloping wall. This has an effect similar to the dissipation of the energy of ocean waves breaking up on a beach.

Finally, the Figure 10 embodiment overcomes the disadvantage of unilateral thrust by forming the closed chamber 104 as a "Y" to divide the stream at 106 and 108 before jetting the two streams into a tank 110 from opposite directions, to produce a mutually cushioning effect.

Claims

1. A pressure surge relief system for a liquid transporting pipeline, the system comprising: a surge relief line adapted for connection at one end to a pipeline and connected at the other end to a closed receptacle; a surge relief flow-blocking member in said relief line, said surge relief member being conditioned to open quickly to large capacity flow when the pressure of fluid in said relief line reaches a predetermined safe limit above normal pipeline pressure; means for modulating fluid flow through said surge relief member; and means in said receptacle for smoothly stopping flow.

2. A system according to claim 1, wherein said relief line and receptacle are substantially free of friction members which would create a back pressure to impair operation of said flow-blocking member in use of the system.

3. A system according to claim 1 or 2, wherein said receptacle comprises a generally cylindrical, closed tank, said other end of said relief line extending into said tank along the axis thereof and having a discharge opening at the downstream end thereof.

4. A system according to claim 1 or 2, wherein said receptacle comprises a generally cylindrical tank, a discharge opening at the downstream end of the relief line being directed along a tangent of said tank.

5. A system according to claim 4, wherein said downstream end of said relief line is disposed adjacent one end of the cylindrical tank.

6. A system according to claim 1 or 2, wherein the other end portion of said relief line extends into said receptacle with a plurality of perforations being provided in the part of said relief line within said receptacle.

7. A system according to claim 6, wherein the downstream extremity of said other end portion of the relief line is imperforate over a portion of its length so that liquid flowing thereinto entraps a volume of impact-cushioning gas.

8. A system according to claim 6 or 7, wherein the total area of said perforations is greater than that needed to accommodate liquid flow therethrough so that gas will be drawn therethrough in use of the system.

9. A system according to claim 6, 7 or 8, wherein said perforations comprise a series of slots around, and spaced along the length of, said other end portion.

10. A system according to claim 6, 7, 8 or 9, including means closing off the end of said other end portion within said closed receptacle.

11. A system according to claim 1,2, 6, 7, 8, 9 or 10, wherein a U-bend is provided in said relief line between said flow-blocking device and said other end portion.

12. A system according to any one of claims 6 to 11, wherein said receptacle is in the form of a substantially cylindrical tank with said other end portion of the relief line extending along the axis thereof, said receptacle axis being displaced from the vertical plane of the axis of the flow-blocking device.

13. A system according to any preceding claim, wherein a charge of gas under pressure is contained in said receptacle.

14. A system according to claim 1 or 2, wherein said receptacle is formed of progressively larger cross-section along its length.

1 5. A system according to claim 1 or 2, wherein said receptacle extends in a plurality of bends to cause multiple direction changes in fluid flowing therein.

16. A system according to claim 1 or 2, wherein said receptacle is elongated and upwardly sloping so that fluid flowing therein impacts against a sloping wall.

1 7. A system according to claim 1 or 2, including a pressure-responsive member movable in said receptacle and exposed to impact by fluid surging in said relief line; and yieldable means biasing said pressure-responsive member in opposition to said impact.

1 8. A system according to claim 17, wherein said pressure-responsive member comprises a bladder in said receptacle, said yieldable means comprising a gas under pressure.

1 9. A system according to claim 17, wherein said pressure-responsive member comprises a piston member slidable in said receptacle.

20. A system according to any preceding claim, wherein a return line is connected to said surge relief line and adapted for communication to the pipeline, a valve being provided in said return line.

21. A system according to claim 20, wherein said valve is a check valve conditioned to enable flow automatically in one direction only toward said pipeline.

22. A system according to claim 20 or 21, wherein a float check valve conditioned to enable flow of liquids only in the direction of the pipeline is provided in said return line.

23. A system according to any preceding claim, wherein said flow-blocking member comprises: an expansible tube valve including a circular barrier extending across a flow passageway; an expansible tube stretched around said barrier; and a control pressure fluid around said tube so that said tube stretches away from said barrier to allow flow when pressure in said relief line overcomes said control pressure.

24. A system according to claim 23, wherein said control pressure is set at a level well above normal pipeline pressure but within safe limits of the pipeline.

25. A pressure surge relief system for a liquidtransporting pipeline, such system comprising: a surge relief line adapted for connection at one end to a pipeline and connected at the other end to a closed receptacle which is precharged with gas at a set pressure level; and a surge relief valve in said relief line, said surge relief valve being conditioned to open quickly to large capacity flow when the pressure in said relief line increases to a level above a predetermined safe limit above normal pipeline pressure and to close fully when relief line pressure decreases to a level below said predetermined limit.

26. A system according to claim 25, including means in said receptacle for absorbing energy of a liquid flowing therein.

27. A system according to claim 26 or 27, wherein said valve is an expansible tube valve including a circular barrier extending across a flow passageway; an expansible tube stretched around said barrier; and a control pressure fluid around said tube so that said tube stretches away from said barrier to allow flow when pressure in said relief line overcomes said control pressure.

28. A system according to claim 27, wherein said control pressure is set at a level well above normal pipeline pressure but within safe limits of the pipeline.

29. A method of relieving a surge of liquid pressure in a pipeline comprising the steps of: monitoring pressure increases in the pipeline; when pressure increases to a predetermined safe level, drawing off a quantity of liquid just sufficient to maintain pressure in said pipeline within a safe design range; diverting said quantity of liquid to a closed receptacle; and dissipating the energy of said quantity of liquid to bring it to a state of quiescence.

30. A method according to claim 29, wherein a charge of gas is introduced into said closed receptacle so as, when compressed by said quantity of liquid, to be operative to force liquid back into the pipeline when pressure in the pipeline has subsided.

31. A method of relieving a surge of pressure in a pipeline substantially as herein described with reference to the accompanying drawings.

32. A pressure surge relief system for a liquidtransporting pipeline, such system being constructed and arranged to operate substantially as herein described with reference to and as illustrated in Figs. 1 and 2, or any one of Figs. 3 to 10 of the accompanying drawings.

33. A pipeline provided with a surge relief system as claimed in any one of claims 1 to 28 or claim 32.