WO2017137497A2

WO2017137497A2 - Pump

Info

Publication number: WO2017137497A2
Application number: PCT/EP2017/052879
Authority: WO
Inventors: Leif Arne TØNNESSEN; Paolo ROMANELLO; Stein FØLKNER
Original assignee: Fmc Kongsberg Subsea As
Priority date: 2016-02-12
Filing date: 2017-02-09
Publication date: 2017-08-17
Also published as: WO2017137497A3; NO341249B1; NO20160237A1

Abstract

The present invention provides a pump comprising a housing (1), the housing comprises a first fluid section (2) and a second fluid section (3) separated by a reciprocating pump element (4), wherein the first fluid section (2) comprises an inlet/outlet (5) for a drive fluid; the second fluid section comprises an inlet (6) and an outlet (7) for a liquid, each of the inlet and the outlet comprises a one-way valve (8,9), such that a liquid may enter the second fluid section (3) via the inlet (6) and is forced to exit the second fluid section (3) via the outlet (7) during use; and the reciprocating pump element (4) is movable between a first position, wherein the volume of the first fluid section (2) is minimized and a second position, wherein the volume of the first fluid section (2) is maximized; wherein the pump comprises damping means (4,10,13) arranged such that the reciprocating pump element encounters an increased motion resistance when approaching the first and the second position, respectively, during use.

Description

PUMP

Technical field of the invention The present invention relates to the field of reciprocating pumps, and more specifically to reciprocating pumps using gas or liquid as drive fluid.

Background Subsea gas compression is an important building block in the realization of subsea factories. The produced well stream from subsea gas fields will typically contain some amount of free liquid in form of water, oil condensate and chemicals for hydrate inhibition. Although there exist some concepts for direct well stream compression by use of wet gas compressor, the traditional and most efficient boosting method is based on liquid/gas separation, pumping of liquid and

compression of gas. Conventionally, the liquid is boosted by an electric pump.

Simplified compression systems are known from the prior art, wherein the need for electric pumps, with power and utility system, are removed without compromising on power efficiency.

These known solutions are based on utilizing the compressed gas at the discharge of a subsea compressor system to boost the liquid in a reciprocating/fluctuating sequence in a reciprocating pump. During the development of such a solution the applicant found that pressure pulsations will occur in off-design situations if not mitigated by special methods for control and/or design of the system. Mitigation of pressure pulsations may be achieved by use of accumulators and/or complicated monitoring systems and active valve regulation of inflow and outflow. The valve regulation may be based on measurements of flow, liquid level or the piston position in the pump, and would require complicated auxiliary systems.

For most reciprocating pump systems that do not comprises a pulsation damper (e.g. an accumulator), the following must be in place in order to have a non-pulsating system

1. at least three pumps with overlapping sequences.

2. the velocity-gradients of inflow to pump and outflow from pump must be below a certain limit.

Systems with a pulsation damper may be stable for a given design condition.

However, for subsea systems typically having large variations in operation conditions, it is difficult to size pulsation dampers to provide sufficient damping in the full operation range. Alternatively the damper characteristic must be actively controlled and thereby further increase the system complexity.

Conventional reciprocating pumps will have a defined piston movement decided by the shape and speed of a crank- shaft, or by a defined flow and volume of a hydraulic power medium. This defined piston movement ensures predictable piston position versus time, and hence predictable pump inflow and outflow.

Reciprocating gas operated pumps in subsea compression systems are relatively novel solutions with only a few demonstrator test setups and no commercial deliveries. No published information exists on methods for controlling the pump inflow and outflow for such subsea pumping systems.

For subsea reciprocating gas operated pumps, the principles of operation are as follows:

Pump Charging Sequence: A valve opens that relieves the gas pressure of the operating gas, and the liquid overpressure facilitates liquid flow into a pump chamber. The filling cycle ends when the pump is full or when the gas-relief valve closes.

Pump Discharging Sequence: A valve opens that provides high pressure operating gas into the pump chamber, and liquid is displaced out of the pump. The discharge sequence ends when the pump chamber is empty or when the high pressure gas valve closes.

The challenge with these pumping sequences is that the piston position versus time is unknown, and for some cycles the pump chamber will be either full or empty with a certain time-delay before the next cycle start. This time-delay may cause loss of overlap with the other pump-chambers in the pump system. Also the liquid flow- rate may be very high at the time the pump chamber is suddenly full or empty, with resulting high inertial forces due to deceleration. The effect of loss of overlap and high fluid inertial forces, may independent of each other, result in harmful pressure peaks.

Based on the prior art there is a need for subsea boosting of liquids which avoids a dedicated electric power supply system, and with minimum requirements to auxiliaries and active control. The goal of the present invention is to provide a fluid operated reciprocating pump for subsea boosting of liquids which will not require the use of complicated control systems or accumulators, and which will secure stable operation in a wide operation range for the boosting system.

Summary of the invention

The present invention is defined by the appended claims, and in the following: In a first aspect, the present invention provides a pump comprising a housing, the housing comprises a first fluid section and a second fluid section separated by a reciprocating pump element, wherein

the first fluid section comprises an inlet/outlet for a drive fluid; the second fluid section comprises an inlet and an outlet for a liquid, each of the inlet and the outlet comprises a one-way valve, such that a liquid may enter the second fluid section via the inlet and is forced to exit the second fluid section via the outlet, during use; and

the reciprocating pump element is movable between a first position, wherein the volume of the first fluid section is minimized (and/or the volume of the second fluid section is maximized) and a second position, wherein the volume of the first fluid section is maximized (and/or the volume of the second fluid section is minimized); wherein the pump comprises damping means arranged such that the reciprocating pump element encounters an increased motion resistance when approaching the first and the second position, respectively, during use.

Preferably, the reciprocating pump element encounters the increased motion resistance when approaching the first and the second position after passing the halfway point. The halfway point is the position at which the reciprocating pump element is halfway between the first and the second position.

Alternatively, the damping means are arranged such that:

the reciprocating pump element encounters an increasing motion resistance when approaching the first and the second position, respectively; or the reciprocating pump element encounters an increasing resistance after passing the halfway point and before arriving at the first and the second position, respectively; or the speed of the reciprocating pump element is reduced after passing the halfway point and approaching the first and the second position, respectively; or the resistance encountered by the reciprocating pump element when said pump element is moved towards the first or the second position after passing the halfway point will gradually increase; or the resistance encountered by the reciprocating pump element, when said pump element is moved towards the first and the second position, respectively, after passing the halfway point, during use, is gradually increased; the resistance encountered by the reciprocating pump element is gradually increased before said pump element reaches the first and/or the second position after passing the halfway point, during use. - a resistance encountered by the reciprocating pump element is gradually increased until said pump element reaches the first and the second position, respectively, after passing the halfway point, during use. the speed of the reciprocating pump element is gradually reduced before said pump element reaches the first and the second position, respectively, after passing the halfway point, during use.

The inlet/outlet for the drive fluid may comprise a separate inlet and a separate outlet, i.e. constitutes two separate fluid pathways connected to the first fluid section, or the inlet and the outlet are identical, i.e. constitutes a single fluid pathway connected to the first fluid section.

In a preferred embodiment, the drive fluid is a gas or a liquid. In one embodiment of the pump, the reciprocating pump element encounters a gradually increased motion resistance after passing the halfway point and

approaching the first and the second position.

In one embodiment of the pump, the damping means comprises at least one fluid flow-restricting element arranged to gradually decrease a cross-section of a fluid flow path through which a gas or liquid must pass during use, when the

reciprocating pump element moves towards the first position and the second position, respectively.

In one embodiment of the pump, the at least one fluid flow-restricting element comprises a restricting body, and at least a part of the restricting body has a gradually decreasing circumference in a cross-section being parallel to a transverse cross-section of the fluid flow path. The gradually decreasing circumference of the restricting body may decrease in a direction towards the fluid flow path.

In one embodiment of the pump, the largest circumference of the part of the restricting body having a gradually decreasing circumference is substantially equal to the circumference of the transverse cross-section of the fluid flow path.

In one embodiment of the pump, the reciprocating pump element comprises a piston, arranged in a bore, or a membrane. In one embodiment of the pump, the fluid flow-restricting element is connected to the reciprocating pump element.

In one embodiment of the pump, the fluid flow-restricting element comprises the piston.

In one embodiment of the pump, the damping means comprises at least one elastic element providing an increasing resistance against the movement of the

reciprocating pump element when the reciprocating pump element has passed the halfway point and approaches the first or the second position.

In one embodiment of the pump, the at least one elastic element is a spring having at least one end attached to an internal surface of the first or second fluid section or to the reciprocating pump element.

In one embodiment, the pump comprises at least one valve for controlling the pressure of the drive fluid in the first fluid section, and/or for controlling the flow of the drive fluid into or out of the first fluid section.

In a second aspect, the invention provides a pump system comprising at least three pumps according to any of the embodiments described above. The use of at least three pumps provides a sufficient overlap of the pump sequences to avoid unwanted pressure spikes. The pump system is advantageously arranged for boosting a liquid in a subsea gas compression system. In a third aspect, the present invention provides for the use of a pump according to the first aspect in a pump system for boosting liquid in a subsea gas compression system.

In one embodiment of the pump system, each of the inlets/outlets for a drive fluid is connected to a drive fluid distributing valve, and each of the inlets for a liquid is connected to a common inlet pipe and each of the outlets for a liquid is connected to a common outlet pipe, wherein the drive fluid distributing valve is arranged to drive the pumps with overlapping sequences. Overlapping sequences ensures that each pump starts its liquid charging sequence before the liquid charging sequence of a preceding pump has ended. Short description of the drawings

Embodiments of the present invention are described in detail by reference to the following drawings: Figs. 1-5 are cross-sectional views of various embodiments of a reciprocating pump according to the invention, wherein the damping means are based on a restricting a fluid flow path.

Figs. 6-7 are cross-sectional views of various embodiments of a reciprocating pump according to the invention, wherein the damping means are based on elastic elements.

Fig. 8 is a cross-sectional view of an embodiment of a reciprocating pump according to the invention, wherein the damping means are obtained by a combination of restricting a fluid flow path and the use of an elastic element.

Fig. 9 is a cross-sectional view of an embodiment of a reciprocating pump according to the invention, wherein the reciprocating pump element is a membrane. Fig. 10 is a perspective view of a pump system comprising four reciprocating pumps according to the invention.

Detailed description of the invention

Several embodiments of a reciprocating pump according to the invention are shown in figs. 1-9. Corresponding technical features in the various embodiments have been given the same reference numbers. All embodiments of the invention operate by the same principle as discussed above, i.e. the working cycle comprises:

A pump charging sequence, wherein a gas-relief valve (not shown in the figures) opens and relieves the gas pressure of the operating gas (i.e. the drive fluid) in the first fluid section 2 of the pump via an inlet/outlet 5 (the inlet/outlet may be the same port or two different ports), and a liquid overpressure ensures that the liquid flows into the second fluid section 3 of the pump, see all figs. The pump charging sequence ends when the second fluid section 3 is full or when the gas-relief valve closes; and

A pump discharging sequence, wherein a high-pressure gas valve (not shown in the figures, the gas-relief valve and the high-pressure gas valve are not necessarily two separate valves, and the function of the valves may for instance be provided by a three-way valve) opens such that a high-pressure operating gas is provided into the first fluid section 2 of the pump, and liquid is displaced out of the second fluid section of the pump. The discharge sequence ends when the second fluid section is minimized or when the high- pressure gas valve closes.

Although the gas pressure in the pump charging/discharging sequences are described as being controlled by a gas-relief valve and a high-pressure gas valve, the fluctuating gas pressure driving the pump may be provided by any other suitable means.

The embodiments of figs. 1-9 show various solutions for obtaining a damping of the reciprocating pump element 4, 14. By providing the reciprocating pump with means for damping the motion of the reciprocating pump element (i.e. damping means), the following advantages are obtained:

The stroke duration is extended, such that the liquid flow exiting the pump during the pump discharging sequence will not come to a full stop with a time delay before the pump charging sequence start. By use of the damping means, the "time delay" in the cycle: Charging - time delay - discharging- time delay - charging etc. is reduced or removed; and

The hard stop that occurs if a reciprocating pump element reaches its end position at high speed is avoided.

The embodiments of the invention described below provide a reciprocating pump design having a built-in tolerance for operational variations, and no system measuring and control system is required for a smooth operation. Figs. 1 and 2 show two similar embodiments of a reciprocating pump. The reciprocating pump in fig. 1 comprises a piston 4 (i.e. a reciprocating pump element) separating a first fluid section 2 and a second fluid section 3. The piston is arranged in a bore and is movable between a first position and a second position. In the first position, the volume of the first fluid section 2 is minimized (i.e. the piston is in its end position in the direction of the first fluid section 2) and the volume of the second fluid section 3 is maximized. In the second position, the volume of the first fluid section 2 is maximized (i.e. the piston is in its end position in the direction of the second fluid section 3) and the volume of the first fluid section 2 is maximized. The piston comprises damping means in the form of a fluid flow- restricting element 10 having a first restricting body 10a and a second restricting body 10b. Both the first and the second restricting body has a gradually decreasing circumference, in a cross-section being parallel to a transverse cross-section of the fluid flow path l ib arranged in the second fluid section 3, and which decreases in a direction towards the fluid flow path. The first and the second restricting body are arranged on opposite sides of the fluid flow path 1 lb. The fluid flow-restricting element 10 is situated on the side of the piston facing the second fluid section 3, and is able to gradually decrease the cross- section of the fluid flow path 1 lb through which a pressurized liquid must pass during the pump charging sequence and the pump discharging sequence.

During the pump charging sequence, the high pressure of the gas (i.e. the drive fluid) in the first fluid section is lowered by opening a gas-relief valve and the piston 4, and consequently the fluid flow-restricting element 10, moves in the direction of the first fluid section 2, such that the pressurized liquid enters the second fluid section 3 through the one-way valve 8 (e.g. a non-return check valve, flap valve etc.). When the piston 4 passes the halfway point (i.e. the position halfway between the first position and the second position) and moves toward the first position, the second restricting body 10b will gradually decrease the fluid flow path l ib (i.e. gradually decrease the area of a cross-section of the fluid flow path). By decreasing the fluid flow path, the piston will encounter an increased motion resistance when approaching the first position and the speed of the piston will decrease. In this manner, the stroke duration is extended, and the piston will not come to a hard stop when reaching the first position.

During the pump discharging sequence, the gas-relief valve is closed and a high- pressure gas valve is opened. The piston 4 will then move in the direction of the second fluid section 3, such that the pressurized liquid exits the second fluid section 3 through the one-way valve 9. When the piston 4 passes the halfway point and moves toward the second position, the first restricting body 10a will gradually decrease the fluid flow path 1 lb. Similar to the effect during the pump charging sequence, the piston will encounter an increased motion resistance when

approaching the second position and the speed of the piston will decrease.

The embodiment of fig. 1, provides a hydraulic damping effect, both during the pump charging sequence and the pump discharging sequence, since the fluid flow path 1 lb gradually restricted during both sequences is for the liquid.

The embodiment of fig. 2 provides the damping effect by use of the same type of fluid flow-restricting element as shown in fig. 1. However, in case of the reciprocating pump in fig. 2, the fluid flow-restricting element is situated on the side of the piston facing the first fluid section 2, and is able to gradually decrease the cross-section of the fluid flow path 11a through which a pressurized gas must pass during the pump charging sequence and the pump discharging sequence. The feature of having the fluid flow-restricting element arranged in the first fluid section 2 provides a pneumatic damping effect, both during the pump charging sequence and the pump discharging sequence, since the fluid flow path 11a gradually restricted during both sequences is for the gas. The remaining features of the embodiment of fig. 2 are similar to the embodiment of fig. 1.

In a further embodiment, not shown, a reciprocating pump may advantageously comprise a combination of both the hydraulic dampening of fig. 1 and the pneumatic dampening of fig. 2. In such an embodiment, the piston will comprise both a fluid flow-restricting element (as shown in figs. 1 and 2) situated on the side of the piston facing the first fluid section and a fluid flow-restricting element situated on the side of the piston facing the second fluid section.

A reciprocating pump, providing a pneumatic dampening during the pump charging sequence and a hydraulic dampening during the pump discharging sequence, is shown in fig. 3. In this case, the piston comprises a first restricting body 10a, situated on the side of the piston facing the second fluid section 3, and a second restricting body 10b, facing the first fluid section 2. During the pump charging sequence, the second restricting body 10b will gradually decrease the fluid flow path 11a arranged in the first fluid section, thus providing a pneumatic damping effect. During the pump discharging sequence, the first restricting body 10a will gradually decrease the fluid flow path l ib arranged in the second fluid section 3, thus providing a hydraulic damping effect. In addition to the two one-way valves 8,9 for controlling the inflow/outflow of the liquid, the reciprocating pump comprises a first one-way valve 15a bypassing the fluid flow path 11a in the first fluid section 2 and a second one-way valve 15b bypassing the fluid flow path l ib in the second fluid section 3. The first one-way valve is closed during the pump charging sequence, but is open during the pump discharging sequence to prevent under-pressure from restricting piston movement in the start of the pump

discharging sequence. The second one-way valve provides the same effect during the start of the pump charging sequence. In the reciprocating pump shown in fig. 4, the piston 4 is also a fluid flow- restricting element. During the pump charging sequence, the piston 4 will gradually decrease the fluid flow path provided by the inlet 5 in the first fluid section 2, thus providing a pneumatic damping effect. Correspondingly, during the pump discharging sequence, the piston 4 will gradually decrease the fluid flow path provided by the outlet 5 in the second fluid section 3, thus providing a hydraulic damping effect. Similar to the reciprocating pump shown in fig. 4, the piston of the pump in fig. 5 is also a fluid flow-restricting element. However, in fig. 5 the fluid flow paths, which may be decreased, is provided by the first set of through-going openings 12a and the second set of through-going openings 12b arranged in the pipe element 16. The through-going openings are preferably longitudinally extended along the axis of the piston movement. The inner bore of piston 4 (i.e. the bore through which the pipe element 16 passes) have an internal sealing/barrier 17, such that the first set of through-going openings 12a and the second set of through-going openings 12b are not in fluid communication. The pipe element passes through the piston. During the pump charging sequence, the piston 4 will gradually decrease the fluid flow path provided by the first set of through-going openings 12a in the first fluid section 2, thus providing a pneumatic damping effect. Correspondingly, during the pump discharging sequence, the piston 4 will gradually decrease the fluid flow path provided by the second set of through-going openings 12b in the second fluid section 3, thus providing a hydraulic damping effect.

In the reciprocating pumps shown in fig. 6 and 7, the damping effect is obtained by use of springs 13 (i.e. elastic elements) arranged in the first fluid section 2 and/or in the second fluid section 3.

In the embodiment of fig. 6, one end of a single spring 13 is connected to the side of the piston facing the first fluid section and the other end connected to an internal surface of the housing. When the piston is at the halfway point between the first and the second position (as defined above), the spring is neutral (i.e. the spring does not provide any force on the piston). During the pump charging sequence, the movement of the piston 4 causes the spring to compress, thus providing a gradually increased resistance against the piston movement, i.e. a mechanical damping effect. When the piston moves in the opposite direction, i.e. during the pump discharging sequence, the spring is extended and will thus also provide a mechanical damping effect.

In the embodiment of fig. 7, multiple springs 13a, 13b are arranged both in the first fluid section 2 and in the second fluid section 3. The first set of springs 13a is arranged in the first fluid section 2 having their first end connected to an internal surface of the housing, and the second set of springs 13b is arranged in the second fluid section 3 having their first end connected to an internal surface of the housing. During the pump charging sequence, the movement of the piston 4 causes the first set of springs 13a to compress, thus providing a gradually increased resistance against the piston movement, i.e. a mechanical damping effect. Correspondingly, when the piston moves in the opposite direction, i.e. during the pump discharging sequence, the piston causes the second set of springs 13b to compress. In the reciprocating pump shown in fig.8, the damping means comprises a spring 13a, 13b and a fluid flow-restricting element 10 arranged in each of the first fluid section and the second fluid section. The fluid flow-restricting element is

constituted by a restricting body 10a, 10b, wherein one end of the spring is connected to the restricting body and the other end is connected to an internal surface of the housing. The restricting body comprises a gradually decreasing circumference in a cross-section being parallel to a transverse cross-section of a corresponding fluid flow path 1 la, 1 lb. During the pump charging sequence, the piston 4 will first push towards the restricting body 10a which in turn will compress the spring 13a. The effect of having both a spring and a fluid flow-restricting element is that a combination of both pneumatic and mechanical damping is achieved during the pump charging sequence. Correspondingly, a combination of both hydraulic and mechanical damping is achieved during the pump discharging sequence.

The embodiments of figs. 1-8 have in common that the reciprocating pump element is a piston 4. However, the present invention is not in any way restricted to piston- based reciprocating pumps. An alternative embodiment, wherein the reciprocating pump element comprises or is a membrane 14 or diaphragm is shown in fig. 9. The membrane separates the first fluid section 2 and the second fluid section 3, and comprises a fluid flow-restricting element 10 having a restricting body. The restricting body comprises a gradually decreasing circumference and is able to restrict both the fluid flow path 11a arranged in the first fluid section 2 and the fluid flow path 1 lb arranged in the second fluid section. During the pump charging sequence, the fluid flow-restricting element 10 of the membrane 14 will gradually decrease the fluid flow path 1 la in the first fluid section 2, thus providing a pneumatic damping effect. Correspondingly, during the pump discharging sequence, the fluid flow-restricting element 10 of the membrane 14 will gradually decrease the fluid flow path l ib in the second fluid section 3, thus providing a hydraulic damping effect.

Although the embodiments of figs. 1-8 are described by use of gas as the drive fluid entering/exiting the first fluid section 2, i.e. a gas/liquid pump, the pump according to the invention may also use a liquid as the drive fluid.

A pump system featuring four pumps according to the invention is shown in fig. 10. Each of the four pumps has the inlet/outlet 5 for a drive fluid connected to a drive fluid distributing valve 18 (or valve arrangement). An example of a suitable drive fluid distributing valve is shown for example in Norwegian patent no. 332878. In other embodiments, the fluid distributing valve may be any type of valve

arrangement able to provide the pumps with the drive fluid in a required sequence as described below. Further, each of the four pumps has the inlet 6 for a liquid connected to a common inlet pipe 19 and the outlet 7 for a liquid connected to a common outlet pipe 20. The drive fluid distributing valve 18 is arranged to provide drive fluid pressure pulses to the pumps in overlapping sequences. The overlapping sequences are timed such that each pump starts a liquid charging sequence, i.e. liquid enters the inlet 6, before the liquid charging sequence of a preceding pump has ended.

Claims

A pump comprising a housing (1), the housing comprises a first fluid section (2) and a second fluid section (3) separated by a reciprocating pump element (4), wherein

the first fluid section

(2) comprises an inlet/outlet (5) for a drive fluid; the second fluid section comprises an inlet (6) and an outlet (7) for a liquid, each of the inlet and the outlet comprises a one-way valve (8,9), such that a liquid may enter the second fluid section (3) via the inlet (6) and is forced to exit the second fluid section

(3) via the outlet (7) during use; and

the reciprocating pump element (4) is movable between a first position, wherein the volume of the first fluid section (2) is minimized and a second position, wherein the volume of the first fluid section (2) is maximized; wherein the pump comprises damping means (4, 10, 13) arranged such that the reciprocating pump element encounters an increased motion resistance when approaching the first and the second position, respectively, during use.

The pump according to claim 1, wherein the reciprocating pump element (4) encounters the increased motion resistance when approaching the first and the second position after passing the halfway point.

The pump according to claim 1, wherein the reciprocating pump element encounters a gradually increased motion resistance after passing the halfway point and approaching the first and the second position.

A pump according to any of the claims 1 to 3, wherein the damping means comprises at least one fluid flow-restricting element (10,

4) arranged to gradually decrease a cross-section of a fluid flow path (1 la, l lb,

5,7, 12a,12b) through which a gas or liquid must pass, during use, when the reciprocating pump element (4, 14) moves towards the first position and the second position, respectively, preferably after passing the halfway point.

A pump according to claim 4, wherein the at least one fluid flow-restricting element (10) comprises a restricting body (10a, 10b), and at least a part of the restricting body has a gradually decreasing circumference in a cross- section being parallel to a transverse cross-section of the fluid flow path

6. A pump according to claim 5, wherein the gradually decreasing

circumference of the restricting body (10a, 10b) decreases in a direction towards the fluid flow path (11a, l ib).

7. A pump according to any of the preceding claims, wherein the reciprocating pump element comprises a piston (4), arranged in a bore, or a membrane (14).

8. A pump according to any of the claims 4 to 7, wherein the fluid flow- restricting element is connected to the reciprocating pump element.

9. A pump according to claim 4, wherein the fluid flow-restricting element comprises the piston.

10. A pump according to any of the preceding claims, wherein the damping means comprises at least one elastic element (13) providing an increasing resistance against the movement of the reciprocating pump element when the reciprocating pump element approaches the first and/or the second position, respectively.

11. A pump according to claim 10, wherein the at least one elastic element is a spring (13) having at least one end attached to an internal surface of the first or second fluid section, or to the reciprocating pump element (4).

12. A pump according to any of the preceding claims, comprising at least one valve for controlling the pressure of the drive fluid in the first fluid section (2).

13. A pump system comprising at least three pumps according to any of the preceding claims.

14. The pump system according to claim 13, wherein each of the inlets/outlets (5) for a drive fluid is connected to a drive fluid distributing valve (18), and each of the inlets (6) for a liquid is connected to a common inlet pipe (19) and each of the outlets (7) for a liquid is connected to a common outlet pipe (20), wherein the drive fluid distributing valve (18) is arranged to drive the pumps with overlapping sequences.

15. The use of a pump according to any of claims 1- 12 for boosting a liquid in a subsea gas compression system.