WO2017137498A1

WO2017137498A1 - Pump

Info

Publication number: WO2017137498A1
Application number: PCT/EP2017/052885
Authority: WO
Inventors: Leif Arne TØNNESSEN
Original assignee: Fmc Kongsberg Subsea As
Priority date: 2016-02-12
Filing date: 2017-02-09
Publication date: 2017-08-17
Also published as: NO20160240A1

Abstract

A pump comprises a housing (1) with a first fluid section (2), a second fluid section (3) and at least one third fluid section (4) separated by a reciprocating displacement assembly (5), wherein the reciprocating displacement assembly (5) comprises a first displacing element (6) and a second displacing element (7). The first displacing element (6) has a first pressure surface (8) in fluid contact with the first fluid section (2) and a second pressure surface (9) in fluid contact with the second fluid section (3). The second displacing element (7) is operatively connected to the first displacing element (6) and comprises at least one third pressure surface (10) in fluid contact with the at least one third fluid section (4). The first fluid section (2) comprises an inlet/outlet (11) for a drive fluid; the second fluid section (3) comprises an inlet/outlet (12) for a fluid; and the at least one third fluid section (4) comprises an inlet (13) and an outlet (14) for a liquid, each of the inlet (13) and the outlet (14) comprising a one-way valve (15, 16). The pump further comprises a valve assembly (23) for controlling the pressure of the drive fluid in the first fluid section (2) and the second fluid section (3).

Description

PUMP

Technical field of the invention The present invention relates to the field of reciprocating pumps, and more specifically to reciprocating pumps using pressurized gas or liquid as the 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 some concepts for direct well stream compression by use of wet gas compressor exist, 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. In current pump systems a number of design limitations are present. For instance, the pressure boost of the liquid is restricted by the pressure of the compressed gas and the liquid to be boosted must also have a minimum initial pressure in order to charge the pump chamber prior to discharge. Reciprocating gas-operated pumps in subsea compression systems are relatively novel solutions with only a few demonstrator test setups and no commercial deliveries.

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 goal of the present invention is to provide pumps and pump systems avoiding at least some of the disadvantages of the prior art.

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, a second fluid section and at least one third fluid section separated by a reciprocating displacement assembly, wherein

the reciprocating displacement assembly comprises a first displacing element and a second displacing element;

o the first displacing element having a first pressure surface in fluid contact with the first fluid section and a second pressure surface in fluid contact with the second fluid section; and

o the second displacing element is operatively connected to the first displacing element and comprises at least one third pressure surface in fluid contact with the at least one third fluid section; - the first fluid section comprises an inlet/outlet for a drive fluid;

the second fluid section comprises an inlet/outlet for a fluid;

the at least one third 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 at least one third fluid section via the inlet and is forced to exit the at least one third fluid section via the outlet during use; and

the reciprocating displacement assembly is movable between a first position, wherein the volume of the first fluid section is minimized and a second position, wherein the volume of the first fluid section is maximized; wherein the reciprocating displacement assembly is movable between a first position, wherein the volume of the first fluid section is minimized and the volume of the second fluid section and one of the at least one third fluid section is maximized, and a second position, wherein the volume of the first fluid section is maximized and the volume of the second fluid section and one of the at least one third fluid section is minimized, and wherein the pump comprises a valve assembly for controlling the pressure of the drive fluid in the first fluid section (2) and the second fluid section (3).

In an advantageous embodiment of the pump, the inlet/outlet of the second fluid section is for a fluid able to provide pressure to the second pressure surface, i.e. a drive fluid, thus facilitating the charging of the at least one third fluid section during use.

In an advantageous embodiment of the pump, the area of the first pressure surface is different from the area of the at least one third pressure surface, such that the pressure of the liquid exiting the outlet of the at least one third fluid section is different from the pressure of the driving fluid in the second fluid section during use. In other words, the first pressure surface can be smaller or larger than the at least one third pressure surface, such that the pressure of the liquid exiting the outlet of the at least one third fluid section is lower or higher than the pressure of the driving fluid in the second fluid section during use.

The possibility for applying different first pressure surface areas compared to the third pressure surface area provides an opportunity to specifically adapt pressure of the liquid in the third fluid section to the need of the application.

In an embodiment of the pump, the volume of the second fluid section and one of the at least one third fluid sections is maximized in the first position, and minimized in the second position.

In an embodiment of the pump, the first displacing element comprises a piston or a membrane, and the second displacing element comprises a piston or a membrane, wherein the first displacing element and the second displacing element is

operationally connected and/or are parts of a single unit, such that the first and the second displacing element will move in the same direction during use.

In an embodiment of the pump, the first displacing element comprises a piston or membrane having a first cross- sectional area, and the second displacing element comprises a piston or membrane having a second cross-sectional area, wherein the size of the first cross-sectional area is different from the size of the second cross- sectional area. In one embodiment, the first cross-sectional area is larger than the second cross-sectional area.

In yet an embodiment, the pump comprises two separate third fluid sections arranged at opposite ends of the reciprocating displacement assembly, each third fluid section comprises an inlet and an outlet for a liquid, and the second displacing element comprises two third pressure surfaces, each of the third fluid sections in fluid contact with one of the third pressure surfaces, i.e. one of the third fluid sections is in fluid contact with one of the third pressure surfaces, while the other third fluid section is in fluid contact with the other one of the third pressure surfaces. The two third pressure surfaces are advantageously arranged at opposite ends of the reciprocating displacement assembly.

In an embodiment of the pump, the drive fluid is a gas or a liquid.

In a further embodiment of the pump, the first pressure surface is larger than the at least one third pressure surface, such that the pressure of the liquid exiting the outlet of the at least one third fluid section is higher than the maximum pressure of the driving fluid in the second fluid section during use.

In an embodiment of the pump, the reciprocating displacement assembly comprises at least one piston arranged in a bore and/or at least one membrane dividing a fluid chamber, the fluid chamber may comprise the first and the second fluid section and/or the second and the third fluid section.

In a second aspect, the invention provides a pump system comprising at least one pump according to the first aspect.

In one embodiment, the pump system comprises a drive fluid source having a low- pressure section and a high-pressure section, a liquid source and a valve assembly, wherein the inlet/outlet of the first fluid section is fluidly connected to the high- pressure side and the low-pressure side of the compressor via the valve assembly;

the inlet/outlet of the second fluid section is fluidly connected to at least the low-pressure side of the compressor; and

the inlet of the third fluid section is fluidly connected to the liquid source; wherein the valve assembly is arranged such that the pump may be operated by providing alternating high-pressure and low-pressure drive fluid to the first fluid section.

In one embodiment of the pump system, the differential pressure between drive fluid in the low-pressure section and drive fluid in the high-pressure section is provided by a gas compressor, a liquid pump, or at least one drive fluid flow- restriction. When the differential pressure is provided by a drive fluid flow- restriction, the high-pressure section is upstream the low-pressure section. In an embodiment of the pump system, the inlet/outlet of the second fluid section is fluidly connected to the high-pressure section and the low-pressure section via the valve assembly, and the valve assembly is arranged such that alternating high- pressure and low-pressure drive fluid may be provided in the second fluid section. The alternating high-pressure and low-pressure drive fluid in the second fluid section is 180 degrees phase delayed compared to the alternating pressure in the first fluid section, i.e. the pressure in first fluid section is high at the same time as pressure in the second fluid section is low (and vice versa).

In an embodiment of the pump system, drive fluid and liquid is provided by a gas/liquid separator.

In one embodiment, the pump system comprises at least three pumps according to any of the embodiments above, and a valve assembly arranged to drive the pumps with overlapping sequences.

The inlet/outlet for the drive fluid arranged in the first fluid section, and the inlet/outlet for a fluid in the second fluid section, may comprise a separate inlet and a separate outlet, i.e. may constitute two separate fluid pathways connected to the first fluid section and the second fluid section, respectively, or the inlet and the outlet are identical, i.e. constitute a single fluid pathway connected to the first fluid section and the second fluid section, respectively. In one embodiment, the pump system 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.

The pump and pump system according to the invention are preferably a subsea pump and a subsea pump system, respectively. The pump and/or pump system may advantageously be part of a subsea gas compression system.

In a third aspect, the present invention provides for the use of a pump according to the first aspect or a pump system according to the second aspect for boosting a liquid in a subsea gas compression system.

In a fourth aspect, the present invention provides a method of boosting liquid in a subsea gas compression system comprising the step of providing a pump according to the first aspect, boosting the pressure of a liquid in the third fluid section by introducing a high-pressure gas in the first fluid section. In one embodiment, the method comprises the step of introducing a liquid into the third fluid section by introducing a high-pressure gas into the second fluid section.

The term "pressure surface" is intended to define a surface on which an applied force/pressure will contribute to the movement of the reciprocating displacement assembly.

The term "third fluid section" may also be defined as a "liquid section".

Short description of the drawings

Embodiments of the present invention are described in detail by reference to the following drawings:

Fig. 1 is a schematic view of a prior art pump system featuring a reciprocating pump.

Fig. 2 is a schematic view of a reciprocating pump, and a pump system, according to one embodiment of the invention.

Fig. 3 is a schematic view of a reciprocating pump, and a pump system, according to one embodiment of the invention. Fig. 4 is a schematic view of a reciprocating pump, and a pump system, according to one embodiment of the invention.

Figs. 5- 10 are schematic views of various embodiments of reciprocating pumps according to the invention.

Detailed description of the invention Several embodiments of a system and a reciprocating pump according to the invention are shown in figs. 2- 10. Corresponding technical features (i.e. features providing the same or similar technical effect) in the various embodiments have been given the same reference numbers. For illustrative purposes, a pump system featuring a prior art reciprocating pump 24, a compressor 18 and a gas/liquid separator 17 is shown in fig. 1. The prior art pump comprises a drive fluid section 2 and a liquid section 4 separated by a piston 5 having a drive fluid pressure surface 8, in fluid contact with the drive fluid section, and a liquid pressure surface 10, in fluid contact with the liquid section 4. The area of the drive fluid pressure surface 8 is equal to the area of the drive fluid section, and based on the principles described below, it is clear that the pressure of the boosted liquid exiting outlet 14 may not exceed the pressure of the drive fluid.

Both the prior art pump in fig. 1 and the 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 25 opens and relieves the gas pressure of the operating gas (i.e. the drive fluid) in the drive fluid section 2 (i.e. the first fluid section) of the pump via an inlet/outlet 11 (the inlet/outlet may be the same port or two different ports), and the liquid flows into the liquid section 4 (i.e. the third fluid section) of the pump, see all figs. It should be noted that the pump according to the invention, as opposed to the prior art pumps, is not necessarily dependent on a liquid overpressure to ensure a liquid flow into the liquid section 4 in the charging sequence, see description below. The pump charging sequence ends when the liquid section 4 is full or when the gas-relief valve closes; and

A pump discharging sequence, wherein a high-pressure gas valve 26 (the gas-relief valve 25 and the high-pressure gas valve 26 are not necessarily two separate valves, and the function of the valves may for instance be provided by a three-way valve or any suitable valve assembly/arrangement) opens such that a high-pressure operating gas is provided into the drive fluid section 2 of the pump, and liquid is displaced out of the liquid section 4 of the pump. The discharge sequence ends when the liquid section (i.e. third fluid section) is minimized or when the high-pressure gas valve 26 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, i.e. a valve assembly 23, the fluctuating gas pressure driving the pump may be provided by any other suitable means.

The embodiments of figs. 2-7 show various embodiments of a reciprocating pump according to the invention, while two systems comprising a reciprocating pump according to the invention is shown in figs. 8- 10. All the embodiments of a reciprocating pump according to the invention have the advantage of being able to boost the liquid in the third fluid section 4

(corresponding to the liquid in the liquid section 4 described for the prior art pump of fig. 1) to a higher pressure than what is possible with a prior art pump using the same drive fluid pressure. Depending on the system and valve assembly, use of the reciprocating pump according to the invention will also remove constraints regarding the liquid column (or pressure) required for introducing liquid into the pump during the charging sequence. The operation of the embodiments of figs. 2-7 is described by use of the terms high-pressure drive fluid and low-pressure drive fluid. The required drive fluids may for instance be obtained by connecting the inlet/outlet of the first and second fluid section of a pump to the low-pressure side and the high-pressure side of a compressor, i.e. a source of low-pressure drive fluid and high-pressure drive fluid. The pump is connected to the source via a valve assembly able to control the drive fluid flow. The systems in figs. 8- 10 show various options for connecting the pumps to a drive fluid source.

A first embodiment of a reciprocating pump according to the invention is shown in fig. 2. The pump comprises a housing 1 having a first fluid section 2, a second fluid section 3 and a third fluid section 4. The first fluid section corresponds to the drive fluid section, and the third fluid section corresponds to the liquid section, described in connection with the prior art pump in fig. 1. The first, second and third fluid sections are separated by a piston 5 (i.e. a reciprocating displacement assembly) having a first piston element 6 (i.e. a first displacing element) and a second piston element 7 (i.e. a second displacing element). The first piston element 6 has a first pressure surface 8 in fluid contact with the first fluid section 2 and a second pressure surface 9 in fluid contact with the second fluid section 3. The second piston element 7 extend from the first piston element (i.e. is operatively connected to the first piston element) and comprises a third pressure surface 10 in fluid contact with the third fluid section 4. The first fluid section comprises an inlet/outlet 11 for a drive fluid, the second fluid section comprises an inlet/outlet 12 for a drive fluid and the third fluid section comprises an inlet 13 and an outlet 14 for a liquid to be boosted/pressurized. Both the inlet and the outlet of the third fluid section have a one-way valve 15, 16, such that a liquid may enter the third fluid section 4 via the inlet 13 and is forced to exit the third fluid section 4 via the outlet 14.

During the charge sequence, as described above, the pressure of a drive fluid in the first fluid section 2 is lowered/released via the inlet/outlet 11 , and a liquid to be boosted enters the third fluid section 4 via the one-way valve 15. Depending on the system in which the pump is used, the inlet/outlet 12 of the second fluid section may be connected to receive a low-pressure drive fluid or a high-pressure drive fluid during the charge sequence. In the latter case, the high-pressure drive fluid will exert a force on the second pressure surface 9, thus allowing the pump to be charged independent of the initial pressure of the liquid to be boosted (i.e. the pump is not dependent on a high liquid column to force the liquid into the third fluid section 4). During the discharge sequence, as described above, the pressure of a drive fluid in the first fluid section 2 is increased via the inlet/outlet 11, and the boosted liquid exits the third fluid section 4 via the one-way valve 16. The high-pressure drive fluid in the first fluid section acts on the first pressure surface 8, and the third pressure surface 10 of the second piston element 7 acts on the liquid. Since the first pressure surface has a larger area than the third pressure surface 10, the pressure boost of the liquid is larger than the pressure of the high-pressure drive fluid in the first fluid section. During the discharge sequence, the second fluid section is connected to a source of low-pressure drive fluid, such as the low-pressure side of a compressor.

An embodiment, wherein the piston 5 in fig. 2 is replaced by a

membrane/diaphragm assembly (i.e. a reciprocating displacement assembly) is shown in fig. 3. The membrane assembly comprises a first membrane element 6 (i.e. a first displacing element) and a second membrane element 7 (i.e. a second displacing element). The first and second membrane element is operatively connected by the rod 27. An embodiment, wherein the reciprocating displacement assembly comprises a combination of a first piston element 6 and a second membrane element 7 is shown in fig. 4. Similar to the pump in fig. 3, the first and second element is operatively connected by the rod 27. An embodiment of a double-acting pump is shown in fig. 5. In this embodiment, the pump features a first fluid section 2, a second fluid section 3, and two separate third fluid sections 4, 4' . The piston 5 functions as a reciprocating displacement assembly. The two separate third fluid sections 4, 4' are arranged on opposite ends of the piston 5. The piston comprises a first piston element 6 (i.e. first displacing element) and a second piston element 7. The first piston element has a first pressure surface 8, in fluid contact with the first fluid section 2, and a second pressure surface 9, in fluid contact with the second fluid section 3. The first and the second pressure surface have the same area. The second piston element has a pair of third pressure surfaces 10,10' , each of the third pressure surfaces in fluid contact with a third fluid section. Each of the first and the second pressure surface has an area larger than any of the third pressure surfaces. In use, both the inlet/outlet 1 1 of the first fluid section and the inlet/outlet 12 of the second fluid section are connected to a source of high-pressure drive fluid and low-pressure drive fluid. When high- pressure fluid is provided in the first fluid section 2 and low-pressure fluid is provided in the second fluid section 3, a liquid to be boosted will enter one of the two third fluid sections 4' (i.e. a charge sequence) and exit the other third fluid section 4 (i.e. a discharge sequence). The pump is thus driven by alternating between a high-pressure and a low-pressure drive fluid in the first and the second fluid section, and a combined discharge/charge sequence is thus obtained during each stroke of the piston 5. Similar to the embodiments of figs. 2-4, the piston 5 acting as the reciprocating displacement assembly of the double acting pump in fig. 5 may be replaced by membranes or any combination of membranes and pistons as shown in figs. 6 and 7. The function of the embodiments of figs. 6 and 7 is similar to the function of the pump in fig. 5.

In fig. 6, the reciprocating displacement assembly comprises a first membrane element 6 (i.e. a first displacing element) separating the first fluid section 2 and the second fluid section 3, and a second membrane element 7 (i.e. a second displacing element) comprising a first membrane 28 and a second membrane 29. The first membrane 28 separates the first fluid section 2 and one of the two third fluid sections 4' , while the second membrane separates the second fluid section and the other one of the two third fluid sections 4. The first membrane and the second membrane are operatively connected to each other and to the first membrane element via a rod element 27.

In fig. 7, the reciprocating displacement assembly comprises a first piston element 6 separating the first fluid section 2 and the second fluid section 3, and a second membrane element 7 comprising a first membrane 28 and a second membrane 29. The first membrane separates the first fluid section and one of the two third fluid sections 4' , while the second membrane separates the second fluid section and the other one of the two third fluid sections 4. The first membrane and the second membrane are operatively connected to each other and to the first piston element via a rod element 27. The reciprocating pump according to the invention may advantageously comprise damping means to avoid detrimental pressure pulses. Suitable damping means are disclosed in the Norwegian patent application having the title "Pump" which has the same filing date, and was filed by the same applicant, as the present application. The pumps in figs. 2-7 may be used in various systems for boosting a liquid. Such systems are shown in figs. 8- 10.

The system in fig. 8 comprises a reciprocating pump similar to the one shown in fig. 2 (in this specific system, the pump features a single inlet/outlet 12) a gas compressor 18 and a gas/liquid separator 17. The first fluid section is connected to both the high-pressure side 20 and the low-pressure side 19 of the gas compressor 18 via the valve assembly 23. In this system, the valve assembly comprises a high- pressure gas valve 26 and a gas-relief valve 25. By controlling the valves of the valve assembly, the pressure of the drive fluid in the first fluid section may be alternated. The second fluid section is directly connected to the low-pressure side of the gas compressor to freely allow charge/discharge of drive fluid from the second fluid section during operation of the pump. As described for the pump in fig. 2, a system featuring said pump allows for boosting of a liquid to a pressure above the maximum pressure of the drive fluid due to the ratio between the first pressure surface and the second pressure surface. The system in fig. 9 comprises a reciprocating pump as shown in fig. 2, a gas compressor 18 and a gas/liquid separator 17. The first fluid section 2 and the second fluid section 3 are connected to both the high-pressure side 20 and the low-pressure side 19 of the gas compressor 18 via the valve assembly 23. In this system, the valve assembly comprises two high-pressure gas valves 26,26' and two gas-relief valves 25,25' for controlling the inflow/outflow of drive fluid to the first fluid section and the second fluid section, respectively. By controlling the valves of the valve assembly, the pressure of the drive fluid in the first fluid section and the second fluid section may be alternated. By allowing high-pressure fluid to the second fluid section during the charge sequence (i.e. the sequence in which liquid is introduced to the third fluid section), the pump is no longer dependent on the liquid having a certain minimum pressure (i.e. not dependent on a certain liquid column in order for the liquid to enter via the inlet of the third fluid section). The technical effect is also described in connection with the pump in fig. 2. In addition to the ability to operate with a low upstream liquid column, the system also allows for boosting of a liquid to a pressure above the maximum pressure of the drive fluid due to the ratio between the first pressure surface and the second pressure surface, as described for the system in fig. 8.

The system in fig. 10 comprises a double acting reciprocating pump 24, as shown in fig. 5, a gas compressor 18 and a gas/liquid separator 17. The first fluid section 2 and the second fluid section 3 are connected to both the high-pressure side 20 and the low-pressure side 19 of the gas compressor 18 via the valve assembly 23. As shown for the system in fig. 9, the valve assembly comprises two high-pressure gas valves 26,26' and two gas-relief valves 25,25' for controlling the inflow/outflow of drive fluid to the first fluid section and the second fluid section, respectively. The pump comprises two separate third fluid sections 4,4' , as described above for the pump in fig. 5. By controlling the valves of the valve assembly, the pressure of the drive fluid in the first fluid section and the second fluid section may be alternated. The technical effect obtained by the system is described in connection with the pump in fig. 5. In addition to the ability to operate with a low liquid column and boosting a liquid to a pressure above the maximum pressure of the drive fluid due to the ratio between the first pressure surface and the second pressure surface, as described for the system in fig. 9, the system in fig. 10 may also allow for providing the liquid to two separate liquid lines if required (in the present system, the liquid exiting the outlets of the third fluid sections via the separate liquid lines 21 is combined in a common liquid line 22).

The pump and the corresponding systems according to the invention are highly advantageous when a multiphase stream, i.e. liquid and gas, is to be pressurized. After separating the initial multiphase fluid stream into a liquid and a gas, the gas is pressurized by a compressor 18 and the liquid is pressurized by the pump. As described above the compressor discharge gas is used as the drive fluid in the pump, and the liquid may be boosted to a pressure above the pressure of the drive fluid gas due to the pressure amplification of the present invention. Downstream the separate pressure boosting of the gas and the liquid, the two phases are commonly comingled for further transportation. Without the pressure amplification of the present invention, the boosted liquid exiting the pump would have had an initial pressure equal to or lower than the gas exiting the compressor, thus requiring a pressure drop between the compressor discharge and the comingling point of the gas and the liquid. By avoiding the required pressure drop the system designer will have more freedom to optimize the process/system with respect to layout and efficiency.

The valve assembly may be any type of valve arrangement able to provide the required alternating drive fluid pressure. A suitable valve assembly may for instance be a distributor valve as disclosed in Norwegian patent no. 332878. Although the embodiments of figs. 1- 10 are described by use of gas as the drive fluid entering/exiting the first fluid section 2 and the second fluid section 3, i.e. a gas/liquid pump, the pump according to the invention may advantageously also use a liquid as the drive fluid. To summarize, the present invention has at least the following main advantages in view of the prior art:

Pump discharge pressure can be tailored to the need of the application. Pressure can be smaller than, equal to or higher than the pressure of drive fluid. The invention enables the use of fluid operated reciprocating pumps for systems where required pressure for pumped fluid is higher than the pressure of the fluid used as driver fluid. Such system may for instance be a compression system with dedicated separate flowlines for gas and liquid. Pump discharge pressure in the prior art is similar to the pressure of the supplied drive fluid. For compression systems where gas is used as drive fluid for the liquid, liquid will get the same pressure as the supplied drive gas. In case the liquid shall be commingled with the gas for multiphase transport to host, the liquid will need some overpressure in order to flow into the gas flowline. In the prior art this is solved by incorporating a restriction in the gas path upstream the liquid/gas commingling point. Such restriction would not be needed for the described invention.

The pump according to the invention may operate without any requirement for a minimum liquid column. The only requirement for operation of the pump is a differential pressure in the flow path of the fluid used as the driver fluid. This differential pressure can be facilitated by a compressor, a pump or flow

restriction. A low available differential pressure in a flow path used for drive fluid can be compensated by the pump design, i.e. by increasing the size of the first pressure surface, the required piston/membrane force and flow capacity can be met even if low differential pressure is available.

The maximum flow capacity of prior art pumps is limited by system layout and cannot be increased by pump sizing initiatives. The flow into the pump in the pump charging sequence is decided by the overpressure of the liquid at pump inlet versus the drive fluid exhaust pressure. The flow out of the pump in discharging sequence is decided by the difference between pump discharge pressure (equal to pressure of supplied drive fluid) and liquid delivery pressure. For conventional compression systems with multiphase flowlines this differential pressure required for pump discharge is facilitated by a restriction (e.g. provided by a cooler) between the location where drive gas is branched out of the main gas flow and the location where liquid is branched in to the main gas flow for multiphase transport. For compression systems, the max capacity is normally limited by liquid column in upstream scrubber and restrictions in pipes between scrubber and pump inlet. For some applications it might be required with a high vertical distance between the scrubber and the pump in order to meet the required capacity.

Although the invention is described in detail by its use in gas compression systems, the conceptual advantages where the pump discharge pressure can be tailored for the application and the removal of prior art requirement to an upstream overpressure at pump inlet versus drive fluid low pressure section, make the innovation attractive also for other applications, such as:

Circulation of cooling medium in subsea process system, by exploiting available subsea fluid that can be utilized as driving fluid.

Prior art:

Various equipment (in particular electrical equipment) in subsea process systems require cooling by a circulating cooling medium. In prior art, the circulation of cooling medium is provided by an electric pump. In some cases the power requirement for the circulation pump is higher than what normally can be extracted from a control umbilical, and a dedicated power string may be required. In case a soft starter or variable frequency drive is required, this further complicates the system.

For separation over several stages, the innovation can be used to dispose the separated fluid of a late stage into the separated fluid of a prior stage. A pressure boosting is required due to pressure reduced fluid pressure at later stage separation versus fluid at prior stages.

Prior art

The commingling of separated fluid from a late separation stage with a fluid from a prior separation stage is conventionally done with electric pumps or ejectors. Electric pumps have the disadvantage of requiring an electric drive system, and ejectors have the disadvantage of poor efficiency, challenges with pressure control and mixing of drive fluid with boosted fluid. The latter is a disadvantage if drive fluid and boosted fluid needs to be of different fluid type, e.g. if water is used for ejecting oil into oil flowline.

Claims

A pump comprising a housing (1), the housing comprises a first fluid section (2), a second fluid section (3) and at least one third fluid section (4) separated by a reciprocating displacement assembly (5), wherein

the reciprocating displacement assembly (5) comprises a first displacing element (6) and a second displacing element (7);

o the first displacing element (6) having a first pressure surface (8) in fluid contact with the first fluid section (2) and a second pressure surface (9) in fluid contact with the second fluid section (3);

o the second displacing element (7) is operatively connected to the first displacing element (6) and comprises at least one third pressure surface (10) in fluid contact with the at least one third fluid section (4);

the first fluid section (2) comprises an inlet/outlet (11) for a drive fluid; the second fluid section (3) comprises an inlet/outlet (12) for a fluid; the at least one third fluid section (4) comprises an inlet (13) and an outlet (14) for a liquid, each of the inlet and the outlet comprises a oneway valve (15,16), such that a liquid may enter the at least one third fluid section (4) via the inlet (13) and is forced to exit the at least one third fluid section (4) via the outlet (14) during use; and wherein the reciprocating displacement assembly (5) is movable between a first position, wherein the volume of the first fluid section (2) is minimized and the volume of the second fluid section and one of the at least one third fluid section is maximized, and a second position, wherein the volume of the first fluid section (2) is maximized and the volume of the second fluid section and one of the at least one third fluid section is minimized, and wherein the pump comprises a valve assembly for controlling the pressure of the drive fluid in the first fluid section (2) and the second fluid section (3).

The pump according to claim 1, wherein the area of the first pressure surface (8) is different from the area of the at least one third pressure surface (10), such that the pressure of the liquid exiting the outlet of the at least one third fluid section (4) is different from the pressure of the driving fluid in the second fluid section during use.

3. The pump according to claim 1 or 2, wherein the first displacing element comprises a piston or a membrane, and the second displacing element comprises a piston or a membrane, wherein the first displacing element and the second displacing element is operationally connected and/or are parts a single unit.

The pump according to any of the preceding claims, wherein the first displacing element comprises a piston or membrane having a first cross- sectional area, and the second displacing element comprises a piston or membrane having a second cross-sectional area, wherein the first cross- sectional area is different from the second cross-sectional area.

The pump according to any of the preceding claims, comprising two separate third fluid sections (4,4') arranged at opposite ends of the reciprocating displacement assembly, each third fluid section comprises an inlet (13, 13') and an outlet (14, 14') for a liquid, and the second displacing element (7) comprises two third pressure surfaces (10, 10'), each of the third fluid sections (4,4') in fluid contact with one of the third pressure surfaces (10, 10').

The pump according to any of the preceding claims, wherein the drive fluid is a gas or a liquid.

The pump according to any of the preceding claims, wherein the area of the first pressure surface (8) is larger than the area of the at least one third pressure surface (10), such that the pressure of the liquid exiting the outlet of the at least one third fluid section (4) is larger than the pressure of the driving fluid in the second fluid section during use.

A pump system comprising at least one pump according to any of the preceding claims.

The pump system according to claim 8, comprising a drive fluid source having a low-pressure section (19) and a high-pressure section (20), a liquid source (17) and a valve assembly (23), wherein the inlet/outlet (11) of the first fluid section (2) is fluidly connected to the high-pressure side and the low-pressure side of the compressor via the valve assembly;

the inlet/outlet (12) of the second fluid section is fluidly connected to at least the low-pressure side of the compressor; and

the inlet (13 of the third fluid section is fluidly connected to the liquid source; wherein the valve assembly is arranged such that the pump may be operated by providing alternating high-pressure and low-pressure drive fluid to the first fluid section.

10. The pump system according to claim 9, wherein the differential pressure between drive fluid in the low-pressure section (19) and drive fluid in the high-pressure section (20) is provided by a gas compressor (18), a liquid pump, or at least one drive fluid flow restriction.

11. The pump system according to claim 9 or 10, wherein the inlet/outlet (12) of the second fluid section is fluidly connected to the high-pressure section and the low-pressure section via the valve assembly, and the valve assembly is arranged such that alternating high-pressure and low-pressure drive fluid may be provided in the second fluid section.

12. The pump system according to any of claims 9 to 11, wherein a drive fluid and a liquid is provided by a gas/liquid separator (17).

13. The pump system according to any of the claims 9- 12, comprising at least three pumps according to any of the claims 1-8, and a valve assembly (23) arranged to drive the pumps with overlapping sequences.

14. The use of a pump according to any of the claims 1-7, or a pump system according to any of the claims 8- 13, for boosting a liquid in a subsea gas compression system.