Discharge pressure actuated pump

Abstract

A pump has a pump barrel formed from a larger diameter section and a smaller diameter section. Each section has a biased piston moveable within the section and the pistons are connected together to form a variable volume chamber between the pistons. As the connected pistons move toward the larger diameter section, a volume of fluid is moved through an inlet valve into the variable volume chamber of increasing volume. When the pistons are moved toward the smaller diameter section, a differential volume of fluid is discharged from the variable volume chamber of decreasing volume through a discharge valve into a discharge conduit. The pistons are actuated to move within the pump barrel by application and release of pressure at a remote end of the discharge conduit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are partial longitudinal sectional views of a pump according to an embodiment of the invention, first and second pistons positioned in a pump barrel connected to a single conduit and biasing means for storing energy to return the pistons located below the first piston, more particularly,

FIG. 1A illustrates an idle position wherein an outlet valve and an inlet valve are in a closed position;

FIG. 1B illustrates the first position wherein the first and second pistons are moved causing the inlet valve to open and a variable volume chamber between the first and second pistons to be charged with fluid; and

FIG. 1C illustrates a second position wherein the first and second pistons are moved causing the outlet valve to be be opened, the fluid being displaced from the variable volume chamber, pumping a differential volume created by the variable volume chamber into the conduit above the pump barrel;

FIG. 1D is a cross sectional view along section A-A, according to FIG. 1A;

FIGS. 2A-2C are partial longitudinal sectional views of a pump according to one embodiment of the invention, the biasing means being positioned between the first and second piston in the variable volume chamber, more particularly,

FIG. 2A illustrates an idle position wherein an outlet valve and an inlet valve are in a closed position;

FIG. 2B illustrates the first position wherein the first and second pistons are moved causing the inlet valve to open and a variable volume chamber between the first and second pistons to be charged with fluid; and

FIG. 2C illustrates a second position wherein the first and second pistons are moved causing the outlet valve to be opened, the fluid being displaced from the variable volume chamber, pumping a differential volume created by the variable volume chamber into the conduit above the pump barrel;

FIG. 2D is a cross sectional view along section B-B, according to FIG. 2A;

FIGS. 3A-3C are partial longitudinal sectional views of a pump according to one embodiment of the invention, the biasing means being positioned in the variable volume chamber, the inlet valve and outlet valve being housed in a third chamber fluidly connected to the variable volume chamber, more particularly,

FIG. 3A illustrates an idle position wherein an outlet valve and an inlet valve are in a closed position;

FIG. 3B illustrates the first position wherein the first and second pistons are moved causing the inlet valve to open and a variable volume chamber between the first and second pistons to be charged with fluid; and

FIG. 3C illustrates a second position wherein the first and second pistons are moved causing the outlet valve to be opened, the fluid being displaced from the variable volume chamber, pumping a differential volume created by the variable volume chamber into the conduit above the pump barrel;

FIG. 3D is a cross sectional view along section C-C, according to FIG. 3A;

FIG. 4A is a partial longitudinal sectional view of a pump according to FIG. 1A, the biasing means being a Belleville spring;

FIG. 4B is a partial longitudinal sectional view of a pump according to FIG. 1A, the biasing means being a coil spring; and

FIG. 5 is a partial longitudinal sectional view of a pump according to FIGS. 1a-1c positioned in a wellbore, the pump having a single conduit extending to surface for producing accumulated liquids from the wellbore, gas being produced to surface in an annulus between the conduit and the wellbore.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the invention are disclosed herein in the context of a fluid device, or pump, particularly useful in the production of fluids through a single discharge conduit extending from surface to a subterranean zone of interest. Description in this context is in no way intended to limit the scope of the invention to fluid devices for use in a subterranean wellbore, the device being equally applicable for remotely actuating and pumping fluids from any fluid source to a discharge in a variety of contexts, including from a sump, lake or pipeline.

Having reference to FIGS. 1A-1D, 2A-2D, 3A-3D, 4A, 4B and 5 and in a wellbore context, a subterranean zone of interest or fluid source F (FIG. 5) is located remote from the surface where the fluid, such as a liquid, is to be produced. A discharge conduit 1 having a liquid discharge end 2 at surface 3 extends downhole to an inlet end 4 in fluid communication with the fluid source F. A fluid apparatus or pump 10, according to an embodiment of the invention, is fluidly connected at the inlet end 4 for pumping liquid from the fluid source F to surface 3 as a result of an actuating pressure P being applied to the discharge conduit 1, typically at surface 3.

Having reference to FIGS. 1A-1D and 2A-2D, the pump 10 comprises a pump barrel 11 having a first barrel section 12 and a second barrel section 13 the first and second sections 12,13 being fluidly connected therebetween. The first barrel section 12 is in fluid communication with the fluid source F and the second barrel section 13 is in fluid communication with the discharge conduit 1. A diameter of the first barrel section 12 is greater than the diameter of the second barrel section 13. A pump piston comprises a first piston 14 housed within the first barrel section 12 for axial movement therein, and a second piston 15 housed within the second barrel section 13 for axial movement therein. The first and second pistons 14,15 are connected therebetween and spaced apart by a connector such as a rod 16, forming a variable volume chamber 17 therebetween which changes volume as the pistons 14,15 are actuated to concurrently move axially within the barrel sections 12,13. As the pistons 14,15 move towards the first barrel section 12, the variable volume chamber 17 increases in volume and as the pistons 14,15 move towards the second barrel section 13, the variable volume chamber 17 decreases in volume.

More particularly, a differential volume is created when the connected pistons 14,15 are actuated to move toward the first larger diameter barrel section 12 which permits a larger volume of fluid to enter the variable volume chamber 17 than the chamber 17 will contain when the connected pistons 14,15 are subsequently actuated to move toward the second smaller diameter barrel section 13. Reciprocating movement or stroking of the pump pistons 14,15 in the pump barrel 11 creates the differential volume which is forcibly discharged from the variable volume chamber 17 to the discharge conduit 1 on each pump stroke.

More specifically, an inlet one way or check valve 18 is positioned at an inlet end 20 of the pump barrel 11 to permit the flow of fluid from the fluid source F into the variable volume chamber 17. A discharge one way or check valve 19 is positioned at a discharge end 21 of the pump barrel 11 to permit the flow of fluid from the variable volume chamber 17 to the discharge conduit 1.

Having reference again to FIGS. 1A-1D and 2A-2D and in one embodiment, the inlet check valve 18 is located in the first piston 14, and the discharge check valve 19 is located in the second piston. In one embodiment, the inlet check valve 18 and the discharge check valve 19 are ball valves.

In use, to actuate the pump 10, pressure is cyclically exerted at a discharge end 22 of the discharge conduit 1. The connected first and second pistons 14,15 are actuated to move from an idle position (FIG. 1A) to a first inlet position (FIG. 1B) wherein the first and second pistons 14,15 are moved toward the inlet end 20 of the pump barrel 11, typically a downhole movement in the context of a wellbore pump. To complete the pumping cycle, the first and second pistons 14,15 move to a second discharge position (FIG. 1C), returning to the discharge end 21 of the pump barrel 11.

In the idle and discharge positions, fluid pressure at the inlet check valve 18 causes the inlet check valve 18 to close. As the first and second pistons 14,15 are moved to the first inlet position, the volume in the variable volume chamber 17 becomes larger. The inlet check valve 18 is caused to open and fluid L from the fluid source F adjacent the inlet end 21 of the pump barrel 11 is caused to be sucked into the variable volume chamber 17 through the inlet check valve 18.

Optionally, the inlet and discharge valves 18,19 can form the pistons 14,15 which sealably engage the barrel 11 or the inlet and discharge valves 18,19 can be supported in a piston housing. As shown, each piston 18, 19 comprises a cylindrical housing 23 having ports 24 formed therein for conducting fluids from the inlet and discharge check valves 18,19 through the pistons 14,15.

Biasing means 25 acting between the pump pistons 14,15 and pump barrel 11 to store energy as the first and second pistons 14,15 are moved downhole to the inlet position. Preferably, the biasing means 25 is a spring, pressurized bellows, elastomeric element or the like. As shown, the spring 25 can be a spring washer, such as a Belleville spring (FIGS. 1A-4A), or, as schematically represented in FIG. 4B, a coil spring or the like.

Thus, when the force of the actuating pressure P applied to the discharge conduit 1 and acting at the second piston 15 exceeds the combined force of the pressure at a fluid source F and the spring 25 biasing, the pistons 14,15 are caused to move to the inlet position, typically downhole in the context of a wellbore. Release of the actuating pressure P permits the spring 25 to release stored energy and causes the pistons 14,15 to move to the discharge position, typically uphole in the context of a wellbore.

As the pistons 14,15 are caused to move to the discharge position, the volume of the variable volume chamber 17 becomes smaller resulting in a differential volume, being the difference in volume of the variable volume chamber between the inlet and discharge positions. The inlet check valve 18 is caused to close and as the volume of the variable volume chamber 17 becomes smaller, the discharge check valve 19 is opened and the differential volume is discharged into the discharge conduit 1. Cyclically repeating the application and the release of pressure P at the discharge end 22 of the discharge conduit 1, results in fluids being pumped from the fluid source F, through the pump 10 and into the discharge conduit 1 for eventual transport to a discharge 2, such as at surface 3.

In an embodiment of the invention a hydraulic circuit (not shown) may be used to apply actuating pressure P at the discharge end 22. Alternately, actuating pressure P may be applied using a positive displacement pump, such as a plunger pump (not shown).

In one embodiment of the invention shown in FIGS. 1A-1C, the biasing means 25 is housed in the pump barrel 11 between the first piston 14 and a stop 26 formed adjacent the inlet end 21 of the pump barrel 11. An inlet port 27 is formed in the stop 26 to permit fluid L from the fluid source F to enter the pump 10. As the pistons 14,15 are moved to the inlet position, the biasing means 25 is compressed by the pistons 14,15 against the stop 26, thereby storing energy in the biasing means 25. When the actuating pressure P is released at the discharge end 22 of the discharge conduit 1, the biasing means 25 acts between the stop 26 and the pistons 14, 15 to move the pistons 14,15 to the discharge position. Preferably, the biasing means is a spring 25.

In one embodiment as shown in FIGS. 2A-2C, the biasing means 25 is positioned in the variable volume chamber 17 between the second piston 15 and a stop 28 formed adjacent a lower end 29 of the second barrel section 13. One or more ports 30 are formed in the stop 28 to permit passage of the rod 16 and for the flow of fluids L therethrough between the first and second pump sections 12,13. Further, the rod 16 is hollow to aid in moving fluids from the inlet valve 18 to the discharge valve 19.

In one embodiment shown in FIGS. 3A-3D, the pump barrel 11 further comprises a bypass passageway 40 for forming a second chamber 41 which is fluidly connected to the variable volume chamber 17. The inlet valve 18 is positioned at an inlet end 42 of the second chamber 41 in fluid communication with the fluid source F. The discharge valve 19 is positioned at a discharge end 43 of the second chamber 41 in fluid communication with the discharge conduit 1. A port 44 is formed between the variable volume chamber 17 and the second chamber 41 and between the first and second pistons 14,15. As actuating pressure P is applied at the discharge end 22 of the discharge conduit 1 and the discharge valve is in the closed position, the pistons 14, 15 are caused to move to the inlet position and the inlet valve 18 is opened for admitting fluid L to the second chamber 41 and throughport 44 to the variable volume chamber 17. As the actuating pressure P is released at the discharge end 22 of the discharge conduit 1, the inlet valve 18 is caused to close, the pistons 14,15 are biased to the discharge position by the biasing means 25 and the discharge valve 19 opens for discharging the differential volume of fluid from the second chamber 41 into the discharge conduit 1. Ports 24 are not required in the pistons 14,15 in this embodiment as fluid flow is directed throughport 44.

The biasing means 25, like the previous embodiments, may be housed in the same manner in the variable volume chamber 17 or in the pump barrel 11 below the first piston 14.

Actuation of the pump 10 is accomplished remotely through the application and release of pressure at the discharge 21 and therefore a prime mover is not required to be situated at or near the pump in the wellbore. Further, where a plurality of wells are situated in close proximity, the plurality of wells could be connected hydraulically to a single source of cyclic pressure for operating the plurality of wells.

Where the fluid source F is positioned substantially vertical and up to about a 60 degree inclination relative to the discharge 21, ball and seat valves are suitable for use as the inlet and discharge check valves 18,19. However, where the fluid source F is positioned substantially horizontal to the discharge 21, such as in a horizontal pipeline, spring loaded check valves may be more suitable for use as the inlet and discharge valves 18,19.

One particular use as shown in FIG. 5, wherein embodiments of the invention are particularly well suited, is the deliquification of gas wells. A distal end of a single conduit, such as a tubing string 114, is fit with a pump 110 according to an embodiment of the invention. The pump 110 is lowered into a wellbore 111 of a gas well and forms an annulus 112 between the conduit 114 and the wellbore 111. The discharge end 122 of the conduit 114 is positioned at surface 3. The pump 110 is positioned adjacent a zone of interest 115 where liquid L co-produced from the gas-producing formation accumulate and, which if left in the wellbore 111, would eventually hinder or stop gas production. Gas G is typically produced through the annulus 112 from the zone of interest 115 to surface 3. Actuation pressure P is cyclically applied and released at the discharge end 122 of the conduit 114 such as through a hydraulic circuit or a positive displacement pump. The actuation pressure P acts at piston 15 of the pump 110. The pump 110 is actuated, as discussed herein, to produce accumulated liquids L to surface 3 through the conduit 114 thereby reducing any hydrostatic head caused by the accumulation of the liquids L in the wellbore 111 and permitting production of the gas G through the annulus 112.

Actuation of the pump 110 can be continuous or intermittent. If operated continuously, the pump 110 removes even small accumulations of liquid L. Alternatively, the pump 110 can be operated intermittently on a fixed (similar to continuous) or a dynamically controlled periodic basis. Typically, a controller would activate the pump 110 either at regular predetermined intervals based on historical liquid accumulation for a particular reservoir type, or dynamically in response to a remote sensor which is able to sense a predetermined volume of fluid accumulation. In either case, actuation of the pump 110 would typically require very low power, such as can be provided by such as by a natural gas powered engine in remote locations not accessible to a utility grid or using an electric motor where electricity is available. Further, an accumulator on a hydraulic circuit or a flywheel on a plunger pump drive may be used to conserve energy.

EXAMPLES

A variety of configurations of embodiments of the pump 110 disclosed herein have been modeled for use in wellbore casings of different diameter. The various configurations are shown in Table A.

	TABLE A
	Units	1	2	3	4	5
Outlet barrel bore API	inches	1.5	2.25	1.5	2.25	1.5
Inlet barrel bore API	inches	2.25	2.75	2.75	3.25	3.25
Outlet barrel bore, metric	mm	38.1	57.15	38.1	57.15	38.1
Inlet barrel bore, metric	mm	57.15	69.85	69.85	82.55	82.55
Outlet barrel x-section area	mm2	1140	2564	1140	2564	1140
Inlet barrel x-section area	mm2	2564	3830	3830	5349	5349
Ratio of inlet to outlet areas		2.250	1.494	3.361	2.086	4.694
Depth of pump	m	500	500	500	500	500
Static head on pump w. water column	Bar	50	50	50	50	50
Static force on outlet piston	N	5695	12814	5695	12814	5695
Pressure applied at surface	Bar	80	90	130	150	100
(target ~3x static at pump)
Additional force on outlet piston	N	9112	23066	14808	38443	11391
Total force on outlet piston	N	14808	35880	20503	51258	17086
Ratio static to pressurized P at pump		2.60	2.80	3.60	4.00	3.00
Belleville spring #		D5025425	D633135	D63313	D80364	D80363
Height	mm	3.9	4.9	4.8	6.2	5.7
Thickness	mm	2.5	3.5	3	4	3
Cone height (H − t)	mm	1.4	1.4	1.8	2.2	2.7
# disks per stack		2	3	2	3	2
Height of one disk stack	mm	6.4	11.9	7.8	14.2	8.7
75% force, one stack	N	9063	15025	12356	21400	11919
75% force, stacked disks	N	18126	45075	25072	64200	23838
(max deflection)
75% deflection, one disk stack	mm	1.05	1.05	1.35	1.65	2.025
Static (initial) deflection	mm	0.330	0.299	0.307	0.329	0.484
One disk stack
Ratio, initial to 75% deflection		0.314	0.284	0.227	0.200	0.239
Total deflection with applied pressure	mm	0.858	0.836	1.104	1.317	1.451
Ratio, operating to 75% deflection (target		0.82	0.80	0.82	0.80	0.72
80%)
Effective stroke one disk stack	mm	0.528	0.537	0.797	0.988	0.968
Target stroke length	mm	500	500	750	500	750
Volume of fluid pumped per stroke	mm3	712196	633063	2017889	1392739	3157403
Volume of fluid pumped per stroke	bbls/d	0.712	0.633	2.018	1.393	3.157
Cycles per minute		6.0	6.0	6.0	6.0	6.0
Volume of fluid pumped per day	m3/d	6.2	5.5	17.4	12.0	27.3
Volume of fluid pumped per day	bbls/d	38.8	34.5	109.8	75.8	171.9
# disk pairs to achieve target stroke length		947	931	941	506	775
Total # disks		1894	1861	1881	1012	1550
Total disk height	mm	6062	11074	7337	7186	6743

As discussed above, the volume of the variable volume chamber 17 is greater when the pistons 14,15 are in the inlet position than when the pistons 14, 15 are in the discharge position. Various arrangements can result in this characteristic including the embodiments of FIGS. 1A-3D wherein the first piston 14 and first barrel section 12 have a larger diameter than the second piston 15 and second barrel section 13. A connecting rod 16 fixes the spacing of the first and second pistons 14,15. An advantage includes maximizing the barrel diameter for inserting into a wellbore or other annular constraint at the fluid source F.

Another example of an arrangement causing a differential swept volume includes replacing the fixed connecting rod 16 with an axial movement multiplier between the first and second pistons 14,15 such that the axial movement of the first piston is augmented relative to the second piston. A simple mechanical lever with an offset fulcrum would suffice.

Further, the inlet and discharge valves 18,19 can be integrated with the pistons 14,15, as shown in FIGS. 1A-2C or as shown in FIGS. 3A-3C, one or both can be located in a second chamber 41 positioned along a sidewall of the pump barrel 11 and fluidly connected thereto through a port 44 between the first and second pistons 14,15 to the variable volume chamber 17 therebetween.

Claims

1. A fluid apparatus comprising: a pump barrel having a first barrel section in fluid communication with a fluid source and a second barrel section in fluid communication with a discharge conduit, the first barrel section having a diameter greater than the second barrel section, the first and second barrel sections being fluidly connected therebetween;a first piston housed in the first barrel section for axial movement therein;a second piston housed in the second barrel section for axial movement therein;means connecting between the first and second pistons for concurrent axial movement within the pump barrel between an inlet position and a discharge position, the first and second pistons being spaced apart for forming a chamber of variable volume therebetween;biasing means for biasing the first and second pistons to the discharge position;an inlet check valve to permit fluid to move from the fluid source to the variable volume chamber; andan outlet check valve to permit fluid to move from the variable volume chamber to the discharge conduit,wherein when an actuating pressure sufficient to overcome the biasing means is applied to the second piston through the discharge conduit, the outlet valve closes and the first and second pistons move to the inlet position and increase the variable volume chamber by a differential volume, opening the inlet valve and permitting the flow of the differential volume of fluid from the fluid source through the inlet valve into the variable volume chamber; andwhen the actuating pressure is released, the biasing means returns the first and second pistons to the discharge position for displacing the differential volume of fluid from the variable volume chamber, closing the inlet valve and opening the outlet valve for discharging the differential volume of fluid through the outlet valve to the discharge conduit.

2. The fluid apparatus of claim 1 wherein the inlet check valve is positioned in the first piston and the outlet check valve is positioned in the second piston.

3. The fluid apparatus of claim 1 wherein the means for connecting the first and second pistons is a rod.

4. The fluid apparatus of claim 1 wherein the fluid source is a zone of interest in a wellbore; and wherein the inlet position is downhole and the discharge position is uphole.

5. The fluid apparatus of claim 1 wherein the biasing means is a spring.

6. The fluid apparatus of claim 1 wherein the biasing means is positioned within the first barrel.

7. The fluid apparatus of claim 1 wherein the biasing means is positioned within the second barrel.

8. The fluid apparatus of claim 1 wherein the biasing means is connected between the pump barrel and at least one of the first or second piston.

9. The fluid apparatus of claim 1 further comprising a bypass passageway forming a second chamber extending between an inlet end in fluid communication with the fluid source and an outlet end in fluid communication with the discharge conduit, the second chamber being fluidly connected to the chamber of variable volume, wherein the inlet check valve is positioned at the inlet end of the bypass passageway and the outlet check valve is positioned at the outlet end of the bypass passageway.

10. The fluid apparatus of claim 9 wherein the third barrel section is fluidly connected to the variable volume chamber through a port.

11. The fluid apparatus of claim 9 wherein the biasing means is positioned within the first barrel.

12. The fluid apparatus of claim 9 wherein the biasing means is positioned within the second barrel.

13. A fluid apparatus comprising: a pump barrel having a first barrel section in fluid communication with a fluid source and a second barrel section in fluid communication with a discharge conduit, the first barrel section having a diameter greater than the second barrel section, the first and second barrel sections being fluidly connected therebetween;a first piston housed in the first barrel section for axial movement therein;a second piston housed in the second barrel section for axial movement therein;a connector between the first and second pistons for concurrent axial movement within the pump barrel between an inlet position and a discharge position, the first and second pistons being spaced apart for forming a chamber of variable volume therebetween, the first and second pistons being biased to the discharge position;an inlet check valve to permit fluid to move from the fluid source to the variable volume chamber; andan outlet check valve to permit fluid to move from the variable volume chamber to the discharge conduit,wherein when an actuating pressure is applied to the second piston through the discharge conduit sufficient to overcome the biasing force applied to the first and second pistons, the outlet valve closes and the first and second pistons move to the inlet position and increase the variable volume chamber by a differential volume, opening the inlet valve and permitting the flow of the differential volume of fluid from the fluid source through the inlet valve into the variable volume chamber; andwhen the actuating pressure is released, the first and second pistons are biased to the discharge position for displacing the differential volume of fluid from the variable volume chamber, closing the inlet valve and opening the outlet valve for discharging the differential volume of fluid through the outlet valve to the discharge conduit.

14. The fluid apparatus of claim 13 wherein the inlet check valve is positioned in the first piston and the outlet check valve is positioned in the second piston.

15. The fluid apparatus of claim 13 wherein the connector between the first and second pistons is a rod.

16. The fluid apparatus of claim 13 wherein the fluid source is a zone of interest in a wellbore; and wherein the inlet position is downhole and the discharge position is uphole.

17. The fluid apparatus of claim 13 wherein the first and second pistons are biased to the discharge position by a spring.

18. A method for producing accumulated liquids from a gas well comprising: positioning a fluid apparatus in the wellbore and forming an annulus therebetween, the apparatus havinga pump barrel having a first barrel section in fluid communication with a fluid source and a second barrel section in fluid communication with a discharge conduit, the first barrel section having a diameter greater than the second barrel section, the first and second barrel sections being fluidly connected therebetween;a first piston housed in the first barrel section for axial movement therein;a second piston housed in the second barrel section for axial movement therein;a connector between the first and second pistons for concurrent axial movement within the pump barrel between an inlet position and a discharge position, the first and second pistons being spaced apart for forming a chamber of variable volume therebetween, the first and second pistons being biased to the discharge position;an inlet check valve to permit fluid to move from the fluid source to the variable volume chamber; andan outlet check valve to permit fluid to move from the variable volume chamber to the discharge conduit,wherein when an actuating pressure, sufficient to overcome a biasing force, is applied to the second piston through the discharge conduit, the outlet valve closes and the first and second pistons move to the inlet position and increase the variable volume chamber by a differential volume, opening the inlet valve and permitting the flow of the differential volume of fluid from the fluid source through the inlet valve into the variable volume chamber; andwhen the actuating pressure is released, the first and second pistons are biased to the discharge position for displacing the differential volume of fluid from the variable volume chamber, closing the inlet valve and opening the outlet valve for discharging the differential volume of fluid through the outlet valve to the discharge conduit;producing gas to surface through the annulus, liquid accumulating in the wellbore adjacent the distal end of the conduit;cyclically applying an actuating pressure at the discharge conduit such that when the force of the actuating pressure is greater than the force exerted by the biasing means and a force of pressure at the fluid source, the discharge valve operates to the closed position, the first and second pistons move to the inlet position and the inlet valve operates to the open position for charging the accumulated fluids from the wellbore into the variable volume chamber; andreleasing the actuating pressure so that the first and second pistons are biased to return to the discharge position, the inlet valve moving to the closed position, the discharge valve moving to the open position and pumping the differential volume from the variable volume chamber through the discharge valve to the discharge conduit.

19. The method of claim 18 further comprising continuously alternating applying and releasing the actuating pressure.

20. The method of claim 18 further comprising intermittently alternating applying and releasing the actuating pressure.

21. The method of claim 18 further comprising: sensing an accumulation of liquid; andcyclically applying and releasing the pressure to pump the differential volume into the discharge conduit.

22. The method of claim 18 wherein the actuating pressure is applied to the discharge conduit by a hydraulic circuit.

23. The method of claim 18 wherein the actuating pressure is applied to the discharge conduit by a plunger pump.