WO2019239105A1

WO2019239105A1 - A reciprocating pump

Info

Publication number: WO2019239105A1
Application number: PCT/GB2019/051483
Authority: WO
Inventors: Eric Stamper
Original assignee: Combined Pumps Limited
Priority date: 2018-06-15
Filing date: 2019-05-30
Publication date: 2019-12-19
Also published as: GB201809832D0; EP3807535A1

Abstract

The invention relates to reciprocating pump (10) comprising actuating and discharge cylinders (12, 20); an actuating piston (14) mounted for reciprocating movement within the actuating cylinder (12); and a discharge piston (24) coupled to the actuating piston (14) and mounted for reciprocating movement within the discharge cylinder (20). The reciprocating movement of the discharge piston (24) successively draw fluids into, and discharges fluid from, the discharge cylinder (20). A fluid discharge valve (30) is provided at an end of the actuating cylinder (12). The fluid drainage valve (30) includes a valve pin (32) which protrudes into the actuating cylinder (12) to enable its periodic actuation by the actuating piston (14). By periodically striking the valve pin (32) the fluid drainage valve (30) is actuated (i.e, opened) thus permitting the escape, via a drain hole (34), of any fluids which have collected within the base of the actuating cylinder, e.g, water build-up arising from moist air introduced from a supply source.

Description

A RECIPROCATING PUMP

The present invention relates to a reciprocating pump and particularly, though not exclusively, to a dual-piston reciprocating pump.

Dual-piston reciprocating pumps provide a substantially constant fluid output. They typically include opposed pistons, which are successively actuated, such that during discharge of one of the pistons, the other piston is being charged, and vice versa. Joining the two outlets of the pistons into a common fluid line facilitates constant fluid flow into the line (during the cycle of the pump), giving a constant flow output. A pneumatically⁷ actuated pump includes a main actuating piston mounted between, and coupled to, opposing discharge pistons. It is pneumatically controlled for reciprocating motion, to alternately charge and discharge the pistons

A number of historical problems prevalent in dual-piston reciprocating pumps were addressed in a previous pump devised by the inventor as disclosed in European patent publication No. EP1,775,469A2 For example, the pump disclosed therein enabled flow characteristics within the pumps to be varied or customised for different applications. Maintenance issued were also simplified by providing modular components which could be easily swapped out to avoid the entire pump being taken offline.

Despite the improvements afforded by the aforementioned pump, the inventor has devised yet further technical improvements leading to an enhanced flow rate efficiency of a dual piston reciprocating pump.

With the above in mind, it is an object of the present invention to provide a dual-piston reciprocating pump that provides an improvement in efficiency vis-a-vis comparable prior art pumps.

According to a first aspect of the invention, there is provided a reciprocating pump comprising:

an actuatin cylinder; an actuating piston mounted for reciprocating movement within the actuating cylinder;

a discharge cylinder; and

a discharge piston coupled to the actuating piston and mounted for reciprocatin movement within the discharge cylinder to successively draw fluid into, and discharge fluid from, the discharge cylinder;

characterised in that a fluid discharge valve is provided proximate a base of the actuating cylinder, the fluid discharge valve including a valve pin protruding into the actuating cylinder to enable its periodic actuation by the actuating piston.

It will be appreciated that by periodically striking the valve pin of the fluid d ischarge valve, the valve is actuated (i.e. opened) thus permitting the escape of any fluids which have collected within the base of the actuating cylinder, e.g. water build-up arising from moist air introduced from a supply source.

Optionally, the pump additionally comprises:

a second discharge cylinder; and

a second discharge piston coupled to the actuating piston and mounted for reciprocating movement within the discharge cylinder, to successively draw fluid into the second discharge cylinder whilst discharging fluid from the first discharge cylinder, and to discharge fluid from the second discharge cylinder whilst drawing fluid into the first discharge cylinder.

It will be appreciated that, in this w^ray, the pump can achieve a substantially constant fluid output.

Optionally, the first and second discharge cylinders are located at opposing first and second ends of the pump. lt will be appreciated that this facilitates effective transfer offeree from the actuating piston to the discharge pistons. Optionally, inlet and discharge flow paths are provided in the discharge cylinder such that the, or each, discharge piston successively draws fluid into, and discharges fluid from, the discharge cylinder via the respective flow paths

Optionally, the inlet and discharge check valves are located in said respective inlet and discharge flow paths, each check valve having a valve body, a valve seat, and a shuttle closure biased against the valve seat to close its corresponding flow path.

Optionally, a central fluid conduit is attached to, and movable with, each shuttle closure within its flow path at an end thereof remote from the valve seat.

Optionally, each shuttle closure is orientated such that its respective valve seat is located at a fluid-supply side thereof.

Optionally, each central fluid conduit is provided with circumferentially arranged perforations along at least part of its length.

Optionally, each shuttle closure is provided with a central concavity; and an end of the central fluid conduit extends into the central concavity.

It will be appreciated that the concavity at least partially surrounds an end portion of the central fluid conduit.

Optionally, the shuttle closure is provided with at least one axial fluid communication pathway extending between the central concavity and the surrounding flow path.

It wall be appreciated that the axial fluid communication pathway provides a bi-directional flow path between the discharge cylinder, internally through the central fluid conduit, through the perforations, through the annular flow paths, and to/from a fluid destination/ source. Optionally, a resilient biasing means is located at respective fluid-exit sides of each shuttle closure to bias it against the valve seat

Optionally, the resilient biasing means is mounted externally around the central fluid conduit with its opposing ends biased against opposed shoulders on the shuttle closure and the valve body.

Optionally, the resilient biasing means is a coil spring.

It will be understood that the actuating piston operates in response to a fluid pressure force provided by an actuating fluid. Preferably, the actuating fluid is a gas, and may be a pneumatic fluid, particularly, compressed air. Aiternativeiy, the actuating fluid is a liquid.

Optionally, the pump comprises a main valve assembly for controlling supply of actuating fluid to the actuating piston.

It will be appreciated that the main valve assembly may be adapted for controlling supply of actuating fluid successively to one end of the actuating cylinder acting on one face of the actuating piston, whilst permitting discharge of actuating fluid from the other end of the actuatin cylinder acting on an opposite face of the actuating piston. In this way, the actuating fluid acts successively on opposing faces of the actuating piston, to cause the reciprocating movement. A suitable main valve assembly is disclosed in European patent publication No. EP1,775,469A2 which is hereby incorporated by reference.

According to a second aspect of the present invention, there is provided a reciprocating pump comprising:

an actuating cylinder;

an actuating piston mounted for reciprocating movement within the actuating cylinder;

a discharge cylinder; a discharge piston coupled to the actuating piston and mounted for reciprocating movement within the discharge cylinder to successively draw fluid into, and discharge fluid from, the discharge cylinder via respective inlet and discharge flow paths; and

inlet and discharge check valves located in said respective inlet and discharge flow paths, each check valve having valve body and a valve seat, and a shuttle closure biased against the valve seat to close its corresponding flow path;

characterised in that a central fluid conduit is attached to, and movable with, each shuttle closure within its flow path at an end thereof remote from the valve seat.

Optionally, each shuttle closure is provided with a central concavity; and an end of the centra l fluid conduit extends into the central concavity.

It will be appreciated that the concavity at least partially surrounds an end portion of the central fluid conduit

Optionally, the shuttle closure is provided with at least one axial fluid communication pathway extendin between the central concavity and the surroundin flow path.

Optionally, a resilient biasing means is located at respective fluid-exit sides of each shuttle closure to bias it against the valve seat.

Optionally⁷, the resilient biasing means is mounted proximate the centra l fluid conduit with its opposing ends biased against opposed shoulders on the shuttle closure and the valve body.

Optionally, the resilient biasing means is a coil spring. It will be appreciated that the axial fluid communication pathway therefore provides a bi- directional flow path between the discharge cylinder and fluid source/supply containers. For example, starting at a fluid source for the inlet check valve, the fluid applies a force against the surface of shuttle closure proximate the valve seat sufficient to overcome the closing force of the resilient biasing means. Once the check valve is opened fluid travels: (i) annularly around the exterior surface of shuttle closure; then (ii) axially inw^rards through the fluid communication pathway formed through the side wall of the shuttle closure; then (iii) axially inwards through the perforations formed through the side w^ra!l of the central fluid conduit; then (iv) internally along the central fluid conduit into the discharge cylinder.

Once the discharge piston reverses direction, the inlet check valve is subject to a fluid back- flow whereby fluid travels internally along the central fluid conduit from the discharge cylinder in a longitudinal direction. Here is applies a relatively large force on the shuttle closure over a relative small surface area within its central concavity; and distributes that force axially, symmetrically and outwardly through the circumferential perforations formed through the side wall of the central fluid conduit; and through the fluid communication pathway formed through the side wall of the shuttle closure. The initial concentra tion of fluid force within the central confines of the central concavity assists the resilient biasing means in closing the shuttle closure. Furthermore, the controlled axial distribution of that force outwardly from the central concavity serves to stabilise the shuttle closure along a central axis during its closing motion. Tests have shown that this arrangement contributes towards a circa. 15% improvement in the closure efficiency of both inlet and discharge check valves used in reciprocating pumps.

Whilst the fluid-flow sequence has been described above with reference to the inlet check valve, it will be appreciated that an equivalence fluid flow sequence is experienced at the discharge check valve.

Optionally, a fluid discharge valve is provided proximate a base of the actuating cylinder, the fluid discharge valve including a valve pin protruding into the actuating cylinder to enable its periodic actuation by the actuating piston. It will be appreciated that by periodically striking the valve pin of the fluid discharge valve, the valve is actuated (i.e. opened) thus permitting any fluids which have collected within the base of the actuating cylinder to escape.

Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

Figure 1 is a part-sectional front view of a reciprocating pump (through line A-A in Fig 2), showing its dual discharge piston within its discharge cylinder and its actuating cylinder and piston; and also incorporating a main valve assembly for driving the actuating piston;

Figure 2 is an end view of the pump of Figure 1 showing inlet and discharge passages at one end of the discharge cylinder;

Figure 3a is a part-sectional vie w of an actuating cylinder fluid discharge valve (see portion C of Fig 1) in a fully closed position;

Figure 3b is a part-sectional view of an actuating cylinder fluid discharge valve in a partially open position;

Figure 3c is a part-sectional view of an actuating cylinder fluid discharge valve in a fully open position;

Figure 3d is a part-sectional view through line U-U in Fig 1 showing a fluid discharge drain hole associated with the fluid discharge valve;

Figure 4a is a part-sectional view from beneath the reciprocating pump of Fig. 1, revealing inlet and discharge check valves at the opposite ends of its discharge cy linder; Figure 4b is a part-sectional view (of portion J shown in Fig. 4a] showing the relative positions of the inlet and discharge check valves when the discharge piston is in a retracted state;

Figure 4c is a part-sectional view showing the relative positions of the inlet and discharge check valves when the discharge piston is in an extended state;

Figure 5 is a part-sectional view showing the fluid-flow directions through the inlet and discharge check valves at one end of the discharge cylinder;

Figure 6 is a part-sectional view showing the in-flow and back-flow⁷ directions through the inlet check valves (i.e. portions P and N from Fig. 4b) at opposite sides of the discharge cylinder; and

Figure 7 is a schematic representation showing an array of sixteen reciprocating pumps according to the present invention with a skid structure.

As shown in Figs. 1 and 2, a reciprocating pump 10 in accordance with an embodiment of the present invention comprises an actuating cylinder 12 and an actuating piston in the form of a drive flange 14, which is mounted for reciprocating movement between opposing side walls 13 within the actuating cylinder 12. A main valve assembly 16 is adapted for controlling supply⁷ of an actuating fluid (e.g. compressed air) successively⁷ to opposite ends of the actuating cylinder 12 via supply lines 18a, 18b in a known manner. The pump 10 also comprises a discharge cylinder 20 which arranged to reciprocate within a pair of opposed end housings 22 connected to opposite sides of the discharge cylinder 12. A discharge piston 24 is coupled to the drive flange 14 and is mounted centrally therethrough such that its opposing ends can reciprocate within the discharge cylinder 20. In doing so fluid can be successively introduced into, and discharged from, the discharge cylinder 20 ends via check valves 26a, 26b as described in more detail below with reference to Figs. 4a-c.

A fluid discharge valve 30 is provided in a lower part of a side wall 13 of the actuating cylinder 12 proximate its base. As shown in Figs. 3a-c, the fluid discharge valve 30 comprises a T-shaped (when viewed in section) valve pin 32, the narrowest part of which extends through an aperture 34 formed through the wall of the actuating cylinder 12. An annular shoulder 36 of T-shaped valve pin 32 is provided with an annular fluid seal 38. A coil spring 40, urges the T-shaped valve pin 32 towards actuating cylinder 12 to thereby close the fluid discharge valve 30 wfien the fluid seal 38 abuts against a corresponding shoulder 12s formed in the wall of the actuating cylinder 12 (as shown in Fig. 3a).

In use, as the drive flange 14 approaches the end of a reciprocation cycle within the actuating cylinder 12 it strikes the narrow end of the T-shaped valve pin 32. As best shown in Fig. 3a, narrow end of the T-shaped valve pin 32 extends partially into the actuating cylinder 12 and so the drive flange 14 abuts against it before it reaches the wail of the actuating cylinder 12 which defines the end of its reciprocation cycle.

Continued movement of the drive flange 14 (to the left as viewed in Figs. 3a-c) applies an opening force to the T-shaped valve pin 32 which overcomes the opposite closing force imparted by the coil spring 40. The opening force causes the annular shoulder 36 and its corresponding annular fluid seal 38 to begin to move away from the corresponding shoulder 12s formed in the wall of the actuating cylinder 12, thus partially opening the fluid discharge valve 30 as shown in Fig. 3b.

Once the drive flange 14 reaches the end of its reciprocation cycle within the actuating cylinder 12, i.e. by abutting against its side wall 13, the distal end surface of the narrow end of the T-shaped valve pin 32 lies in the same plane as the side wall 13. When in this position, the T-shaped valve pin 32 is fully retracted such that the fluid discharge valve 30 is fully open (as shown in Fig. 3c) this allowing fluid (and any air) to be discharged from the actuating cylinder 12 via a drain hole (see Fig. 3d) associated with the fluid discharge valve 30, the fluid passing annuiariy around the annular fluid seal 38 and the T-shaped valve pin 32.

As the drive flange 14 commences a new reciprocation cycle and moves away from its side wall 13 the fluid discharge valve 30 progressively doses under the action of its spring 40. As shown in Fig. 1, two fluid discharge valves 30 may be provided at opposite side walls 13 of the actuating cylinder 12 to alternately open and close and thus ensure that there Is no performance inhibiting build-up of fluid within the actuating cylinder 12 (i.e. potentially arising due to temperature fluctuations introducing moisture into the actuating compressed air supply of the main valve assembly 16).

Check valves pairs 26a, 26b are provided proximate the opposite distal ends of the discharge cylinder 20 as shown in partial section within Fig. 4a (in which the discharge piston 24 is retracted from the discharge cylinder 20) and Fig. 4b (in which the discharge piston 24 is extended into the discharge cylinder 20). Each check valve pair comprises an inlet check valve 26a through which fluid is introduced into the discharge cylinder 20; and a discharge check valve 26b through which fluid is discharged from the discharge cylinder 20. Each check valve 26a, 26b comprises a shuttle closure 50 which is urged towards an annular valve seat 52 by a coil spring 54. At its forward end, each shuttle closure 50 comprises an angled annular sealing surface 56 for engaging against annular valve seat 52. At its rear end, each shuttle closure 50 comprises a central concavity 58 into which is received the end of a central fluid conduit 60. The end portion of the central fluid conduit 60 is provided with a circumferential array of openings or perforations 62. Axial fluid communication pathways 64 extend axially through each shuttle closure 50 to provide a fluid-pathway between its central concavity 58 and the annular space surrounding its exterior surface.

The fluid-flow paths through each check valve 26a, 26b will now be described with reference to Figs. 5 and 6. in Fig. 5, the inlet check valve 26a is shown on the left-hand side of the discharge cylinder 20 and the discharge check valve 26b is shown on the right-hand side of the discharge cylinder 20. In use, as the discharge piston 24 retracts (to the left a viewed in Fig. 5) the inlet check valve 26a opens as shown by the spacing between the annular sealing surface 56 of its shuttle closure 50 and the corresponding annular valve seat 52. Simultaneously, the discharge check valve 26b closes as shown by the abutment of the annular sealing surface 56 of its shuffle closure 50 and the corresponding annular valve seat 52. With reference firstly to the inlet check valve 26a shown on the left-hand side of the discharge cylinder 20 in Fig. 5, the arrows show the prevailing fluid-flow direction whereby fluid is drawn from a source (not shown) through the inlet check valve 26a and out into the discharge cylinder 20. Movement of the drive flange 14 (see Fig. 1) causes a retraction of the discharge piston 24 within the discharge cylinder 20. This creates a pressure imbalance either side of the inlet check valve 26a such that the pressure applied against the outer front surface of its shuttle closure 50 is able to overcome the opposing force exerted by the coil spring 54 which otherwise tends to close the shuttle closure 50 against its annular valve seat 52. As the shuttle closure 50 disengages from the annular valve seat 52 fluid is then able to pass into the annular space within the inlet check valve 26a which surrounds the shuttle closure 50. Incoming fluid then travels axially towards the central concavity⁷ 58 via the fluid communication pathways 64 formed in the shuttle closure 50; and into the central fluid conduit 60 via its circumferential array of perforations 62. Once inside the central fluid conduit 60, fluid travels longitudinally therethrough and exits from the inlet check valve 26a into the discharge cylinder 20

Opposing movement of the drive flange 14 (see Fig. 1) causes the discharge piston 24 within the discharge cylinder 20 to begin to move downwards, i.e. opposite to the direction of the arrow in Fig. 5. This creates a pressure imbalance either side of the discharge check valve 26b such that the pressure applied against the inner front surface of its shuttle closure 50 is able to overcome the opposing force exerted by the coil spring 54 which otherwise tends to close the shuttle closure 50 against its annular valve seat 52. As the shuttle closure 50 disengages from the annular valve seat 52 fluid is then able to pass into the annular space within the discharge check valve 26b which surrounds the shuttle closure 50. Discharging fluid then travels axially⁷ towards the central concavity 58 via the fluid communication pathways 64 formed in the shuttle closure 50; and into the central fluid conduit 60 via its circumferential array of perforations 62. Once inside the central fluid conduit 60, fluid travels longitudinally therethrough and exits from the discharge check valve 26b towards a fluid destination (not shown).

As the movement of the drive flange 14 - and hence the discharge piston 24 - moves from its fully retracted position and begins to extend back towards the check valves 26a, 26b the fluid-flow through the inlet check valve 26a ceases and the fluid-flow through the discharge check valve 26b commences. During this transition stage the inlet check valve is subject to a fluid back-flow whereby the fluid flow direction reverses

As shown schematically in Fig 6, the normal prevailing fluid flow within the check valves 26a, 26b is represented by the right-to-left arrows whereas the back-flow is represented by the !eft-to-right arrows in the particular example shown, the inlet check valve 26a is shown (from the opposite side than is shown in Fig 5) and hence the back-flow originates within the discharge valve 20. However, it will be appreciated that the principle of operation is the same for the discharge check valve 26b

During the transition stage the fluid back-flow reverses the fluid flow direction within the central fluid conduit 60 such that it travels towards, and impacts centrally upon, the inner rear surface of the shuttle closure 50 within its central concavity 58. The fluid back-flow is then forced axially out through the circumferential perforations 62 and into the fluid communication pathways 64 formed through the shuttle closure 50

It will be appreciated that by concentrating the fluid back-flow against a relatively small surface area central portion of the rear surface of the shuttle closure 50 within its central concavity 58, this provides an assistive closing force to the shuttle closure 50 which takes effect quicker than that of the coil spring 54 Accordingly, the inlet check valve 26a is closed more quickly than w^?ouid otherwise be the case to eliminate, or at least minimise, the escape of fluid back- flow through the inlet check valve 26a.

Furthermore, the axial and symmetrical circumferential distribution of fluid back-flow through the circumferential perforations 62 and into the fluid communication pathways 64 enhances the balance of the shuttle closure during its rapid closing motion thereby providing a more rapid and effective seal of against the valve seat 52 to eliminate, or at least minimise, the escape of fluid back-flow through the inlet check valve 26a Tests have shown that this arrangement contributes towards a circa. 15% improvement in the closure efficiency of both inlet and discharge check valves 26a, 26b used in reciprocating pumps 10 according to the present invention As shown in Fig. 7, a plurality of reciprocating pumps 10 according to the present invention can be connected together on a skid structure 100. In the particular embodiment shown, the skid 100 can carry sixteen reciprocating pumps 10 sharing four main valve assemblies 16 each for controlling supply of an actuating fluid (e g compressed air) to four such pumps 10. It will be appreciated that such pump arrays can be scaled in accordance with any required application. In one particular application, each pump within the skid 100 is capable of pumping approximately five thousand barrels of fluid per day at pressures ranging from approximately 20 bar to 1,375 bar. Modifications and improvements may be made to the foregoing without departing from the scope of the present invention as defined by the accompanying claims.

Claims

1 A reciprocating pump comprising:

an actuating cylinder;

a discharge cylinder; and

a discharge piston coupled to the actuating piston and mounted for reciprocating movement within the discharge cylinder to successively draw fluid into, and discharge fluid from, the discharge cylinder;

characterised in that a fluid discharge valve is provided at an end of the actuating cylinder, the fluid discharge valve including a valve pin protruding into the actuating cylinder to enable its periodic actuation by the actuating piston

2 A reciprocating pump according to claim 1, further comprising:

a second discharge cylinder; and

a second discharge piston coupled to the actuating piston and mounted for reciprocating movement within the discharge cylinder, to successively draw fluid into the second discharge cylinder whilst discharging fluid from the first discharge cylinder, and to discharge fluid from the second discharge cylinder whilst drawing fluid into the first discharge cylinder

3 A reciprocating pump according to claim 2, wherein the first and second discharge cylinders are located at opposing first and second ends of the pump.

4 A reciprocating pump according to any preceding claim, wherein inlet and discharge flow paths are provided in the discharge cylinder such that the, or each, discharge piston successively draws fluid into, and discharges fluid from, the discharge cylinder via the respective flow paths

5 A reciprocating pump according to claim 4, wherein the inlet and discharge check valves are located in said respective inlet and discharge flow paths, each check valve havin a valve body, a valve seat, and a shuttle closure biased against the valve seat to close its corresponding flow path

6. A reciprocating pump according to claim 5, wherein a central fluid conduit is attached to, and movable with, each shuttle closure within its flow path at an end thereof remote from the valve seat.

7. A reciprocating pump according to claim 6, wherein each shuttle closure is orientated such that its respective valve seat is located at a fluid-supply side thereof

8. A reciprocating pump according to claim 6 or 7, wherein each shuttle closure is provided with a central concavity; and an end of the centra l fluid conduit extends into the central concavity.

9. A reciprocating pump according to claim 8, wherein the shuttle closure is provided with at least one axial fluid communication pathway extending between the central concavity and the surroundin flow path.

10. A reciprocatin pump according to any of claims 6 to 9, wherein each central fluid conduit is provided with circumferentially arranged perforations along at least part of its length.

11. A reciprocating pump according to any of claims 5 to 10, wherein a resilient biasing means is located at respective fluid-exit sides of each shuttle closure to bias it against the valve seat.

12. A reciprocating pump according to claim 11, wherein the resilient biasing means is mounted proximate the central fluid conduit with its opposing ends biased against opposed shoulders on the shuttle closure and the valve body.

13. A reciprocating pump according to claim 11 or 12, wherein the resilient biasing means is a coil spring.

14. A reciprocating pump according to any precedin claim, wherein the pump comprises a main valve assembly for controllin supply of actuating fluid to the actuating piston.

15. A reciprocating pump comprising:

an actuating cylinder;

a discharge cylinder;

a discharge piston coupled to the actuating piston and mounted for reciprocating movement within the discharge cylinder to successively draw fluid into, and discharge fluid from, the discharge cylinder via respective inlet and discharge flow paths; and

16. A reciprocating pump according to claim 15, wherein each shuttle closure is orientated such that its respective valve seat is located at a fluid-supply side thereof.

17. A reciprocating pump according to claim 15 or 16, wherein each shuttle closure is provided with a central concavity; and an end of the central fluid conduit extends into the central concavity.

18. A reciprocating pump according to claim 17, wherein the shuttle closure is provided with at least one axial fluid communication pathway extending between the central concavity and the surrounding flow path.

19. A reciprocating pump according to any of claims 15 to 18, wherein each central fluid conduit is provided with circumferentially arranged perforations along at least part of its length.

20. A reciprocating pump according to any of claims 15 to 19, wherein a resilient biasing means is located at respective fluid-exit sides of each shuttle closure to bias it against the valve seat.

21. A reciprocating pump according to claim 20, wherein the resilient biasing means is mounted externally around the central fluid conduit with its opposing ends biased against opposed shoulders on the shuttle closure and the valve body.

22. A reciprocating pump according to claim 20 or 21, wherein the resilient biasing means is a coil spring.

23. A reciprocating pump according to any of claims 15 to 22, wherein a fluid discharge valve is provided proximate a base of the actuating cylinder, the fluid discharge valve including a valve pin protruding into the actuating cylinder to enable its periodic actuation by the actuating piston.

24. A reciprocating pump according to any of claims 15 to 23, further comprising: a second discharge cylinder; and

a second discharge piston coupled to the actuating piston and mounted for reciprocating movement within the discharge cylinder, to successively draw fluid into the second discharge cylinder via an inlet flow path, whilst discharging fluid from the first discharge cylinder via a discharge flow path; and to discharge fluid from the second discharge cylinder via a discharge flow⁷ path, whilst drawing fluid into the first discharge cylinder via an inlet flow path.

25. A reciprocating pump according to claim 24, wherein the first and second discharge cylinders are located at opposing first and second ends of the pump.

26. A reciprocating pump according to any of claims 15 to 25, wherein the pump comprises a main valve assembly for controlling supply of actuating fluid to the actuating piston.