US20040206229A1

US20040206229A1 - Additive injection device

Info

Publication number: US20040206229A1
Application number: US10/742,792
Authority: US
Inventors: James Morrison
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-12-23
Filing date: 2003-12-23
Publication date: 2004-10-21
Also published as: US20040118274A1; US6862972B2; CA2453999A1

Abstract

A reciprocating drive system that utilizes energy available from pressure changes in flowing fluid systems, said drive used for the injection of additives into pipelines. The reciprocating drive includes a hydraulic accumulator having a gas chamber connected to a gas pipeline to contain said gas, whereby variation in the elevated pressure of the pipeline is reflected in the pressure applied to an incompressible fluid in a hydraulic conduit. The drive is capable of recycling gas used to drive the system back into pipelines.

Description

This is a Continuation-in-part of U.S. patent application Ser. No. 10/326,406[0001]

BACKGROUND OF THE INVENTION

The present invention relates to a reciprocating drive system.

FIELD OF THE INVENTION

Reciprocating drive systems are often used to drive ancillary equipment, such as a pump that may be used for the injection of additives into pipelines.

DESCRIPTION OF THE PRIOR ART

It is frequently necessary to inject an additive into a well or pipeline. These installations are often located in remote locations so the systems must be self-contained. Due to road conditions in some remote locations, chemicals which ate injected into pipelines cannot be transported to the site for months at a time, and standard sources of power to run the injection system may not exist. Examples of additives that might be injected into pipelines include; chemicals for the prevention of line freezing due to hydration, chemicals that disperse waxes or asphaltene, and chemicals that prevent corrosion of pipelines. Therefore, there is a need for pipeline injection systems that offer both dependable and accurate metering, as well as having the capability to operate without traditional sources of power.

A number of different types of systems are available on the market for the injection of chemicals into remote pipelines or wellheads. Many of these systems utilize the natural gas carried by a pipeline as a prime mover. Use of natural gas for this purpose, however, is fraught with numerous problems.

A first problem with this type of system is that the natural gas used to drive the system is exhausted into the atmosphere, as the majority of these systems are unable to recover the gas. Pipeline natural gas open contains high levels of hydrogen sulphide, which is toxic and harmful to the environment. As a result, a number of governmental regulations have recently been put in place to restrict the release of natural gas into the environment. Further, the loss of natural gas to the environment represents a substantial, cumulative economic loss to operators.

An additional problem of using gas driven systems is a difficulty in controlling the mass of additive injected per unit of time. Gas driven systems suffer in performance due to the high compressibility of gas. Specifically, such systems are often typified by erratic piston motion, and as a result valve damage can also occur. Further, injection systems are required to operate efficiently at as low a pressure as possible so as not to restrict movement of gas within pipelines any more than necessary.

An alternative form of injection uses air/oil hybrid systems, but these are also characterized by specific deficiencies. Such systems often experience a loss of oil caused by the reciprocating motion of a piston rod. As a result of the oil loss, gas can replace oil in the system. Mixing of gas and oil in this manner causes a frothing of the oil component of the system, which can lead to erratic and uncontrolled movement of the piston rod used to inject the additive.

It is therefore an object of the present invention to obviate or mitigate the above disadvantages.

SUMMARY OF THE INVENTION

A reciprocating drive for use with a gas pipe line carrying gas at an elevated pressure, said drive comprises a drive rod, a pair of fluid motors each having a reactive surface acting on the rod to move the rod in opposite directions upon application of fluid pressure thereto. A valve is connected between the pipeline and the motors and operable to direct gas from the pipeline to one or other of the motors. A reversing mechanism acts on the valve to change periodically the setting of the valve and reverse direction of movement of the rod. The speed control device controls the rate of movement of the drive rod. The speed control device comprises a body of incompressible fluid disposed in a pair of chambers interconnected by a hydraulic conduit. Each of the chambers includes a cylinder and a piston moveable within the cylinder upon movement of the rod to vary the volume of the chamber. The chambers are arranged relative to one another such that a decrease in the volume of one of the chambers causes a corresponding increase in the volume of the other of the chambers The speed control valve further comprises a flow control valve located in the conduit and a hydraulic accumulator connected to the conduit. The accumulator has a first chamber in communication with said conduit to contain the uncompressible fluid and a second chamber connected to said gas pipeline to contain the gas, whereby variations in the elevated pressure of said pipeline are reflected in the pressure applied to the incompressible fluid in the conduit.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings wherein: [0011]
FIG. 1 is a schematic representation of a pipeline additive installation. [0012]
FIG. 2 is a schematic representation of the components used in the system of FIG. 1. [0013]
FIG. 3 is a representation of an alternative embodiment of the components used in FIG. 2. [0014]
FIG. 4 is a further embodiment of the component shown in FIG. 3. [0015]
FIG. 5 is a representation of the alternative configuration of cylinders shown in FIG. 2. [0016]
FIG. 6 is a sectional view of a further embodiment. [0017]
FIG. 7 is a view similar to FIG. 6 of a yet further embodiment, and [0018]
FIG. 8 is a view of a portion of FIG. 7 on an enlarged scale.[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring therefore to FIG. 1, a pipeline additive system generally indicated [0020] 10 is connected to a pipeline 12 at a location where there is a pressure drop in the pipeline such as that provided by a restriction such as an elbow indicated at 14. The elbow 14 provides a pair of spaced locations along the pipeline such that there is a small but discernible difference in pressure of gas in the line. The system 10 includes a reservoir 16 containing a supply of additives and connected to a pump 18 through a supply line 20. A check valve 22 controls the direction of flow from the reservoir 16 to the pump 18. The pump 18 discharges the additive through a supply line 24 and check valve 26 to the pipeline 12. The pump 18 is driven by a drive assembly 28 that utilises the pressure of the gas or fluid in the pipeline 12 as its motive force. A supply line 30 is connected between the pipeline 12 and drive assembly 28 and an exhaust line 32 is similarly connected between the drive assembly and the pipeline 12. The connection of the supply line 30 and exhaust line 32 is at respective ones of the spaced locations along the pipeline such that there is a discernible pressure difference between the two locations.
Further details of the drive assembly and pump may be seen from FIG. 2. The [0021] pump 18 is a reciprocating pump having a cylinder 34 with an elongate internal chamber 36. A piston rod 38 is slideable within the chamber 36 to induce fluid through the supply line 20 and expel it from the discharge line 24, which are in communication with the chamber 36.
The [0022] piston rod 38 extends from the pump 18 through a pair of actuators 40, 42. The actuator 40 has a cylinder 44 within which the rod 38 slides and a piston 46 secured to the rod 38. The piston 46 divides the cylinder 44 into a pair of chambers 48, 50. The actuator 42 similarly includes a piston 52 secured to the rod 38 and defining a pair of chambers 54, 56.
The [0023] piston rod 38 carries a pair of adjustable stops 58, 60 that co-operate with a toggle mechanism 62 to actuate a valve 64. The valve 64 is a two position four way valve that controls the supply of gas from the inlet 30 to respective ones of the chambers 50, 54 and similarly connects the chambers 50, 54 to the exhaust line 32.
The [0024] chambers 48, 56 are connected to one another through branch conduits 66, 68 that are each connected to an accumulator 70. Adjustable flow restrictors 72, 74 are included in the branch conduits 66, 68 respectively to control the flow of fluid between the chambers 48, 56 through the accumulator 70. The accumulator 70 has a gas chamber 76 that is connected through a branch conduit 80 to the inlet 30 and a hydraulic chamber 78. The pressure in the gas chamber 76 thus corresponds to the pressure supplied to the inlet of the valve 64. The chambers 48, 46 and the hydraulic chamber 78 of the accumulator 70 are filled with an incompressible hydraulic fluid, typically an oil, so that movement of the rod 38 causes displacement of fluid between the chambers 48, 56 and 78. A pair of check valves 85 are connected in parallel to the flow restrictors 72, 74 and allow flow from chamber 78 to respective ones of the chambers 48, 56. A relief valve 88 is also provided in each of the lines 66, 68 to provide protection from over pressure of the system.
The [0025] supply line 30 includes a filter 82 and pressure regulator 84 to control fluctuations in the pressure supplied to the valve 64. A back pressure valve 86 is connected in the exhaust line 32 to inhibit reverse flow of gas through the valve assembly.
In operation, with the components in the relative position shown in FIG. 2, the [0026] piston rod 38 is fully retracted from the chamber 36 which is filled with the additive drawn from the reservoir 16. Pressure from the inlet 30 is supplied through the valve 64 to the chamber 54 and the chamber 50 is connected through the exhaust line 32 to the lower pressure zone of the pipeline. The pressure difference between the chamber 54 and chamber 50 induces movement of the piston rod 38 to expel fluid from the chamber 36. The rate of movement of the rod 38 is controlled by the flow rate through the restrictor 72, 74 which is proportional to the pressure differential applied across the restrictors. Any variation in volume between the chambers 48 and 56 is accommodated by compression of the gas in the chamber 76. As the piston rod moves to expel fluid from the chamber 36 through the discharge 24, the abutment 58 contacts the toggle 62 and moves the valve 64 into its alternative position. In that position, the higher gas pressure is applied to the chamber 50 and the chamber 54 connected to the exhaust 32. The direction of movement of the rod 38 is thus reversed causing the chamber 36 to again expand and draw additive into the chamber 36. The rate of movement of the piston rod 38 again is controlled by the flow of fluid through the branch conduit 66, 68 to maintain the speed at the desired rate. The reciprocal motion will continue to dispense the additive from the chamber 36 at each reversal utilising the gas supplied in the pipeline in a closed system. The stroke length of the piston rod may be adjusted by positioning the abutments 58, 60 at different locations along the piston rod 38 between the two actuators 40, 42 to co-operate with the toggle 62 at different points during the stroke.
Because the rate of movement of the rod is determined in part by the pressure difference across the restrictor [0027] 72, 74 it is necessary to prevent variation in the rate of movement due to fluctuations of the gas pressure within the line, which are in turn supplied to the chambers 50, 54. Variations in the gas pressure are transmitted through the branch conduit 80 to the gas chamber 76 and thereby cause a corresponding increase in the pressure in the fluid chamber 78. Thus, an increased pressure in the drive chambers 50, 54 due to an increase of pressure in the supply line 30 will cause a corresponding increase in the chamber 78 and maintain the pressure differential across the restrictor 72, 74 constant. The rate of movement of the piston rod 30 therefore remains constant and the volume of additive dispensed per unit of time can be maintained. The application of the gas pressure effectively pressure compensates the flow control valves provided by restrictors 72, 74 to maintain a substantially constant flow rate.
The [0028] check valves 85 permits air that may be trapped in the chambers 48, 56 to be vented to the accumulator chamber 78. The relief valves 88 are set above the typical system pressure, for example to 600 psi to permit relief of the system. This may occur for example, with an increase in ambient temperature when the system is shut off and could otherwise result in seal failure.
In the above embodiment, each of the branch conduits contains a [0029] restrictor 72, 74. However, as shown in FIG. 3 in which like components will be denoted with like reference numerals with a suffix “a” added for clarity, a single variable restrictor 74 a is included in the branch conduit 68 a The single restrictor 74 a may be used to control the flow of fluid through the accumulator 70 a and branch conduit 66 a. Again the pressure in the chamber 76 a is adjusted with variations of the inlet 30 to maintain the pressure differential across the restrictor 74 a, substantially constant.
A further embodiment is shown in FIG. 4 which permits control of the speed at different rates in opposite directions. In the embodiment of FIG. 4, the [0030] branch lines 66 b, 68 b are interconnected by a pair of cross flow lines 100, 102. Each of the cross flow lines 100, 102 includes a check valve 104 and a variable flow restrictor 74 b. The check valves 104 are oppositely facing and that inhibits flow in opposite directions through each of the lines 100, 102. The accumulator 70 b is similarly protected by a pair of check valves 106. In this embodiment the accumulator 70 b acts as a pressurized reservoir. The accumulator 70 b provides fluid to chambers 48 b and 56 b as these chambers lose fluid during operation. The accumulator 70 b ensures that fluid that is lost during operation is replaced in order to ensure that gas and fluid are not mixed. The flow through each of the restrictors 74 b may be adjusted independently and therefore the rate of movement of the piston rod 38 in each direction may be different.
In the embodiments shown in FIGS. 2, 3 and [0031] 4 the actuators 40, 42 have been shown in spaced relationship with the toggle mechanism 62 located between. Other arrangements of the actuator may be utilised as shown in FIG. 5. In FIG. 5a, the rod 38 extends through both sides of the actuator 42 to provide for double acting power transfer on both advance and retraction.
In the arrangement shown in FIG. 5[0032] b, the actuators 40, 42 abut each other on opposite sides of a partition 90 and the toggle mechanism 58, 60 may be moved externally of the actuators 40, 42. The partition 90 separates the oil, chambers 48, 56 with the gas chambers 50, 54 outboard of the partition. The rod 38 may extend through the cylinder 42 similar to 5 a as shown in ghosted outline. In a further arrangement shown in FIG. 5c, a pair of actuators 40, 42 are supplemented by an additional actuator 110 to provide additional surface area to move the piston rod 38 in each direction, thereby providing more power (Force×Distance). The actuators 40, 42 are arranged as in FIG. 5a with oil chambers 48, 56 and gas chambers 68, 66 respectively. If preferred, the gas chambers may be incorporated in a single actuator with oppositely acting oil chambers paired on the other actuators.
In a further embodiment shown in FIG. 6, the [0033] gas chambers 48, 54, 56 actuators 40, 42 are combined with a diaphragm device 200. The device 200 has an external housing 201 and an internal diaphragm 203 to which the rod 38 is secured. The oil chambers 48, 56 are similarly combined in separate hydraulic dashpot 202. The dashpot 202 includes a piston 204 sliding in a cylinder 206 and connected through ports 208 to the accumulator 210. Pipeline pressure is applied to the accumulator through conduit 80 c. The restrictors 72, 74 are incorporated in valve block 212 located in the body of the accumulator 210.
Flow of gas to the opposite sides of the [0034] diaphragm 200 is controlled by a valve block operated through stops 58 c, 60 c to reverse the porting of the valve. The function of the device is similar to that described above, with the diaphragm 200 providing the reciprocal motive force and the pipeline pressure acting through the conduit 80 c to maintain flow through the restrictors in valve block 212 at the required rate.
In a further embodiment shown in ghosted outline in FIG. 6, additional diaphragm devices are attached to the drive system in order to provide a greater surface area. Addition of one or more diaphragms to the disclosed embodiment is preferred if: (1) a greater force is required to operate the drive, for example if a large volume chemical injector is driven or (2) if a lower supply differential exists that requires a greater surface area to obtain a desired force, [0035]
In the above embodiments, the [0036] valve 64 is shown as two position four way valve operated through the toggle mechanism. The use of the toggle mechanism provides a progressive movement of the valve between its two possible positions and in certain circumstances it has been found that stalling of the drive may occur due to internal function and/or the reduction of the differential pressure available. Should the valve stall in an intermediate position, restarting of the drive as the differential pressure returns is difficult. In order to overcome these difficulties a valve actuating mechanism as shown in FIG. 7 has been utilised. Like reference numerals will be used to denote like components with a suffix d added for clarity.
Referring therefore to FIG. 7, a drive assembly [0037] 28 d is connected to a pump 18 d through a shaft coupling 300. The coupling 300 is formed at one end of the drive shaft 38 d that extends through a dash pot assembly 202 d to a diaphragm 203 d. The diaphragm 203 d is located in a housing 201 d. A piston 204 d is secured within a cylinder 206 d located within the dash pot 202 d with the chambers 48 d, 56 d defined on opposite sides of the piston 204 d. The chambers 48 d, 56 d are connected through internal passageways and valving to accumulator 210 d. The internal valving functions to provide the control described in FIG. 2 through 4 above and need not be described in further detail at this time,
A [0038] bi-stable valve assembly 64 d is mounted on the diaphragm device 200 d on the opposite side to the dash pot 202 d. The valve assembly 64 d includes a bistable valve actuator mechanism 302 and a valve assembly 304. The valve actuator mechanism 302 is connected to an extension of the shaft 38 d through a lost motion coupling 306. The coupling 306 includes an axial slot 308 formed in the shaft 38 d and receiving a pin 310. The pin 310 is mounted to a cylindrical nose 312 of a carrier 314. The shaft 38 d is a sliding fit within the nose 312 and may slide relative to the carrier 314 within the limits of the slot 308. The nose 312 has an end face 316 that faces an end wall 318 of an enclosure 320 within which the carrier 314 is located. The carrier 314 is formed as two housings 322, 324 separated on a common radial plane. The housing 322 has an end face 326 that is oppositely directed to the end face 316 of the nose 312. The end face 326 faces an end wall 328 of the enclosure 320 so that the carrier 314 may move axially within the enclosure 320 between the limits imposed by the end walls 328, 318.
The [0039] carrier 314 supports a conical spring 330 whose periphery 332 is secured between the two housings 322, 324. The conical spring has a radially inner aperture 334 that is secured to a valve actuator 336. The conical spring is a bistable element formed from a planar annulus of spring steel that is pre-stressed to adopt a conical free body configuration. Such springs are available commercially under trade name “Clover Disk” and provide a pair of stable position, one on each side of a median plane.
The [0040] valve actuator 336 extends to a valve housing 338 of the valve assembly 304 and carries a valve spool 340. As can best be seen in FIG. 8, the spool 340 has a central land 342 with oppositely directed scaling faces 344, 346. The land 342 is located in a central distribution chamber 348 to which the pressure port 30 d is connected. The chamber 348 has a pair of radial walls 350 extending radially inwardly to overlap the sealing faces 344, 346 respectively. The spool 340 extends through a central aperture in the walls 350 to supply chambers 352 that communicate with respective supply ports 354, 355 respectively. The supply ports 354, 355 are connected to opposite sides of the diaphragm 203 b. Exhaust chambers 360, 361 are located on either side of the supply chambers 354, 355 and carry lip seals 362 that are selectively engageable with the spools 340. The terminal, portions of the spool 340 have lands 364 that are engageable with the lip seals 362 when the spool is in one of its extreme positions. The exhaust chambers 360 are connected to the exhaust line 32 d which is also connected to the gas side of accumulator 210 d through port 80 d.
In operation, the [0041] supply line 30 d supplies high pressure gas to the central distribution chamber 348. The sealing face 344 of land 342 is in sealing engagement with the radial wall 350 and so prevents flow from the high pressure port to the supply port 354 in that position, the land 364 is clear of the lip seal allowing the supply port 354 to communicate past the lip seal to the exhaust chamber 361 and exhaust line 32 d. The chamber 348 is in communication with the supply port 355 that therefore supplies pressure fluid to one side of the diaphragm 203 b. In this position, the lip seal 362 is scaled against the land 364 to isolate the exhaust port 360 from the supply port 355. The pressure differential applied to the diaphragm 203 b thus initiates movement of the shaft 38 d toward the pump 18 d. The movement causes displacement of the piston 204 b within the cylinder 206 d and thereby transfers hydraulic fluid from one side of the piston to the other in a controlled manner.
During the initial movement, the [0042] slot 308 slides past the pin 310 to leave the carrier 314 in the position shown in FIG. 7. The carrier 314 is held in that position by the action of the conical spring 330 which applies a force to the valve actuator 336 to hold the scaling face 344 against the face 350. The bias of the spring 330 thus holds the face 350 of the carrier 314 against the end face 328 and maintains the carrier 314 in a stable position.
As the [0043] shaft 38 d continues its movement, the terminal portion of the slot 308 engages the pin 310 and initiates movement of the carrier 314 away from the wall 328. As the valve actuator 336 cannot move axially with the carrier 314, the spring 330 is moved over center causing the actuator 336 to slide axially and move the sealing face 346 into engagement with the radial face 350. In that position, the actuator 336 cannot move any further and the bias within the spring 330 causes the carrier to move axially until the end face 316 engages the end wall 318. The inherent bias in the spring 330 thus acts on the valve actuator 336 to maintain the land in sealing engagement. In that position, the supply port 354 is sealed from the exhaust port 32 d by virtue of the lip seal 362 and is connected to the pressure port 30 d. Similarly, the supply port 355 is isolated from the pressure port but connected to the exhaust port 360 by disengagement of the lip seal 362 from the land 364. The pressure differential across the diaphragm 203 b is thus reversed causing a return movement of the shaft 38 d. The lost motion device 306 permits return motion until such time as the pin 310 is again engaged and the over center action of the spring 330 causes a reversal of the spool and carrier.
It will be seen that the actuating mechanism [0044] 28 d provides a bias of the valve into one of two positions with the reversal of direction being effected in an immediate manner. This ensures that the valve cannot occupy a position in which both sides of the diaphragm are connected to a common port and this ensures that the pressure differential available will be applied across the diaphragm to effect movement of the pump.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A reciprocating drive for use with a gas pipe line carrying gas at an elevated pressure, said drive comprising a drive rod, a pair of fluid motors each having a reactive surface acting on said rod to move said rod in opposite directions upon application of fluid pressure thereto, a valve connected between said pipeline and said motors and operable to direct gas from said pipeline to one or other of said motors, a reversing mechanism acting on said valve to change periodically the setting of said valve and reverse direction of movement of said rod, and a speed control device to control the rate of movement of said drive rod, said speed control device comprising a body of incompressible fluid disposed in a pair of chambers interconnected by a hydraulic conduit, each of said chambers including a cylinder and a piston moveable within the cylinder upon movement of said rod to vary the volume of said chamber, said chambers being arranged relative to one another such that a decrease in the volume of said chambers causes a corresponding increase in the volume of the other of said chambers, said speed control valve further comprising a flow control valve located in said conduit and a hydraulic accumulator connected to said conduit, said accumulator having a first chamber in communication with said conduit to contain said incompressible fluid and a second chamber connected to said gas pipeline to contain said gas, whereby variations in said elevated pressure of said pipeline are reflected in the pressure applied to said incompressible fluid in said conduit.

2. A device according to claim 1 wherein a pair of flow control valves are located in said conduit and said accumulator is connected between said valves.

3. A drive according to claim 1 wherein said flow control valve is adjustable.

4. A drive according to claim 1 wherein said valve includes a return line connected to said gas pipeline to return gas vented from said motors.

5. A drive according to claim 4 wherein said reversing mechanism is carried by said rod and is adjustable relative thereto to vary the stroke of said drive rod.

6. A drive according to claim 1 wherein said drive motors comprise a diaphragm device having a housing and an internal diaphragm or piston to define each of the drive motors, said rod being connected to said diaphragm from movement therewith.

7. A drive according to claim 6 wherein said valve is connected to said pipeline at spaced location to establish a pressure differential between gas supplied to said motors and gas vented by said motors.

8. A drive according to claim 1 wherein said drive motors each include a piston to provide said reactive surface and a cylinder in which said piston is moveable.

9. A drive according to claim 8 wherein said piston is secured to said rod.

10. A drive according to claim 9 wherein said drive and chamber are located in a common actuator with said cylinder disposed on one side of a common piston and a chamber disposed on the opposite side of said common piston.

11. A drive according to claim 1 wherein said reversing mechanism includes a bistable actuator acting between said valve and said rod.

12. A drive according to claim 1 wherein said bistable actuator is connected to said rod through a lost motion mechanism.

13. A drive according to claim 12 wherein said bistable actuator includes a carrier and a valve actuator, each moveable of which is moveable between a respective pair of abutments, interconnected by a bistable element operable in each of two positions to bias said carrier and valve actuator against a respective pair of said abutments and hold said valve in one of two positions.

14. A drive according to claim 13 wherein said bistable element is a conical spring element.

15. A drive according to claim 13 wherein said valve includes a land having radial sealing surfaces and a moveable within a chamber between a pair of sealing faces.

16. A drive according to claim 15 wherein said faces provide abutments of said actuator.

17. A reciprocating drive comprising a drive rod, a pair of fluid motors each having a reactive surface acting on said rod to move said rod in opposite directions upon application of fluid pressure thereto, a valve to control flow of fluid to said motors and a reversing mechanism acting on said valve to change periodically the setting thereof and reverse the direction of movement of said rod, said reversing mechanism including a bistable actuator acting between said valve and said rod.

18. A drive according to claim 17 wherein said bistable actuator is connected to said rod through a lost motion mechanism.

19. A drive according to claim 18 wherein said bistable actuator includes a carrier and a valve actuator, each moveable of which is moveable between a respective pair of abutments, interconnected by a bistable element operable in each of two positions to bias said carrier and valve actuator against a respective pair of said abutments and hold said valve in one of two positions.

20. A drive according to claim 19 wherein said bistable element is a conical spring element.

21. A drive according to claim 19 wherein said valve includes a land having radial sealing surfaces and a moveable within a chamber between a pair of sealing faces.

22. A drive according to claim 21 wherein said faces provide abutments of said actuator.