GB2259969A - Valve Assembly - Google Patents

Abstract

A valve assembly for isolating a pressure gauge from a pressurised oil filled cable conduit in order to test the gauge has a valve member (25) with an axial passage (28) for allowing communication between a pressure port (12) and a gauge port (16) in the housing (7, 8). An actuating member (32) is depressible to shut off the passage and isolate all ports (Fig 26) then upon further depression to move the valve member (25) to allow communication between the gauge port (16) and a drain port (20) (fig 2C). Movement of the valve member and the actuating member may be resisted by resilient means (40, 42) and the valve may be provided with means to prevent inadvertent operation such as a screw-down collar (123, fig 4). The assembly may incorporate a depressible selector shuttle (288, fig 4) which can be locked down for selectively allowing communication between the pressure port and the gauge port so that depression of the actuating member will connect the pressure and drain ports to allow a sample of oil to be taken from the drain port for analysis. <IMAGE>

Description

VALVE ASSEMBLY This invention relates to a valve which has particular utility in testing systems under pressure.

Underground power cables generally run along conduits in which they are.surrounded by oil at elevated pressure. Gauges are provided at intervals along the power line, to monitor the pressure, and these are usually arranged to give various alarm signals if the oil pressure falls below certain predetermined minimum values.

If one of these pressure gauges registers substantially the same pressure for a prolonged time interval, there is a danger that it may seize up so that when there is a pressure drop this is not recorded by the gauge and no alarm signal is given. There is therefore a need for a simple way of testing the pressure gauges in such a system at regular intervals.

According to the present invention there is provided a valve assembly including a valve shuttle and a valve body having first second and third ports wherein the valve shuttle is displaceably mounted in the body so as to be movable from a first operating position in which the first and second ports are in fluid communication and the third port is closed off to a second operating position in which the second and third ports are in fluid communication and the first port is closed off. Such a valve assembly can be connected between a system maintained at elevated pressure and a pressure gauge, and which can be actuated to isolate the gauge from the high pressure source and connect it instead to a drain line, thereby creating an artificial pressure drop.If the gauge reacts to this pressure drop and gives the required alarm signal or signals, it can be seen to be working properly, and reconnected to the high pressure system.

The valve in accordance with the present invention preferably operates in two stages, first isolating the pressure gauge from the high pressure system and then connecting the gauge to the drain line. The valve is thus essentially a three-way valve to which three lines are connected and which is movable between a first operating position in which it connects a first line to a second line, via an intermediate position in which all three lines are closed off from one another to a second operating position in which the first line remains closed off and the second line is connected to the third. The valve may be actuable to move between the intermediate position and the second position at a variable rate, thus producing a controllable pressure drop.

Advantageously movement of the shuttle from the intermediate to the second operating position is against the force of a resilient biasing means. Such an arrangement will provide an operator with a feel for when movement towards the second operating position is occurring.

Preferably the valve assembly includes main and secondary valve chambers with which the first and second ports respectively communicate and which can be selectively placed in fluid communication via a passage in the valve shuttle. Such an arrangement reduces the amount of machining that the valve body requires and facilitates blocking of the said fluid communication by means of a shuttle actuation member.

Fluid flow through the shuttle passage is advantageously controllable shuttle actuation means which is movable to seal off the said passage. A single movement of a shuttle actuation means is thereby possible to commence movement of the shuttle and seal off the passage through the shuttle.

Preferably the actuation means is further movable to urge the shuttle towards the second operating position, so that a single movement of the shuttle actuation means can be used to move the valve assembly from its first to its second operating position.

In order to reduce the number of seals that are required in the valve assembly the valve shuttle preferably includes a sealing surface against which the shuttle actuation means is sealable and which is sealable with respect to the valve body to close off the third port.

In order to improve the sealing off of the third (drain) port when the assembly is in its first and customary operating position the valve shuttle is preferably biassed towards the first operating position.

For convenience and speed of operation the shuttle actuation means is preferably manually depressable.

To improve the fluid communication between the first and second ports (main and gauge ports) the shuttle actuation means is preferably biassed away from the valve shuttle.

So as to reduce the valve body machining required and to facilitate valve construction the valve assembly preferably includes first and second body members in which the first and second valve chambers are located and a sleeve which sealingly engages both body members and against which the valve shuttle is sealable.

The valve assembly preferably further includes stroke preventing means for selectively preventing movement of the shuttle away from the first operating position.

In order that the valve assembly may be used to either test a gauge as described above or alternatively extract a sample of oil from the pressure line for analysis the valve assembly preferably also includes an independently actuable selector device through which fluid communication between the first and second ports is selectively allowable. The analysis of such a sample of oil contained in a cable duct can indicate problems with the cables before they become apparent from their operating characteristics.

The selector device preferably includes a locking mechanism for locking it in a position which permits fluid flow through the selector between the first and second ports. With such an arrangement single handed operation of the valve assembly is possible even when both the selector and the three-way valve have to be actuated in order to extract a sample of oil for analysis.

In order to reduce the number of components and body machining required and to facilitate construction, the valve body preferably includes first and second body members in which the second shuttle member is slideable and a sleeve which sealingly engages both body members and against which the second shuttle member is sealable.

Two preferred embodiments of the invention will now be described with reference to the accompanying drawings wherein: Figure 1 shows schematically how a valve in accordance with the invention connects a high pressure line to a pressure gauge and also shows a drain line to which the pressure gauge may be connected; Figures 2a to 2c respectively illustrate, in a slightly simplified schematic form, the three possible positions of a valve in accordance with the invention; Figure 3 is an exploded view showing in detail the construction of the components of a valve in accordance with the invention; ; Figure 4 shows in a slightly simplified schematic form a valve assembly in accordance with the invention which includes a selector device for controlling flow between a first pressure port and a second gauge port, in the configuration shown the first pressure port is in communication with a second gauge port via the three way valve only; Figures 5 shows the device illustrated in Figure 4 in an alternative actuation position in which communication between the first pressure port and the second gauge port is possible via the three way valve and the selector; Figure 6 shows the device illustrated in figure 4 in a further actuation position in which communication between the first pressure and the third drain port is possible via the selector device; Figure 7 is an exploded view showing in detail the construction of the valve assembly illustrated in Figures 4 to 6;; Figure 8 is a plan view of the locking mechanism associated with the selector device shown in Figures 4 to 7; Figure 9 is a vertical section on the line XX of the locking mechanism shown in Figure 8.

Referring first to Figure 1, there is shown an underground cable conduit 10 of the type used for enclosing underground power lines. For simplicity, the lines themselves are not shown. The cables within the conduit 10 are surrounded by oil at an elevated pressure. This pressure is monitored by a pressure gauge 18 connected to the conduit 10 by pressure line 1, and gauge line 2 through a valve 14. If the oil pressure in the conduit 10 falls below a predetermined minimum operating pressure, this pressure drop should be recorded on the gauge 18, which may in turn give an alarm signal such as a flashing light or a buzzer. It may also send an alarm signal to a central control station.

In order to ensure that the gauge 18 has not stuck and might thus fail to react to a pressure drop, it must be tested from time to time. This is done by switching the valve 14 to a position in which the pressure line 1 is closed off at its outer end and the gauge 18 is connected via a drain line 3 to a drain 22. This should cause the gauge 18 to register a drop in gauge pressure. If it does not, the gauge will be shown to be defective, and to require repair or replacement.

A slightly simplified version of a valve suitable for operating the system shown in Figure 1 is shown in Figures 2a to 2c. The valve is shown in cross-section in each case.

Figure 2a shows the normal operating position of the valve, with a pressure port 12 communicating with gauge port 16 to connect the cable duct to the pressure gauge as in Figure 1.

A valve housing is provided which comprises an upper valve housing 8 and a lower valve housing 7. Lower valve housing 7 accommodates a valve shuttle member 25 in a cylindrical main valve chamber 24 into which pressure port 12 opens. Upper valve housing 8 accommodates an axially reciprocable valve stem 32 in a secondary valve chamber 34 into which gauge port 16 opens. Valve stem 32 slidingly engages a stem channel 35 in the upper valve housing 8. A double 0 ring seal 57 is provided on the valve stem 32 to prevent fluid loss between the valve stem 32 and the stem channel 35. A drain port 20 in the lower valve housing 7 opens into the interior of the valve assembly in a region between the main and secondary valve chambers.

Confronting faces of the lower and upper valve housings 7 and 8 are complimentarily grooved in order to clampingly locate an annular sleeve 46 concentrically with respect to the valve chambers 24 and 34. The annular sleeve 46 is sealed to the upper and lower valve housings by 0 ring seals 48, and its lower end acts as a valve seat 52 for a main seal 26 which is located at the upper end of the valve shuttle member 25. In the position shown in Figure 2a, the valve shuttle member closes off the drain port 20. The region into which the gauge port 20 opens is sealed off at one end by engagement of the main seal 26 between the valve seal 52 and the shuttle 25 and at the other end by engagement of an 0 ring seal 27 with lower housing 7.The shuttle member has a central axial passage 28 through which the main and secondary valve chambers 24, 34, and hence the pressure port 12 and gauge port 16 can communicate with one another in the position shown in Figure 2a. A shuttle spring 40 may be provided to bias the shuttle member 25 into the position shown in Figure 2a, or it may be held there by the elevated pressure from the pressure port 12.

A reduced diameter end portion of the valve stem 32 passes into an enlarged portion of the passage 28 of the shuttle member 25, allowing some clearance around it. The valve stem 32 includes an integral annular projection 36 which, in the normal operating position shown in Figure 2a, is biassed into a position in which it bears against the axial end surface of secondary valve chamber 34. Again, the bias may be provided by a valve stem spring 42 or by the elevated pressure prevailing in the cable duct and communicated to the secondary valve chamber by the pressure line.

The valve stem 32 is in the form of a plunger which can be actuated from outside the valve when it is desired to test the pressure gauge. Such actuation moves the stem downwards as shown in Figure 2b.

As the stem is pushed, it reaches an intermediate position shown in Figure 2b in which an annular shoulder 50 of the valve stem 32 abuts the main seal 26 of the shuttle member 25, thus closing off the pressure port 12 and main valve chamber 24 from the gauge port 16 and secondary valve chamber 34. At that point, the pressure registered on the gauge immediately before the closure should be retained on the gauge.

As the stem 32 is pushed further, the valve reaches its second operating position as shown in Figure 2c. Here the annular shoulder 50 has pushed the shuttle member 25 and main seal 26 clear of its valve seat 52 formed by the lower end of annular sleeve 46. The gauge port 16 communicating with the pressure gauge is placed in communication with the drain port 20 via the secondary valve chamber 34 and a narrow annular passage 54 between the valve stem 32 and the annular sleeve 46. The drain port 20 is at atmospheric pressure, and oil drains from the secondary valve chamber 34 and gauge port 16 until the pressure gauge indicates zero gauge pressure, in other words atmospheric pressure. The gauge will then also actuate any alarm signal intended to indicate a drop below a predetermined gauge pressure.If the pressure gauge is found to be working satisfactorily, the stem 32 is released and the valve returns to the position shown in Figure 2a with the pressure gauge once again showing the pressure prevailing in the cable conduit.

The stem 32 may be pushed beyond the position shown in Figure 2b at any required rate to produce a controlled drop in pressure in the gauge. If required, the valve may be moved to an intermediate position, between the positions shown in Figures 2b and 2c, to produce a partial pressure drop, and then returned to the position shown in Figure 2b at which point the intermediate pressure registered on the gauge should be retained. A stroke preventing collar 23 is threadedly engaged with the valve stem 32 above the upper housing and can be screwed down over the valve stem in order to prevent inadvertent actuation of the valve.

Figure 3 shows an exploded view of an actual valve in accordance with a preferred embodiment of the invention.

Parts shown in Figure 3 which are equivalent to parts shown in Figure 2 are indicated using the same numeral. The valve includes upper and lower valve housings 8 and 7 which may suitably be made of brass. A first cylindrical bore in the lower housing constitutes the main valve chamber 24.

Coaxial with the main valve chamber is a cylindrical bore in the upper housing which constitutes the secondary valve chamber 34.

Extending from the secondary chamber 34 to the outside of the valve housing is a stem channel 35, which is coaxial with the two valve chambers and of smaller diameter than either of them.

A first radial bore 9 extends from the main valve chamber 24 to the outside of the lower valve housing. This includes a tapped outer portion (pressure port 12) of enlarged diameter to receive a nut 60 and an olive 61 for connecting the bore 9 to the pressure line from a cable conduit.

A second radial bore 11 extends from the main valve chamber 24 to the outside of the lower valve housing. This includes a tapped drain port 20, and is provided with a nut 60 and olive 61 for connecting the bore 11 to the drain line.

A third radial bore 13 extends from the secondary valve chamber 34 to the outside of upper valve housing 8.

The bore 13 has a widened threaded outer portion (gauge port 16) to receive a nut 60 and olive 61 for connecting the bore 13 to the gauge line.

Tapped holes 75 are formed in the upper housing 8, for securing the lower housing 7 to the upper housing 8 by means of machine screws 77.

A mounting plate 80 is screwed to the lower end of the lower housing after the valve has been assembled by means of countersunk machine screws 82 which engage the threaded bores 84 in the lower housing. The mounting plate 80 projects outwardly from the lower housing 7 towards and away from the viewer in a manner not shown in the figures and is used to secure the valve to a wall.

Grooves 78 and 79 in the lower and upper housings clampingly locate the annular sleeve 46 when the housings are screwed together.

A valve stem 32 extends through the secondary valve chamber 34 and the stem channel 35 and projects outwardly from the valve housing. A cap 86 with a colour coding tag 85 is secured to the end of the stem, suitably by means of a screw thread. The valve is actuated by pushing this cap.

A stroke preventing collar 23 is threadedly engageable with the valve stem 32 below the cap 86. Two 0 ring seals 57 pass round the valve stem 32 and bear against the inner surface of stem channel 35 to prevent leakage of oil along the channel from the valve chambers.

An integral annular projection 36 is provided on the valve stem 32 which serves to limit upward movement of the valve stem. The valve stem is biassed upwardly by the action of valve stem spring 42 which is compressed between the annular projection 36 and the annular sleeve 46 when the valve is assembled.

The valve shuttle member 25 is accommodated in the main valve chamber 24 and has located at its upper surface the annular main seal 26. In the normal operating position of the valve, the shuttle bears with this seal against the lower end of the annular sleeve 46.

The shuttle member 25 is axially slidable within the main valve chamber 24, and carries an O-ring seal 27 so as to effect a fluid-tight seal between the outer cylindrical wall of the shuttle member and the inner wall of the valve chamber.

The shuttle member contains an axial passage 28 to allow fluid communication between opposite ends of the shuttle member. Radial passages 29 are also provided in order to facilitate flow from the main valve chamber into the axial bore.

At its end remote from the seal 26, the shuttle member 25 has an annular recess 68, which accommodates one end of the shuttle spring 40 which biases the shuttle member towards its sealing engagement with the annular sleeve 46.

The components of the valve may be of any suitable conventional material such as brass, steel or aluminium, depending on the intended use. The various sealing elements may suitably be of polytetrafluoroethylene (PTFE) or a fluoroelastomer such as Viton (Registered Trade Mark) or any other rubber which is'compatible with oil.

The means for connecting the various lines to the valve housings are shown in Figures 3 and 7 and comprise nuts 60 and olives 61, any suitable means may however be used.

The operation of the valve shown in Figure 3 is identical to the operation of the slightly schematically illustrated valve shown in Figures 2a, b and c.

A second embodiment of the invention is shown in Figures 4 to 9 which includes a selector device 270 (shown on the left hand side of Figures 4 to 7) and will be referred to as the combination assembly 100. The three way valve 271 shown on the right hand side of these Figures is identical in construction and operation to the three way valve described with reference to Figures 2 and 3. For this reason like parts are designated with the corresponding numeral increased by 100, and the construction and operation of the three way valve is only referred to where it is necessary in order to explain the operation of the combination assembly including the selector device.

Referring to Figure 4 the combination assembly includes housing an upper housing 208 and a lower housing 207. These housings differ from the three way valve housing members 8 and 7 in that they are wider and contain a selector upper chamber 299 and a selector lower chamber 298 respectively. The lower and upper selector chambers 298 and 299 are in fluid communication with the main and secondary valve chambers 124 and 134 respectively via an internal pressure line passage 286 and an internal gauge line passage 285 respectively. An annular sleeve 290 which sealingly engages both housing members with 0 ring seals 287 is clampingly located between complementary grooves formed in the confronting faces of the housing members 207 and 208 concentrically with the selector chambers.A selector shuttle member 288 is biassed upwardly by selector shuttle spring 289 so as to urge selector seal 291 into engagement with the lower end of the annular sleeve 290, thus preventing fluid communication between the upper and lower selector chambers 299, 298. An upper end of the selector shuttle 288 slidingly engages selector channel 235 in the upper housing 208. A double 0 ring seal 284 is provided on the selector shuttle 288 to prevent fluid loss from the selector upper chamber 299 between the selector shuttle and the upper housing.

Where the selector shuttle extends outwards from the upper housing 208 a locking mechanism 292 is provided.

This mechanism is seen more clearly in Figures 8 and 9 which are an enlarged plan view of the locking mechanism alone and a cross section on the line XX of Figure 8 respectively.

A collar 294 provided with two arcuate undercut regions 296 is bolted to the upper housing 208 by two bolts 283. A substantially rectangular slot 297 extends through the upper surface of the collar 294. Attached to the upper end of selector shuttle 288 with a nut 295 is a cross member 293 which is downwardly displaceable in slot 297 against the bias of selector shuttle spring 289. A selector handle 282 is connected to the upper extremity of the selector shuttle 288. By pressing the handle 282 downwards and twisting it anti-clockwise the cross member 293 will engage the undercut regions 296 of the collar and thus be prevented from returning upwardly when the handle is released.

When the selector shuttle 288 is in its 'up' position as shown in Figures 4, 8 and 9 fluid communication between the selector upper and lower chambers 299 and 298 will be prevented. In this configuration the three way valve 201 will operate exactly in as described above with reference to straightforward gauge testing. When the valve shuttle 125 is not depressed (see Figure 4) the gauge port 116 will be in fluid communication with the pressure port 112, and when the valve shuttle 125 is fully depressed (combination assembly not shown in this configuration) the gauge port 116 will be in fluid communication with the drain port 120 for gauge testing and the pressure port 112 will be blocked off.

When the selector shuttle 288 is locked in the 'down' position (see Figures 5 and 6) fluid communication is possible between the pressure port 112 and the gauge port 116 through the selector 270. When the valve shuttle 125 is not depressed (see Figure 5) the pressure port 112 will be in fluid communication with the gauge port 116 both through the selector 270 and through the valve 271.

Communication through the valve 271 takes place via the axial bore 128, the secondary valve chamber 134 and the internal gauge line passage 285.

Leaving the selector shuttle 288 in the 'down' position the valve shuttle 125 may now be depressed to the position shown in Figure 6 (intermediate position of the valve 271 equivalent to position 2b has not been illustrated). In this configuration the gauge port 116 is in fluid communication with the pressure port 112 via the pressure line passage and the selector device 270 and with the drain port 120 via the gauge line passage 285, the secondary valve chamber and annular passage 154. The combined result of which is that the pressure port 112 is in fluid communication with the drain port 120. Thus by depressing the valve shuttle 125 when the selector shuttle 288 is in the 'down' position a sample of fluid can be removed from the drain line for analysis.

The valve 271 can therefore either be used for connecting the gauge port to the drain port for gauge testing as described above or connecting the pressure port to the drain port for drawing off a fluid sample, depending on the position of the selector 270.

A stroke preventing collar 123 is threadedly engaged with the valve stem 132 above the upper housing and can be screwed down as seen in figures 4 and 5 over the valve stem in order to prevent inadvertent actuation of the valve Figure 7 shows an exploded view of an actual combination valve assembly equivalent to the combination valve assembly shown in Figures 4 to 6. Parts shown in Figure 7 which are equivalent to parts shown in Figures 3 to 6 are indicated using the same numerals.

The combination assembly 100 includes an upper housing 208 and a lower housing 207, which contain cylindrical bores which constitute the secondary and main valve chambers 134 and 124 respectively. Extending radially from the main valve chamber 124 are first and second bores 109 and 111 which terminate at a pressure port 300 and a drain port 302 respectively. Nuts 160 and olives 161 are provided for sealing pipes to these ports.

The components of the three way valve 271 incorporated into the combination assembly are identical to the components used in the three way valve illustrated in Figure 3. The three way valve includes a shuttle spring 140, a valve shuttle 125 with internal axial and radial passages 128, 129, an external 0 ring seal 127 and a main seal 126 located at its upper surface. An annular sleeve 146 with two 0 ring seals 148 is provided which is engageable in grooves 303 formed concentrically with respect to the valve chambers in the confronting surfaces of the upper and lower housings. A valve stem 132 with an annular projection 136 is slidingly engageable with a stem channel 135 in the upper housing 208 against which it is sealed with double 0 ring seal 157.The valve stem is a loose fit in annular sleeve 146 and is biassed upwardly by stem spring 142 against the upper housing 208 when the assembly is constructed. A cap 186 with a colour coding tag 185 is connectable to the upper end of the valve stem.

The left hand sides of the upper and lower housings 208 and 207 contain cylindrical bores which constitute the selector upper and lower chambers 299 and 298 respectively.

The selector upper chamber 299 is connected by an internal gauge line passage 285 to the secondary valve chamber 134, and by a radial bore 308 to gauge port 301. A nut 160 and olive 161 are provided for sealing this port to a pipe.

The selector lower chamber 298 is connected by an internal pressure line passage 286 to the main valve chamber 124.

An annular sleeve 290 with two 0 ring seals 287 is provided which is engageable in grooves 304 formed concentrically with respect to the selector chambers in confronting surfaces of the upper and lower housings. A selector shuttle member 288 with an annular projection 306 is slidingly engageable with a shuttle channel 310 in the upper housing 208 against which it is sealed with a double O ring seal 284.

The selector shuttle is a loose fit in the annular sleeve 290 and is biassed upwardly by selector shuttle spring 289 when the assembly is constructed. The shuttle spring bears on an annular projection 306 of the selector shuttle adjacent to the upper surface of which an annular selector seal 291 is located.

A locking mechanism 292 is provided at the upper end of the selector device for selectively locking the selector shuttle in the 'down' position. The locking mechanism includes an undercut collar 294 connected to the upper housing 208 with bolts 283. A cross member 293 is connected to the selector shuttle near its upper end by a nut 295 and a selector handle 282 is connected to the topmost part of the shuttle.

The upper and lower housings 208, 207 are connected by machine screws 312 which pass through longitudinal bores 318 in the lower housing and engage tapped holes 313 in the upper housing. A mounting plate 315 is then secured over the lower end of the lower housing and secured there by countersunk machine screws 316 which engage tapped holes 317 in the lower housing. The mounting plate 315 projects outwardly from the lower housing 207 towards and away from the viewer in a manner not shown in figure 7 and is used to secure the combination assembly to a wall.

Materials suitable for fabricating the combination assembly from are the same as those discussed above with reference to the three way valve assembly.

The operation of the combination assembly shown in Figure 7 is identical to that of the schematically represented combination valve which is described above, with reference to figures 4, 5 and 6.

Although two particular embodiments of the invention have been described it is envisaged that various modifications may be made to the design without departing from the scope of the invention as defined by the claims.

Claims (18)

1. Valve assembly including a valve shuttle and a body having first second and third ports wherein the valve shuttle is displaceably mounted in the body so as to be movable from a first operating position in which the first and second ports are in fluid communication and the third port is closed off to a second operating position in which the second and third ports are in fluid communication and the first port is closed off.

2. Valve assembly as claimed in claim 1 wherein between the first and second operating positions an intermediate position is provided in which all three ports are closed off.

3. Valve assembly as claimed in claim 1 or claim 2 wherein movement of the valve shuttle from the intermediate to the second position is against the force of a resilient biasing means.

4. Valve assembly as claimed in any preceding claim wherein the first and second ports can be placed in fluid communication via a passage in the valve shuttle.

5. Valve assembly as claimed in claim 4 wherein fluid flow through the passage is controllable by shuttle actuation means which is movable to seal off the said passage.

6. Valve assembly as claimed in claim 5 wherein the actuation means is further movable to urge the shuttle towards the second operating position.

7. Valve assembly as claimed in claim 5 or claim 6 wherein the valve shuttle includes a sealing surface against which the shuttle actuation means is sealable and which is sealable with respect to the valve body to close off the third port.

8. Valve assembly as claimed in any one of claims 5 to 7 wherein the shuttle actuation means is biassed away from the valve shuttle.

9. Valve assembly as claimed in any one of claims 5 to 8 wherein the shuttle actuation means is manually depressable.

10. Valve assembly as claimed in any preceding claim wherein the valve shuttle is biassed towards the first operating position.

11. Valve assembly as claimed in any preceding claim wherein the valve body includes first and second body members in which main and secondary valve chambers respectively are located which communicate with the first and second ports respectively.

12. Valve assembly as claimed in claim 11 further comprising a sleeve which sealingly engages both body members and against which the valve shuttle is sealable.

13. Valve assembly as claimed in any preceding claim including stroke preventing means for selectively preventing movement of the shuttle away from the first operating position.

14. Valve assembly as claimed in any preceding claim including an independently actuable selector device through which fluid communication between the first and third ports is selectively allowable.

15. Valve assembly as claimed in claim 14 wherein the selector device includes a second shuttle member which is biassed towards a position in which fluid flow through the selector is prevented.

16. Valve assembly as claimed in claim 14 or claim 15 wherein the selector device-includes a locking mechanism for locking it in a position which permits fluid flow through the valve assembly between the first and third ports.

17. Valve assembly as claimed in claim 15 or claim 16 wherein the valve body includes first and second body members in which the second shuttle member is slideable and a sleeve which sealingly engages both body members and against which the second shuttle member is sealable.

18. Valve assembly substantially as hereinbefore described with reference to and as shown in Figures 1 to 3 or Figures 5 to 9.