EP0378595A1

EP0378595A1 - Fluid system

Info

Publication number: EP0378595A1
Application number: EP19890904595
Authority: EP
Inventors: Kenneth Edward Hopkinson
Original assignee: Cubit Ltd
Current assignee: Cubit Ltd
Priority date: 1988-06-22
Filing date: 1989-04-11
Publication date: 1990-07-25
Also published as: GB8814858D0; WO1989012782A1; DK47290A; FI900891A0; GB2232750A; JPH03502964A; GB9003931D0; DK47290D0; AU3417389A

Abstract

Un système hydraulique comprend un dispositif de régulation d'écoulement (120), des moyens (128) pour amener un fluide d'amont au dispositif de régulation d'écoulement (120), et un moyen (112) consommateur de fluide, le dispositif de régulation d'écoulement (120) comportant une chambre (148), une admission vers la chambre (148) en provenance du moyen d'amenée de fluide (138), une pluralité de sorties en provenance de la chambre (148), dont l'une comporte une sortie primaire (121) à travers laquelle du fluide est acheminé vers le moyen (112) consommateur de fluide, les ou au moins une des sorties restantes comprenant chacune une sortie secondaire à laquelle est associée une soupape (142, 143). Lorsqu'elles sont actionnées dans un premier état, les soupapes (142, 143) permettent au fluide de se diriger depuis la chambre (148) jusqu'à un moyen de décharge (143) en passant par la sortie associée, et lorsqu'elles sont actionnées dans un second état, les soupapes (141, 142) au moins réduisent la quantité de fluide passant par la sortie secondaire pour aboutir au moyen de décharge (143).A hydraulic system includes a flow control device (120), means (128) for supplying upstream fluid to the flow control device (120), and fluid consuming means (112), the device flow control (120) comprising a chamber (148), an inlet to the chamber (148) from the fluid supply means (138), a plurality of outlets from the chamber (148), one has a primary outlet (121) through which fluid is conveyed to the fluid consuming means (112), the or at least one of the remaining outlets each comprising a secondary outlet with which a valve (142, 143) is associated ). When actuated in a first state, the valves (142, 143) allow the fluid to flow from the chamber (148) to a discharge means (143) through the associated outlet, and when they are actuated in a second state, the valves (141, 142) at least reduce the amount of fluid passing through the secondary outlet to reach the discharge means (143).

Description

Title: Fluid system

Description of Invention

This invention relates to a fluid system and more particularly but not exclusively to a fluid system for steam raising. Steam raising systems are known in which there is a flash boiler to which water is fed by a pump. The flash boiler provides steam under pressure to a steam accumulator from where steam may be drawn off for use. The capacity of the pump needs to be matched to the capacity of the flash boiler for efficient production of steam.

According to one aspect of the invention we provide a fluid system comprising a flow control device, means to supply a head fluid to the flow control device, and a fluid using means, the flow control device comprising a chamber, an inlet to the chamber from the fluid supply means, a plurality of outlets from the chamber, one of which comprises a primary outlet through which fluid is fed to the fluid using means and the remaining outlet or at least one of the remaining outlets comprising a secondary outlet having a valve means associated therewith, the valve means being operable in a first condition to permit fluid to be directed from the chamber through the associated outlet to a relief means and being operable in a second condition to at least reduce the amount of fluid passing through the or the respective associated outlet to the relief means.

Thus in a system embodying the invention, there is no need to match the output of the pump, where the means supplying the head of fluid is a pump, to the capacity of the fluid using means but rather by controlling the amount of fluid passing to the relief means, via the secondary outlet or outlets, the amount of fluid passing to the fluid using means can be adjusted to match the capacity of the fluid using means. Preferσbly, each of the Drimary and secondary outlets have identically sized injector nozzles, sized such that the pressure in the chamber of the flow control device is always greater than the pressure downstream of the flow control device to the fluid using means. Thus any build up of pressure in the fluid using means for example as steam is produced when the fluid using means is a steam raising means, can be arranged not to have any significant effect on the pressure in the chamber and on the performance of the pump, where provided.

Preferably the pressure in the chamber is arranged always to be at least twice the maximum pressure downstream of the flow control device to the fluid using means, even with the or all of the valve means associated with the or each of the secondary outlets being open.

It has been found however, that because the back pressure can be very high, particularly where the fluid using means is a flash boiler, it can be advantageous to provide a restriction to the fluid flowing through the relief means, which increases as the back pressure on the primary outlet increases, so that the build up of back pressure does not affect the amount of fluid passing to the fluid using means through the primary outlet.

Thus in accordance with a second .aspect of the invention, we provide a method of operating a fluid system in accordance with the first aspect of the invention, comprising the steps of sensing a parameter of the system to which the fluid using means contributes, and as the magnitude of the parameter increases, restricting the amount of fluid passing through the relief means.

In accordance with a third aspect of the invention we provide a method of operating a steam raising system which comprises means to supply a head of fluid to a fluid using means which comprises a flash boiler, the method comprising controlling the flow rate of water fed to the flash boiler to achieve "dry" steam, "wet" steam or hot water as required.

The methods of the second and third aspects of the invention may incorporate any of the features of the fluid system according to the first aspect of the invention. -3-

The invention will now be described with reference to the accompanying drawings in which:

FIGURE I is a diagrammatic illustration of a fluid system in accordance with the present invention,

FIGURE 2 is a detailed illustrative view of part of a flow control device for use in the system of Figure I .

FIGURE 3 is a block diagram illustrating the fluid system of Figure I , but modified.

FIGURE 4 is a detailed illustrative view of a restrictor device for use in the modified fluid system of Figure 3.

Referring to figure I , a fluid system for steam raising is shown which comprises a steam accumulator I 10 from which steam may be drawn off for use, via a supply line I I I .

Steam is supplied to the steam accumulator 1 10 from a fluid using means comprising a flash boiler 1 12 which includes a heating coil I 15 through which water passes as it is heated by a burner 1 16, in the present invention an oil or gas burner, the products of combustion passing from the flash boiler 1 12 by a flue 1 17. A suitable electrically heated flash boiler could alternatively be used.

Water is supplied to the flash boiler I 12 from a flow control device 120 which will be described in detail hereinafter, the device 120 having a primary outlet 121 , and in this example, a pair of secondary outlets 122, 123.

The primary outlet 121 is connected to a supply conduit 124 through which water may pass from the flow control device 120 to the flash boiler 1 12.

Water is fed to the flow control device 120 from a positive displacement high capacity pump 128 which is connected to a water source 129 which in this example, comprises a vessel (reservoir) having an inlet 1 30. The level of water within the vessel 1 29 is controlled by a float control valve 131 , and a conduit 132 connects the vessel 129 to the pump 128.

The pressure of steam within the steam accumulator I 10 is sensed by a sensing means 133 which provides a signal to a control means 1 34 which is arranged to operate as described below.

Each of the secondary outlets 122 and 123 is connected to a respective condensate return or relief means 143 which may direct water back from the flow control device 120 to the reservoir 129. The primary outlet 12 ! in this exαmple, has a valve means 140 associated therewith which may be omitted if required, and each of the secondary outlets 122, 123, also has a respective valve means 141 , 142 associated therewith.

When the flash boiler 1 12 is operated, the valve means 140 associated with primary outlet 121 is opened or opened fully so that water may flow from a chamber 148 of the flow control device 120 to the flash boiler 1 12 and thus be heated at it flows through the coil 1 15, by the burner 1 16.

Also when the flash boiler 1 12 is operated the valve means 141 and 142 of the secondary outlets 122, 123, may be opened or closed so that the amount of water available for feeding from the chamber 148 to the flash boiler 1 12 can be matched to the capacity of the flash boiler 1 12.

It will be appreciated that the amount of water present in the steam produced by the flash boiler 1 12 will depend on the amount of water being fed into the flash boiler 1 12.

Thus for example, where dry steam is required, the valve means 141 , 142 may be opened fully, so that, in this example, only a third of the water fed by the pump 128 to the chamber 148, passes into the flash boiler.

If only wetter steam is required, one of the valve means 141 , 142 may be closed, or partially closed, or each of the valve means 141 , 142 may be partially closed, so that more water is fed into the flash boiler 1 12, without the capacity of the pump 128 being altered.

The system can be operated to produce hot water only by closing both valves means 141 , 142, so that ail the water being fed to the chamber 148, passes through the primary outlet, to the flash boiler 1 12.

To deactivate the flash boiler 1 12 for example in response to the sensor means 133 sensing a threshold pressure in steam accumulator 1 10, the valve means 140 may be closed and valve means 141 , 142 opened, so that all of the fluid fed to the chamber 148 of the flow control device 120 from the pump 128 will be directed back to the reservoir 129 via the relief means 143, and the fuel or power supply to the burner I 1 would be cut off.

A sensor means 135 may be provided to sense flue temperature or the temperature elsewhere within the system, and to cut off the fuel or power supply to the burner 1 16 in the event of an excess temperature being reached.

In the example described, the valve means 140 to 142 are preferably each solenoid operated valves which may have a valve member movable between α fully open condition and a fully closed condition. In this case, the control means 134 would need to be arranged simply to energise or de- energise a coil of the solenoid valves 40 to 42 to open or close the valves.

Thus an operator may select the quality of steam, or hot water required, by operating the control means.

To balance the system, each of the outlets 121 , 122 and 123 has an injector nozzle 136 so that the pressure in the chamber 148 is greater, preferably at least twice as great, as the fluid pressure downstream of the device 120, for example in conduit 124, or in the relief means 143. The injector nozzles 136 may be identically sized, and the valve means 141 , 142 (and 140) may be capable of being fully opened when energised, or fully closed only.

It will be appreciated that the system described with reference to figure I is shown by way of example only and there are many modifications which can be made. For example, if desired, the flow control device 120 may have only one or more than two secondary outlets to achieve a desired versatility i.e. different quality or dryness, of steam. Further, if desired, the steam accumulator I 10 may be omitted altogether in which case steam pressure in a supply conduit such as conduit 1 I I may be sensed by sensor 133.

Alternatively, one or each of the valve means 140 to 142 may be a variable condition valve so that the actual quantity of water passing through the valve may be varied. In this latter case, the control means 134 may need to be a sophisticated control means which may incorporate a programmable microprocessor which is capable of responding to different sensed conditions by adjusting the degree to which each of the valve means 140 to 142 are opened or closed to achieve an appropriate balance of water being fed to the flash boiler I 12 and being returned to the reservoir 129.

The pump 128 which supplies a head of fluid to the chamber 148 may be a fixed displacement pump or a variable displacement pumD in which latter case the control means 134 may also be capable of adjusting the displacement of pump 128. Alternatively, the pump 128 may be of fixed displacement but be of variable head, but in each case the control means 1 34 may be operable to adjust the capacity of pump 128 to match the water supply to flash boiler 1 12 with a desired output. For example, as the back pressure exerted by the steam pressure in coil 1 15, acts on the primary outlet 121 , the capacity of the pump 128 could be increased. A preferred modification to achieve this is described hereinafter with reference to Figures 3, 4 and 5. If desired, one or more further outlets may be provided from the flow control device 120, as indicated In figure I , at 138, in dotted lines. The outlet 138 is permanently open, but has an injector nozzle 136 like all of the other outlets 121 , 122, 123. However, the outlet 138 has no associated valve means. Provision of the outlet 138 ensures that in the event of all of the valve means 140, 141 , 142 being closed, the pump 128 can continue to run without excess pressure building up in the line between the pump 128 and the flow control device 120.

The size of injector nozzles 136 for each of the outlets 121 , 122, 123, and 138 where provided, must be selected to achieve a desired pressure within chamber 148, and if more outlets are provided from the flow control device, the sizes of all of the injector nozzles 136 would have to be changed so that the amount of water fed to the flash boiler 1 12 will match the capacity of the flash boiler I 12 to produce a desired quality (i.e. dryness) of steam, or hot water.

Thus the system described provides a flexible fluid system capable of producing "dry" steam, "wet" steam, or hot water, by adjusting the amount of water being fed to the flash boiler 1 12 by opening and closing the valves 140, 141 , and 1 2 as necessary.

Referring to figure 2, part of a practical version of flow control device 120 of the fluid system of figure I is shown.

The part shown is a modular unit which provides part I48 only of the chamber 148 which is a drilling through a mono block 150, there being an inlet to the chamber 148 at one end of the block 150 to which water is fed from the pump 128 or from another chamber part I48 provided by an adjacent mono block 150.

Extending from the chamber part I48a is an injector nozzle outlet 151 which terminates with an accurately sized injector nozzle 136, suitably sized to achieve a desired pressure drop between chamber 148 and a supply chamber 153 into which the injector nozzle 152 injects fluid at pressure.

Access to the injector nozzle 136 for maintenance purposes is achieved by removing an access plug 154 In the block 150.

The supply chamber 153 is connected via a further conduit 155 which comprises a further drilling in the block 150, with an outlet, in the present example the primary outlet 121 which is in use, connected to the flash boiler 1 12 via conduit 124. in the conduit 155 there is provided the valve means 140 which, when in an open condition as shown, permits the fluid to pass along conduit 1 55 to the outlet 1 21 , and when closed, prevents fluid passing along conduit 155 to the outlet 1 21.

The valve means 140, as mentioned above, is solenoid operated, there being a valve member 1 56 which engages with a valve seat 1 57 when the valve means 140 is closed.

It can be seen that a by pass passageway 1 58 intersects with conduit 1 55 and extends to a relief conduit 143' which comprises a further drilling in the block 150 generally parallel with the chamber part 148a. A secondary valve means 1 60 is provided along passageway 158 and which when opened, permits fluid to flow from the passageway 155 to the relief conduit 1 3', and when closed prevents fluid passing to the relief conduit 143'.

The secondary valve means 160 associated with primary outlet 121 , is also solenoid operated by the control means 1 34 and comprises a valve member 16 1 which is moved into engagement with seat 1 62 to close the valve. The primary valve 140 and secondary valve 1 60 may be arranged to operate in tandem so that when valve means 140 is open so that fluid is supplied to the outlet 121 , the secondary valve means 1 60 is closed so that no fluid can pass to the relief means 143. However, when the primary valve means 140 is closed, for example when it is desired to shut down flash boiler 1 1 2, the secondary valve means 160 may be opened to permit fluid which passes into passageway 155 to then pass into the relief conduit 143 via the by pass 158 and be returned to the reservoir 129.

If desired therefore when the valve means 40 is closed, the by pass 1 58 may be operated as a secondary outlet.

It will be appreciated from the description of the system I 10 above that the provision of the secondary valve means I 60 is not required to operate the system in accordance with the invention but in the preferred embodiment, Is provided to enable the system to be operated alternatively as described above.

Instead of a pair of valve means 140, 1 60 which may operate in tandem, a diverter valve may instead be provided. The valve means 140, 1 60 or the diverter valve could be external to the block 150 in another embodiment.

The block 1 50 is preferably coupled end to end with two further similar blocks, in order to provide the system of figure 3, with the chamber part 148a in block 150 communicating with the corresponding parts of the chamber 148 in the other two blocks and the relief conduit 143' communicating with corresponding relief conduit 143' of the other blocks. Each of the other two blocks provides a secondary outlet like passageway 158 to permit fluid to be vented to the relief conduit 143'.

The relief conduit 143' in this embodiment comprises a common relief means for each of the primary and secondary outlets, and extends to the reservoir 129.

The valve means 160 for the secondary outlet provided by one of the adjacent blocks 150, would operate as valve means 141 to allow fluid to pass to the relief conduit 143' when the valve means 141 is open and the valve means 160 for the secondary outlet provided by the other adjacent block 150 would operate as valve means 142 to allow fluid to pass to the relief conduit 1 3' when the valve means 142 is open. Each secondary outlet has an injector nozzle 136. Thus neither of the additional two blocks 150 need have an outlet such as outlet 121 in block 150, or an associated valve means like valve means 140. Preferably though, each of the blocks are identical to block 150 and have an outlet such as outlet 121 and an associated secondary valve means similar to primary valve means 140 which, when the system is operating in accordance with the present invention, remains closed.

In each case, the purpose of providing the adjacent modular blocks 150 with secondary valve means is to enable the "secondary outlet" blocks to be alternately operated as a "primary outlet" block, with the respective outlet corresponding to outlet 121 being a by pass connected to the flash boiler I 12. The pair of valves of each block may be operated in tandem but the secondary valve means 160 of the block containing the primary outlet 121 would be closed, and the secondary valve means similar to valve 140 of the valve blocks having the secondary outlets 122, 123, would be closed in order to operate the system in accordance with the invention.

Where an outlet such as outlet 138 is provided in the flow control device 120, a further mono block may be provided, at one end of one of the other blocks 150, in which each of the valve means 140, 160 may be omitted, or retained in an open condition.

In each case, the chamber 148 could be closed at one end and connected at the other end to the conduit from pump 128, and the relief conduit 143' could be connected at one or both ends, to return the fluid to reservoir 129. If desired, each of the outlets similar to outlet 12 ! of blocks 1 50 instead of being closed, or connected to flash boiler I 1 2, may be connected to respective further flash boilers to increase the capacity of the system when required .

In this event, the system will operate substantially as described in our co-pending International application published under No. WO 88/04 390.

Various modifications may be made without departing from the scope of the invention. For example, although as described with reference to figure 2, a preferred form of flow control device 120 has been described, which is simple to manufacture, other configurations are no doubt possible.

Although the invention has been described with reference to a fluid system comprising a steam raising system, it will be appreciated that the invention may be applied to any other fluid using system in which case the fluid using means may not be a boiler, the fluid may not be water, and the parameter sensed need not be pressure.

Referring now to figures 3 to 5, a modified system is il lustrated for particular use for steam raising.

Because the steam pressures which can be achieved by the flash boiler I 12 are so high, it has been found that the back pressure on the primary outlet 121 can reduce the amount of fluid passing to the flash boiler I 1 2 from the chamber 148 as the steam pressure builds up, and an increasing amount of fluid passes from the chamber 148 to the relief means 143 even though the injector nozzles 136 are all identically sized.

Hence the relief means 143 which in figure 3 is a common relief means 143 to each of the secondary outlets 122, 123, as with the device 120 shown in figure 2, has a restrictor device 165 incorporated therein, the device 165 restricting the amount of water which can return to the reservoir 129 through the relief means 143 by an increasing amount as the steam pressure produced by the flash boiler 1 12 increases.

The restrictor device 165 is shown illustratively in more detail in figures 4 and 5 and comprises a body 170 having an inner chamber 1 71. A first inlet opening 172 is provided to which a conduit 1 6 of the relief means 143 is connected, and a second opening 1 73 to which a further conduit 1 68 of the relief means 143 is connected, the conduit 168 extending to the reservoir 129.

The chamber 171 is closed at one end by a cap 174 which has an opening 175 for a purpose to be explained. Within the chamber 171 a valve member 176 is provided which is sfidable along the chamber 171 and is maintained in sealing engagement with a chamber wall by a pair of 0 rings 177.

The valve member 176 has at one end thereof, a nose 178 which, when the member 176 is in the position shown in figure 4, is in sealing engagement with a seat 179 surrounding the opening 172, to entirely close the opening 172. However the nose 178 has an opening 180 which communicates with chamber 171 via a pair of narrow transverse passageways 181 , so that even when the nose 178 is in engagement with the seat 179, fluid flow through the restrictor device 170 is not entirely prevented.

When the nose 1 78 of member 176 moves out of engagement with the valve seat 179 towards the position shown in figure 5, an increasing amount of fluid can flow into chamber 171 and hence through outlet 173 to the reservoir 129, as outlet 173 becomes increasingly uncovered by the valve member 176.

The opening 175 is connected via a conduit 167 to the steam accumulator 100, or at least to a line downstream of the flow control device 120 to or from the flash boiler 1 12. Thus steam pressure produced by the boiler 1 12 acts on the valve member which moves as a piston within the chamber 171 , to urge the member 176 towards and into engagement with the valve seat 179.

Thus the restriction to fluid flow through the reiief means 143 to the reservoir 129 Increases in proportion to the back pressure produced by the flash boiler 1 12 so that the amount of water passed into the flash boiler 1 12 from the primary outlet 121 of the flow control device 120, remains generally constant irrespective of the back pressure of steam at the primary outlet 121.

Of course, the size of conduit 167 and valve seat 179 have to be selected and the diameter of valve member 176 chosen so that the valve member 176 moves appropriately in response to increased steam pressure.

Any alternative restrictor device which restricts the amount of fluid passing from the flow control device 120 back to the reservoir 129 in proportion to the build up of steam pressure produced by the flash boiler I 12, could be employed, but preferably the restrictor device operates in response to a a means which senses steam pressure, like conduit 167. In each case, in place of a pump 1 28, any other means for supplying a sufficient head of fluid to the flow control device 1 20 may be provided.

In another embodiment, solenoid operated valves 140- 142 (and 1 60) are not provided, but each secondary outlet at least, is provided with a manually operable valve, or an electrically operable valve not connected to a control means 1 34.

The valves 140- 142, 160, may be manual ly fully opened, or closed, or set in position so that the flash boiler 1 1 2 will produce either dry steam, when the amount of fluid passing to the flash boiler I 1 2 is suitably matched to the capacity of the boiler I 12, or wet steam when more fluid is directed to the flash boiler, or even hot water, when even more fluid is directed to the boiler I 12. Thus the quality of steam for any particular application may be achieved by operating and/or setting the manual/electrical valves.

The features disclosed in the foregoing description, or the accompany¬ ing drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A fluid system comprising α flow control device, means to supply a head fluid to the flow control device, and a fluid using means, the flow control device comprising a chamber, an inlet to the chamber from the fluid supply means, a plurality of outlets from the chamber, one of which comprises a primary outlet through which fluid Is fed to the fluid using means and the remaining outlet or at least one of the remaining outlets comprising a secondary outlet having a valve means associated therewith, the valve means being operable when in a first condition to permit fluid to be directed from the chamber through the associated outlet to a relief means and being operable when in a second condition to at feast reduce the amount of fluid passing through the or the respective associated outlet to the relief means.

2. A system according to claim ! wherein each of the primary and secondary outlets have identically sized injector nozzles.

3. A system according to claim I or claim 2 wherein the injector nozzles of each of the primary and secondary outlets are all sized such that the pressure in the chamber of the flow control device is always greater than the pressure downstream of the flow control device to the fluid using means.

4. A system according to claim 3 wherein the pressure in the chamber is arranged always to be at least twice the maximum pressure downstream of the flow control device to the fluid using means, with the or all of the valve means associated with the or each of the secondary outlets being open.

5. A system according to claim I or claim 2 wherein means are provided to restrict the amount of fluid passing to the relief means which increases as back pressure on the primary outlet increases.

6. A system according to any one of the preceding claims wherein the primary outlet has its own valve means which may be closed.

7. A system according to any one of the preceding claims wherein the first operating condition of the valve means of the or each secondary outlet is in αn open condition wherein fluid may flow through the secondary outlet to the relief means, and the second operating condition is a closed condition in which fluid is at least substantially prevented from passing from the chamber through the respective secondary outlet.

8. A system according to any one of claims I to 7 wherein at least one further outlet from the chamber is provided with no valve means associated therewith.

9. A system according to claim 8 wherein the or each further outlet from the chamber has an injector nozzle sized identically with the primary and secondary outlets.

10. A system according to any one of the preceding claims wherein the valve means associated with the or each secondary outlet is a hand operated valve.

1 1. A system according to any one of claims I to 10 wherein the or each of the valve means comprises a solenoid operated valve.

12. A system according to claim 10 or claim I I wherein the or at least one of the valve means associated with a secondary outlet, is of the type which is openable to varying degrees, for fine control of the amount of fluid being directed to the relief means.

13. A system according to any one of the preceding claims wherein the relief means comprises a conduit which directs fluid from the valve of the or each secondary outlet to a reservoir from which fluid may be drawn to supply the head of fluid.

14. A system according to claim 13 wherein the reservoir comprises a vessel having an inlet from a fluid source, the amount of fluid permitted to enter the vessel by the inlet, being controlled by a valve, sensitive to the level of fluid within the vessel.

15. A system according to any one of the preceding claims wherein the system is a steam raising system, the fluid using means comprising a flash boiler.

16. A system according to claim 15 wherein a steam accumulator is provided to which steam is fed from the flash boiler.

17. A system according to any one of the preceding claims wherein the or at least one of the secondary outlets having a by pass associated therewith so that when the valve means is operated in the second condition, fluid is directed through the second outlet to the fluid using means.

18. A system according to claim 17, wherein the or each by pass has a second valve means associated therewith which moy be operated in tandem with the first mentioned valve means, the second valve means, when opened or fully opened, permitting fluid to pass through the second outlet, the fluid using means connected to the by pass whilst the first valve means is closed or fully closed.

19. A system according to claim 17 wherein the valve means comprises a diverter valve to direct fluid to the relief means when operated in the first condition, and to the by pass and hence to the fluid using means when operated in the second condition.

20. A system according to any one of the preceding claims wherein the primary outlet has a by pass associated therewith, with associated valve means to enoble at least a proportion of the fluid passing through the primary outlet instead to be directed to the relief means.

21. A system according to any one of the preceding claims wherein a part of the chamber and the or each valve means associated with the or each secondary outlet comprises a modular unit comprising a single block, adapted to be coupled to other similar units to achieve a fluid system having any desired number of outlets.

22. A fluid system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

23. A method of operating a fluid system in accordance with any one of the preceding claims, comprising the steps of sensing a parameter of the system to which the fluid using means contributes, and as the magnitude of the parameter increases, restricting the amount of fluid passing through the relief means.

24. A method of operating a steam raising system which comprises means to supply a head of fluid using means which comprises a flash boiler, the method comprising the flow rate of water fed to the flash boiler to achieve "dry" steam, "wet" steam or hot water as required.

25. A method of operating a fluid system substantially as hereinbefore described with reference to the accompanying drawings.

26. Any novel feature or novel combination of features disclosed herein and/or shown in the accompanying drawings.