GB2077393A

GB2077393A - Pneumatically Assisted Relief/Safety Valves

Info

Publication number: GB2077393A
Application number: GB8018753A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-06-07
Filing date: 1980-06-07
Publication date: 1981-12-16
Also published as: GB2077393B

Abstract

A pressure relief/safety valve is provided with a supplementary pneumatic actuator which defines two pressure chambers 3A, 3B, one on each side of a piston or diaphragm 26 and each having means YA, YB for connecting the chambers into a servo- control system which supplies pneumatic pressure to one or other of the chambers to inhibit or promote opening of the valve. A lost-motion connection between the valve spindle 13 and the control rod 24 of the supplementary actuator permits operation of the valve without pneumatic assistance, should the servo-control system fail. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Control Systems for Pneumatically Assisted Relief or Safety Valves This invention relates to fluid pressure systems incorporating relief or safety valves such as are employed to relieve excessive pressure in pipe lines, processing vessels and like equipment subjected to fluid pressure.

In accordance with the invention there is provided a fluid pressure relief system comprising a spring loaded relief or safety valve, a double acting actuator having a displaceable member capable of response to pneumatic pressure applied or removed above or below it instigated by a pilot device, an operative connection between the relief or safety valve and the said member, the latter on changes of pneumatic pressures applied to it, assisting the closing or opening of the valve. The operative connection is so disposed that on failure of the pneumatic supply the relief or safety valve may operate in the conventional manner independently of the actuator.

For a better understanding of the invention and to show means of carrying it into effect, embodiments will be described by way of example with reference to the accompanying drawings, in which: Fig. 1. Is a diagrammatic representation of a fluid pressure relief system according to the first embodiment of the invention, with single pilot control.

Fig. 2. Is a diagrammatic representation of a fluid pressure relief system according ta the second embodiment of the invention, similar in operation to Fig. 1., but provided with dual parallel pilot control in continuous operation and with automatic stand-by pneumatic supply to anticipate normal supply failure.

Fig. 3. Is an axial sectional view of a relief or safety valve with associated actuator incorporated in the system, the actuator being mounted above the normal valve spring.

Fig. 4. Is an axial section and external view of a three-way pneumatic supply/exhaust relay valve to function with single pilot on/off control.

Fig. 5. Is an axial section and external view of a three-way pneumaticsupply/exhaust relay valve to function with dual pilot on/off control.

Fig. 6. Is a front elevation view of a pneumatic pilot on/off control of the system.

Fig. 7. Is a side view of the pilot on/off control as Fig. 6.

Fig. 8. Is a third embodiment of the system showing an axial section of a relief/safety valve with actuator mounted beneath the normal valve spring.

Fig. 9. Is a fourth embodiment of the system showing a diagrammatic representation of a fluid pressure relief system as Fig. 1. but incorporating boosted relay valves to minimise pneumatic time lag when the control equipment is mounted remote from the relief or safety valve or when several relief or safety valves are operated from the same pneumatic equipment.

Referring to Fig. 1 to 9 the first illustrated embodiment as Fig. 1 comprises pneumatically operated fluid pressure relief system incorporating a relief or safety valve 1 mounted on a pressure vessel 2 and arranged to relieve excessive fluid pressure within the said vessel.

The valve 1 is fitted with a double acting pneumatically operated diaphragm type actuator 3 which is mounted above the valve spring as an integral part of the valve. The actuator 3 is adapted so that upper or lower chambers 3A or 3B respectively may be fed with air or gas pressure or exhausted through their respective relay valves 4A or 4B which are controlled by the action of the pilot device 5A to either supplement the closing load of the relief or safety valve or assist in opening it in accordance with fluid pressure conditions in the vessel 2.

The relief valve 1 may comprise, as shown in Fig. 3, a body 6 having an inlet 7 and an outlet 8, the inlet being fitted with a tapered nozzle 9 provided at its upper end with a seating 10 engageable by a valve disc 11 carried in a hollow housing 12 to which is axially screwed an articulated spindle 1 3. The seating area is optionally balanced by a bellows 1 5 on the discharge side of the valve. The spindle lower end 1 3A is axially directed by guide 14 within the bellows and remote from the discharge of the valve. The valve disc 11 is loaded by the valve spring 1 6 constrained between spring lower washer 1 7 and upper washer 18 the closed loaded valve disc 11 normally sealing the relief valve from discharging fluid from the pressure system.The lower washer 1 7 rests on a peripheral shoulder on the spindle at the function of the lower and upper spindle ends 13A and 13B. The upper washer 18 abuts onto the lower end of adjusting bolt 19 which is axially screwed into the upper flange of the spring bonnet 20, the adjusting bolt being capable of vertical adjustment to vary the spring loading on the valve disc. The spindle 13 extends upwards into the actuator intermediate 21 and at its upper end screws adjustably into the lower end of the coupling bridle 22, being locked in position by grub screw and friction washer 0.

The actuator 3 secured above the intermediate 21 is defined into upper and lower chambers 3A and 3B respectively by the displaceable diaphragm assembly 26. The actuator rod 24 is reduced in diameter as it enters the diaphragm assembly 26 and forms a joint with the diaphragm lower plate, thence, passing through, it terminates in a screwed end fitted with nut 28 the upper end of the actuator rod forming a bolt to secure the diaphragm assembly together. The rod extends downwards from the diaphragm assembly passing through the combined guide and "0" ring type stuffing box 25, continuing slideably through the ferrule 23. The actuator rod terminates at its lower end with a peripheral shoulder 24A.The lower end face of the rod is engageable with the top end of the valve spindle rod 13 whilst the annular shoulder face is engageable with the corresponding annular face of the bridle ferrule 23.

The two positions defined within the bridle, firstly the spindle end and secondly the annular face of the ferrule, permit vertical movement of the actuator rod shoulder 24A by an amount equivalent to the natural lift of the relief or safety valve disc plus a small clearance. This movement less the clearance is the distance the diaphragm springs 27A and 27B will raise the actuator diaphragm assembly and the actuator rod shouldered end 24A on supplementary pneumatic pressure failure.

Normally, the actuator rod end in contact with top end of the valve spindle, will be transmitting supplementary loading from the actuator to the relief or safety valve disc 11 but when, on pneumatic failure, it is raised to an inoperative position, as described, it will allow the coupling bridle 22 and appended valve spindle and disc to lift in the conventional manner, free of the actuator.

A screwed top cap 32 closes the upper chamber 3A and provides means to relatively adjust the diaphragm springs.

The pneumatic supply/exhaust 3-way relay valve 4A shown in Fig. 4 comprises a body 34 fitted with a diaphragm assembly 36 located in a pressure tight chamber 37 formed between body and spring case 35. A spindle 38 attached to the diaphragm assembly at 39 extends into the body and terminates in a valve disc extending through a port in the dividing web of the body and terminates in a soft doubie-faced valve disc 40. A body bottom plug 41 forms a lower valve seating and lower pipe entry. The horizontal lower face of the body dividing web forms the upper valve seating.

Pneumatic pressure normally applied to the underside of the diaphragm assembly from the on/off pilot control acts against the adjustable spring 42 and forces the valve disc onto the upper seat and closes the port between the body side connections and opening the bottom port. When, on the pilot operating, the pneumatic pressure is removed, the valve disc closes the lower port and opens-the side connections to each other.

The supply/exhaust 3-way valve in Fig. 5 with dual diaphragm 36A and 36B control is used when two on/off pilot controls 5B and 5C as Fig.

2, are in continuous parallel service. The description is similar to the normal valve with exception that two diaphragm assemblies, pneumatically isolated from each other, are employed and the valve spindle is rigidly connected to both. The spring adjustment at 43 is such that when both pilot outputs are piped to the 3-way relay valve the valve disc is in the upper position sealing off the side connections from each other and opening the lower port. On the removal of either or both signals from the pilot the valve disc reverses position, opening the upper port and side connections to each other and closing the lower port. A spring adjustment indicator may be fitted at 44 to facilitate conversion from dual to single diaphragm operation.

The on/off pilot control 5A in Fig. 1 and also in Figs. 6 and 7, comprises a sensing element, shown as a bourdon tube 45 connected through a capillary tube 46 and thence by pipe line 47 to a pressure vessel 2, Fig. 1 so as to transmit the fluid pressure from the vessel to the sensing element. - The lower end of the bourdon tube is fixed to the pilot casing 48. The free end holds an adjustable hook 49 which, with increasing fluid pressure deflects upwards and at the set point lifts a flapper 50 about hinge 51 so as to uncover the outlet of a nozzle at 62.

The pneumatic elements of the pilot are mounted on a horizontal carriage 53 hinged by a flexible fulcrum 54 to a mounting block 54A also fixed to the casing. The remote end of the carriage is supported by a differential screwed adjusting bolt 55 which, is itself, supported by the casing and which can swing the carriage about its fulcrum sufficiently to maintain an operative relation between flapper with the lifting hook over the deflection of the bourdon tube through its working range.

Mounted at the fulcrum end of the carriage is a unit comprising a block 57 which carries a nozzle 58, the body of which is screwed into a block and which incorporates a metering orifice 52 the block being carried upon a bellows the whole forming an axially flexible unit expanding with applied pneumatic pressure. At the remote end of the carriage is another block unit 59 connected pneumatically to the first described. The latter block, however, carries a radius arm 60, hinged at 61 which, at its far end is secured to the nozzle block and maintains a fixed relationship between the two blocks. This latter block also carries the hinged flapper spaced above the radius arm and parallel with it at such a height that its lower face can locally rest squarely on the face of the nozzle outlet at 62 thereby sealing off the pneumatic units to the atmosphere.There is no nozzle in this latter block unit.

The sensing element of the pilot device may be of diaphragm, bellows, force balance or mechanical type, as distinct from the bourdon tube shown in Fig. 6. Further, the sensing element and associated pneumatic equipment may be arranged to operate in the reverse sense to that already described and on a falling signal from an integrally related fluid system.

Normally air/gas pressure, usually at 20 psig, Fig. 1 is fed to the inlet connection of the pilot 63 through regulator R1, Fig. 1 via a pressure gauge 64 to the nozzle block by means of flexible tube 65. This pressure also passes to the flapper hinge block. Both these units are therefore extended due to the same air/gas pressure. However, the axial movement of each unit is controlled by a cantilever spring 66 and 69 located parallel to and below the carriage through a bridle 67 and 70 connecting its block to its spring. The length of each spring and therefore its spring rate, is adjusted at 68A and 71 A by the position of the slideable clamp 68 and 71 anchoring the fixed end of the spring to the carriage.

At the pilot set point the bourdon tube hook slightiy lifts the flapper away from the face of the nozzle. Due also to the throttling effect of a metering orifice 52, the pressure in the two bellows, falls rapidly causing some controlled collapse of the units. The immediate effect is to simultaneously increase the leakage gap between flapper and nozzle, both directly at the nozzle and also by lowering the flapper hinged end which, using the hook as a pivot, swings the flapper further from the nozzle.

This compounded feed-back is rapid and effective. The outlet pressure, shown on the outlet pressure gauge 73 immediately fails to zero, the pressure collapse being transmitted via flexible tube 72 to the supply/exhaust 3-way relay valves diaphragm chamber 37 through pilot outlet connection 74. The diaphragm assembly of each valve drops due to thrust of spring 42 changing the valve disc from closing the upper port to closing the lower port, thereby opening the former.

The second embodiment is shown in Fig. 2. It functions as the foregoing Fig. 1 but by the use of three way supply/exhaust relay valves 4C and 4D as described and shown in Fig. 5 both with dual diaphragm operation, feeding the relief or safety valve actuator, enables two pilots 5A and 5B to be simultaneously in circuit, in parallel, fed by regulators R1 and R4 to mitigate against pilot failure.

Normally, the control spring in these relay valves is set to oppose the added pneumatic output pressures from the two pilots. On either pilot operating when the vessel fluid pressure rises to the set point, the three-way relay valves will function due to the fall of pilot output pressure on the relevant control diaphragm and so open the relief or safety valve irrespective of whether the remaining pilot functions or not.

The relief or safety valve will close pneumatically when both pilot output pressures have recovered.

With the arrangement shown in Fig. 2 it is possible to isolate either pilot by means of isolating valves b for inspection or servicing and by resetting the three-way relay valve spring maintain the relief valve in single pilot assistance as Fig. 1. The relay valves 4C and 4D may be duplicated and fitted with by-passes and isolating cocks to permit servicing whilst maintaining pneumatic relief or safety valve operation.

A stand-by air supply may be brought to a control valve 75, normally closed but opening on failure of usual pneumatic supply, the stand-by supply being isolated from the failed supply line by means of check valve 76.

The arrangement of Fig. 2 thus provides a means of minimising relief or safety valve pneumatic assistance failure.

The sequence of pneumatically assisted relief operation is as follows, Fig.1: A suitable pneumatic supply is connected to the pilot 5A through pressure regulator R1 also to the actuator uppper chamber relay valve 4A through regulator R2 and to lower chamber relay valve 4B through regulator R3. These regulators are not necessarily set to identical discharge pressures.

The relief or safety valve spring 19, Fig. 3, is set to the desired relieving pressure, as is the setting of the pilot 55, Fig. 6.

Under normal conditions in the fluid pressure system 2 protected by the relief valve, the pilot 5A is inoperative and the constant pneumatic pressure supply through regulator R1 is built up through the pilot to the diaphragm chamber 37 of relay valves 4A and 4B shown on Fig. 4. This diaphragm pressure holds the valve disc 40 in the up position closing the upper part and opening the lower admitting air or gas pressure through ports Z and Y of relay valve 4A to the actuator upper chamber 3A, at YA, similarly opening the lower chamber 3B at YB to atmosphere through ports Y and Z of relay valve 4B. Thus a downward thrust is applied to the relief valve disc 11 Fig. 3 through the actuator rod 24 and valve spindle 13, applying a supplementary closing pressure to the valve disc above that determined by the valve spring 1 6.

On the fluid system pressure rising to the pilot set point through connection 47, Fig. 1, the pilot sensing element 45, Fig. 6, deflects upwards at its free end, the hooked attachment lifting the flapper and uncovering the nozzle orifice 62 causing a rapid decay of the pilot output pressure, due, as already explained to the metered feed back system. This pressure fall is transmitted to the relay valves diaphragm, reversing the position of the valve disc 40 by the thrust of the diaphragm spring, closing the lower port Z and opening port X. The pneumatic pressure is thus exhausted from the actuator upper chamber 3A through relay valve 4A, ports Y and X. Simultaneously pneumatic pressure is fed to the actuator lower chamber 3B through ports X and Y of relay valve 4B.

This reversal of pressure conditions in the actuator raises the actuator diaphragm assembly, the actuator rod shoulder 24A contacts the bridle ferrule 23 and the consequent upward thrust transmitted through the valve spindle 13 to the valve disc 11 together with the upward thrust on the disc due to the high fluid pressure in the system at 7 rapidly opens the valve to its full left and excess fluid is discharged from vessel 2.

The relief or safety valve will close upon the fluid system pressure falling to a point determined by the pilot adjustment. This closing pressure is always lower than the relieving set pressure, the difference between them being called the blowdown. Pilot adjustment of blowdown is by restricting the movement of the feed-back bellows and is effected by positioning the clamps 68 and 71 to vary the lengths of cantilever springs 66 and 69, Fig. 6.

At the pilot blowdown setting the free end of the element returns to its normal position, releasing the flapper to seal the nozzle outlet orifice. The pilot output to the relay valves control diaphragm is thus restored, raising the valve disc 46, Fig. so closing the upper port and opening the lower. This opens port Z to Y in relay valve 4A to repressurise the actuator upper chamber 3A and exhaust the lower chamber through port Y to Z in relay valve 4B so assisting the relief valve to sharply close by the combined downward thrust of both actuator and valve spring on the valve spindle.

Under relief valve pneumatic assistance full valve lift is attained with zero fluid pressure accummulation. Also, in practice, the relieving pressure setting may be closer to the normal fluid working pressure than is usual with conventional operation.

Further, by pneumatic assistance as described, more effective valve tightness is attained under fluid pressure operating conditions than by spring loading alone.

A third embodiment in Fig. 8 shows an alternative form of double acting actuator applied to a relief or safety valve. This design, unchanged in principle from that already described in Fig. 3 is to provide more ready access to valve spring adjustment and replacement. it also provides for rapid resetting of spring within its designed range by predetermined amounts. This is effected by a stack of concentric washers mounted beneath the adjusting bolt head which may be varied in number and/or thickness, the height of the stack determining the effective length of the adjusting bolt when screwed down with the head contacting the stack.

Such a relief or safety valve, Fig. 8 may comprise a body 6 having an inlet 7, and an outlet 8, the inlet being fitted with a tapered nozzle 9, provided at its upper end with a seating 10 engageable with a valve disc 1 carried in a holder 12 to which is axially screwed an articulated spindle 13. The seating area is optionally balanced by a bellows 15 on the discharge side of the valve. The valve spindle extends axially upwards through bottom guide 14 before passing slideably through the hollow centre hub 29 of the displaceable diaphragm member 26, beyond which it passes into the spring bonnet 20 terminating in a screwed length carrying the spring lower washer and spring adaptor 17. The spring 16 is confined between lower washer and upper washer 18.The upper washer abuts on to the lower end of the adjusting bolt 1 9 which is axially screwed into the removable top plug 20A of the spring bonnet 20.

The upper length of the adjusting bolt carries beneath the head a stack of removable concentric washers 31 which individually serve to effect limited changes in spring adjustment.

The combined intermediate 21 and actuator lower chamber is mounted directly on to the valve body top flange. The displaceable diaphragm member assembly 26, bolted together by centre hub 29 and nut 28, separates upper chamber 3A from lower chamber 3B and is axially directed by the integral extensions of the hub sliding through combined middle guide and "0" ring stuffing box 25 and combined upper guide and "0" ring stuffing box 30. These stuffing boxes isolate the actuator chambers from each other and from the intermediate and spring bonnet.

Normally, the diaphragm member is forced downwards by supplementary pneumatic pressure fed to the actuator upper chamber 3A at YA, thrusting the centre hub lower end annular face against the coupling shoulder joining the spindle lower end 1 3A to the upper end 13B. This thrust provides additional closing force upon the valve disc 11 over and above that of the valve spring 16.

On pneumatic supply failure the diaphragm assembly is moved upwards by the action of springs 27A and 27B into a neutral position equivalent to the natural lift of the valve disc plus a small clearance before the upper annular face of the centre hub contacts the lower face of the spring lower washer. In this position the actuator cannot affect the free movement of the valve spindle when the relief valve operates in the conventional manner.

On pneumatic operation, as described previously, the actuator upper chamber 3A is exhausted at YA to atmosphere and pressure is fed to the lower chamber 3B at YB, the rising diaphragm assembly centre hub upper face contacts the spring lower washer thence exerting an upward thrust opposing that of the spring on the valve disc and so assisting the valve to open.

These pneumatic operations are exactly as described in the sequence of pneumatic operation.

It has been shown that the fluid upthrust beneath the valve disc, is the determining factor in relief or safety valve functioning. However, the reactive forces on the valve disc assembly due to escaping fluid is a major factor in the correct functioning of such valves especially when conventionally operated. This factor, though no part of this specification, can cause irregular valve operation when a relief valve is maladjusted. The pneumatically assisted system does give prompt and effective overriding control beyond conventional operation and is advantageous when plant cannot be shut down for nicety of relief valve adjustment.

The actuators on Figs. 3 and 8 may have piston or bellows type displaceable member as distinct from the diaphragm type shown. Also, the chambers may be isolated by bellows instead of "0" ring type stuffing boxes.

A fourth embodiment is shown in Fig. 9 for service when the small capacity of the standard three-way relay valves is inadequate. It would be used when several large actuators have to be operated from a single pilot or when the pneumatic equipment must be remote from the relief valve. From Fig. 9 and using same notation as Fig. 1, the output from the relay valve normally controlled direct from the pilot is taken to the control diaphragm of the larger capacity booster relay valve instead of feeding direct to the actuator chamber. These boosters are arranged to have the same output sequence as the single relays in Fig. 1. The cascaded relays 4A and 4AA serving the upper chamber 3A are identically ported, i.e. supply Z, useful output Y and exhaust X, whilst those serving the lower chamber 3B are ported; relay valve 4B, supply X, useful output Y, exhaust Z and booster relay 4BB, supply Z, useful output Y and exhaust X.

A further embodiment, not illustrated, would be to provide the relief or safety valve with manual lifting gear to contact the lower face of bridle 22, Fig. 3 and to make the actuator supporting structure of skeletal design ensuring that the valve disc upper face and the valve spring are both open to atmosphere. These features are to meet boiler code requirements for steam boiler safety valves, the principle and designs of the pneumatic features and equipment being as previously described.

Claims

1. A fluid pressure system comprising a relief or safety valve, an actuator having a displaceable member, which member is capable of applying force and movement through operative parts when fed with pneumatic pressure, in the first instance, to partially load the valve member or disc and, when pneumatic pressure supplies to the actuator so direct, to remove the loading force and further supply a force opposite in sense to the original to assist in opening the valve, the said operative parts being so devised to permit the valve to function freely and independently when the actuator is inoperative due to pneumatic failure.

2. A system as claimed in Claim 1, wherein the relief or safety valve comprises a valve member or disc, a spindle on which the valve member is mounted and a spring for urging the valve member onto an associated seating, said spindle being extended axially through and beyond the valve housings into the actuator displaceable member and being so coupled that the member, in practice, being a diaphragm assembly or piston, is able to functionally position itself axially along the extended spindle to permit the actuator to operate under the influence of pneumatic pressure but not restrict the independent functioning of the relief or safety valve when pneumatic supplies are unavailable.

3. A system as claimed in Claim 2 wherein the relief or safety valve loading is mainly by an adjusting bolt compressing the valve spring but which also provides a supplementary pneumatic loading independent of the spring.

4. A system as claimed in Claim 3 wherein means is provided on the valve spindle to apply a removeable supplementary load.

5. A system as claimed in Claim 1 ,wherein the actuator displaceable member, shown as a diaphragm assembly, is axially guided and so arranged that the pneumatic pressures and discharge pressurefrom the relief of safety valve are each and all isolated from the other but that the valve spindle is free to move independently of the diaphragm assembly when the relief or safety valve functions without external pneumatic assistance.

6. System as claimed in Claim 5 wherein the actuator diaphragm assembly in combination with the valve spindle and/or associated parts provide a lost motion device to permit both pneumatic assistance or independent relief or safety valve operation.

7. A system as claimed in any foregoing claim wherein the actuator has a casing disposed when the relief or safety valve is in a normal upright position above the valve body and either above or below the spring bonnet so as to define on one side of the displaceable member an upper air or gas space and on the other side a lower air or gas space, both of which being serviceable to promote the functioning of the valve.

8. A system as claimed in Claim 7 wherein the means is provided for directing pneumatic pressure to the actuator upper chamber and simultaneously exhausting the lower chamber to provide a downward supplementary force thereby providing additional valve tightness beyond the valve spring setting. The same means is provided to exhaust the said upper chamber to atmosphere and simultaneously direct pneumatic pressure to the actuator lower chamber such changes as described assisting the relief or safety valve to open at the pre-set fluid system pressure to its full extent without simmer or warm and to close positively with blowdown as desired.

9. A system as claimed in Claim 7 or 8, wherein the means is provided for directing pneumatic pressures to the actuator instigated by means of an on/off pilot control with an adjustable differential between the pilot operating and shutting down in terms of the fluid pressure in the vessel or like, over practically the full range of the instrument.

10. A system as claimed in Claim 9, wherein the pilot control as a means of instigating pneumatic pressure to the actuator comprises a sensing element which may be a bourdon tube or bellows or mechanical means which is responsive to changes in pressure, temperature or other conditions in a vessel or the like to be protected by a relief or safety valve, said sensing element being arranged to co-operate with associated pneumatic gear to provide at a desired relieving pressure or other condition a signal for controlling delivery of pneumatic pressure, through specialised relay valves, a pneumatic signal to the actuator for operation of the relief or safety valve when the desired relieving pressure is reached in the vessel to be protected.

11. A system as claimed in Claims 8 to 10, wherein 3-way pressure operated relay valves are provided to respond to signals from a pilot device to admit and/or exhaust a separate pneumatic supply to the upper and lower chambers of an actuator to provide, in the first instance a downward force to supplement the setting of the spring of a relief or safety valve and then, when so directed to exhaust such force and simultaneously provide an upward force to assist in the opening of said relief or safety valve. Further, on receiving the proper signal from the pilot device to restore the initial pressure conditions to the actuator chambers in order to close the relief or safety valve.

12. A system 'as claimed in Claims 9 and 10, wherein the sensing element is so arranged that when the pre-set condition is attained in the vessel or like to be protected is reached a flapper is engaged by the deflection of the sensing element free end causing the flapper to lift from the face of a nozzle, where under normal conditions it forms a closure on the pilot pneumatic system. This initial lift causes a pressure drop across a metering device, which precedes the nozzle, which drop is transmitted to feed back elements provided to amplify the initial flapper lift and thereby amplify and cause a rapid fall in the discharge pressure beyond the metering orifice. Adjustment is provided to vary the ratio of effective flapper lift to sensing element deflection.

13. A system as claimed in Claim 11, wherein the means of amplifying the effect of pressure fall in the discharge from the constant pressure pneumatic supply to the pilot due to the displacement of a flapper under the influence of a sensing element is provided by feed back elements. Such elements comprise a block or housing supported by a collapsible bellows which in turn reacts upon a carriage. A fall of pneumatic pressure causes collapse of the feedback bellows such collapse being controlled by the action of adjustable length cantilever springs, the element carrying the nozzle directly increases the gap at the nozzle outlet and the element located at the fulcrum end of the flapper draws the flapper downwards and due to the lifting hook on the sensing element acting as a fulcrum, rapidly further increases the gap at the nozzle causing a drastic fall in the pilot pneumatic discharge.

14. A system as claimed in Claim 13 wherein the sensitivity of the pilot feed-back elements is controlled by adjustable cantilever springs which may also be set to influence the rate of pilot recovery, the length of fluid system blowdown and the closing of the relief or safety valve.

1 5. A system as claimed in Claim 12, wherein the pilot pneumatic elements are mounted on a carriage hinged at one end and raisable or lowerable at the other by means of a screw adjustment to alter the operating point of the pilot.

1 6. A system as claimed in previous claims wherein the sensing element of the pilot is connected by tube to the interior of the vessel or the like under control so that the sensing element will be actuated in accordance with the change in condition within the vessel or the like.

1 7. A system as claimed in previous claims where the sensing element may be such that pressures, temperatures, liquid level or any such variables may be controlled utilising pressure connections as Claim 16 or mechanical or closed circuit level transmission to the pilot from the sensing source.

1 8. A system as claimed in previous claims wherein the relief or safety valve may open at identical set pressure whether pneumatically assisted or conventionally operated, the former means providing full valve disc lift with nil fluid pressure accummulation.

1 9. A system as in all foregoing claims as illustrated on Figs. 1 to 9.