GB2060140A

GB2060140A - Fluid Control Valve

Info

Publication number: GB2060140A
Application number: GB8029905A
Authority: GB
Original assignee: Northern Engineering Industries PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 1979-10-02
Filing date: 1980-09-16
Publication date: 1981-04-29
Also published as: GB2060140B

Abstract

A valve for use as a pressure control valve or as a flow control valve. In a pressure control valve the characteristic of the valve control gas spring in a dome 12 is compensated by a variable orifice at which a pressure is derived and fed back to the control diaphragm 20 of the valve via a tube 28. The orifice is defined by an annular member 26 movable against a preloaded spring 88 and a second orifice is defined between the member and the wall of a passage 24 in which it moves. The second orifice opens only after the preload is overcome by the force of fluid flowing through the member after passing through the valve seat. The valve can handle liquids or gases and is applicable to the accurate control of pressure or flow rate in the chemical and process industries. <IMAGE>

Description

SPECIFICATION Valve The invention relates to valves and particulariy though not exclusively to valves for controlling the pressure of liquids and gases.

Control valves of one known kind have a valve member the position of which is controlled by a control element such as a diaphragm or piston, which is responsive to pressure of the fluid being controlled (for example responsive to the outlet pressure).

The control element is also subject to a counter force provided by a spring or by a volume of pressurised fluid at the side of the element remote from that exposed to the fluid controlled.

The application of such valves in industrial processes is limited by the fact that the valve characteristic for example the relationship between flow through the valve and the fluid pressure at the outlet of the valve is dominated by the inherent characteristic of the spring (or pressurised fluid equivalent).

The action of the valve is dependent on movement of the valve member between the fully closed and fully open positions and many applications demand an overall valve characteristic which is different from that dictated by the force/movement characteristic of the control spring or fluid equivalent.

It has been proposed in a pressure regulating valve of the pressure reducing kind partly to compensate for the spring characteristic by the use of a pitot tube having a variable orifice at one end, the other end communicating with one side of the diaphragm of the valve, as described in British patent specification No. 1033196, but many modern applications of control valves demand very high pressure differentials which may be as high as 6000 pounds per square inch (420 kg/cm2) and at the same time demand a very close approach to exactly constant outlet pressure at all flow conditions, which is either impossible to achieve with known valves; or can be achieved only where additional and complicated feedback control loop systems are used to control the valve and/or the flows through it.

The object of the invention is to provide a valve which has an overall operating characteristic which meets the requirements of at leastsome of such applications in industry without the need to use those additional complications.

A valve according to the invention is characterised by a member movable within a passage through which, in addition to a first flowpath provided by a first orifice at which a pressure is derived to compensate for the effect of the spring characteristic, a second flowpath is possible provided by a second orifice defined between a surface of the movable member and a surface of another member and is further characterised by a spring acting on the movable member and pre-loaded, the second orifice opening or enlarging only after the pre-load is overcome.

Several forms of valve will now be described by way of example to illustrate the invention with reference to the accompanying drawings, in which Figure 1 is a vertical section through a first embodiment of valve in which certain parts including the valve member and control element are shown in their leftward halves in positions corresponding to the fully closed valve condition, and in their rightward halves in positions corresponding to the fully open position; Figure 2 is similar section through the upper part of a second embodiment of valve; Figures 3 to 6 are traces recorded to show working characteristics in which outlet pressure in pounds per square inch is plotted vertically against flow-rate through the valve shown in Figure 1 plotted horizontally as the equivalent pressure drop across a standard measuring orifice; and Figures 7 and 8 are vertical sections through parts of third and fourth embodiments of valve, respectively.

Figure 1 shows a pressure reducer valve for passing air at a minimum inlet pressure of 70 kg/cm2 (1000 pounds per square inch) and a maximum outlet pressure of 66.5 kg/cm2 (950 p.s.i.).

The valve comprises a cast steel body 10; a cast steel dome 12 mounted on the body; a valve member 14 vertically reciprocable in the body 10; a valve seat 1 6 in the body 10; a push-rod 18; a control element in the form of a diaphragm 20 mounted at its peripheral margin between the body 10 and the dome 12; a cylindrical housing 22 being part of a compensator means defining a passage 24; a flared tubular member 26 longitudinally movable within the passage 24; and a tube 28 defining a duct 30 extending from within the member 26 to a plate 32 through which the push-rod 1 8 slidably extends and which is mounted on the body 10 beneath the diaphragm 20. The plate 32 has slots 33 in its upper face.

The body 10 has a screwed inlet port 34 and a screwed outlet port 36 which receives a threaded spigot of the compensator means 22.

The valve member 14 is hollow and has an annular seal pad 38 engageable with the seat 1 6 and is biassed upwardly by a compression spring 40 into engagement with the push-rod 1 8. The valve member is of the balanced type having an insert 42 which has a through-bore (not shown) so that the forces on the valve member 14 due to pressure on the valve member 1 4 are equalised.

The inner diameter of the valve seat 16 is equal to the smaller outer diameter of the valve member 14 so that when the valve is closed the pressure within the valve seat acting to produce a downward force on the valve member 14 is exactly counterbalanced by the same value of pressure acting on an equal area of the valve member 14 to produce an equal upward force.

The push-rod 18 is urged upwardly by the spring 40 into engagement with a plate 50 which is held against the underside of the diaphragm 20 by a conical compression spring 52.

The interior of the dome 1 2 communicates with the inlet of the valve body 10 via a bore system 54 which includes manually adjustable valve plugs 56, 58. The interior of the dome 12 can be pressurised by the admission past the valves of air from the inlet, then the valves are closed to trap the pressurised air or gas in the dome 12.

The dome 12 contains a plate 60 lying on top of the diaphragm 20 and having a control bore 62 leading to radial slots 64 at the underside of the plate 60, so that the pressurised air or gas in the dome 12 acts on the upper side of the diaphragm 20.

The plate 32 has a tapped through-aperture at 66 into which the screwed end of the tube 28 is fitted.

The duct 30 within the tube 28 opens into a chamber 67 between the plate 33 and the diaphragm 20.

The tube 28 extends through the outlet 36 of the body 10 and its other end at 68 defines, with the adjacent inner surface of the member 26, an annular orifice 70.

The orifice 70 is variable in dependence upon the position of the member 26 and upon the shape of the flared end portion 72 of the member 26, which is designed to produce the desired characteristic of operation of the valve as explained below.

The downstream portion 74 of the member 26 is cylindrical so that the member 26 has a throat 76 intermediate the ends of the member 26 so that the member 26 acts as a Venturi of fluid passing through it.

The member 26 has an annular frusto-conical end face at 80 at its flared end portion which is engageable with a corresponding fruso-conical internal surface 82 of the wall of the passage 24.

A second annular variable orifice 83 is thus defined between the face 80 and the surface 82.

The member 26 has a first and second external annular shoulders 84 and 86 and a compression spring 88 is trapped between the first shoulder 84 and an annular spider 90 secured in the outlet end of the extension 22. The spring 88 is installed so as to be pre-loaded to urge the member 26 to the right and to hold the face 80 against the surface 82 until, as explained below, the spring force is overcome by the force on the member 26 arising from pressure upon it.

The leftward movement of the member 26 is limited by engagement of the second shoulder 86 with a tubular stop 92, which is secured to the spider 90 and which supports the member 26 for sliding movement through the stop 92.

Operation Before discussing the operation of the valve described above, it is useful to consider the mode of operation, which is generally well-known, of a known type of valve from which the tube 28 and the extension 22 with the components within it would be absent. In such a known valve the plate corresponding to the plate 32 would have through-apertures allowing the air pressure at the outlet side of the valve member to act on the underside of the diaphragm.

For a given mass flow rate through the valve the valve member assumes a particular position such that the outlet pressure is held at some value less than the inlet pressure, depending on the set pressure within the dome 12.

At this condition, the downward force on the diaphragm due to the set dome pressure equals the upward force on the diaphragm due to the valve outlet pressure acting on the underside of the diaphragm.

At a different value of mass flow rate through the valve, the valve member must assume a different position in order to produce the required pressure reduction and the diaphragm too will assume a different position. There will therefore be a change in volume of the air or gas, in the dome resulting in a change of set pressure in the dome so that the downward force on the diaphragm will be different.

Thus, for the new balanced condition, the outlet pressure must be different from what it was formerly.

Over the flow range plotted as abscissa the outlet pressure plotted as ordinate, the valve characteristic is a generally straight, but slightly inclined, line.

The demands of many industrial applications are such that constant, or very much more closely constant, pressure under all flow conditions is required, calling for a horizontal or very nearly horizontal characteristic in the valve.

The valve described as an example of practice of the invention has a greatly improved characteristic which is horizontal or very nearly so.

As flow through the valve increases pressure on the member 26 increases until the force on the member 26 overcomes the pre-set load in the spring 88 and the member 26 moves to the left until the total pressure and flow parameters stabilise.

At the all steady condition assuming maximum mass flowrate the member 26 is in a position such as shown in the lower half of the extension 22 (Figure 1).

The valve member 14 and the diaphragm 20 are in the corresponding positions shown at the right-hand half of Figure 1.

Some of the flow through the extension 22 passes through the orifice 83 and around the outside of the member 26. The remainder passes through the member 26 and because of the Venturi effect in the throat 76 there is a loss of pressure in the throat.

It is that reduced pressure which acts on the underside of the diaphragm by transmission of the pressure through the tube 28. The balanced condition is achieved by that reduced pressure.

If the flowrate decreases, the member 26 experiences a reduction in pressure and force and the load in the spring 88 forces the member 26 to the right until the reduction in apertures of the variable orifices 70 and 83 produce a resultant pressure on the member 26 to give a new balanced condition.

Less air now flows around the member 26 and more flows through the Venturi throat 76 so that the loss of pressure in the throat is greater; in other words the pressure in the throat 76 is slightly less than formerly and a slightly decreased upward force acts on the diaphragm 20.

The diaphragm is now in a slightly lower position to achieve balance and the valve member 14 similarly is in a slightly lower position.

The result is that, at that new balanced position, the outlet pressure is equal to, or very close indeed to, the original output pressure. At all operating flow rates the output pressure is kept at, or very close to, the same value because of the compensating effect of the member 26 and tube 28.

The profile of the flared portion 72 is carefully chosen to give correct pressures beneath the diaphragm at all conditions.

In tests with air at low flowrates using only low inlet pressure to ensure a fully open valve condition, a standard type C2 valve available from IV Pressure Controllers, Spur Road, North Feltham Trading Estate, Middlesex TW14 OSZ England, produced a pressure drop of 0.6 kg/cm2 (9 p.s.i.) from start of flow to full flow using an inlet pressure of 21 kg/cm2 (300 p.s.i.) and a nominal outlet pressure of 7 kg/cm2 (100 p.s.i.). This characteristic is shown as the recording No. 1 in Figure 3. Another recording (No. 2) is also shown in Figure 3 in which the inlet pressure was 14 kg/cm2 (200 p.s.i.). In both tests the flowrate was equivalent to 760 Nm3/hr. (450 standard cubic feet per minute) at atmospheric pressure.

The invention provides a marked improvement in such performance. Initial tests were carried out using the same C2 valve modified as shown in Figure 1 (but without the seal 200 described below).

Figures 4 and 5 show recordings using air at inlet pressures of 14 kg/cm2 (200 p.s.i.) and 21 kg/cm2 (300 p.s.i.), respectively with outlet pressure of 7 kg/cm2 (100 p.s.i.) in each case and the same flowrate as in Figure 3. The pressure drop was only some 0.1 kg/cm2 (1.5 p.s.i.).

The rate and pre-load of the spring 88 affects the characteristic of the valve as shown by further initial tests which gave the recordings shown in Figure 6. The flowrate was as before in all cases.

The uppermost recording was at an inlet pressure of 14 kg/cm2 (200 p.s.i.) and an outlet pressure of 7 kg/cm2 (100 p.s.i.). In the middle recording the same spring was used but the inlet pressure was changed to 21 kg/cm2 (300 p.s.i.). In the lowermost recording another spring was used having a lower rate similar to that of the spring used when the recordings shown in Figures 4 and 5 were made. The inlet pressure in this last test was 21 kg/cm2 (300 p.s.i.) and the outlet pressure was again 7 kg/cm2 (100 p.s.i.).

In the upper two recordings the spring has a rate of 1.4 kg/cm (8.0 pounds per inch) and a pre load of 1.8 kg (4.0 Ibs) and in the lowermost recording the spring had a rate of 3.6 kg/cm (20 Ibs/inch) and a zero pre-load.

The valve as described has an additional advantage in that the diaphragm 20 is not exposed to spurious forces arising from transient dynamic effects during the operation of the valve.

These affects if they arise, occur immediately downstream of the valve member 14.

The pressure acting on the underside of the diaphragm 20 is solely that present at the end 68 of the tube 28 as derived from the immediately adjacent Venturi throat 76 of the member 26. No transient dynamic effects arise at that point; or else any arising are negligible. Consequently, the valve is relatively more stable in operation than known valves for similar use in which the control element such as a diaphragm is exposed to transient dynamic effects.

Figure 2 shows part of a modified valve in which instead of a dome containing pressurised air the body carries a cover 100 containing a compression spring 102 arranged between a lower abutment 103 and an upper abutment 104 adjustable to change the load on the spring 102 by a screw 106.

The lower abutment 103 acts on a diaphragm 20A analogous to diaphragm 20 shown in Figure 1 and the remainder of the valve which is not shown or described in exactly similar to that already described with reference to Figure 1; the operation of the valve is also similar to that already described.

For applications where the overall valve characteristic is required to be different the member 26 may have a profile differing from that shown (which is one such that the relationship between movement of the member 26 and change in the size of the orifice 70 is linear) but chosen to suit such different characteristics; in any case the profile may be different depending on whether gas, liquid or a mixture of gas and liquid is being handled.

The valve may be other than a pressure reducer; and it may be required instead to control flowrate or obey some relationship dependent on both pressure and flow.

In some cases it may be necessary to provide additional damping of the movement of the member 26. That can be achieved by the use of an O-ring as shown at 200 in Figure 1, which increases the friction between the member 26 and the stop 92.

Alternatively, an O-ring could be fitted in an external groove in the member 26 and engage the inner surface of the stop 92.

The O-ring is an optional feature. The member 26 is subject to frictional damping by its sliding engagment with the stop 92 if the O-ring is omitted and in many cases such friction may be sufficient. The initial test mentioned above was performed without an O-ring and gave no evidence that the use of an O-ring or other additional damping was necessary.

The diaphragm 20 may be of metal instead of rubber.

In another modification, the Venturi throat may be formed by a profiled member within a hollow tubular member. For example, the profiled member may surround the end 68 of the tube 28 and be mounted or formed thereon.

In another modification, the desired pressure variation at the end 68 may be achieved by an orifice effect not dependent on the Venturi effect; however, the orifice arrangement whether Venturi or otherwise will be designed to produce the necessary automatic compensation in pressure fed back to the diaphragm to compensate for the limitation in the control force (spring, fluid volume or other control means) acting on the diaphragm or other control element.

The traces shown in Figures 3 to 6 are of open loop form because of a hysteresis effect in the valve operation caused by friction acting on the moving parts. The effect of friction is that, as the flow through the valve increases the relationship between outlet pressure and flow is slightly different from that as the flow decreases. The typical hysteresis loop represents the energy expended in overcoming friction as the valve parts move.

The frusto-conical profile at the surface part 82 may if preferred be instead a surface of revolution based on a curved "generator"; or may be of some other shape. The shapes of the inner surface of the part 72 and of the surface part 82 are so chosen together with the characteristic of the spring 88 and the mutual positioning of the parts forming the orifices that the desired compensation is achieved. The orifice 83 has a characteristic matched to that of the orifice 70.

The orifice 83 may be either completely closed (as shown in the upper part of Figure 1) or in other applications it may be arranged to be open, as shown in Figure 7, for flow valves between zero and that at which the pre-load in the spring 88 is overcome.

In Figure 7 the member 26 is shown as having an end flange 300 engaging the stop 92.

The orifice 83 is defined by the cylindrical inner surface of the housing 22 of the compensator means.

During the final stage of the movement of the member 26 the orifice 83 does not increase in area. However, the orifice 70 does continue to increase in area during the final stage. There is thus a further reduction in pressure in the duct 30 during the final stage which causes a further increase in total mass flow through the valve and the compensating means, including an increase in mass flow through the orifice 83.

Thus, the increasing load in the spring ensures an increasing differential across the compensating means which boosts the transfer of pressure from the valve inlet to the input to the compensating means to maintain the final output pressure constant.

In other applications, the stop 92 can be arranged to ensure that the leftward travel of the member 26 (as seen in Figure 1 for example) ceases when the area of the orifice 83 first attains its maximum value so that no increase in area of the orifice 70 is allowed beyond that point.

For other applications the orifice 70 may be arranged to remain at constant area during part of the movement of the member 26. For example, the cylindrical inner part of the member 26 may be arranged to move past the end 68 of the tube 28 during the initial stage of movement of the member 26 so that the orifice 70 is constant in area until the flared inner surface moves past the end 68.

As already mentioned, the member 26 may move only after a predetermined flow rate is reached, the spring 88 having a preload to ensure that action. In that case the flow rate at which the orifice 83 opens is such as to ensure adequate pressure drop at the end 68 of the tube 28.

Typically, the opening of the orifice 83 ensures that such pressure drop is maintained but at the same time excessive pressure drop in the tube 28, which would cause over-compensation, is prevented.

Figure 6 shows examples in the upper two traces of over-compensation, the spring being too strong and so causing excessive limitation of the opening of the orifice 83. This in turn led to excessive pressure drop in the tube 28 and overcompensation in the final output pressure characteristic of the valve.

Many more variations of overall characteristic are available to the designer and they cannot be enumerated here.

The movement of the member 26 may be opposed for certain applications by the action of more than one spring. The springs may act successively so as to produce a progressive cumulative action for example; or two or more springs may act in unison throughout the movement; or some other sequence or combination may be used as required for the application.

The compensating means in typical arrangements such as that shown in Figures 1 and 2 be such that the orifice 70 will produce an adequate pressure drop at the initial point of operation with small flow through the compensating means and such that proper control is maintained throughout the working range of flow, so that the desired overall valve characteristic is achieved.

The overall characteristic may include compensation for pressure loss or change in a relatively long pipe. In such a case (not shown) the valve body 10 is connected to one end of the pipe and the compensator means is connected to the other end of the pipe. The tube 28 may then be replaced by a tube which after it leaves the housing 22 passes outside the pipe and runs outside the pipe until it enters the body 10. Such a tube may be flexible for convenience. Pressure loss in the pipe can be accurately compensated for as an alternative to compensation at a point immediately adjacent the valve body.

The compensator means decribed with reference to Figures 1 and 2 can be readiiy made very robust. This is important for applications where, on close down of a pressurised system in which the valve is installed, the pressure upstream of the valve is vented completely and relatively quickly. The stored pressure downstream of the compensator means now acts in reverse on the compensator means and the member 26 in particular must for such applications be capable of withstanding very high differential reverse pressures (e.g. 6000 p.s.i.

(420 kg/cm2) or more). The compensator means in such applications also withstand full forward pressure differentials, which may be correspondingly high.

The invention is not limited to pressure reducing applications. It is also applicable to back pressure control applications as shown in Figure 8.

In that application the valve member 314 is positioned above the valve seat 31 6 and is acted on by a spring 317 engaging the plate 350. The valve member 314 is acted on by a push-rod 318, which Is connected to a piston 31 9 forming the movable wall of a chamber 367.

The tube 328 communicates with the chamber 367 and with the interior of the flared end of the member 326 of the compensator means (corresponding to the member 26 described above).

The function of the compensator means is analogous to that already described save that the tendency of the valve inlet pressure to rise as flow increases is compensated for so as to maintain the pressure at the inlet 334 constant.

The pressure at the outlet side of the valve is sensed via the bores 335, 337.

In a further embodiment, (not shown) the two variable orifices of the compensator means are defined each partly by respective movable members. One member is similar to the member 26 being hollow and having a flared end adjacent the tube 28 and being movable against a spring.

However, no path for fluid between the member and the wall of the passage in which it moves is available.

Instead a second passage is provided leading from the valve outlet and re-joining the passage (in which said one member moves) downstream of the member.

In the second passage there is a second member arranged to move against a second spring and a second orifice is defined between the second member and the wall of the passage in a manner analogous to the provision of the orifice 83.

The two springs must be closely correlated or be identical and the two members' movements must be similarly correlated. Preferably, the members are mechanically limited.

Other variations of construction providing two orifices affording flow paths in parallel are possible.

Instead of the housing 22 being immediately adjacent the outlet of the valve body 10, it may be connected to one end of a pipe, the other end of which is connected to the outlet of the valve body 1 0. The housing and the parts within it, together with such pipe and the valve body and its contents are regarded in this specification as forming the valve.

Claims

1. A valve comprising a valve member the position of which is controlled by a control element such as a diaphragm or piston and the valve member being subjected to force provided by a spring or a volume of pressurised fluid, in which the effect of the characteristic of the spring or fluid volume is compensated by pressure which acts on the control element and which is derived at a permanently open variable orifice partly defined by a member which is movable subject to a spring or equivalent in response to changes in flow rate through the valve characterised in that the movable member (26; 326) is movable within a passage (24) through which in addition to the first flow path provided by the first orifice (70) a second flow path is possible provided by a second orifice (83) defined between a surface 80 of the member (26; 326) and a surface (82) of another member (22) and further characterised in that the spring (88) is preloaded and in that the second orifice (83) opens or enlarges only after the preload is overcome.

2. A valve according to claim 1, characterised in that the size of the first orifice (70) continues to increase throughout the entire movement of the member (26; 326) against the force of the spring (88).

3. A valve according to claim 1 characterisedin that the size of the first orifice (70) is constant during part of the movement of the member (26; 326) against the force of the spring (88).

4. A valve according to claim 1,2 or 3 characterised in that the first orifice (70) attains its maximum value at the same time as the second orifice (83) attains its maximum value.

5. A valve according to any preceding claim characterised in that the second orifice (83) remains closed until the pre-load in the spring (88) has been overcome.

6. A valve according to any claim of claims 1 to 4, characterised in that the second orifice (83) is always open.

7. A valve according to any preceding claim characterised in that during part of the movement of the member (26; 326) the size of the second orifice (83) remains constant.

8. A valve according to any claim of claims 1 to 5 or according to claim 7 as dependent upon any claim of claims 1 to 5 characterised in that the pre-load in the spring (83) is reacted between engaged surfaces (80, 82) which are separable to define the second orifice (83) after the pre-load is overcome.

9. A valve according to claim 6, characterised in that the pre-load in the spring (83) is reacted between engaged stops (92,300) independently of the surfaces (80, 82) defining the second orifice (83).

10. A valve according to any preceding claim characterised in that one surface defining the first orifice (70) is a surface of revolution of a curve about an axis parallel to the direction of flow and is on a part (72) of the member (26; 326).

11. A valve according to claim 10, characterised in that the first orifice (70) is defined in part of the range of movement of the member (26; 326) by an interior cylindrical surface of the member (26; 326).

12. A valve according to any preceding claim characterisedin that the second orifice (83) is defined in part by a frusto-conical surface (82) relative to which the member (26; 326) is movable.

13. A valve according to any preceding claim characterised in that the second orifice (83) is defined in part of the movement of the member (26; 326) by an interior cylindrical surface of a housing (22) within which the passage (24) extends.

14. A valve according to any preceding claim characterised in that the valve member (14) is controlled by only one control element (20).

15. A valve according to any claim of claims 1 to 13 characterised in that the valve member (14) is controlled by two control elements one of which is responsive to the inlet pressure of the valve and the other of which (319) is responsive to the compensating pressure fed to it by the duct defined by the member (328).