GB2239689A

GB2239689A - Pressure reducing valve with static balance

Info

Publication number: GB2239689A
Application number: GB8928552A
Authority: GB
Inventors: Bernard Goodwin; Stephen Hurst
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1989-12-18
Filing date: 1989-12-18
Publication date: 1991-07-10
Also published as: GB8928552D0

Abstract

A pressure reducing valve comprises a valve body (1) having a through bore, the bore being divided by a wall (4a) having an aperture (4b) therethrough. Fluid flowing in the bore experiences a pressure drop from the inlet side (1a) to the outlet side (1b) of the aperture. A valve member (40) is adjustable with respect to the aperture to alter the pressure drop and fluid flow rate. The valve member is operatively connected to a static balance means (2) which comprises a piston (8) responsive to the pressure difference across the valve member. A hydraulic actuator (3) may also be provided to control the position of the valve member, the actuator also 7 being dependent on differences in fluid pressure detected by means of a Venturi downstream of the valve. <IMAGE>

Description

PRESSURE REDUCING VALVE This invention relates to pressure reducing valves such as are used in the water industry and, more particularly, to the automatic control of such valves.

Pressure reducing valves (PRV's) are well known.

They comprise a body having a through bore with a valved aperture therein. Liquid flowing through the bore passes through the aperture and experiences a pressure drop from the upstream to the downstream side of the aperture.

Adjustment of the valve member with respect to the aperture increases or decreases the pressure drop and decreases or increases, respectively, the fluid flow rate through the valve.

Hydraulic control systems have been developed to provide control pressures for automatic control of a PRV for example to maintain a particular downstream pressure for varying flow rates. In the water industry, for example, automatically controlled PRV's are used to prevent pressure build-up downstream of a valve when water demand falls such as overnight. The valve is designed to close as water demand falls, so preventing a downstream pressure build-up.

Conversely, when water demand increases, the PRV opens to increase flow rate and maintain downstream pressure.

Whilst conventional automatically controlled PRV's are reasonably satisfactory, they mostly have the disadvantage of employing small flow-sensitive orifices in the hydraulic control system. The system operates using the fluid (eg. water) which is being valved, and it is not uncommon for these small orifices to become occluded or blocked resulting in malfunction or failure of the control system. To overcome the problem, frequent maintenance of the system is necessary and this is expensive.

Furthermore, in the commonly used PRV's the valve member can be subjected to quite a substantial imbalance of forces especially when there is a large pressure drop across the valved aperture in the housing. Thus, the control system has to provide quite high control forces in order to make small adjustments in the position of the valve member, and the provision of precisely correct relatively large forces is itself a demanding requirement which is not always satisfactorily met in conventional systems.

We have now devised some improvements in the hydraulic control systems of PRV's whereby disadvantages in prior art systems are reduced or overcome. In particular, we have found that the use of small flow-sensitive orifices in the systems can be completely avoided, and that the use of high control forces can be avoided.

In accordance with a first aspect of the invention, there is provided a PRV wherein the valve member is operatively connected with a static balance means arranged to compensate substantially for unbalanced forces acting on the valve member in the valve housing. The static balance means does not employ fluid flow, and the balancing force it provides varies and is substantially equal to the imbalance at any time on the valve member arising from the different pressures (inlet pl and outlet P2) in the valve.

In one preferred arrangement, the static balance means comprises a piston slidable in a chamber, the piston being operatively linked to the valve member and being subjected to the difference in pressure across the valve member to provide an appropriate balancing force in dependence thereon. Conveniently, inlet pressure (P1) is applied to one side of the piston and outlet pressure (P2) to the other side.

It will be appreciated that by using a static balance which is flow-dependent, there is no requirement for small flow-sensitive orifices to generate pressures. The pressure(s) applied to the static balance can, for example, be taken directly from the inlets and outlets (or thereabouts) of the PRV. Furthermore, by providing a balancing force which varies automatically so that it always substantially equals the imbalance on the valve member, the forces needed to move the valve member are small. This is in sharp contrast to prior art arrangements where a single force was used to move the piston and in so doing overcome any imbalance forces thereon. By separating the two components of force, in accordance with the present invention, only very low energy control forces are needed in the valve actuator to move the valve member and thus control the pressure reduction.

There are various types of valve actuator which can be used in accordance with the present invention to actuate the hydraulically balanced valve member. For example, low energy hydraulic, electric or pneumatic actuators can be used. An electronic control with a linear actuator can, for example, be employed, the system including sensors for valve inlet and outlet pressures and valve member positions, a computer and a linear actuator. Such an arrangement could be powered from the mains or'from batteries.

In accordance with a further feature of the invention, we have devised a hydraulic valve actuator for controlling the hydraulically balanced valve member. The preferred hydraulic valve actuator of the invention is, like the balancing means, a pressure-operated actuator which does not use fluid flow but only fluid pressure. Further, it does not require small flow-sensitive orifices to create the pressures which it utilises.

In a simple form, the valve actuator comprises means for applying to the valve member a force dependent on the outlet pressure of the PRV, so that as the outlet pressure drops the valve member is moved to open the valve further, and as the outlet pressure rises the valve member is moved to close the valve, to achieve an equilibrium position. Preferably, the force dependent on the outlet pressure is balanced against an adjustable control such as a spring, so that the actual operational pressures of the valve can be controlled and pre-set as required.

In many instances, and particularly where the PRV is being used to control a water supply at a downstream point remote from the valve, the valve actuator will also include means responsive to the valve flowrate (Q). For example, a venturi can be provided downstream, and the pressure difference between the inlet and the throat thereof used as a measure of flow rate. This pressure difference can be communicated to the valve actuator in such a way that, as the pressure difference (flow rate) changes, an additional force is applied to the valve member to open or close as appropriate.

The generation of a force from a pressure is most conveniently achieved by the use of one or more pistons in cylinders, and the pistons can be directly linked to the valve member.

There are many conventional PRV's installed in water supply mains. In many of these, the valve member is mounted on an elongate rod (valve stem) which extends out of the valve housing. Also, the conventional control systems are mounted externally of the valve. In accordance with a further feature of the present invention, a control system of the invention can be retro-fitted to a conventional PRV of the above type. In particular, a static balance means of the invention can be operatively connected to the end of the valve stem rod of the PRV, in order to apply a balancing force thereto. The balance means will normally also be connected to the inlet and outlet valve pressures by simple tubing.Further, a valve actuator control of the invention can also be fitted by operatively connecting it also to the valve stem rod, and providing fluid pressure connections as necessary. It is an important aspect of the present invention to be able to retro-fit the balance means and, if desired, also the valve actuator means to a conventional PRV, and the invention includes such items per se for this purpose.

In order that the invention may be more fully understood, reference is made to the accompanying drawings, wherein: Figure 1 is a schematic vertical sectional view of a prior art PRV with associated control system; and Figure 2 is a schematic vertical sectional view of one embodiment of PRV of the invention.

The prior art PRV illustrated in Figure 1 comprises a valve body 1 having an inlet chamber la and an outlet chamber ib separated by a wall 4a having therein an aperture 4b. A valve member 40 is movable with respect to the aperture 4b to open and close the same. The valve member 40 comprises a circular sealing plate 41 being of a diameter larger than that of the aperture 4b and having a crenellated ring 42 extending from one side thereof through the orifice 4b. Indentations 43 in the ring 42 serve as openings for allowing fluid communication between the inlet and outlet chambers la, ib. The size of these openings varies with the position of the ring 42 relative to the aperture 4b, being largest when the ring 42 is retracted from the aperture 4b to its fullest extent.Full closure of the valve occurs when the sealing plate 41 abuts the rim of the aperture 4b. From the side of the plate 41 opposite to that bearing the ring 42, extends an integral connecting rod 44 and piston 5. The rod 44 and piston 5 are accommodated within a branch lc of the valve body 1 which branch is capped by a separable end wall Id. A link rod 6 internal with the piston 5 extends out of a sealed aperture in end wall id.

Control of the valve member 40 is effected by a hydraulic control system external to the valve body 1 supplying control pressures p3 to the outer surface 5a of the piston 5. The hydraulic control system comprises a valve by-pass conduit 30 having a valve-cock 25 at each end thereof. A small fixed orifice 20 generates a flow rate differential which is modulated by a pilot valve 24. The pilot valve 24 is actuated by movement of a diaphragm 22 which is subjected on one side thereof to the pressure P2 of the fluid in the outlet chamber ib of the valve, and on the other side thereof to the compression force of an adjustable spring 23.The control pressure p3 generated in an intermediate portion of the by-pass conduit 30 is fed by a branch conduit having a needle cock 21 disposed therein, into the valve branch ic to be applied to the outer surface 5a of the piston 5.

In the PRV of Figure 2, the valve itself is essentially the same as that shown in Figure 1 and like numerals indicate like parts. The description thereof will not be repeated. However, in Figure 2, the control system embodies various features of the present invention which will now be described.

As illustrated in Figure 2, the control system comprises a hydraulic static balancing actuator 2 and a hydro-mechanical valve actuator 3, both operatively linked to the valve member 40 via the link rod 6 which projects externally of the valve body 1.

The static balancing actuator 2 is formed of a piston chamber 29 having a piston 8 therein subjected on a first side 8a thereof, co-directional with the outer face 5a of piston 5, to a valve inlet chamber pressure pl, and on a second side thereof to the valve outlet chamber pressure P2.

Conduits 9 and lOb respectively supply pressure pl and pressure p2 to the piston chamber 29 from positions external to, but immediately adjacent the respective chambers la and ib, or from positions within la and ib.

Whereas in the prior art control system the outer face 5a of piston 5 is subjected to a differential pressure p3, in the system of the present invention, this pressure is substituted by the outlet chamber pressure P2 fed via a conduit 10a. Outlet pressure p2 now acts on the valve piston 5 and member 40 combination from opposing directions, thereby eliminating a substantial component of the force imbalance acting across the valve piston 5 and member 40 and due to differences between pl and P2. The remaining imbalance, due to the difference in area of piston 5 and member 40, is compensated for by piston 8 which is dimensioned to match said difference in area.Movement of piston 8 is transmitted to the valve member 40 via a rod 28 integral with the piston 8 and inter-engaged with the link rod 6 by a universal linkage 7.

Balancing by the above set up reliably eliminates at least 90% of the force imbalance across the valve member 40. This statistic may be improved upon if change of momentum effects within the valve body 1 can be accurately estimated and compensated.

The valve actuator 3 comprises a pair of piston chambers 31 and 32 having first 11 and second 12 pistons therein, respectively. A rod 33 integral with each of pistons 11 and 12 extends axially therethrough and interengages with the rod 28 of piston 8 by means of a further universal linkage 7. First piston 11 is subjected to pressure p2 alone, acting on a first side 11a thereof which side is co-directional with outer face 5a of piston 5.

Pressure P2 is supplied to piston 11 via a conduit lOc.

Second piston 12 is subjected on a first side 12a thereof, co-directional with face 5a, to a throat pressure p4 of a venturi flowmeter (not shown) located downstream of the valve. A second side of the piston 12 is subjected to an inlet pressure p5 of the venturi flowmeter. Pressures p4 and ps are supplied to opposite sides of piston 12 via conduits 14 and 13, respectively. A pre-compressed control spring 15 is provided such as to bias the rod 33 outwardly of the valve body 1.

In operation the first valve actuator piston 11 will be urged inwardly, toward the valve1 by the pressure P2 acting unbalanced thereon. First piston 11 thus biases valve member 40 to the closed position against the opposite bias of spring 15. Further, the second piston 12, subject to the pressure differential across the venturi flowmeter, also serves to oppose piston 11, urging valve member 40 to an open position.

When the fluid pressure p2 in the valve outlet chamber 1b rises following a decline in fluid demands, and hence in downstream flow, the difference between the fluid pressures pl and P2 in the inlet and outlet chambers la and 1b of the valve, respectively, will be prevented from directly causing a force imbalance across the piston member 40 and piston 5 by the static balance actuator 2, as previously explained. The increased p2 will, however, urge first valve actuator piston 11 inwards to consecutively shift rod 33, rod 28 and link rod 6 inwards, closing the openings 43 of valve member 40, and thereby reducing supply flow through the valve.The second valve actuator piston 12, acting against piston 11, will simultaneously be subjected to a lesser force because the flow through the downstream venturi meter will have declined resulting in a reduced pressure differential across the venturi. When flow downstream of the PRV through the venturi commences to increase, the force acting across piston 12 will correspondingly increase urging the valve member 40 open.

Although the above described preferred embodiment is directed to use of hydro-mechanical valve set actuating means 3, the same role may be performed by an electronic actuator comprising pressure sensors, a sensor for the extent of opening (x) of the valve, a C.P.U. to process the information and a stepper motor to actuate the valve.

Another possible alternative to use of a hydraulic valve set actuating means is a pneumatic actuator.

An important advantage of having a static balance actuator in an otherwise electronically controlled valve lies in the fact that operating forces necessary to move the valve are much reduced. Consequently large electric motors are not required enabling the systems to rely on battery power alone.

Claims

CLAIMS:

1. A pressure reducing valve comprising: a valve body having a through bore; an aperture in the bore such that fluid flowing through the bore passes through the aperture and experiences a pressure drop from the inlet side to the outlet side of the aperture; and a valve member adjustable with respect to the aperture to alter the pressure drop and the fluid flow rate through the valve, the valve member being operatively connected to a static balance means which provides a varying balancing force to compensate substantially for unbalanced forces acting on the valve member, the balancing force being substantially equal to the imbalance on the valve member at any time arising from the different inlet and outlet pressures in the valve.

2. A valve according to claim 1, wherein static balance means comprises a first piston slidable in a chamber, the first piston being operatively linked to the valve member and being subjected to the difference in pressure across the valve member to provide the balancing force in dependence thereon.

3. A valve according to claim 2, wherein inlet pressure to the valve is applied to one side of the first piston and outlet pressure from the valve is applied to the other side of the piston.

4. A valve according to any preceding claim, wherein said valve body additionally comprises a branch extending perpendicularly to said through bore, a valve piston slidable in the branch and adapted to move with the valve member, one side of said valve piston forming a wall of the through bore such that inlet pressure is applied to said one side of the valve piston, and outlet pressure being applied to the other side of said valve piston.

5. A valve according to claim 4, wherein a first rod extending from said first piston and a link rod extending from said valve piston are connected through a universal linkage.

6. A valve according to claim 1,2,3 or 4, which also includes a pressure-responsive actuator for controlling the position of the valve member of the pressure reducing valve.

7. A valve according to claim 6, wherein said actuator is a pneumatic actuator.

8. A valve according to claim 6 ,wherein said actuator is an electronic actuator.

9. A valve according to claim 8, comprising an electronic control, means for sensing the inlet pressure and the outlet pressure and the valve member position, and a linear actuator electronically operable in response to the sensing means to control the pressure reducing valve.

10. A valve according to claim 6, wherein said actuator is a hydraulic actuator.

11. A valve according to claim 10, wherein the actuator comprises means for applying to the valve member a force dependent on the outlet pressure, such that as the outlet pressure drops the valve member is moved to open the valve further, and as the outlet pressure rises, the valve member is moved to close the valve to achieve and equilibrium position.

12. A valve according to claim 11, wherein the force dependent on the outlet pressure is balanced in the actuator against an adjustable biasing force.

13. A valve according to claim 12, wherein said adjustable biasing force is provided by a spring.

14. A valve according to any of claims 10 to 13, wherein the actuator additionally comprises means responsive to the fluid flow rate through the valve.

15. A valve according to claim 14, wherein said responsive means comprises a venturi downstream of said valve the difference in venturi pressure between the inlet of the venturi and the throat thereof being communicated to the actuator such that an additional force is applied to the valve member to move the valve member in response to said difference in venturi pressure.

16. A valve according to claim 15, wherein said additional force is generated from said difference in venturi pressure by means of at least one piston linked to the valve member and slidable in a cylinder in response to said difference in venturi pressure.

17. A hydraulic valve actuator for controlling the hydraulically balanced valve member of a valve as claimed in any of claims 1 to 5, wherein the actuator is arranged to be coupled to the valve member and the actuator is responsive to fluid pressure.

18. An actuator according to claim 17, which comprises means for applying to the valve member a force dependent on the outlet pressure of the valve.

19. A static balance means adapted to be operatively linked to a valve member in a pressure reducing valve, the static balance means providing a varying balancing force to compensate substantially for unbalanced forces acting on the valve member and comprising a first piston slidable in a chamber, said first piston being adapted to be connectable to said valve member and being responsive to the difference in pressure across the valve member.