GB2405957A

GB2405957A - Pressure reducing valve system

Info

Publication number: GB2405957A
Application number: GB0320619A
Authority: GB
Inventors: Stuart William Trow; Martyn Redman
Original assignee: FLUID CONTROLS U K Ltd
Current assignee: FLUID CONTROLS U K Ltd
Priority date: 2003-09-03
Filing date: 2003-09-03
Publication date: 2005-03-16
Also published as: GB0320619D0

Abstract

A pressure reducing valve system is disclosed that modulates its outlet pressure such that the pressure increases as flow rate increases, for example to compensate for head losses arising from flow in a water supply installation. The system includes a pressure reducing valve (PRV) 10, a pilot valve 12 for operating the PRV, and a pressure feedback stage 40. The pressure feedback stage 40 includes a gate valve 42 that creates a head loss that increases with flow through the PRV. The head loss causes water to flow in a bypass pipe 44 that incorporates a venturi 46. The pilot valve 12 is controlled by pressure at the venturi 46. This pressure decreases as flow rate increases, thereby causing the pilot valve 12 to increase the output pressure of the PRV as flow rate increases.

Description

PRESSURE REDUCING VALVE SYSTEM

This invention relates to a pressure reducing valve system, particularly, but not exclusively, for use in a hydraulic installation such as a water supply and distribution system. For convenience the invention will be described below with reference to a particular application in water supply but it will be appreciated that its uses are not so limited and, indeed, that it has wide applicability in fluid systems generally.

In the supply of water from source, i.e. mains supply, to a multiplicity of end users, it is conventional practice to pass the supply through a PRV, which may for example, reduce pressure from a mains pressure of about 70 metros to 40 metros. T his is desirable as a lower operating pressure greatly reduces leakage rate and the frequency of burst mains.

T,RVs are designed to control to a pre-set constant outlet pressure irrespective of flow rate and the inlet pressure. Selfregulation is achieved with a pilot loop surrounding the main valve. This externally mounted loop constructed of small bore pipe is connected from the inlet of the 1,RV to a chamber in the main valve referred to as the control space and then usually through a tee connection to the outlet of the valve. Between the inlet and the control space is a restriction usually of the fond of an orifice plate and between the control space and the outlet is a pilot valve.

A typical prior art pilot valve has a simple spring loaded valve arrangement whereby a spring is caused to act against a diaphragm plate fixed to a diaphragm. On the other side of the diaphragm a rigid connection is made between the diaphragm plate and a yoke.

: ' The bottom of the yoke holds a seat onto which water from the upstream pilot loop pipe work is discharged through a nozzle. The water then proceeds to the outlet of the main valve such that the total pressure acting beneath the pilot diaphragm is equivalent to the :.: . 25 outlet pressure. The position of the main valve throttling element is determined by the :: volume ol water in the control space, which is governed by the pilot valve so as to maintain a constant outlet pressure.

After the pilot valve has been set to control to a particular outlet pressure, the summation of forces resulting from those acting across the main valve elements and those occurring in the pilot valve are such that with a steady flow through the main valve, the pilot valve seat assumes a fixed position away from the nozzle and a small flow continually passes through the pilot loop.

When a change in flow takes place, this could be caused by either a change in consumer demand or a change in the inlet pressure, the outlet pressure acting beneath the pilot valve diaphragm causes the position of the pilot valve seat to alter. In the case of an increase in the now the outlet pressure initially falls and the pilot valve opens. This causes water to be discharged from the control space with the net effect that the main valve opens and the outlet pressure increases. However, as the outlet pressure increases the force acting beneath the diaphragm also increases and when the seat has returned to its previous position the volume of water in the control space will become constant. The force exerted by the spring in the pilot valve and the outlet pressure will then have returned to their pre-set values. Conversely, if the flow rate falls, the outlet pressure initially rises: this causes the pilot valve to close and water then enters the control space with the net effect that the main valve closes and the outlet pressure falls. However, as the outlet pressure falls the force acting beneath the diaphragm also falls and when the seat has returned to its previous position the volume of water in the control space will become constant. The force exerted by the spring in the pilot valve and the outlet pressure will then have resumed to their pre-set values. This continuous process enables the PRV to regulate to a set outlet pressure for changes in flow rate and the inlet pressure. :e

20 In a water supply network, flows change quite markedly throughout the course ofthe day, peak demand typically occurring around breakfast time and the lowest demand occurring . during the early hours ofthe morning. Pipe friction losses governed by the flow rate lead ë to a loss of pressure and the pressure downstream of the PRV must be set at a sufficient level to overcome pressure losses between the PRV and the consumer at a peak flow, when the greatest friction losses occur. With a fixed set outlet pressure, the pressure as . set by the PRV is greater than required at all flows below the peak flow and therefore leakage rates and susceptibility to bursts is greater than need be. Furthennore if an exceptional demand occurs, such as a fire fighting load, pressures could fall below minimum requirements to ensure a satisfactory supply of water to all consumers.

It is therefore desirable for the PRV to increase its outlet pressure in some predetermined manner with flow rate (so-called "modulation"). Various attempts have been made to achieve this fond of control. These have typically required addition of a remote sensing pilot and either a fixed orifice plate or an in-line venturi to the valve system, thereby increasing its complexity and cost. These components arc designed to provide a specific modulation vs. flow relationship and cannot be readily adapted to meet different requirements: they marmot therefore be kept as standard stock items. Moreover, such systems tend to operate slowly and not particularly smoothly.

It is an aim of this invention to provide a pressure reducing valve system that is less complex and better in operation than known systems.

From a first aspect, this invention provides a pressure reducing valve system comprising: a pressure reducing valve (PRY) having a flow inlet, a flow outlet, and a control inlet; a pilot valve for controlling the pressure reducing valve, the pilot valve having an inlet connected to the flow inlet of the pressure reducing valve, a control outlet connected to the control inlet of the pressure reducing valve, and a pressure sensing line; and a pressure feedback stage on the outlet of the pressure reducing valve, the pressure feedback stage operative to apply to the pressure sensing line of the pilot valve a pressure that corresponds to the outlet pressure of the pressure reducing valve, reduced by a value that increases as flow through the pressure reducing valve increases.

Thus, the behaviour of the pilot valve is modified by the reduction of pressure on the pressure sensing line such that it causes the outlet pressure of the pressure reducing valve to rise, to compensate for the reduction in the pressure that it senses, as the flow through .

. the valve system increases. :

Most typically, the valve system is configured such that the amount by which the pilot . :.: .. valve causes the PRV to increase pressure as flow rate increases is greater than the .. , 25 increase in head loss within the valve system as flow rate increases.

The pressure feedback stage may include a venturi, the flow through which increases as the flow through the pressure reducing valve increases. In such embodiments, the sensing line oi the pilot valve typically monitors pressure at the venturi. For example, the venturi may be connected across a flow restriction in the outlet of the pressure reducing valve. Thus, the head loss that occurs as flow passes through the restriction causes some ofthat flow to be diverted into the venturi. In such case, the flow restriction may be such that the restriction it places on the fowls variable, thereby providing control over the flow through the venturi. For example, the flow restriction may include a (preferably adjustable) gate valve.

Most typically, the pilot valve is adjustable to enable the outlet pressure of the valve S system to be set as required. Advantageously, commissioning means can be provided, operable such that the pressure feedback stage retunes to the pilot valve a pressure that is substantially the outlet pressure ofthe PRV. This can be useful during commissioning of the valve system.

According to a second aspect, the invention provides a pressure reducing valve system comprising: a pressure reducing valve (PRY) having a flow inlet, a flow outlet, and a control inlet; a pilot valve having a control inlet and a control outlet that is connected to the control inlet of the pressure reducing valve, for controlling operation of the pressure reducing valve, and a pressure feedback stage on the outlet side of the pressure reducing valve, including a flow restriction in the outlet of the pressure reducing valve and a venturi connected across flow restriction, the venturi being connected to the control inlet of the pilot valve, wherein the pressure feedback stage is operative to control the pilot valve . according to the pressure head across the flow restriction, and the flow restriction is :. variable, thereby permitting control over the outlet pressure of the pressure reducing - valve system.

The flow restriction forces flow through the venturi, which varies with the pressure head ë across the flow restriction and therefore the flow rate through the PRV. The control inlet ..e -.

Of the pilot valve is connected to the venturi, so that the outlet pressure of the PRV is modulated according to demand. Since the flow restriction is variable, the degree of modulation for a particular flow rate can be adjusted.

The pressure reducing valve system can therefore be kept as a stock item, which can be adjusted on-site to provide a degree of flow modulation that is matched to the requirements at that particular location. The system can also be adjusted in-situ to take account of any changes to the downstream water distribution system affecting the head loss vs. demand relationship, including for example new building developments, and mains replacement and reint'orcement. The variable flow restriction can also be opened fully in abnonnal circumstances (e.g. for fire fighting) to restore the Pull capacity of the water main, whereas a fixed orifice creates a permanent obstruction to the flow.

The simplicity of this invention means that it is suitable for retrofitting to existing pressure reducing pilot valves. Therefore, from a further aspect, this invention provides a pressure feedback stage suitable for fitting to an existing pilot-controlled PRV to implement an embodiment of the first or second aspects of the invention. i The pressure feedback stage may be provided as a kit, and might typically include some or all of a valve (e.g. a gate valve), a bypass pipe for connection to the valve, a venturi within the bypass pipe, and fitting and commissioning instructions that direct a user to apply the kit to an existing PRV.

Embodiments of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which: Fig. I is a side cross-section diagrammatic view of a pressure reducing valve system, including a main valve and a pilot valve, according to a first embodiment of the invention; and -e Figure 2 is a graph illustrating the output pressure against flow through a valve system embodying the invention. .

The embodiment that will be used to illustrate the invention is a pressure reducing valve system for controlling pressure in a water distribution network, but it will be understood : .: ,' that its principles can be applied to control of pressure in other applications and of other fluids.

A pressure reducing valve system embodying the invention is shown in Fig. 1. The system includes a main pressure reducing valve (PRY) 10 and a pilot valve 12. The PRV 10 is conventional and includes an inlet l and an outlet 20 which are connected to one another through a valve seat, and a valve body that is mounted on a valve stem t'or movement towards and away from the valve seat. A portion 22 ofthe valve stem projects from the PRV 10 to give a visual indication of'the position of the valve body. The valve body is mounted in a diaphragm, above which there is a biassing spring and a chamber known as the control space. The position of the valve body and hence the pressure reduction produced by the PRV 10 is determined by the volume of water in the control space. Since the internal components of the PRV are conventional, they are not shown in Figure 1.

The pilot valve 12 is a constant flow pressure reducing pilot. The pilot valve has an inlet connected through a pipe to the inlet 18 of the PRV, and a control outlet 32 that is connected through a pipe to the control space of the PRV. The control outlet can add water to or remove water *om the control space, thereby controlling the pressure j reduction effected by the PRV. The pilot valve also has a sensing outlet 34 which, in a conventional configuration, would be connected to the outlet 20 of the PRV. The pilot valve may optionally be of the type disclosed in WO-A99/23544 of the present applicant, or may be of a conventional nature. : As will be well understood, the operation ofthe pilot valve in a conventional PRV system is such that it will adjust the amount of water in the control space to maintain the pressure of the control outlet 32 close to a fixed value.

In this embodiment, a pressure feedback stage 40 is connected to the outlet 20 of the PRV, such that water flowing through the PRV also flows through the pressure f eedback stage 40. The pressure feedback stage 40 includes an adjustable knifegate valve 42 through which the greater part of the water leaving the PRV passes. The degree of opening ofthe knifegate valve 42 can be controlled by a control wheel 48. A bypass pipe . . 44 is connected in parallel with the gate valve 42. There is, within the bypass pipe 44, a venturi section 46, this being a constricted section of reduced diameter. A small diameter tap pipe is connected to the venturi section, the tap pipe being connected to the control outlet ofthe 32 ofthe pilot valve 12.

. - . Consider now operation of the pilot as water flows at a constant rate through the valve l system described above. If the knifegate valve 42 is fully open, there is a minimal pressure drop across it, and there is ahnost no flow in the bypass pipe 44. Therefore, the pressure measured by the pilot valve 12 is substantially the same as the pressure in the outlet of the PRV 10. The system will therefore operate as a conventional pilot controlled PRV.

Now, if the knifegate valve 42 is partially closed, there will be a head loss across it, with the result that the two ends of the bypass pipe are at different pressures, thereby causing water to flow through the bypass pipe 44. In the r egion of the venturi 46, the water flows at a greater speed than in the rest of the bypass pipe 44, with the result that its pressure is reduced in that region. This pressure reduction is communicated to the control outlet of the pilot valve 12. In response to this reduction in pressure, the pilot valve l 2 adjusts the PRV l 0 to increase the pressure of its output.

If' the rate at which water is flowing through the system increases, the pressure drop across the knifegate valve 42 will also increase, and this in turn will cause more water to i flow through the bypass pipe 44. This increases the magnitude of pressure reduction in the region of the venturi 46 which has the effect of causing the pilot valve 12 to increase the output pressure of the PRV l O. Thus, the valve system causes the output pressure to be increased as the rate of flow through it increases. : During operation, there is a continuous flow of water from the inlet 30 to the sensing outlet 34 ofthe pilot valve 12 (this is the pilot loop discussed above). The pilot loop flow is re-combined with the main flow of water through the valve system at the venturi 46.

For successful operation, the reduction in head loss across the T,RV 10 must exceed the increase in head loss across the knifegate valve 42 as the flow increases. The extent to

I

: which the outlet pressure is modulated is detennined by the setting of the knifegate valve . 42. The greater the restriction imposed by the knifegate valve 42, the greater the head loss across it for a given flow rate, and therefore, the greater the depression that will . occur at the venturi 46 for a given flow rate. I lowever, closing the knifegate valve increases the head loss when the PRV l O is fully open. In practice, the system can be configured such that the head loss caused by the knifegate valve 42 is a small fraction -. 1 (for example, less than approximately 25%) of the increase in pressure resulting from modulation of the outlet pressure. As illustrated in Figure 2, it is possible to obtain different modulation profiles by altering the setting of the knifegate valve 42.

To aid commissioning of the valve system, first and second ball valves 50, 52 are provided in the bypass pipe 44 upstream and downstream ofthe venturi 46, and first and second pressure gauges 54,56 are provided respectively upstream and downstream of the venturi 46 and at the outlet ol' the knifegate valve 42 respectively. The procedure for commissioning the valve system, with the knifegate valve 42 initially fully open, is as follows.

1. Close the first ball valve SO. (The valve system will now operate as a conventional pilot-eontrolled PRV system.) 2. Commission the pilot valve 12 as in a standard PRV system, using the second pressure gauge 56 to set the downstream pressure.

3. Use a pressure logger to log operation of the PRV for a suitable period (e.g. I - 2 weeks) to achieve a profile setting for flow modulation. j 4. From the output of the logger, detcnnine the time of day at which peak demand is likely to occur.

5. At a time of peak demand, open the first ball valve 50.

6. With the knifegate valve 42 still fully open, adjust the pilot valve 12 until the second pressure gauge 56 indicates the required pressure at a time of low demand.

7. Gradually close the knifegatc valve 42 until the pressure indicated by the second pressure gauge 56 is the required peak demand pressure.

The first ball valve 50 can, if necessary, be partially closed to restrict flow in the bypass pipe 44, for example, in the event that cavitation occurs within the venturi 46.

-..DTD: Since the PRV 10 and the pilot valve 12 can be conventional items, it will be understood . that the components that make up the pressure feedback stage 40 can readily be applied I. 20 to a conventional pilot controlled PRV to implement further embodiments of the invention.

I- 1 :: Various modifications of the invention arc possible. For example, instead of using a I constant flow pressure reducing pilot valve and combining the flows through the pilot I valve 12 and the bypass pipe 44 in the throat of the venturi 46, a remote sensing pilot may be used, which is controlled by taking the pressure in the venturi 46 back to the diaphragm on the pilot valve. As a further modification, a second pilot valve may be used to limit the pressure increase with increasing demand.

Claims

Claims 1. A pressure reducing valve system comprising: a. a pressure

reducing valve (PRY) having a flow inlet, a flow outlet, and a control inlet; b. a pilot valve for controlling the pressure reducing valve, the pilot valve having an inlet connected to the flow inlet of the pressure reducing valve, a control outlet connected to the control inlet of the pressure reducing valve, and a pressure sensing line; and c. a pressure feedback stage on the outlet ofthc pressure reducing valve, the pressure feedback stage operative to apply to the pressure sensing line of the pilot valve a pressure that corresponds to the outlet pressure of the pressure reducing valve, reduced by a value that increases as flow through the pressure reducing valve increases.
2. A pressure reducing valve system according to claim 1 configured such that the amount by which the pilot valve causes the PRV to increase pressure as flow rate increases is greater than the increase in head loss within the valve system as flow - ..

rate increases. e.
3. A pressure reducing valve system according to claim I or claim 2 in which the . . pressure feedback stage includes a venturi, the flow through which increases as the flow through the pressure reducing valve increases.
4. A pressure reducing valve system according to claim 3 in which the sensing line : : of the pilot valve monitors pressure at the venturi.
5. A pressure reducing valve system according to claim 3 or claim 4 in which the venturi is connected across a flow restriction in the outlet ofthe pressure reducing valve.
6. A pressure reducing valve system according to claim 5 in which the flow restriction is such that the restriction it places on the flow is variable, thereby permitting control over the flow through the venturi.
7. A pressure reducing valve system according to claim 5 or claim 6 in which the restriction includes a gate valve.
8. A pressure reducing valve system according to any preceding claim in which the pilot valve is adjustable, enabling the outlet pressure of the valve system to be set as required.
9. A pressure reducing valve system according to any preceding claim further compri sing commissioning means, operable such that the pressure feedback stage returns to the pilot valve a pressure that is substantially the outlet pressure of the PRV.
10. A pressure reducing valve system comprising: a. a pressure reducing valve (PRY) having a flow inlet, a flow outlet, and a control inlet; b. a pilot valve having a control inlet and a control outlet that is connected to the control inlet of the pressure reducing valve, for controlling .. operation of the pressure reducing valve, and : . . c. a pressure feedback stage on the outlet side of the pressure reducing A-.

valve, including a flow restriction in the outlet of the pressure reducing valve and a venturi connected across flow restriction, the venturi being connected to the control inlet of the pilot valve, wherein the pressure feedback stage is operative to control the pilot valve . according to the pressure head across the flow restriction, and the flow restriction :: is variable, thereby permitting control over the outlet pressure of the pressure reducing valve system.
1 1. A pressure reducing valve system substantially as above described with reference to the drawings.
12. A pressure feedback stage suitable for fitting to an existing pilotcontrolled PRV to Implement a pressure reducing val ve system according to any preceding claim
13. A pressure feedback stage according to claim 12 provided as a kit that includes some or all of a gate valve, a bypass pipe for connection to the valve, a venturi within the bypass pipe, and fitting and commissioning instructions that direct a user to apply the kit to an existing PRV. . : ... ä.e; 1 A. a

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