GB2448086A

GB2448086A - Pilot valve

Info

Publication number: GB2448086A
Application number: GB0809214A
Authority: GB
Inventors: John Henry May
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-03-26
Filing date: 2007-03-26
Publication date: 2008-10-01
Anticipated expiration: 2027-03-26
Also published as: GB2448086B; GB0809214D0

Abstract

A pilot valve for controlling operation of a hydraulically-actuated pressure reducing valve includes a housing and a first valve member 28 and seat 29 mounted in the housing for movement relative thereto in response to the outlet pressure of the pressure reducing valve. Movement of a second valve member, comprising a nozzle 24 fixed to the inside of a toroidal ring 25 discharging flow from a chamber within the second valve member onto the seat 29, is caused by differential pressure applied between diaphragms 22 and 23 reacting against a resiliently biased member 16. Control by the pilot valve is dependent on the relative positions of the first and second valve members.

Description

PILOT VALVE

This invention relates to a pilot valve for controlling operation of a hydraulically actuated pressure reducing valve (PRV) . For convenience, the invention will be described below with reference to a particular application in water supply hut it will be appreciated that its uses are not so limited and indeed, that it has wide applicability in fluid systems generally. The invention also relates to a PRV system including a pilot valve and a PRy.

In the supply of water from source, i e. mains supply, to a multiplicity of end users, it is common practice to pass the supply through a PRV, which may, for example, reduce pressure from a mains pressure of 50 Metres to a constant outlet pressure of 20 Metres. This is desirable as a lower pressure greatly reduces leakage rate, diminishes consumption and reduces the frequency of burst mains.

PRV's are designed to control to a pre-set constant outlet pressure, regulating the degree of throttling to compensate for varying higher inlet pressures and varying flow rate. PRV's comprise of two valves working in unison.

The main valve, connected directly into the water main, comprises of a throttling element usually, but not necessarily, of a globe design comprising of a plug and a seat. The throttling element is directly connected to an hydraulic actuator usually, but not necessarily, connected directly above the throttling element. The position of the actuator and throttling element is controlled by the volume of water contained in the control chamber of the actuator. In turn, the volume of water in the control chamber is determined by a second, smaller valve, known as the pilot valve, connected into a small bore pipe loop surrounding the main valve between inlet and outlet.

This externally mounted pilot loop, constructed of small bore pipe, is connected from the inlet of the PRV to a control chamber and then usually through a T-connection to the outlet of the PRy. Between inlet and the control chamber is a restriction commonly in the form of an orifice plate and between the control chamber and the outlet is a pilot valve.

The volume of water in the control chamber determines the degree of throttling of the PRV which is governed by the setting of the pilot valve. A typical prior art pilot valve is of the form of a simple spring loaded diaphragm design.

The force exerted by a pre-settable compression spring or resiliently biased member acting against a diaphragm plate fixed to a diaphragm, is balanced by the resultant of force terms arising from the outlet pressure acting over the opposite side of the diaphragm, the pressure from the control chamber acting with the spring force from a nozzle over a seat rigidly connected to the diaphragm, the force due to the change in momentum of the jet issuing from the nozzle and the weight of the moving parts.

For any given inlet pressure greater than the outlet pressure setting and flow through the PRV the main valve assumes the degree of throttling required for the prevailing conditions. Under these conditions, a small but continuous flow passes through the pilot loop but no flow enters or leaves the control chamber. The pilot valve is partially open and is said to be in equilibrium.

In control if, for example, an increase occurs in the flow, the instant effect is to cause a reduction in the outlet pressure. This reduction in outlet pressure, however, changes the force balance in the pilot valve, wherein the spring force becomes greater than the outlet pressure acting over the pilot diaphragm and causes the pilot valve to open.

The effect is to cause an increase in flow in the pilot loop and, due to the restriction of the orifice, a reduction an pressure occurs on entry to the control chamber. The resultant forces from the inlet, outlet and control chamber pressures are such that a volume of water is forced to leave the control chamber wherein it passes through the pilot valve and into the outlet connection to the water main. This action causes the main valve to open and the outlet pressure increases until the pilot valve assumes its equilibrium position.

Conversely, if there is a reduction in flow the instant effect is to cause an increase in the outlet pressure. Under this condition the pilot valve closes and water issuing through the pilot loop is diverted into the control chamber causing the main valve to close until the outlet pressure is restored to its set point and again the pilot valve assumes its equilibrium position PRV's control to a set pressure, therefore it is implicit that equilibrium occurs at exactly the same opening position irrespective of the magnitude of the set outlet pressure.

it follows, therefore, that although the set outlet pressure is usually set by directly altering the force exerted by the resiliently biased member acting over the diaphragm in the pilot valve. It is also possible to change the outlet pressure by the method of altering the position at which the equilibrium position occurs. Altering the position at which the equilibrium position occurs has a great advantage in that the force required to change the set pressure is only a fraction of that required to alter the force exerted by the spring.

Generally pilots do not employ this method to change pressure. It has however, been recognised by both Miller US3896843 1975 and May W009923544 1999 and has been utilised in this application.

In water distribution there is considerable benefit in causing a PRV to alter the outlet pressure as a function of system demand. Increasing outlet pressure as demand increases and reducing outlet pressure as demand diminishes thereby enables compensation for friction losses with the result of providing lower and more uniform system pressure with the benefit of lower levels of leakage and a reduction of burst frequencies. This is known as flow modulation.

Various methods have been employed to achieve flow modulation but all are compromised.

Electrical methods to alter the setting of a pilot, valve are complex, requiring signals from pressure sensors and flow meters and an electronic processing capability as well as a power supply. These can be divided into two groups, direct arid indirect.

Direct methods alter the force exerted by the spring through an electrical actuator and necessitate a continuous supply of mains power.

Indirect methods utilise the inlet pressure to control the volume of water in an additional chamber so arranged as to directly alter the compression of the pilot valve spring.

Both of these techniques are discrete control systems which can lead to instability between the PRV and distribution system and are expensive to set up and maintain.

Hydraulic methods carry the advantage of continuous control and are not necessarily prone to induce instabilities but necessitate the generation of a significant differential pressure by use of a secondary device in the form of an orifice plate or venturi fitted upstream or downstream of the PRy. The secondary device may necessitate civil works to install and carries the disadvantage of introducing additional head loss which may not be acceptable in periods of peak demand.

An example of an hydraulic device can be found in May GB2267141 1993.

It is an object of the invention to provide a pilot valve that mitigates these problems and that preferably enables control to be achieved without the need for pressure transducers, flow meters, electronics or a separate pilot chamber or the use of an additional device to generate a differential pressure. This necessitates either elimination or diminishment of forces opposing the movement of the nozzle.

This invention utilises the equilibrium principal but in an arrangement which negates the drawbacks of prior art as described below.

Miller US3 896843 1975 invention is more suited Lo high pressure oily hydraulics than piJot valves as used in water distribution. The arrangement disclosed relies heavily on the movement of shafts through 0' rings and as such necessitates that a sufficient out of balance force occurs to overcome 0' ring break away forces, thus negating this approach.

May W009923 544 1999 did not account for the fact that in the arrangement described, leakage would take place between the cylinder walls of the adjustable nozzle vent and nozzle housing without the addition of a sealing arrangement.

Thus an additional force would be required to overcome the sealing mechanism in the pilot valve seat actuator and the nozzle vent and nozzle housing.

In water supply and distribution, the effectiveness of control mechanism such as those described by Miller US3 896843 1975 and May W009923544 1999 is dependent upon the ease with which movement can take place across 0' ring seals.

Small particulate matter is commonplace in water supply systems and readily builds up between a shaft and an 0' ring. Pilot valves incorporating one or more 0' rings are adversely affected losing sensitivity and control accuracy, sometimes within a matter of hours and are not suitable when attempting to gain control with small pressures such as those generated by pitot tubes.

As an alternative method to achieve a flow modulating duty May W009923544 1999 proposed the use of the main valve position to modulate the pilot valve. This proposed method, however, necessitates unique cam profiles for each application and if there was a change to the supply head flow characteristic, the desired modulating control would not he achieved. This could lead to loss of supplies. Furthermore, if the upstream pressure increased, as may be the case if upstream supplies were switched, it would he possible for the outlet pressure to diminish with flow. These deficiencies render May W009923 544 1999 unsuitable as a general purpose controller.

Neither of these pilot valves proposed by May W009923544 1999 or Miller (US3896843 1975 are suitable for the object of this invention which is to cause the PRV to reliably modulate the outlet pressure accurately in response to changes in flow under typical water distribution conditions.

The present invention overcomes the aforesaid problems enabling continuous hydraulic modulation to be achieved with accuracy, without additional head loss, without dependence on a consistent supply characteristic and is very tolerant to small particulate matter found in potable water supplies.

According to the present invention, there is provided a pilot valve for controlling the operation of a hydraulically-actuated pressure reducing valve (PRy), the pilot valve including a housing, a first valve member mounted in the housing for movement relative thereto in response to the outlet pressure of the PRy, a second valve member mounted in the housing for movement relative thereto, and an actuator driven by a differential pressure for adjusting the position of the second valve member relative to the housing; the pilot valve being designed to control operation of the PRV according to the relative positions of the first and second valve members.

Because the adjuster acts on the second valve member, allowing its position to be adjusted relative to the housing, only a small adjusting force is needed to change the outlet pressure. The outlet pressure of the PRV can therefore he adjusted very easily and precisely and made responsive to the dynamic pressure in the water mains via the pressure signal generated by a pitot tube sensing the main flow.

According to one preferred embodiment of the present invention, the pilot valve seat is rigidly connected by means of a stirrup to an upper diaphragm.

Advantageously, the pilot valve inciudes a nozzle communicating flow from the conLrol chamber on to the pilot valve seat such that the position of the nozzle relative to the upper diaphragm is responsive to the rate of flow in the water main.

Advantageously, flow from the control chamber passes through an additional chamber housed in the second valve member comprising of counter opposing pistons over diaphragms such that changes in the control chamber pressure due to changes in the inlet pressure have no effect on the control of the outlet pressure.

Advantageously, the degree of modulation is determined by movement of the second valve member due to the reaction force generated by the differential pressure produced across the additional chamber of the second valve member between downstream pressure and the pitot signal pressure against a second resiliently biased member opposed by a reaction plate secured against the pilot valve body.

Advantageously, the second valve member is axially guided between a location hole in the reaction plate and a sleeve contained in the end chamber.

Advantageously, the degree of flow modulation can be set by controlling the magnitude of the differential pressure across the second valve member via an externally mounted adjusting valve between the pitot pressure and outlet pressure connections to the pilot valve body.

Advantageously, the working pressure range can be set by introducing adjustable mechanical stops limiting the axial movement of the second valve member.

Advantageously, the rate of change of outlet pressure can be controlled by use of a needle valve installed between the pitot tube and the end chamber.

Advantageously, the diaphragms may be of a rolling diaphragm design to enable sufficient unimpeded travel to realise a substantive flow modulating duty.

Advantageously, the port and diameter of the connection between the pilot valve and the outlet pressure connection can be increased to compensate for pipe work pressure losses.

To enable the pilot valve to modulate at very low flows, where the pitot pressure is less than the distance between the nozzle and diaphragm furthermost from the nozzle, it is preferable that the pilot valve is installed and operates in a horizontal position.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings.

Fig. 1 is an arrangement of a fluid system comprising a main valve, a pilot valve according to the invention and a pitot tube.

Fig. 2 is a cross-section through the pilot valve according to the invention.

A typical arrangement is shown in Fig.l. Here the main valve 1 is connected to the pilot valve 2 in a conventional manner saving the connection of the pitot tube 3 expanded for clarity.

Flow through the main valve 1 is throttled between the valve seat 4 and plug 5 which is in turn connected to the main valve actuator 6.

The pilot loop, comprising of small bore pipe work, connects the upstream tapping point 7 to a filter 8 and orifice 9 to the control chamber 10.

The flow from the control chamber 10 is then communicated to the inlet tapping point 11 of the pilot valve and is ported away from the tapping point 12 to the downstream tapping point 13 of the main valve via increased diameter small bore pipe work.

The pitot connection 14 is connected to the tapping point 15.

The differential pressure generated between the static pressure Hd and dynamic pressure Hd+U2/2g reacts against the light spring 16 causing the pilot valve nozzle 17 to move in accordance with the main flow passing through the main valve.

As the nozzle moves because the equilibrium position remains constant, the compression in the main pilot valve spring 18 varies the PRV outlet pressure in accordance with the main flow.

The degree of flow modulation is controlled by the setting of the valve 19.

The pressure from the control chamber is completely balanced by the counter opposed pistons 20 acting over diaphragms 22 and 23.

The range of modulation possible is enhanced by the use of rolling diaphragms 21, 22 and 23 which afford frictionless sealing and separation of the chambers.

A detailed cross section of the pilot valve according to the invention is shown in Fig 2.

The central pipe 23 communicating flow from the control chamber of the main valve is caused to discharge onto the through a nozzle 24 onto the seat 28 by means of a pipe ring 25.

Flow into the central pipe 23 via the chamber bounded by pistons 20 is guided by the location hole in the spring reaction plate 26 and the sleeve 27.

The seat of the pilot valve 28 is rigidly connected to the upper rolling diaphragm 21 by a stirrup assembly 29.

The outlet pressure with no modulation is set by opening valve 19 thus equalising pressures between the diaphragms 22 and 23 and adjusting the spring compression cap 30.

The degree of modulation required is achieved by progressive closure of valve 19 until the desired pressure has been achieved for the prevailing conditions in the main.

The rate of change of the outlet pressure is determined by the degree of restriction of the needle valve 31.

A mechanical stop 32 can be used to preset the minimum outlet pressure.

Additional stops to set the maximum outlet pressure are readily introduced by controlling the movement of the second valve member.

Claims

Claims 1. A pilot valve for controlling the operation of a

hydraulically-actuated pressure reducing valve (PRy), the pilot valve including a housing, a first valve member mounted in the housing for movement relative thereto in response to the outlet pressure of the PRy, a second valve member mounted in the housing for movement relative thereto, and an actuator driven by a differential pressure for adjusting the position of the second valve member relative to the housing, the pilot valve being designed to control operation of the PRV according to the relative positions of the first and second valve members.
2. A pilot valve according to claim 1, and further comprising a main pilot valve spring connected to said housing by way of a spring compression cap, a stirrup assembly and a first diaphragm.
3. A pilot valve according to claim 2, wherein said first valve member includes a valve seat rigidly connected to said housing by way of said stirrup assembly and said first diaphragm.
4. A pilot valve according to claim 3, wherein said second valve member includes a nozzle for discharging fluid from a chamber of said pressure reducing valve onto said valve seat.
5. A pilot valve according to claim 4, wherein fluid conveyed to said nozzle is by way of a pipe ring terminating with said nozzle directed inwardly of said ring.
6. A pilot valve according to any preceding claim, wherein said second valve member includes two counter-opposed pistons and respective second arid third diaphragms which bound a second valve member chamber.
7. A pilot valve according to claim 6, and further comprising a resiliently biased member located between one of said counter-opposed pistons and a reaction plate attached to the body of the pilot valve housing.
8. A pilot valve according to claim 7, wherein said second valve member is axially guided by location holes in said reaction plate and a sleeve contained in an end chamber of said pilot valve.
9. A pilot valve according to any preceding claim, and further comprising an externally mounted adjustable valve for controlling the magnitude of the differential pressure across said second valve member.
10. A pilot valve according to claim 8 or claim 9 as appended to claim 8, and further comprising one or more mechanical stops limiting the axial movement of the second valve member.
11. A pilot valve according to any one of claims 6 to 10 as appended to claim 2, wherein said first, second and third diaphragms are of a rolling diaphragm form.
12. A system comprising an hydraulically-actuated pressure reducing valve (PRV) and a pilot valve, said pilot valve controlling the operation of said pressure reducing valve, the pilot valve including a housing, a first valve member mounted in the housing for movement relative thereto in response to the outlet pressure of the PRy, a second valve member mounted in the housing for movement relative thereto, and an actuator driven by a differential pressure for adjusting the position of the second valve member relative to the housing, the pilot valve being designed to control operation of the PRV according to the relative positions of the first and second valve members.
13. A system according to claim 12, and further comprising a pitot tube for measuring the dynamic head velocity in said pressure reducing valve.
14. A system according to claim 13, and further comprising a needle valve located between said pitot tube and an end chamber of said pilot valve, said end chamber being bounded by said housing and an element of said second valve member.
15. A system according to any one of claims 12 to 14, wherein said pilot valve is located in a pilot loop comprising small bore pipework connecting an upstream tapping point of said pressure reducing valve to an inlet tapping point of said pilot valve.
16. A system according to claim 15, wherein the connecting pipework between an outlet pressure tapping of said pilot valve and a downstream tapping point of said pressure reducing valve is of larger diameter than other said connecting pipework in said fluid system.
17. A system according to any one of claims 12 to 16, wherein said pilot valve is arranged in a horizontal position in said fluid system.
18. A system according to any one of claims 12 to 17, wherein said system is a fluid system.
19. A system according to claim 18, wherein said fluid system is a water supply system.
20. A pilot valve as substantially hereinbefore described with reference to the accompanying drawings.
21. A system as substantially hereinbefore described with reference to the accompanying drawings.