GB2267141A - Flow modulator system for pressure reducing valves - Google Patents

Abstract

A flow modulator system for a pressure reducing valve includes means to sense changes in flow and actuate the valve in response thereto. An orifice plate (3), having an orifice of diameter not less than half the diameter of the flow path, creates a first differential pressure in the flow path downstream of the valve (2), which is dependant on the flow rate of fluid in the flow path. A bypass tube (13) of smaller cross-sectional area is connected between a low pressure zone (4) at a distance D/2 (where D is the pipe diameter) downstream of the orifice plate (3) and a point (5) between the valve and the orifice plate (3) at a distance from the orifice plate at least equal to the diameter of the flow path. The bypass tube is provided with a venturi (7) to create a second differential pressure. The pressure at a low pressure zone (8) of said venturi is transmitted to the valve (2) as an indication of the flow rate in the flow path. <IMAGE>

Description

MODULATION SYSTEM FOR PRESSURE REDUCING VALVES The present invention relates to pressure reducing valves.

More particularly it relates to means for sensing automatically such changes in the flow as require a change in the valve setting. Ideally, the change is effected automatically by the sensing means.

The present invention is particularly applicable to the water supply industry, although other uses can be foreseen. In water supply distribution systems, leakage of water from the system may be of the order of 30% or more of all the water supplied. However it is now known that a reduction in the water pressure will reduce the level of leakage, and in fact that for a given reduction in pressure, the reduction in leakage is even greater proportionately.

Water supply pipes are generally buried in the ground and small seepages cannot easily be detected. A more major leak may be detected but in order to repair it, the pipe will have to be excavated, with consequent disruption above ground.

A reduction in pressure of the water supply will cut down leakage losses, reduce consumption, especially wasteful consumption, and will reduce the frequency of burst pipes.

All this will prolong the life of the system and reduce labour costs in repairing the system.

However, there are. certain disadvantages inherent in a reduction or pressure. Obviously, the pressure in the system must. be sufficiently high for a user remote from the source of supply to have a reasonable pressure at the point of use. More importantly, the demands on the system may vary greatly. At some times, a majority of the consumers connected to the system may require a supply and the pressure in the system as a whole must be sufficient to supply this need. However, at other times, such as during the night, very few consumers will require the water supply and the pressure could, with adequate safety, be reduced.Most control valves currently in use provide a constant downstream pressure irrespective of flow rate, and this constant pressure must be sufficient to supply the critical point with water whatever the demand.

In order to meet this problem of varying demand, there have been developed flow modulated pressure reducing valves, in which the pressure is increased in response to an increased flow, and vice versa.

In some forms of these flow modulated PRVs (pressure reducing valves) the degree of closure of the valve is controlled by means of a diaphragm operated by a pressure differential between a point in the system at which the pressure is determined by the flow rate and another point at which the pressure is normal.

In one system, the pressure differential is created by means of an orifice plate in the flow path, but this system has the disadvantage that the system has a limited modulation range. The pressure differential created is not sufficient to control the PRV with any degree of accuracy. In order to increase the modulation range, the orifice must be made smaller and this will eventually reduce the effective flow capacity of the pipeline. In other systems, the orifice plate is replaced by a venturi tube. However, it is still not possible to obtain high modulat n ranges without incurring high flow resistance ailci coiiseyuciillal UC tion o e flow of water.

It is an object of the present invention to provide a system for controiling a flow modulation pressure reducing valve having an improved high modulation range without an undesirable check in the flow of water.

According to the present invention there is provided a flow modulator system for a pressure reducing valve in a water supply system comprising means to sense changes in flow and actuate the valve in response thereto; characterised in that said means comprise an orifice plate arranged in the flow path downstream of the valve and having an orifice of diameter not less than half the diameter of the flow path, thereby to create a first differential pressure which is dependant on the flow rate of water in the flow path without limiting substantially the flow rate of the water therethrough; and in that the system also comprises a bypass tube of smaller cross-sectional area connected between a low pressure zone created by said orifice plate at a distance therefrom substantially equal to half the diameter of the flow path, and a point between the valve and the orifice plate at a distance from the orifice plate at least equal to the diameter of the flow path; and a venturi tube in said by pass tube to create a second differential pressure greater than said first differential pressure and dependant thereon, the pressure change at a throat of said venturi tube being transmitted to the valve as an indication of the flow rate of the flow path.

An embodiment of the invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 shows diagrammatically a system embodying the invention; FIGURE 2 is an elevation of a compact embodiment of differential pressure creation means including an oiiJ:ic plate aiict vciiLui lute; FIGURE 3 is a longitudinal cross-sectional view through the embodiment of Figure 2, taken along the line x-x thereof; and FIGURE 4 is a scrap view of an alternative inlet arrangement for the venturi tube of Figure 3.

The system is placed in a water main 1 delivering water in the direction of arrow A to a plurality of consumers. Into the system is incorporated a pressure reducing valve 2, which may be any type capable of varying the downstream pressure in such a manner that a constant head of pressure can be maintained at a remote consumer location, regardless of demand from all consumers. The pressure at this critical remote point in the water distribution system, over a wide range of flow rates, determines the operation of the PRV 2, and can be determined from the flow rate immediately downstream of the PRV 2.

Downstream of the PRV 2 is an orifice plate 3. A bypass tube 6 of comparatively small diameter compared to that of the main pipe 1 is connected between points 5 and 4 respectively upstream and downstream of the orifice plate 3. In this bypass tube is positioned a venturi 7. A tube 9 is connected to the throat 8 of the venturi 7. Tube 9 is connected to the PRV 2 where the pressure in the tube causes actuation of the valve in response to changes in that pressure.

In a typical example of such a system, a mains water pipe 1 of internal diameter D conveys water, a substantially incompressible liquid, in the direction of arrow A to a plurality of consumers. The pressure of the water supply is determined by pressure reducing valve 2 in dependence on the flow rate of water downstream thereof, the flow rate being a measure of the demand at any one period of time.

The orifice plate 3 is set in the main flow line and has an orifice which is generally dimensioned slightly greater than D/2. However, this orifice diameter may need to be changed, possibly quite considerably, depending on the expected range of flow rates, pressures and D. The'exact diameter is calculated to maintain flow rate as high as possible and give a small but quantifiable net pressure loss across it. This net pressure loss is typically of the order of 2-3 meters of lead.

A bypass tube 6 leads from the main pipe 1 at point 5 upstream of the orifice plate 3 and returns to it at point 4 downstream of the orifice plate 3. Points 4 and 5 are so located as to reflect the maximum pressure drop across the orifice plate 3. Thus the exit 4 of the bypass tube 6 should be, as closely as possible, a distance of D/2 from the orifice plate 3 (the distance being measured from the median line of the orifice plate and the centre of the exit 4). Entrance 5 to the bypass tube 6 is located at a distance upstream of the orifice plate at least equal to D.

The bypass tube 6 contains a venturi 7 in its inlet leg.

The venturi 7 could be located elsewhere in the bypass tube 6, but is preferentially distanced from a bend in the tube since this causes secondary flows in the liquid downstream of the bend and these swirling flows may affect the performance of the venturi. For this reason, the inlet leg is preferred.

A tube 9 is taken from the throat 8 of the venturi 7 to the PRV 2. The venturi 7 is so configured that the pressure modulation of 2 to 3 meters between points 4 and 5 is amplified to a pressure modulation of 20 to 30 meters within tube 9, which in turn gives accurate control of PRV 2.

This can be achieved with a venturi throat 8 diameter of 5 Lo 7ttint. WlL r'ducci SCC Ycction oE Llie venturi 7 may have an angle of 7 to 12%. The diffuser section may have an angle in the region of 2h%. This angle gives optimum modulation but can cause the venturi to be somewhat long.

It is desirable that the bypass tube is as short as is possible. In order to shorten the venturi length, it is possible to use a cropped diffuser. That is one which expands at an angle in the region of 4 until the passage has a ratio of cross sectional area to that of the throat of at least 4:1. At such a point the passage -widens instantaneously to its full width.

Accordingly , pressure measurements taken at the throat of this venturi may be used to give a good modulation range to the PRV without the main flow of water being restricted in any way by the orifice plate.

It is, of course, possible to permit higher gains by adding in further venturis, each connected to the throat of a preceding one, but in most circumstances, this should be unnecessary.

The by-pass loop containing the venturi 7 may include a flow control valve, an orifice or other means which could be adjusted to alter the modulation characteristic of the system as required. This enables the system to be detuned to avoid fluctuations in pressure at the critical point.

This is preferably located in the return leg 13 of the bypass tube 6.

In the embodiment shown in Figures 2 and 3, an orifice plate 10 spanning the pipe 1 has a plurality of circular apertures 11 arranged around a central point.

In the embodiment shown the bypass tube 6 is arranged through the centre of the orifice plate 10 in order to give a more compact arrangement. The tube 6 connects across the orifice plate 11 which serves to create a differential pressure between zones upstream and downstream of the plate. The effect of this differential pressure is amplified by the venturi tube 7, and the pressure at the throat thereof is used, via pipe 9, to control the PRV 2. For example the orifice plate may create a pressure differential of 3-4 metres, while the smaller venturi tube may produce 30-40 metres of pressure differential.

Figure 4 shows an alternative form of inlet for the venturi tube 7. As shown, the inlet end is blocked and water enters through a number of angled radial apertures 12.

In the embodiments of both Figure 3 and Figure 4, the venturi tube has preferred angles from the axis at upstream and downstream sides of the throat of 10.50 and 2.50 respectively.

One further advantage of a system embodying the present invention is that it is not prone to blockage by particulate matter carried by the water supply. The orifice plate has an aperture of such 'diameter that any particles carried by the supply will pass through it easily.

Claims (5)

C L A I M S:

1. A flow modulator system for a pressure reducing valve in a water supply system comprising means to sense changes in flow and actuate the valve in response thereto; characterised in that said means comprise an orifice plate arranged in the flow path downstream of the valve and having an orifice of diameter not less than half the diameter of the flow path, thereby to create a first differential pressure which is dependant on the flow rate of water in the flow path without limiting substantially the flow rate of the water therethrough; and in that the system also comprises a bypass tube of smaller cross-sectional area connected between a low pressure zone created by said orifice plate at a distance therefrom substantially equal to half the diameter of the flow path, and a point between the valve and the orifice plate at a distance from the orifice plate at least equal to the diameter of the flow path; and a venturi tube in said bypass tube to create a second differential pressure greater than said first differential pressure and dependant thereon, the pressure change at a throat of said venturi tube being transmitted to the valve as an indication of the flow rate of the flow path.

2. A flow modulator system for a pressure reducing valve as claimed in claim 1, characterised in that the valve is automatically operated according to a pressure differential between the pressure at the low pressure zone of the venturi tube and a reference pressure.

3 A flow modulator system for a pressure reducing valve as claimed in either claim 1 or claim 2, characterised in that a second venturi is provided further to increase modulation and is located to sense the pressure differential between the low pressure zone of said venture tube and a reference pressure.

4. A flow modulator system for a pressure reducing valve as claimed in any one of the preceding claims characterised in that the bypass tube further comprises a flow restriction means which is so adjustable as to ensure that the pressure at a critical point of a water supply system fed through the PRV varies only within predetermined limits.

5. A flow modulator system for a pressure reducing valve in a water supply system substantially as described herein with reference to the accompany drawings.