GB2219349A - Smoothing flow in pipes - Google Patents

Abstract

In order to smooth the flow of fluids in a pipe 12, a pressure vessel 10 is connected to the pipe through a T-joint so that the pressure vessel has an air space 22 above a liquid (normally water) space 24. A flow-restricting orifice 20 is fitted between the pipe run and the pressure vessel to allow a restricted rate of flow into and out of the vessel. A pressure switch dispersed in the air space 22 may control the operation of an intermittently operating pump feeding the pipe 12. <IMAGE>

Description

TITLE: Flow Control in Pipes This invention relates generally to the controlling of fluid flow in pipes and in particular to the reduction or elimination of the effect known as "water hammer" in piping systems, and relates to apparatus for installation in a piping system, to inter alia reduce water hammer.

Water hammer occurs when a tap or valve in a pipeline carrying water or any other substantially incompressible fluid is closed causing a pressure pulse to travel backwards up the pipe. where the frequency of this pulse corresponds to the resonant frequency of a section of the pipe, then that section will resonate causing the undesirable characteristic noise known as water hammer.

According to the invention there is provided apparatus for installation in a piping system, the apparatus comprising a closed vessel having an orifice at one end forming a restricted passage into the vessel, and means for connecting the vessel to a pipe run through a T-joint.

It has been found that the installation of such apparatus in a piping system is beneficial in reducing water hammer. It appears that the size of the orifice is critical. As a general rule, the orifice size can be set at a percentage of the internal diameter of the pipework making up the installation. Preferably the diameter of the orifice is between 8% and 14% of the pipe diameter with a figure of 12% being the preferred percentage. For domestic installations with 15mm diameter pipe, the orifice preferably has a diameter of between lmm and 2mm. A particularly preferred diameter is l.8mm.

The vessel needs to be soundly constructed to be able to accommodate surges in the mains water pressure which may itself be as high as 120 psi (8.3 bar). The orifice may be formed by a separate drilled plate fitted in the neck of the vessel, and the vessel itself forms a surge chamber.

The invention also provides a method of reducing water hammer in a piping system, the method comprising the steps of connecting a closed vessel to a pipe run in the piping system through a T-joint, the vessel communicating with the piping system through a small orifice and being arranged so that the volume enclosed by the vessel is above the orifice.

In another aspect, the invention provides a method of pressurising a piping system fed by an intermittently operating pump, wherein the pump is switched on and off by a pressure switch arranged in a closed vessel, the closed vessel being connected to a pipe run in the piping system through a T-joint, communicating with the piping system through a small orifice and being arranged so that the volume enclosed by the vessel is above the orifice.

In yet another aspect, the invention provides a piping system comprising an intermittently operating pump, a holding tank, a pipe through which water can be pumped by the pump to the tank, a closed pressure vessel connected to the pipe through a T-joint and communicating with the pipe through a small orifice, and a water outlet connection between the pump and the holding tank, the vessel having an air space and including a pressure sensitive switch arranged in the air space, the switch being set to switch the pump on and off when predetermined maximum and minimum pressures are sensed.

The invention will now be further described, by way of example, with reference to the accompanying drawing, in which Figure 1 is a schematic view of a first form of apparatus in accordance with the invention; and Figure 2 is a schematic view of a second form of apparatus in accordance with the invention.

Figure 1 shows a closed vessel forming a surge chamber 10 connected to a pipe run 12 through a T-joint 14. The pipe run is exposed to a source of pressurised fluid, for example mains water pressure as indicated by the arrow 1G, and has a tap or valve 18. Although the chamber 10 is shown connected into a straight run of piping between the mains pressure and a valve, this is not the only position in which it can be arranged, and this will be discussed in more detail later in this specification.

Between the chamber 10 and the pipe 12 is an orifice plate 20 with a small orifice in it of between lmm and 2mm diameter for a domestic installation. The connection between the mouth of the chamber and the pipe run 12 can be made using any convenient plumbing technique, for example using a conventional T-joint with compression or other fittings at the ends.

In use, when the chamber is connected up to a pipe run, the water pressure in the pipe will enter the chamber 10.

Because the chamber will initially contain air, the water pressure will drive water through the orifice and compress the air in the chamber until the air pressure and the water pressure inside the chamber are in equilibrium. When this happens the chamber will have an air volume 22 and a water volume 24. The air volume 22 is then totally enclosed.

water hammer with its associated noises and shocks to the pipework typically occurs in a pressurised water system when flow is stopped abruptly by the rapid closure of a valve or tap 18. It occurs principally in long straight runs of piping to which quick closing valves or taps are connected, and is most common in areas of high water pressure.

The reason for these noises is that flowing water has kinetic energy due to its weight and velocity, and when flow is halted abruptly, this energy imparts to the water the effect of a battering ram inside the piping. If the pipe is relatively weak, it may burst or develop leaks, eg due to the displacement of compression fittings. when the pipe is strong, the kinetic energy is converted to pressure energy which is developed in the process of compressing the water and distending the pipe. This reaction, compression and distention sets up pressure waves which travel back and forth along the runs of piping at the speed of sound in water, and cause the pipe wall to expand and contract in rhythm with the rise and fall of pressure inside the piping.

Water hammer may occur in any stretch of pipe work between the location of the valve and the connection to mains water pressure and not necessarily close to the valve itself.

Thus, the surge chamber shown in Figure 1 can also be located anywhere in the piping system and not necessarily in the run where water hammer occurs. In order to mitigate or remove the water hammer effect, it is necessary to remove the pressure surge from the pipe, and this can be done by locating the surge chamber anywhere in the piping system.

The presence of the surge chamber has the following result.

As already mentioned, the air and water pressures in the surge chamber are in equilibrium while the piping system is in a static state and no flow takes place. ##ben a tap is opened on the piping system, the static pressure will drop due to the water flow and friction characteristics in the piping and this will cause a corresponding change in the level of water in the surge chamber. On closing the tap the water pressure will rise and will start to flow back into the surge chamber.

If there is unrestricted flow into the surge chamber then conditions could arise where instead of the chamber absorbing the pressure change it could turn into a resonant chamber, magnifying the pressure pulse and thereby causing even greater problems. However if, as in accordance with the invention, the water flow in and out of the chamber is via a restricted orifice then any large change to the conditions in the pipe will take place over a finite time such that resonance is highly unlikely.

The requirement of the orifice is to allow rapid passage of the compressed volume of water following a valve closure, but not so large as to permit a resonant chamber effect.

It can be calculated from a knowledge of the additional volume caused by the pressure compression (and this additional volume can itself be calculated) how large the orifice needs to be for a particular piping system at a particular pulse pressure. Such calculations show that an orifice diameter of 2mm would enable the pulse pressure in a conventional domestic plumbing system to be dissipated in under 0.1 seconds under most circumstances. The calculations also show that the orifice size could be reduced to about lmm, and this would still produce a workable arrangement, but an orifice of this size would be prone to blockage from water carried particles. Of course the higher the flow rate through the orifice, the greater will be the self-cleaning effect of the water passage and this feature is important.

The free air volume contained in the surge chamber 10 and available to absorb the generated pulse should be a minimum of 100 times the pulse volume. Thus at a high water supply pressure of 120 psi (8.3 bar) this free volume requires to be 5 cubic inches (80 cubic centimetres). In a convenient embodiment, a cylinder can be used as the surge chamber, the cylinder having an internal diameter of 75mm and an internal length of 175mum.

When water is run from a tap, the pressure in the supply pipe falls as a result of the flow using energy in creating turbulence and also as a result of pipe wall friction. When the tap is closed the pressure will rise sharply, as a result of the requirement to dissipate the flow energy and also as a result of the restoration of equilibrium between the pressure in the pipe and the supply pressure.

Without a surge chamber as shown in this specification, this pressure rise will be rapid. If however a surge chamber with an orifice in its entry pipe is installed, the air inside the chamber will be pressurised during the time the taps are closed. when the taps are opened, there will be a pressure reduction in the system and some water will be expelled due to the pressure differential across the orifice whilst water is flowing in the pipe. A gentle return to the static equilibrium pressure will be produced as the water flows back into the cylinder through the restricted orifice once the taps are closed.

In this way, the introduction of a very simple piece of apparatus through a very simple connection, into an existing or a new pipe work system allows the ready reduction or even elimination of undesirable water hammer.

It is preferred that the orifice plate 20 is a separate component, so that it can be removed and replaced if it becoirtes worn and so that plates with different size orifices can be available if required to meet the requirements of different piping systems.

Figure 2 shows an alternative use for the apparatus of the invention. Here the pipe run 112 extends from a pump 32 in a well 30 to a header tank 34 which provides a gravity feed for a water supply flowing from the tank through an outlet 113 to consumer units (not shown). The tank end of the pipe run ends in a shut-off valve 36 controlled by a ballcock 37.

At the top end of the chamber 110, in the air space 122, there is a pressure operated switch 38. This switch is connected through a line 40, an electrical control unit 42 and a second line 44 to the pump 32 so that the pump is switched on when the pressure in the air space drops below a predetermined level and is switched off when the pressure rises above this level.

In use, when water is drawn off from the tank 34, the valve 36 will open and the stored pressure in the air space 122 of the chamber 110 will feed water back into the pipe run 112.

This will result in a pressure drop in the space 122 causing the pump to be switched on. when the header tank 34 is full again, the float 37 will close the valve 36 but the pump 32 will continue to run until the pressure in the space 122 rises to switch off the pump.

The advantage of this system over the use of a float operated switch on the header tank is that it becomes possible to provide a pressurised water offtake 46, closed by a valve 4, through which pressurised water can be drawn off, normally at a higher pressure than could be provided by the gravity tank 34.

The presence of the orifice 120 in the surge tank 110 results in a delay being introduced into the operation of the switch 38 to prevent rapid switching of the pump 32 as the system pressure varies.

The use of this surge chamber therefore allows a pressurised outlet to be obtained from an unpressurised pipe system without making the pump switching respond to every incremental pressure change in the system.

Claims (14)

Claims:

1. Apparatus for installation in a piping system, the apparatus comprising a closed pressure vessel having an orifice at one end forming a resticted passage into the vessel, and means for connecting the vessel to a pipe run through a T-joint so that the orifice is between the pipe run and the interior of the vessel.

2. Apparatus as claimed in Claim 1, wherein the orifice has a diameter of between 8t and 14% of the pipe diameter making up the pipe run.

3. Apparatus as claimed in Claim 1, and for use in a domestic plumbing system, wherein the orifice has a diameter of between imam and 2mm.

4. Apparatus as claimed in Claim 3, for use with a plumbing system having 15mm diameter pipe, wherein the orifice diameter is between 1.7mm and 2mm.

5. Apparatus as claimed in any preceding Claim, wherein the pressure vessel will withstand internal pressures up to 10 bar.

6. Apparatus as claimed in any preceding Claim, wherein the vessel has a neck at said one end, and the orifice is formed by a separate drilled plate fitted in the neck of the vessel.

7. A method of reducing water hammer in a piping system, the method comprising the steps of connecting a closed vessel to a pipe run in the piping system through a T-joint, the vessel communicating with the piping system through a small orifice and being arranged so that the volume enclosed by the vessel is above the orifice.

8. A method as claimed in Claim 7, wherein the orifice has a diameter of between 8% and 14% of the diameter of the pipe making up the piping system.

9. A method as claimed in Claim 8, wherein the piping system is a domestic plumbing system and the orifice has a diameter of between lmm and 2mm.

10. A method of pressurising a piping system fed by an intermittently operated pump, wherein the pump is switched on and off by a pressure switch arranged in a closed pressure vessel, the closed vessel being connected to a pipe run in the piping system through a T-joint, communicating with the piping system through a small orifice and being arranged so that the volume enclosed by the vessel is above the orifice.

11. A piping system comprising an intermittently operating pump, a holding tank, a pipe through which water can be pumped by the pump to the tank, a closed pressure vessel connected to the pipe through a T-joint and communicating with the pipe through a small orifice, and a water outlet connection between the pump and the holding tank, the vessel having an air space and including a pressure sensitive switch arranged in the air space, the switch being set to switch the pump on and off when predetermined minimum and maximum pressures are sensed.

12. Apparatus for installation in a piping system, substantially as herein described with reference to Figure 1 of the accompanying drawings.

13. A piping system substantially as herein described with reference to Figure 2 of the accompanying drawings.

14. A method of reducing water hammer in a piping system substantially as herein described with reference to Figure 1 of the accompanying drawings.