HOW DOES A REDUCING VALVE WORK?
 1) Here is a typical Broady pressure reducing valve as it leaves the factory. Note, it is shipped in the open position. The internal valve seat is held open by a pressure spring pushing down.
Compression is applied to the spring by an adjusting screw working on the spring button. When you order a reducing valve with a delivery “set” pressure of 40 PsiG, for example, the adjusting screw is turned down to exert 40 PsiG downward “push”.
2) Now, for purposes of this illustration, let us assume we install this Broady reducing valve in the simplest of installations. We hook a pipe up to the reducing valve inlet to bring in water at 80 PsiG. Then we hook a pipe up to the outlet side of the reducing valve and on the end of the outlet pipe we install a tap.
Lets assume the reducing valve is set to deliver water at 40 PsiG and that the tap is closed. To better illustrate what happens we’ve also installed a pressure gauge on the inlet side and the outlet side of the reducing valve.
3) As the water comes in the inlet pipe at 80 PsiG it flows into the reducing valve, through the open seat, up under the diaphragm and on through the outlet pipe stopping at the closed tap.
Since to regulator is set at 40 PsiG, just as soon as the water pressure (going through the valve) builds up under the diaphragm to 40 PsiG, the downstream pressure spring force is overcome and the reducing valve closes tightly. Result: 80 PsiG on the upstream side of the reducing valve and 40 PsiG on the downstream side. The condition just described is known as “dead-end” or a “no-flow” condition.
4) Now let’s see what happens when flow starts. By slightly opening the tap the “captive” 40 PsiG water will start to flow out. The moment flow starts, pressure under the diaphragm will start to flow out and start to fall off to below 40 PsiG which, in turn, will cause the pressure spring to again open the valve seat to allow more water to enter.
It should be readily understood that it takes a slight pressure drop under the diaphragm to get the reducing valve to open when flow first starts. Obviously, if the tap is shut off again the flow stops, once the pressure at the outlet side of the reducing valve and under the diaphragm reaches 45 PsiG the valve will close off.
5) The previous illustrations, of course, are over simplified. In most actual installations the reducing valve will supply many “taps” i.e toilets, showers, sinks, etc, and countless other industrial applications, and they will be turning on and off intermittently. Thus varying the flow requirements on the reducing valve quite widely.
Looking at the sketch again, let’s see what happens when the flow requirements do vary.
By turning on the tap gradually (from a small trickle to full open) and noting the gauge reading on the outlet side of the reducing valve, it will be immediately noted that as the flow increases the outlet pressure decreases.
This characteristic of all the simpler self-actuated type reducing valves can be easily understood by recalling that it’s the flow or the opening of the tap and the resulting lowering of pressure under the reducing valve’s diaphragm that opens the valve initially. Quite obviously, as the flow is increased more and more, pressure under the diaphragm will fall lower and lower, which in turn allows the pressure spring to push open the valve seat wider and wider in an effort to satisfy the flow demand. This condition can theoretically keep going until the absolute maximum flow (for the design of the reducing valve) is reached and conceivably the outlet pressure has fallen off to 0 PsiG.
6) On actual jobs, of course, the flow requirements through a reducing valve will vary all the way from a little trickle to a large volume under peak load conditions. It’s important to realise, therefore, that the outlet or downstream pressure coming out of the reducing valve will also vary somewhat with these flow variations. That’s why it’s said the simpler type reducing valve will hold the desired reduced or “set” pressure within reasonably close limits.
That’s why Flowstar offers so many different types of pressure reducing valves, since the criteria of goodness and also cost in a reducing valve is its ability to deliver the required rate of flow while simultaneously “holding” the desired reduced or “set” pressure. Furthermore, it also becomes apparent why in any critical job it is always necessary to have some ideas as to the flow rate (or capacity) required.
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