Typical pilot operated pressure reducing valve station

 pressure reducing valve

An individual valve can be used to give substantial reductions in pressure, but if the turndown is greater than 10 to 1, consideration should be given to using two reducing valves in series giving more accurate control.

A reducing valve will modulate from its maximum capacity down to zero load, when it will shut. However, if the valve is to work under low load conditions for much of its life there may be a good case for fitting two smaller valves in parallel.

If the low load is 10% (or less) of the maximum load then two valves are preferred. Valves in parallel are also used where it is vertical that steam supply is not interrupted.This arrangement ensures proper control of pressure when either valve is being overhauled.