I was stood adjacent to a steam heated boiling pan in which a toffee mixture was being heated. The wonderful smell of mint humbugs, my favourite sweet, was making it difficult to concentrate on what the Factory Engineer stood next to me was saying.

Look, he said, see what happens when I turn on the second boiling pan, the pressure in the line falls and the toffee mixture will not then boil as vigorously as I would like. I have checked the pipe sizing from the flow charts you sent to me and they are fine, so I think the Steam Pressure Reducing Valve, (PRV), I bought is simply just not big enough.

I understood his concern and then looked closely at the PRV and identified it as a Basic Diaphragm Operated Reducing Valve of excellent quality and reputation and that was where the problem lay. Not in the quality of Reducing Valve, but in the basic type and principle of operation. See Fig 1 on RELATED PDF.

With one boiling pan in operation, reducing the steam pressure from 80 psig to 25 psig and an estimated load of some 200 lb/hr, the secondary set pressure was holding steady at 25 psig. See Fig 2 on RELATED PDF.

When the second boiling pan was turned on there was an increase in demand for steam, now some 400 lb/hr. However to satisfy that additional requirement the actual valve itself, within the PRV, has to open wider to permit the increased flow to pass. This will only happen if the secondary set pressure falls, caused by the steam rapidly condensing as it enters the jacket of the boiling pan, so permitting the spring force on the diaphragm to become greater than the opposing set pressure and in turn moving the valve to a more open position. See Fig 3 on RELATED PDF.

In this instance the steady flow of steam has caused the secondary pressure to stabilise at 22 psig, a typical example of what many Engineers refer to as DROOP. See Fig 4 on RELATED PDF.

I explained this to the Factory Engineer and offered him the choice of accepting the control limitations of such a Basic PRV, or changing this particular PRV for a Pilot Operated Self Acting Reducing Valve which would negate the effect of DROOP.

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