The Engineer I was stood next to watching the toffee mixture boil in the Steam Heated Boiling Pans, understood why, when steam was turned on to heat the second pan, (see Understanding Inaccurate Control of Steam Pressure - Part 1), the Basic Direct Acting Pressure Reducing Valve, (PRV), could not hold the secondary pressure constant as the Steam flow rate increased.

What happens then if the incoming steam pressure before the PRV falls, he asked, as sometimes, such as first thing in the morning when there is a big demand for steam, I find the steam pressure to the pans all over the place!

I asked him to imagine that the PRV had been set against an inlet pressure of 80 psig and to reduce to 25 psig. If the inlet pressure then dropped to 60 psig, the force of that steam which had been pushing against the Valve Head to try and close it, becomes less than the combined force, set by the spring on the diaphragm and the secondary, (downstream), pressure. As a consequence the diaphragm is pushed by the now larger spring force to move the valve head further open and this in turn allows more steam to flow through the valve. The downstream pressure then actually rises, which in fact is the opposite of what you would have imagined should happen.

Some PRV manufacturers refer to this as The Regulation Characteristic, or because of what actually happens, The Inverse Regulation Characteristic. In practice many such PRVs have a typical Regulation Characteristic of 20:1. That is a drop of 20 psi, will give rise to an increase of 1 psi, or vice versa.

One way of trying to cancel out the effect of the impact of the Regulation Characteristic, is to install two basic direct acting PRVs in series, see Fig 5 (on attached PDF).

Using an example of the inlet pressure to the first valve falling from 80 psig to 60 psig, but with this valve set to reduce to 40 psig, as can be shown from this illustration, the second PRV is now only faced with a small change of 1 psig to the inlet pressure, and as a consequence the final desired secondary pressure of 25 psig is almost constant.

However, using two valves in this way can be an expensive solution and when supplying steam to a number of units, it is more cost effective to select a Pilot Operated Reducing Valve, which will also provide much closer control of steam pressure.

In effect this form of construction is a little like having two valves in series, with the secondary steam pressure now being controlled by a small Basic PRV that only has to handle a low flow rate of steam sufficient enough to operate the main valve.

The Engineer of the Toffee Factory, was understandably a little sceptical of the need for a Pilot PRV which would cost more than the simple type currently in use. His next question to me was therefore understandable. Why does the Pilot PRV perform better than my basic valve?

I had already explained the characteristic of Droop, (see Understanding Inaccurate Control of Steam Pressure - Part 1), but could now with the aid of a diagram show why the effect of Droop was virtually cancelled out when using a Pilot Valve to open a Main Valve, see Fig 7 (on attached PDF).

This diagram shows that the Flow Rate of steam passing through a small pilot valve is extremely small, yet this volume of steam is sufficient to provide the motive force which will control the opening and closing of the main valve within the PRV. As a consequence the amount of Droop, is almost negligible and so there will be no discernible impact upon the Pilot PRV.

Of course, I continued with my explanation, There are other factors involved which contribute to accurate control of steam pressure than just installing a Pilot Operated PRV, but more about that in part 3.