CONTROLLING THE SUPPLY OF STEAM - SURPLUSSING VALVES

Very often when efficiencies in Production give rise to an increase in the demand for Process Steam, attention turns to the Boiler and its rating. Invariably questions are asked about its size and this leads on to an examination of the costs involved in replacing it with a boiler having a greater output.

However in many cases such large capital costs can be avoided by paying attention to the control of the Steam Supply. For example by simply staggering the use of certain items of Plant, large peak demands on the boiler can be avoided. This is though easier said than done, and in order to avoid conflict between Engineering and Production it can be beneficial to utilise a hidden gem in a range of Control Valves that will take some of the stress out of such a tricky situation. We are speaking of the use of a "Surplussing Valve or what is called a Back Pressure Regulator".

The first thing to note before we describe the function of such a valve is that large sudden demands on a Boiler will cause carry over, together with a drop in the operating steam pressure and as a consequence, variable temperatures in the steam supplied to the Process. The Surplussing Valve will help prevent such occurrences, though cannot become a substitute for another Boiler when additional pieces of new plant call for a large increase in Steam.

Most Engineers are familiar with a self operating Pilot Controlled Steam Reducing Valve, (See our Technical Tips). A Surplussing Valve is in effect from the same family of Valves and looks almost identical to a Pilot Operated Reducing Valve. However whereas in the Reducing Valve the Pilot Valve is normally open at start up, in a Surplussing Valve the opposite is the case, with the Pilot Valve being closed at start up. A look at a typical example of the application of a Surplussing Valve, as shown in Sketch 1, will assist in understanding how the Valve works.

SEE Sketch 1 (PDF)

You will see that the Valve is sensing the Primary, upstream pressure, and that the Valve is closed. As the Boiler Pressure rises after start up, the steam pressure is sensed on the underside of the Pilot Diaphragm within the Valve. Gradually the pressure rises to the point where it is able to push back the Pilot Diaphragm against the Spring holding the Pilot closed, so allowing the Pilot Valve to open. This permits steam to flow to the Main Valve mechanism, usually a piston; causing the Main Valve to open and so Steam commences to flow into the pipe system. By setting the spring you are able to control the pressure at which steam is allowed to flow to the Process. Therefore if there is a sudden large demand upon the Boiler for Steam, due usually to several operatives turning on Plant at the same time, the Surplussing Valve will close as the Primary Steam Pressure begins to fall, so preventing large carry over from “flattening” the Boiler.

SEE Sketch 2 (PDF)

Typically with the use of such valves you are also able to prioritise which item of Plant is critical and should always have steam supplied to it, while other applications, in this example steam heating to the Factory, take second place.

Surplussing Valves may also be utilised in other applications, for example controlling the pressure at which some Flash Steam Systems can operate, or for controlling the pressure in a Condensate Return System.

Contact Control Specialities 1-800-752-0556 if you wish to discuss such an application.