The ins and outs of steam control valves

When dry saturated steam comes into contact with a surface at a lower temperature than itself (let’s take the example of a jacketed cooking vessel) it will give up its latent energy and returns to condensate. Upon condensing, the steam’s volume reduces by approximately 1673 times; depending on the steam pressure. This reduction in volume results in a localized pressure reduction. Now imagine that the steam to the jacketed vessel is supplied and controlled by a steam control valve. A pressure difference will now exist across the control valve, and steam will naturally flow from the higher pressure side of the valve (valve inlet) to the lower pressure side (valve outlet and jacketed vessel).

When the control valve is fully open, the outlet pressure and subsequent pressure difference will assume a value whilst the valve balances out the required condensation rate of the process, with the rate the steam supplied. Remember it is the process that decides the amount of steam a process requires, not the valve. The valve however is responsible for rate of steam flow allowed to flow to the process and this is governed by the valve size and the pressure drop. If the valve fitted is too small, the pressure drop across the valve will be such that any further reductions in outlet pressure will not increase the flow; this condition is called “critical pressure”. At and beyond the critical pressure condition, the valve is said to be “choked”. For dry saturated steam, choked flow is achieved when the outlet pressure is 58% of the absolute inlet pressure.

Big valve or small valve, the condensation rate required remains the same; the question is can the valve deliver?