11.1.1 Pressure control valves
These valves are designed to greatly reduce the pressure of the hot steam so that its conditions correspond better to those inside the condenser.
The simplest way of creating a large pressure drop across a valve is by the conversion of heat energy to velocity (kinetic energy) by passing the steam through a small orifice. Downstream from the orifice, turbulence accompanied by shock waves reconverts the velocity into heat energy. The change in energy for a conventional single-orifice valve is shown in Fig 2.81.
Although this is a satisfactory means of creating a pressure drop, the shock waves created produce a large amount of undesirable throttling noise. Consequently, valves have been developed that produce the same pressure drop but with considerably less noise, Fig 2.82 shows a typical example. In such a valve, the reduction in pressure is obtained through the principle of 'adiabatic flow with friction'. Steam is passed through a number of restrictions called a disc stack. The stack provides a complicated flow path which results in the dissipation of energy through a high loss of head rather than through Shockwaves (Fig 2.81). The flow area inside the stack is gradually increased towards the downstream end. This helps maintain an almost constant steam velocity over the expansion caused by the pressure drop. Generally, these valves are capable of going from fully-closed to fully-open within three seconds and from fully-open to fully-closed in five seconds.