TYPES OF VALVES
Figure 28 Relief Valve
Relief valves are typically used for incompressible fluids such as water or oil. Safety valves are
typically used for compressible fluids such as steam or other gases. Safety valves can often be
distinguished by the presence of an external lever at the top of the valve body, which is used as
an operational check.
As indicated in Figure 29, system pressure provides a force that is attempting to push the disk
of the safety valve off its seat. Spring pressure on the stem is forcing the disk onto the seat.
At the pressure determined by spring compression, system pressure overcomes spring pressure
and the relief valve opens.
As system pressure is relieved, the valve closes when spring
pressure again overcomes system pressure. Most relief and safety valves open against the force
of a compression spring. The pressure setpoint is adjusted by turning the adjusting nuts on top
of the yoke to increase or decrease the spring compression.