There are two components linked with safety valve. 1.
Tonnage 2. Pressure.
When the inlet static pressure rises above the set pressure
of the safety valve, the disc will begin to lift off its seat. However, as soon
as the spring starts to compress, the spring force will increase; this means
that the pressure would have to continue to rise before any further lift can
occur, and for there to be any significant flow through the valve.
The additional pressure rise required before the safety
valve will discharge at its rated capacity is called the overpressure. For
steam boilers, a small overpressure is required, usually 3% or 5%. For most
other applications, 10% overpressure is required. The allowable overpressure
depends on the standards being followed and the particular application. For
compressible fluids, this is normally between 3% and 10%, and for liquids
between 10% and 20%.
Generally up to 5 TPH boiler set and reset pressure difference
should be below 5% and over 5 TPH set and reset pressure difference between 5%
to 10%.
Once normal operating conditions have been restored, the
valve is required to close again, but since the larger area of the disc is
still exposed to the fluid, the valve will not close until the pressure has
dropped below the original set pressure. The difference between the set
pressure and this reseating pressure is known as the “blowdown” and it is
usually specified as a percentage of the set pressure.
Boiler drum safety valve set pressure will be approximately
drum design pressure x 1.07%. 7% varies according to designer.