Relief & Safety Valves

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 Relief & Safety Valves:

Relief & Safety valves prevent equipment damage by relieving accidental over-pressurization of fluid systems.
The main difference between a relief valve and a safety valve is the extent of opening at the set point pressure.
A relief valve gradually opens as the inlet pressure increases above the set point.
A relief valve opens only as necessary to relieve the over pressure condition.
A Safety valve rapidly pops fully open as soon as the pressure setting is reached.
A Safety valve will stay fully open until the pressure drops below a reset pressure.
The reset pressure is lower than the actuating pressure set point.
The difference between the actuating pressure set point and the pressure at which safety valve resets is called blowdown.
Blowdown is expressed as a percentage of the actuating pressure set point.
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 is often distinguished by the presence of external lever at the top of the valve body, which is used as an operational check.

Working of Relief Valve:



Relief valve use a steel spring as a means to automatically open when pressure reach unsafe levels.
These valves can be adjusted and regulated to pop off when internal pressure exceed predetermined settings.
Once internal pressure return to operational levels, the relief valve closes.

Construction of Safety Valve:



It consists of a cast iron body having two branch pipes.
Two separate valves are placed over the valve seat.
A lever is placed over the valve by means of two pivots.
The lever is held tight at its proper position by means of a spring.
One end of spring is connected with the lever while other end with the body of the valve.
The valve is kept on it seats with help of spring force.

Working Principle of Safety Valve:

In the normal condition, the downward force due to spring is higher than upward force applied by steam. 
The valve is closed due to spring force. 
When steam pressure exceeds the normal limit, upward force due to steam pressure is become higher than downward force due to spring. 
Thus the valves are lifted from their seats opening the passages for steam to release out of boiler.



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