Table of Contents

Safety Valve | Function ,types, Construction ,Working

Safety Valves

Functions:

Safety valve is used to guard the boiler against the excessive high pressure of steam inside the drum. If the pressure of steam in the boiler drum exceeds the working pressure then the safety valve allows blow-off the excess quantity of steam to atmosphere. Thus the pressure of steam in the drum falls. The escape of steam makes a audio noise to warm the boiler attendant.

Locations:

Safety valves are located on the top of the boiler.

Types of Safety Valves:

There are four types of safety valve:

Dead weight safety valve.
Spring loaded safety valve
Lever loaded safety valve
High steam and low water safety valve
Spring loaded safety valve:

Constructions:

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:

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.

There are three kinds of Safety Valves

Spring-loaded Pressure-relief Valves
Spring-loaded Pressure-relief ValvesGenerally, the safety valveｓ are referred to the Spring-loaded Pressure-relief valves, the most common type ( See the left figure ). The load of the spring is designed to press the “Disc” against the inlet pressure. Depending on the fluid type, such as steam, gas or liquid, we are offering a Bellows model to clear the back pressure effect.

Pilot-operated Pressure-relief Valves

Pilot-operated Pressure-relief ValvesPilot-operated Pressure-relief Valves are composed of Pilot Assy and Main Valve. Although Spring-loaded Pressure-relief valves adopts the force of the spring against the inlet pressure, the reliving pressure and reseating pressure of the Pilot-operated Pressure-relief Valves is controlled by Pilot Assy, which acts nearly as same as Spring-loaded Pressure-relief valves. There is no adjusting function in the Main Valve.
Pilot-operated Pressure-relief Valves have larger size variations compared to Spring-loaded type, which is applied in a severe condition such as high pressure.

Dead-Weight Pressure-relief Valves

Dead-Weight Pressure-relief ValvesIn case the design pressure of the pressure vessel is set at very low pressure, Dead-weight Pressure-relief valve adjusts relieving pressure only by the disc weight. The Vacuum relief valve has been adopting this functional characteristic which absorbs the pressure when the inside of the pressure vessel falls into a negative pressure.