What is the safety valve? Construction | Working | Why do we need two Safety valve for Ship’s Boiler?
A device used to protect the boiler from over pressurization for the safe working operation of the device. The safety valve is one of the most critical pieces of equipment mounted on the boiler for removing the excess pressure for safe work operation of the boiler. Here in the boiler, we have two Safety valves mounted on the boiler drum.
Why do we need two safety valves?
Two Safety valves are fitted on boilers as a precaution. Each safety valve is designed in such a way that even if the boiler is firing at full throttle still the safety valve opens and releases the excess pressure and if in case one of the safety valves falters the other safety valve will be there.
The difference between both safety valves is 0.5 bar. Let’s consider the normal working pressure of the boiler is 12 bar and the valve is set at 16 bar but in case the first valve doesn’t lift then the other valve will automatically lift at 16.5 bar pressure of the boiler and releases the extra pressure up to 8 bar or 9 bar then valve set back.
Safety valves consist of a lever that is placed over the valve and is used to control the spring pressure. A spring is placed inside the valve, one end of the spring is connected to the lever, and the other end is connected to the body of the valve. The valve is kept on its seats with the help of spring forces.
Working of safety valve:
As shown in the diagram, let’s consider a boiler generating steam pressure “P” acting on an area “a” of diameter “d”. So the net force acting on the valve is a product of the pressure acting on the boiler and the area. Since the spring is also exerting a constant downward force on valve seat “Kx”.When the net force exerted by steam on the valve seat is equal to or less than the Spring Force then the safety valve will not lift up, when the steam is not exerting pressure on a small area of diameter “d”.
But when steam pressure exerts pressure on the diameter of valve seat “D” then the net force acting by steam on the valve seat is greater than the spring force so the safety valve will lift until the excess pressure drops back down again.
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