i. Safety Valve
As you know steam is generated in the boiler at a particular pressure. There are possibilities that the steam pressure in the boiler continues to rise and may exceed the value for which the boiler is designed. This may happen due to many reasons,such as, if the water in the boiler shell decreases or the feed water pump fails or the burner continues to supply heat even if the required steam pressure is reached or the steam demand is decreased.
If such thing happens the boiler shell will get overheated and may explode and result in great damage. To avoid this thing to happen, the boiler is provided with a safety device, which is known as the Safety Valve.Function of Safety Valve: The function of a safety valve is to safeguard the boiler from the hazard of pressures higher than the design value. It prevents the steam pressure in a boiler from exceeding a prefixed value. If the inside pressure exceeds the maximum working pressure, this valve opens and automatically discharges the steam from the boiler.
When once the excess steam is vented out and the pressure in the boiler is reduced to the working pressure the safety valve resets to its closed position automatically.The safety valve is located at the highest point in the steam space.
Types of Safety Valves and their construction
The safety valves may be classified into two groups:
a) Weight loaded safety valves
b) Spring loaded safety valves
Dead Weight Safety Valve: The weight loaded safety valve is commonly known as Dead weight safety valve.In this valve the weight, in the form of cylindrical cast iron discs, is placed directly on the valve. The valve is made of gunmetal and rests on a gunmetal seat. It is secured on the top of a vertical cast steel pipe. The pipe is bolted to the mounting block which is riveted to the top of the shell. The valve is threaded into a large cast iron casting which is like a vertical pipe like cover. This cover pipe carries the cylindrical disc shaped weights in it. Thus the total load on the valve is the sum total of weight of the valve itself, the weight of cover pipe and the weights placed on the cover pipe. When this load is greater than the force due to steam pressure acting on the valve, the steam will not escape. When the force due to steam pressure exceeds the load on the valve the valve is lifted up from its seat and the steam will escape. The lift of valve is controlled by set screws.Control and Safety Devices for Steam Boilers
Spring Loaded Safety Valve: If the valve in a safety valve is loaded with a spring instead of dead weights, then it is known as a spring loaded safety valve. A spring loaded safety valve .The spring used in these valves is in the helical form with round or square wires.The wires are made of steel. The load due to steam pressure acts along the axis of the spring. In the tensile load designs the spring of the safety valve is elongated when the excess steam pressure acts on the valve. Whereas, in the compressive load designs the spring of the valve is compressed when the excess steam pressure acts on it. As soon as the excess steam is released, and the pressure reduces below the working pressure, the spring brings back the valve to the valve seat and closes the valve.
The spring-loaded safety valves possess many advantages over the dead weight safety valves. These could be listed as below:
i) A spring safety valve eliminates the need of heavy weights.
ii) A spring safety valve offers easy examination of it and is easy to maintain.
iii) A spring-loaded safety valve is not affected due to jerks or movement.
As you know steam is generated in the boiler at a particular pressure. There are possibilities that the steam pressure in the boiler continues to rise and may exceed the value for which the boiler is designed. This may happen due to many reasons,such as, if the water in the boiler shell decreases or the feed water pump fails or the burner continues to supply heat even if the required steam pressure is reached or the steam demand is decreased.
If such thing happens the boiler shell will get overheated and may explode and result in great damage. To avoid this thing to happen, the boiler is provided with a safety device, which is known as the Safety Valve.Function of Safety Valve: The function of a safety valve is to safeguard the boiler from the hazard of pressures higher than the design value. It prevents the steam pressure in a boiler from exceeding a prefixed value. If the inside pressure exceeds the maximum working pressure, this valve opens and automatically discharges the steam from the boiler.
When once the excess steam is vented out and the pressure in the boiler is reduced to the working pressure the safety valve resets to its closed position automatically.The safety valve is located at the highest point in the steam space.
Types of Safety Valves and their construction
The safety valves may be classified into two groups:
a) Weight loaded safety valves
b) Spring loaded safety valves
Dead Weight Safety Valve: The weight loaded safety valve is commonly known as Dead weight safety valve.In this valve the weight, in the form of cylindrical cast iron discs, is placed directly on the valve. The valve is made of gunmetal and rests on a gunmetal seat. It is secured on the top of a vertical cast steel pipe. The pipe is bolted to the mounting block which is riveted to the top of the shell. The valve is threaded into a large cast iron casting which is like a vertical pipe like cover. This cover pipe carries the cylindrical disc shaped weights in it. Thus the total load on the valve is the sum total of weight of the valve itself, the weight of cover pipe and the weights placed on the cover pipe. When this load is greater than the force due to steam pressure acting on the valve, the steam will not escape. When the force due to steam pressure exceeds the load on the valve the valve is lifted up from its seat and the steam will escape. The lift of valve is controlled by set screws.Control and Safety Devices for Steam Boilers
Spring Loaded Safety Valve: If the valve in a safety valve is loaded with a spring instead of dead weights, then it is known as a spring loaded safety valve. A spring loaded safety valve .The spring used in these valves is in the helical form with round or square wires.The wires are made of steel. The load due to steam pressure acts along the axis of the spring. In the tensile load designs the spring of the safety valve is elongated when the excess steam pressure acts on the valve. Whereas, in the compressive load designs the spring of the valve is compressed when the excess steam pressure acts on it. As soon as the excess steam is released, and the pressure reduces below the working pressure, the spring brings back the valve to the valve seat and closes the valve.
The spring-loaded safety valves possess many advantages over the dead weight safety valves. These could be listed as below:
i) A spring safety valve eliminates the need of heavy weights.
ii) A spring safety valve offers easy examination of it and is easy to maintain.
iii) A spring-loaded safety valve is not affected due to jerks or movement.
ii. Fusible Plug
Function of Fusible Plug: Each furnace of the boiler is required to be fitted with a fusible plug. It is used to protect the boiler against the damage due to overheating for the reasons of low water levels. Its function is to extinguish the fire in the furnace of a boiler when the water level in the boiler falls to an unsafe extent and prevent the explosion which may take place due to overheating of the furnace plate. The plug is made of a fusible alloy. It melts away at high temperatures and releases the steam over the grate, thus quenching the fire.There are many designs of fusible plug.
iii. Water level Indicator
Function: The function of water level indicator in a boiler is to show the level of water in the boiler. If the water level in the boiler falls below a minimum level, the operator should make arrangements either to supply more water or to stop firing the fuel. There are two such indicators mounted on the boiler to ascertain the level of water in it. They are also called as water gauges.
Function of Fusible Plug: Each furnace of the boiler is required to be fitted with a fusible plug. It is used to protect the boiler against the damage due to overheating for the reasons of low water levels. Its function is to extinguish the fire in the furnace of a boiler when the water level in the boiler falls to an unsafe extent and prevent the explosion which may take place due to overheating of the furnace plate. The plug is made of a fusible alloy. It melts away at high temperatures and releases the steam over the grate, thus quenching the fire.There are many designs of fusible plug.
iii. Water level Indicator
Function: The function of water level indicator in a boiler is to show the level of water in the boiler. If the water level in the boiler falls below a minimum level, the operator should make arrangements either to supply more water or to stop firing the fuel. There are two such indicators mounted on the boiler to ascertain the level of water in it. They are also called as water gauges.
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