Boilers are high pressure and temperature systems used for generating steam to drive steam turbines for electricity generation. If, due to any reason of operation or malfunction in equipment or controls, the operating pressure in the system goes above the safe limit of the material of construction, it can cause catastrophe. To prevent such catastrophes, safety valves are provided in the boiler at various locations.
Safety valve locations
The boiler drum holds both water and steam in large quantity. It is the thickest component in the system, and the component that most needs protection against pressure increases above the limit. There are normally two to six safety valves provided in the drum depending upon the capacity.
The superheater outlet will have one to three safety valves on either side of the boiler. There will be an electromatic relief valve on the superheater pipe in addition. This valve will be set at lower pressure than the lowest set safety valve on the superheater.
The reheater pipes both at the inlet and outlet side will also have safety valves which can range from two to eight both in the inlet and outlet of the reheater put together.
Types of safety valves
Boiler safety regulations in countries like America and India permit only non-assisted type safety valves. Spring loaded safety valves are popular. These springs are selected by safety valve designers to have high repeatability on set pressure and reseating pressure.
Another type of safety valve in use is the torsion bar safety valve, which are used by some boiler designers.
In some countries like Germany, the use of assisted type safety valves is permitted. The pressure raise is sensed by pressure sensors and the safety valve is opened to relieve the excess pressure.
Set pressure and Capacity
Depending upon the design pressure of boiler, the various safety valves in the boiler are set to open at a particular pressure called the "set pressure." Until the system pressure reaches the set pressure, the valve will not open or leak. This is achieved by the safety valve designers by adopting a patented disk design. Once the valve pops or opens, the pressure in the system may still raise. Because of this, the safety valves are designed to go full open within an overpressure level of 3%. The safety valve will close once the system pressure reduces to what is called the "reseat pressure." The valve will have to be set in such a way that the reseat pressure is 95% of the set pressure. The percentage of this is called the "percentage blowdown" and is limited to a maximum of 5% to avoid wastage of steam produced. However the spring loaded safety valves cannot be set for a blow down lower than 3% as this will make the valve chatter.
The capacity of the safety valve is the quantity of steam it can relieve when the valve is fully open. The safety valves in a boiler are so selected that the capacity of safety valves on drum and superheater put together will be more than the 100% generation of the boiler. The superheater safety valves are also selected in such a way that they will float before the drum safety valves; otherwise the superheater can get damaged due to overheating of the tubes.
The reheater safety valves are also selected and sized as is done in the boiler system. These valves are commissioned using the HP – LP bypass system of the turbine.
The safety valve setting forms a part of the commissioning team of the boiler. Once the safety valve is set, they are not adjusted or disturbed, except to again be floated when they are serviced or at a regular interval of more than a year as a test.