An industrial boiler is a piece of equipment that generates steam or other vapours at pressures of more than 15 psi (pounds per square inch) for external use. The boiler heats liquids (usually water) using electricity, combustible gases (natural gas) or waste heat from another industrial process. Because the risk of over pressure is high, boiler code requires the use of safety valves. The American Society of Mechanical Engineers (ASME) has defined the requirements for a boiler safety valve in Section I of the Boiler and Pressure Vessel Code.
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Safety valves are mechanical devices containing a metal disc that is held in place by the compression force of a metal spring. As internal pressure increases in a boiler, the spring force is overcome and the safety valve pops open, relieving the pressure. Internal guides must be provided in the valve to ensure consistent operation. The spring's full lift compression cannot exceed 80 per cent of resting deflection (not compressed). The disk or seat of the safety valve must be fastened to the body to prevent lifting. There must also be a drain below the seat to allow any condensing liquid to drain out.
The choice of the materials of construction of a boiler safety valve is important to the proper operation of the device. The guide is a close-fitting surface and must have resistance to corrosion as well as the spring. The nozzle and disk (seat) of the safety valve must be compatible with the service fluid (water/steam in most boilers) to avoid corrosion. Cast iron disks and nozzles are prohibited. All materials used must be qualified ASME materials or be an approved equivalent.
Testing and Certification
Boiler safety valves are devices that must be tested to ensure correct operation. The set pressure (pressure at which the boiler pressure overcomes the spring and relieves) must comply with tolerances set by ASME (percentage of set pressure). The blowdown pressure (difference between pop pressure and reseating) must also meet ASME tolerances (percentage of set pressure). The allowable over pressure requirement is used to determine the safety valve's design and capacity and must be certified by the valve manufacturer.
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