EEI MASS Practice Test 15

How a Pressure Relief Valve Protects a Boiler

¹A boiler stores a large amount of energy in the form of hot water and steam under pressure. If the pressure inside rose without limit, the drum or tubes could eventually rupture, releasing that energy all at once. Operators and control systems are the first line of defense against overpressure, but controls can fail and people can make mistakes. The final protection is a device that needs no power and no operator: the pressure relief valve, often called a safety valve on steam service.

²Most boiler safety valves are spring-loaded. A disc sits on a seat at the top of a nozzle connected to the steam space, and a heavy spring presses the disc down. Steam pressure pushes up on the underside of the disc. As long as the spring’s force is greater, the valve stays shut. The pressure at which the upward push of the steam equals the spring’s force is called the set pressure. It is fixed when the valve is adjusted. At least one safety valve on the boiler is set no higher than the boiler’s maximum allowable working pressure, the highest pressure the boiler is designed to carry.

³When boiler pressure reaches the set pressure, the disc begins to lift. On steam service the valve is designed to pop open rather than ease open gradually. As the disc rises, escaping steam fills a chamber around the disc and pushes on a larger area, which lifts the disc quickly to its full height. Steam then rushes out through the discharge piping, and pressure in the boiler begins to fall. The valve does not close again at the set pressure. It stays open until the pressure has dropped somewhat below it, a difference known as blowdown. Blowdown keeps the valve from chattering, or rapidly opening and closing, which would damage the seat.

⁴Because a relief valve may sit unused for months, it depends on careful maintenance. Its discharge piping must be supported and kept clear so that escaping steam can leave freely and be carried to a safe location. Nothing may ever be done to hold the valve shut, and its setting should be changed only by qualified people following the plant’s procedures. A valve that leaks steam between lifts should be reported, since the leakage gradually cuts the seat.

⁵A relief valve is a last resort. It protects the boiler only if the plant treats it as a device that must work perfectly the one time it is truly needed.