InspectAPedia offers a guide to inspecting, testing and repairing boiler pressure relief valves. The guide includes images of different types of pressure valves and their failure mechanisms.
Boiler pressure valves are used to prevent a boiler from exploding. A spring-loaded diaphragm holds the valve closed until the boiler pressure gets too high. It then opens, allowing steam or water to escape from the boiler through a discharge tube. The valve and its pipes have to be correctly matched to the boiler's operating temperature and pressure. Common failure modes include leaking or corrosion, incorrectly located or bypassed valves and blocked or damaged discharge tubes.