A pressure relief valve is a type of safety valve which is used to control or limit the pressure in a system. Pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. The pressure is relieved by allowing the pressurized fluid to flow from an auxiliary passage out of the system. The relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. Pressure relief valves are vital safety devices which are used in a variety of machines. These devices depend on pressurized vessels or are themselves pressurized systems used to protect during times of overpressure, especially within hydraulics and liquid service machines. They perform this mighty task by automatically opening further when the pressure increases and closing when the pressure returns below its opening pressure. Many devices like brakes, pilot lines, and hydraulic cylinders rely on pressure relief valves. Water pressure relief valves are often used by private and municipal water supply companies, these machines are used in pumps for a variety of different reasons including firefighting, high-rise buildings, water towers, drinking water applications, and water tanks. All of these mentioned outlets are vulnerable to high water pressures, i.e, those that exceed 200 pounds per square inch. High pressure can damage machines and harm the workers from using them. Water pressure reducing valves work with the devices above and remove excess pressure by opening up, thereby minimizing the pressure.
Direct-acting relief valves are normally closed, pressure limiting valves which are used mainly to protect hydraulic components from pressure transients. When the pressure at the inlet reaches the valve setting, the valve starts to open to tank, throttling flow to limit the pressure rise. These valves are smooth and quiet, essentially zero leaks, dirt tolerant, immune to silting and are very fast.
Pilot operated relief valves are used for emergency relief during overpressure events. Traditionally the valve is normally held closed by a spring or similar mechanism which presses a disk or piston on a seat, which is forced open if the pressure is greater than the mechanical value of the spring. The valve is held shut by piping a small amount of the fluid to the rear of the sealing disk, with the pressure balanced on either side. A separate actuator on the piping releases pressure in the line if it crosses a threshold. This releases the pressure on the back of the seal, causing the valve to open.