Safety Valves and Relief Valves

A safety or relief. valve is a very important protective device. It is designed to avoid accidents by relieving pressure when something goes wrong in the system.

Safety valves and relief valves respond to high pressure in a pipe system. These valves relieve the pressure before it can cause an accident or damage.

Safety valves are used for protection on steam and gas production facilities. Relief valves are generally used on liquid production systems.

Safety valves are commonly called PSV's (Pressure Safety Valves). Relief valves are commonly called PRV's (Pressure Relief Valves).

A safety valve on a gas processing system is usually connected to a flare vent system. The gas escapes to a flare where it is burnt. A safety valve on a steam system will vent to the atmosphere.

Safety valves are set to open (pop) completely at a pre‑set pressure. This pre‑set pressure can be adjusted. The safety valve will stay open until the pressure drops back to the pre‑set lower pressure. The valve should then shut immediately.

The outlet ports on safety valves and relief valves are always larger than the inlet ports. This allows the excess pressure to flow through the valve faster and bring the pressure down to normal quickly.

The safety valve is spring loaded. The spring is adjusted before it is installed. At the pre‑set pressure the spring loaded disc is forced open. It is designed so that the flow pushes the disc fully open. It stays in the fully open position until the pressure falls to below the pre‑set pressure. The spring then snaps the disc to the closed position where it is firmly seated.

The relief valve is normally spring loaded. When the pressure goes above the pre‑set pressure the disc does not go fully open immediately. The amount it opens depends on the amount of excess pressure. The more excess pressure there is, the more the valve opens.

When the pressure falls to the pre‑set pressure, the valve will close gently. There is no loss of liquid because the liquid that escapes is recycled to the suction of the pump or inlet of the vessel it came from, These valves are generally used as relief valves for the thermal expansion of liquids in pipelines.