PSV is a pressure safety valve and PRV is a pressure relief valve. Both are used for process safety. So it is important that an engineer must know about PSV and PRV. They make the decision about which should be used where should be used. The design and components of PSV and PRV are very similar.
• The PSV will fully open suddenly when the pressure achieved the set point whereas the pressure relief valve is the valve used to secure the pressure in the vessel especially when incompressible fluid flows and it should be opened in proportional with the pressure condition and therefore the opening of the valve is not sudden.
• In normal operations, the PRV is closed and no fluid passes through it. But when the pressure in the line exceeds the limit, the valve opens to relieve the pressure. This protects the other machineries such as motors, pumps, and actuators from becoming damage from the high pressure. Once the relief valve fails due to some reason the safety valve opens.
• For safety valves, there are limitations based on design and operating pressure. The relief valve are designed in such a way that once they release the pressure they close back and they continue to operate while safety valves. Hence they can be reset.
• In some cases, like fail-safe and shut down equipment, safety valves are used for permanent shutdown. Relief valves are used mainly for operational requirements.
• Safety valves can be used manually while this is not possible for relief valves.
• In most of the vapor and steam systems, safety valves are used whereas in fluids or compressed air systems.
• Most of the relief valves release the pressure before the setpoint or at the high-pressure condition and circulates it to the system while safety valve releases the steam vapours into the atmosphere.