Pressure Safety Valve / Pressure Relief Valve

Pressure safety valve is sometimes known as Pressure Relief Valve. Its purpose is to prevent a pressure in a pressurized vessel or line to go above the set point by venting to atmosphere or diverting the flow to an alternate channel.

It can be used for all mediums including air, liquids, gases, water and oil. 

As it is considered a critical component, some authorities (eg. SINGLAS) may require periodic testing to certify that it is in working order. One of the common tests involve a ‘pop test’. 

Selection of a safety valve involves sizing and material selection.

What is a Pressure Safety Valve

Safety Valves or Relief Valves protect pressure vessels from overpressure by opening when the set pressure is reached. 

It is commonly used in:

  • Pressure Vessels
  • Storage Tanks
  • Boilers
  • After a pump
  • Before entering process to protect process equipment
There is a subtle difference between a safety valve and relief valve. A safety valve has a characteristic ‘pop’ sound when the valve opens to quickly discharge the process fluid. The fluid is usually discharged to the atmosphere. Whereas in a relief valve, the discharge is in proportion to the overpressure. The fluid is usually returned back to the system

Parts of a Safety Valve

1. Nozzle – Connection of the safety valve

9. Adjusting Screw – This is where you adjust the set pressure

4. Cap – Prevents access to the adjustment screw

11. Spring – Holds the disc down to close the safety valve

5. Disc – Blocks the orifice to close the safety valve

Sizing of Safety Valves: Basics

Sizing of a safety valve involves complex calculations to determine the required orifice size. It depends various factors such as the type of fluid, temperature, maximum set pressure and the required relief capacity of the safety valve. This ensures that during an emergency, the safety valve is able to vent enough fluid to reduce the pressure.

Determining the Set Pressure

The set pressure usually depends on the maximum working pressure of the equipment or line the safety valve is supposed to protect. Ideally, it should be set around 10% below this point. 

Calculating the orifice size

The calculation of the orifice size is usually done using software. Some manufacturers also provide a capacity chart to determine the required orifice size. 

Contact us for help in selection and sizing of a safety valve here

Maintenance & Calibration Procedure

It is important and sometimes mandatory by law to perform calibration and testing of the safety valve. The purpose is to ensure that the safety valve is in proper working condition and will not fail when it needs to discharge pressure. 

In Singapore, it is mandatory for all PSV installed at boilers and pressure vessels to undergo recertification regularly. This is regulated by SINGLAS.

A safety valve may fail due to:

  • Repeated opening and closing
  • Dust, rust and other debris from the pipe
  • PSV has remained closed for long periods of time
  • Normal wear and tear

Calibration Procedure

Performing calibration and servicing requires a proper test bench that can deliver air or water at different pressure to perform the required tests. During a pop test, the pressure is slowly increased until the safety valve opens slightly. This has to be done carefully to prevent damage to the valve. A leakage test must also be carried out to ensure that the valve is not leaking.