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
- After a pump
- Before entering process to protect process equipment
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
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.