Pressure Safety Valve or safety valves as the name implies is a type of pressure relief valve used to protect pressure vessels from excessive pressure, characterized by a rapid opening or a pop action once it reached the set pressure.
I usually encounter PSV in gas industries manufacturing cryogenics like liquid oxygen, argon, nitrogen, and carbon dioxide. Installed in permanent tanks or mobile tanks (in trucks).
I also see it in the food industry used in storage tanks for oil.
PSV is used specifically on safety concern. It is simple in use but one of the very important parts of safety. Only powered by the fluid pressure to do its safety job, it does not use electrical power. This makes it the last line of protection when every other device fails.
The main purpose is to open and release pressure when it reached a set pressure and then back to close position when pressure level normalized.
PSV is comparable to a pressure switch where it is triggered also when it reached a set pressure. It is in the ON and OFF state.
The Difference Between a Safety Valve and a Relief Valve
When I was first exposed in this type of pressure instrument, I thought Pressure Safety Valves (PSV) is the same with Pressure Relief Valves or PRV, I did not know that they are different in some ways.While both terms are used interchangeably, below are some of the main differences:
A pressure safety valve (PSV) or safety valve is a pressure relief valve:
> used mainly on a gas-filled tank ( cryogenics like liquid oxygen, argon, nitrogen)
> that opens rapidly (pop action) and fully when reached the set pressure is reached.
> where released gas is vented or discharged in the atmosphere or open air
A relief valve is a pressure relief valve (PRV):
> used in a liquid filled tank
> that will open gradually or in a proportion with the increasing pressure
> where the released liquid is discharged back to the system
Why Do We Need to Calibrate a PSV?
Every manufactured PSV has a set pressure engraved on its body. A set pressure that we need to verify to ensure that the valve will perform its function when needed.
Below are some of the reasons why we need to perform calibration and testing of PSV regularly:
To verify that is still within the set pressure.
As a part of preventive maintenance to maintain proper working condition
A PSV is purely mechanical, it has a strong spring inside that is perfectly designed to give the required set pressure. One of these mechanical parts (like the disc) can wear which can affect the set pressure.
Exposure to contaminants like dust or debris coming out with the fluid can affect the resealing or closing of the disc after release which may result in a leak.
Sometimes, the closed position of the PSV disc where it did not open or activate for a longer period has the tendency to stick (stuck-up). This affects the set pressure. Thus, it is a good way to exercise the valve.
Different Classifications of Pressure Safety Valves
Safety relief valves are classified as:
Conventional type safety relief valve
Balanced bellows type
Pilot operated
Power actuated
Temperature and pressure actuated
You can read more about the different types of safety valves. Visit below link.
pressure-relief-valve-safety-vacuum-relief-valve
back-to-basics-pressure-relief-devices
Pressure Safety Valve Calibration Setup
The calibration setup is the same with the pressure gauge, the only difference is that PSV has no display to be seen.
There are 2 setups you can implement.
1. By using a pneumatic pump as the pressure source. Connect the module and the valve as shown in the photo.
2. By using nitrogen gas or compressed air as the pressure source. You can watch the below video for a simple demonstration.
PSV calibration is done by comparing the set pressure, which is the rapid releasing action (popping) of the PSV, to the displayed value of the reference standard. The pressure relief valve testing procedure to verify the accuracy of output pressure (set pressure) is the same.
If you want to know other types of valve testing to determine its full performance like leak testing and flow characteristics, you may need to refer to ISO 4126-1.
Pressure Safety Valve Calibration or Testing Reference Standards Needed:
A pressure module or a test gauge
PR235 as a display for pressure module
A pressure source like a compressed gas, nitrogen gas or a pneumatic pump
Set of Fittings
Calibration Procedure:
Determine the set point of the pressure gauge. Be aware of this set point to anticipate the opening of the valve while controlling the pressure source.
After performing the set up above, ensure that there is no leak.
Increase the pressure until a sudden release or pop action is observed.
Check the displayed pressure reading in the standard and record in your Measurement Data Sheet (MDS).
Slowly decrease the flow of the pressure then observe the closing of the valve, this is the reseating pressure value- record it
Repeat procedure 2 – 3 up to 3 times.
Do not forget to label and seal to prevent unauthorized adjustment.
PSV Calibration Frequency
PSV calibration frequency is based on the performance of your safety valves. A 1-year interval is ok as per my experience but I also see PSV calibration interval up to 3 years. As I said, it depends on how it performed based on its history. As an initial interval, start to 1 year then increase it based on its performance.
