One of the simplest but most important instruments that I encounter which are used not for displaying or monitoring the process, but for safety, is the pressure safety valves.
When I was just starting out as a cal tech, looking at it installed in a tank makes me curious about its usage. You cannot see any actions or output display as a sign of its operation.
In this post, I will present the following:
- What is a safety valve?
- Its difference with a relief valve
- Why do we need to calibrate a Pressure Safety Valve?
- PSV Calibration Setup and procedure
- The 3 stages to observe during calibration.
- How to verify a safety valve?
- How to adjust a safety valve?
What is a pressure safety valve (PSV)?
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.
Read more about safety valves in this link: more about safety valves
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
I have read a good article about its differences, Read more here:
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 that may result in a leak.
- Sometimes, the closed position of PSV disc where it did not open or activated 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 Safety Valve Calibration Setup
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 Procedure
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.
Pressure Safety Valve Calibration or Testing Reference Standards Needed:
- A pressure module or a test gauge
- Fluke 754 as a display for pressure module
- A pressure source like a compressed gas, nitrogen gas or a pneumatic pump
- Set of Fittings
- 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.
The 3 stages to observe during Safety Valve Calibration.
- set pressure – this is the pressure reading when the valve will pop or release a pressure rapidly. Tolerance is usually 3% of the set pressure.
- overpressure – This is pressure above-set pressure where the valve will open fully. It has a tolerance of up to 10% above the set pressure.
- closing pressure – this is known as the reseating pressure. It is the pressure reading when the valve will fully close and stop releasing.
This can be best understood under the term blowdown pressure which is the difference between the set pressure and reseating pressure.
Because of the rapid popping action during the discharge, it is hard to notice the difference of set pressure and overpressure. Without the tolerance specified, set pressure and overpressure reading are the same, mostly for results higher than set pressure.
We are using a 10% tolerance or the tolerance specified by the manufacturer or as per the requirement of the user as the basis for a pass or failed verification.
How to Verify the Reading of the PSV if it is Within the Specified Set Value?
As per international standards ISO 4126-1, the tolerance limit during safety valve testing or calibration is +/- 3% of set pressure.
Example: set pressure is 25 Bar
The tolerance limit of the PSV is 3% of the set value, simply multiply the set value by 0.03.
> 25 X 0.03 = 0.75
The reading should be within
> 25+/-0.75 or (24.25 to 25.75)
If the reading is not acceptable or out of specs, you need to perform an adjustment. Read further below.
How to Adjust a Pressure Safety Valve?
Since a PSV is purely mechanical. It can be repaired or adjusted. The adjustment is simple, you just need the right tool.
PSV has a set pressured that is determined by the strength of the spring inside it. The more the spring it is compressed, the stronger or higher the pressure it can withstand or create.
In order to adjust the set pressure, we need to change the compression and/or the elongation of the spring by rotating the adjusting screw located just above the spring.
3 Steps to perform Adjustments:
- Removing the bonnet cover, a ring with a screw will be exposed
- Loosen the ring-like lock screw to access the adjusting screw
- Once the lock is loosened, turn the adjusting screw ( clockwise or counter-clockwise) to adjust at the desired range. See photo below.
Simply rotate the screw until the desired range is achieved. But be careful not to over rotate, you may damage the spring.
Pressure safety valve or PSV is the last line of protection for all pressured vessel or tanks from over pressure using only the system pressure as the source of power. In this post, I have presented what is a safety valve, Its difference with a relief valve, why do we need to calibrate a Pressure Safety Valve, PSV Calibration Setup and procedure, The 3 stages to observe during calibration,how to verify a safety valve and how to adjust a safety valve inlcuding the main parts of a PSV.
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