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 pressure 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.
Watch below the step-by-step assembling of a pressure safety valve and learn more about its parts and how it is positioned inside the PSV body.
step-by-step assembling of a pressure safety valve
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.
Conclusion
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.
