A Simple Pressure Safety Valve Calibration Procedure

with 26 Comments
a PSV installed in tank

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:

  1. What is a safety valve?
  2. Its difference with a relief valve
  3. Why do we need to calibrate a Pressure Safety Valve?
  4. PSV Calibration Setup and procedure
  5. The 3 stages to observe during calibration.
  6. How to verify a safety valve?
  7. 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.

A Pressure Safety Valve Installed in a liquid nitrogen tank

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.

A Safety Valve installed in Oil Tank

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:

Difference between a relief valve and a safety valve


Why Do We Need to Calibrate a PSV?

Parts of a pressure safety valves- a dismantled safety valve
A dismantled Pressure Safety Valve showing the main parts

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:

  1. To verify that is still within the set pressure.

  2. As a part of preventive maintenance to maintain proper working condition

  3. 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.

  4. 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.

  5. 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:

  1. Conventional type safety relief valve
  2. Balanced bellows type
  3. Pilot operated
  4. Power actuated
  5. Temperature and pressure actuated

You can read more about the different types of safety valves. Visit below link.

  1. pressure-relief-valve-safety-vacuum-relief-valve
  2. back-to-basics-pressure-relief-devices


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.

Safety valve calibration setup using a pneumatic pump.
Calibration Setup using a pneumatic pump

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:

  1. A pressure module or a test gauge
  2. Fluke 754 as a display for pressure module
  3. A pressure source like a compressed gas, nitrogen gas or a pneumatic pump
  4. Set of Fittings

Calibration Procedure:

  1. 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.
  2. After performing the set up above, ensure that there is no leak.
  3. Increase the pressure until a sudden release or pop action is observed.
  4. Check the displayed pressure reading in the standard and record in your Measurement Data Sheet (MDS).
  5. Slowly decrease the flow of the pressure then observe the closing of the valve, this is the reseating pressure value- record it
  6. Repeat procedure 2 – 3 up to 3 times.
  7. Do not forget to label and seal to prevent unauthorized adjustment.

The 3 stages to observe during Safety Valve Calibration.

  1. 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.
  2. 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.
  3. 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:

  1. Removing the bonnet cover, a ring with a screw will be exposed
  2. Loosen the ring-like lock screw to access the adjusting screw
  3. Once the lock is loosened, turn the adjusting screw ( clockwise or counter-clockwise) to adjust at the desired range. See photo below.
Adjusting a safety valve
Adjusting a Pressure Safety Valve
adjusting a bigger safety valve
Adjusting a bigger Safety Valve

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.

Thank you for visiting my site.

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Best Regards,

Edwin

26 Responses

  1. Sudarshan Chandrasekhar
    | Reply

    Thanks for a great valuable piece of advice.

    • edsponce
      | Reply

      Hi Sudarshan,
      You are welcome. Thank you for reading my post.

      Best Regards,
      Edwin

  2. mshalaby
    | Reply

    Excellent
    But there is a question
    What are the factors affecting the calibration PSV
    I mean, is there a difference between PSV calibration in liquids and gases in terms of set point or no difference?

    • edsponce
      | Reply

      Dear Sir,
      You are very welcome. I tried calibrating a PSV that uses gas as a fluid with water but with a minimal difference or negligible effect in terms of set points. But when you do this, make sure to blow or dry the PSV before returning. Some valves like the one used in liquid nitrogen can freeze the water inside which can cause problems.

      So, as a good rule to follow, calibrate a PSV with a fluid that is the same with its actual usage. If a PSV is used in a gas fluid, calibrate also using a gas pressure source. If PSV is used for water as the fluid, use also water as the medium for calibration.

      Hope this helps,
      Edwin

  3. Owais
    | Reply

    Great information in one place.
    Thanks

    Regards,

    Owais

    • edsponce
      | Reply

      Hi Owais,
      You are welcome. Thank you for reading my post.
      I appreciate your comment.

      Regards,
      Edwin

  4. Adeoye Adewale
    | Reply

    I am going to calibrate my first ever PSV thanks to this article. Previous works with PRV over the years but never encounter a PSV. Good job.

    • edsponce
      | Reply

      Hi Adeoye,
      I am glad this post has helped you in some way. Thank you for the comment.

      Best regards,
      Edwin

  5. mohammed abdullah
    | Reply

    could you please explain what is the acceptable difference between the set pressure and the reseat pressure? and what is the governing standard for this acceptance?

    • edsponce
      | Reply

      Hi Mr. Mohammed,

      Reseating pressure or the closing pressure can be best understood as the blowdown pressure, it is the difference of the set pressure and reseating pressure.

      The acceptable reseating pressure (blowdown pressure) is below:
      1. For gas (maximum), it is 15 % or 0.3 bar, whichever is greater;
      2. For liquid (maximum), it is 20 % or 0,6 bar, whichever is greater;

      Blowdown = set pressure – reseating pressure

      For example:
      set pressure = 20 bar
      Reseating pressure = 19.8 bar
      Therefore : blowdown pressure = 20 – 19.8 = 0.2 bar

      The governing standard for this is ISO 4126-1:2013

      I hope this helps.
      Thank you for reading my post.
      Edwin

  6. Elsakka
    | Reply

    thanks for the valuable post
    could you give us a brief about PSV preservation? and does the preservation overcome the need for calibration?

    • edsponce
      | Reply

      Hi Mr. Elsakka,

      Thank you for visiting my site.
      My basic understanding of PSV Preservation means to increase its life span where the PSV is working as intended for a longer period of time. In my opinion, performing calibration is one way to preserve or increase the life span of the PSV.

      During calibration, we preserve the PSV because we perform:
      1. Preventive maintenance by cleaning the PSV and performing a leak test.
      2. a simple test on its functionality like opening and closing the valve
      3. performing calibration to determine the accuracy of its output
      4. verification if it meets established specifications
      5. adjustment if needed

      Other preservation method includes shielding or having a good installation design of the PSV during installation in order to avoid:
      1. too much vibration
      2. exposure to contaminants like dust
      3. exposure to temperature (too high or low temperature) that exceeds its specifications

      As per the above importance of PSV calibration, preservation cannot overcome the need for calibration. What I can suggest is that after implementing the preservations above which include calibration, perform an assessment.

      How to assess you might ask? Gather all the calibration certificates of the PSV, and record the history of its calibration results. Based on the collected results, where you observed that the PSV are within tolerance or passing the specifications, for example within a 3 year period, then you can increase the calibration interval for every 3 years.

      Save your data, this will be your evidence if ever an auditor will question it.

      If you need to know how to understand a calibration certificate, visit my post in this LINK.

      Hope this helps.

      Edwin

  7. John
    | Reply

    The factory will send the PSV calibrated. Which standard I can find that I must calibrate the PSV in site In?. From the factory to the plant there are more than 200 Km

  8. Tony
    | Reply

    Hi guys,

    Very interesting procedure.
    But obviously you need to unmount the PSV to test it.

    If there is only one PSV on a gas tank, how would you test it without releasing all the gas to atmosphere
    I would have said to add a 3-way valve between the tank and the PSV to switch on the pneumatic pump whenever you want but then it’s not safe anymore if you forgot to put the 3-way back in position

    Thanks for your advices

    • edsponce
      | Reply

      Hi Tony,
      Thank you for reading my post.

      Yes, this procedure is performed where the PSV is removed or unmounted from its installation.

      As per my experience, the majority of the PSV that I calibrated have an isolation valve. You just need to close the isolation valve then safely remove the PSV. For PSV without an isolation valve, we will wait for the customer to empty the tank before we perform the calibration.

      Appreciate your comment.

      Thanks and regards,
      Edwin

  9. Rafael Michael
    | Reply

    Thank you Sir for your valuable Input.
    I have a certain question regarding the calibration/testing of the pressure safety valve;
    If someone wants to work within this field (calibration of the valves) and needs to be internally certified for that. What the authorization/destination he has to get the certificate from !?
    Thank you for your reply in advance.

    • edsponce
      | Reply

      Hi Rafael,
      You are welcome.
      I do not know if it is available in your location but you can look for training provider under the sponsor of a national lab in your area, or a government established training center which specialized in Instrumentation and Control where Calibration is one of its core subjects.

      Some private laboratories are also offering this kind of training under the field pressure calibration.
      You can also try to inquire to some suppliers or manufacturers of the valves.

      Try to look in your location.

      I hope this helps,
      Edwin

  10. sam seth
    | Reply

    How to use equipment in psv calibration

    • edsponce
      | Reply

      Hi Sam,
      What equipment are you referring to?

  11. RAVICHANDRAN M
    | Reply

    IS IT REQUIRED RECALIBRATION , WHEN PRV WAS POPPED AND RESEATED AND FOUND NO LEAK ?.

    • edsponce
      | Reply

      No it is not required. That is what a PSV does. The need to recalibrate arises when the PSV will popped at more or less than the set value and/or tolerance.

  12. kheddaoui
    | Reply

    Hi sir
    what about for the tolerance of the PSV if set pressure less than 5 bar.

    • edsponce
      | Reply

      Hi Kheddaoui,
      There is tolerance engraved usually in the body of the PSV, most small range PSV that I encounter has a tolerance of 5% of the set pressure.
      If you follow a tolerance of 5%, then: 5×0.05 = 0.25 bar, 5-0.25 = 4.75, therefore, your lowest acceptable reading should not drop below 4.75 Bar.

      I hope this helps.

      Thank you for visiting my site.

      Edwin

  13. David
    | Reply

    Hello, my question is with the rate of the pressure being applied. It has been my experience that the rate at which I apply the inlet pressure affects the cracking pressure of the PSV being tested. The slower the inlet pressure is applied it seems to produce a lower cracking point. Have you experienced this and if so what what would you suggest.

    Thanks

    • edsponce
      | Reply

      Hi David,

      Thanks for visiting my site.

      Yes, I have experienced it but as per my observation, it has no significant effect during calibration. The only effect that I observed is that popping sound is stronger compare to lower pressure flow. But as per accuracy, I did not yet experience any significant effect.

      We are using nitrogen gas in a tank with a regulator wherein you can constantly control the flow of the fluid until it reaches the PSV set pressure.

      I hope this helps.,
      Edwin

  14. Javed Gulzar
    | Reply

    Any person can share a Risk assessment for calibration of PSV,s

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