A Simple Pressure Safety Valve Calibration Procedure

with 60 Comments

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

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

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?

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 which may result in a leak.

  5. 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:

  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

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.

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 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:

  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.

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. How to implement this? Visit my other post in this link >> calibration frequency

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 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:

  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.


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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Best Regards,


60 Responses

  1. Thanks for a great valuable piece of advice.

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

      Best Regards,

  2. mshalaby
    | Reply

    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?

    • 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,

  3. Owais
    | Reply

    Great information in one place.



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


  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.

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

      Best regards,

  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?

    • 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.

  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?

    • 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.


  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

    • 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,

    • Alex
      | Reply

      if your vessel only has 1 PSV you need to take the vessel off-line before you can isolate the PSV, otherwise your vessel will be unprotected!

      As the vessel is off-line anyway removing the PSV physically should become your first option as it will be far simpler/faster/easier.

  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.

    • 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,

  10. sam seth
    | Reply

    How to use equipment in psv calibration

    | 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.

    • 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.


  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.


    • 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.,

      • taufiq abd rashid
        | Reply

        Hi sir..what is actually the min required size for the nitrogen tank for calibration of the PSV? Is the tank size also depending on the size of the PSV/ set pressure? Can u give simple calculation to calculate the tank size?

        If we are using water or oil for calibration of the PSV, are we also require the tank during calibration?

        • Hi Taufiq,
          I am not an expert regarding the calculation of tank sizes, I am only focused on the calibration of PSV. But as per my observation, the type of PSV is based on the size of the tank, the output pressure, the type of fluid (gas or liquid) it contains, and other more factors.

          During calibration of PSV, it needs to be isolated and removed from the tank. We only use air or nitrogen gas and water for calibration.

          Best regards,

  14. Javed Gulzar
    | Reply

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

  15. Manikantan
    | Reply

    Hi, Nice work…
    If my PRV Pops during operation due to over pressure and re-seat properly, then there is a requirement to bring down the PRV down and re-Calibrate the same.

    Which is the standard says about this??


    • Hi Manikatan
      I am not aware of a standard that requires recalibration after a successful opening and closing during overpressure as you have mentioned. This is the normal operation of a PRV as far as I know unless it has a different design of operation in which the manufacturer manual is the best reference.

      The reasons why we need to calibrate the PRV (or any other instruments) are:
      1. When it has reached its calibration due date
      2. After adjustment
      3. Required by authorities
      4. Out of specifications after verification
      5. Suspected deviations on its performance during operation
      6. Scheduled preventive maintenance

      If there are no other acceptable reasons, then I do not see why you need to recalibrate the PSV.

      If ever you had seen a related standard (or a manual), I appreciate it if you can comment back.

      Thank you for visiting my site.
      Best regards,

  16. Bayu Kuntoro
    | Reply

    Hii…. Thanks for nicekind article and information. Very helpful for all of us.
    My question is , How long interval for inspection and testing performance for High pressure and Low Pressure PSV on Boiler Unit ??( consist of Steam drum, HP drum etc).

    • Hi Bayu,
      You are welcome.

      There are no set standards regarding the interval of inspection or testing. The interval depends on the following:
      1. The procedure of the user as per experience or company policy
      2. The recommendation of the manufacturer
      3. The reliability of the PSV that depends on its performance based on the available data or history. If you encounter an out of tolerance within the calibration period, you can decrease the next testing interval, or you can increase if it is always within tolerance every time you perform testing.

      I hope this helps. Thanks for reading my post.

      Best Regards,

  17. Juandre
    | Reply


    Do you receive any documentation after calibration?

    • Hi Juandre,
      What kind of documentation, can you clarify more your question?

      Thanks and regards,

  18. Thanks for sharing valuable information. Is the PSV test is carried out by connecting through XP2i interface ?

    • Hi Dime,
      You are welcome.
      The PSV is calibrated using only a regular calibrator.

      Thanks for reading my post.


  19. Khoi
    | Reply

    How we identify the reseating pressure of PSV?Any criteria for this pressure??

    • Hi Khoi,
      Reseating pressure can be best identified by taking blowdown pressure, which is the difference between the set pressure and the reseating pressure.

      As per ISO 4126-1, blowdown pressure should not exceed the manufacturer specifications and within the limits of 2% to 15% of set pressure for compressible fluids and 2.5% to 20% for incompressible fluids.

      I hope this helps,

    • Premkumar
      | Reply

      Dear sir

      Please guide me how much bubble accepted for gases , oil , water , steam of PSV & psv setting like PTFE or metal to metal setting .as per safety standards .
      Thank you.

      • Hi Premkumar,
        Sorry to inform you but that is out of my knowledge. I hope you can find your answers.

        Thanks for visiting my site.

      • Alex
        | Reply

        For PRVs refer to API 527, i don’t think they differentiate between PRV vs PSV.
        Soft seats/PTFE are not permitted to leak when checked at 90% of set pressure.
        Metal seat valves depend on several variables.

        • Hi Alex, Thanks for providing the answer for Premkumar.

          Have a safe day!

  20. Etim sunday
    | Reply

    thanks sir im really bless by ur lessons God bless u sir pls I want to ask a question can we use prv in place of psv? or can we use psv in place of prv? thanks sir r

    • Hi Etim,
      You are welcome.
      Their use depends on the fluid used and the requirements of the system. I did not try yet to use them in place of each other. But you should not interchange their usage, they are designed for each fluid used to maintain safety. Do not risk it.

      Thanks for reading my posts.


    | Reply

    hola tengo que realizar calibraciones internas de mis patrones de temperatura con otro patrón de referencia que tengo, pero tienen la misma resolución , entonces que criterio puedo utilizar para argumenta que mi patrón de referencia es valido para calibrar mis patrones de trabajo?

  22. Moditha
    | Reply

    Hi, I need to know the pressure regulator use in the range of 0-6000 psi, however the local calibration firm inform the maximum pneumatic pressure value is 1500 psi that they can calibrated. The usage the gas tanks are rated at 2000 psi which is higher that the maximum calibration value. There is no any alternate calibration firm to cover the 0-6000 psi. What can be done for such situation if the pressure gauge need to be calibrated for accuracy?

  23. Yash Pal
    | Reply

    Hi Edsponce,

    First thanks for great post and it is very helpful and valuable.
    Could you please, also tell the popping time if there is any specified.

    • Hi Yash,
      There is no time specified for a popping time that I am aware of, it is based on the pressure level on when the valve will pop or close.

      Thanks for reading.

  24. Muhammad Waleed
    | Reply

    Hi Edwin! Your website is of great use, well-done.
    could you please tell why requirement of calibration arises on safety valves once it has been calibrated? or what are the factor that causes a calibrated safety valve to operate out of range after a certain time?

    • Hi Muhammad,
      Thank you.

      Safety valves need to be recalibrated because of its design.
      Safety valves during calibration are set to desired range. But the following are some factors that can affect its accuracy after a certain time:
      1. Since a valve is purely mechanical, which opens and closes every time a pressure limit is reached, there is a wear and tear, especially the disc and spring.
      2. Dust or other particles can build-up that can cause leakage.
      3. The spring can be over-stressed once a strong pressure is encountered.
      There are other reasons but these are what I usually observed.
      4. A schedule re-calibration is also one way to perform preventive maintenance and avoid any stuck-up or leakage problems.

      Best regards,

  25. Yash Shah
    | Reply

    Hi sir,
    I am Calibration Engineer.
    Once a time , I took a Safety Valve for Calibration and Mount On Hydraulic Pressure Comparator.
    when I Increase The Pressure , the safety Valve Pop up at set Pressure.
    But, The Problem is , It was a Decrease the Pressure.
    But Not stable Pressure After Seat Down(Pressure Slowly Decreasing).
    So , What is the Problem In Safety Valve.

    I Check at Least 15 Safety Valve but Same Issue repeated(My Master Instrument was Working Good).
    So , Is This A same Issue To All?
    I want To know.

    • Hi Yash,
      I am not sure if I understand your concern but a non-stable decreasing pressure happens if you use a non-continuous pressure source like a hand pump. It is not the issue for all as per my experience.

      You may want to use a pneumatic pressure source like liquid nitrogen gas if the pressure range is ok.

      But the important thing is you have a good performance for the pop-up pressure or set pressure and then it is closing at the expected closing or reseating pressure.

      I hope this helps, thank you for visiting my site.

  26. Avdell
    | Reply

    Thanks a lot for your enormous effort, it’s appreciated, I went and read all the comments and the replies, and checked all the links in the article and I can assure you you made learning much more straightforward,
    I have one question tho, about reseating pressure, if the pressure keeps decreasing you said this could be due to the use of a manual pump of a non-continuous pressure source,
    what’s a continuous and non-continuous pressure source?

    • Hi Avdell,

      Pressure in an open valve keeps on releasing until it decreases and reached the reseating pressure to finally close, therefore, using a non-continuous pressure source is difficult to detect reseating pressure.

      I refer to the hand pumps as the non-continuous pressure source because you need to manually pump the pressure depending on the speed of the hand, while the continuous source is the gas, specifically the nitrogen gas in a tank that supplies a continuous pressure once its valve is open.

      I hope this helps.
      I appreciate your comments and thank you as well for visiting my site.


  27. Daniyal Ahmed
    | Reply

    Can we calibrate a safety valve of PN40 pressure rating that originally comes with an 11 barg set pressure to a higher 28 barg set pressure?

    • Yes, I believe so since the maximum pressure it can handle is 40 bar. But to be sure, check the manufacturer’s specifications.

      Thanks for visiting my site,

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