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PSV Sizing

Sizing of pressure relief valve is clearly explained in API STD 520 part 1. Some formula is provided for calculation the orifice size either for vapor or liquid application, critical or non critical condition.

My friend, there is no difficult thing in sizing PSV as long as all data required is already provided. The all we need is just inputting the data in the formula or in the spreadsheet :D , then select the appropriate standard orifice size ( refer to API STD 526)

The following pictures, show the formula in API STD 520

For non critical condition, the outlet pressure has impact in sizing orifice area but for critical condition is not. In critical condition, the outlet pressure is lower than the critical flow pressure therefore there will be no impact to the sizing.

Note this, in critical condition, the flow through the nozzle will be influenced by the inlet pressure (relieving pressure). In non critical condition, the flow will depend on the differential pressure between inlet and outlet. (Relieving pressure – back pressure).
Use correct formula !

For you who looking for the sizing example, click the following link.
1. Gas/Vapor Critical and an example from Leser
2. Gas Subcritical and an example from Leser
3. Steam and an example from Leser
4. Liquid, an example from Leser

Thank you for your attention.


  1. Terimakasih mas. Saya jadi tercerahkan memahami rumus PSV. Dan terimakasih link-nya.

  2. Thank you very very much for such great posts and easy to understand informatio!! :) I just started working as a Process Engineer and I'm starting figure out all this. Now I finally feel comfortable with the entire PSV sizing, factors and just how the system works! Thanks!

  3. Hi Triyanto, Thanks for all of these. May I know what is the unit for all of these formula? Moreover, Is it any difference in the calculation for the orifice area sizing for conventional type and balance bellow type TRV in terms of back pressure ? As I know, balanced type TRV is not effected by the back pressure. Could I set it as zero? Hope you can reply me as soon as possible.

    1. - The A unit is in "inch2" - please check the the API 520 part 1

      - For critical condition, back pressure will impact only for Kb factor. Since in critical condition case, flowrate will only affected by the inlet pressure(relieving pressure). For balance below type of safety valve Kb = 1 (=no need correction factor, since there is any 'balance below' part of safety valve to avoid any back pressure impact from flare header)

  4. Hi,
    Thanks for the great details about valve, i have bookmarked your blog for the future updates.

    We are leading manufacturers company engaged in Solenoid Valve, Get Valve, Pressure Relief Valve

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  6. I have worked out the required orifice area/diameter, but I am now looking for a formula to workout the flow rate through the sized orifice as it will be larger than the calculated requirement