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Relief Valve Sizing

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#1 sau1a86


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Posted 14 September 2020 - 02:14 AM

Can anyone please help me with the basics of how to go about a Relief valve sizing ? any solved examples would also do  

#2 breizh


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Posted 14 September 2020 - 02:32 AM


Download the engineering document using this link . You will find a bunch of good information about sizing , more important is to define the scenario requiring the PSV :



To me you need to consult knowledgeable people , senior peers among your organization .

Good luck


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Edited by breizh, 14 September 2020 - 03:51 AM.

#3 Subhendu


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Posted 30 September 2020 - 04:48 AM



Pressure relief sizing is a vast subject, and is very difficult to capture the same in a post. To size a relief valve you need to follow the below steps

  1. Firstly, you need to identify all plausible causes of overpressure, that may or may not have a relief load. For list of possible causes of overpressure please refer to API Std 521. Afterwards, you have to calculate the relief loads associated with each possible cases.
  2. Next refer to API Std 520 Part 1, where different sizing equations are provided for liquid relief, vapor relief or two-phase relief. Based on these equations you will be able to calculate the required orifice area corresponding to each relief case.
  3. Next refer to API Std 526 for selection of orifice letter (D having the smallest and T having the largest orifice area) based on required orifice area. However, please note that orifice letter thus selected can vary from the PSV vendor recommendation because of the difference in various constants used in the sizing equations. 

Also, please be advised that type of PSV also affects the design. As a general rule of thumb, if backpressure is less than 10% of PSV set pressure, you can use conventional PSV, unless there is significant variation in the superimposed backpressure, > 3%, in that case balanced bellow valves are to be used.

If difference between the operating pressure and design pressure is less than 10%, then you need to use a Pilot type PSV.

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