PSV Installation-Guide

My friend, Let me share a simple material about PSV installation. However it will only focus on designing the system.  This is become my first posting in this year. Hopefully, this material is useful for you in developing the system around PSV.

My friends, in developing or reviewing the P&ID, especially in PSV system, please consider the following ;

1. PSV normally be installed close to protected equipment.
The safety valve is installed for protecting the equipment, so that the closer to the equipment is better. For example, from safety point of view, to install the PSV directly at vessel is better than at line outlet vapor.

2. PSV for vapor application at vessel -shall be connected to the vessel in the vapor space higher than HHLL.

3. If any demister, PSV shall be connected to the vessel  at below the demister since there is  any potential blockage of demister, and if the PSV be installed at downstream demister, it will not protect the vessel in that case.

4. PSV may be installed at outlet vapor line. For this case, take care with the 3% pressure drop limitation.

5. For inlet line, please consider the total pressure drop of inlet line less than 3% of set pressure of the PSV.
This requirement (3%) is to avoid chattering on PSV. 

6. Inlet PSV shall "free draining" to avoid liquid accumulation. 
Please consider put NOTE on the P&ID, and as process engineer, you should check the final design by Piping engineer.

7. Block valve at inlet line ( if any) shall be LO and LC for spare PSV.
Block valve at outlet line (if any ) shall be LO

8. if discharged to ATM ( please indicate on P&ID "ASL" (at safe location) instead of ATM). The block valve is not required. The weep hole to be provided at lowest point of discharge pipe to drain the liquid.

9. Outlet line to be designed with Mach No < 0.7 
Check the momentum ( Rho v2 ) if the value of (Rho v2) > 200.000 Pa..please take care since it may be vibration. Usually Piping engineer will also check the requirement of piping support.
Back pressure to be considered based on PSV type.
Noise for emergency case typically more than 85 dB is still accepted. 
Check the two phase flow pattern, if there is any slug flow - please take care, you must state in the P&ID so that Piping Engineer will consider the strengthening of support.

10. Outlet line shall only have downward elevation change to the flare sub-header.  

11. Consider minimum distance from by pass block valve to discharge line for heavy hydrocarbon service.

12. Lateral connection to be designed with 45 deg or 30 deg to the header.

13 Please consider, Other system may have specific requirement. Think about it by yourself. 

I think that all, that I can share to you by today.

My friend, how are you today? It’s been long time I don’t update my blog, really sorry guys. I was very busy doing my project – urea plant in Palembang. Thank you very much for many of you who've contacted me via email and give me feedback and much-much spirit for me to keep updating this blog ‘again’

Thank so much for support me guys, I love you.

I hope this weekend give you all the best of everything,


  1. PERTAMAX Reekk!!
    Oww finally.. the brand new post for this year (2014) has been released.
    First of all, thanks for updating this great blog. emm, i have some concern about this

    topic since currently i am doing an expansion/revamping project of my plant resulting

    some modification:
    1. PSV normally be installed close to protected equipment.
    Fist of all, we can see the Original PFD (Existing) in:

    and the Modification PFD (Revamping) in:

    (*sorry i am not be able to show the picture directly in this comment)

    In the contrary with this statement, the PSV is located even in the Upstream line of

    the protected Equipment (Low Temperature Separator) from fire. I feel strange with this

    configuration, but the contractor neglict my opinion on relocate the PSV to vessel or

    at lest to the vapor line of te vessel.
    and for Oversize PSV that previously designed for Full Flow (i don't really know about

    this case) changed to Fire ( req 1 D 2 but the available is 3 K 4). Chattering

    phenomenon will be occur. How could we overcome this issue without changing the

    available PSV (3 K 4)? or there is no way to resolve this but change the PSV is a must.

    6. Inlet PSV shall "free draining" to avoid liquid accumulation.
    I have a problem with this criteria. Actually, the PSV in compressor system is located

    at low elevation. since the Pipe Rack (where the Flare Header sit down to) is 4 meters

    in height, the compressor skid should be higher and by concrete-ing the skid, and this

    is a "very costly" way. because the skid is a big area, could you imagine that? so

    that, our team and the contractor agree to let the discharge PSV "free draining-free" /

    "free draining less", moreover there is a pocket line to flare header, yes pocket line.

    Since we believe that the Compressor Area is a Liquid-Free area. So there won't be a

    liquid trapped in the pocket line since there is a "liquid free guarantee" from

    compressor area. The liquid comes from the others PSV won't flow to the pocket line due

    to 45 deg lateral connection to flare header. So it is safe. Besides, we agree with

    this configuration since the existing compressor area PSV is just the same, pocket line

    in discharge of PSV that absolutely "free draining-free". I wonder, if it is an

    exception? as we know, there are always exceptions in many things in this world.. hehe.

    what do you think guru?

    7. Block valve at inlet line ( if any) shall be LO and LC for spare PSV.
    Block valve at outlet line (if any ) shall be LO. what is the "spare PSV" stands for?

    the PSV Series 100% design pressure, 105% design pressure, so on...?? would you kindly

    explain the reason on LC-ing the spare PSV? or how about CSC-ing this spare PSV? is it


    9. as i read from another source, the noise limitation for intermittent service is 115

    dBA instead of 85 dBA. is it true?

    10. Outlet line shall only have downward elevation change to the flare sub-header. Is

    there any standards for the range value for the slope? my plant flare header line has a

    1:5000 slope untill Flare Pot/Flare KO drum, and 1:200 from flare pot to flarestack.

    huhuhu, that's all. Thanks in advance guru.. :)

  2. Many thanks, your blog is great, you should add more articles !

  3. Welcome back!
    I suggest one more point:
    Spare PSV to be provided for equipment without spare, but no spare PSV is required for equipment with spare.

  4. Dear Tholabul

    Normally, Spare PSV is provided on any single equipment which is not having spare equipment. Spare PSV is provided with LC isolation valve to avoid proces fluid expouse directly to PSV. While main PSV is provided LO isolation valve to enable constant availability of main PSV for protection. However, when main PSV is taken out for maintenance and calibration, spare PSV is kept online by arranging its isolation valve as LO.

    Further, when we are using PSV in boiler service, we are not allowed to use isolation valve on PSV suction by standard. In such case PSV is directly located on vessel and since you can not isolate PSV, you dont provide spare PSV also in such case.

    3% criteria for PSV inlet line sizing is very important. However, some designers dont follow it for PSV sizing for firecase. Fire scenario is emmergency scenario. During fire scenario, anywhy there would be demage to piping, PSVs and PSV will be replaced if envisaged.

    Do you Agree??

  5. Great blog and great posting! Readers might also enjoy Steve Mackay’s engineering blog and free technical resources available there.

  6. hi
    one question.
    how to selection of design temperature? and hot to know of design temperature?

    1. For Hot Design temperature : The maximum inlet relieving temperature or equipment design temperature whichever greater.
      - sometimes one prefer to consider fire case and another one not consider.

      For Cold design temperature : Minimum outlet temperature or minimum equipment design temperature whichever lower.


  8. Yes, I think there is a large element of wishful thinking.
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  9. Great Post to learn PSV Installation .The added Pictures are Really helped me a lot to understood .


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  10. Dear Sir , Kindly Clarify me the Design Temperature , am having the Doubt in That ????


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  11. Thanks for ypur advices!
    Just for information: in Russia we never install block valves on PSV-lines (according our standards).

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  15. Thank you very much for this post

  16. Very useful posting!

  17. Sir, I have a question may not be related to your topic but please help me. Usually, control valve have one block valve to by-pass it. Why the by-pass valve always be installed near the inlet of flow? Thank you for your sharing!

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