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Dealing with Pressure Relief Valve

  • 1.  Dealing with Pressure Relief Valve

    Posted 08-22-2019 16:46
    .How to deal with pressure relief valve? I was given to "evaluate the safety and mechanical of a high pressure relief valve in low pressure steam header that the pressure relief valve is rated at 190 C and it's currently operating at 225 C. Is doing any changes important? Re run the PRV calculations." To do this, what do I need? It's an industry issue.

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    Abdulla Alishaq
    Doha
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  • 2.  RE: Dealing with Pressure Relief Valve

    Posted 05-11-2020 08:30
    Pressure relief valve sizing is a critical part of ensuring a safe process.  The process engineer must understand both the normal and emergency operating scenarios (upstream conditions) as well as the down stream scenarios (flare header shared by other processes, shared flare stack, neutralizer solution scrubber, etc).   This can be a terribly complex combination of scenarios that would challenge the most experienced process engineer.  It is not an assignment to be given lightly nor its assessment to be taken without intense scrutiny.  Calculating an orifice size for set of given upstream and downstream pressures for a single PRV is the most trivial part of this calculation.

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    Scott Love PE
    Fellow
    Bartlesville OK
    [ConocoPhillips (retired)]
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  • 3.  RE: Dealing with Pressure Relief Valve

    Posted 19 days ago
    There's a lot of issues involved here in your problem statement, and I agree with Scott Love's response.  Unless you're willing to jump onto a huge learning curve in order to get this done correctly, then a popular phrase is "You may not even know what you don't know".  I teach a training class for one full day and I warn people we barely scratch the surface.

    The fact that normal operation is above the rated temperature of the PSV is a concern, but it may not be the end of the world--try contacting the actual vendor to have conversations about this.  The risk may be the PSV opens a little sooner, bro it may be the body of the PSV isn't rated for this condition.  Complete failure is unlikely.

    Hysys has a tool embedded called Safety Analysis Tool that's useful for sizing--the hard part is determining the relief rate required.

    I have one client in particular that intentionally assigned 2 very similar projects to two different contractors for PSVs--the only difference essentially was location by about 20 miles.  The project was to size the PSVs for standardized compressor skids and a flare header.  One contractor came up with a 12" flare header, and the other with an 18" flare header (and I tell this story a lot but with 2 different numbers to hide the real numbers, but the magnitude of the numbers is very true).  Client went to both, basically interviewed why it needed to be that big--and then after that they decided "We need to do this in house all the time from now on".

    Who loses?
    The consultants with real expertise (myself, and many others around the nation that I know)
    The contractor world in general.
    The client--they now have to train someone to be come an expert in the field which, in my humble opinion, takes a minimum of 2-3 years of doing this day in and day out, and you're not really an expert until year 4-5 of doing this as a day to day job.

    PSVs are a perfect niche market for the role oof a consultant.  If a company / owner has a fleet of over 500 PSVs, that's where I consider it as "It may be necessary to have an internal expert", but below that, inefficiency in learning curve and experience is difficult to justify.

    For your steam header here's a few considerations:
    1). Loss of desuperheating fluid at the let down station
    2). The letdown valve fails open--and your can consider friction losses in piping to/from the valve
    3). If there's a vessel in your low pressure system that your PSV protects, then obviously fire
    4). Any number of tube rupture scenarios on the low pressure steam users where there are exchangers
    5). System blocked in and letdown station still operates normally (you cannot take credit for the fact that the instrumentation responds and closes it in)
    ....and other considerations.

    The calculations are not terrible at that point for a steam system...just keep reading through API 520/521 and you'll find a lot of answers.

    What your PSV is NOT intended to protect is water hammer events in the line--transient events are not intended to be protected from by PSVs.  This gets into opening time and more...

    All for now, thank you!

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    Eric Parvin PE
    Owner / Process Manager
    Highlands Ranch CO
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