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Unfired Vessels ASME Section VIII -- Establishing Relieving Capacity

Dale_3 Member Posts: 57
All - I'm hoping someone can point me in a good direction regarding establishing the required relieving capacity of relief valves on unfired pressure vessels. E.g. Steam to hot water shell and tube HX's I have had conversations with multiple P.E.'s and gotten multiple answers. I thought about buying this part of the ASME code but I'm not rich. Unfortunately for me it drives me crazy to not understand something and getting multiple mixed signals on this has me crazier than normal. Where I work you will see anything from tiny valves on big systems to big valves on tiny systems. I'm hoping there is a website or something out there cheaper than ASME books that might lead me to the light. Any and all feedback with welcome. Thanks in advance --- Dale


    EBEBRATT-Ed Member Posts: 13,810
    feeling the pain. it would be nice if the ASME stuff was available at a decent price but it is not.

    If it is a manufactured system (shell & tube) call the equipment manufacturer. Any tank or a piece of equipment "built up" in the field would be a problem.

    Other resources would be the State Boiler Inspector. Another would be an ASME fabricator or boiler repair shop........but they usually do what the boiler inspector tells them to do.

    Not a lot of information available on this. Long calculations and too much math for me.

    Not Knowing drives me nuts too
  • Jamie Hall
    Jamie Hall Member Posts: 21,541
    Agree with @EBEBRATT-Ed here. I know how to do the math. I know the physics involved. I could look up the necessary characteristics of the materials and the system. I could do it, if I had to for some obscure reason. I'm not a bit surprised that you are getting different answers from different PEs -- or that you see odd valves (apparently over or under size) on different installations. There are a lot of variables to consider (even the material properties of the pressure vessel gets into it!, never mind the media in question and the power of the input and the geometric configuration!).

    Not going there.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ratio
    ratio Member Posts: 3,509

    … Other resources would be the State Boiler Inspector. …

    This. My experience with inspectors (with, TBH, a few notable exceptions) is that they want you to do it right, & if they think you're trying to do the right thing they're happy to help. Call 'em out for a consultation, tell them that you have a question about your install & want to go over it with them.

    NB: I do not deal with state inspectors very often.

    EBEBRATT-Ed Member Posts: 13,810
    I like to know the exact answer myself. Getting there is sometimes too painful. Oversize it and walk away
  • Dale_3
    Dale_3 Member Posts: 57
    Thanks for the feedback. The State Fire Marshall already has an e-mail. I understand there are a lot of variables to be answered on this subject but in my situation I have 1 steam source "a power plant", typical steam PRV's with upstream relief valves feeding shell and tube HX's that are basically built the same way and all doing nothing but heating water with anything from 5 to 60 PSIG of steam. From my vantage it really appears to be simple but I've got people telling me how complicated the math is and others saying just put any old valve on it and your good. My pea brain wonders if any of them really know more than I do. :smile: I understand you have to start with what failure scenario you will follow but after that can the math really be that hard?

    @EBEBRATT-Ed I understand where you're coming from. The problem is when I read instructions I see them saying over and over again to NOT grossly oversize the valves for a few reasons that make sense.

    Thanks again - Dale
    EBEBRATT-Ed Member Posts: 13,810
    If you know the up stream and down stream steam pressure the mfg of the prv can tell you how much steam it will pass, that's all you have to relieve.

    If your relieving steam pressure select a relief valve that will pass that much steam at the pressure you want. (probably 15 psi) I disagree oversizing this valve will do no harm. Its not a control valve that controls its a safety oversizing will only affect your wallet.

    If your relieving water get the prv capacity of the steam valve and convert lbs of steam to btus. get a water relief that will relieve those btus at the pressure you want
  • Dale_3
    Dale_3 Member Posts: 57
    @EBEBRATT-Ed I've got the steam side covered and used to do what you described regarding PPH of the prvs to btuhs for relief sizing but I have systems that PEs are installing 3/4" valves on that have 5000 PPH +/- prvs and those numbers don't work for me.
    Thanks for the feedback.