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Asme Versus Api


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#1 kdal

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Posted 14 December 2006 - 09:58 AM

Am sure that this has probably been asked and discussed before but please excuse my ignorance.
I have a PSV datasheet which specifies the required relieving rate in mass flow rate (kg/h). This then leads to the calculated orifice area and thus the selected API orifice and then the rated capacity is backcalculated for the selected orifice size and entered (again in mass flow) on the datasheet.

I also have the certification for the same PSV but the flow units are volumetric (litres per min). It is certified and stamped (I believe) ASME code.

Now the problem for me is that it is not easy to show that the certification is consistent with the datasheet because the units are inconsistent. The explanation from the vendor is that the ASME certification is done to the standard unit of measurement as specified by ASME (in this case l/min for gas).

Is this normal industry practice? Does this now mean that I have to convert units back to kg/h to confirm that the certification is correct? Seems to me like the certification should be transparent to any layman looking at it and it shouldn't require additional calculation to determine whether it's consistent or not. Any thoughts?

Cheers

#2 pleckner

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Posted 14 December 2006 - 07:15 PM

I can answer all the questions in your post all at once by simply saying "YES" to all of the above.

You can easily change your calculations to volumetric units as you should know the density of the relieving fluid at relieving conditions. ASME also shows you how to make your gas equivalent to the standard test medium, which just happens to be air for systems relieving any vapor/gas other than steam. If you have a copy of the ASME BPV Codes you will find this procedure. If not, ask the vendor to send you the procedure; it really isn't complicated and you will do just fine.

#3 kdal

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Posted 15 December 2006 - 02:10 AM

Phil,

Thanks. As a Process Engineer it's not a problem for me to convert between the mass and volumetric flows. My concern (and I should have made it clearer) is that I'm working on a job with several hundred PSVs and we're at the stage where the inspectors and QA folks are crawling all over the kit as it's being installed and they get very jumpy when (in their eyes) documentation doesn't tie up. I'm putting on my QA hat and thinking that if I saw a datasheet calling for X kg/h and then a certificate saying that the device was good for y l/min then I might be asking questions about the adequacy of the device. It might then be down to the Process guy to clarify the point for ALL such devices. Why is the certification not transparent? i.e. why isn't the test certificate in the same units of measurement as the vendor datasheet? Is this common? Vendor is telling me that the certification units of measurement are "standard" for ASME (and by implication will not be altered).

kdal

#4 pleckner

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Posted 15 December 2006 - 12:56 PM

The vendor is correct, ASME dictates everything about the stamp, there is no interpretaion or changes allowed from their rules. The stamped capacity is based on volume flow of air if the relieving fluid is a gas/vapor. If the relieving fluid is steam, then the stamp capacity will be in terms of mass units for STEAM. If the relieving fluid is a liquid, the stamp capacity will be in terms of liquid volumetric flow for WATER. This is just the way it is, sorry.

I can sympathize. I recommend you set up a spreadsheet and do a quick conversion.

#5 kdal

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Posted 18 December 2006 - 02:03 AM

Phil,
I feel a little sheepish now! Your reply, coupled with another discussion with our PSV vendor, has shown me the error of my ways. Of course, the ASME certification is simply the verfication of the bore size and what it can pass at the ASME test conditions and it's not at all related to the Process relieving fluid, capacity or conditions. Yes it is possible to do the conversion but it is unneccessary because the ASME certification is for orifice size and not meant to certify the vendor calc to arrive at the chosen orifice size. I feel a little dumb but slightly more knowledgable now. Thanks for your time.
kdal

#6 pleckner

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Posted 19 December 2006 - 07:33 AM

Don't feel sheepish. This is a confusing topic for those not knee deep in the details on a more regular basis.

But, I don't agree that the certification is not related to the real process conditions. To determine pipe sizes (3% Rule on the inlet to the PSV and for built-up back pressure on your discharge piping) you must use the rated capacity of the PSV in terms of your gas/vapor/liquid, not in terms of the test medium.

One last thing, I hope you gave the vendor the "required" orifice and didn't leave that calculation up to them! They should receive from you the required capacity in terms of flow and properties and the miniumum orifice size that will do the job. You can even specify the PSV you want if you wish.




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