"Re-rating" a non-pressure vessel

[boofi]

Hi all,

I've got an induced air flotation device to skim oil from our waste water. It's a standard design, almost like a skid, installed about 30 years ago. There is no documentation save for a drawing, no stamp, and no data sheet. I asked the OEM if they had any calculations or anything and they've come up empty. It does have an MAWP but it looks like it was written in. 1/2 oz. per inch squared, which is like .031 psig. We're guessing it used to be rated for atmospheric, but some peripheral piping was upgraded that required a higher MAWP of the IAF. Maybe. I've been asked to evaluate if we can re-rate this IAF as the operations folk are operating above the prescribed MAWP anyway. However, as this thing was built to no code and no calculations exist, I'm not sure where to start. It's not even circular or rectangular. The cross section has multiple vertical/horizontal sections and corners. I looked at API 510 section 7.7 but even then it says "3) Perform design calculations based on applicable codes and standards." Can't do that since I don't know what code to go by. We could do an FEA analysis if we contract it out I guess. Does this thing even need to have a rating? Anybody have any advice?

Thanks for reading my ramble!

 

[marty007]

You say the operations are running above the labelled MAWP. What pressure are they operating at? 1psi? 10? 100?

 

[boofi]

lol they're running about around 1-3" WC. So like 0.072 psig. I feel like these numbers are so small they're negligible. I'm fairly new to the job but how is it even possible for an instrument to get accurate readings that small?

 

[EdStainless]

Considering that variation in atmospheric pressure is often +/-0.2psia (not counting storms) it makes me wonder what you are actually measuring.

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P.E. Metallurgy, Plymouth Tube

 

[Caloooomi]

Did you build it yourselves for your own use?

 

[LittleInch]

This would not fall under anyones definition of a pressure vessel. Therefore while I have no idea what you are looking at, the best alternative is some sort of proof test. Either shut off all the inlets and outlets and pressure to 1.5 times what your max pressure is or fill it above your max level to induce the same sort of pressure ( a few inches wc). Then you can safely "certify" its ability to withstand the max operating conditions

Yes you can get instruments to measure inches WC. All pressures are guage so it doesn't matter what the atmospheric pressure is, its the difference to atmospheric pressure that you're measuring.

Or just not bother.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

"how is it even possible for an instrument to get accurate readings that small?"

You can do it with a ruler. I'm no longer surprised to meet young engineers who do not know what a manometer is or how to read one. Manometers are why we use inches of water as a pressure unit. Bear in mind that if your tank is one foot tall and full of water, the pressure at the bottom of the tank is 12 inches of water. Understanding small details such as this is what leads to "good engineering judgement". For future reference it would be good to always remember that 1 psi is roughly 24 in. H2O, and when you think this there should be a picture in your head of something full of water 24" deep, not a picture of a number written on paper.

https://en.wikipedia.org/wiki/Pressure_measurement.