safe operating pressure vs. ASME MAWP rating 3

[StockTank2]

i have a vertical suction scrubber on a nat gas compressor unit; can I run at or below MAWP? or need to operate at RDP (about 90% of MAWP) - thank you.

 

[GenB]

You can perfectly run at MAWP if you do not have to deal with a Safety Relief Valve opening at that pressure, else you have to run safely under the safety valve setting which cannot be above the MAWP.

 

[LittleInch]

Also depends on the number and accuracy of any instrument pressure alarms, trips and the degree of pressure fluctuation. The main issue normally is as GenB notes, is the accuracy of the set point of the Pressure relief valve.

BTW what is "RDP". Not an abbreviation I've come across before. - Reduced duty Pressure??

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[viper3982]

RDP is a common term for oil industry. Rated Discharge Pressure. You have to use a table in the API 11P based on MAWP.

 

[moltenmetal]

If the device is an ASME vessel, it must be protected by a relief valve set no higher than the MAWP. Maximum operating pressure is therefore a fraction of the MAWP determined by the nature and performance of the relief device. At 0.9 x MAWP, relief valves typically start to "simmer" and generally process units can't tolerate that leakage. A prudent margin will vary by the nature of the process and the hazard associated with the leakage.

 

[StockTank2]

thanks y'all very much; on cylinders we are advised to replace spring-loaded with pilot-operated PSV's, i just wasn't sure if there was protocol common on other vessels.

 

[StockTank2]

moltenmetal (et al.) one follow-up; if we neede to run for example 620psi (on very steady regulator) into a 645 MAWP vertical scrubber and cannot tolerate any leakage via PSV, any design suggestion or is it just not feasible; thanks

 

[moltenmetal]

If it is an ASME VIII vessel and requires a relief device, then the answer to that question is no- it is not feasible. It is too close for any relief device you may select.

 

[LittleInch]

stock tank - you will need to get a very accurate pilot operated relief valve. If you allow 15psi for the relief valve set point under tolerance, you're talking about 2%. Not many suppliers will guarantee that, but you might find one if you look hard enough and pay enough money. The best I've normally been able to get is 3%.

It is very tight though so although theoretically feasible, it may not be practical to apply it when you consider any alrms or trip settings as well.

600 or 610 psig might fly, but 620???

viper - thanks - I'm in the oil game but never come across that term before. Don't understand why you don't use MAOP or MOP, but that's how it goes sometimes.

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