The plant's compressed air system o

[narivich]

Greetings.
The plant's compressed air system operates at 6.8 bars. A new air receiver was built to ASME code for 100 PSI AWP and tested to 150 PSIG. How much can it be operated away from the 100 AWP, towards 150 PSI? Considering 1 bar diffential for cut-in/ cut-out.

Thank you

narivich

 

[The Obturator]

The answer is the AWP at 6.8 bar (should read MAWP = Maximum Allowable Working Pressure). It was probably hydrostatically tested at x1.5 as per ASME VIII. You should also have a Pressure Relief Valve set at the MAWP to prevent overpressure.

Because it was tested at x1.5 MAWP with a different medium, does not mean it is cleared to work at that pressure (are the other components in the system qualified for the higher pressure?).

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[marty007]

If the vessel is stamped with an MAWP of 100 psi, then you are not allowed to operate above 100psi.

The relief valve should be set to relieve at 100psi.

This is a fairly low pressure, and the vessel is likely able to operate at a pressure above 100psi. However, to do this you would need to have the vessel re-rated for a higher MAWP. This would involve preparing calculations for a higher pressure to ensure nothing needs to change with the geometry, re-hydrotesting the vessel, then having the re-rating documented (in North America this would be done as an alteration under the rules of the National Board Inspection Code (NBIC), and signed off by an Authorized Inspector).

Cheers,

 

[marty007]

One observation - ASME VIII-1 only requires the hydrotest to be performed at 1.3 x MAWP x stress ratio.

As this is an air-receiver, I assume the design temperature is fairly low, so the stress ratio is likely 1.0. The original hydrotest likely only needed to be done at 100psi x 1.3 = 130psi.

Cheers,

 

[MJCronin]

Was the "brand new" air receiver purchased by a Certified PMP or MBA or some other member of plant management ?

Can you get a "trade in" for a receiver with an MAWP of 150 psig ?

If you decide to use the 100 psig MAWP air receiver, what are you going to do about the safety valve that will be constantly lifting ? You cannot install a 150 psig safety valve without risking an accident ....


MJCronin
Sr. Process Engineer