4 replies to this topic

[Luigi]

I have to add an air receiver to the air system of the plant, to increase the volume of the system, and I need some help in sizing the safety valve for the receiver.

In general, I think that the design criteria for the safety valve for an air receiver would be to pass the compressor air capacity in case the control system fails to stop the compressor when the pressure is reached. Is this true?

Under this assumption, the case of failure in the control system is already taken care of by the existent safety valves. Should I assume that at the same time, the vessels (there is more than 1) will be blocked from the system and I should provide relief capacity for the whole system? If the latter is true, would that mean that if I add a compressor, do I need to change the relief valves of all the receivers to a larger capacity?

A consequence of the above mentioned is that the supplier of a compressor package, would need to have information of any existing installation to provide the right relieving capacity.

Thanks for any input.

[Art Montemayor]

Luigi:

The rules, standards, and most often the law state: A pressure vessel (particularly an ASME stamped vessel) has to have a pressure relief valves attached to it to protect it from over-pressure.

Additionally, in the case of a reciprocating compressor, the controlling, worse case scenario for an over-pressure event is the “Blocked Discharge” case where the compressor is kept running while the discharge piping of the machine is blocked off (usually by a shut off valve).

If you have a properly-engineered existing air supply, the discharge of the existing compressor is already protected by a PSV sized to relieve the total operating capacity of the compressor. If you add additional air receiver to the air system but keep the same compressor capacity, you are already protected by the existing PSV. However, since ASME states that you must install a PSV to protect each vessel, you must either put another PSV on the new air receiver or provide calculations that show that the original PSV can protect all the air receivers – something I don’t think you want to do. It’s far more practical and expedient to simply place a separate PSV on the new air receiver, set at the MAWP (which I believe is the same as the existing air receiver MAWP). You really don’t need the second PSV, but you may have to obey the ASME ruling – which is instituted for a very good reason.

The safe relief capacity is the same whether you use one air receiver or one hundred. It’s the capacity of the compressor that you are relieving, not the capacity of the air receivers. Therefore, if you add additional compressors, you must add additional PSV relief capacity – either larger PSVs or more PSVs.

[Luigi]

Thank you for your answer.

[pleckner]

I would make the design pressure of the receivers 10 psig or so higher than the comperssor PSV set pressures and have this new PSV set at this design pressure. Unless you can determine any additional relieving scenarios for the receivers (perhaps they are in a fire zone), then just put on a minimum PSV (3/4" x 1") and be done with it and call the scenario thermal expansion. In reality, you'll never have to be concerned about them again except for standard preventive maintenance and inspection.

Art is correct in that if this is a stamped vessel then it requires protection. You have several options but the least expensive and less time consuming is what I've discussed above.

[JoeWong]

I would make the design pressure of the receivers 10 psig or so higher than the comperssor PSV set pressures and have this new PSV set at this design pressure. Unless you can determine any additional relieving scenarios for the receivers (perhaps they are in a fire zone), then just put on a minimum PSV (3/4" x 1") and be done with it and call the scenario thermal expansion. In reality, you'll never have to be concerned about them again except for standard preventive maintenance and inspection.

Art is correct in that if this is a stamped vessel then it requires protection. You have several options but the least expensive and less time consuming is what I've discussed above.

I would consider the system as follow :

Reciprocating Compressor
PSV at reciprocating compressor should be designed for Blocked Discharge full flow relief.

Centrifugal Compressor
Compressor chamber and discharge separator will be fully rated and PSV at centrifugal compressor may only design for gas expansion.

So for both cases, we don't have to worry about it.

As for PSV on the Air receiver, i would agree with Mr Montemayor existing receivers will have it own PSVs for protection. No much worry. However, the air receiver may be large (subject to demand and storage time), the PSV may needs to design for gas (air) expansion in case of fire (unless fire scenario has been ruled out).

I would make the air receiver design for at least 105% of the comperssor PSV set pressure and have this new PSV set at this design pressure to avoid both PSVs chatter.

regards,

JoeWong