6 replies to this topic

[QuantumG12]

Hello, 

I have ran into a problem choosing a set pressure for a PSV on a KO vessel. The vessel has a demister pad, so a relief device is suggested as per API standards.

 

We have ruled out an overfilling scenario due to the presence of liquid level alarms and drainage, and the only possible overpressure scenario is a blocked-in flow. 

 

Operating P = 55 psig

Operating T = 125 F

MAWP  = 270 psig

 

Now it would make sense to have a set pressure at the MAWP of 270 psig. However, the fluid to be relieved condenses at approximately 205 psig. At this point, the presence of liquid in the vessel would activate liquid level alarms and shut the process down (the process would shut down before ever reaching anything close to the set pressure). 

Should the PSV still be sized at a set pressure of 270 psig for a flashing fluid, despite the fact that it would realistically never see that scenario? Or should the set pressure be lowered so that it can be sized for a more realistic overpressure scenario (vapor phase only), even if it is below the vessel MAWP. 

 

Thanks in advance.

[Bobby Strain]

We can't offer any advice with such scant information.

 

Bobby

[latexman]

With blocked-in flow, is pressure rise relatively fast such that heat transfer is limited and adiabatic compression is approximated? If so, temperature will rise. Or is pressure rise relatively slow such that heat transfer occurs and isothermal compression is approximated?

 

Does/can the source of the pressure exceed 270 psig?

[QuantumG12]

With blocked-in flow, is pressure rise relatively fast such that heat transfer is limited and adiabatic compression is approximated? If so, temperature will rise. Or is pressure rise relatively slow such that heat transfer occurs and isothermal compression is approximated?

 

Does/can the source of the pressure exceed 270 psig?

Latexman,
I appreciate the quick response. 
For this scenario, isothermal compression is approximated, so the operating temp of 125 F is considered the relieving pressure. The source pressure does not exceed 180 psig.

Thanks.
 

[QuantumG12]

We can't offer any advice with such scant information.

 

Bobby

Bobby,
 

Please refrain from commenting on the forum if it provides no academic or professional value. Many of us are on here trying to better our engineering practices and learn from professionals like yourself but are instead met with dead-end responses such as this. 
It would be most beneficial to ask for what additional information is needed to make progress on the topic, or perhaps ask for clarification if I failed to communicate my question effectively.

Thanks.

[latexman]

The source pressure does not exceed 180 psig.

If the source pressure cannot exceed MAWP (270 psig) there really is no sizing scenario, right?

So if "the only possible overpressure scenario is a blocked-in flow" and that scenario proves to not be a sizing basis, that leads to somewhat of a dilema. One either has to provide a PSV per ASME Code or meet ASME UG-140, Overpressure Protection by System Design, which, IMHO, is much more onerous than providing a PSV.

Is there no fire case? In my world, something that volatile is usually flammable or combustible. Will the KO vessel ever be solvent washed for some reason in the future? Or put into another service, perhaps in the service of the most abundant flammable or combustible on the plant site? Sorry, with no details, I'm grasping at straws here, but this is what I do when trying to forecast the unknown future use of a piece of process equipment.

[Bobby Strain]

Just as you are free to seek advice, we are free to give it. Sometimes you won't like it.

 

Bobby