Process Vessel Design

It will only get to full vacuum if there is no liquid in it, Boss. Design for full vacuum.

I do not know so this is a bit of an educated guess and making a whole heap of assumptions.

If a vessel is designed for full vacuum it is capable of working down to full vacuum for an extended period. My guess is that this will involve significant safety factors for the welding and hoop stresses.

If a vessel is specified as Emergency Vacuum my assessment of that is that due to exceptional circumstances a vacuum may be drawn in the sour water flash drum. I would expect that in this event the process will be shut down and the vacuum in the flash drum relieved.

This then begs the question as to why two different approaches have been used.

Firstly cost - I would expect full vacuum to be more expensive in the build stage than emergency vacuum.

Secondly consequence - if you get a leak of air into a sour water flash drum it is annoying but not dangerous (I think). My experience in oil and gas means that when I see the phrases reflux / distillate drum I am thinking of hydrocarbons and leakage of air into a hydrocarbon vessel is obviously highly dangerous. If though these drums are part of the sour water stripper then maybe this isnt such a risk.

Some more information would be helpful

Has this plant been subjected to a formal HazOp Study? What does the study report have to say about the topic?

You need to speak to your process engineer to discover what exactly occurs when the sour gas 'flashes' in the drum, and the consequences/ effect on the process. Thereafter, you may wonder why you posed the question.

I have specified the design of many ASME VIII pressure vessels. I have participated in development of a design criteria for many types of equipment.

In the USA, under the ASME code, a vessel may be designed for a partial or "full-vacuum". But, it is unwise to design for anything but a full vacuum, for almost all vessels, in the opinion of many in this field.

The term "emergency vacuum" means nothing to the ASME vessel designer. He can use no higher allowable stress if the condition rarely occurs. The vacuum design will be the same if the vessel undergoes a daily vacuum condition or the once-in-fifty-year event.

It is important to understand how vacuum design is performed and the strong controlling effect of the vessel diameter.

Vessel design books discussing theory such as Bednar, Harvey and Moss are a good investment (pay for them !) if you plan to stay in this field.

I join the question

Thanks a lot for all the inputs.