Design pressure of vessel

[AtleK]

A short question:

I'm considering design pressure of various vessels in an oil&gas utility system. The vessel in question is protected by a pair of PSV's which relieves to flare. The flare system has a design pressure of 10 bar. Should the vessel's design pressure be set to the same pressure, 10 bar? The inlet to the vessel will have a maximum of 4 bar, a recycle flow from the vessel and back via a pump and a heat exchanger can make a delta P of 4 bar.

Anyone who knows where to find some general guidelines on design pressure?

 

[MJCronin]

ASME VIII requires vessels to be protected by a qualified pressure relief device.

The device must be set at the MAWP of the vessel. The MAWP is defined in Section VIII

The vessel must be designed to the MAWP (Maximum Allowable Working Pressure) as specified by the vessel purchser.

http://wiki.answers.com/Q/What_is_Maximum_Allowable_Working_Pressure_definition

 

[MJCronin]

Consider a re-post in the Boiler & Pressure Vessel forum....

 

[AtleK]

Thanks MJCronin, I have read a few standards on this and believe I have a fair understanding of the subject MAWP/design pressure - I was searching for a more philosophical approach perhaps; that the flare system determines the design pressure of a vessel.

And it must be so, the flare system may very well be at 10 bar, and 'my' vessel would need to relieve the pressure build up in it.

So, I have looked more closely into the ASME VIII and also API 520/521, but I am not at ease yet. How will the design pressure I set in one place affect another area? How can I optimize the design?

 

[europipe]

You have to set the safety pressure to the max. design press. of your vesselsystem.
The des. press. of the flareheader is set to the highest of one of the other users (that 10 barg.).
Mostly the flarehdr. has an oper. press. of 0 to 0.5 barg.

 

[chemebabak]

Atlek,

Your question is, "What criteria should I use to specify the design pressure of a new vessel?" This is completely independent of the flare header design pressure.

This design pressure is specified based on the maximum pressure that the vessel will see in process. That is, if the maximum pressure that the vessel will operate is ~15 barg at 350°C, then the design pressure would be 15 barg at 350°C plus a design margin on pressure and temperature. There is no rule for defining a design margin. This is arbitrary and should be based on your best engineering judgement.

The design pressure and temperature are used to calculate the vessel wall thickness. If the margin is too great, then the wall could be much thicker than is required. In other words, the wall could be much more expensive. If the margin is too low, then the fluctuation of the pressure might increase enough to open the PSV. A general rule of thumb that you could consider is to add a 10% margin for pressure and 30°C for temperature, however you should consult with an experienced engineer before making a decision.

 

[AtleK]

chemebabak: then I am very relieved (oh, yes) - my consideration was that, since the vessel could see 10 bar in the flare system, the pressure inside the vessel could build up to 10 bar. Superimposed from the flare system! But since it is completely independent, I assume the PSV acts as a non return valve. Or...?

That was the question that was hidden in my original post.

 

[sheiko]

I second Chemebabak.

Consider this resource:

http://www.standard.no/en/Sectors/Petroleum/NORSOK-Standard-Categories/P-Process/P-0012/

"We don't believe things because they are true, things are true because we believe them."

 

[chemebabak]

Atlek, please consider that I do not know much about your process or flare design. Since it is designed for 15 bar (225 psig) then I imagine that this is the HP flare header. Many flare headers only see ~15 psig. One reason that a flare header could have such a high design pressure would be in the event of a refinery wide fire. Some refineries build both a low and high pressure flare header.

For vessel design pressure you need to consider the design margin. That is, the PSV will not open on back pressure to damage the vessel. Nevertheless, for PSV routing to flare you should consider back pressure. Yes, the PSV will only open when the vessel pressure reaches the PSV set point, however the PSV could fail if the back pressure is too high. You need to consult with an engineer familiar with this refinery before selecting routing to the flare.