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Thursday, January 3, 2008

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There was a interesting question raised elsewhere and i find it interesting to share with my lovely reader...
Maximum recommended wall temperature (Tw) for carbon steel vessel would be about 593 C or 1100 F as quoted in API Std 521. This value is material dependent.
Would it be accurate to say that the high "Design Temperature" could/should be used for Tw ?
My opinion would be :
There are many factors affecting fire type, heat flux, etc and wall temperature would be varies.

Heat Flux Varies
Pressure containing equipment exposing to fire, the wall temperature is very much subject to type of fire, relative distance, momentum of fire, how a fire flame impinging equipment, what type of material on fire, etc and the results is heat flux would be localized.

Low Heat Transfer
Low heat transfer between wall and vapor would leads to wall temperature far higher than the gas temperature.

Environment factor such as humidity, wind condition, etc affect the heat transmission.

As above factors are difficult to quantify, thus wall temperature of equipment would be difficult to be estimated. Personally i would always recommend to put extra efforts in fire detection, prevention and evacuation of risk instead concentrating in heat flux, wall temperature for gas expansion case. Read more HERE

However, for Pressure relief device load, i would consider to reduce the set pressure as low as reasonable possible and use the conservative approach such as taking the gas temperature at relieving condition even though it is higher than the nominated design temperature, worst F factor, etc.

It is not cost effective to consider maximum recommended wall temperature (Tw) as design temperature of the system. Otherwise 90% (if not 100%) of plant design temperature would be maximum recommended wall temperature (Tw).

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posted by Webworm, 10:46 PM


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