Here is something that I learned. This is a part of an
article that I read at
http://www.cheresources.com/asiseeit1.shtml. Visit the
link for the full article.
"MAWP and Design Pressure
In paragraph 1.2.3.2 (b), API 520 defines maximum allowable working pressure (MAWP) as
"⦠the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure."
The operative word here is "completed". The vessel is completed when a fabricator, according to the code laid down by ASME, has designed it. The vessel's fabricator, not the Process Engineer, determines MAWP. (Some may try to stretch my definition of "completed" to mean that the vessel is also erected in place. Not quite because the certified vessel drawings, which are delivered way before the vessel is, contains this information).
In the same paragraph, API 520 says that the MAWP is normally greater than design pressure. The Process Engineer usually sets the design pressure at the time the vessel specification is being written. The design pressure is the value obtained after adding a margin to the most severe pressure expected during normal operation at a coincident temperature. Depending upon the company the engineer works for, this margin is typically the maximum of 25 psig or 10%. The vessel specification sheet contains the design pressure, along with the design temperature, size, normal operating conditions and material of construction among others. It is this document that will eventually end up in a fabricator's lap and from which the mechanical design is made."
Hope this helps.
Best Regards
Kannan
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