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MAWP Temp and Pressure

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fgdchile

Mechanical
Jun 22, 2016
1
Recently at one of our plants we had a 5 year PHA revalidation. During this PHA the 3rd party facilitator question one of our vessels that operates at 780psig @108F to 115F base on ambient conditions. The name plate on the vessel indicates design condition of 1440psig @ 100F. What should be the right course of action for this vessel, is rerating the vessel to cover the 10 to 20 degree difference in temperature required? the relief valve on this vessel is set to 950psig.
 
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Re-rate the vessel based on a revised temperature. It is unlikely that the design conditions will need to change much based on allowable stress. However, you may need to de-rate the pressure based on the flange pressure rating - I am guessing that you are using stainless steel flanges (1440 psi @ 100°F is the flange rating for Group 2.1, 2.3, 2.4, 2.5, 2.6, 2.7, 2.9, and 2.11 materials). It's purely a paperwork exercise, but should be completed for due diligence.
 
Although TGS4's advice is sound, there is a possible alternative: Chances are that the temperature being measured is the process fluid.

Now for the assumption, since this was not clearly stated in the original post: Let's presume that this is a VIII Div. 1 vessel. I'll also presume that the vessel is not insulated.

Keep in mind that the design temperature for a Div. 1 vessel is the average thru-wall temperature of the steel. [Note that this is not the case with all codes, e.g. B31.3.] Thus, depending on circumstances you may find that you can measure the outside metal temperature of the vessel (after sun exposure on a hot day), assume that the inside surface metal temperature is the same as that measured for the process fluid, and average the two. In some parts of the world this will make things worse, in others it will make them better.
 
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