3 replies to this topic

[surendra12564]

ASME SECTION VIII DIV 1 stated an 10 % allowable over pressure over set pressure to achieve full lift of a single relief valve for blocked case . If i set the set pressure as  maximum allowable working pressure(MAWP), the accumulation and over pressure is same and it is 10% over MAWP. 

My question is, the system is designed for MAWP and we are allowing an accumulation of 10% over MAWP to achieve full lift. How does the system can take this excess pressure over MAWP , if at all thickness is governed by only MAWP at co incident temperature.

For example  my system is designed for a pressure of 10 kgsc and 120 degree Celsius. and thickness calculations done accordingly. I want to protect the system for blocked case, i installed a single relief vale  with a set pressure of 10 kgsc. As i said ASME SECTION VIII DIV 1 allows  an accumulation of 10% over MAWP for full lift, for this example it is 11 kgsc. But my system is designed ( Thickness calculation) for only 10 kgsc, how can the system will take 11 kgsc pressure ??

[latexman]

ASME Code has a safety factor of 3-4 built into it.  Don't worry; be happy. 

[Art Montemayor]

surendra:

 

I highly recommend that you take Latexman's concise and terse response as an important one.  His comment is meant to be taken in jest.  He is precisely correct in his statement and I only take this time to add to his valuable advice.

 

It is very important that you learn and apply what his statement and recommendation implies.  I am certain that he is assuming that you have invested as much time and effort in the application of safety relief devices as he has, and for that reason I am attaching a document I prepared some time back in replying to a student that had problems with the same topic.  You rightfully show concern for the accumulated pressure realized in a pressure relief valve and you correctly give importance to the MAWP.   However, I believe you lack the knowledge of  how the MAWP is calculated and under what correct basic data and assumed conditions.  You also seem to not be aware of the usual and normal way a pressure vessel is subjected to a required hydrostatic test.  A hydro is far in excess of the MAWP.  So, can this be???

 

Experienced and knowledgeable engineers, such as Latexman, don't have disdain for the value of a pressure vessel's MAWP.  On the contrary.  They are aware and mindful of the precise conditions under which it is calculated, its specific intentions, and how it applies to engineering design and operations.  Please read the attached and you will further appreciate what he has stated.

 

 MAWP as Applied to PSVs.docx   107.56KB   146 downloads

[surendra12564]

Thanks to both of you for throwing light on the subjected matter