6 replies to this topic

[Sherif Morsi]

Hi,

 

When sizing PSV, we account for 10% overpressure for blocked outlet cases and 21% for fire cases. Why are these values set up as such other than they are stated in the code(s)?

 

Thanks,

Sherif

[fallah]

Sherif,

 

Which code(s) and how amount?

[Sherif Morsi]

Fallah,

 

in ASME, the overpressure for fire case is 21%. Also, it is a common practice to have the 10% and 21%. I am just trying to find out the core reason behind these values

 

Sherif

[fallah]

Sherif,

 

The allowable overpressure of 10% and 21% more than MAWP of a pressure vessel are code requirements (21% came from 10% overpressure more than 110% i.e. 1.1*110%) and common practices are following the ASME code in this regard. But 10% or 21% itself is the optimum value more than MAWP (might be a little bit more or less) prevents the pressure relieving devices to be larger than normal size also allows the maximum operating pressure of the pressure vessel to be close enough to its design pressure.  

Edited by fallah, 20 May 2014 - 12:33 PM.

[Sherif Morsi]

I believe the 110% is the 10% more than the MAWP. Right?

 

Why adding another 10% though?

 

Sherif

[fallah]

Sherif,

 

My answers are in red color as follows:

 

 

I believe the 110% is the 10% more than the MAWP. Right?

 

Actually, in general 110% is 10% more than PSV set pressure. If set pressure would be equal to MAWP, then 110% is 10% more than MAWP as maximum overpressure.

 

Why adding another 10% though?

 

For fire case another 10% to be added for lowering the PSV size because in most fire cases it can be supposed the engulfed vessel along with the PSV would be destroyed.

 

[Sherif Morsi]

Thanks Fallah