4 replies to this topic

[aroon]

Hi all,

 

Is anybody having any information or basis of the following ASME criteria's?

 

Why is the allowable overpressure / accumulation over MAWP/set pressure only 21% for relief device sizing in the event of fire scenario? What is the significance of this number "21"?

 

Why is the allowable overpressure / accumulation over MAWP/set pressure only 10% for relief device sizing in the event of other design scenarios?

 

As per ASME Section-1 for boiler relief device sizing, the criteria is only 6%. Why it is so?

 

Any supporting help or the documents will be highly appreciated. Thanks in advance for your valuable help.

[fallah]

aroon,

 

What included in below link may help you out:

 

http://www.cheresour...v-set-pressure/

 

For 6% overpesure limitation in ASME Sec I which is for multiple PSVs, the same criteria of 21% overpressure in fire case can be applied; i.e. 1.03*1.03= almost 1.06

 

If you would need another clarification please let's know...

[aroon]

Thanks Fallah for your swift response.

 

Explanation of margin over the margin can be understood, but I would like to understand very basic of this concept since this concept is developed long back. Is the basis related to rule of thumb (3% and 10% over design pressure) or any study had been carried out by ASME to prove these margins?

 

Sorry to be very much curious to know about much details. However, my mind is not getting satisfied with these margin answers. Why 3% and 10%? Why not any other figures? Many of the things we have studied and experienced during our engineering tenure are having concrete basis, but I am not understanding why ASME didn't elaborate more on the basis of all such a things.

 

Best regards,

Aroon

[TS1979]

Allowable overpressure is a code requirement instead of rule of thumb. The overpressure requirement is more relevant to PSV vendors because PSV vendor must meet this requirement – need to be certified. I guess these numbers were based on very knowledgeable people’s experience.

 

The overpressure allowable is due to the characteristics of the PSV. No valve can go from full close to full open instantly. During the process of PSV lifting from full close to full open, due to the area limitation, the vessel pressure may continue building up. The code requires the overpressures for the blockage case 10%, fire case 21%, and boiler 3% of setting pressures.

 

Fire case in reality is a rare case, higher overpressure allowable will reduce the valve size.

[fallah]

Hi,

 

If a PSV being sized for pressure relieving equal to set pressure, the PSV (bigger in size) theoretically would be opened, without any overpressure in its upstream, till source of overpressure is removed. But the PSV may practically be operated in unstable manner due to probable flactuations in set pressure which easily affects balancing situation of the PSV in relieving condition.
 

Now looking at another view: when the vessel reach the set pressure and the PSV begins to lift, pressure is being relieved, so vessel internal pressure is going to be decreased but at the same time source of the overpressure is pressurizing the vessel! Then if PSV lifting has to be continued the system pressure must increase. For this reason the PSVs are allowed having an overpressure, let say 10%, to reach full lift. This experimental 10% margin for overpressure in ASME code is relatively small and conservative in safety standpoint, because up to 1.3-1.5 times of the design pressure yield stress wouldn't be exceeded and on the other hand such overpressure most likely would occur no more frequently than the hydrostatic test.

 

Thus, the PSV would be sized based on the same set pressure but with 10% (16% for multiple and 21% for fire) overpressure then relieving pressure to be 110% of the set pressure in PSV balancing situation and will lead to the smaller PSV and stable operation (without chattering and ....).

Edited by fallah, 27 February 2014 - 01:49 AM.