2 replies to this topic

[sakthivelg1]

Dear friends,

                  Please find in API 520 Table 5. In additional valve, since the set pressure equal to 105%MAWP, the over pressure is reduced to 16% from 21%. But relieving pressure is shown same for first valve & additional valve i.e 135.7 psia.

 

For Additional valve,

Relieving pressure should be 105 x 1.16 = 121.8 psig i.e 136.5 psia

 

Can any one, explain why is relieving pressure of both first valve & additional valve are shown same? .if the relieving pressure need to be same then, the over pressure should not be 16%. which is correct?

 

[fallah]

Hi,

 

The overpressure is the pressure increase over the set pressure of a PSV up to 1.21 times of MAWP for fire contingencies.

 

Relieving pressure is equal to the valve set pressure plus the overpressure.

 

Then the maximum relieving pressure is fixed to 1.21 times of MAWP and when the set pressure would be increased to 105% of MAWP, it is obvious that the overpressure should be reduced to 16% from 21% (1.05*1.16= almost 1.21).

[aroon]

Ignore 16% term, which is actually rounded value. Believe on the term 21% as per ASME and shall be based on lowest set (Primary) PSV (if second PSV is 105% of primary PSV). Use relief pressure (121 psig = 135.7 psia) for sizing and calculations of both PSVs. Main thing is PSV full lift is generally achieved with overpressure range of around 7 to 10% and this additional overpressure is actually the credit as per ASME based on contingency to avoid oversizing of the valves ...

 

In such a type of cases, always try to think conservative with respect to safety and avoid taking additional credit over an above ASME limit .....