2 replies to this topic

[rohollah]

Dear experts

As per fig 30 from- API 520 part1 seventh edition- back pressure correction factor for balanced bellows psv
with 21%overpressure is Kb=1
in the other hand the unbalanced portion of the disc can be affected by back pressure.
so, why Kb=1 for 21%overpressure?

regards
Rohollah

[rahul22yadav]

Dear,
I have tried to answer your doubt in two steps as mentioned below may be it may help you;
1. Balanced bellow PSV is suitable for application where the back pressure during relieving does not exceed 50% of the set pressure, hence the effect of back pressure (the effect of unbalanced portion) on PSV capacity is taken care of by correction factor upto 50% for balanced bellows(i.e. by increasing the orifice area). However beyond that (50% over pressure) the force on unbalanced portion of balanced bellow PSV will be higher and will considerably reduce the capacity of PSV.
2. For 21% over pressure that is for fire case Kb=1 upto 50% back pressure as per API (the balanced bellow PSV limit). For such a high over pressure the opening force on the inner side of disc (during relieving) is so high that the capacity of the PSV is not affected by the back pressure force on unbalanced portion of the PSV (upto 50% back pressure).

Regards,

[fallah]

Rohollah,

In the case you mentioned the overpressure value would be such that, let say as per manufacturers experiences, there is no closing force on the unbalanced portion of the valve disc, then the K_b can be considered equal to 1.

Fallah

Edited by fallah, 23 July 2012 - 02:26 AM.