4 replies to this topic

[ayan_dg]

For a single relief valve protecting a pressure vessel , the maximum allowable accumulation is 121% of vessel MAWP for relief due to fire and 110% of vessel MAWP in all other relief scenario. My question is why for relief due to fire an allowance of 21% provided , why it is not 110% for all cases ?

[ankur2061]

ayan_dg,

The links below should give you the answer:

http://www.eng-tips....=213081&page=13

http://www.cheresour...sure-fire-case/


Regards,
Ankur.

[fallah]

ayan_dg,

Actually, PSV orifice size for 21% overpressure is a little bit lower than the size for 10% ovrpressure with the same relief load.

Because in most fire cases it is assumed the relevant vessel is going to be failed at the end of the event, this lower size would be considerable in economical point of view. As a matter of fact, keeping spare PSV of fire case in warehouse supports this concept.

Anyway, I think 121% for fire case originates from considering 10% overpressure in addition to first 10% overpressure to be considered in the cases other than fire; i.e., 1.1*110% = 121%.

Regards

[ayan_dg]

fallah:

Yes, I agree with you that in Fire case the orifice size will be less than the other cases, provided relief rate is same in all cases. My question is exactly that. Suppose the vessel MAWP is 100 barg (also, it is the setpoint of the PSV). In fire case, one will allow the pressure inside the vessel to go up to 121 barg and in other cases one is not allowed it to go over 110 barg. What is so special with fire case? If the vessel can sustain a 121 barg pressure during fire case, it will be able to handle that pressure in other cases as well.

[fallah]

In both cases PSV begins to open at 100 barg (set point) and the difference is just in full lift point that in fire case would happen at 121 barg and in other cases at 110 barg.

Actually, assuming the vessel being isolated in the case of a fire, we have no inflow and a heat source provided. The actual flow of the relief will vary with time, but will always need more obverpressure (21%) than normal overpressure condition (10%) to keep the PSV open; otherwise, we may face with repeated opening and closing leading to the PSV to chatter.