Plant Blowdown Starting Pressure

[maddocks]

We've been instructed to blow down the facility to the lesser of either 700 kPag or 1/2 the design stress of the equipment in 15 minutes or less.

Does anyone know if the starting point is system design pressure (i.e., MAWP) or normal operating pressure? API RP521 is somewhat vague in this manner and it's not real clear. It does make a significant difference because of the large margin between normal and MAWP in some of the vessels.

 

[dcasto]

I'd design for the normal pressure. If it takes 16 minutes to get to the desired point from MAWP, so what. 15 minutes is just an arbritary number to begin with anyway.

There is more concern for rate to me. If you have to blowdown a million Kg in 15 minutes that is one thing versus 1000 Kg. Most chemical plant limit the rate to a point of either a flare limit or if to atmos, 10,000 kg/hr.

 

[maddocks]

Thanks dcasto - I'm with you on this one and was hoping you'd jump in on this discussion but I have no way of reaching you directly. I'm thinking the same thing - but the client is being somewhat conservative with design pressure as a starting point.

We're still limiting flare tip mach to 0.5 so no concerns there and we still do detailed piping pressure drop calc's to check backpressure on the system.

 

[Buchi]

Hi Maddocks,

I think the clients Design practice should take precedence on this. Exxonmobil, for instance, requires that you start blowing down from the maximum operating pressure. I have encounted clients to prefers PSHH set point, which can be like 90% of the MAWP.

But for compressors, you have to start from the settle out pressure.

Buchi

 

[MortenA]

I think it depends on if the blow down case is a fire case - if so normally pahh could be the starting point - and remember to allow for additional boil off. I think HYSYS can do this for you.

Best regards

Morten