Pressure Relief catch tank 2

[tmagee]

I am having trouble finding requirements for sizing a Catch Tank. I have been asked to replace an existing atmospheric catch tank which has 2-6" incoming headers, each collecting rupture disk discharge lines from numerous reactors of various sizes. The tank has an 8" vent to atmosphere. Is there a code or rule to follow? 1 1/2 times the largest reactor? NFPA 30 does not reference sizing. Thanks,

 

[Chance17]

There is no simple answer to your quesion.
If rupture disk activation causes the reactors to empty into the tank (disk at bottom of vessel)- then 1.5 reactor volume is an OK design.

Typically one looks at the contingency that causes the rupture disk to activate and make sizing decisions based on actual relieving rates.
Also typically the disk is at top of vessel.
For example assume liquid overfilling by a pump with 100-gpm capacity causes the disk to activate.
My current client assumes 30-minutes fill time.
This is the time assumed for operators to detect the problem and take corrective actions.
So the drum size would be 3,000 gallons with extra allowances for the heel and top (basically 85% working volume).

Another factor to consider - Is it objectionable for the tank to overflow ?? It is usually very objectionable.
I'd be very liberal with design factors to make the tank bigger.

In my experience I have seen several problems with atmospheric tanks being overpressured by safety device activation.
If the tank is rated for several inches of water pressure, it is easy for rupture disk activation to have enough vapor load to overpressure the tank.
I would recommend using a tank with several psig rating.
50-psig would be ideal.
Make sure the vapor can get out of the 8" vent without overpressure of the tank.

 

[StoneCold]

Yitbos
Has given you good advice. He has covered about all that I would tell you. Make sure that you have a drain that you can use to get the material out. Possibly a solvent addition nozzel to flush the tank and a manway would be nice. Our units are 1.5 x reactor volume and 50 psi. We also have cyclones on the top to separate gas / liquid and vent to a 50 ft tall stack.

Regards
StoneCold