2 replies to this topic

[Azam]

I wish to obtain information on sizing criteria for vacuum breakers on vessels.

Any inputs in this regard would be highly appreciated.

[hollerg]

The proper criteria is determined by the outflow rate and the allowable pressure differential

A useful approach is to size the nozzle as large as the largest outlet nozzle. Be sure to consider the use of the largest nozzle low on the vessel being used as a drain. Often folks are in a hurry to drain the tank, after filling up the vessel for pressure check and may use a valve not designated as the drain.

1) The largest outflow rate usually will be the gravity drain rate not the maximum pump rate from the largest nozzle.

2) I usually check that rate against twice the maximum hydraulic rate with control valves having max trim or end of pump curve for the maximum impeller

3) Be sure to consider the case where a source of vacuum (other than draining the tank may occur). For instance a pump may normally keep positive pressure in the line between the tower's sump and the recieving tank, however if the pump goes down, a backflow can occur. Once the line is clear of liquid, flow will usually go sonic in that line.

4) Add the normal withdrawl rate with the rate of inbreathing from diurnal cycling.

Note that groth makes a freely downloadable program that will compute the size for items 2 and 4. You will need to perform 1 and 3 yourself to see what is the controlling rate.

Vacuum breakers should provide the required capacity at the vacuum rating of the vessel(api tanks primarily), unless you are certain you have a safety factor. This means that the set point will not usually be equal to the design vacuum rating.

[Azam]

Dear Hollreg

Thanks for your detailed input. I really appreciate that. Thanks once again.

m_azam