Vacuum Breaker on Bath Heaters/Expansion Tanks

[AkDsMechEngr]

I am trying to relocate an existing vacuum breaker from the bath on a shell and tube heat exchanger to the expansion tank. There are no calcs that I can find for the previous VB installation and no specific guidelines in ASME Section VIII. There is a blanket gas system that maintains 15 psig on the expansion tank. I am trying to figure out if I can change the VB size from 1-1/2" to 3/4". I've tried running calcs to determine my capacity, but I'm not sure what my P2 is for the calcs to determine required flow rate. Anyone have any suggestions?

 

[StoneCold]

AkDsMechEngr
What you need to do is determine how much vacuum your tank can take. That is your minium P2 (maximum vacuum). Then you can choose a comfortable P2 and design the VB flow to keep the tank at this pressure or higher. The more involved part of the calcuation is determining the Rate of flow required to avert the vaccum condition. The rate of flow of a VB may be due to rapid cooling of a vapor or flow of material out of a tank with no way to replace the volume removed. In your case it will depend on what scenario you are looking at.