Vacuum Breaker Design?

[jproj]

Are there any guidelines or calculations out there for the sizing of a vacuum breaker on a pressure vessel? The only thing I've seen are recommendations....

1) size to pass air equivalent to the vessel volume in 1 minute (say 50 ft3 vessel - size to pass 50 cfm).
2) size based on outlet capacity of the vessel:
45,000 - 100,000 lb/hr = 2" vacuum breaker (etc.)

The problem I have with the first criteria is the pressure differential... how do you go about calculating the air flow when you don't know the internal pressure (it's obviously not going to be constant)? The problem with the second is that I don't know how they cam up with the flow ranges.

Specifically, I'm looking for how to determine the required size to relieve the vacuum created from sudden steam condensation (assuming steam fills the entire vessel volume).

Any help / references are greatly appreciated.

jproj

 

[quark]

I will do it this way.

Total volume of steam will be the pressure inside the vessel times the total volume of the vessel(from this you can calculate the mass of steam inside the vessel). Check the specific volume of water at the operating conditions and so total volume of water. Now the difference gives you the aeration required in the vessel at the same rate of steam condensation.

This is the method I follow (mind you it is an approximation) with steam sterilizers.

Regards,

 

[jproj]

Correct me if I'm wrong, but isn't the mass of the steam inside the vessel just the vessel volume times the steam density at the operating pressure? --> ft3 * (lbm/ft3)steam = lbm (steam)

I take it that I would then multiply that number with the specific volume of saturated liquid at the operating pressure to get the volume of air required. --> lbm (steam) * (ft3/lbm)liquid = ft3 (air required to fill the void).

So, given this, I still only have a volume, not a rate. Is a 1 minute time frame reasonable? Also, once I do have a rate (volume/time frame), there's still the issue of either specifing a velocity through the vacuum breaker (which would let you determine the pressure differential), or specifying a pressure differential (which would let you calculate the velocity).... Any thoughts?

jproj

 

[quark]

You are right. I just complicated the method of doing the calculations. Directly calculating from density is the easiest thing. For volume calculation, you should consider the difference of tank volume and the water volume and not just the water volume.

You can calculate the vacuum created with isothermal conditions as an approximation. Otherwise, your designed negative pressure value should give you the pressure differential.

Is the condensation process so instantaneous? 1 minute can be ok, for aeration takes place as soon as the valve opens and this may not create problems with vacuum vis-a-vis the structural strength of the vessel.

Regards,