P trap under vacuum

[bhooper]

I am working on a project right now involving a mist collector and a p trap. The blower on the mist collector is pulling 20" vacuum. The problem lies in this p trap setup. My co-workers and I cannot come to a consensus as to what the proper proportions should be to prevent the drain from backing up and spilling inside. At current the theory is that the difference between the inlet and outlet is 110% of the vacuum level, and that the height of the upstroke should also be 110-120% of the vacuum level. My personal thoughts have been that the 20" of vacuum is displacement and that over 20" on the inlet side should hold regardless of how much fluid is in a zero state.

Any help is appreciated, I'm tired of this debate...

 

[rmw]

Twenty inches of what vacuum? 20" WC? 20" Hg? 20" of what ever liquid the moisture separator is collecting? If the fluid in your "P" trap is the same as the fluid of your measurement system, then 20 inches of P trap plus a little (10% extra) should be just fine.

If it is 20" of mercury, you better get your Fluids text book and start reading up.

rmw

 

[bhooper]

Sorry, it's 20" of H20, or rather a water/oil mixture that is something close to 80/20.

So the 20" needs to be above where the water would be at atmophere correct? Does it matter how much is stored in the bottom of the U portion assuming that there is another 22" to go vertically?

 

[rmw]

Not knowing the density of the oil portion of the 80/20 mix, it is hard to determine, but you should be able to calculate it. If it were, say pure oil, and an oil with a sp gr of 0.7, you would have to increase your column height by 20/0.7 to more like 28.5 inches. So your answer is some where in between depending on the properties of the constituent fluids and the resulting overall Sp Gr.

rmw

 

[bhooper]

I am told that the combined fluid has a close enough specific gravity of 1.