Vacuum system calculations 1

[keekooceeaou]

Hi,

I have a vacuum system for sludge draining.
The sludge from an open tank (so atmospheric pressure) is drained by rubber hose connected with pipe. Then it goes to liquid-vapor separator and the gas goes to the vacuum pump.

1. Assuming, that there is constant pressure of 0.1 bar abs. in pipe system, I need to find out at what velocity is the liquid flowing in rubber hose. How to calculate that?
2. When fluid level in tank lowers, the mixtrure of fluid and air will be sucked in. How should I calculate velocity then?
3. Finally, what if there is no liquid at all and only air is sucked into my system? How to calculate velocity of gas in pipe and pressure drops, at a given pump output?

Thanks from advance for any help.

 

[BigInch]

1. For pipe flowing full of liquid, use the Darcy equation and, I recommend, the Churchill friction factor equation, or the Colebrook friction factor, if you like doing iterations to find the friction factor.

2. With liquid and gas in in a pipe, you will have to resort to a 2-Phase flow method, such as the Lockhart-Martinelli correlation. The LM correlation relates 2 phase flows to 1-phase equation results, using correlations depending on various proportions of liquid and gas flow in the pipe and the hose configuration. Allow yourself a good safety factor in the prediciton of head losses and flowrates using 2-phase flow correlations.

3. Given a gas volumetric flowrate, always at a specific condition such as standard temperature and pressure of 15ºC and 1 atm, that volumetric rate must be converted to the volumetric flowrate at actual inside-the-pipe flow conditions, here, 0.1 Barg and whatever your particular (absolute) temperature may be. Assuming air at typical ambient temperatures and low pressure ranges, that can be approximated using Charles Law, so V2 = V1 * P1 * T2 / T1 / P2. Once you have the converted volume at STP to actual temperature and pressure, you can divide that volume by the cross-sectional inside-area of the hose to get the actual gas velocity inside.

BigInch-born in the trenches.

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