Fluid Flow calculations

[Phil182]

I' looking at calculating the theoretical flow rate of water through a vertical pipe. This pipe is modeling the suction pipe of a vacuum truck. I'm doing this in an attempt to prove a higher vacuum in the tank is better than a higher flow rate of air being removed from the tank to create the vacuum.
I've attached a diagram with a simplified expectation of what I am expecting to find. Pump A cannot reach a high vacuum, but has a higher flow rate at free air, pump can achieve a higher vacuum but a lower flow rate at free air.
So I am using Bernoulli equation using the pressure difference as 1 Bar at the bottom of the pipe, and 100 mBar at the top, where the pipe enters the vacuum tank. This assumes that the pipe is totally filled with water. I'm trying to remember from uni what the effects of friction loss from the pipe walls will be. Is this the only other thing I need to take into account when finding the theoretical flow rate at the tank inlet? Does anyone know the formula for friction loss? Cheers

 

[pipesnpumps]

Phil,
You have the head loss at the suction hose inlet and the friction loss in the hose. Since the vacuum tank isn't full of water, there is no velocity related losses at the exit of the hose (it exits into the gas headspace).

Use darcy-weisbach to calculate the friction losses:

http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_646.html

 

[Phil182]

Thanks alot for that. I now have a new problem. I need to compare two pumps, and one will suck up water into the vacuum tank the same way as you would with drinking through a straw, it would all be liquid in the pipe. The other pump cannot create enough vacuum in the tank to lift only water in the pipe, and so has to allow air in the pipe aswell, much the same way as when you are at the end of your drink, and are sucking air through the straw aswell as the drink. Is it possible to calculate flow rate with a mixture of water and air? density would have changed, but I basically need to know how quickly the tank can be filled when lifting from different depths.

 

[BigInch]

2-phase flow is a bit trickey, becasuse you must know or assume some ratio of gas to liquid stream before calculating pressure drop. You might have to look at quite a few ratios from 0 to 1. There are a number of different 2-phase flow correlations you can use to get an idea of what you can expect to see as explained here,

http://books.google.es/books?id=e0G...v=onepage&q=2 phase flow correlations&f=false

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http://virtualpipeline.spaces.liv

 

[katmar]

You could lower the density in the riser pipe by bleeding air into it near the bottom, or you could use you vacuum pump as a blower and blow air into the bottom of the pipe. If you do it this second way (without drawing a vacuum in the truck) you would have to extend the suction pipe lower into the supply sump.

This type of application is known as an "air lift pump". Unfortunately it is empirical knowledge and it is hard to design an installation without having hands on experience of a very similar installation. Googling will find you a few references, and even some YouTube videos of air lift pumps in action.

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