Combing flows before pumping

[snyper]

I have two tanks, one with slurry, one with water. I wish to combine the fluids and feed a single centrifugal pump.

The slurry flowrate is fixed, while water is added at a particular ratio to bring the mixture temperature and density below a set value. (Ratio is normally around 4 parts water to 3 parts water, but varies).

Ideally I would mix the slurry and water in a third agistated mix tank, and pump from there.

But is there an option to mix the fluids in-line? ie tee the lines together before the pump suction, and control ratio via an automated control valve (probably on the water line). Would the heads in the tanks need to be equal for this to work?

 

[jistre]

Sure. Static mixers. They're essentially a pipe spool with screw-like inserts in them and a side nozzle to introduce stream 2 into stream 1. As the fluid passes through the mixer, the inserts mix up the fluid with no moving parts. However, if you're not pushing the two fluids through the mixer with a pump on each stream, then you'll have a much harder time regulating the flow of each fluid due to variations in tank fill height, density, viscosity, etc. That alone may warrant the third mix tank.

Of course, mixing slurry may be too ambitious for a non-powered mixer. You'd have to check with the companies that make these. I just ordered some a couple of days ago from Chemineer.

 

[jistre]

Another option, without having to buy a new tank is to blend and agitate the slurry to the proper density and temperature in its tank by adding water directly to it instead of mixing in a third tank.

 

[JLSeagull]

With flow measurements, sum the flows and control the ratio using the flow through the valve against the total.

 

[snyper]

Thks for your comments. I am yet to check the effectiveness of in-line mixers under gravity flow, and with varying mix ratios.

Aside from the getting adequate mixing - one scenario may have my water tank significantly elevated above my slurry tank. How will the difference in heads affect the flow at my mixing point? Is it easy to work out what my ratio would be, given tank heads, densities, pump flowrate and pipe sizes?

 

[BigInch]

Yes its easy to calculate those things, but maybe not so easy to control. As your suction head changes with fluid elevation in the tank and the density and perhaps viscosity of the mixture being pumped changes, flowrate and power consumption will also change. On a constant speed pump, and if the suction head is kept constant, a new flowrate for a lighter, or denser and more, or less, viscous mixture will (probably) eventually be reached. You might be able to let the flowrate go up and down with the mixture, but if you can't allow that, you might need some type of flow control to maintain a constant flowrate, if that is important to you. If you expect very wide changes in density or viscosities, and a constant flowrate is needed, a variable speed control for the pump driver might be in order, for slighter variations in density a flow control valve could be considered to hold a constant flowrate.

http://virtualpipeline.spaces.msn.com

 

[dickon17]

To help visualize this problem, what ballpark flow rates are we discussing here? How is the slurry flow rate fixed? Is the slurry abrasive?