Positive Displacement Pumps

[more22]

Colleagues,
I have a situation in which I am pumping slurry material with a positive displacement pump (progressive cavity type) into an existing header in which a centrifugal pump is already pumping water. So the combined stream will be a water/slurry mixture.
I am concerned the PD pump will cause the centrifugal pump to operate back on its curve and allow improper injection of the needed water flowrate.
Will the PD pump transfer different slurry flowrates at the same discharge pressure, or will the discharge pressure vary with the slurry flowrate?
Please advise on whether a pressure control system is needed.

 

[quark]

PD pumps give constant volume flowrate irrespective of the system resistance(in simple terms, pressure).

This is a common practice for dosing chemicals in water or any other fluid.

 

[sshep]

More22,

I see you posted twice. In keeping with quark's advice I think this is the appropriate thread to continue so I repeat:

Even if your PD pump were just pumping water, your system head curve (as seen by the centrifugal pump) will shift to the left by the flowrate of the PD pump. The centrifugal pump will therefore shift back on its curve. In addition, the system head curve may "curve up" more than plain water as a result of the slurry taking more pressure drop than water- further shifting the pump back on its curve. These effects may be negligible, or important depending on the flow rates and size of your header. This should be a fairly easy process to analyze. I take it that you are not actually controlling the centrifugal pump water flow such as with a control valve. If you are controlling the injected water flow you probably don't have a problem.

What could be more difficult to deal with is that should your header velocity be low, solids will settle out until the header flow area is eventually reduced and the velocity gets high enough to keep future solids suspended. Such an effect will definitely put your centrifugal pump back on it's curve. There is no pressure control system that I can imagine which will be able to help you with that. Check for a healthy reynold number to assess. If this effect looks likely you need either more head (to account for loss of flow area), more flow (to get velocities up), or both. Another alternative is to provide plenty of clean-out nozzles to service your pipe when it starts plugging- a good idea with slurry service in general.

best wishes, sshep