The quick answer is that since a rotometer is a variable area device, the pressure drop is basically reflective of the weight of the float and insensitive to flow. best wishes, sshep
Like sshep said, the weight of the float IS the back pressure. Well, almost. There will be some small secondary losses in the end-fittings, just due to changes in geometry. If the rotameter also has a valve, then the pressure drop can be huge, but will be a function of the valve opening.
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 desired water flowrate.
Will the PD pump transfer different flowrates at the same discharge pressure, or will the discharge pressure vary with the PD flowrate?
Please advise on whether a pressure control system is needed.
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.
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.
