Mixing pumps setup opinions

Hi all, about to set up a flow loop in our garage that will be mixing oil and water for some water cut testing. In the past we've basically put a hand valve right after the pump to control rate (ghetto). This time we're going to have one pump per phase. We were going to use control valves behind each pump to control individual rates, but now that I'm getting more familiar with drives, thinking I'll put a small VFD on each pump instead and control rate that way. The VFDs are much cheaper than the control valves for the size pumps we're using.

Anyway, wondering if we should still have valves in place or if I should just rely solely on the VFDs to control rate for each phase. Any opinions from previous experiences would be appreciated.

Are you using a positive displacement pump?

g.mccormick said:

Are you using a positive displacement pump?

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No, it'll be centrifugal I believe. This is why I asked, didn't even think about that. Things will be far from linear on a centrifugal pump which means the VFD control will probably suck, yes?

VFDs on centrifugal pumps are established technology and can operate with stability and good control accuracy. You don't want your control tuning to be too aggressive, though. When one pump increases flow it can push the other pump back on its curve, causing it to speed up, pushing the first pump back on its curve, and so on, leading to hunting. If you tune for a slow response you shouldn't have any problems.

Tom Jenkins said:

VFDs on centrifugal pumps are established technology and can operate with stability and good control accuracy. You don't want your control tuning to be too aggressive, though. When one pump increases flow it can push the other pump back on its curve, causing it to speed up, pushing the first pump back on its curve, and so on, leading to hunting. If you tune for a slow response you shouldn't have any problems.

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Agree. If you had a pdp, it is easier since a feedforward can be calculated (ie. pump displaces X gal/rev, you need y gal/min, speed feedforward is y/x rev/min). Also, like Tom mentions with centrifugal pumps one will effect the other as it pushes head around. Take Tom's suggestions to heart (he knows much better than I) and tune loops for a slow response.

Thanks guys!