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[Art Montemayor]

Tom:

By your questions I can tell you’re not familiar with pumps nor have you had hands-on familiarity with them. That’s OK. We all have had to go through the learning process. The real important thing I want to leave you with is that you should not feel constrained or any less an engineer simply because you haven’t had the exposure to pumps that you need. The opportunity will come in due time and you’ll see then how really simple and common sense they are and how they operate. Now for your questions:

!) The speed at which a centrifugal pump is driven is changed with ease – in fact, VSDs (Variable Speed Drives) are becoming common place as drivers on these pumps in order to control their delivery capacity and save on energy at the same time. In the old days – when steam power was “King” – it was common place to have most centrifugal pumps driven by steam turbines – and the speed could be varied at will. Of course, you must realize that when you change the speed of a centrifugal pump you are also changing the head characteristics as well as the capacity. This is an aspect peculiar to a centrifugal device and not to a positive displacement type of pump such as a piston type. You can vary the speed of an electric motor drive in several ways; through a multi-pole motor or a VSD.

2) You also change the speed of a positive displacement pump in the same manner. In the specific case of a piston type of pump, you also have the option to apply a “Reeves” drive – which is a variable pitch belt drive that attaches itself between a constant-speed induction electric motor and the pump coupling.

3) There actually is no need for a so-called “performance curve” for a piston type of positive displacement pump. The displacement is directly proportional to the speed or to the stroke, so all you do is create a spread sheet relating the capacity at the different settings. All this assumes that your valves are seating properly and sealing 100%. There is always some slippage (or leakage) through the valves, but you can document empirical losses on an actual application and apply this field data to the operation.
Normally, the displacement of positive displacement pumps is varied in step changes – not differentially (as in a VSD).

I hope the above answered all your questions and concerns.