In our R&D test facilities we require water at very low flowrates (10 - 300 kg/h) and high pressure (50barg) for a series of experiments. We plan to use a CAT piston pump with VSD. However we still cannot achieve the lowest flowates (10kg/h). Therefore to achieve the lowest flows I plan to fit a bypass system on the pump outlet. I know this will be a waste of energy but it's research work only and won't be used that often.

My question is where does the throttling valve go. On the main line or on the bypass line? My thoughts are that it should go on the main outlet line, as that is where you want to control the flow. But my colleague thinks it should control the bypass line.

We put a CV in the by-pass line to control pressure and a CV in the main line to control flow.  Each loop affects the other so tuning will be critical.  Pressure surges will be present if you do nothing about them.  Stand pipes or surge dampeners on pump inlet and outlet will lessen them.

I'd agree it goes on the bypass line.  Being a positive displacement pump, if you put it on the main line downstream of the pump, you will simply overpressurize that portion of the line.  Flow rate won't decrease simply by increasing backpressure.  

If you put the valve on the bypass line, as you say "the pressure will just sort itself out".  Control of the bypass valve still needs to be integrated with the VSD though.  Valve control might best be done only once the VSD hits the lower limit of its speed.

For what you are trying to do, a pressure control valve on the bypass line would probably be best.  The flow meter would go in the main line downstream of the bypass branch with a globe or needle valve downstream to control the flow to the test.

As stated above, a PD pump will flow what it flows.  The needle valve will control flow to your test, and the control valve will ensure the pressure you need and automatically dump the required additional flow.

Most of our applications are multiple destination feed points.  We've used a CV in the by-pass line to control pressure and a CV in the main line (at each destination) to control flow for many, many years.  If we have a system with a single destination, we set it up the same way.  Same control code and everything, because one of these days someone will want to tap into the header and use it somewhere else!  And, we always put a spare nozzle in the header and a flanged off line run tee at the end of the line.

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