Closed Loop Centrifugal Pump Performance on VFD's; is it in Series or Parallel?

Looking forward to reading your report.....

This is not a simple calculation.....

A real engineering company responsible for the original design of this system will provide detailed computer calculations to the owner. These calculations, typically done by computer simulation, will cover all reasonable modes of system operation.

AFT fathom is one of many software products that have been in use for decades for this type of piping system analysis.

http://www.aft.com/products/fathom

However, whenever an owner sells a building, it is customary for the new maintenance department to discard any and all plans, drawings, manuals, calculations and any other bit of useful information. By doing this they are not responsible, in any way for the performance of any system.

If there is any system problem from that point on, they call in for the help of others.... because they have no documents

The process is repeated every time the building/factory is sold...

What role do you play here ?

Real Engineering company? detailed Computer calculations? Who wrote the software? Who wrote the algorithms in the drive that reports the draw? Who made the gauge? What speed is the motor really running? What is the real power factor on the motor? How clean is the power really? I could go on, but you lost me with your faith in software of any kind. I could say that in some instances of my manual tests of automated systems I find a flaw in the process but that would be false. In EVERY case I find a flaw. this particular challenge became interesting by the shear diversity of answers by smart people. Quoting basic laws of hydraulic fundamentals would be useful if they were being ignored, but they are not. there is something to this specific set-up and i aim to get to the bottom of it..

If pump 1 is rated 150' TDH and the suction header is pressurized at 150' will it not then produce 300' TDH total? No, the pump can only produce the rated 150' TDH

You have left out the most important key elements:

1. Pump inlet and suction header pipe size.

2. Pump outlet and discharge piping size.

3. System discharge header and distribution systemr pipe size.

4. System dynamic restrictions: (Quantity of valves, sizes of valves, type of valves, number of bends, elbows, change of direction fittings, flow & temperature device restrictions, etc.)

5. If you operate the pumps in series the output flow is pretty much limited to the maximum output of a single pump.

6. If you operate the pumps in parallel and the intake, discharge and distribution header is of adequate size it is possible to reach a flow rate equal to all pump flow ratings added together minus dynamic losses.

It would help to see a diagram of the system. Why do many pumps and control valves? How many chillers, how many users? How will the suction pressure be 150', don't you have a surge/expansion tank? Why control valves and variable speed?

OK, the system curve will not change with varying pump speed. The pump curves will change approximately according to the pump affinity laws.

q₁ / q₂ = n₁ / n₂

dp₁ / dp₂ = (n₁ / n₂)²

hp₁ / hp₂ = (n₁ / n₂)3

And the operating point will be at a different point on the system curve, unless the control valves force it back.

As MacG said, not generally a good idea to restrict the suction of a pump.

Running the pumps all at once will increase the pressure at the return header. If you try these pumps in series you will need a separate high pressure pump for the gland packing.

For a closed-loop system there should be a connection for make-up/expansion, either from a header tank or an expansion vessel. The best place to connect this as close to the pump suction as possible. This ensures nowhere in the loop falls below atmospheric pressure, which could cause cavitation or air ingress.

As others have said, you should not be throttling the pump suction.

If you have the pump Q/H curve, you can plot the curve for pump combinations in parallel or series. In parallel, multiply the flow per pump at a given head by the number of pumps. In series, multiply the head per pump at a given flow by the number of pumps.

Sir,

Your question looks more like mathematical swiss cheese.

• "...control valves on the discharge..." then "...control valve on the suction." ??
• Centrifugal pumps and fans typically follow similar rules, often called the "fan laws" that are fairly reliable for modest changes in speed. However, a 50% change in speed is likely to be outside their reliability.
• The pressure on the return header is a variable depending on where on the header you are. You give so few specifics that any answer is based on assumptions. The return header, depending on your use of the words, may be the suction header.
• If by "in series" you mean the first pump's discharge is the second pump's suction and the second discharges into the third..., then --if the pumps are identical and the piping is identical-- the pressure drops across each one can be close to equal, but that scenario is highly unusual.

Good luck with your testing. I hope somewhere in your experience that you learn what you are doing.

--JMM