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Controlling two serial connecten pumps in a distributed heating system

I would like to put a question to any of the expert readers in the field of pump operation. We are modifying the distributed heating system in the city of Amsterdam in the Netherlands. Due to this we want to implement two serial connected pumps with equal pump curves in order to be able to overcome the hydraulic resistance of the system during peak demand conditions. Apart from the hydraulic issues involved I would like to receive expert or experienced opinions about how to control serial connected pumps. I am considering some kind of split range control solution for the speed control of the first and the second pump. I would like to implement an automatically controlling system, where with low demand only the first pump is being energized and controlled, whereas when a higher demand is wanted, the second pump will be switched on as well and controlled together with the first pump. Now I know that pumps need to be operated on a certain range, situated on both sides of the BEP ( Best Efficiency Point ). What I would like to learn or to know is how we can choose our switching points in order to get a smooth operation without any pressure transients in the hydraulic system.
I have been consulting all kind of literature about pumps and systems, but apart from the information how to establish the resulting pump curve with two serial connected pumps, there was no additional information about how to implement a working and reliable concept. I do hope to hear from you. Any leads to the right literature or hints are more than welcome.

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Permalink Reply by Jack Condon on February 3, 2010 at 8:21am: i believe you are trying to maintain a near constant system pressure at varying flows? one approach to consider for two pumps in series with varying flow requirements is to run the first pump at a single speed sized for your minimum system requirements and running the second pump off a VFD with a pressure signal. as your flows increase and system pressure drops, the VFD will speed up the second pump to maintain the system pressure you require. of course each pump should be sized to match the required maximum flow you will see, and produce at least half the total required head at this maximum flow (plus some appropriate safety factor). this approach limits your higher cost VFD control portion to one pump. the first pump will have no problem pumping through the second pump in series even when it is not required to be running, although i would probably design the system so it is providing some head even at your low flow condition.; ▶ Reply to This

Permalink Reply by nobuyoshi yonekura on March 5, 2010 at 5:13pm: Hello, Karel Postulart san

This problem is seemed difficulty and complex.
Then,　following two questions;

①Was this problem solved 　or　 is the hint still necessary?
②Do you know the characteristic of the Water (pump)mill though the characteristic of Water pump might be knowing?
If flowing quantity in the second pump is a strong field, the first pump becomes a water(pump) mill.
It is necessary to arrange these correlations　etc.; ▶ Reply to This

Permalink Reply by ferdinand lizabeth on April 30, 2010 at 10:18am: Hello,
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lizbeh
Email.lizabethferdinand@yahoo.fr; ▶ Reply to This