HW system balancing
HW system balancing
(OP)
Greetings all - I apologize for recent lack of activity as life's been busy.
On a recent project, folks expected a certain design differential pressure across hot water pumps and the system had less friction than anticipated. So to make up for that lack of friction and to recover pressure across the pumps, balancing valves at the immediate outlet of the pumps were throttled back to achieve the pump D/Ps shown on the schedules.
Somebody help me think this out... are we doing ourselves a favor by pinching flow at the discharge of each pump? If the pressure across pumps was 45 psid and we now have 60 psid, could the most remote load now not be satisfied?
Bottom line: is there a real purpose to balancing valves right at pump discharges (I can see their purpose on branches of multiple branch systems).
Thanks for your thoughts on this. -CB
On a recent project, folks expected a certain design differential pressure across hot water pumps and the system had less friction than anticipated. So to make up for that lack of friction and to recover pressure across the pumps, balancing valves at the immediate outlet of the pumps were throttled back to achieve the pump D/Ps shown on the schedules.
Somebody help me think this out... are we doing ourselves a favor by pinching flow at the discharge of each pump? If the pressure across pumps was 45 psid and we now have 60 psid, could the most remote load now not be satisfied?
Bottom line: is there a real purpose to balancing valves right at pump discharges (I can see their purpose on branches of multiple branch systems).
Thanks for your thoughts on this. -CB





RE: HW system balancing
If your sizing of valves is generously done and pump delivers more flow rate (as suggested by your required dp and design dp) you require to control the individual balancing valves to a greater extent and this leads to awkward flow control as the dp across the individual balancing valves increase.
You need not have to worry if your prime aim of installing a balancing valve at pump discharge is to correct the increased flow due to less resistance in the piping system.
PS: Pump characteristics before and after installation of balancing valve will help us getting the picture clearer.
Regards,
RE: HW system balancing
I believe this is worth doing (usually); over the life of the installation you will pay (energy costs) for pumping excessively.
Regards,
Brian
RE: HW system balancing
RE: HW system balancing
First off, pumping is variable speed based on loop D/P - sorry for not mentioning this originally. As observed, pumps are two in parallel at about 40 Hz. It looks like during the winter, loop D/P is reduced because loads are more wide-open than anticipated (blame it on low airflow, infiltration, whatever) so the D/P across pumps is lower, indicating that the water flow is higher for the given RPM... Under that condition, I now throttle my pump outlet balancing valve to raise the D/P to design value.
To me, this is counterintuitive and counterproductive. I've reduced my pumping capability so I might not have the flow I need at remote loads...
If my pumping system has VFDs controlling loop D/P and stages right based on speed, why do I need to throttle the outlet flow at each pump??
RE: HW system balancing
In your case the dp across the headers will take care of the fluid requirement and can dictate your pump accordingly. A balancing valve on pump discharge is redundant if you have VS Pumping application with dp across headers, in any case. Moreover, it will create troubles in your case.
Yes, you are right.
PS: Just check whether your pump develops the increased resistance due to increased flow. This might have caused some misunderstanding.
Regards,