HW system balancing

[ChasBean1]

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

 

[quark]

Sizing of the balancing valves might be a critical factor in your case and your cause will be assisted if you have balancing valves at the pump discharge and at individual branches as well.

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,

 

[briand2]

If design flow rates are not met (i.e. total flow is either under or over design) after a proportional balance has been achieved throughout the entire system, the only way of achieving design figures is to adjust the pump delivery. This is typically done by varying pump speed (by changing pulley sizes, for instance). Revisiting each branch / terminal balancing valve is not practicable. In your case, it may be that the pump was fixed speed and so the only way of getting the design flow rates is to take out excess head at the pump / main balancing valve.

I believe this is worth doing (usually); over the life of the installation you will pay (energy costs) for pumping excessively.

Regards,

Brian

 

[lilliput1]

Yes you need to throttle the flow. It would be better to cut the impeller but yes you can throttle the flow with a balancing valve. This will reduce the hp requirement (vs at the wide open flow greater than design) and make sure the heat exchanger can produce the design leaving water temperature. It is not recommended to use a combination check and balance valve for this purpose because if the check valve portion breaks, you will have to drain the piping system. Use a separate silent check valve and a separate balancing valve. You can use DeZurick eccentric type plug valve with differential pressure measurment ports for balancing of large piping system too big for circuit setter type balancing valves.

 

[ChasBean1]

Thanks for your responses!

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??

 

[quark]

You didn't still say whether you have individual balancing valves or not. If I presume two way actuated valves only then you are dead right. Restricting the pump dp may starve the farthest air handling units.

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,