Flow rate through a balancing valve or control valve

steamfitter

On a pump curve, as the flow rate increase, the pressure decreases or vice-versa.
How about a PRV, or a plug cock? When water is throttled the pressure drops downstream, but does the flow rate increase or decrease?
Just trying to clarify any differences or similarities between hydronic circulators and zone valves as I watch Dan Holohans's "Classic Hydronics" DVD set for the 3rd time. Great stuff!

Jamie Hall

A valve -- or any other obstruction like an elbow -- will have a pressure drop associated with the flow through it. Very roughly, the greater the pressure flow, the greater the pressure drop. The flow, however, neither increases nor decreases. It can't. That is, of course, assuming a constant pressure upstream. That's where you are seeing the pressure drop downstream.

Now... put that valve in a pumped system, such as might be a hot water loop, and you have to contend with the pump's characteristics, too. Let's say, for simplicity, that the system consists of a pump and a loop of pipe with a valve in it. The pipe with valve wide open has a certain pressure drop; the pump will create a flow corresponding to that pressure drop according to the pump's characteristic curve. Now if you partially close the valve, the pressure drop increases, and the pump's operating point moves on its curve to a lower flow.

So the valve itself doesn't affect the flow, except by altering the pump's performance due to the increased resistance to flow.

EBEBRATT-Ed

What @Jamie Hall said is spot on

steamfitter

Thank you Jamie for the explanation!
If a plug valve on the discharge of a pump is partially closed, then the pressure drop increases and the pump' s flow is affected.
Pump curves generally show a decrease in flow rate with a corresponding increase in pressure. Does throttling down the plug valve raise the pressure between the pump outlet and the valve?
Is the flow rate lowered downstream of the plug valve?
Am I confusing the relationship between the pump curve and the balancing valves on the discharge of centrifugal pumps?

Jamie Hall

steamfitter said:

Thank you Jamie for the explanation!
If a plug valve on the discharge of a pump is partially closed, then the pressure drop increases and the pump' s flow is affected.
Pump curves generally show a decrease in flow rate with a corresponding increase in pressure. Does throttling down the plug valve raise the pressure between the pump outlet and the valve?
Is the flow rate lowered downstream of the plug valve?
Am I confusing the relationship between the pump curve and the balancing valves on the discharge of centrifugal pumps?

The pressure differential across the valve increases as you close it. Does this translate into a higher pressure between the pump outlet and the valve? In a closed loop it would, all else being equal.

The flow rate changes as the operating point of the pump changes, both upstream and downstream of the valve. The valve does not and cannot, by itself, change the flow rate. X gpm going into the valve, X gpm coming out. It just changes where the pump is operating.

The pump curve is an inherent characteristic of the pump. No valves will change the pump curve. They may change where the pump is operating on the curve, however.

I might add that as soon as you get two or more loops, with perhaps balancing valves on each loop, things can get rather interesting to calculate. When you get into even fancier setups, with two or more pumps plus some valves and a bunch of fittings, as in, for instance, primary/secondary piping, calculating what's actually happening can get really messy. Not impossible at all. Just messy. Since nothing operates in isolation!

steamfitter

Thank you again Jamie for clear and concise responses to my questions, which are often awkward for me. As I try to wrap my head around the understanding of how the components and systems work I realize more and more everyday how complex they can become.
I have a deeper respect for design engineering and can't thank you enough for your generous instruction!
Best,
Manny

hot_rod

Know too that a balancing valve is a parasitic device, needed many times but often misused or not adjusted correctly.

Ideally a pump sized exactly to the load or an ECM circulator which has a lot more intelligence to adjust to changing loads.