Working Principle of Dynamic Balancing Valves

[PS]

Hi Professionals,

I'm having few troubles in understanding the functioning of balancing valves.

Balancing valves are used to balance the flow among the load per design. We use static (manual) balancing valves in constant flow applications.

But, when it comes to the usage of dynamic (automatic) balancing valves, it is said that the valve will maintain constant flow under varying pressure conditions. I'm struck up here with the question: varying inlet pressure or varying downstream pressure or varying differential pressure across the valve?

If these valves are going maintain the SET flow (50 GPM, say) at all times, what will happen when the 2-way control valve is 50% opened? Will the balancing valve in the system try to maintain 50 GPM? If so, the purpose of 2-way valve becomes void. If NOT, the statement "the valve will maintain constant flow under varying pressure conditions" will contradict the actual function.

Now, recently we are using PIBCVs in hydronic piping. I have read that PIBCVs solve the purpose of both control and balancing in a single fit. Even here, I had a doubt that if the demand is 50%, what will be the flow across the valve? My guess, in case of PIBCVs, was there should be some setting in the control port that orders the balancing port that at 50% demand the balancing port should allow 50% (apprx) of SET flow rate. Am I right?

Please do help gaining more understanding about their usages. Also, please do suggest few articles explaining the purpose of each type of valves.

Thanks in advance for the suggestions.

 

[EnergyProfessional]

Balancing valves are just adjustable resistances. if pressure increases, flow increases.
what you want are pressure-independent control valves (PICV) from Belimo etc.

If the valve is closed 50%, flow is reduced. That is the point. It is a throttle. PICV are ball valves and have laser-cut balls. the opening is shaped to give you 50% heating or cooling capacity at 50% open (this is not 50% flow). Much easier to control and select. no field adjustment required. Also cheaper since one PICV replaces the control valve and the balancing valve and no need to balance.

 

[PS]

Thanks @EnergyProfessional.

The usage of PIBCV is pretty clear. I understood. Even in the constant flow applications, the scenario of adding resistance is very easy to infer and the usage of manual (pressure dependent) balancing valves is understandable.

But still, I am NOT getting any clues that how a system is balanced at partial load conditions when automatic (pressure independent) balancing valves are used. (Please note I'm NOT referring to PIBCV here. I;m referring to the pressure independent valve that only balances but NOT controls). Because the control valve will try to reduce the flow since there is a decrease in demand. On the other hand, since the automatic balancing valves are SET with the pre-defined flow, they will try to increase the flow to the coils. Won't this create instability in the flow?

 

[EnergyProfessional]

You don't have to balance the system anymore with PICV. All you have to do is use static pressure reset (uses valve positions to create a setpoint) and a system flowmeter (to set the upper limit for pressure reset).

with just a static limiter you mean, you may as well use a PICV and just leave it open all the time. I'm not sure where you use them at all, if you don't want to control a flow. Maybe tell us what your application is.

 

[PS]

Thanks @EnergyProfessional.

Actually I'm NOT using any PICVs in my project. But I'm using 2-way floating point control valves and automatic balancing valves in the AHU valve packages. Now, at design conditions, everything seems to be perfect and I can understand what happens at each and every branch in the pipe network.

But, when the system is operating at partial load conditions, say 50%, how will these balancing valves perform? Obviously, the requirement of flow will be less than the design conditions. How will the balancing valves respond to this reduction in flow?

While commissioning, I do remember that I've SET the balancing valves at 75%open, 65% open, 85%open,etc... based on the flow requirement and looking into the performance chart of those valves. In other words, I've SET the balancing valves for 100 GPM flow at design conditions. What will happen at 50% load? And my Control Valve at 50% load will also be partially opened. How the response of balancing valves will be in this situation?

I hope I have put my questions in a more clear manner. Please do ask for any clarifications.

Thanks!

 

[LittleInch]

It might be best to sketch your system but as described I think your auto balancing valves will simply go 100% open because they are themselves control valves which are trying to maintain a certain flow. As flow goes below their set point, they just keep opening.

If flow is being controlled then by other means then their job is done and they will only come back into play when the flow exceeds their set point.

As soon as you put two control valves in series controlling on the same thing ( flow) then only one will really work. If the set points are too close then you can get instability.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EnergyProfessional]

Sounds whoever designed this tried to do what a PICv does, but with two devices and in a manner that doesn't really work well controlling. You probably have hunting of valves.
Best course of action rip out what you have and install PICV. i works just perfectly, at 25% open position your coild will have about 25% capacity, at 50% it will have ~50% and so on.

I also wouldn't use floating control in this day and age - all modulating. Really not a price difference anymore as it used to be.

 

[ChasBean1]

Great question PS. I think these devices give you the throttling range based on stem/disc position while limiting the flow in their fully open position. Think of a gas cylinder with a regulator. In a 2,000 psi cylinder you can accurately regulate the air outlet down to 20 psig and still throttle the outlet to 0.5 lpm, 5 lpm or 10 or 20 lpm via a needle valve on the outlet of the regulator.

The only drawback with these devices is that your pressure source has to be higher (e.g., if you have a pumping system that might have maintained 20 psid, it might have to be raised to 30 psid) to keep all of these regulating devices satisfied.

It's basically better control at the cost of higher pumping energy.

 

[lukaiENG]

To counter ChasBean1's last point - the rationale behind using these valves is that because you can control (limit) the flow to the various coils in the system to only what is required - you are pushing less water and therefore doing less pumping power. Yes, you might need to maintain a slightly higher pressure but this should be offset by the decrease in flow rate.

A secondary benefit to these valves is that by not putting excessive flow through the coils you are able to maintain higher deltaTs across your heat rejection plant and therefore have higher CHWr temperatures/higher efficiency/more hours of economization. In most systems configurations, this should be a significant energy efficiency savings.

 

[LittleInch]

PS,

you seem to have got confused by titles.

"...when automatic (pressure independent) balancing valves are used. Please note I'm NOT referring to PIBCV here. I;m referring to the pressure independent valve that only balances but NOT controls"

Errrr, if a valve is automatic and balances, it IS a control valve. How else does it work??

However you then state you've set valves at a certain percent open which seems to indicate that these are manual valves, hence dependant on pressure difference to control flow to a certain amount.

Chasbean confuses the issue by using as an example tow valves controlled by different things. The first is controlled by pressure, the second regulates flow.

At partial load (partial load of what?) the flow will clearly change, but you haven't described your system well enough for anyone to figure out and you keep changing the system.

PLEASE draw this system and indicate which valves are automatic and which manual and if automatic, what they control on - pressure or flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChasBean1]

"Chasbean confuses the issue by using as an example tow valves controlled by different things. The first is controlled by pressure, the second regulates flow."

Sorry to confuse the issue. Have no idea what a "tow valve" is, and not sure what is meant by any part of this post.

 

[LittleInch]

two, not tow.

Using small keyboard. ...

Seems clear to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PS]

Thanks All.

@LittleInch: "Errrr, if a valve is automatic and balances, it IS a control valve. How else does it work??" I guess balancing is very different from controlling. Balancing is done based on the resistance to flow where as controlling is done based on the controlled parameter (say, Temperature at conditioned space).

Attached here is the scheme of the circuit. Please do help me gain understanding of the system.

http://files.engineering.com/getfil...83-86a5-0362fb0764ce&file=IMG_20170614_212159

[1].jpg

Thanks in advance for the support.

 

[LittleInch]

Hmmm

I still can't really understand what this PI balancing valve is doing - Is it a simple manual valve? or is it a modualating valve with some sort of control setting . (PI I understand as a " Pressure Independent??). If the valve is independent of pressure then it needs to be controlling on something else, whether this is pressure drop across the said valve or flow or temperature or some sort of control input.

"I guess balancing is very different from controlling."
This is the heart of the problem - No it's not. Balancing is simply another way of saying flow control. you state you set the valve to 85% "corresponding to 100 gpm". Therefore you are controlling flow to 100 gpm.


"Balancing is done based on the resistance to flow where as controlling is done based on the controlled parameter (say, Temperature at conditioned space)."

Yes, but then what does the "control valve" do to control flow based on its input - it opens or closes and restricts the flow - EXACTLY the same same thing a balancing valve does.

tip here - FORGET the names you are calling all these valves. There are only two types of valves here - on/off valves and control valves. control valves control flow by closing and creating a pressure drop which varies with flow. No flow, no pressure drop. That's it - all the other names you give these valves is irrelevant, the y ALL do the same thing, just have different inputs.

Simple mechanical valves like you have are only capable of a single input normally. My understanding is that your balancing valves are set up to limit flow to a set value ( in your example 100gpm. If flow is less that that then they open wide, so if you have a second valve in series ( your "2way modulating valve") and flow reduces to < 100 gpm, the balancing valve will simply open up as it's flow setting isn't being exceeded.

Does that help?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PS]

Thanks @LittleInch.

I can understand the last part. Even I was having a similar thought. But I still feel there are differences between balancing and controlling. I can understand balancing is another form of controlling but I do feel the type of control which we do in HVAC is different from balancing.

"Yes, but then what does the "control valve" do to control flow based on its input - it opens or closes and restricts the flow - EXACTLY the same same thing a balancing valve does." - My understanding is: balancing valve tries to maintain a constant flow; whereas control valve try to supply water per the demand, it can either increase or decrease.

"Simple mechanical valves like you have are only capable of a single input normally. My understanding is that your balancing valves are set up to limit flow to a set value ( in your example 100gpm. If flow is less that that then they open wide, so if you have a second valve in series ( your "2way modulating valve") and flow reduces to < 100 gpm, the balancing valve will simply open up as it's flow setting isn't being exceeded." - Even I thought of similar justification. But I was NOT sure whether this justification or right or not.

 

[LittleInch]

We're getting there....

The difference I think you're trying to express is that the control set point for balancing is a fixed set point (flow or differential pressure) which the integrated controller in the valve limits max flow whereas the other valve responds to a variable input.

In many systems these inputs would be combined into a control system with various low select or "overrides" in the control system controlling a single valve.

" My understanding is: balancing valve tries to maintain a constant flow; whereas control valve try to supply water per the demand, it can either increase or decrease." sort of - balancing vale limits flow to a certain value, but like all valves, they can't create more flow - that's the job of the pump. Once they are fully open then the flow is dependant on other elements.

ditto your "control valve", can only supply water up to the limit imposed by the balancing valve to stop one unit taking more than it's share of a supply. Too much water through one unit will starve the others of supply and lead to all sorts of problems.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PS]

@LittleInch,

"The difference I think you're trying to express is that the control set point for balancing is a fixed set point (flow or differential pressure) which the integrated controller in the valve limits max flow whereas the other valve responds to a variable input." - Exactly this is the point which I understood with my experience.

" sort of - balancing vale limits flow to a certain value, but like all valves, they can't create more flow - that's the job of the pump. Once they are fully open then the flow is dependant on other elements." - I feel this would the apt answer to my questions.

Thanks for the help!! Thanks a lot!

 

[lilliput1]

If the PI balancing valve you are using maintains constant flow independent of pressure then you have a constant flow system requiring 3 way control valve that diverts the flow of chilled water either to bypass or through the coil with common return through the PI balancing valve. This constant flow system is a waste of energy compared to variable flow. Instead you should use a differential pressure control valve to maintain differential pressure between a each branch supply and the branch corresponding return main. The valve being located upstream of the coils and control valves in the branch will limit the upstream pressure to each control valve in the branch to keep them operating within their control range.

 

[ChasBean1]

Please re-read my 6/14/17 post. Balancing by any type of valve is only useful when all loads are fully open. Pressure regulating balancing valves require more pumping power and add to operating cost, although provide better control under maximum demand conditions that do not normally exist.

 

[PS]

Thanks @lilliput1 and @ChasBean1.