balancing valve with isolation valve

I will go into more detail. Each of the four new similar cooling towers has a flow rate of 3088 gpm (195 l/s), which is balanced by a DRV (double regulating balancing valve) in each circuit. On each input line of the CT, there is a BV (butterfly valve) with a motorized isolation valve that works like an on/off control. My question is why it would put in two fittings like a BV and a DRV instead of just one BV with a motorized regulating actuator that is the right size? A BV that is the right size and has automation to control the flow rate can get rid of the need for a DRV, which cuts down on capital costs and pressure drops that aren't necessary. Why choose BV when there's a big price difference between BV and a globe valve? The nozzle drop is 34 kpa, so the total PD is 70 kpa. The Kv of BV will be 839, which is 195*36/sqrt70 kpa. During the process, the automation will automatically control the flow, and the same valve can be used to shut off the flow. I really value your helpful feedback.

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

Can you share process drawing?

Daren!

In general whilst butterfly valves can be used for flow control, they are not great in terms of accuracy and this also requires constant flow feedback. Can be done this way for sure. Reasons for doing it this way could be cost, experience, personal preference, company policy, they built the las tone that way...

Once the system is balanced you leave the balancing valve where it is and then just use the BF valve for on/off.

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hi,LittleInch-At BELIMO Dubai, a technical guy actually made this suggestion. The butterfly valve was listed as having a control function on the Belimo website's online valve selection page.


In the absence of a suitable automation, mark the valve position at the design flow rate, and position the valve manually even if automation fails. However, I concur with you that the globe valve is the best for regulating the flow.
However, the point, in my opinion, is: why use energy wasteful pressure creators?



The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

Because you have 4 units which need equal flow.

Ideally the one with the highest friction losses would have the regulating valve fully open and then close the others.

However people often oversize pumps so you need to throttle them a bit to not flow too much through your coolers.

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Also: If you get a response it's polite to respond to it.

You could do this with one valve, yes, but the pressure drop you need to add to achieve the desired flowrate is the same if added by an isolation valve + manual balancing valve or by a single control valve. In other words there is no energy being wasted compared to a single valve setup.

As known a butterfly valve with a nearly closed position will very often come into cavitation range. Anyway the practical regulation range (allowable throtteling) of a butterfly valve is limited. THe quality of the BF valve should also come into consideration (costruction and closing areas precision, geometry and material quality).

Conclusion: The regulating might be solved as you suggest. But, in my opinion, regulating by a butterfly valve should be a bit down on the option list! Better to use a motorized regulating valve suitable for closing. Not necessarily cheaper in cost, but perhaps more dependable and cheaper over time.

Another thought. Have the regulating of the pump(s) speed directly been considered, without regulating/balancing valves? Especially if the feed is by two or more pumps it could be sensible to balance the load between the pumps.

hi,GBTorpenhow
You made a good point. Thanks. I just wanted to say that the balance valve plus the isolation is the same as a single isolation because the single valve closes to get the PD for the design flow. When it comes to the pump, there are no energy saves. But if the goal is to vary the flow as part of optimization and decrease the number of cooling towers on duty to meet demand, it only needs one modulating BV to do both jobs, such as balancing and isolating. For example, if only two of the four towers are needed, the other two towers are cut off and the adjusting BV is changed to match the flow.

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

All depends on whether your valve incorporates any from of flow measurement.

Double regulating balancing valve can be different things to different people.

but maintain more or less the same DP across the valve once set correctly and you'll get more or less the same flow.

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

hi,gerhardl
Belimo butterfly valves, in my opinion, differ from other kinds. Butterfly valves for regulating chilled water are not promoted by any manufacturer in my limited experience. The globe valve is typically used for control or balance. You pointed out that the virtually closed position of the BV will result in cavitation. In a variable flow system, I agree that the balancing valve is unnecessary. Thanks

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

hi LI, yes, balancing valve will have in/out test point and possible to check the PD and measure the flow rate is easy. But BV, generally don’t have test point, for this additional points to be implemented. Thanks

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-