I have been performing a cooling water system calc in Excel. The system is being pressure balanced using butterfly valves. I have always heard that in real life it is incredibly difficult to actually balance a cooling water system this way. I am wondering if the calculations that I am performing would actually give me something that sort of resembles the actual system. Say I put in the pipe length, size, fittings, flows that I want to go through and use the butterfly valves to take the excess pressure drop I need to balance everything, would the flow rates that I have specified resemble what would happen out in the field? Thanks for any help!

If you want good flow regulation, a butterfly valve is generally a poor choice.  The flow characteristics are just not suited to such a task.  That's why one should specify a control valve if you want really close regulation.

A better choice than the butterfly is a globe valve for reasonably steady flow.  I would estimate, as a rough rule of thumb, that the valve absorbs at least 30% of the total DP available.

It's very hard to provide advice when the details of the problem are not known.

Butterfly valves are very good in your application.  Water is a terrible thing to waste, but you waste nothing in a cooling water system.  The use of control on a water exchanger is a matter of saving energy downstream of the water cooled exchanger, on the process side.  For example, sub cooling reflux isn't the most energy efficient thing, so some controls are good.  Getting the maximum cooling on a refrigeration condensor is good, so flow control isn't required other than lowering the flow so that other water users have some water.

The butterfly valve is a poor choice especially where large pressure drop (K value) is required to balance a branch. A butterfly valve has a very non linear characteristic vs its angular position. For a small angle from full open there is little increase in pressure loss. As the disc goes beyond around 45 degrees, the pressure drop (K value) increases dramatically. Hence the valve can be hard to set. An additional issue is that the flow is diverted against the side of the pipe and will remain that way. This can lead to local erosion of the pipe. Hence the reason why it is not recomended to use a butterfly valve as a balancing valve.

Regarding the method of calculating the hydraulic network, I have actually programmed the Hardy-Cross method, which is a "beginner's" method, not too accurate because of the restrictive assumptions.  This can be implemented in Excel (very tedious even for small problems; besides the spreadsheet would be a unique solution to a given network - therefore of no generic value).  There are, needless to say, much better commercial programs available at modest cost.

Your real problem, of course, will be finding a proper way to characterize the hydraulic resistance of partially open butterfly valves.  The pressure drop changes dramatically over just a few additional degrees of opening when you are in the effective regulation range for this type of valve.  Unless you spend umpteen hours "fine-tuning" the parameters, I wouldn't be too optimistic about the results, as far as matching field results, even if you have programmed correctly.

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