Flow Coefficient for 2 different valves installed in parallel flow paths
Flow Coefficient for 2 different valves installed in parallel flow paths
(OP)
The flow coefficient can be used to calculate the predicted pressure drop across a valve. How do you predict the pressure drop if you have two valves in parallel that are both open but the valves have different flow coefficients?





RE: Flow Coefficient for 2 different valves installed in parallel flow paths
There is no theoretically rigorous way to calculate this simple-looking arrangement. We have to fall back to empirical equations. I've tried (several times) to iterate on the flow until the two dP's were identical to 6 decimal places and the dP I calculated was far different from reality. The empirical control valve equations (which include Cv) are a tiny step better than simply making up a number. The revisions that try to account for exit losses are a couple of little steps less robust.
The equations we use are good enough to size a valve ±20% with some windage built in. They aren't bad at predicting cavitation. Third decimal place calcs are way beyond them.
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist
RE: Flow Coefficient for 2 different valves installed in parallel flow paths
RE: Flow Coefficient for 2 different valves installed in parallel flow paths
What is the actual purpose of your excercise?
If you look at the definition of flow Q and Cv (see for instance http://www.fnwvalve.com/FNWValve/assets/images/PDF... ) you will remember that Cv is a measured unit, measured under a given set of conditions. Imagine now an open vessel with a filling keeping the water at a constant level = constant pressure, and this pressure equal to the required pressure for the cv measurement. With free outlet, and not connected, your two valves will then run full flow with different innards and give two different cvs'. This is your starting point.
If you now instead run theese valves in a parallell, connected to a common outlet, and over a number of other devices, restrictions and regulators, you put in a vast number of possible variations. The main variations are related to the rest of the pipeline and what happens after your two original valves. Influences are for instance the pipelines' total cpacity, resistance at different flows, outlet capacity and variations. In addition comes possible throtteling after the valves, layout where the one valve with higest afterpressure will influence flow of the other, the ratio of the one cv to the other, etc. etc.
See the mathematical difficulties?! I would go for Zdas04s' measurement solution, or at least a part test mimicking the actual connection and further throtteling, and test at a number of flow-points to get a grphic curve.
Accuracy by theoretical solution? No experience, but Zdas04 is usually correct!
RE: Flow Coefficient for 2 different valves installed in parallel flow paths
Now the OP should say whether he/she knows how to apply the iterative method in order to solve his/her configuration. Frankly this is the only way I'd go in order to make a prediction of the flows and pressure drops, without starting the flow and putting in a gauge to use David's words.
RE: Flow Coefficient for 2 different valves installed in parallel flow paths
I would joint GerhardL with the question: What is accrual purpose of your exercise? What is the problem and what is your trying to achieve? Can you give use a bit more information (without confidential data)?
The reason I would like more info is there are different ways to answer to your question, to solve your issue. Most likely there other people who might face your problem already, some might have even a solution.
Kurt
RE: Flow Coefficient for 2 different valves installed in parallel flow paths
Pick one Cv value for one valve, find the pressure drop for a given flow. Using the same pressure drop, find the flow possible in the other valve. Add the flows.
you must get smarter than the software you're using.
RE: Flow Coefficient for 2 different valves installed in parallel flow paths
Q = Cv / √(SG/ΔP) (by definition)
ΔP is the same for valves in parallel, so
QTOTAL = Q1 + Q2 = Cv1 / √(SG/ΔP) + Cv2 / √(SG/ΔP)
= (Cv1 + Cv2) / √(SG/ΔP)
and therefore CvCOMBINED = Cv1 + Cv2
There was a similar discussion over Cvs in series in the thread thread378-326544: Calculating Flow Coefficient Through 2 Valves in Series that gave a link to the formulas for series and parallel valves on the Engineering Toolbox web site. Note that the symbols used on that web site may not be familiar to some engineers, but the differences are highlighted in the referenced thread.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Flow Coefficient for 2 different valves installed in parallel flow paths
RE: Flow Coefficient for 2 different valves installed in parallel flow paths
you must get smarter than the software you're using.