3-way ball valve Cv
3-way ball valve Cv
(OP)
I have an application where a 4" 3-way ball valve has a Cv of 840 in the 'through' position. Knowing that 3-way plug valves are notoriously poor when it comes to Cv, I found that the 4" plug valve has a value of 160.
Since we're having issues with the ball valve I wanted to compare what the effect would be with a plug valve. The difference in Cv's represents ~80% reduction.
Suppose I were to replace the ball valve with the plug valve. The existing flow rate is 350 gpm of a starch slurry (1.15 SG).
My first inclination is to evaluate the individual pressure drop expected as follows:
ball valve: 350gpm*1psi/840gpm ~ 0.4 psi
plug valve: 350gpm*1psi/160gpm ~ 2.2 psi
The additional pressure drop of about 1.8 psi would yield a drop in flow that would hardly be seen when applied to the pump performance curve. Yet when you consider the 80% difference in Cv, you'd think that the overall flow would be affected, i.e., 350*0.20 ~ 70 gpm?
My gut tells me to consider the individual pressure drop and that there would be little impact on capacity. Am I going wrong anywhere?
Since we're having issues with the ball valve I wanted to compare what the effect would be with a plug valve. The difference in Cv's represents ~80% reduction.
Suppose I were to replace the ball valve with the plug valve. The existing flow rate is 350 gpm of a starch slurry (1.15 SG).
My first inclination is to evaluate the individual pressure drop expected as follows:
ball valve: 350gpm*1psi/840gpm ~ 0.4 psi
plug valve: 350gpm*1psi/160gpm ~ 2.2 psi
The additional pressure drop of about 1.8 psi would yield a drop in flow that would hardly be seen when applied to the pump performance curve. Yet when you consider the 80% difference in Cv, you'd think that the overall flow would be affected, i.e., 350*0.20 ~ 70 gpm?
My gut tells me to consider the individual pressure drop and that there would be little impact on capacity. Am I going wrong anywhere?





RE: 3-way ball valve Cv
350 350 gpm
840 160 Cv := gpm/sqrt(psid)
0.417 2.188 sqrt(psid)
0.645 1.479 psid
Mike Halloran
Pembroke Pines, FL, USA
RE: 3-way ball valve Cv
The specific gravity is 1.15. The formula I used was Q = Cv*sqrt((psid/SG)).
I got the following:
ball valve = 0.209 psi
plug valve = 5.5 psi
That represents about 11 ft head to be added to the pump discharge. From the performance curve the resultant flow would be about 300 gpm. This would be about a 15% difference in capacity.
Do you agree? If so, this helps in the evaluation.
Thanks,
Jack
RE: 3-way ball valve Cv
... totally apart from the difficulties in evaluating and balancing the Cv's, let me add to the confusion by mentioning that in my experience it is in most cases better to use two separate regulating valves (for instance v-ports), and if necessary two additional separate on/off valves.
This both to obtain better and more accurate regulation and better thightness (when closed) and longer standtime/less maintenance with cheaper, and more easily found on the market standard components.
... just a thought!
Good luck!
RE: 3-way ball valve Cv
Adding that,
ball plug
350 350 Q, gpm
840 160 Cv := gpm/sqrt(psi)
0.417 2.188 sqrt(psid)
0.645 1.479 psid (water)
0.417 2.188 sqrt(psid/SG=1)
1.15 1.15 SG slurry
0.479 2.516 sqrt(psid/Sgslurry)
0.692 1.586 psid (slurry)
One of us is doing this somewhat less than perfectly right.
It could well be me; I will admit to considerable rust.
Mike Halloran
Pembroke Pines, FL, USA
RE: 3-way ball valve Cv
The definition of Cv is
Cv = Q x sqrt(SG/ΔP) Q in USgpm and ΔP in psi
Rearranging for ΔP gives
ΔP = (Q/Cv)2 x SG
This gives me 0.20 psi for the ball valve and 5.50 psi for the plug valve at 350 USgpm.
It is quite possible and reasonable for this to give a change of only 15% in the flow rate because there can be considerable pressure drop in the rest of the piping, and this will remain constant - or even decrease slightly if the flow rate decreases.
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RE: 3-way ball valve Cv
Mike Halloran
Pembroke Pines, FL, USA
RE: 3-way ball valve Cv
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RE: 3-way ball valve Cv
RE: 3-way ball valve Cv
RE: 3-way ball valve Cv
The TFE seats on the existing 3-way ball valve are not lasting more than 3 months before we start to get leakage by. This creates a plugging problem downstream as the solids in the slurry will settle out.
My plan is to go with a similar ball valve but with UHMW polyethylene seats that is supposed to withstand abrasive wear.
The application is a suspended starch slurry (~35% solids) that in our view is not particulary abrasive. If anyone has any other thoughts I'd be glad to listen.
Thanks for asking.
jp
RE: 3-way ball valve Cv
What other seat materials are available?
RE: 3-way ball valve Cv
RE: 3-way ball valve Cv
Now that the particular problem have beenn pinpointed:
1. Two valves for this purpose is almost always far better than one.
2. If ball valve: is inside V-port (not ball - shell form with V) possible/available? (Always better than normal ball for regulation)
3. I suspect starch solutions could gather and have peculiar properties (behave like solids and be highly abrasive) under certain circumstances (?)
4. In addition to flow velocity reduction, if possible: ceramics, or possibly other materials with long standtime for solid suspensions, should be preferred.
RE: 3-way ball valve Cv
You mention a V-port above (Note 2). Remember this is not a throttling application. It merely diverts flow 90 deg from the straight through flow pattern.
The idea of two 2-way valves is good but is somewhat problematic in this case due to space and control system available points, etc.