Ball Valve Bore vrs Cv 1

[foaman]

Can anyone tell me why the max Cv of ball valves from different manufactires varys so much for valves of the same size?

For example, one manufacture of a 3.0" ball valve has an inside bore of 71.9mm and calaims their max Cv is 195. Another manufacturer of similar construction claims that their 3.0" ball valve with and ID of 65mm has a max Cv 640.

 

[rconner]

The comparative values you supply are not intuitively obvious (you might want to check the vendors' intended units for this coefficient).

 

[belowzero]

You have to understand what goes towards making the CV figure in design terms.
There is not just ball bore to take into account, the length of the valve, the run in and run out to the ball, stem holes and intrusions, sharp machined edges, casting finish etc etc.
The dimensions you state look like a full bore and a reduced bore, you should be comparing like for like, regardless of the figures. As a reduced bore WILL always have poorer flow in comparison to a full bore, even if taking short pattern against long pattern.
B

 

[rconner]

The way I read it (assuming units are the same) the manufacturer with the smallest bore appears to be claiming drastically better flow/less headloss than that of the larger bore (while I don't know enough to challenge this strongly, this does not have at least obvious basi,s and I suspect there may be also some skepticism of the original inquirer resulting in th epost to begin with.
This is why I wonder if by any chance the first valve is giving units e.g. in gal/time unit, and the second in something else (e.g. liters/time also questioning units)?

 

[cliff6361]

Fellows, Cv is a coeficient factor that has no units! The units associated with Cvs are the base for which a flow factor is determined. These units do not carry to the Cv factor.

What can have a bearing on the value is the interior shape of the flow passage and valve length. Think of a full port ball valve as a piece of pipe of the same lay length as the valve. The Cv should be very close to this piece of pipe.

Standard or reduced port however is differant. These have a reduced section coming in, smaller ball ID and an enlargement going out over the valve length. Thereore the Cv facter will be differant in this cases.

 

[katmar]

One way to get a handle on this is to look at the generic K values that are given for ball valves in fluid mechanics texts. For a 3" full bore valve a typical K would be around 0.07. Converting this to a Cv gives 1017 (Cv = 29.9 x d^2 / sqrt(K) ).

Typically a reduced bore ball valve has a bore one size less than the nominal pipe size, so 65 mm for a 3" valve is about right for a 2.5" ball. Typically you could expect a K of about 0.47 and therefore a Cv of 392.

Based on these values I would say your first value of 195 is a typo and it should be 1095. But 71.9 mm isn't what I would call "full bore" and 1095 is probably a bit high.

The second value of 640 is about typical of the overstatement of many valve manufacturers and I would regard around 400 as more realistic.

Another way to get an estimate is to regard the ball as a "thick orifice" and use those formulas.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rconner]

It sounds like most of us are at least in agreement there is a problem here! (cliff6361, is the definition e.g. at

http://www.engineeringtoolbox.com/flow-coefficients-d_277.html,

that appears to mention Imperial units, incorrect?)

 

[gerhardl]

Hello all, rconner has already given the correct answer.

Both Kv and Cv values are not unitless factors, but will contrary always require that the units and conditions must be given.

The whole purpose of both Kv and Cv are to give the actual measured flow through a valve at given conditions: pressure loss and type of fluid (water) and temperature held constant, and flow amount (expressed in volumetric units per time unit) given as the measured result.

For normal ball valves, with exact equal opening, the figure will not be very significant different from make to make for new, undamaged valves.

Different actual openings (reduced bore opening in fact where only nominal opening data is given) will of course give the wrong result if you take the nominal opening trying to figure out the flow.

So: check the actual units and conditions standing behind the Cv's you have been given, check that the opening diameters are the real minimum openings, and third ask if the result is measured figures or if the company has given an approximation based on measurements of valves of diameters near, but not equal, to the ones you have.

The comparison can only be made correctly if all these factors are known. It might well be that several of the possible explanations given by others are part of the 'real' answer to your question.

 

[belowzero]

Units/conditions - CV = Imperial KV = Metric. There are numerous publications and internet pages. Different calcs/factors for Gas and fluid. I must admit both look a little dubious for the CV's they are giving, a 'Jamesbury' 3" Full bore is 1160 and reduced bore 350, these are very reliable figures and a good basis for comparison.
B