11 replies to this topic

[Brightonnk]

Hey everyone, so Im currently doing some calculations regarding cv values for ball valves. I have devised a method that now calculates the flwo coefficient for most ball valves however I have discovered there is a new type of ball valve which instead of a parallel bore, it has a hollow ball- allowing the flow to not go ina straight line. Does anyone have any knowledge on how to even begin calculating the pressure drop in that ball? do i use intergration or are there any other equations that would do this for me?

Thanks in advance

[Brightonnk]

Does anyone know anything about hollow ball valves?

[Art Montemayor]

Brightonnk:

ALL ball valves are hollow. That is the only way you can pass fluid through or across them.

Please be specific in your description. Submit a drawing, a photo, a sketch or a reference in the internet where we can see such valve type. In the 52 years I've been an engineer in the field, I have seen just about every type of valve devised - good and bad. I don't understand your description and haven't the foggiest idea of what type or make of valve you refer to.

[Brightonnk]

Hi Art,
Sorry for the confusion- Im still a student and probably am not the confident in explaining things.
The 'hollow' ball I am referring to is ... http://files.enginee...le=AA000002.pdf

something like this ball [see url]
...As supposed to that ball having a parallel bore. I can easily calculate the pressure drop and then cv value when the valves bore is straight but when the ball is a like a shell I am having difficulty to get a precise accurate calculation.

Thanks

[Steve Hall]

Here's a better picture. I don't know that you can expect to accurately calculate the pressure drop through this ball, especially at partially closed positions. The manufacturer should provide Cv values based on flow tests.

(Source: http://www.wellball....=38&pc_parent=9)

Attached Files

•  hollow-ball-1.jpg   7.5KB   7 downloads

Edited by Steve Hall, 04 December 2012 - 10:23 AM.

[Brightonnk]

Hi steve, The project im doing at the moment is to calculate cv vlaues for a range of valves. the valves come with cv values but the values are an unreliable source. So i was just wondering how i would go about calcualting the cv value from first principles. but if you say its not accurately done then that answers my questions.
Thanks

[Brightonnk]

also the ball valves I have are on/off valves so I am only calcuating the valves at open position (100%)

[Art Montemayor]

Brightonnk:

As Steve has stated, it is normal practice for manufacturers to supply Test-based C_V values for each of their valve types and sizes. C_V values, to my knowledge, are not "calculated"; they are developed from actual test data - normally with the valve 100% open (unless otherwise indicated as to how open the valve is measured at).

I don't understand how you would be calculating the C_V values for valves.

Thanks to Steve, I now can see that spherical valves are being fabricated as spheres - instead of solid, drilled balls. I would suspect the C_V values for this type of valve to be at least equal to that of a full bore ball valve. I am assuming that the turbulance caused by the excess space inside the sphere does not deter from the C_V value of the full-flow effect of this type of valve. However, the manufacturer should be the expert on this and should be able to furnish you will all their acquired C_V values for their valves.

[Brightonnk]

cv values might not usually be calculated, but I am calculating them- its a project to run alongside my degree with an established valve manufacturer.

They have cv values which come from the manufacturers but they don't belive them to be correct. So i am calculating and then later validating my calculations with testing. So obviously I will find out the cv value from testing the valves but as part of my project i am calculating/predicting the cv values.
So i was just wondering if there was a specific way to do so. But thank you for replies and I will take note of them

[henryleung]

From my understanding, the Cv value is an empirically determined value. It is a empirical factor to determine pressure drop at fitting/valve etc. One way to calculate it (and the only way I can think of) is by CFD and assume certain surface condition on the inner wall of the valve. I guess you can draw your ball valve using CAD and then put it in to a CFD simulation software package to calculate the Cv that way.....To theoretically calculate the Cv you must solve the navier-stokes equation with your ball valve geometry and surface condition..

[kkala]

I think http://www.cheresour...id-ball-valve/'> http://www.cheresources.com/invision/topic/16938-spherical-shell-ball-vs-solid-ball-valve/ can be also useful to this topic.

[Brightonnk]

henryleung: Thanks for that information, CFD is something I have looked into alogn with actual testing using a flow loop. Would I get the same sort of results from testing and CFD? or is one more reliable than the other?

Thanks