Ball Valve Non Linearity
Ball Valve Non Linearity
(OP)
I am modelling a system with a ball valve for emergency shutdown. In one reference document the term "non linearity" is used to describe the type of ball valve. I take it that this is the relationship of the open area or Cv to % rotation of the stem. Factors betweeen 3 and 7 have been quoted.
I am looking for a text that explains how the linearity factor can be applied to the Cv vs % open characteristics.
I am looking for a text that explains how the linearity factor can be applied to the Cv vs % open characteristics.





RE: Ball Valve Non Linearity
RE: Ball Valve Non Linearity
RE: Ball Valve Non Linearity
http://www.eng-tips.com/viewthread.cfm?qid=280761
RE: Ball Valve Non Linearity
Thanks for the link. I was aware of this thread as I had participated in it.
What I need to understand is this term "non-linearity" in relation to a ball valve.
My take is that the valve is controlled in opening and closing by a PID loop and thus the valve Cv vs % open can be characterized when Cv vs time data is used in a transient analysis. However it may perhaps be used to express the physical characteristics of the valve.
What I don't understand is the "factor" associated with this non-linearity. I am after an expression that can be used to adjust the valve Cv vs time in a transient model.
I have asked my client but the answers are slow in coming back. I have searched the web before posting the question. have contacted valve manufacturers.
RE: Ball Valve Non Linearity
"Non-linearity" in terms of a ball valve's flow coefficient versus position, simply means that there's not a constant relation between the two.
That is, for example, that when opening a ball valve, the first 10% of position change will result in the Cv value increaseing to 40% of the full open Cv value...while moving the valve the last 90 to 100% open only increases the Cv value say from 99% to 100%.
This is as opposed to a linear globe valve type control valve, where the percentage open and percent Cv value track almost one for one.
So in a Cv versus time model, you can curve fit the valve chart into a into a polynomial or exponential equation, and assume a valve closing speed, and there you derive a Cv versus time equation to use however you want.
RE: Ball Valve Non Linearity
RE: Ball Valve Non Linearity
somebody posted this document a while back -- I thought it was helpful and kept a copy.
Patricia Lougheed
******
Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.
RE: Ball Valve Non Linearity
Good on ya Patricia, I looked all over and couldn't find my copy anywhere.
Stanier, your ball valve may vary. However, it's a matter of making your own curve-fit to the diagrams on page two of vpl's post. Then adjust based on your own testing if needed for accuracy.
Folks have used this method to measure flow in situations that do not require great precision.
Good on ya,
Goober Dave
RE: Ball Valve Non Linearity
... Note and question? 'Non -linearity closing' by ESD could that not also shoow to an 'outside' influence throtteling or slowing the actuator to obtain a fast first-step closing (say down to about 80% closed) and the rest (second step) slowed down to avoid water/fluid hammer?
Is it clear in the factual description that this common closing down sequence above is not the 'non linear' movement referred to, as all ballvalves, more or less, will have the non-linear flow-through as described in the attached curves, and it seems a bit special to describe or specify details in a common ball-valve's flow-through curve as 'a requirement' if this is the case ?
RE: Ball Valve Non Linearity
it would appear that this non linearity factor to describe a ball valve performance is not in common usage. I shall return to the source of the report and try and gather an explanation. When I do I will post same here.
RE: Ball Valve Non Linearity
rmw
RE: Ball Valve Non Linearity
Despite the fact ball valves are not intended for control purposes and are should be managed fully open or closed, in some instances (when performing transient flow analysis) it could happen that variation of Cv vs open % becomes important.
In this link you can get further info
h
Nonlinearity could also be interpreted as the relation existing between minor losses through the valve and velocity (and consequently flow).
h = K*v^2/(2g)