Butterfly Valve Series

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Authority of Butterfly valve when used as control valve

2010-11-17

When trying to install a two-way control valve of 450mm size and larger, we usually end up using motorized butterfly valves (globe type is very expensive). The application is chilled water system, Plate Heat Exchanger.
Butterfly valves also have HUGE capacity for size compared to globe control valves, limiting them to 60 degrees cuts the effective Cv in half, and if you have put a 60-degree cam in the positioner (or its electronic equivalent if the positioner is digital)  then you have all the points of resolution in the 60-degrees that you would have had over 90 degrees. 
If a butterfly valve is operating within its delta-P limits then it is really only limited by rangeability.  Butterfly valves just don't have great rangeability.  25:1 is a working number. But you don't NEED more than 10:1 in this application.   Rangeability for the sake of this discussion is the 60-degree Cv divided by the minimum controllable Cv.  HPBVs get a little wonky as the disc srarts to engage the seat.  By comparison, globe valves tend to have 50:1, Characterized ball valves are published with 300:1, and eccentric rotary valves are published with rangeabilities of greater than 100, but I have seen over 3000:1 in the lab with 'em.
The big trick with a HPBV or any rotary valve for that metter is to make sure there is no lost motion in the linkage.  THe valve has to attach to the actuator with a clamped coupling, and the positioner has to read the valve/sctuator stem directly-not through a linkage or an attached device.  


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