can butterfly valves be used for throttling

- May 18, 2019-

1. Butterfly valves have an equal-percentage characteristic out to about 60 degrees open.  They throttle just fine BUT THEY ARE LIMITED IN THEIR APPLICATION.  (Every valve has limits)

Butterfly valves have a huge Cv for their size.  Thus they have a reputation for having a quick-open characteristic.  They don't- but if you just slap a line-size butterfly valve in a line without benefit of the thought process, you will likely swamp the system and say "Dagnabit! I tol' y'all them butterfly valves wasn't no good" or other similar intellectual analysis.  If a typical globe valve for control is a size smaller than the line it is in, isn't it reasonable to expect that a much higher capacity valve would be even further reduced?  

Limitations of butterfly valves: They have a low value of Fl- also characteristic of other high-capacity valves.  Put too much differential pressure across them in water service and they are likely to begin cavitating before a globe valve would.  Is this a problem in a balancing valve? Probably not-since a balancing valve is just for...balancing, which is just creating a small restriction to match a similar small restriction elsewhere in the system.  

Rangeability can be similar to a globe valve, but it is necessary to use a gear operator..A handle with a 10-notch plate doesn't really allow for much fine tuning.  Automated valves neet to hae the actuator torque characteristic matched to the valve's torque characteristic.  And a rubber-seated butterfly valve (typical for a balancing application) requires a lot of torque to break the vane out of the seat.  I mentioned 60-degrees earlier: a good place to stop opening the valve for reasons other than capacity.  There is a dynamic torque peak at about 75 degrees where the flow becomes attached to the vane.  The center of pressure is ahead of the shaft-so the valve wants to close.  Don't open past 60 degrees for modulating servixe and this won't be a problem.  Past the torque peak the torque drops off rapidly to zero at 90 degrees open. Some sources report this as a torque "reversal" but the reversal occure in the first derivative of the torque function.   If your actuator is too "soft" it is possible to overshoot and go into a nasty dynamic.   Hence few butterfly valves offered with 3-15 pneumatic diathragm actuators these days, since a diaphragm actuator has the dynamics of a waterbed.  Nice stiff rack and pinion, Scotch yoke, or electric actuators work fine for butterfly valves.  High-Performance (aka double-offset) butterfly valves have reduced breakout torque as well as reduced dynamic torque.  Triple offset valves have even less breakout torque but require huge seating loads to compress the metallic seal ring.  The big advantage of a triple offset valve is that there are no elastomers in the valve so it can withstand extended temperature range.