Butterfly valve? The butterfly valves I know close in 90 degrees.

Aren't you looking at the data of a globe valve. These valves have a correlation opening x flow rate.

Steven van Els
SAvanEls@cq-link.sr

Large butterfly valves could have gears and such in the spindel but normally a butterfly valve will close in 90 deg - so unless the gear is really funny the closure of the valve will also be linear.

This does however not mean that the "resistance" introduced by closing the valve will increase linear. As i recall it the flow will only decrease slowly when close and then drop abruptly when the valve is almost fully closed (the last 20-30 % introduceses almost all of the pressuredrop)

Best Regards

Morten

the valve is definitely a butterfly valve.  The info I have (from North American Combustion Handbook) agrees with Morton's info (it states the valve has 100% flow when 60% open, but that the flow drops off sharply in the last 30% of closure).  Thanks for your help.

I am unable to get your problem . can you give me more details.
Regards

the valve is outlet to a heat exchanger.  the heat exchanger requires 5000 gpm under certain conditions.  The post-maintenance acceptance criteria for the valve is 22 to 26 turns.  There is not normally flow instrumentation on the line.  The correlation given (1 turn = <176 to 213 gpm>) was derived from a computer simulation.  Using a linear approach, I don't come up with 5000 gpm at 22 turns.  Not being a valve expert, and given the information on valve characteristics, I don't think the linear approach is right.  What I'm looking for is  a (1)  a correlation for butterfly valves that would compare number of turns to flow rate.  this correlation can include other variables than what I have - because I'm also looking for
(2)  what other information do I need to obtain?  I'm thinking gear ratio, percent open at 100% flow, and percent open under normal throttled conditions.  But do I need anything else???

thanks

I believe the chart in the following link has exactly what you are looking for in regards to the throttling characteristics of butterfly valves.

http://www.me.cc.va.us/dept/ietech/water_wastewater/distance_learning/courses/CIV240/butterfly_valves3.htm

Regulating the flow is done by increasing the resistance through the passage of fluid.
This can be done by globe valves, or ball valves with a special bore. These valves have a defined correlation valve opening x flow. This correlation can be linear or some defined function.

Gate valves and butterfly valves are used for shut-off purpose not regulating flow. The correlation valve opening x flow is definitely not linear.
I suspect that your valve has some mechanism and you have to relate turns x valve opening, but to extend that to the low rate would be very difficult

Steven van Els
SAvanEls@cq-link.sr

This one's pretty easy.  You just need the Cv curve for the valve and an understanding of the flow process. The Cv curve for the type of butterfly valve is available from the manufacturer.  Enertec can definately get it for you. You can also find it on most manufacturers web-sites.

If you don't already know this, Cv is called the flow coefficient.  For liquids, you can quickly calculate the flow rate knowing the differential pressure (DP).  Flow rate (gpm) equals the Cv times the square root of DP.  DP units are psi.

For a simple start, assume the DP is constant, regardless of the valve position.  It probably is not, and there are ways to account for this and have a more accurate prediction of flow rate.  However, it would be a textbook long response.  Using the equation above, make a curve of valve position versus flow rate.

Now, the gear operator requires a certain number of turns for 90 degrees rotation of the valve.  You can operate it count the number of turns or get it from a product catalog.
Just rplace the valve position on the lower axis of the curve with the equivalent number of turns to get to that position.  Now you can quickly estimate the number of turns to get a certain flow rate through the valve.

I hope you find this helpful.

BR

As BCD said, go back to the manufacturer.  A generalized Cv versus opening for a butterfly valve is fine for rough work but I get the feeling you want something more accurate and I'd want to start off with the manufacturer's curve.

In response to another post, butterfly valves can be used for throttling with the current designs.  Some of them are designed for essentially on/off service but that isn't universally ture.

This is an update to this question.  Based on the information received here, I went back to the utility and questioned their values - and found out they had made the grossly inaccurate assumption that percent open = percent flow.  They're now redoing some calculations to get a more accurate number.  Thanks to everyone who responded on this.

Patricia Lougheed

Are they going to refer to it as the VPL correction factor?

So the gate valve has definitely not a linear relation of flow x opening

Steven van Els
SAvanEls@cq-link.sr

Butterfly valves are meant for quick open and tight shut off only.And I agree with svanels about the Butterfly valve's function.
My thinking is,If The Handwheel is connected to a gearbox(It has to be!)Then The Disc's degree of opening depends on the module and pitch of the worm gear inside the gear box.
Now,The gear box is selected on the Basis of torque value of the valve.Each gear box has a unique value of Torque range of Operation beyond which It cannot be used.For eg.:-
consider a series of gear boxes called eng series then you name it as eng0-150,eng150-250,eng 250-350,eng350-450 etc.The numerical value being the torque range the paricular gear box can operate.Suppose you have a valve having a torque of 275kgm,the gear box selection is done by multipleing a factor of 1.3 to the torque which is 1.3*275=357.5kgm so then the eng350-450 gear box is selected for the valve. Sometimes for reasons of economy the gear box is selected with the minimal factor of safety then the eng250-350 gear box fits in.In the former case you have a larger gear box than neccessary but are assured of it's safe function;In the latter case the gearbox is much smaller and you are saving money too  but the flip side is the gear box is not design worthy for higher pressures.Kindly check with your supplier as to how the selection was made.
Also,Find out if the Handwheel is designed for the particular gear box only.
I am sure this info. will help you.

Hi,

I have seen a lot of words in the replies but....

First the operator:
a 15" you say (funny size not a 14"or 16"?) defenitely requires a gearbox. Depending on the forces required to open & close and the dynamic torque (!) there is a certain gear selected.
Any supplier can tell you the number of turns for a full open to close travel, so for the 90 degrees output.
>>>>Turns


The flow characterisitic for a traditional butterfly valve is equal percentage and tends towards the linear line as it is more off set (double or tripple eccentic BFV's). It will stay more eq% than linear.
There is a characteristic and if not there are definately steps per 10 degree steps available from the supplier.
This must give you a good estimate of where you want to set your valve position for you given flow.
>>>> Flow

There are plenty of inaccurancies in this. What is the medium, downstream/upstream behaviour...how stable is the system, are there more consumers on that same flow

are you sure you do not prefer a flow sensing device and a control valve instead.
Other than some say, you can control with BFV's but this is for Flow control. Not for pressure control while the recovery factor for BFV is high.

Good luck !