Butterfly Valves fundamentals

[Outlaw500]

Does anyone know if you can calculate the dynamic torque required to operate a butterfly?

I'm designing my own butterfly valve for a special application in air handling.

It is about 96 inches in diameter and has about 40 inches of water column across the valve at closed state.

What is the best way to go about calculating opening / closing torques required. In looking at the AWWA spec's I'm supposed to get the valve coefficients from the manufacturer. In this case I am the mfg...

Thanks for the help.

 

[bcd]

Hello,

The standard way to calculate dynamic torque is to run a flow test and measure the torque required to rotate the valve at each position. Steps are taken to account for direction of rotation and you then mathmatically determine a dynamic torque coefficient. From this, you can use this coefficient on geometrically similar valves to estimate the torque at various positions. You need to know the differential pressure for each open. To get the DP, you would need to know the Cv, which considering you have a new design, you may not know that either.

Dynamic torque is a basically a form of fluid drag. The incomming flow hits the leading edge of the disc and generates a local high pressure area. The net force from this area times the distance to the centerline of the shaft gives you dynamic torque. Fluid drag is linearly proportional to density, which is why dynamic torque in butterfly valves is not a problem with gas. It sometimes (very rarely) is a problem with liquid. Most of the time, the seating / unseating torque is several times greater.

If you need an estimate, you could use a manufacturers Cv data, calculate DP assuming constant upstream pressure. Then concervatively use the 1/2 the disc area (upstream side) to calculate a force. Determine the center of pressure for that area and multiply it by the force. You will get a very conservative dynamic torque value.

To get overall valve torque, you will need to include bearing friction and seal friction.

However, considering the size of your valve, the low density media, and low DP, dynamic torque will be far less than the torque needed to seat the valve.

 

[Outlaw500]

I'm considering a clearance seat on this damper because my leakage percentage I need to hold isn't that great.

I found the following formula to check the leakage rate does it look correct?

Q = 636 x A(P x Delta P / Temp )^.5

Where:

Q = Flow Rate CFM
A = Area of gap in square inches
P = Inlet pressure (psi)
Delta P = Differential pressure across damper (psi)
Temp = Process air in Degrees Rankine

 

[insult2injury]

One of the AWWA standards has a calculation procedure defined I believe.

I2I