Cavitation Free Discharge

[konrads]

Hi

I have a problem where a DN 700 butterfly valve that has been specified on the discharge from a 9 metre high ground tank. The flow from the valve discharges into an initially empty pipe. Currently I am trying to determine the discharge velocity and flow rate for a head of 9 metres with varying degrees of valve opening angles. I have located a number of equations referring to sigma and FL coefficients but have had little success in rationalising the results. I would appreciate any assistance or previous experience

 

[yogibear1]

Generally, butterfly valves are not used for regulation, they are for shut off. Throttling service may, depending upon the style and method of attaching the sealing elements to the body and the disc, rip the valve internals apart. That said, butterflys can regulate if you consider the valve expendable. But the range of throttling is narrow; one has to close the valve considerably before much happens.

Velocity will also vary with tank head. If the tank is relatively large compared to the flow out, then the head change will be minimal. You can calibrate the valve and determine flow rate by plotting the time to fill a known volume - the empty pipe, at differing valve positions.

 

[plainwater]

what exactly is the purpose of exercise?

 

[konrads]

As detailed above, the storage tank discharges into an ocean outfall. Initially the ocean outfall is empty and fills before full operation. The period that the line takes to fill is in the order of 5 to 10 minutes. Under these initial conditions free discharge is experienced. I am attempting to determine the discharge rate into the line and calculate the rate at which the line fills.

 

[Bbird]

The flow relation for any valve can be written as

V=Cd*square root(2gH)

Where Cd = Discharge coefficient , range from about 2 to 0
V= velocity of delivery pipe
H =head loss across the valve

A loss coefficient (reciprocal of Cd) is also used and has a range of 0 to infinitive. Many valve manufacturers can supply the valve characteristic (curve showing Cd varying with opening angles). Wylie and Streeter book “Fluid Transients” also gives typical values at the Appendices.

It appears your problem may be more complicated than it sounds. If you can tell the line is empty and later fully filled by a 700NB valve I would bet your valve is discharging into a pit. This is because air has to be displaced as the outfall pipe is gradually filled. The air must travel back to the tank if there is no pit and the pipe is continuous. Having a pit means the water between the pit and the ocean flows by gravity and your valve problem is confined between the tank and the pit.

The complication of your valve assessment is the difficulty in establish the head loss, as the water level in the pit (if it is there) could be rising while the water level could be dropping inside the ground tank . Thus Yogibear1’s comment of varying head in the tank applies. The valve in this case should have a short pipe dips into the water of the pit. Alternatively your valve may be discharging into the atmosphere over the pit for which the head inside the ground tank to the discharge point will be your head loss H.

If you can establish the head loss with the valve at known opening angle, you can get the discharge reasonably accurately (for engineering purpose) using the published discharge coefficients.