Applications of Concentric vs Eccentric-Disc Butterfly Valves 1

[6813248]

I wish to know the respective areas of applications for concentric-disc butterfly valves as oppose to that of off-centre disc butterfly valves.
Is butterfly valves used in water supply distribution mains? If yes shold it be concentric or eccentric type?

 

[TerryOsser]

Concentric butterfly valves are OK for utility service, these are soft seated or rubber lined valves. Generally disc is permanently in contact with seat.Eccentric butterfly valves (i.e. have an offset) have a cam effect, and so disc does not wear on seat through all rotation of disc. Triple offset designs eliminate vitually all seat wear, and are torque seated. They are the dog's danglies. Offset designs can be metal seated, and for these zero leakage can easily be obtained.
Traditionally butterfly valves are used in utility services (water, etc). This stigma is slowly being shaken off (supplier now call the high performance beasties rotary process valves)and now they are giving ball valves a run for there money in other applications.
For yours (water dist. mains) a rubber lined concentric would give average/good performance. A double offset valve would give superior performance, but is more expensive. Better (performance)still (and with the addtional cost implications!) is the triple offset, but I think unnecessary. these are now used widely in the North Sea for hydrocarbon service upto & including Class 600 (NPS 3 & above)for isolation. What was the question??!!

 

[6813248]

TerryOsser, thanks for the prompt reply. Is it true that concentric butterfly valves are designed for bi-directional flows and that this is their advantage over eccentric ones?
In water distribution mains, how does one decide between sluice (or gate) valve and butterfly valve?
Is it true that for flow control, butterfly valves are better than sluice valve?
Is it true that concentric butterfly valves are better for tight-shut?
Hope you or anyone could help.

 

[TerryOsser]

Both are for bi-directional flow. Due to the fact of the eccentricity of the eccentric butterfly valve it has a preferred direction, the force required to open the valve when the flow is in the non-preferred direction is slightly higher. In clean water service they are the most economical valve.
A metal seated valve will last longer and has the potential for less problems than a soft seated or rubber lined valve.
The choice is down to all your required parameters. Butterfly valves are cheaper than gate valves, and can be of wafer type.
It is generally accepted that gate valves are used for isolation (on-off) valves whereas butterfly valves are both throttling and isolation.
Tight shut off - with a metal seated valve (i.e. gate, aslthough you can get soft seated gate) tight shut off is obtainable, but the testing standards allow for some leakage.
Soft seated valves should always be zero leakage, unless the seat is damaged.
Bear in mind this is all from an Oil & Gas background / experience. Hope this helps