Which should be operated more gate or butterfly valves.

[Habee]

I'm incharge of a valve maintenance program for a water distribution system. We go through the systems and operate every valve. My boss has asked me which valve should be operated more frequently, a gate valve or a butterfly valve. I can not find any data on this subject.

 

[zdas04]

In a moderate-pressure water distribution system, the reason for exercising a valve is to clear the seating surfaces of scale and on rising-stem valves to re-hydrate any packing that is not externally lubricated. A gate valve has a much larger capacity to clear scale (more force applied normal to the seating survace), but a much larger need to rehydrate the packing. The butterfly valve can only apply crush force to a scale build-up and won't be very effective if the scale has substantial depth.

My feeling is that if the gate valve has greased packing then I'd exercise the butterfly valves 4-5 times more often than the gate valve. If the stem packing in the gate valve has a tendency to fail through drying-out I'd cut that back to exercising the butterly 2-3 times more often.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

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[JimCasey]

Supplemental to what David has said, A butterfly valve especially in what seems described here as an AWWA application will >usually< be rubber-lined and as such the scale will fracture and dislodge when the valve cycles. It is advantageous with a butterfly valve to install with the stem horizontal, and for the vane to open so that the bottom edge moves upstream. This will fracture/dislodge the layer of sediment/scale at the bottom of the pipe and the particles will be flushed away by the initial high-velocity flow as the valve begins to open. Another advantage of mounting BFVs with Horiz shafts is that the bottom bearing is not sitting in a puddle of sediment and the valve lasts vastly longer.

I have never had personal experience with a gate valve that does not leak, and crushing scale between the seats and disc will not contribute to improving the surface finish hence leak resistance of the valve.

 

[rsv]

Gate valves are often used on petrochem plants as primary isolation valves in case of emergency as, they can be relied on to close when required even after being left for long periods in the open position.

 

[stanier]

If the gate valve had an inside screw as often used in the water industry frequent operation is desirable to stop corrosion build up on the thread.

 

[rsv]

I didn't think anyone bought valves with inside screws?