Difference of Lug type and Wafer type Butterfly Valves

What is the difference between Lug type and Wafer type Butterfly Valves?

The difference between a Wafer and Lug Type Butterfly valve is only the mounting scheme.

Lug type valve has lugs that are used to mount the valve to lugs that are attached to the flanges on the ends of sections of the pipe so that each length of pipe can be removed and replaced independently. 

Wafer type valves don't have attachment lugs, though they may have alignment lugs. They are sandwiched between the flanges on the ends of two lengths of pipe. To replace either length of pipe requires shutting the system down.

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