The difference between the lug and wafer type butterfly valves and when are these specified? I can't seem to find a clear definition of each.
In the refining and O&G production industry, exposed bolts can expand and be a leakage risk during a fire.
A wafer type is therefore only installed in flammable service if protected by rigid fire insulation.
Lug type valves are used in end of pipeline service.
Lug types can be threaded or with the threads drilled out.  The advantage of drilling out the threads is ease of installation/removal (removing galvanized bolts and re-installing can cause galling which requires re-cutting the thread).  Installing a single through-bolt is easier.
The area near the shaft typically will have two threaded holes on either side at 0° and 180° for machine bolts.
Wafer are cheaper than lug.
Double flanged butterly valves available in triple offset designs are the same price as lug and easier to install/remove but have longer face-to-face dimension.

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