Butterfly valves have been used extensively for several years in many projects all over the world and proved its capability in performing its function because they are less expensive and easy to install comparing with other isolation valves types (e.g. gate valves).
Three types are commonly used in respect to the installation namely: Lug type ,Wafer type and double-flanged.
Lug type has its own tapped holes (female threaded) which allow the bolts to be threaded into it from both sides.
This allows the dismantling of any side of the piping system without removal of the butterfly valve in addition to keeping the service on the other side.
It's also important to note that you don't need to shut down the entire system in order to clean, inspect, repair, or replace a lug butterfly valve (you would need to with a wafer butter valve).
Some specifications and installation do not consider this requirement especially at critical points like pumps connections.
Double flanged butterfly valves could also be an option especially with larger diameter pipes (below example shows 64 in Diameter pipe).
My advice: Check your specifications and installation in order to assure that the wafer type is not installed at the critical points on the line which might need any type of maintenance or repair during the service life instead, use the lug type for our range of piping in building services industry.If you have certain applications with big diameters ,you might think about double flanged type.