Lug type butterfly valves have threaded inserts at both sides of the valve body. This allows them to be installed into a system using two sets of bolts and nuts. The valve is installed between two flanges using a separate set of bolts for each flange. This setup permits either side of the piping system to be disconnected without disturbing the other side.
A lug type butterfly valve used in dead end service generally has a reduced pressure rating. For example a lug type butterfly valve mounted between two flanges has a 150 psi pressure rating. The same valve mounted with one flange, in dead end service, has a 75 psi rating.