Final answer:
The two main types of butterfly valves are wafer and lug. Wafer type valves are used for maintaining a seal against bidirectional pressure differential, fitting between two pipe flanges, while lug type valves have threaded inserts and can be installed with bolts on either side, allowing for one side of the piping system to be disconnected without affecting the other.
Step-by-step explanation:
The two general types of butterfly valves are 1) Wafer and lug 2) Flanged and wafer 3) Lug and flanged 4) Wafer and flanged. The correct answer is wafer and lug type butterfly valves. Wafer type butterfly valves are designed to maintain a seal against bidirectional pressure differential to prevent any backflow in systems designed for unidirectional flow. They are used for applications that require a tight seal, and they fit between two pipe flanges. Lug type butterfly valves, on the other hand, 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 no nuts, as each side of the valve has its own bolts. It is also possible to disconnect one side of the piping system without disturbing the other side with lug type valves.