How are single-sealed butterfly valves and double-sealed butterfly valves distinguished?

Single-Sealed Butterfly Valves

In a unidirectional sealed butterfly valve, the disc must face the direction of media flow when closed; the media flows in only one direction, and the valve body must be marked with an arrow indicating the direction of media flow. Care must be taken to ensure the media flows in the correct direction during installation.

Double-Sealed Butterfly Valves

A bidirectional butterfly valve allows the disc to face either toward or away from the direction of media flow. During installation, there is no need to consider the direction of media flow, and there are no arrows on the valve body indicating the flow direction. The load on the stem of a bidirectional butterfly valve is greater than that of a unidirectional butterfly valve. In design, for butterfly valves of the same diameter and pressure rating, the stem diameter of a bidirectional butterfly valve is larger than that of a unidirectional butterfly valve.