Non-lubricated plug valves generally fall into four main types including lift, movable-seat, resilient-liner, and what other type?

In the context of non-lubricated plug valves, the four main types include lift, movable-seat, resilient-liner, and double-block bleed (DBB) valves. The double-block bleed valve is designed to provide a positive seal in both directions and also allow for bleeding of the line between two isolated block valves. This makes it a critical component in systems where safety and isolation are paramount, especially in applications that must prevent the mixing of fluids or maintain pressure integrity.

The double-block bleed design allows for maintenance and testing without interrupting the operation of the system, enhancing overall reliability and safety. Understanding this type of valve is essential when working with systems where flow control and leak prevention are priorities.

The other types of valves listed in the question (check, globe, and butterfly) serve different functions within fluid control and are not classified as plug valves, which specifically refer to a valve that uses a plug mechanism for flow regulation.