What is the primary function of a plug valve?

A plug valve primarily serves to control the flow of liquid through a pipeline. Unlike other types of valves, it has a cylindrical or conical plug that fits into the valve body. This design allows the valve to be easily rotated to either allow or block flow. When the plug is rotated 90 degrees, it either opens or closes the pipeline, providing a straightforward mechanism for regulating flow.

The ability of a plug valve to rotate allows for quick shut-off, making it efficient in applications where rapid changes in flow are necessary. While it can be useful in dealing with some debris or scale in the pipeline to maintain functionality, the primary purpose remains focused on flow control rather than the specific shearing action described in the incorrect answer choice. Thus, viewing the main role of a plug valve as flow regulation provides a clearer understanding of its design and application.