The primary function of a plug valve is isolation i.e. either fully open or fully closed and the secondary function can be reduction. This is achieved by tapering the valve bore and reducing the aperture in the through port in the middle of the plug.
Plug valves are very similar to ball valves, again quarter turn operation via manual hand lever or other actuating device. The seat to seal interface is created by the polished plug rotating within the valve body.
It can be a straight plug, or a tapered plug depending on the performance required for the process conditions. Energisation is by mechanical force, which is built in to the valve at the assembly stage to the valve. Lubricated and lined plug valves also offer varying degrees of tight shut-off and operating torque requirements.