Lubricated plug valves use a metal tapered plug in a tapered bore in the metal valve body, and there are grease passages machined around the ports in the plug, Before the valve is rotated, grease is injected to float and lubricate the plug, so that the metal-to-metal wear is minimized. Lubricated plug valves are low-end, suitable for such as petroleum services where contamination by the grease is not objectionable. Literature for lubricated plugs can be viewed in the flowserve.com website uder valves/plug-lubricated/ Nordstrom or Serck-Audco.
UNlubricated plug valves use a similar tapered plug but have a self-lubricating polymer sleeve or liner in the body. The sleeve provides a noncontaminating low-friction guiding and sealing surface. Typically the ports are rectangular, but similar in cross-sectional area to the pipe, with gracefully tapered transitions, so that pressure loss is negligible. Unlubricated plug valves can be made is a wide variety of alloys or completely lined, and are an excellent choice for chemical services. They also tend to have excellent resistance to packing leakage because (1) the primary seal is the plug to sleeve seal(2) thre is a secondary pressure-energized diaphragm to seal anything that gets past the primary seal(3) they are inline adjustable for both in-line and stem seal leakage (4) for particularly sensitive applications they are offered with an additional conventional packing box with a lantern-ring so none of the product is released to the environment.
Unlubricated plug valves are available for control (modulating) applications, modified with a triangular port for equal-percent characteristic and capacity to match the application. Although they take a lot of torque to turn, this is overcome by proper actuator sizing and the use of a positioner. The positioner, actuator, and valve stem are all on one axis, and with clamped, lash-free connections the package provides accurate sensitive control at substantially lower cost and package volume than a "globe" control valve.
Eccentric plug valves are sometimes also called rotary globe valves. The eccentricity of the plug allows it to seal against the seatring without sliding contact, so that all metal trim can be used. With all metal parts in contact, Eccentric plug valves are suitable for more severe, higher-temperature and higher pressure control applications than characterized unlubricated plugs, but they are not able to provide dead-tight shutoff.
For literature on Lubricated plug valves visit
and take a look in the Durco literature.
For Eccentric rotary plug valves, also in the Flowserve website, go into the Valtek Literature and take a look at the Maxflo3.
