I am working on updating our specifications for plug valves and have a question.  There seems to be two main types of plug valves, rectangular ported and round ported.  I have heard that round ports are better for plug valves because the head loss is less, and I have heard that rectangular ports are better because they have better sealing characteristics.  In addition, to get a full port size with a rectangular shaped port, you have to go one size larger.
 
Can I ask which type of plug valve you all prefer, and why?  Are there other characteristics of a plug valve that are more important than port shape that I want to be sure and include in my specs?

lubricated and eccentric plug valves are the types of plug valves commonly used.  Lubricated plug valves normally have a cylindrical or tapered plug intersecting the flow with a rectangular port opening.  Round ports can be obtained in the smaller sizes.  Specially formulated greases are used both for lubrication and sealing of lubricated plug valves.  When operated periodically, these valves are relatively easy to operate and provide a tight shutoff, but the plugs may freeze if not operated for a long period of time.  Plug vlaves are especially good for high pressure applications.  Eccentric plug valves are preferable to lubricated plug requirements; eccentric plug valves are also less prone to freeze. 

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.

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