Valves which operate by rotation of a plug member are highly effective, and are of course widely used for many applications.  Wear of relatively moving parts is however a matter of substantial concern in such devices, and may be particularly problematic when the application creates high-pressure loads upon the plug. 
Wear reduction and improved facility of operation have been afforded in valves of this type by providing means for displacing the plug away from its seat before rotation is initiated.  Valves having such a feature are disclosed in various forms in the art, exemplary of which are the following U.S. Pat.  Nos.  2,125,810 to Reed; 2,289,722 and 2,412,529 to Mueller et al; 2,459,682 to Carrie; 2,562,003 to Tratzik; 2,706,997 to Moody (showing a threaded interengagement of the stem with the plug); 2,707,613 to Wheatley; 2,708,094 to Heinen (employs plates which move radially to effect sealing, and locking rollers having two working positions); 2,711,302 to McWhorter (provides escaping pins that move into and out of arcuate tracks and indentations); and 2,988,319 and 4,073,468 to Erwin.  Axial displacement is imparted to the valve plug of German patent No.  714,994 by use of bearing elements, and Lew et al disclose, in Australian specification No.  13980/83 (priority for which is based upon U.S. Pat.  Ser.  No.  372,462, filed Apr. 28, 1982), numerous embodiments of plug valves in which cylindrical ball plugs are used having a hemispherical end and a bore extending at an oblique angle therethrough.  In certain instances (see FIG.  4) the valve of Lew et al is constructed so that the plug is lifted away from its seating surface prior to turning, and in the form shown in FIG.  21 a secondary plug, threadably engaged upon the end of an auxiliary stem, is used for flow throttling.  In Lew et al U.S. Pat.  No.  4,498,371 a direct-acting rotary actuator cylinder is described for use with the type of ball plug valves described, and a commercially available plug-type valve, called the "General Twin Seal Valve", includes mechanical means for freeing the plug before it is rotated from the closed to the open position. 
Despite the prior art activity indicated by the foregoing, a need remains for a valve that is adapted for high pressure applications, which has a rotatable plug that is displaced from its seat prior to opening, and which is highly effective and reliable, durable, and relatively facile and economical to manufacture. 
Accordingly, it is the broad object of the present invention to provide a novel plug valve which is capable of operation under high pressure conditions, and which is highly effective and reliable, durable, and relatively facile and economical to manufacture. 
It is a more specific object of the invention to provide a novel half-turn valve in which the plug is tightly seated in the fully closed position, and in which the stem is turned to displace the plug from its seat and to thereafter rotate it to open the valve. 
A further and even more specific object is to provide such a valve assembly wherein a unique mechanism is employed to afford highly reliable and facile operation.