A plug valve comprising, a valve body having inlet and outlet ports therein, a valve chamber in said valve body between said inlet and outlet ports and intersecting said ports and open at its opposite ends through said valve body,PLUG VALVES a tapered valve plug rotatably mounted in said chamber and having a transverse bore therethrough alignable with said inlet and outlet ports, Forged Steel Valvessaid tapered valve plug having an end face at one end thereof and extending at said one end into one of said chamber open ends, the valve plug being larger in diameter at the other end thereof and extending at said other end into the other of said chamber open ends, Bellows Seal Valvesa reduced diameter valve stem on said other end of said valve plug and extending axially therefrom through said other open end of said chamber to exteriorly of the valve body and defining with said valve plug an annular shoulder, external stem sealing means at said other open end of said chamber in sealing relationship with said valve stem, a valve bonnet secured to said valve body over said other open end of said chamber and in surrounding relationship to said stem, an annular pressure ring in overlying relationship to the stem sealing means, a plurality of equally spaced adjusting screws extended through said bonnet and into engagement with said pressure ring for pressing the stem sealing means into sealing engagement with said stem and to initially adjust the position of said valve plug in said chamber, sealing means at said one end of said valve plug and said one open end of said chamber to seal said one end of said valve plug relative to said valve body, a cap secured to said valve body over said one open end, and an adjusting screw means threadedly extended through said cap and into cooperative association with said end face to urge said valve plug toward said stem sealing means to effect a leak tight seal at the other end of the valve plug. The present invention involves the use of a micrometric or rather finely threaded adjustment screw advanceable axially of the valve plug to compress the top and stem seal thereof. Equalized seating of the plug, and alignment of its port with the ports of the valve body, are functions delegated to a three-point equalizing adjustment means surrounding the stem, as more fully disclosed in my U.S. Pat. No. 3,540,740. Motions of the three-point equalizing adjustment means and of the micrometric plug adjustment screw are interrelated, in that an advancement of said screw to compress the top or stem seal produces in effect a practically imperceptible loosening of the plug relative to its tapered seat in the body bore; however, such minute loosening of the plug is found not productive of leakage if the plug is otherwise properly adjusted, this being due to the "memory" characteristic of the plastic liner incorporated in the valve construction. The invention also is very effective in preventing external leakage when valve is subjected to temperature changes. As valve is heated the relative thick section of Teflon surrounding the plug and lining the body expands approximately 10 times the metal encapsulating body. This causes Teflong to flow and results in a looser fit between body and plug. Plug can still seal downstream due to differential pressure and since external seal means has considerable less thickness of Teflon differential expansion does not effect the external seal. An object of the invention is to provide in a lined plug valve, highly effective yet simple and inexpensive means for preventing both internal and external leakage of fluid-control valves, whether the valve is fully lined or only partly lined. The invention, moreover, is applicable to plug valves having right cylindrical as well as tapered plugs.

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