Sealing without sealant
In a non-lubricated plug valve the tapered plug bears on a polymer surface whose resilience and low friction replace the sealant film of the lubricated type. PTFE deforms slightly under seating load and flows into the microscopic roughness of the plug face, so the seal is continuous over a broad conical band; and because PTFE has one of the lowest friction coefficients of any solid, the plug turns against this loaded surface without galling even though contact pressure is high.
Sleeve mechanics
In a sleeved design the PTFE sleeve is keyed into ribs or recesses machined in the body so it can neither rotate with the plug nor extrude downstream under differential pressure. The taper geometry allows compensation for wear: gland or cover adjustment presses the plug deeper into the sleeve, restoring seating stress after the polymer relaxes. This adjustability is why a sleeved valve retains bubble-tight shutoff over many cycles, but it is also a limit, because over-tightening raises torque sharply and accelerates cold flow.
Lined valve mechanics
In a fully lined valve the fluoropolymer covers every wetted surface, so the shell never sees the fluid and can be ordinary ductile iron. Sealing occurs lining against lining over the taper. Permeation is the design issue: aggressive small molecules migrate slowly through fluoropolymers, so lining thickness, moulding quality and venting of the shell decide service life in hot concentrated acids, and reputable manufacturers will state their lining thickness and spark-test practice.
In-line behaviour
Open, the port gives a straight, low-turbulence passage with no body cavity, so the valve flushes clean and nothing stagnates. Non-lubricated plug valves are on-off valves: throttling exposes the sleeve edge at the port to high-velocity flow, which nibbles and cold-flows the polymer and destroys the seal band. Temperature matters doubly, because the polymer softens and creeps as temperature rises, so pressure ratings fall steeply above about 150 degrees C, well before the metal shell is challenged.
Failure modes
The characteristic failures are cold flow of the seat under sustained load or heat, chemical attack or permeation of the polymer by incompatible media, torque growth from over-adjustment, and solids embedding in the soft seat in dirty service. Media that PTFE cannot resist are few but real, notably molten alkali metals and some fluorinating agents. Each severe duty has an answer, reinforced grades, PFA linings, jacketing or lift-plug action, which is why the medium must be declared precisely.
Why detail matters
Seat retention design determines resistance to pressure surges, polymer grade sets chemical and temperature limits, plug surface finish sets torque and seal life, and gland architecture sets fugitive emissions performance. An enquiry that names medium, concentration, temperature and cycle duty lets the manufacturer pick all four deliberately instead of quoting a generic valve.
