PTFE, graphite and molybdenum disulphide are fine particles suspended in a grease or sealant carrier, and they form a film on sliding surfaces that cuts metal contact, friction and galling under load. PTFE is chemically inert and kind to soft seats, graphite prefers a trace of moisture, and molybdenum disulphide carries heavy loads in dry conditions. The additive supports the base compound, but it never replaces choosing the right grease for the service.
Knowledge ยท Lubrication science
Solid additives in valve compounds: PTFE, graphite and molybdenum disulphide
Solid additives give valve compounds friction control that base oil and thickener alone cannot supply.
1. What a solid additive does
A valve grease or sealant has two main parts, a base oil and a thickener. Some compounds also carry solid additives, which are fine particles held in suspension. PTFE, graphite and molybdenum disulphide are the solids commonly met in valve work.
Under light load, the oil film keeps sliding surfaces apart. When a valve is pressurised and seats clamp down hard, that film gets squeezed very thin. Solid particles then lodge in the surface peaks and keep the metals apart.
This regime is boundary lubrication, and it explains why valve lubrication goes beyond plain oil. A solid additive still needs its carrier to reach the surface. The carrier delivers the solid and keeps it in place.
The same film helps in three places inside a valve. These are the seat faces, the stem and packing area, and the bearings. Each place sees different loads, so each asks different things of the solid.
2. PTFE in valve grease
PTFE is a soft fluoropolymer with very low friction and almost no chemical reactivity. In a valve compound it gives a slippery film that suits stems, trunnion bearings and soft seats. It does not attack elastomers, and it copes with a wide range of media.
These qualities make PTFE grease a common first choice for valves in chemical and gas duty. It is also forgiving when service conditions change over time. If the film breaks down, the residue stays inert rather than active.
The main limitation of PTFE is the load it can carry. The layered solids carry more, so PTFE is seldom the first answer for heavily loaded metal seats. The additive also depends heavily on the carrier that holds it in the film.
Line media can strip ordinary grease from a valve seat and carry the additive away. This failure mode is covered in washout and media resistance.
3. Graphite and molybdenum disulphide
Graphite and molybdenum disulphide are layered crystals rather than soft polymers. Their plates shear easily and cling to metal, so they carry load where an oil film fails. Both are valued for galling protection on metal seats and stem threads.
These two solids behave differently from each other in one important way. Graphite needs a trace of moisture on the sliding surface to perform. Very dry gas can therefore leave it underperforming as a lubricant.
Molybdenum disulphide needs no moisture, which suits dry gas service, though it is less chemically inert. Dry gas, sour gas and gas carrying carbon dioxide raise wider questions about deposits and seal life. These are covered in gas composition effects on valves.
Neither solid suits every service without careful checks up front. In sour service, additives and carriers must resist the gas. Deposits can also interfere with the film, and compatibility belongs at the specification stage.
4. Galling protection and its limits
Galling is a form of adhesive wear on sliding metal surfaces. Under load, surface peaks weld and then tear, transferring metal between seat and ball. The result is scoring, rising torque and, at worst, a seized valve.
Layered solids reduce galling by holding a film between the surfaces. They protect best before damage starts, at assembly, commissioning or routine top-ups. They cannot smooth over scores, corrosion or distortion that already exist.
If a valve already turns with heavy torque, the additive question comes second. The honest next step is a proper assessment of the trim, not another grease. No compound can restore metal that has already torn away.
5. Choosing a compound and verifying it
The additive is only one choice inside a larger decision. Load, media, moisture and temperature point towards different solids. The carrier must also suit the valve and its injection route.
The carrier grade matters as well, as grease consistency grades make clear. Before any injection is recommended, the standard gates come first:
- The valve must have injection fittings in good condition.
- The valve design and service must suit injection. Where OEM documents exist, check them for limits and warranty terms.
- Site rules and the permit-to-work must allow work on the live line.
An improvement after treatment is still not proof of closure. Flow or bubbles that stop after treatment only suggest progress. The confirming step is a seat-leak test under agreed conditions, such as pressure decay.
We formulate a range of valve lubricants in-house for different services and duties.
Common questions
PTFE grease for valves questions, answered directly
Is PTFE grease good for valves?
In many services, yes. PTFE grease suits stems, bearings and soft seats because it is chemically inert and low in friction. For heavily loaded metal seats, a layered solid such as molybdenum disulphide often carries more load.
What does molybdenum disulphide do in valve grease?
It forms a clinging film that keeps metal surfaces apart under high load. That film lowers friction and protects against galling on metal seats and stem threads. Unlike graphite, it works without moisture, which suits dry gas.
Does graphite lubricate in dry gas?
Poorly, on its own. Graphite lubricates at its best with a trace of moisture on the surface, and very dry gas can leave it weak. For dry gas service, molybdenum disulphide or PTFE is usually the sounder choice.
Can PTFE grease stop a valve from passing?
Not reliably on its own. A lubricant film can improve seat sealing slightly, but a passing seat usually calls for a sealant. If the valve still passes after treatment, the next step is a seat assessment.
Unsure which additive suits your valve?
Send the valve details. An engineer replies within 24 hours.
Include valve size and type, service conditions, media and the symptoms you see, and we will advise on the right compound.