Can Carbon Graphite Rings Run Dry? Self-Lubrication Mechanism, PV Limits and the Time Boundary for Dry Friction

This is one of the most frequent questions about mechanical seals, and the answer has two halves: carbon graphite rings can run dry — for a while. "Can" comes from self-lubrication; "a while" comes from the hard limits of heat removal and wear.

I. Where Self-Lubrication Comes From

Graphite crystals are layered: carbon atoms bond strongly within each layer, while layers stack on weak van der Waals forces. During sliding, shear deformation happens between graphite layers, not between graphite and the mating face — the material supplies its own low-shear-strength sliding plane.

Graphite also adheres weakly to most metals, so a thin transferred graphite film forms on the counterface early in sliding, further reducing friction. Typical dry friction coefficient for carbon graphite is 0.04-0.15, and thermal conductivity of 70-150 W/(m·K) exports friction heat quickly — together the physical basis of short-term dry running.

Two Prerequisites for Reading the Data

1.Impregnation type matters: resin fills the pores, and the resin phase on the wearing surface has worse friction behavior than graphite, raising the dry coefficient

2.The counterface matters: pairing with silicon carbide or carbon works well; rough or galling-prone metals struggle to hold the transfer film

II. PV Value: The Boundary Metric

Dry-running capability is not described as yes/no but by PV value: face pressure (MPa) times sliding velocity (m/s), i.e. friction power input per unit area per unit time.

• Resin-impregnated carbon graphite typically allows dry-running PV in the 1-2 MPa·m/s class
• Selection practice keeps working PV at 1/2 to 1/3 of the limit — a 2-3x safety factor
• The first consequence of exceeding it is not instant cracking but face temperature rise: temperature climbs → graphite oxidation wear accelerates → surface roughness degrades → friction coefficient rises further, a vicious cycle

The flush-plan requirements in API 682 exist precisely to guarantee adequate lubrication and cooling in the seal chamber, pulling the seal out of dry-running risk.

III. The Time Boundary for Dry Friction

There is no universal constant; the boundary depends on three variables: how far the initial PV sits below the limit, how well heat conducts through the shaft sleeve, and ambient temperature.

• Low-PV duty (low-speed agitator shaft seals): graphite can tolerate tens of minutes to a few hours dry
• Medium/high-PV duty (centrifugal pumps): allowable dry running is usually minutes — for start-stop transients or brief loss of liquid
• In any duty, face squealing, face temperature above 200℃ or smoke from the seal chamber means stop immediately

"Dry-running capable" in equipment design should be read as "short-term dry-running capable". Pump selection and piping should avoid prolonged deadheading; where liquid lubrication is genuinely unavailable (e.g. CIP cleaning windows in food and pharma equipment), specify PTFE-impregnated dry-running grades.

IV. When Not to Count on Dry Running

1.Duty where pressure × velocity approaches the material PV limit

2.Low-boiling media prone to flashing and loss of liquid film at the faces

3.Frequent start-stop duty — every start is a dry-friction shock, and accumulated damage builds faster than in continuous dry running

Huahao Sealing offers custom machining and recommends resin, PTFE or metal impregnation based on actual PV and required dry-run duration. Dry-running capability is calculated, not trial-and-error: compute PV first, check the safety factor, then pick the grade.