- ▸Selection order: media and impregnation first, then temperature limit, then PV value and face pressure check — reversing this order usually means rework
- ▸Temperature criteria: carbon graphite works above 600°C in non-oxidizing media; in oxidizing atmospheres limit to about 350-400°C; resin-impregnated parts are capped by the resin's glass transition
- ▸Above roughly 1.5 MPa of media pressure a balanced design is needed; exceeding the PV limit is the leading cause of premature wear
- ▸Dry friction coefficient 0.04-0.15 and thermal conductivity 70-150 W/(m·K) are why graphite suits high-speed dry running and thermal shock conditions
- ▸Shaft diameter sets bore, OD and wall thickness; below 3 mm wall thickness, check compressive strength (typically 150-300 MPa)
The core logic of carbon graphite seal ring selection fits in one sentence: media determines impregnation, temperature determines the base material, pressure and PV value determine the structure, and shaft diameter determines dimensions. Work through these four steps and the right grade emerges on its own. This article walks through the criteria for each step.
Step 1: Media Determines Impregnation
Media is the first input. The carbon graphite matrix itself is highly chemically inert; the weak link is the impregnant, so judging media is really choosing an impregnant.
- Acids, alkalis, salts and strongly corrosive media: furan resin impregnation, the widest chemical resistance spectrum
- Mildly corrosive media, water, oils: phenolic resin impregnation, the best value
- Combined corrosion resistance and toughness: epoxy resin impregnation
- Hot oil service above 400°C: all resins fail — antimony or other metal impregnation is mandatory
- Food and pharmaceutical sanitary duty: PTFE impregnation, no extractables
Key point: obtain the medium's composition, concentration and solids content first. Slurries with high solids also demand wear resistance, where antimony or babbitt impregnation clearly outperforms resins on hardness.
Step 2: Temperature Sets Base Material and Impregnation Limits
Read temperature on two lines. The graphite base itself serves long-term above 600°C in non-oxidizing media — the reason it beats nearly every polymer sealing material. But in oxidizing atmospheres (air, steam, oxygen-bearing media) graphite starts oxidizing noticeably around 450°C, so usage is generally capped at 350-400°C.
The impregnant has its own lower limit: common phenolic, epoxy and furan resins serve long-term in the 150-200°C range. Beyond that the resin softens and decomposes, pores reopen, and leakage follows. So "metal impregnation for high temperature" is not a preference — it is a hard constraint.
Step 3: Pressure and PV Value Check
Face pressure and PV value (face pressure times mean face sliding velocity) determine wear rate.
1.Media pressure below about 1.5 MPa: an unbalanced design suffices — simpler and cheaper
2.Above about 1.5 MPa: use a balanced design to reduce the load factor B and keep face pressure in range
3.Check PV value: actual PV must not exceed the allowable PV for the chosen material pair; exceeding it accelerates wear and can thermally crack the face
With a dry friction coefficient of 0.04-0.15 and thermal conductivity of 70-150 W/(m·K), carbon graphite tolerates far higher PV values than PTFE or elastomers — the reason it is the first choice for high-speed pumps and agitators.
Step 4: Speed and Shaft Diameter Set Dimensions
Speed affects two things: the velocity term in PV and the dynamic ring's tracking ability. At high speed the liquid film is harder to maintain and self-lubrication matters more.
Notes on shaft diameter and dimensions:
- Wall thickness generally should not be less than 3 mm; thinner rings lose flatness stability and installation strength
- For large-diameter rings verify compressive strength — typically 150-300 MPa for carbon graphite, higher when antimony-impregnated
- Check running clearances against shaft diameter and speed in handbooks: too tight risks seizure, too loose allows wobble
Summary: A Selection Checklist
1.List media composition, concentration and solids → choose impregnation
2.Confirm media temperature and atmosphere → match base and impregnant temperature limits
3.Calculate face pressure and PV value → choose balanced or unbalanced
4.Size to shaft diameter and gland cavity → set ID, OD, wall thickness and tolerances
Following these four steps avoids most selection errors. Huahao Sealing was founded in 2006 with its factory in Lu'an, Anhui. We customize non-standard carbon graphite seal rings to customer drawings and run full-process quality inspection on every batch. If selection is uncertain, send the five parameters — media, temperature, pressure, speed and shaft diameter — and a material recommendation usually comes back within one working day.