- ▸PV = face pressure × mean sliding velocity; it is the first input to selection — compute PV, then choose the pair
- ▸The standard high-PV pairing is carbon graphite (soft ring) against silicon carbide (hard ring), balancing low friction, heat conduction and solids resistance
- ▸PV capability varies strongly with impregnation: resin-impregnated is lowest, antimony-metal-impregnated highest, by several times
- ▸Selection keeps working PV at 1/2 to 1/3 of the material's limit — a 2-3x safety factor
- ▸High-PV duty requires flush cooling; graphite's 70-150 W/(m·K) exports heat, but the heat needs somewhere to go
- ▸Carbon graphite's thermal conductivity is orders above typical seal materials — precisely why it appears in high-PV pairings
Material selection for mechanical seals does not start by picking a material and checking it afterwards; it starts by computing PV. PV compresses pressure, speed and face size into one number — the most direct measure of the thermal load on the pair.
I. Compute PV First: Three Inputs, One Formula
PV = face pressure (MPa) × mean face velocity (m/s).
1.Face pressure comes from media pressure, balance ratio and spring loading, commonly 0.3-0.6 MPa in design
2.Velocity comes from speed and mean face diameter, commonly 5-30 m/s in pump seals
3.Their product measures friction power density
As an illustration (not a specific project): 0.4 MPa × 20 m/s = 8 MPa·m/s — several times the dry limit of resin-impregnated graphite (1-2 MPa·m/s), demanding liquid lubrication and cooling plus a high-PV grade.
II. How to Pair: Soft Against Hard
The mainstream high-PV pairing is carbon graphite against silicon carbide.
1.Graphite soft ring: self-lubricating, good bed-in, 70-150 W/(m·K) conduction carrying the heat out
2.SiC hard ring: extreme hardness, solids and chemical resistance, low friction against graphite
3.Graphite-on-graphite soft/soft pairs serve only low-PV clean duty; hard/hard (SiC/SiC) resists wear but handles dry running and impact poorly, mainly for solids-laden viscous media
The pairing logic: let the easily replaced soft ring take the wear, let the hard ring preserve face geometry.
III. The PV Capability Ladder of Graphite Grades
Same carbon graphite, different impregnation, different PV capability.
- Resin-impregnated: general-purpose, best corrosion resistance, lowest PV capability
- Antimony-impregnated: higher strength and conductivity, compressive strength in the 250-300 MPa class, highest PV capability for high-PV solids duty
- PTFE-impregnated: dry-running and chemical-duty specialist with lower friction
- Carbon graphite compressive strength spans 150-300 MPa as the base, tuned by impregnation and formulation
IV. Safety Factor and Cooling
Laboratory PV limits must be derated in engineering: working PV at 1/2 to 1/3 of the limit.
Supporting measures matter equally in high-PV duty:
1.Flush plans to pull seal chamber temperature down (the API 682 plan system exists for this)
2.A heat-removal path — however good graphite's conduction, heat trapped in the chamber still runs away
3.Adjustable face loading, fine-tuned after trial runs based on temperature rise and leakage
Huahao Sealing computes PV from the customer's media, pressure, speed and shaft size and recommends the impregnation grade; custom orders start with the selection calculation, not the machining dimensions. One wrong step in high-PV selection and all downstream precision is wasted.