Mechanical Seal
General / GeneralDefinition
A mechanical seal is a device that seals the rotating shaft of a pump where it exits the casing, preventing process fluid from leaking to the atmosphere. Unlike gland packing (which deliberately allows a small drip for lubrication), a mechanical seal achieves near-zero visible leakage by maintaining a precision-ground interface between a rotating face (attached to the shaft) and a stationary face (fixed in the pump). It is the standard shaft sealing method for the vast majority of centrifugal pumps in oil, gas, chemical, and power generation service.
Working Principle
Two extremely flat, smooth faces — one rotating with the shaft, one stationary in the pump bore — are pressed together by spring force and process pressure. A thin fluid film (0.0001 to 0.001 inch thick) between the faces provides lubrication and cooling. This film is the actual seal — it prevents solids from entering the interface while allowing a microscopic amount of fluid to vaporize at the face edges, which is the 'leakage' (typically a few drops per day, invisible as vapor). The seal must absorb shaft axial and radial movement, vibration, and thermal expansion while maintaining face contact. Elastomer O-rings or bellows provide secondary sealing between the faces and their respective mounting surfaces.
Key Components
| Rotating face | Carbon-graphite or silicon carbide ring mounted on the shaft |
| Stationary face | Silicon carbide or tungsten carbide ring fixed in the seal chamber |
| Springs | Maintain face contact; single coil, multiple, or wave spring designs |
| O-rings / Elastomers | Secondary seals: EPDM, Viton, perfluoroelastomer (Kalrez) |
| Sleeve | Protects the shaft; seal components mount on the sleeve, not the shaft |
| Gland plate | Bolts to the pump; houses the stationary face and flush connections |
Types & Variants
| Type | Configuration | Leakage | Best For | Limitation |
|---|---|---|---|---|
| Single seal | One pair of faces | Near-zero (vapor) | Clean, non-hazardous fluids | No backup if faces fail |
| Double seal (tandem) | Two seal pairs in series | Zero to atm | Toxic, flammable fluids | Requires buffer fluid system |
| Double seal (back-to-back) | Two seal pairs opposing | Zero to atm | High-pressure, abrasive | Requires barrier fluid at higher pressure |
| Cartridge seal | Pre-assembled unit | Varies by type | All services; easy installation | Higher initial cost |
Materials
Rotating face: carbon-graphite (general purpose, low cost), silicon carbide (abrasive and high-temperature), tungsten carbide (high pressure, severe service). Stationary face: silicon carbide (most common), tungsten carbide, or alumina ceramic (economy). O-rings: EPDM (water, dilute chemicals), Viton/FKM (hydrocarbons, general chemical), perfluoroelastomer/Kalrez (aggressive acids, high temperature), Aflas (amines, steam). Metal components: 316SS standard; Hastelloy C-276 for corrosive service; titanium for seawater.
Applicable Standards
API 682 (Mechanical Seals for Centrifugal Pumps) — the primary standard defining seal types, arrangements, materials, and testing for petroleum and petrochemical service. API 682 Type A (pusher seal), Type B (metal bellows), Type C (elastomer bellows). ISO 21049 (identical to API 682 for international use). ASME B73.1 covers seal chamber dimensions for chemical pumps. STLE (Society of Tribologists and Lubrication Engineers) provides lubrication and wear guidance for seal face materials.
Applications
Refinery pumps: single seals with API Plan 11 (self-flush) on hydrocarbon service; double seals with API Plan 52/53 on toxic (H2S) or flammable services. Chemical pumps: PTFE or perfluoroelastomer O-rings for aggressive acids; double seals for carcinogenic fluids (benzene, vinyl chloride). Water and wastewater: economical single seals with carbon vs. ceramic fa