Elastomer Bellow Seals: When to Use Them (and When Not To)

An elastomer bellow seal is a non-pusher mechanical seal design that uses a moulded rubber bellows instead of a spring plus dynamic O-ring. The bellows both loads the faces and absorbs shaft movement, giving the seal excellent tolerance for fouling, hang-up and small amounts of dry running. This makes it a workhorse in HVAC, wastewater, sanitary and light chemical services.

How the design works

The elastomer bellows is bonded or clamped to the shaft at one end and to the rotating face at the other. As the pump runs, three things happen inside the bellows:

1. It supplies the axial closing load that keeps the faces in contact.
2. It absorbs radial run-out and lateral shaft movement without dragging on the shaft.
3. It seals between the rotating face and the shaft — no O-ring sliding, no fretting, no hang-up.

The net result is a much more forgiving seal on services that would clog or hang a pusher design.

Where elastomer bellows win

• HVAC circulator pumps — decades of proven service in building water loops.
• Wastewater and effluent pumps — resists stringy debris, low-solids slurries and grease.
• Clean-water booster pumps — low cost, easy installation, long life.
• Submersible pumps — compact, tolerant of shaft misalignment.
• Low-pressure chemical pumps — with a compatible elastomer (EPDM, Viton, NBR).

Common OEM cross-references include John Crane Type 21 / Type 6A, Sepco 500 series and Chesterton 155.

Limits to be aware of

Elastomer bellows are the right choice for a wide range of duties but they are not universal:

• Pressure limit typically 10 bar (145 psi). Above this, pick a cartridge pusher or metal bellow.
• Temperature limit depends on elastomer: NBR to 100 °C, EPDM to 120 °C, Viton to 200 °C.
• Chemical compatibility is dictated by the elastomer chosen — always cross-check with a compatibility table.
• Not suitable for highly aggressive solvents that swell or attack the bellows. For those, choose a PTFE bellows or metal bellows design.

Elastomer material selection

Elastomer	Best for	Avoid
NBR (nitrile)	Water, oils, hydraulic fluid	Ozone, aromatics, ketones
EPDM	Hot water, steam, alcohols	Petroleum oils
Viton (FKM)	Fuels, mild acids, chlorinated	Ketones, esters, amines
Aflas	Aggressive chemicals, high temp	Aromatics

When to choose something else

• Solvent or aggressive-chemical service — use metal bellows.
• Above 10 bar — use a cartridge pusher seal.
• Zero-emissions or toxic services — use a double cartridge with API Plan 53.
• Compressors — use a dry gas seal per API 692.

For a broader map of every seal family, see our mechanical seal types guide.

Ordering an elastomer bellow seal

Send us the pump make and model, the shaft diameter, the operating temperature, and the fluid. We stock the John Crane Type 21 / Type 6A equivalents in the common shaft sizes and can ship replacements within a week. Request a quote.