What is the difference between KF, CF and ISO vacuum flanges?

KF flanges seal on an elastomer O-ring compressed by a centring ring and a clamp, and are hand-tightened; they suit rough and fine vacuum to about 10⁻³ mbar. CF flanges use a knife-edge against an oxygen-free copper gasket, are bolted, and are bakeable to about 400 °C, making them the standard choice below 10⁻⁶ mbar. ISO-K and ISO-F flanges are larger bore fittings sealed with an O-ring, closed either by claw clamps (ISO-K) or bolts (ISO-F), and are common on load-locks and pump inlets.

Can a centring ring or O-ring be reused?

A centring ring can be reused while its locating tabs are intact, but an O-ring should be replaced whenever a joint is opened for critical work. Elastomer O-rings take a permanent set, absorb solvent, and harden with heat; a ring that sealed last month may leak after the next bakeout. Copper gaskets for CF flanges are single-use without exception, because the knife-edge crushes the copper once and a second compression will not seal reliably.

How tight should a vacuum flange be tightened?

KF clamps are tightened by hand or with a light wing-nut until the O-ring is evenly compressed — roughly 5 to 10 N·m — and never with a long lever, which distorts the flange face. CF bolts are tightened in a star pattern in two or three passes to the gasket maker's figure, typically 15 to 25 N·m for CF35 and 25 to 35 N·m for CF63. Overtightening is more common than undertightening and causes the leak it is meant to prevent.

Vacuum Flanges, Clamps and Centring Rings Singapore

Q: Does JWRESEARCH supply vacuum flanges and clamps in Singapore?

Yes. JWRESEARCH supplies KF, CF, ISO-K and ISO-F flanges, blank flanges, adaptors, tees and crosses, together with aluminium clamp sets, chain clamps, centring rings, copper gaskets, Viton and Buna-N O-rings, and bolt sets, delivered across Singapore with guidance on standard, size and gasket material.

Published: {{NEW_ARTICLE_DATE}} | By JWRESEARCH | Category: Laboratory Equipment

A vacuum system is only as good as its joints. Pumps and chambers get the attention, but each flange decides whether the system reaches its base pressure — and where a system fails to hit target, the leak is usually a flange assembled with the wrong gasket, an old O-ring, or a clamp tightened by feel.

JW RESEARCH SUPPLY supplies vacuum flanges, clamps and centring rings to Singapore laboratories across the KF, CF and ISO families. This guide covers which standard suits which pressure, the sizes and torque figures that matter, and the habits that keep joints leak-free.

The Three Flange Families

The families differ in how they seal, and the seal sets the pressure limit far more than the metal does. KF compresses an elastomer O-ring between two flat faces, centred by a ring; CF crushes a copper gasket against a knife-edge with bolts; ISO uses a larger O-ring, closed with claw clamps or bolts.

Family	Seal	Typical Sizes	Practical Pressure	Bakeout
KF (NW)	Elastomer O-ring + centring ring, clamp	DN 10, 16, 25, 40, 50	Atmospheric to 10⁻³ mbar	About 150 °C
CF	Oxygen-free copper gasket, knife-edge, bolts	DN 16, 25, 35, 50, 63, 100	To 10⁻⁹ mbar and below	About 400 °C
ISO-K	O-ring + claw clamps	DN 63, 100, 160, 200, 250	Atmospheric to 10⁻⁶ mbar	About 150 °C
ISO-F	O-ring + bolted	DN 63, 100, 160, 200, 250	Atmospheric to 10⁻⁷ mbar	About 150 °C

Specification Reference

Parameter	Typical Value	What It Affects
Flange material	304 or 316L stainless; aluminium on ISO-K	Corrosion resistance and outgassing
Clamp material	Aluminium alloy or 304 stainless	Weight, thermal expansion, cost
O-ring material	Viton (FKM), Buna-N (NBR), silicone	Chemical compatibility and temperature limit
O-ring hardness	70 Shore A standard, 75 for high vacuum	Sealing force and resistance to compression set
Centring ring	Aluminium or stainless, with or without mesh screen	Alignment and protection of the pump from debris
KF clamp torque	5 – 10 N·m hand-tight	Even O-ring compression without flange distortion
CF bolt torque	15 – 25 N·m (CF35); 25 – 35 N·m (CF63)	Gasket seating and repeatable seal
Leak rate target	< 1 × 10⁻⁹ mbar·L/s helium (CF)	Whether the base pressure target is reachable

Choosing the Standard

Work from the pressure you need, not the pressure on the pump's datasheet. Rotary-vane and diaphragm pumps used for filtration or rotary evaporation rarely go below 10⁻² mbar, where KF is adequate. Once a diffusion or turbomolecular pump is involved, CF belongs on every permanent joint.

A useful rule for mixed systems: KF on anything opened weekly, CF on anything opened yearly or less. That keeps gasket consumption low, pump-down short, and avoids the common failure of a chamber rebuilt entirely in KF that never reaches its specified vacuum.

Clamps and Centring Rings

The centring ring centres the two faces so the O-ring seats evenly, and on pump inlets it often carries a screen that keeps fragments out of the pump. Replace a ring with a damaged tab or torn screen.

Aluminium clamp sets are light and fine for glass and small stainless assemblies. Stainless clamps hold torque better on joints cycled hot and do not gall against a stainless flange. Chain clamps are the practical answer for ISO-K joints from DN 160 upward.

Gaskets and O-Rings

Viton is the default for vacuum work: low outgassing, tolerant to about 200 °C, and resistant to most laboratory solvents. Buna-N is cheaper and fine for water-ring and rough pumps but degrades faster with solvent vapour. Silicone has the widest temperature range but the highest permeability — a poor choice where base pressure matters.

Copper gaskets for CF joints are single-use. The knife-edge work-hardens the copper on first compression, and a second use will not seal reliably — the most common cause of a chamber that will not pump down after maintenance.

Assembly and Leak Checking

Wipe flange faces with isopropanol and lint-free tissue, inspect the sealing land for scratches, and fit a new O-ring with a film of vacuum grease. Seat the centring ring squarely and tighten by hand in two stages; for CF, tighten bolts in a star pattern to the specified torque.

Helium test any joint that has been opened, and record the reading. A joint reading 10⁻⁸ mbar·L/s today and 10⁻⁶ next quarter means the O-ring has taken a set — replace it before it fails an experiment.

Practical Notes for Singapore Laboratories

Humidity above 80 per cent is normal here, and it changes two things. Condensation on a cool flange can bridge insulation gaps, so keep high-voltage and gauge connections clear of bare metal; and elastomer O-rings stored in an open drawer absorb moisture and swell, so keep spares sealed against sunlight and ozone.

Hold stock of the two or three sizes used on site — a missing DN 25 centring ring can idle a system for a week.

Frequently Asked Questions

Can I join a KF flange to a CF flange?
Only through an adaptor. The sealing methods are incompatible, so a KF-to-CF adaptor is the standard route; it keeps the CF side bakeable while the KF side stays quick to open.

Why does my KF joint leak after a bakeout?
Almost always the O-ring: heat takes a compression set and the ring no longer fills the groove. Replace the O-ring after any bake above 120 °C, and check the clamp has not loosened as the aluminium expanded.

Are aluminium clamps suitable for high vacuum?
They seal fine but hold torque less consistently than stainless on joints that are heated repeatedly. Use stainless clamps and bolts on CF and on any KF joint that is baked.

Does JWRESEARCH supply vacuum flanges and clamps in Singapore?
Yes — KF, CF, ISO-K and ISO-F flanges, blanks, adaptors, tees and crosses, plus clamp sets, chain clamps, centring rings, copper gaskets, O-rings and bolt sets, delivered across Singapore.

Contact JWRESEARCH

JW RESEARCH SUPPLY — laboratory equipment and consumables, Singapore.
Tel / WhatsApp: +65 8039 3660
Email: SINGAPORE@JWSHY.COM
Website: www.jwresearch.cn