Which vacuum valve should I use between a flask and a rotary vane pump?

Use a bellows-sealed angle or right-angle valve with a KF25 or KF40 flange. The bellows keeps the atmosphere out of the process path, the angle body gives a straight-through bore with good conductance, and the valve can be closed slowly so a flask is not dragged towards the pump in one movement. A ball valve is cheaper but has a smaller bore and a stem seal that will eventually permeate.

What is the difference between KF and CF flanges?

KF is a quick-release flange sealed by an elastomer O-ring compressed by a clamp and centring ring; it is hand-tight, reusable and suited to rough and fine vacuum down to about 10 to the minus 7 millibar. CF is a knife-edge metal-gasket flange bolted in a full circle, bakeable to 300 degrees Celsius or more and used for high and ultra-high vacuum where elastomer permeation and outgassing would set the base pressure.

Why does my vacuum line pump down slowly after I add fittings?

Every added elbow, tee, narrow bore and valve reduces conductance, and in the molecular flow regime conductance falls with the cube of the diameter, so a short length of small tubing can dominate the pumping speed. Use the largest bore that fits, keep runs short and straight, avoid unnecessary 90 degree elbows, and confirm the improvement with a rate-of-rise test rather than assuming the pump is at fault.

Vacuum Valves and Fittings Singapore

Q: Does JWRESEARCH supply vacuum valves and fittings in Singapore?

Yes. JWRESEARCH supplies manual and pneumatic ball, butterfly, gate, diaphragm and bellows-sealed angle valves, KF and CF flanges, centring rings, clamp sets, O-rings, metal gaskets, flexible bellows hose, tees, crosses and adaptors, delivered across Singapore with advice on flange sizing and line conductance.

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

A vacuum line is only as good as its joints. The pump may be oversized and the flask clean, but one hardened O-ring or one ball valve with a 6 mm bore will hold the system at a pressure the experiment cannot tolerate. Valves and fittings get the least attention and decide whether a pump-down takes four minutes or forty.

At JW RESEARCH SUPPLY, we supply vacuum valves, KF and CF flanges, centring rings, clamps, O-rings, bellows hose and adaptors to chemistry, pharmaceutical, materials, semiconductor and teaching laboratories across Singapore. This guide covers the valve types, the two flange systems, and the conductance arithmetic that explains most slow pump-down complaints.

Valve Types and Where They Belong

Valve choice follows three questions: pressure range, how clean the process path must be, and whether a person or an actuator turns it.

Valve Type	Typical Range	Seal	Best Use
Ball valve	Atmosphere – 10⁻³ mbar	PTFE seat, elastomer stem	Isolation on roughing lines, coolant and utility
Butterfly valve	Atmosphere – 10⁻³ mbar	Elastomer	Large-bore isolation, throttling a big pump
Gate valve	Atmosphere – 10⁻⁷ mbar	Elastomer or metal	Full-bore isolation with high conductance
Diaphragm valve	Atmosphere – 10⁻⁸ mbar	PTFE or elastomer diaphragm	Corrosive gas and solvent vapour, contamination-free
Bellows-sealed angle valve	Atmosphere – 10⁻⁹ mbar	Welded bellows, metal or elastomer seat	High vacuum isolation, no sliding stem seal
Right-angle valve	Atmosphere – 10⁻⁹ mbar	Bellows	Compact routing where an elbow would be needed
Needle / metering valve	Atmosphere – 10⁻² mbar	Elastomer or PTFE	Controlled bleed, venting and backfill

Specification Reference

Parameter	Typical Value	What It Affects
Body material	Brass, stainless steel 304 or 316L	Corrosion resistance, outgassing rate, cost
Nominal bore	KF10 10 mm, KF16 16 mm, KF25 25 mm, KF40 40 mm, KF50 50 mm	Conductance and achievable pump-down time
Flange standard	KF (ISO-KF), CF (ISO-CF), ISO-K, 1/4 – 1 in NPT	Whether parts from different suppliers will mate
Leak rate	1 × 10⁻⁹ mbar·L/s (elastomer), 1 × 10⁻¹⁰ mbar·L/s (metal)	Base pressure and helium leak-test acceptance
Max differential pressure	1 – 10 bar (bellows valves), 16 bar (ball valves)	Safety when venting against a closed valve
Temperature rating	80 °C elastomer seat, 150 °C PTFE, 300 °C metal (CF)	Bakeout and autoclave-adjacent lines
Actuation	Manual handwheel, pneumatic, solenoid, motorised	Interlocks and unattended operation
Cycle life	10,000 – 100,000 cycles (bellows)	Replacement interval on a busy manifold

KF and CF: Two Systems, Two Purposes

KF flanges are the workhorse of rough and fine vacuum: two flat faces trap an elastomer O-ring, a centring ring holds it concentric, and a wing-nut clamp squeezes the assembly by hand. They open fast, are reusable and hold to about 10⁻⁷ mbar with Viton, but they are not bakeable and will permeate under solvent vapour.

CF flanges are for high and ultra-high vacuum: a knife edge on each face bites into a copper or aluminium gasket, closed with a full circle of bolts torqued in sequence. With no elastomer in the path the joint bakes to 300 °C and holds to 10⁻¹⁰ mbar or better, at the price of time — a CF joint is not something to open between runs.

Never force the two together: a KF clamp on a CF pair, or an O-ring in place of a metal gasket, gives a joint that passes a quick test and fails a week later.

Conductance: Why the Line Is Slower Than the Pump

In molecular flow the conductance of a tube falls steeply as diameter drops, so a short length of narrow bore, an unnecessary elbow or a partly open valve can each cost more pumping speed than the pump's own specification suggests. Use the largest bore the flange allows, keep the run short, prefer a right-angle valve body over a sharp 90° elbow, and avoid a valve narrower than the tube it sits in.

When pump-down is slow, measure before replacing hardware. A rate-of-rise test — isolate the chamber and record pressure for ten minutes — separates a real leak, outgassing and a conductance limit: a rise that flattens is usually outgassing, a linear rise is usually a leak.

Assembly, Sealing and Leak Checking

Wipe flange faces clean, fit a new O-ring if the old one is glossy, hardened or flattened, and tighten clamps evenly. A little vacuum-grade grease extends Viton life; silicone grease on a CF gasket is a contaminant, not a lubricant. Support long runs so the tubing's weight does not lever a joint open.

After assembly, helium leak check the joints you disturbed, or run an isolated rate-of-rise test if no detector is available. Keep centring rings, O-rings and one spare valve per critical position — a two-minute swap beats any specification sheet.

Frequently Asked Questions

Can I use a ball valve for high vacuum?
Not reliably. The stem seal relies on elastomer and the bore is usually smaller than the flange, so both leak rate and conductance suffer. Use a bellows-sealed angle valve above 10⁻³ mbar.

How tight should a KF clamp be?
Hand-tight with a wing nut or a light turn with a clamp tool. If the O-ring is squeezed flat or the flange faces touch metal to metal, the clamp is over-tightened and the ring should be replaced.

Do I need a pneumatic valve?
Only when pressure, a timer or a controller has to operate the valve without a person present — for interlocks, automatic venting or unattended overnight runs.

Does JWRESEARCH supply vacuum valves and fittings in Singapore?
Yes — ball, butterfly, gate, diaphragm and bellows angle valves, KF and CF flanges, clamps, centring rings, O-rings, metal gaskets, bellows hose and adaptors, 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