A vacuum ball valve is a quarter-turn ball valve built to hold a seal when the pressure inside the line is far below atmospheric. One 90° turn isolates a pump, a foreline or a gauge, and in the open position the full bore leaves the gas path almost untouched. It is a sound choice for rough and medium vacuum, and the wrong choice for a baked ultra-high vacuum chamber.
This guide explains how a vacuum ball valve works, which vacuum levels it can realistically handle, where it does better than a gate or bellows valve and where it does not, and what to write on a request for quotation (RFQ) so the valve you receive fits the line it goes into.
What Is a Vacuum Ball Valve?
A vacuum ball valve is a ball valve whose seats, stem seal, body joints and end connections are chosen for vacuum service instead of positive pressure. The working principle is the same as any ball valve: a ball with a bore through it turns a quarter turn between open and closed. What changes is the direction of the load and the type of connection.
In a water or air line, the medium pushes outward and a leak is usually visible. In a vacuum line, atmosphere pushes inward from every side, and a leak shows up only as a slow pump-down or a pressure that stops falling. That is why a vacuum ball valve usually differs from a general-purpose ball valve in four places:
- End connections. Vacuum flanges such as ISO-KF (also written KF, NW or QF) or ISO-K, instead of pipe threads. ISO-KF is a small clamp flange; ISO-K is the larger clamped flange series.
- Seats. Usually PTFE (polytetrafluoroethylene), a fluoropolymer with low friction and wide chemical resistance.
- Stem seal. A seal chosen for vacuum, often PTFE packing, sometimes with an added fluoroelastomer O-ring (FKM, widely sold under the Viton brand).
- Internal cavity. A route that lets the space around the ball pump down with the line, so it does not release trapped air later. The section on the cavity below explains why.
At Yzng Trong, our stainless steel vacuum valves are cast 316 stainless steel (ASTM A351 CF8M) with PTFE seats and a vacuum-rated stem seal. KF ends run from NW16 to NW50, and ISO-K ends come in ISO80 and ISO100.
How a Vacuum Ball Valve Works
Turning the stem 90° rotates the ball so its bore either lines up with the pipe (open) or sits across it (closed). Open, the valve behaves like a short straight tube. Closed, the ball presses against two seat rings and the seats carry the seal. For vacuum service, three things deserve a closer look: the open path, the closed seal and the cavity around the ball.
Open: a straight, full-bore path
On a full-port valve, the bore through the ball matches the pipe bore. This matters more in vacuum than in liquid service, because conductance (how easily gas passes through a component at low pressure) drops quickly when a passage narrows. A full-port ball valve in a foreline keeps the line’s effective pumping speed close to what it would be without the valve. VACUUBRAND, for example, describes its VKE stainless steel ball valve as giving unimpeded passage over the whole nominal cross section, and suggests ball valves for load locks for that reason.
Closed: the seats carry the seal
With the ball closed, the higher pressure on one side pushes the ball against the seat on the vacuum side. PTFE seats conform slightly to the polished ball and close the leak path. Each time the valve turns, the seats wipe across the ball. MKS Instruments explains on its heated vacuum ball valve page that this wiping action makes ball valves less sensitive to particle contamination than other vacuum valves, and that they are used to isolate scrubbers and traps downstream of rough pumping systems.
The cavity around the ball
A floating ball valve has a small enclosed space between the ball and the body. If that cavity is sealed off while the line is evacuated, it stays near atmospheric pressure, and when the valve turns again the trapped air is released into the system. When you compare vacuum ball valves, ask each supplier how the cavity is relieved.
What Vacuum Level Can a Vacuum Ball Valve Handle?
Most vacuum ball valves are built for rough and medium vacuum, from atmospheric pressure down to about 10⁻³ mbar. Some specially prepared and tested designs go further into high vacuum. A PTFE-seated ball valve does not belong in ultra-high vacuum. The standard names for these ranges come from ISO 3529-1:2019, summarized below as published in the European Spallation Source (ESS) Vacuum Handbook.
| Vacuum range (ISO 3529-1:2019) | Absolute pressure | Is a vacuum ball valve a fit? |
|---|---|---|
| Low (rough) vacuum | Atmospheric pressure to 1 mbar | Yes. This is the most common use, on roughing lines and pump isolation. |
| Medium (fine) vacuum | 1 mbar to 10⁻³ mbar | Yes, with a valve rated for the range. Check the stated leak rate. |
| High vacuum (HV) | 10⁻³ mbar to 10⁻⁸ mbar | Only specially prepared and tested valves, and usually on the pump side, not as the chamber valve. |
| Ultra-high vacuum (UHV) | 10⁻⁸ mbar to 10⁻¹¹ mbar | No. PTFE seats and elastomer seals are not used at this level. |
| Extreme-high vacuum (XHV) | Below 10⁻¹¹ mbar | そうだ。 |
Three independent sources point the same way. VACUUBRAND positions its VK ball valves as isolation devices for the rough and fine vacuum range. Flowserve’s bulletin says standard off-the-shelf ball valves can serve industrial vacuum, under favorable conditions, down to about 10⁻² mmHg (roughly 0.013 mbar), while specially prepared and tested valves have been used through the high vacuum range. The ESS handbook prohibits all plastics in its ultra-high vacuum systems, which rules out a PTFE seat there.
Watch the units. Vacuum is specified in absolute pressure. A gauge reading such as −0.98 bar is relative to the local atmosphere, so near sea level it means roughly 30 mbar absolute. That is still rough vacuum. If a drawing only says “full vacuum” or “−1 bar”, ask the process owner for the absolute working pressure before you choose a valve.
Advantages of a Vacuum Ball Valve
The main advantages of a vacuum ball valve are full-bore flow, fast quarter-turn operation, tolerance of dirty gas and easy maintenance. Those strengths match foreline and pump isolation, which is where ball valves are most often found in vacuum systems.
- Full bore, little conductance loss. An open full-port valve adds almost no restriction, so pump-down time stays short.
- Quarter turn, easy to automate. Opening or closing takes 90°. An ISO 5211 mounting pad, the international interface between a quarter-turn valve and its actuator, lets a pneumatic or electric actuator bolt on directly.
- Tolerates deposits. The seats wipe the ball on every stroke, which helps on exhaust lines from coating and deposition tools.
- Visible position. A lever in line with the pipe means open; a lever across the pipe means closed.
- Serviceable in place. On a three-piece body, the center section unbolts while the end caps stay clamped in the line, so the ball and seats can be cleaned or replaced without disturbing the flanges.
Where Other Vacuum Valves Are a Better Fit
A vacuum ball valve is the wrong choice for ultra-high vacuum, for systems baked or run above the seat temperature limit, for throttling, and as the main valve between a high-vacuum chamber and its high-vacuum pump. In those positions another valve type is usually better.
- Seal tightness. Soft seats and elastomer seals leak and permeate more than bellows or metal seals. In VACUUBRAND’s own catalog, its KF ball valves list a leak rate of 1×10⁻⁶ mbar·l/s, while its bellows-sealed high vacuum valve lists 1×10⁻⁹ mbar·l/s.
- Temperature. The PTFE seats set the ceiling. Our vacuum ball valves are rated −20 °C to +180 °C at the valve body, and some catalog vacuum ball valves are rated lower (VACUUBRAND lists 80 °C maximum gas temperature for the VKE). Check the process and bake-out temperature first.
- Throttling. A soft-seated ball valve is an on/off valve. Holding it partly open to slow a pump-down wears the seat edge.
- Chamber isolation in high vacuum. We build our vacuum ball valves for fore-line and pump isolation, not as a high-vacuum chamber gate, and our product pages state that limit.
- Trapped cavity. Without cavity relief, the space around the ball can release air into the system when the valve turns.
The table below shows where each common vacuum valve type usually sits, so you can see whether a ball valve is the right tool for your position.
| バルブの種類 | Where it usually sits | Choose it when |
|---|---|---|
| Vacuum ball valve | Forelines, roughing lines, pump and gauge isolation, load locks, exhaust to scrubbers and traps | You need a full-bore on/off point in rough or medium vacuum, the gas may carry particles, or the valve will be automated |
| Gate valve | Between a process chamber and its high-vacuum pump | The bore is large and the chamber runs in high vacuum |
| Bellows-sealed angle or inline valve | High vacuum lines, gas inlets, leak-sensitive isolation | A low leak rate and minimal outgassing matter more than a straight-through path |
| Butterfly valve | Compact inline isolation on KF lines | You want high conductance in a short face-to-face length |
Where Vacuum Ball Valves Are Used
Vacuum ball valves are used wherever a vacuum system needs a simple, full-bore on/off point, which is mostly on the pump side of the system. Typical positions include:
- Foreline and roughing-line isolation between a rotary vane, scroll or screw pump and the system, so the pump can be serviced without venting everything upstream.
- Gauge isolation, to fit or remove a gauge or protect it during venting.
- Load locks and sample entries, where the full bore keeps pump-down fast.
- Freeze dryers and vacuum ovens, between the chamber and its pump.
- Exhaust lines on CVD and PVD tools (chemical and physical vapor deposition), ahead of scrubbers and particle traps.
- Helium leak detection setups, as the on/off valve between a test piece or calibrated leak and the detector.
If a line has to switch between two paths, a 3-way L-port or T-port vacuum ball valve can replace two separate valves. Our three-piece manual vacuum valve with KF NW16–NW50 or ISO80 ends covers most hand-operated isolation points in this list.
How to Choose a Vacuum Ball Valve: 8 Selection Tips
Choose a vacuum ball valve by working outward from the system: absolute working pressure, valve position, flange, seals and temperature, bore, operation, leak rate and body construction. Each tip below becomes one line on your RFQ.
1. Start from the absolute working pressure
Give the lowest absolute pressure the valve will see, in mbar, Pa or Torr. That single number tells the supplier whether a standard vacuum ball valve will do or whether you need a prepared valve or a different valve type.
2. Decide where the valve sits
Foreline, pump isolation, gauge isolation and load locks suit a ball valve. If the valve will sit directly between a high-vacuum chamber and a turbomolecular or cryogenic pump, look at gate valves instead.
3. Match the flange to the line
The ISO-KF connection is defined in ISO 2861 and DIN 28403. As Leybold describes the ISO-KF system, it joins two symmetrical flanges with a centering ring, an O-ring and a clamping ring tightened by hand, with no tools. Match the size exactly: on our manual three-piece valve, NW16 corresponds to 1/2″, NW25 to 1″, NW40 to 1-1/4″ and NW50 to 1-1/2″ and 2″. For larger bores, ISO-K takes over, and our pneumatic vacuum ball valve with NW50 KF or ISO-K DN80/DN100 ends covers the 2″ to 4″ range.
4. Check seat, seal and temperature
PTFE seats are the usual choice. Confirm the stem seal design and the temperature rating of the whole valve, including any actuator. On our pneumatic vacuum valve, for example, the valve is rated −20 °C to +180 °C, but the actuator is rated −20 °C to +80 °C, so the actuator sets the limit if the valve sits on a warm line.
5. Specify full port
Write “full port” on the RFQ and ask for the bore diameter on the dimension sheet. A reduced-port valve narrows the line and lowers its conductance.
6. Choose manual or actuated, and the fail position
Manual levers suit points that are set and left. For timed pump-down sequences, a pneumatic actuator opens and closes on a PLC signal. Decide what the valve must do if air is lost: a double-acting actuator stays where it was, while a spring-return actuator moves to a preset open or closed position. Our guide to spring return vs double acting actuators covers that choice in detail. Our pneumatic vacuum valves run on a 4 to 8 bar air supply.
7. Ask for a leak rate number and the test method
“Vacuum tight” is not a specification. Ask for the leak rate in mbar·l/s (or Pa·m³/s), and ask whether it applies to the body to atmosphere, across the closed seat, or both; VACUUBRAND, for instance, lists the two separately. Flowserve notes that a quantitative leak rate certificate requires a helium mass spectrometer test, so ask how and how often the supplier tests.
8. Pick the body construction for maintenance
A three-piece body adds two bolted body joints but lets you rebuild the valve in the line. A two-piece body has fewer joints and comes out of the line for service. Our article on industrial ball valves with fewer leak paths compares body types joint by joint.
Common Mistakes When Specifying a Vacuum Ball Valve
Most problems with vacuum ball valves start on the order sheet, not in the valve. These six come up most often.
- Ordering a threaded water-service ball valve with “vacuum” in the notes. Taper threads rely on sealant and add two leak paths. Specify KF or ISO-K ends.
- Writing “−1 bar” instead of an absolute pressure. The supplier cannot tell rough vacuum from high vacuum without the absolute number.
- Putting a ball valve where a high-vacuum gate valve belongs. On the chamber side of a turbomolecular pump, leak rate and outgassing matter more than cost.
- Using the valve to throttle pump-down speed. Use a separate metering or soft-start valve for that.
- Leaving the fail position blank on a spring-return actuator. State normally closed or normally open on the order.
- Accepting a leak-tight claim without a number. Ask for the value, the gas, and the test method in writing with the quote.
よくある質問
Can a regular ball valve be used for vacuum?
Sometimes, within limits. Flowserve’s vacuum service bulletin says standard off-the-shelf ball valves can work on industrial vacuum under favorable conditions down to about 10⁻² mmHg, which falls in the medium vacuum range. The weak points of a general-purpose valve are threaded ends, a stem seal designed mainly for outward pressure, and a body cavity that may hold air. For anything beyond coarse rough vacuum, or where pump-down time matters, a valve built for vacuum with KF or ISO-K flanges and PTFE seats is the safer choice, and a stated leak rate is the only way to be sure.
What is the difference between a vacuum ball valve and a gate valve?
Both give a straight path when open, but they are built for different positions. A gate valve slides a flat plate across the bore and is the usual choice for isolating a high-vacuum chamber from its turbomolecular or cryogenic pump, especially in large sizes. A vacuum ball valve turns a ball a quarter turn and usually sits on the pump side, on forelines, roughing lines, gauges and load locks. Ball valves tolerate particle-laden gas well because the seats wipe the ball on every stroke, while gate valves are chosen for large bores and high vacuum.
What does a leak rate of 1×10⁻⁶ mbar·l/s mean?
It is the quantity of gas that passes the seal, expressed as pressure times volume per second. A leak of 1×10⁻⁶ mbar·l/s into a sealed 1-litre volume would raise its pressure by 1×10⁻⁶ mbar every second, or about 0.09 mbar per day. In a 10-litre volume the rise would be ten times smaller. Catalog KF ball valves for rough and fine vacuum often quote values around this level, while bellows-sealed high vacuum valves quote around 1×10⁻⁹ mbar·l/s. Always ask whether a figure applies to the body or across the closed seat, and how it was measured.
Are KF and NW flanges the same thing?
Yes, in practice. KF, NW, QF and ISO-KF are different names for the same small clamp flange, defined in ISO 2861 and DIN 28403. Two symmetrical flanges are joined with a centering ring that carries an O-ring, and a hand-tightened clamp holds them together without tools. Common sizes are NW16, NW25, NW40 and NW50, where the number is the nominal bore in millimetres. Above NW50, systems usually move to the larger ISO-K flange. Order the centering ring and clamp in the same size as the valve ends.
How do you maintain a vacuum ball valve?
Maintenance mostly concerns the seats and the stem seal. Watch for the first signs of wear: a pump-down that takes longer than it used to, or pressure that rises with the valve closed. Before any work, isolate and vent the section. On a three-piece valve, the center section unbolts while the end caps stay clamped in the line, so the ball and seats can be cleaned or replaced without disturbing the flanges. Wipe sealing surfaces with a lint-free cloth, and replace any nicked or flattened O-ring on the flange centering rings at the same time.
Summary
A vacuum ball valve is a quarter-turn, full-bore isolation valve with seats, stem seal and flanges chosen for vacuum. It works best in rough and medium vacuum on the pump side of a system: forelines, pump and gauge isolation, load locks and dirty exhaust lines. It is not a substitute for a gate or bellows valve in high vacuum chamber isolation, and it has no place in a baked UHV system. To choose one, state the absolute pressure, the valve position, the flange size, the temperature, full port, the operation and fail position, and a leak rate with its test method.
If you send us those lines, we will propose a valve for each position in your system. Yzng Trong has manufactured stainless steel ball valves in Taiwan since 1999 under an ISO 9001 quality system; you can read more about our factory and certifications, or send your vacuum valve specifications for a quote.