A coolant system that has air trapped in the piping is a system that delivers inconsistent coolant to the drill tip. Air in the coolant causes erratic pressure, reduced cooling at the cutting edge, and poor chip evacuation. Vacuum breakers and air release valves are the simplest components in the coolant system — a float, a seat, and a vent — but when they fail, the symptoms are blamed on pumps, filters, and everything except the tiny valve that stopped working.
Valve Types
Vacuum Breaker Types
| Valve Type | Function | Operating Principle | Typical Location | Standards |
|---|
| Atmospheric vacuum breaker (AVB) | Prevents vacuum in piping — admits air when pressure drops below atmospheric | Spring-loaded poppet or check disc — opens when internal pressure < external pressure | High points in supply piping — top of vertical risers | ASSE 1001 — no backflow prevention |
| Pressure vacuum breaker (PVB) | Prevents vacuum and provides backflow prevention | Spring-loaded check valve + air inlet valve — opens on vacuum, closes on flow | Make-up water lines — coolant supply lines | ASSE 1020 — provides backflow protection |
| Spill-resistant vacuum breaker (SRVB) | Same as PVB but no external spillage during operation | Similar to PVB with internal spillage containment | Where water spillage from PVB discharge is unacceptable | ASSE 1056 |
| Hose vacuum breaker | Prevents coolant drainage from elevated hose when flow stops | Check valve at hose end — opens to admit air when flow stops | End of coolant supply hoses — drill head connections | ASSE 1079 |
Air Release Valve Types
| Valve Type | Function | Operating Principle | Typical Location | Flow Capacity |
|---|
| Manual vent valve | Manual air release — operator opens to bleed air | Simple ball valve or needle valve at high point | Filter housing top — pump discharge high point | N/A — manual operation |
| Automatic float-type air release valve | Automatic release of accumulated air — closes when coolant reaches valve | Float rises with coolant level — seals vent when coolant present — opens when air pocket forms | High points in piping — highest point in system | Small orifice (1–6 mm) — continuous air release during operation |
| Combination air release / vacuum breaker | Releases air under pressure — admits air under vacuum | Dual function — air release float + vacuum breaker spring | Largest valves — at system high points | Large air release (for filling) + small automatic release (for operation) |
| Kinetic air release valve | High-capacity air release for pipeline filling | Large float — rapid opening during filling — closes when liquid arrives | Main distribution pipes — long horizontal runs | Very high (for filling) |
Operating Principles
Air Release Valve Operation
| Condition | Float Position | Valve State | Result |
|---|
| Pipe full of coolant — no air | Float at top — pressed against seat | Closed — no air release | Normal operation — no air in system |
| Air accumulates at high point | Air pocket forms — coolant level drops | Float drops with coolant level | Air pocket occupies top of valve body |
| Float drops below seat | Float no longer sealing | Open — air escapes through vent | Air is released from system |
| Coolant rises to replace air | Coolant enters valve body as air escapes | Float begins to rise | Air continues to escape until coolant reaches valve |
| Coolant reaches float | Float contacts seat | Closes — vent sealed | Air removal complete — system operational |
Vacuum Breaker Operation
| Condition | Valve Position | Result |
|---|
| Normal operating pressure (positive pressure) | Closed — spring holds disc on seat | Normal operation |
| System drained — coolant column in vertical pipe | Negative pressure develops as coolant tries to drain | Vacuum breaker opens — admits air |
| Pump stops — elevated hose drains | Vacuum forms in hose — hose could collapse | Vacuum breaker opens — hose pressure equalizes |
| Air admitted — vacuum relieved | Internal pressure equalizes with atmospheric | Spring closes valve |
Installation Locations
| Location | Valve Type Required | Why | Valve Size |
|---|
| Highest point in coolant supply piping | Automatic air release valve (float type) | Air rises to the highest point — this is where it must be released | 1/2" to 3/4" NPT |
| High points on long horizontal pipe runs | Air release valve at each high point | Long runs accumulate air at each rise in the pipe profile | 1/2" NPT |
| Filter housing top | Manual vent valve or automatic air release | Air trapped in filter housing after element change — must be vented for proper flow | 1/4" to 1/2" NPT |
| Pump discharge — high point | Combination air release/vacuum breaker | Air at pump start — vacuum when pump stops | 3/4" to 1" NPT |
| Top of vertical risers (piping going up) | Vacuum breaker + air release | Vacuum forms when coolant drains back — air accumulates at top during filling | 3/4" to 1" NPT |
| End of coolant supply hose (machine connection) | Hose vacuum breaker | Prevents coolant draining from elevated hose when disconnected | Hose thread size |
| Make-up water line connection | Pressure vacuum breaker (PVB) | Required by plumbing code — prevents backflow of coolant into water supply | Per pipe size |
| Coolant tank — vent | Atmospheric vent (open pipe) — not a valve | Allows air to enter and exit tank during filling and draining | Per tank vent spec (typically 2–4" pipe) |
Sizing
| Pipe Size (Supply Line) | Air Release Valve Size | Vacuum Breaker Size | Air Release Flow Capacity | Vacuum Breaker Air Capacity |
|---|
| 1" | 1/2" NPT | 3/4" NPT | 2–5 m³/h (at 1 bar) | 10–20 m³/h |
| 2" | 3/4" NPT | 1" NPT | 5–10 m³/h | 20–40 m³/h |
| 3" | 3/4" NPT | 1" NPT | 8–15 m³/h | 40–60 m³/h |
| 4" | 1" NPT | 1.5" NPT | 10–20 m³/h | 60–100 m³/h |
| 6" | 1" NPT | 2" NPT | 15–30 m³/h | 100–200 m³/h |
Maintenance
Air Release Valve Maintenance
| Task | Frequency | Detail |
|---|
| Visual inspection | Monthly | Check for coolant leakage from vent — check for debris around valve — verify valve body not cracked |
| Clean exterior | Monthly | Wipe valve body — remove coolant deposits — check for corrosion |
| Test operation | Quarterly | Press valve stem (if equipped) — verify air release — listen for air escaping |
| Manual air release | Quarterly | Open manual vent — verify air or coolant released — close — record |
| Disassemble and clean | Annually | Remove valve body — clean float mechanism — clean seat — check O-ring |
| Inspect float | Annually | Check for cracks — pinholes — coolant absorption — replace if damaged |
| Inspect seat | Annually | Check for debris — pitting — wear — clean or replace seat |
| Check vent port | Annually | Verify vent is not blocked — insect/debris screen intact (if equipped) |
| Replace valve | Every 3–5 years | Float degradation — seat wear — spring fatigue |
Vacuum Breaker Maintenance
| Task | Frequency | Detail |
|---|
| Visual inspection | Monthly | Check for leakage — verify valve not stuck open — check for debris |
| Test operation | Quarterly | Create vacuum condition (drain pipe downstream) — verify valve opens — verify closes when pressure returns |
| Clean interior | Annually | Disassemble — clean spring — clean seat — check sealing disc condition |
| Check spring | Annually | Verify spring not corroded — check free length — replace if corroded |
| Check seat and disc | Annually | Clean — inspect for pitting — debris damage — replace if damaged |
| Replace valve | Every 5 years | Spring fatigue — seat wear — internal corrosion |
Troubleshooting
| Problem | Symptom | Likely Cause | Corrective Action |
|---|
| Air release valve leaking coolant | Coolant dripping from vent during normal operation | Float cracked or sunk — debris on seat — float mechanism stuck | Replace float — clean seat — free mechanism — replace valve if damaged |
| Air release valve not releasing air | Air trapped in piping — erratic coolant flow — pump cavitation | Vent port blocked — float stuck in closed position — debris | Clean vent — free float — disassemble and clean |
| Air release valve chattering | Valve opening and closing rapidly — noise | Flow turbulence at valve location — valve installed at elbow — wrong valve size | Move valve to calm location — increase pipe straight run before valve — check sizing |
| Vacuum breaker not opening | Hose collapses when pump stops — coolant does not drain from elevated pipe | Spring stuck — debris in mechanism — valve corroded | Disassemble — clean — lubricate — replace if corroded |
| Vacuum breaker not closing | Air being drawn into system continuously — coolant aeration — pump cavitation | Debris on seat — spring broken — disc damaged | Clean seat — replace spring — replace disc |
| Vacuum breaker leaking under pressure | Coolant leaking from vent during pump operation | Seat damaged — disc not sealing — debris | Clean or replace seat — replace disc — clean debris |
| Noise from air release valve | Hissing or spitting during operation | Normal air release — check if frequency is excessive | If constant hiss: check for air being drawn into system on suction side — repair suction leak |
| Manual vent valve difficult to open | Valve stuck | Corrosion — debris — infrequent operation | Clean — lubricate — operate monthly to prevent sticking |
FAQ
What does an air release valve do in a coolant system?
An air release valve automatically releases trapped air from the coolant piping. Air accumulates at high points in the piping system — air is less dense than coolant, so it rises and collects at every rise in the pipe profile. The air release valve is installed at these high points. Inside the valve, a float rises when coolant is present (sealing the vent) and drops when an air pocket forms (opening the vent). When the vent opens, the trapped air escapes until coolant reaches the valve — then the float rises and seals the vent again. This automatic cycle removes air continuously during operation without operator attention. Without air release valves, trapped air causes: pump cavitation (air in the pump suction reduces pump efficiency and can damage pump internals), erratic coolant flow at the drill tip (air bubbles in the coolant stream create inconsistent cooling and chip evacuation), reduced heat transfer (air does not transfer heat as effectively as liquid coolant), and false pressure readings (air pockets affect pressure gauge readings).
How does a vacuum breaker work and why is it needed?
A vacuum breaker is a simple spring-loaded check valve that opens to admit air into the piping when the internal pressure drops below atmospheric pressure. It is needed in coolant systems to prevent: hose collapse (when the pump stops, coolant in an elevated hose tries to drain down — creating a vacuum that can collapse the hose — the vacuum breaker admits air, equalizing pressure). Coolant drainage from elevated pipes (without a vacuum breaker, coolant in a vertical pipe would siphon out when the pump stops — causing the pipe to empty and requiring re-priming — the vacuum breaker admits air and breaks the siphon). Backflow from machines (when multiple machines share a coolant supply, a vacuum in the supply line can draw coolant from one machine back into the supply header — potentially contaminating the supply). The vacuum breaker opens automatically when vacuum forms and closes when pressure normalizes. It requires no power, no control signal, and no operator action.
Where should air release valves be installed in a coolant system?
Air release valves must be installed at: the highest point in the entire coolant piping system (this is the most critical location — air naturally rises to the highest point — if there is no air release valve there, air will accumulate and eventually block coolant flow). Each high point in the piping profile (every time the pipe goes up and then down, air accumulates at that high point — if the pipe has multiple peaks, each peak needs a valve). The top of filter housings (air trapped in the filter housing after element replacement prevents proper flow — manual or automatic vent needed). The pump discharge high point (air that enters the system through the pump (from cavitation or start-up) needs to be released immediately after the pump). Long horizontal runs ((>) 30 m) — install air release valves every 30–50 m along the run at slight rises. As a general rule: any location where the pipe rises and then falls is a potential air trap and needs an air release valve. The most common installation mistake is having only one air release valve at the building's highest point — but the pipe may rise and fall multiple times between that point and the machine.
How do I know if an air release valve is working?
Signs that an air release valve is working: occasional hissing sound during operation (air is being released — normal — you should hear brief hissing from each valve every few minutes during normal operation). No coolant leakage from the vent (when working correctly, the valve seals when coolant reaches it — zero leakage during normal operation). No air in the coolant stream at the drill tip (if air bubbles are visible in the coolant at the machine — the air release valves may not be working or may be at incorrect locations). Pump runs smoothly with no cavitation (if the pump sounds smooth with consistent discharge pressure, the air release valves are doing their job of removing air from the system). Signs that an air release valve is NOT working: coolant leaking from the vent continuously (float is stuck or damaged — not sealing). No hissing sound ever (valve may be blocked — air is not being released — trapped air is accumulating in the piping). Erratic coolant pressure at the machine (air pockets are moving through the system — pressure fluctuates as air passes the pump). Pump cavitation noise (air is accumulating at the pump suction — not being removed by the air release valves).
What maintenance do vacuum breakers and air release valves need?
Maintenance requirements: visual inspection monthly — check for leaks, damage, debris. Test operation quarterly — for air release valves: press the test stem (if equipped) — air or coolant should release. For vacuum breakers: create a vacuum downstream (drain a section of pipe) — the valve should open audibly. Annual maintenance — disassemble air release valves — clean the float mechanism — inspect the float for cracks or coolant absorption — clean the seat — inspect O-rings and replace if hardened. Disassemble vacuum breakers — clean spring and seat — check spring condition — replace if corroded. Check vent ports for blockage — insects, debris, or dried coolant can block the tiny vent orifice. The most common failure is a coolant-soaked float that no longer floats (the float absorbs coolant over time — becomes heavy — sinks instead of floats — the valve leaks coolant continuously). Replace floats every 3–5 years or at the first sign of leakage. Replace complete valves every 5 years or when cleaning no longer restores proper function.
Vacuum breakers and air release valves are small components with a large impact on coolant system performance. Install air release valves at every high point in the piping — release trapped air automatically. Install vacuum breakers on vertical risers and elevated hoses — prevent vacuum formation and hose collapse. Inspect monthly — clean annually — replace floats every 3–5 years. A properly maintained air release system ensures consistent coolant flow at the drill tip, prevents pump cavitation, and eliminates the pressure fluctuations that cause drilling quality variation. This article reflects industry practice as of 2026.
