Check valves and relief valves are small components with big responsibilities. A failed check valve allows chips to flow backward into the pump. A failed relief valve can allow pressure to exceed hose rating, causing a violent hose burst. These valves must be selected correctly, tested regularly, and replaced on schedule.
Check Valve Types
Check Valve Comparison
| Type | Mechanism | Pressure Range | Flow Range | Crack Pressure | Application |
|---|
| Spring-loaded (poppet) | Spring pushes poppet onto seat | 10–350 bar | 5–200 L/min | 0.1–1.0 bar | General coolant system — most common |
| Spring-loaded (ball) | Spring pushes ball onto seat | 10–200 bar | 5–100 L/min | 0.5–3.0 bar | Small bore applications |
| Pilot-operated | Pilot pressure opens main poppet | 10–350 bar | 20–500 L/min | Variable | High-flow circuits, accumulator isolation |
| Swing-type | Hinged disc swings open with flow | 2–50 bar | 50–1,000 L/min | Very low (< 0.1 bar) | Low-pressure high-flow return lines |
Check Valve Selection
| Application | Recommended Type | Why |
|---|
| Pump outlet (high pressure) | Spring-loaded poppet | Reliable sealing at high pressure |
| Coolant line to drill | Spring-loaded poppet | Prevents chip backflow |
| Return line from machine | Swing-type or spring-loaded | Low pressure drop |
| Accumulator isolation | Pilot-operated | Controlled opening, high flow |
| Bypass line | Spring-loaded ball | Compact, simple |
| Coolant collar supply | Spring-loaded poppet | Positive seal when not drilling |
Check Valve Failure Modes
| Failure | Symptom | Cause | Corrective Action |
|---|
| Leaking (passing reverse flow) | Pressure drops when pump stops, chips in pump | Seat wear, poppet damage, contamination on seat | Clean or replace valve |
| Stuck closed | No flow in forward direction | Contamination jamming poppet, corrosion | Clean or replace valve |
| Stuck open | Reverse flow continues | Broken spring, poppet jammed open | Replace valve |
| Chatter (rapid open/close) | Noise, pressure fluctuation | Flow too low for valve size, wrong spring | Replace with correctly sized valve |
| External leakage | Coolant visible at valve body | Seal/ O-ring failure, cracked body | Replace seals or valve |
Relief Valve Types
Relief Valve Comparison
| Type | Mechanism | Pressure Range | Flow Capacity | Response Time | Application |
|---|
| Direct-acting (poppet) | Spring force directly opposes poppet | 10–200 bar | Low to moderate | Fast (< 10 ms) | Simple overpressure protection |
| Direct-acting (ball) | Spring force directly opposes ball | 10–100 bar | Low | Fast | Low-cost protection |
| Pilot-operated | Small pilot valve controls main valve | 20–350 bar | High | Moderate (20–50 ms) | High-flow systems, accurate pressure control |
| Proportional | Electronic control of pressure setting | 10–300 bar | Moderate to high | Moderate | Variable pressure applications |
Relief Valve Selection
| Application | Recommended Type | Why |
|---|
| Coolant pump outlet (primary protection) | Direct-acting or pilot-operated | Fast response, reliable |
| High-pressure coolant circuit | Pilot-operated | Accurate pressure control at high flow |
| Low-pressure return circuit | Direct-acting | Simple, low cost |
| Accumulator circuit | Pilot-operated | Prevents spurious opening from pressure spikes |
| Coolant collar supply | Direct-acting | Fast response protects seals |
| Bypass filtration circuit | Direct-acting | Low flow, simple protection |
Relief Valve Failure Modes
| Failure | Symptom | Cause | Corrective Action |
|---|
| Stuck open (blowing continuously) | Low/zero system pressure, hot coolant | Contamination on seat, spring broken | Clean or replace valve |
| Stuck closed (no relief) | Pressure exceeds setpoint, potential hose burst | Corrosion, contamination jamming | Replace valve immediately |
| Drift (setting changes) | Pressure gradually changes | Spring fatigue, seat wear | Re-set or replace valve |
| Chatter | Rapid valve cycling, noise | Flow too high for valve capacity, unstable system | Check system design, replace with correctly sized valve |
| External leakage | Coolant visible at adjustment screw or body | Seal failure, cracked body | Replace seals or valve |
Valve Setting Procedures
Relief Valve Setting
| Step | Action | Detail |
|---|
| 1 | Install accurate pressure gauge at valve | Use gauge with range 1.5× expected setting |
| 2 | Ensure system is at operating temperature | Cold coolant changes viscosity and pressure readings |
| 3 | Close isolation valve downstream of relief (if safe) | Or block system to create dead-head condition |
| 4 | Start pump at low speed | Increase speed while monitoring pressure |
| 5 | Observe pressure at which valve opens | Pressure stabilizes at the relief setting |
| 6 | Adjust valve: turn screw IN to increase, OUT to decrease | Small adjustments — 1/8 turn at a time |
| 7 | Re-test after adjustment | Allow pressure to stabilize |
| 8 | Set at 110% of maximum operating pressure | Margin prevents nuisance opening |
| 9 | Lock adjustment screw | Secure with locknut or seal wire |
| 10 | Record setting | Date, pressure, valve ID, operator |
Check Valve Testing
| Step | Action | Detail |
|---|
| 1 | Isolate check valve from system | Close upstream and downstream isolation valves |
| 2 | Apply pressure in reverse direction | Use hand pump or system pressure |
| 3 | Observe pressure gauge | Pressure should hold — any drop indicates leakage |
| 4 | Apply pressure in forward direction | Valve should open at crack pressure |
| 5 | Measure crack pressure | Pressure at which flow begins |
| 6 | Compare to specification | Replace if crack pressure is > 20% above spec |
| 7 | Record results | Date, crack pressure, leakage, valve ID |
Installation Best Practices
Check Valve Installation
| Practice | Reason | Implementation |
|---|
| Install with flow arrow pointing downstream | Prevents reverse installation | Verify arrow direction before installation |
| Install vertically (vertical pipe) with flow upward | Gravity assists closing | Preferred orientation — reduces wear |
| Install horizontally only if necessary | Horizontal may allow disc to hang open | Use spring-loaded type for horizontal |
| Provide straight pipe before valve (5× pipe diameter) | Reduces flow turbulence that affects operation | Straight pipe run upstream |
| Install isolation valves around check valve | Enables removal for testing without draining system | Ball valves upstream and downstream |
Relief Valve Installation
| Practice | Reason | Implementation |
|---|
| Install as close to pressure source as possible | Faster response to overpressure | Within 1 m of pump outlet |
| Install tee-style with branch to relief | Flow enters side, relief is on branch | Prevents debris from settling on seat |
| Provide straight pipe before valve | Reduces turbulence effects on setting | 5× pipe diameter minimum |
| Never install isolation valve between pump and relief | Isolation defeats the safety function | Relief must always be connected |
| Route relief return to tank or low-pressure area | Discharge must go somewhere safe | Visible discharge line |
| Support valve and discharge piping | Prevents pipe stress from affecting setting | Pipe clamps, valve bracket |
Common Valve Problems
| Problem | Likely Cause | Diagnostic Check | Corrective Action |
|---|
| Relief valve opens below set pressure | Spring weakened, wrong spring installed | Remove and test spring force | Replace spring |
| Relief valve does not close after opening | Contamination on seat, poppet damaged | Remove and inspect seat and poppet | Clean or replace |
| Check valve leaks in reverse direction | Seat wear, contamination, spring broken | Pressure test in reverse | Clean or replace |
| Valve body cracks | Overpressure, freeze damage, fatigue | Visual inspection | Replace valve — repair not possible |
| Valve corrodes internally | Wrong material for coolant chemistry | Inspect during maintenance | Upgrade to stainless steel |
| Chatter under normal operation | Valve oversized, system instability | Check flow rate vs valve rating | Install correctly sized valve |
Maintenance Schedule
| Component | Weekly | Monthly | Quarterly | Annually |
|---|
| Check valve (spring-loaded) | — | — | Function test | Replace or rebuild |
| Check valve (swing-type) | — | — | Visual check of hinge | Replace if worn |
| Relief valve (direct-acting) | — | Verify setting | — | Rebuild or replace |
| Relief valve (pilot-operated) | — | Verify setting | — | Rebuild or replace |
| All valve seals | — | Visual for leakage | — | Replace |
| All valve bodies | — | Visual for cracks | — | Replace if damaged |
FAQ
What is the purpose of a check valve in a deep hole drilling coolant system?
The check valve prevents reverse flow when the coolant pump stops. Without a check valve at the pump outlet, chips and coolant from the machine can flow backward through the supply line and into the pump when the pump stops. Chips entering a piston pump cause immediate scoring of the valve plate and pistons. The check valve also maintains pressure in the supply line, preventing air from entering when the system is off.
What is the purpose of a relief valve in a deep hole drilling coolant system?
The relief valve limits maximum system pressure to prevent overpressure damage. If a coolant line becomes blocked (closed valve, clogged drill, chip jam), the pump continues to generate pressure until something fails — a hose bursts, a seal blows, or the pump is damaged. The relief valve opens at a set pressure (typically 110% of maximum operating pressure) and diverts flow back to the tank, preventing the pressure from exceeding safe limits.
How do I set the relief valve on a deep hole drilling coolant system?
Set the relief valve at 110% of the maximum normal operating pressure. To set: install an accurate pressure gauge at the valve, start the pump, gradually increase pressure until the valve opens (pressure stabilizes), adjust the valve screw (IN to increase, OUT to decrease), re-test, and lock the adjustment. For a system operating at 100 bar, set the relief valve at 110 bar.
What causes a relief valve to chatter?
Chatter (rapid opening and closing) is caused by: the valve being oversized for the flow rate (flow is too low to keep the valve stably open), system pressure instability (pump pulsation, other valves cycling), or incorrect spring selection. Chatter causes rapid wear of the seat and poppet. Fix by sizing the relief valve to match the actual system flow, not the pump maximum flow.
How often should coolant system valves be tested?
Check valves: function test quarterly — apply pressure in the reverse direction and verify the valve holds. Relief valves: verify setting monthly — observe the pressure at which the valve opens. Rebuild or replace both valve types annually or whenever they fail testing. For critical systems (high pressure > 100 bar, or safety-related applications), increase testing frequency to monthly for check valves and weekly for relief valves.
Check valves and relief valves are the unsung safety components of a deep hole drilling coolant system. A properly functioning check valve keeps chips out of the pump. A properly set relief valve prevents overpressure failures. Test them regularly, replace them on schedule, and never bypass or disable them. This article reflects industry practice as of 2026.
