The shaft-failure investigation should focus first on liquid lock or a second-stage flow blockage, not on slow gas compression against a closed downstream valve. A positive-displacement compressor can develop destructive torque almost immediately when trapped liquid prevents displacement. With compressible air volume available, pressure normally rises over a measurable interval and a correctly selected, installed, and functional relief path has an opportunity to open.
Failure-mode comparison
The number that matters is the instantaneous second-stage torque, not simply the 16-second elapsed time to the inter-stage high-pressure trip. The trip timestamp marks when the control system detected its limit; it does not reveal when peak shaft torque occurred.
| Candidate condition | Physical mechanism | Expected evidence | Investigative priority |
|---|---|---|---|
| Closed downstream isolation valve with trapped air | Each displacement cycle compresses a finite gas volume. Discharge pressure rises until unloading, shutdown, leakage, or pressure relief changes the condition. | High second-stage discharge pressure, relief-valve activity, rising motor load, and a pressure trace before the trip | Test the valve lineup and protective functions, but a closed valve alone does not explain why protection failed. |
| Liquid-filled second stage or discharge path | Liquid has very low compressibility. A trapped volume produces a steep pressure rise and an abrupt torque impulse before normal pressure protection can relieve enough mass. | Liquid in the separator, second-stage inlet, casing, drains, or discharge piping; impact marks; abrupt torsional failure | Highest priority because it directly explains rapid shaft breakage. |
| Second-stage internal discharge restriction | Incorrectly installed, damaged, or blocked internal discharge valves prevent the second stage from accepting first-stage flow. | Inter-stage pressure rises while second-stage flow remains restricted; valve orientation or condition is abnormal | High priority, particularly after maintenance or internal assembly work. |
| Suction-regulator malfunction | Incorrect regulation moves the stages outside their intended pressure and torque envelope. A shared motor can supply enough torque to overload one stage. | Regulator fails its functional test, linkage is misadjusted, or startup pressure and load do not follow the operating manual | High priority because the event followed suction-regulator maintenance. |
| Pre-existing shaft or coupling defect | Fatigue, misalignment, or an existing crack reduces the torque needed for final fracture. | Fatigue beach marks, fretting, corrosion, misalignment, or progressive damage at the fracture | Retain until metallurgical and alignment checks exclude it. |
Pressure, torque, and protection timing
A screw compressor is a positive-displacement machine. When its outlet is restricted, each cycle attempts to move another trapped volume. Air can compress, so pressure and absorbed torque rise as the trapped gas inventory is reduced. Liquid cannot provide the same compliance. If the second-stage casing or blocked discharge section contains liquid, rotor motion demands either immediate liquid displacement, structural deflection, leakage, or mechanical failure. This is heat and force, not logic.
A pressure relief valve protects the pressure boundary only when pressure reaches its opening condition at the valve inlet and the valve can discharge sufficient mass through an unobstructed path. It is not automatically a shaft-torque limiter. A fast liquid-pressure transient can impose destructive rotor torque before the valve completes its mechanical opening or before liquid reaches the valve. Isolation, blocked sensing passages, incorrect installation, sticking, unsuitable service, or a discharge restriction beyond the protected volume can also defeat the intended protection.
Star/delta starting limits electrical starting current and motor torque during acceleration; it does not prove that the driven train remained below its mechanical torque limit. The transition state at the instant of fracture must come from motor-current records, starter events, speed data, and the starter configuration. The 16-second trip time by itself cannot identify that state.
Quantities and records that decide the case
| Quantity or condition | Why it matters | Where to read or inspect it |
|---|---|---|
| Inter-stage pressure | Shows whether first-stage delivery exceeded second-stage intake capacity | Trip record, pressure switch or transmitter, and controller diagnostic buffer |
| Second-stage discharge pressure | Separates downstream backpressure from an internal second-stage blockage | Local instrument, controller trend, or a temporary test instrument for the controlled test |
| Motor current and starter state | Provides a time-based proxy for absorbed torque and identifies star/delta transition | Motor protection relay, starter event log, or power monitor |
| Liquid inventory | Tests the liquid-lock mechanism directly | Moisture separator, drains, second-stage inlet, casing low points, and discharge low points |
| Relief path condition | Determines whether the protected volume had a usable escape path | Valve inlet connection, isolation position, discharge routing, inspection records, and certified test record |
| Regulator response | Confirms whether the maintained component controlled stage pressure and loading | Bench or functional test against the manufacturer’s procedure |
| Shaft fracture character | Distinguishes sudden torsional overload from progressive fatigue | Preserved fracture faces, dimensional inspection, and metallurgical examination |
Recommended diagnostic procedure
- Quarantine the compressor and preserve the shaft, coupling, internal valves, separator contents, and disturbed piping exactly as found. Protect fracture faces from cleaning, grinding, handling damage, and corrosion.
- Build a second-by-second event timeline from start command through the inter-stage high-pressure trip. Include starter state, motor current, speed if available, regulator command and feedback, inter-stage pressure, and every recorded valve position.
- Verify the complete gas path physically. Trace the first-stage discharge through the moisture separator, second-stage inlet, internal discharge components, second-stage outlet, relief connection, and external isolation valve. A control-system indication is not proof of mechanical valve position.
- Drain and measure liquid from every available low point. Record where each sample came from and inspect the separator and automatic or manual drains for blockage, incorrect lineup, or failure to discharge.
- Remove and inspect the second-stage internal discharge components for reversed installation, obstruction, damage, or restricted movement. Compare orientation and assembly only with the compressor manufacturer’s drawing and operating manual.
- Functionally test the serviced suction regulator separately before another driven test. Confirm its full travel, fail position, feedback, connections, and startup action against the stated operating envelope.
- Audit the pressure relief path from the second-stage protected volume to its termination. Confirm that no isolation or blockage separates the valve from the casing, then obtain an appropriate certified functional test rather than manually disturbing its adjustment.
- Request fracture analysis and verify rotor freedom, bearing condition, coupling selection, and shaft alignment. A sudden final fracture does not exclude an older fatigue crack.
Controlled restart and verification
Do not restart with the damaged train or with liquid, regulator operation, internal valve orientation, and the relief path unresolved. Before a controlled test, restore the machine to the manufacturer’s assembly condition, prove drains clear, confirm the downstream route open, and install calibrated pressure and electrical measurements where the permanent history lacks second-stage discharge pressure.
Trend inter-stage pressure, second-stage discharge pressure, motor current, starter state, and regulator position from before the start command. Use the manufacturer’s startup procedure and shutdown limits. A satisfactory test shows continuous pressure development, correct loading or unloading response, stable current for the operating state, and no unexplained liquid accumulation. Abort the test when any measured value approaches the operating-manual limit or when pressure rises without corresponding discharge flow.
Recurring investigative pitfalls
Assigning the failure solely to the downstream valve skips the more important question: why did the second stage experience enough torque to fracture its shaft? Likewise, the presence of a relief valve does not demonstrate that it protected the trapped volume, opened dynamically, or could pass the material present.
Missing discharge-pressure history prevents a definitive pressure reconstruction. Preserve that uncertainty and close it with synchronized instrumentation during non-destructive testing. Avoid using the trip timestamp as the duration of overload; the damaging impulse may have preceded the recorded trip.
The strongest working path is to inspect for liquid and internal blockage first, functionally test the recently maintained suction regulator, verify the actual relief connection, and then use fracture morphology to distinguish sudden overload from pre-existing damage.
FAQ
How do I tell whether a closed discharge valve broke the compressor shaft?
Reconstruct inter-stage pressure, second-stage discharge pressure, motor current, starter state, and physical valve position on one timeline. A closed valve establishes backpressure, but liquid inventory, the relief path, internal restrictions, and fracture morphology determine why torque reached the shaft’s failure level.
How do I check for liquid lock in a two-stage screw compressor?
Isolate the machine, drain and measure every separator, casing, inlet, discharge, and piping low point, then inspect the drain devices and second-stage flow path. Preserve samples and document their exact locations before dismantling changes the evidence.
How do I verify the pressure relief valve protected the second stage?
Trace an unobstructed connection from the protected second-stage volume to the valve, confirm every isolation device position, inspect the discharge route, and review a certified functional test. Valve presence alone does not verify dynamic protection against a liquid-pressure transient.
Stop the investigation and escalate to the compressor manufacturer’s official support channel when the operating envelope, internal valve orientation, relief arrangement, or shaft torque limit cannot be established from controlled documents. Also escalate before restart when fracture analysis identifies progressive damage or the event reconstruction cannot explain the inter-stage high-pressure trip without exceeding a documented machine limit.