Reciprocating Compressor Valve Engineering

Reciprocating Compressor Valve Engineering

Owned decision: relate valve condition, valve design compatibility and observed performance symptoms without substituting generic tolerances for the serial-specific manufacturer limits. Mandatory asset: Valve Inspection and Compatibility Record.

1. Direct engineering answer

A reciprocating compressor valve is a pressure-actuated flow-control assembly that opens and closes in response to the cylinder-to-manifold differential pressure. A valve defect can reduce delivered capacity, increase discharge temperature, change indicated pressure behaviour and increase mechanical loading. None of those symptoms independently proves valve failure. A sound decision first confirms the operating envelope and instrumentation, identifies the exact cylinder and valve position, then correlates performance evidence with physical inspection and manufacturer-specific acceptance limits. Suction and discharge valves are not assumed interchangeable. The release decision must identify who has authority to inspect, rebuild, replace, test and sign off the pressure-containing equipment.

2. Boundary and relationship to adjacent pages

General compressor overhaul planning, broader reciprocating compressor technology, parts purchasing and machine-wide reliability strategies belong to their respective pages. The page does not prescribe a universal lift, spring force, plate thickness, seat flatness or tightening torque. Such values depend on valve construction, cylinder geometry, gas service, pressure differential, rotational speed and OEM revision. Even an apparently identical valve plate can be wrong if its material, port area, spring arrangement or pressure duty differs. A procurement recommendation remains provisional until the serial-specific parts list and supplier evidence are reconciled.

3. The valve duty cycle

During the suction stroke, cylinder pressure falls below the suction manifold pressure sufficiently to overcome the valve assembly’s opening resistance. Flow enters the cylinder until pressure differential and valve dynamics change. During compression, the suction valve should reseat and prevent backflow. The discharge valve opens once cylinder pressure exceeds discharge-side pressure plus the resistance associated with valve motion and flow. Real events are dynamic: plate inertia, spring response, gas pulsation and pressure losses alter timing. A simple textbook pressure-volume cycle does not predict a particular installed valve’s lift or fatigue life. This distinction matters when interpreting measured temperature, volumetric efficiency and pulsation data.

4. Valve constructions and selection boundaries

Common reciprocating compressor valves use plates, rings, poppets or other guided elements, with springs and guards controlling motion. Designs differ in effective open area, moving mass, pressure drop, contamination sensitivity, repair method and allowable speed. Some constructions are intended for specific gases or demanding cycling conditions; the construction label alone cannot establish suitability. Inspect the actual exploded drawing, assembly code, material schedule and approved repair instruction. When a legacy design has been superseded, obtain the controlled supersession chain and confirm whether associated seat, guard, springs, fasteners or clearances must change together.

5. Suction versus discharge identification

Mark every removed assembly by machine, cylinder, end, stage, valve pocket and flow direction before handling. A multi-stage machine can have valves that look similar but operate at materially different pressure, temperature and differential-pressure conditions. Photograph orientation and record installed part codes before cleaning removes markings. Never rely on colour, apparent diameter or an informal workshop nickname. Confirm port geometry and the seat-to-pocket interface against the correct drawing. A valve fitted in the wrong position can cause abnormal pressure dynamics or mechanical contact even if its outer dimensions allow assembly.

6. Evidence intake before opening

Collect the machine nameplate, frame and cylinder identifiers, serial-specific parts catalogue, prior service reports, operating-hour history, gas or air service description, rated speed, suction and discharge conditions, stage temperatures, load/unload sequence and recent alarms. Preserve time stamps and instrument calibration status. Ask whether the fault emerged after a parts change, maintenance, duty change or prolonged storage. Obtain any vibration or acoustic trend and performance test with stable operating conditions. This history allows the inspector to distinguish a progressive sealing problem from a sudden mechanical fracture or an operating excursion.

7. Operating data quality

A pressure reading without measurement location, units, instrument range and time context is weak evidence. Record whether the reading represents absolute or gauge pressure, whether it was taken upstream or downstream of filters and coolers, and whether it coincided with a loaded operating state. Temperature probes may respond slowly to short events; a handheld surface reading is not a direct gas-temperature measurement. Confirm speed and actual suction conditions before comparing capacity data. For an air compressor, inlet filter restriction and high ambient temperature can change performance without a valve defect.

8. Capacity loss as a symptom

Leakage through a valve that should be closed can recirculate gas and reduce useful cylinder throughput. The observed station flow may also fall because of worn piston rings, excessive clearance, intake restriction, control unloading, incorrect speed or downstream leakage. Compare measured flow at a defined reference basis and pressure with the applicable machine curve. Where a baseline exists, document the deviation and its uncertainty; without a baseline, avoid declaring a percentage of valve-related loss. Valve leakage is a hypothesis until corroborated by cylinder-specific evidence, temperature patterns or physical inspection.

9. Temperature patterns

A leaking discharge valve can return hot compressed gas toward the cylinder during part of the cycle, while a suction-valve fault can alter cylinder filling and compression behaviour. Temperature direction and magnitude depend on stage, instrumentation and process. Rising discharge temperature can also follow higher compression ratio, poor cooling, insufficient ventilation, fouled intercoolers or lubricant problems. Record temperatures at stable duty and compare stages rather than treating one reading as diagnostic. Stop or restrict operation when measured conditions exceed the verified machine limit; do not derive a safe temperature threshold from this article.

10. Noise, pulsation and vibration

Broken valve elements or unstable valve motion may generate abnormal impacts and pressure pulsation. Similar signatures can arise from loose guards, mechanical bearings, foundation defects, liquid carryover and pipe resonance. Collect location-specific observations and, where approved, pressure-pulsation or vibration measurements. Preserve raw traces rather than reducing the investigation to a subjective noise description. Inspection must be planned for isolation and depressurisation; never place hands, tools or sensors near exposed moving or pressurised valve components. Confirm instrument attachment and hazardous-area requirements for the actual installation.

11. Failure mechanisms: fatigue

Repeated valve opening and closing creates cyclic bending, impact and contact stress. A plate or ring may crack at a stress concentration, edge defect or contact point. Fatigue assessment should preserve fracture surfaces before abrasive cleaning, photograph crack origins and examine whether spring condition or lift control contributed. A replacement of the broken element alone can repeat the failure if the seat, guard, unloading device or operating pulsation remains abnormal. Material and fracture conclusions require a competent specialist; visual appearance is not a substitute for metallurgical evidence.

12. Failure mechanisms: contamination

Dust, carbon deposits, corrosion products and lubricant-derived residue can prevent complete seating or restrict element movement. Record deposit distribution before cleaning; it may identify ingress paths, overheating or lubricant carryover. Determine whether contamination originates upstream, within the cylinder or during maintenance handling. A dirty valve is not automatically defective beyond recovery, but cleaning can expose pitting or erosion that changes acceptance. Use only manufacturer-approved cleaning chemistry and methods compatible with elastomers, coatings and materials. Preserve samples when contamination source is disputed.

13. Failure mechanisms: liquid carryover

Liquid entering a reciprocating cylinder can create severe mechanical loads because liquid is poorly compressible. Evidence may include distorted valve elements, fractured guards, abnormal impact marks and concurrent damage to rods or connecting components. Treat suspected liquid carryover as a machine-system event, not simply a valve replacement job. Review upstream separators, drains, intercooler integrity, start-up practices and operating transients. The return-to-service decision requires the qualified owner to consider associated mechanical and pressure-system inspection before restoring load.

14. Failure mechanisms: spring damage

Valve springs influence opening and closing dynamics, but the correct force and free length are design-specific. Inspect for fracture, distortion, corrosion, heat effects and inconsistent orientation. Record spring part identity and position. Do not infer acceptability from a spring’s ability to compress by hand. Where the OEM calls for a force-versus-deflection test, use calibrated equipment and the specified procedure. Replacing a single spring in a matched set may be prohibited by the approved repair instruction; confirm the actual assembly rules.

15. Failure mechanisms: seat and guard

Seat faces, guide surfaces and guards can exhibit pitting, fretting, erosion, distortion or impact marks. A new moving element installed against a damaged seat may continue to leak. Measure dimensions only where the drawing identifies a datum and an allowable limit. Photograph as-found damage with scale and orientation. Distinguish removable deposits from lost parent material. Lapping, machining or reworking a seat can change installed geometry and must not be authorised without an approved repair route and post-work verification.

16. Corrosion and material mismatch

Air moisture, condensate chemistry and process contaminants may corrode valve components. Material selection must account for gas composition, temperature, pressure, fatigue behaviour and any relevant contamination controls. A visually similar stainless or polymer component may have different strength, thermal expansion or chemical resistance. Verify grade and treatment using credible material documentation when required by the design. If the original component has no readable markings, establish identity from controlled procurement or inspection records rather than assigning a grade from appearance.

17. Inspection safety and isolation

A valve pocket may retain pressure even after the main machine is stopped. The work permit should identify energy isolation, depressurisation points, drain paths, stored mechanical energy, hot surfaces and lifting arrangements. Confirm zero-energy state using the site-approved procedure and suitable instruments; do not rely solely on a stopped motor or a closed isolation valve. Use the approved sequence for removing covers and valve assemblies, especially where springs or unloaders may be preloaded. Record the isolation permit reference and qualified personnel responsible for release.

18. Removal and traceability

Assign each valve a unique tag linked to its pocket and orientation. Photograph installed state, fastener arrangement, gasket condition and any witness marks. Record damage discovered during extraction separately from pre-existing evidence. Keep components from different cylinders segregated to prevent accidental mixing. A removed valve with uncertain position is quarantined from reinstallation until identity is reconstructed. Protect mating faces from handling damage; loose components should be packed and labelled so subsequent inspection retains their origin and chain of custody.

19. Cleaning and non-destructive inspection

Select cleaning and examination based on material and manufacturer procedure. Magnified visual inspection can reveal cracks, edge damage, corrosion and deposit patterns; additional methods may be warranted by the component and defect type. Any penetrant, magnetic-particle or other method requires compatible materials, qualified procedures and interpretation. Avoid destructive preparation until photographs and failure evidence are retained. If the component is not permitted to be reused after certain inspection or removal steps, follow the OEM instruction rather than treating a clean appearance as approval.

20. Dimensional inspection

The inspection plan must identify what is being measured, its datum, instrument, calibration record, ambient conditions and applicable tolerance source. Candidate dimensions include element thickness, seat contact geometry, lift-control features, spring characteristics and pocket fit—but only when the specific design permits or requires them. Record actual measured values with units and uncertainty where material. A pass/fail field without the corresponding OEM limit and revision is insufficient. If no verified limit is available, the result is HOLD FOR TECHNICAL DISPOSITION, not an invented pass.

21. Flow-direction and assembly compatibility

Confirm the assembly’s functional orientation and that the correct suction or discharge flow path is maintained. Check the valve seat, guard, element, springs, gaskets, fasteners and any unloading interface as one controlled configuration. A replacement assembly may have the same mounting diameter but different lift, flow area, thickness or actuation geometry. Verify clearances to moving cylinder parts under the correct manufacturer procedure. Do not force installation or alter pocket geometry to make an unapproved assembly fit.

22. Superseded part numbers

An OEM supersession may replace one part code with another while requiring associated changes. Capture the original part code, proposed code, supplier document, revision date and serial applicability. Determine whether the replacement is a complete assembly or a component-level kit, and whether old and new elements may be mixed. Document approved gasket and fastener changes. Where the supplier states a general family fitment without a serial-specific cross-reference, treat it as an inquiry, not technical approval. Preserve the evidence version used for the decision.

23. Gaskets, seals and fasteners

Valve pocket sealing depends on compatible gasket geometry, material and assembly procedure. Reuse restrictions can apply to gaskets, locking features and fasteners. Incorrect thickness can affect the installed position or sealing condition. Confirm the approved replacement kit and tightening sequence; torque values and lubrication conditions are not universal. Record fastener condition, thread engagement and any repair to damaged threads. A leak discovered after reassembly requires controlled investigation rather than repeated tightening beyond a documented limit.

24. Unloader and capacity-control interfaces

Some reciprocating compressors use suction-valve unloaders or other mechanisms that intentionally hold valves open during unloaded operation. A control fault can resemble valve leakage or prevent full capacity even with healthy valves. Inspect actuation identity, travel, command state and response under the approved procedure. Ensure that replacement valve geometry remains compatible with the unloader fingers, lift mechanism or clearance features. Record whether the observed low flow occurred while the compressor was genuinely loaded. Control tests should precede invasive disassembly when safe and diagnostically appropriate.

25. Cylinder and piston-ring interactions

Valve findings should be interpreted alongside cylinder wear, piston rings, packing, clearance volume and connecting-rod condition. A valve can be damaged by another defect, or a healthy valve can be blamed for a cylinder efficiency problem. Review pressure-temperature evidence, leakage paths and overhaul records before concluding causation. Where an internal inspection identifies contact marks or foreign material, expand the scope to the associated mechanical interfaces. A single repaired valve does not certify the whole cylinder or compressor package.

26. Stage-specific diagnosis

For a multi-stage machine, analyse each stage at the same stable operating condition. Record stage suction and discharge pressure, temperature, cooling conditions and relevant intercooler pressure loss. An abnormal interstage pressure can arise from valve leakage, upstream restriction, cooling change or downstream stage capacity. Avoid treating stage pressure direction as an infallible fault locator. Use the actual machine schematic and the engineering relationship between stage mass flow and operating conditions. Test data should include loading state and measurement location to permit review.

27. Performance calculation: capacity deviation

Illustrative example only: a verified reference test indicates 12.0 m³/min at stated inlet reference conditions, while a comparable measured test indicates 10.8 m³/min. Relative shortfall = (12.0 − 10.8)/12.0 × 100 = 10%. This quantifies the observed deviation, not its cause. Before associating the loss with a valve, confirm comparable speed, inlet pressure and temperature, discharge pressure, flow measurement basis and instrument uncertainty. If the two measurements use different reference conditions, first normalise them using an approved method. The 10% result must not be used as a universal repair threshold.

28. Performance calculation: differential pressure

Illustrative example: a clean-condition valve path is reported as 0.12 bar pressure difference and a later comparable measurement is 0.20 bar, giving an increase of 0.08 bar. The comparison is meaningful only if flow, gas density, valve opening state, measurement taps and instrument calibration are comparable. A larger measured difference may indicate restriction, but can also reflect a change in flow or upstream piping. Do not treat this difference as the pressure drop across an individual valve unless the measurement configuration actually isolates that valve. No acceptance limit is implied.

29. Performance calculation: lost output exposure

Illustrative operational calculation: if a process receives 0.9 m³/min less useful air than its validated requirement for 6 loaded hours per day, the apparent shortfall exposure is 0.9 × 60 × 6 = 324 m³/day at the stated flow reference basis. This is a planning quantity, not proof that the compressor valve caused the deficit. It must not be multiplied by a generic energy factor to claim savings. A corrected system evaluation should distinguish unmet demand, compensating operation by other compressors, control response and measurement uncertainty.

30. Evidence strength and fault hypotheses

Rank evidence by direct relevance to the actual valve: controlled OEM configuration documents, calibrated as-found measurements, repeatable machine data and documented physical findings normally carry more weight than supplier catalogue descriptions or anecdotal observations. A fault hypothesis should identify what observation would support or refute it. For example, a suspected leaking discharge valve can be investigated using cylinder-specific temperature trends, authorised inspection and a controlled seat assessment. An unexplained high temperature alone remains an open hypothesis. Record contrary evidence as well as supportive evidence.

31. Repair versus replacement

Repair can be appropriate when the manufacturer permits it, components remain within defined limits and a qualified facility can verify the restored assembly. Replacement may be required for cracking, unacceptable wear, incompatible materials or uncertain identity. Economic evaluation should include planned downtime, repair scope, inspection effort, verification cost and consequence of repeat failure. A low-priced replacement without traceability is not equivalent to a documented assembly. The final disposition must distinguish replace component, rebuild approved assembly, replace complete assembly, investigate system cause and do not reuse.

32. Workshop assembly quality

Before assembly, confirm parts against the controlled bill of materials and verify cleanliness, orientation, spring placement, guides, seating surfaces and approved lubricants or dry-assembly requirements. Maintain a build record that links each critical part to the valve tag. The technician should record any deviation or unexpected fit rather than resolving it through undocumented machining or substitution. Where the manufacturer requires matched elements or special fixtures, use them. Signoff should identify the assembler, inspector, instruction revision and completed checks, not merely a generic “overhauled” stamp.

33. Bench examination and testing

Bench tests may include functional movement, leakage checks or other OEM-defined evaluations. Test pressure, medium, direction, fixture and acceptance criterion must be sourced for the exact design. A bench pass does not necessarily establish dynamic performance at installed speed and temperature. Record the raw measured result and instrument calibration, then state the scope of the conclusion. Where no approved bench procedure is available, do not improvise a pressure test. Escalate to the OEM or authorised technical owner for the correct verification method.

34. Reinstallation and pocket verification

Before installation, confirm pocket cleanliness, seating condition, correct gasket, valve orientation, approved fasteners and unobstructed flow passages. Inspect associated unloaders and clearance interfaces. Use the specified tightening and locking procedure; capture torque records only where the OEM requires them. A pocket with damage outside known limits is an open repair item even if the new valve fits. Complete a position-by-position reconciliation so no removed valve remains unaccounted for and no assembly is installed in an unverified stage or cylinder.

35. Controlled recommissioning

The return-to-service plan should cover isolation removal, guard reinstatement, lubrication and cooling readiness, instrumentation, unloaded start where prescribed, load transition, abnormal-noise checks, leak observations and stable operating data. Establish stop criteria from the actual equipment and site operating instructions. Trend cylinder/stage pressures, discharge temperatures, speed, power and delivered flow as relevant. An acceptable initial start does not replace a sustained duty verification where required. Retain as-left data under comparable conditions so subsequent reliability analysis can distinguish genuine improvement from operating variation.

36. As-found versus as-left comparison

As-found and as-left results should share a defined measurement basis. Record the pre-work symptom, evidence source, observed defect, completed action, replaced part identity and post-work operating conditions. For example, a lower discharge temperature after repair is useful only if discharge pressure, inlet conditions, speed and cooling were reasonably comparable. When comparability is limited, explicitly state it and arrange a later controlled performance test. Do not claim restoration to rated capacity solely because an alarm has cleared.

37. Worked case: intermittent capacity deficit

Illustrative case: a two-stage compressor shows reduced delivery during an apparently loaded period. The initial hypothesis is first-stage suction-valve leakage. The team verifies actual loaded command and engine or motor speed, then checks intake restriction and calibrated stage readings. A first-stage valve inspection finds deposit accumulation on the seating surface, while the valve plate has no visually apparent crack. The dossier records the seat and plate as HOLD pending OEM dimensional and leakage criteria; a cleaned appearance does not authorize reuse. The alternative hypothesis of unloader interference remains open until actuation is checked. This case demonstrates evidence sequencing rather than an asserted root cause.

38. Worked case: same family, different revision

Illustrative comparison: Unit A and Unit B carry the same family label but different serial ranges and valve assembly revisions. Unit A has a documented OEM valve kit and complete service records; Unit B has a visually similar aftermarket assembly with missing material certificate and an uncertain spring specification. Both machines may show a similar temperature symptom, yet their part decisions differ. Unit A can proceed to qualified inspection against its verified limits. Unit B must hold replacement approval until serial applicability and component evidence are established. A common family name cannot bridge the revision gap.

39. Worked case: recurring valve fracture

A valve element fractures soon after replacement. Instead of repeatedly fitting another element, preserve fragments and review assembly records, spring selection, lift-control geometry, unloading sequence, pulsation history and possible liquid ingress. Compare the fracture location with contact marks and any evidence of abnormal impact. Confirm whether the replacement was an approved supersession and whether the full kit was required. Release should be blocked until the technical owner has resolved the likely damaging mechanism or defined an approved controlled test plan. A replacement receipt alone is not closure.

40. Worked case: apparent valve noise

A rhythmic sound is reported near a cylinder after overhaul. The valve is suspected, but initial evidence also shows a loose adjacent pipe support and a change in pulsation with loading. The team first verifies operating safety and establishes the sound location under an approved monitoring plan. The record separates mechanical observations from acoustic inference. If authorised inspection shows intact valve elements and no rejection, the valve is not condemned on sound alone. The pipe support and pulsation condition are assigned to the appropriate owners.

41. Worked case: wrong valve pocket

Two removed assemblies are accidentally placed in a shared tray during maintenance. Their markings are incomplete and they appear identical. The inspection record must flag loss of positional traceability. Installation is withheld until the technician reconstructs identity using controlled dimensions, original photographs, parts documentation and qualified review. If identity cannot be established, the disposition is quarantine and procure or verify suitable assemblies. This protects against the false confidence created by matching external dimensions and avoids silently restoring a potentially incorrect configuration.

42. Worked case: contamination after shutdown

After prolonged shutdown, several valves show rust-coloured deposits and one cylinder produces irregular capacity. The investigation records storage conditions, condensate management, preservation history and deposit location. Cleaning may restore visible movement but cannot prove that seating geometry or springs remain acceptable. Inspect all affected pockets according to the defined exposure, not only the first failed valve. A verified preservation or moisture-control action belongs in the system-level corrective plan. The valve record closes only when individual component disposition and machine-level moisture risks are assigned.

43. Worked case: high discharge temperature

A stage discharge temperature rises following a process pressure increase. The compressor is operated at a different compression ratio than its prior baseline, and a valve fault is suspected. The investigation first confirms the authorised operating envelope, inlet conditions and cooler performance. It then assesses valve evidence without assuming that a temperature rise alone proves leakage. If the new operating duty exceeds documented design conditions, the machine-level duty review takes priority over component replacement. Record whether any valve inspection is still warranted after the operating condition is corrected.

44. Worked case: replacement kit substitution

A supplier proposes a replacement kit containing plate, springs and gasket under one commercial code. The original parts list identifies a different assembly revision and the supplier’s statement does not specify the serial range. The engineer requests a controlled cross-reference, materials, pressure/temperature applicability and any mandatory companion changes. Until verified, the kit remains proposed—not approved. The valve inspection and compatibility record must show who assessed each claim, which source was used and whether any part of the kit is explicitly excluded from the original design.

45. Worked case: bench pass, field failure

A valve passes an approved static bench leakage test but the installed machine still shows abnormal capacity and pulsation. The bench test is retained as evidence of one specific property, not treated as comprehensive proof of dynamic suitability. Reassess unloading, flow dynamics, valve pocket geometry, cylinder clearance, pipe pulsation and actual operating conditions. If the same valve passes static tests repeatedly while failing under duty, escalate the design or system interaction rather than modifying the valve beyond the approved repair instruction.

46. Worked case: uncertain OEM limit

An inspection finds a small amount of seat wear, but the available workshop note lists no acceptance criterion and the drawing revision is unknown. The technician records the actual measurement, instrument and photograph, then sets HOLD FOR LIMIT VERIFICATION. The owner obtains the correct serial-specific source or a documented OEM disposition. No arbitrary “industry standard” value is inserted. If the limit remains unavailable, a new verified assembly may be the safer procurement option, subject to approved compatibility and site risk assessment.

47. Cost and downtime decision

A defensible commercial decision separates confirmed work from speculative savings. Compare repair parts, inspection labour, machine downtime, transport, verification testing and consequences of repeat failure. Where the machine is process-critical, include the cost of temporary air or redundancy only if supported by the site’s actual contingency plan. Avoid claiming a repaired valve will deliver a specified efficiency gain unless a comparable measured test establishes it. A cost model should preserve sensitivity to uncertainty in the fault cause and the availability of approved parts.

48. Procurement acceptance

Purchase descriptions should state the exact valve assembly or approved supersession, compressor serial applicability, stage and pocket, material and design requirements, certificate or inspection records, packaging/preservation conditions and return rights for nonconforming items. Receiving inspection checks identifiers against the approved purchase basis, not merely the invoice. If a supplier offers a “universal” valve, demand documentary evidence of the exact configurations covered.

49. Evidence ownership and escalation

Assign the machine owner to supply operating data and access; the service lead to control isolation, inspection and assembly; the technical reviewer to approve component compatibility and acceptance limits; procurement to maintain supplier documentation; and the commissioning lead to collect as-left test evidence. One person may fill multiple roles where competent and authorised, but no required check should disappear. Unresolved questions must have a named owner and due disposition, not a blank field or implicit acceptance. Preserve source revision and the date of the technical decision.

51. Mandatory asset — Valve Inspection and Compatibility Record

The following sheet is a controlled working template. Record each source identifier, inspection method, measured value, reviewer, unresolved question and final disposition. Empty or unsupported fields are HOLD; they are never interpreted as PASS.

52. Completed illustrative record — two configurations

53. Acceptance and rejection logic

ACCEPT only where exact asset applicability, approved assembly configuration, component inspection and prescribed tests are verified, the as-left operating record is adequate, and all material uncertainties are closed. CONDITIONAL RELEASE requires an explicit authorised risk decision and defined operating restrictions; it is not the default when evidence is missing. REJECT or QUARANTINE when identity is uncertain, the assembly is incompatible, a safety-critical defect remains, or a mandatory limit/test fails. Record the decision authority, date, source versions and next action. A test result may be accepted for its stated scope while the overall machine remains on hold.