Repair and Maintenance of Engineering System

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Maintenance is the combination of all technical and administrative actions intended to retain an item in, or restore it to, a state in which it can perform its required function (BS 3811 / EN 13306).

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This section explains why maintenance is needed, its objectives, and the benefits of a good maintenance plan.

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Reasons for the Need of Maintenance • Wear and tear of moving parts (bearings, gears, seals, belts) due to friction and abrasion. • Fatigue from cyclic loading, vibration and thermal cycling; creep at high temperature. • Corrosion and erosion — chemical/electrochemical attack, cavitation, abrasive flow. • Lubrication failure, contamination (dust, water, chips), overheating. • Loosening of fasteners, misalignment, imbalance caused by vibration. • Ageing and deterioration of materials (rubber, insulation, plastics), obsolescence. • Misuse, overload and operator error; poor installation or design deficiencies. • Safety and statutory requirements (boilers, pressure vessels, lifts, cranes need periodic inspection and certification). • Economic need: the cost of unplanned downtime, lost production and secondary damage is usually far higher than the cost of maintenance.

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Objectives of Maintenance • Maximise availability and reliability of plant and equipment at minimum total cost. • Ensure safety of people, equipment and environment. • Extend useful life of assets and preserve their value. • Maintain product quality and process capability. • Improve energy efficiency and reduce waste. • Keep equipment ready for emergency use (fire pumps, standby generators).

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Benefits of a Good Maintenance Plan Benefit Explanation Increased uptime / availability Fewer unplanned breakdowns; production targets met Lower total cost Less emergency repair, overtime, secondary damage and scrap; optimum balance between maintenance cost and downtime cost Longer equipment life Timely lubrication, adjustment and part replacement delay wear-out Improved safety Hazards (leaks, worn guards, faulty wiring) found before they cause accidents Better product quality Machines hold tolerances; fewer defects and rework Energy efficiency Clean filters, correct alignment and lubrication reduce power consumption Better planning Labour, spares and shutdowns scheduled in advance; controlled spare-parts inventory Regulatory compliance Statutory inspections and records kept up to date Reliable data Equipment history supports root-cause analysis and replacement decisions Customer satisfaction On-time delivery, consistent quality Elements of a Maintenance Plan and Performance Measures • Elements: equipment register/inventory and criticality ranking, history cards, maintenance schedules and checklists, work orders / job cards, spare-parts management (ABC, VED analysis), skilled manpower, tools and budget, documentation, CMMS (computerised maintenance management system). • Availability A = MTBF/(MTBF + MTTR) (inherent availability).

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MTBF = total operating time / number of failures;

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MTTR = total repair time / number of repairs. • OEE (Overall Equipment Effectiveness) = Availability × Performance × Quality; world-class OEE ≈ 85%. • Other KPIs: planned maintenance percentage (PMP), maintenance cost per unit output, backlog, schedule compliance, mean downtime. • Total cost curve: as maintenance effort increases, maintenance cost rises and breakdown/downtime cost falls — the optimum level is where total cost is minimum.

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Maintenance work exposes people to fire, electrical and machine hazards, while equipment is exposed to corrosion.

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This section covers fire protection, electrical hazards, machine-tool safety, types of corrosion and their prevention, protective coating systems and cathodic protection.

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Fire Protection • Fire triangle: heat + fuel + oxygen (fire tetrahedron adds the uninhibited chemical chain reaction).

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Remove any one side to extinguish: starving (remove fuel), smothering/blanketing (remove oxygen), cooling (remove heat), breaking chain reaction (dry chemical powder, halon substitutes). • Use of extinguisher — PASS:

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Pull the pin, Aim at the base of the fire, Squeeze the handle, Sweep side to side. • Detection and fighting systems: smoke detectors (ionisation, optical), heat detectors, flame detectors, manual call points, alarms, sprinklers (glass bulb bursts ≈ 68 °C), hydrants and hose reels, fire exits and emergency lighting, regular fire drills. • Prevention: good housekeeping, safe storage of flammables, hot-work permits for welding/cutting, no smoking zones, proper electrical installations.

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Fire class (IS/EN) Fuel Suitable extinguisher A Ordinary combustibles (wood, paper, cloth) Water, foam, ABC powder B Flammable liquids (petrol, oil, paint) Foam, CO2, dry chemical powder — NOT water jet C Flammable gases (LPG, acetylene) Dry chemical powder; first isolate gas supply D Combustible metals (Mg, Na, K, Ti) Special dry powder (graphite, NaCl based) Electrical Live electrical equipment CO2 or dry powder — NEVER water or foam F (K in USA) Cooking oils and fats Wet chemical Electrical Hazards • Hazards: electric shock, burns, arc flash/arc blast, fire, explosion in flammable atmospheres, secondary falls. • Effect of current through the body (50 Hz AC, approx.): ~1 mA perception; ~10 mA 'let-go' limit (muscle contraction);

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30 mA can be dangerous (hence RCD trip setting);

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50–100 mA can cause ventricular fibrillation (often fatal).

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Severity depends on current, path (hand-to-hand across heart worst), duration and frequency; body resistance falls sharply when wet. • Protection: proper earthing (grounding), fuses/MCBs (overcurrent), RCD/RCCB/ELCB (earth-leakage, 30 mA), double insulation, extra-low voltage (≤ 50 V AC) for hand lamps in damp places, insulated tools, rubber mats and gloves, keeping panels closed. • Safe work practice: isolation + Lockout-Tagout (LOTO) before maintenance, test for absence of voltage ('test before touch'), permit-to-work, only authorised/qualified persons, keep dry, no metal jewellery. • First aid for shock: switch off supply or separate victim with a dry non-conducting object — never touch directly; start CPR if not breathing; get medical help.

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Machine Tool Safety • Mechanical hazards: rotating shafts and chucks (entanglement), in-running nip points (gears, belts-pulleys, rolls), reciprocating and shearing parts, flying chips and broken tools, sharp edges, hot surfaces. • Guards: fixed, interlocked (machine stops when opened), adjustable, self-adjusting; emergency stop buttons (red mushroom head on yellow background); two-hand controls and light curtains on presses. • Operator rules: no loose clothing, ties, rings or gloves near rotating parts; tie long hair; wear goggles and safety shoes; clamp work securely; remove chuck key; stop machine before measuring/cleaning; use brush/hook for chips; never exceed rated speeds. • Grinding wheels: 'ring test' for cracks before mounting, do not exceed marked maximum rpm, keep wheel guard, work-rest gap ≤ 3 mm, stand aside when starting. • Apply LOTO before maintenance; good housekeeping, adequate lighting, training and supervision.

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Types of Corrosion and Prevention Methods • Types (see 1.2 for detail): uniform, galvanic (dissimilar metals), pitting, crevice, intergranular (sensitised stainless steel), stress-corrosion cracking, erosion-corrosion, fretting, selective leaching (dezincification), hydrogen damage, microbiologically-influenced corrosion (MIC), high-temperature oxidation. • Prevention:

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(1) material selection (stainless, Cu-Ni, plastics);

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(2) design — avoid crevices, dissimilar-metal contact (or insulate them), stagnant zones, sharp bends; provide drainage;

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(3) environment control — inhibitors, de-aeration, pH control, dehumidification;

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(4) protective coatings;

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(5) cathodic protection;

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(6) anodic protection (maintain passive film — stainless steel in acids).

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Coating Systems Coating type Examples Notes Metallic — sacrificial Galvanising (hot-dip Zn), sherardising (Zn powder diffusion), Zn/Al metal spraying, Alclad Coating is ANODIC to steel — protects even at scratches Metallic — barrier (noble) Tin plating (tinplate), Ni and Cr electroplating, cladding Coating is CATHODIC to steel — a scratch accelerates corrosion of the exposed steel Conversion coatings Phosphating (paint base), anodising (thick Al2O3 on aluminium), chromating, black oxide (bluing) Improve adhesion and corrosion resistance Organic — paints Primer (zinc-rich, red oxide, zinc phosphate) + intermediate + top coat; epoxy (chemical resistance), polyurethane (UV-resistant top coat), alkyd, coal-tar epoxy (buried pipes) Multi-coat system; dry film thickness (DFT) specified Powder coating Electrostatically sprayed polymer powder, oven cured Tough, no solvents Linings Rubber, glass, PTFE, FRP linings Tanks and pipes handling chemicals • Surface preparation is the most important factor in coating life — most coating failures are due to poor preparation.

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Methods: degreasing, pickling, power tool cleaning, abrasive blast cleaning to Sa 2½ (ISO 8501-1 'near-white metal'). • Coating failures: blistering, peeling (poor adhesion), cracking, chalking (UV degradation), rust creep from damage, holidays (pinholes — checked by holiday detector).

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Cathodic Protection (CP) • Principle: make the structure to be protected the CATHODE of an electrochemical cell so that it does not corrode. • Sacrificial (galvanic) anode system: more active metal connected to structure corrodes instead — Mg (soil, high resistivity), Zn and Al alloys (seawater).

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No external power, simple, low maintenance; limited current — small structures, ship hulls, water heaters, well-coated pipelines. • Impressed current CP (ICCP):

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DC power source (transformer-rectifier) drives current from inert anodes (graphite, high-silicon cast iron, mixed metal oxide (MMO) coated titanium) to the structure.

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Adjustable, high current — long pipelines, large tanks, jetties, reinforced concrete. • Protection criterion for steel: potential more negative than −850 mV vs Cu/CuSO4 reference electrode.

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Monitored through test posts. • Coatings + CP are used together: coating reduces current demand, CP protects coating defects (holidays).

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Beware stray-current corrosion of nearby structures.

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A maintenance strategy defines WHEN and HOW maintenance is done for each item.

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This section covers the factors for choosing a strategy, breakdown, preventive, scheduled and predictive (condition-based) maintenance, Total Productive Maintenance, total planned quality maintenance and condition-monitoring techniques.

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Factors for Choosing a Maintenance Strategy • Criticality of equipment — effect of its failure on production, safety and environment. • Failure pattern — age-related (wear-out) failures suit time-based maintenance; random failures suit condition-based or run-to-failure. • Cost comparison — cost of maintenance vs cost of failure (repair, downtime, secondary damage, penalties). • Availability of redundancy/standby units; detectability of deterioration (is there a measurable warning signal?). • Age and condition of equipment, manufacturer's recommendations, statutory requirements. • Availability of skilled staff, tools, condition-monitoring instruments and spares; operating context (continuous process vs batch).

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Types of Maintenance Strategy Description Suitable when / pros & cons Breakdown (run-to-failure / reactive) Repair only after failure occurs Non-critical, cheap, redundant items (lamps, fans).

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Low planning cost, but unplanned downtime, secondary damage, safety risk, high overtime cost Corrective Restoring an item after a defect is detected (may be planned) Fixes faults found during inspections Preventive (time-based / usage-based) Inspection, lubrication, adjustment, replacement at FIXED intervals (hours, km, cycles) Age-related wear.

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Reduces breakdowns; may cause over-maintenance and replacing good parts Scheduled (planned) Maintenance carried out to a predetermined calendar schedule — routine servicing, annual shutdowns and overhauls Plants with planned shutdown windows; resources prepared in advance Predictive / condition-based (CBM) Monitor condition (vibration, temperature, oil) and maintain only when indicators show deterioration Critical rotating equipment.

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Maximum use of component life; needs instruments and skilled analysts Opportunistic Maintenance done when equipment is down for another reason Reduces extra downtime Design-out maintenance Redesign to eliminate the root cause of repeated failures Chronic problem equipment Reliability-centred maintenance (RCM) Systematic selection of tasks for each failure mode based on its consequences (FMEA-based); originated in civil aviation (Nowlan & Heap, 1978) Complex, safety-critical systems • P-F curve:

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P = point where a potential failure becomes detectable;

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F = functional failure.

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Condition-monitoring inspection interval must be shorter than the P-F interval (typically about half).

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Total Productive Maintenance (TPM) • Developed in Japan (Seiichi Nakajima, JIPM; first at Nippondenso, 1971).

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Involves ALL employees — from top management to operators — in maintaining equipment.

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Goals: zero breakdowns, zero defects, zero accidents. • Foundation:

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5S — Sort (Seiri), Set in order (Seiton), Shine (Seiso), Standardise (Seiketsu), Sustain (Shitsuke). • Eight pillars:

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(1) autonomous maintenance (Jishu Hozen) — operators do daily cleaning, inspection, lubrication, tightening;

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(2) planned maintenance;

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(3) focused improvement (Kobetsu Kaizen);

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(4) quality maintenance (Hinshitsu Hozen);

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(5) early equipment management;

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(6) training and education;

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(7) safety, health and environment;

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(8) office (administrative) TPM. • Six big losses: breakdowns, set-up and adjustment, idling and minor stoppages, reduced speed, start-up (yield) losses, quality defects and rework. • Measured by OEE = Availability × Performance × Quality (world class ≈ 85% = 90% × 95% × 99.9%).

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Total Planned Quality Maintenance • Integrates planned maintenance with quality management (also called total quality maintenance, TQMain, or the quality-maintenance pillar of TPM): equipment conditions that guarantee zero defects are identified, set as standards, and kept within limits through planned inspection and measurement. • Steps: identify quality characteristics → find equipment conditions (4M: man, machine, method, material) affecting them → set standard values → measure and trend → act before conditions drift out of limits (preventive rather than reactive quality control). • Tools:

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QA matrix, QM matrix, PDCA cycle, poka-yoke (mistake-proofing), SPC charts, root-cause analysis, maintenance and quality data integrated in CMMS/ERP.

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Condition Monitoring Techniques Technique Detects / used for Vibration analysis Unbalance (1× rpm, radial), misalignment (2× rpm, high axial), looseness (many harmonics), bearing defects (BPFO/BPFI frequencies), gear mesh problems;

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FFT spectrum analysis Infrared thermography Hot spots in electrical panels and connections, bearings, insulation and refractory failures, steam traps Oil / lubricant analysis Spectrometric wear-metal analysis, ferrography (wear particles), viscosity, water content, TAN/TBN, contamination Ultrasound / acoustic emission Compressed air and gas leaks, bearing lubrication, electrical discharge, valve passing Shock pulse method Condition of rolling-element bearings Motor current signature analysis Broken rotor bars, eccentricity in induction motors Performance monitoring Efficiency, pressure, flow, temperature trends (pumps, compressors, boilers) Visual inspection / borescope, NDT Cracks, corrosion, wall thickness (ultrasonic) — see 7.6

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Reliability engineering predicts and improves the probability that systems work without failure.

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This section covers failure modes, reliability measures (MTBF, MTTF, MTTR) and their interrelations, the bathtub curve, causes and effects of failure, reliability block diagrams, fault tree analysis, event tree analysis, hazard analysis and the fishbone diagram.

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Reliability Measures and Their Interrelations • Reliability R(t): probability that an item performs its intended function without failure for a stated time under stated conditions.

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Unreliability F(t) = 1 − R(t). • Failure (hazard) rate λ = number of failures per unit operating time.

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For a constant failure rate (useful-life period):

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R(t) = e−λt (exponential distribution). • MTBF (mean time between failures — repairable items) = total operating time / number of failures = 1/λ.

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MTTF (mean time to failure — non-repairable items) = ∫R(t)dt = 1/λ for constant λ. • At t = MTBF, R = e−1 = 36.8% — i.e., only about 37% of items survive to the MTBF. • MTTR (mean time to repair) — measures maintainability; repair rate μ = 1/MTTR; maintainability M(t) = 1 − e−μt. • Inherent availability A = MTBF/(MTBF + MTTR).

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MTBF ≈ MTTF + MTTR for repairable systems.

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Operational availability also includes logistic and administrative delays (MDT). • Weibull distribution (shape β, characteristic life η at which 63.2% have failed): β < 1 decreasing failure rate (infant), β = 1 constant (exponential), β > 1 increasing (wear-out).

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Bathtub Characteristic of Failure Region Failure rate Causes Remedies I.

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Early failure (infant mortality / burn-in) DECREASING Manufacturing defects, poor quality control, weak components, installation and commissioning errors Burn-in/running-in, debugging, quality control, acceptance testing, warranty II.

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Useful life (random/chance failures) CONSTANT (lowest) Random overloads, operator errors, chance events, environmental shocks Redundancy, derating, design margins, good operation; exponential law applies;

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MTBF quoted here III.

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Wear-out INCREASING Fatigue, wear, corrosion, ageing, creep, material degradation Preventive replacement/overhaul before entering this region, condition monitoring • Mechanical items often show a short infant region and a pronounced wear-out region; electronic components a long flat useful-life region; software does not wear out (failures come from design faults).

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Failure Modes, Causes and Effects • Failure mode: the manner in which an item fails — fracture, fatigue crack, wear, corrosion, excessive deformation/yielding, buckling, creep, seizure, leakage, loosening, short/open circuit, overheating. • Failure mechanism: the physical/chemical process leading to failure (fatigue crack growth, corrosion, diffusion).

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Failure cause: design deficiency, material defects, manufacturing/assembly errors, improper operation or overload, inadequate maintenance, environment. • Effects: local effect → next higher level effect → end (system) effect on safety, production, environment and cost. • FMEA (Failure Mode and Effects Analysis) — bottom-up, inductive: for each component list failure modes, effects, causes, current controls; score Severity (S), Occurrence (O), Detection (D) on 1–10.

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Risk Priority Number RPN = S × O × D (1–1000); act on highest RPN (and high severity).

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FMECA adds criticality analysis.

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Design FMEA and Process FMEA.

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System Reliability and Reliability Block Diagram (RBD) • RBD: shows how component reliabilities combine logically (not physically) to system reliability. • Series system (all must work):

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Rs = R1 × R2 × … × Rn — always LESS than the weakest component; λs = Σλi. • Parallel (active redundant) system (any one works):

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Rp = 1 − (1 − R1)(1 − R2)…(1 − Rn) — always GREATER than the best component. • Example: two components each R = 0.9 → series 0.81; parallel 1 − 0.1 × 0.1 = 0.99. • Other configurations: k-out-of-n (e.g., 2-out-of-3 voting), standby redundancy (with switching), series-parallel combinations. • Improving reliability: simplify design (fewer parts), use high-quality components, derating (operate below rated stress), redundancy, burn-in, environmental protection, preventive maintenance.

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Fault Tree Analysis (FTA) and Event Tree Analysis (ETA) Feature Fault Tree Analysis (FTA) Event Tree Analysis (ETA) Logic Top-down, deductive — 'how can this happen?' Forward, inductive — 'what happens next?' Starting point An undesired TOP EVENT (e.g., boiler explosion) An INITIATING EVENT (e.g., pipe rupture, fire) Structure Logic gates:

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AND (all inputs needed — P = ΠPi), OR (any input — P = 1 − Π(1 − Pi) ≈ ΣPi); basic events (circles), intermediate events (rectangles), undeveloped events (diamonds) Binary branches for success/failure of each safety function or barrier; probability of each outcome = product along the path Output Minimal cut sets (smallest combinations of basic events causing top event); single-point failures = first-order cut sets; top-event probability Range of outcome sequences (safe shutdown, minor release, major accident) and their frequencies Origin / use Bell Labs, 1962 (Minuteman missile); aerospace, nuclear, process safety Nuclear safety (WASH-1400), process plants, evaluating protective layers • Bow-tie analysis combines FTA (causes, left) and ETA (consequences, right) around a central hazardous event, with preventive and mitigating barriers shown.

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Hazard Analysis and Fishbone Diagram • Hazard: a condition or source with potential to cause harm.

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Hazard analysis techniques:

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PHA (preliminary hazard analysis — early design stage), HAZOP (hazard and operability study — team applies guide words NO/NONE, MORE, LESS, AS WELL AS, PART OF, REVERSE, OTHER THAN to process parameters such as flow, pressure, temperature at each node of the P&ID;), What-if analysis, checklists, JSA/JHA (job safety analysis — break a job into steps, identify hazards and controls), FMEA. • Fishbone (Ishikawa / cause-and-effect) diagram: the problem (effect) is written at the 'head'; main 'bones' are cause categories — 6M:

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Man (people), Machine, Method, Material, Measurement, Mother nature (environment); sub-causes are brainstormed on each bone to find root causes.

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Developed by Kaoru Ishikawa; one of the 7 basic quality tools. • Related root-cause tools:

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5 Whys, Pareto chart (80/20), scatter diagrams, check sheets.

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Risk assessment is the systematic process of identifying hazards, analysing and evaluating the risk they pose, and deciding on controls.

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This section covers the definition and measurement of risk, risk-analysis techniques, risk-reduction measures and resources, and industrial safety.

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Definition and Measurement of Risk • Hazard: anything with the potential to cause harm (a source — e.g., rotating shaft, high voltage, toxic gas).

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Risk: the combination of the likelihood (probability) that harm will occur and the severity (consequence) of that harm. • Risk = Probability (likelihood) × Consequence (severity). • Measurement: qualitative (high/medium/low); semi-quantitative — risk matrix (e.g., 5 × 5: likelihood 1–5 × severity 1–5 → score 1–25 with green/yellow/red zones); quantitative — QRA (frequency per year, individual risk, societal risk F-N curves, expected monetary loss). • ALARP (As Low As Reasonably Practicable): risk must be reduced until the cost of further reduction is grossly disproportionate to the benefit.

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Regions: unacceptable (intolerable), tolerable/ALARP, broadly acceptable (individual risk below ≈ 10−6 per year). • Residual risk = risk remaining after controls are applied.

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Risk Assessment Process • HSE five steps:

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(1) identify hazards → (2) decide who might be harmed and how → (3) evaluate the risks and decide on precautions → (4) record findings and implement → (5) review and update. • ISO 31000 risk-management process: establish context → risk identification → risk analysis → risk evaluation (together = risk assessment) → risk treatment → monitoring and review, with communication and consultation throughout.

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Risk Analysis Techniques Category Techniques Qualitative Checklists, What-if analysis, brainstorming, PHA, HAZID, HAZOP, JSA/JHA, Delphi, SWIFT Semi-quantitative Risk matrix, FMEA / RPN, LOPA (layers of protection analysis), risk indexing Quantitative FTA, ETA, bow-tie, QRA, Monte Carlo simulation, reliability analysis, Bayesian networks, sensitivity analysis, consequence modelling (fire, explosion, toxic dispersion) • Swiss cheese model (James Reason): each defence layer has 'holes' (weaknesses); an accident happens when holes in all layers line up — hence multiple independent barriers. • Heinrich's accident triangle: for every 1 major injury there are about 29 minor injuries and 300 no-injury incidents (near misses) — reporting near misses prevents major accidents.

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Heinrich also attributed about 88% of accidents to unsafe acts, 10% to unsafe conditions and 2% to unavoidable causes; domino theory of accident causation.

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Risk Reduction (Treatment) • Risk treatment options (4 Ts):

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Terminate (avoid the activity), Treat (reduce likelihood or consequence), Transfer (insurance, contracting out), Tolerate (accept/retain residual risk).

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Hierarchy of controls (most → least effective) Examples 1.

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Elimination Remove the hazard completely (do the task at ground level instead of at height) 2.

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Substitution Replace with something less hazardous (water-based paint for solvent paint, lower voltage tools) 3.

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Engineering controls Isolate people from hazard: machine guards, interlocks, ventilation, enclosures, relief valves, barriers Hierarchy of controls (most → least effective) Examples 4.

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Administrative controls Procedures, training, permits-to-work, signage, job rotation, supervision 5.

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PPE (last resort) Helmets, goggles, gloves, respirators, ear protection, harnesses Risk Reduction Resources • Human: trained and competent workforce, safety officers/engineers, safety committees, emergency response teams. • Technical/engineering: safety devices (relief valves, interlocks, alarms, gas detectors, fire suppression), emergency shutdown (ESD) and safety instrumented systems (SIS) with Safety Integrity Levels (SIL 1–4, IEC 61508/61511), redundancy, fail-safe design. • Information: codes and standards, Safety Data Sheets (SDS/MSDS) — 16 sections under GHS, incident databases, manufacturer manuals. • Organisational: safety management system (ISO 45001), safety policy, permit-to-work, audits and inspections, emergency response and evacuation plans, drills, management of change (MOC). • Financial: safety budget, insurance.

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Legal/regulatory: in Nepal, the Labour Act 2074 (2017) includes occupational safety and health provisions;

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ILO conventions and international standards guide practice.

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Industrial Safety and Risk Assessment • Accident causes: unsafe acts (human error, not using PPE, bypassing guards, horseplay) and unsafe conditions (defective equipment, missing guards, poor housekeeping, inadequate lighting, toxic atmosphere). • Safety performance indices:

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Frequency rate FR = (number of lost-time injuries × 106) / man-hours worked;

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Severity rate SR = (man-days lost × 106) / man-hours worked (some standards use × 1000); incident rate (OSHA) = injuries × 200,000 / hours worked. • High-risk activities requiring permit-to-work: hot work, confined-space entry (test atmosphere — O2 19.5–23.5%, flammable and toxic gases), work at height (fall protection above ≈ 2 m), electrical isolation, excavation, lifting operations. • Safety signs (ISO 7010): red — prohibition/fire equipment; blue — mandatory action; yellow — warning; green — safe condition/emergency exit. • Accident investigation → root cause → corrective and preventive action; near-miss reporting; safety audits; ergonomics and occupational health surveillance.

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Non-destructive testing examines materials and components for defects or properties without impairing their future usefulness.

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It is used in manufacturing quality control, in-service inspection and maintenance.

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This section covers ultrasonic testing, dye penetrant testing, magnetic methods (magnetic particle and magnetic resonance), radiography (X-ray and gamma) and eddy current testing.

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NDT vs Destructive Testing • Destructive tests (tensile, impact, hardness on samples) give properties but destroy the specimen.

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NDT can test 100% of production and parts in service, locate defects and measure thickness, saving cost and preventing failures. • Visual testing (VT) is always the first NDT step (with magnifiers, borescopes, gauges). • Personnel are qualified to Level I, II or III (ISO 9712, ASNT SNT-TC-1A).

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Ultrasonic Testing (UT) • Principle: high-frequency sound waves (typically 0.5–25 MHz, commonly 1–10 MHz) generated by a piezoelectric transducer (quartz, PZT, barium titanate) travel through the material and are reflected by discontinuities or the back wall. • A couplant (gel, oil, water) is needed to remove the air gap between probe and surface. • Methods: pulse-echo (single probe — time of flight gives depth: d = v·t/2), through-transmission (separate transmitter and receiver), resonance.

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Normal (straight) probes use longitudinal waves (laminations, thickness); angle probes use shear waves (welds).

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A-scan (amplitude vs time), B-scan (cross-section), C-scan (plan view).

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Advanced: phased array (PAUT), TOFD. • Sound velocity in steel: longitudinal ≈ 5900 m/s, shear ≈ 3230 m/s. • Detects internal defects — cracks, laminations, inclusions, porosity, lack of fusion — and measures wall thickness (corrosion monitoring of pipes, tanks, boilers). • Advantages: deep penetration (up to metres in steel), accurate depth and size, access from ONE side, portable, no radiation hazard, instant results.

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Limitations: couplant and skilled operator needed; difficult on rough, thin, small or complex parts and coarse-grained materials (austenitic welds, cast iron); near-surface dead zone; sensitive to defect orientation; limited permanent record with manual scanning.

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Dye Penetrant Testing (DPT / Liquid Penetrant Testing) • Principle: a low-viscosity coloured or fluorescent liquid enters surface-breaking defects by capillary action and is drawn out by a developer to show an enlarged indication. • Procedure:

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(1) pre-cleaning → (2) apply penetrant and allow dwell time (≈ 5–30 min) → (3) remove excess penetrant → (4) apply developer (white chalky powder that blots penetrant out) → (5) inspect (red dye in white light, or fluorescent dye under UV light) → (6) post-clean. • Works on almost ANY non-porous material — metals (ferrous and non-ferrous), ceramics, glass, plastics. • Advantages: simple, cheap, portable, large areas and complex shapes, high sensitivity to fine surface cracks.

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Limitations: only defects OPEN TO THE SURFACE; not for porous materials; surface cleanliness critical; no depth information; chemical handling.

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Magnetic Particle Testing (MPT) and Magnetic Resonance • Magnetic particle testing: the part is magnetised; a surface or near-surface (up to a few mm) defect causes magnetic flux leakage which attracts fine iron particles (dry powder or wet suspension, visible or fluorescent) forming an indication. • Applicable ONLY to ferromagnetic materials (carbon and low-alloy steels, cast iron, nickel, cobalt) — NOT austenitic stainless steel, aluminium, copper, titanium. • Defects are best detected when perpendicular to the magnetic flux lines, so parts are magnetised in two directions: circular magnetisation (current through part/central conductor) reveals LONGITUDINAL defects; longitudinal magnetisation (coil or yoke) reveals TRANSVERSE defects.

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Parts are demagnetised afterwards. • Advantages: fast, cheap, detects near-surface defects and cracks under thin coatings.

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Limitations: ferromagnetic only, orientation dependent, demagnetisation needed, limited depth. • Magnetic resonance testing (as listed in the syllabus): nuclear magnetic resonance (NMR) techniques place the specimen in a strong magnetic field and detect radio-frequency signals from nuclei (mainly hydrogen) — used to evaluate moisture content, porosity, curing and ageing of polymers, rubbers, composites and concrete, and MRI imaging.

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It is not a common method for detecting cracks in steel; for ferromagnetic metals the standard magnetic NDT methods are MPT and magnetic flux leakage (MFL — pipeline and tank-floor inspection).

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Radiographic Testing (RT) — X-ray and Gamma Ray • Principle: penetrating radiation passes through the part; areas of lower density or thickness (voids) absorb less radiation and appear DARKER on the film/detector; denser inclusions (e.g., tungsten) appear lighter. • X-rays: produced by an X-ray tube (electrons striking a tungsten target); energy adjustable; can be switched off; needs electric power.

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Gamma rays: from radioactive isotopes — Iridium-192 (common for steel ≈ 10–60 mm), Cobalt-60 (thick sections, high energy), Selenium-75; portable, no power needed, but always emitting (needs shielded container). • Recording: film, computed radiography (CR), digital radiography (DR); computed tomography (CT) for 3-D.

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Image quality checked with IQI (penetrameters) — wire or hole type. • Detects volumetric internal defects: porosity, slag inclusions, shrinkage, incomplete penetration in welds and castings.

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Planar cracks are detected only if roughly PARALLEL to the beam; laminations are hard to detect. • Advantages: permanent record, most materials, shows defect shape and size in 2-D.

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Limitations: ionising radiation hazard, needs access to BOTH sides, expensive, slow, depth of defect not directly shown. • Radiation safety: time, distance (inverse square law), shielding (lead, concrete); dosimeters (TLD, film badges); controlled areas; licensed operators.

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Eddy Current Testing (ECT) • Principle: an AC coil induces circulating eddy currents in an electrically conductive material (electromagnetic induction); cracks, conductivity or thickness changes alter the eddy-current flow and hence the coil impedance, which is measured. • Skin effect: eddy currents concentrate near the surface; standard depth of penetration δ = 1/√(πfμσ) — decreases with higher frequency, conductivity and permeability. • Uses: surface and near-surface cracks (aircraft skins, rivet holes, wheels), tube inspection of heat exchangers and condensers (bobbin probes), conductivity measurement and alloy/heat-treatment sorting, coating thickness (non-conductive paint/anodising on conductive base). • Advantages: fast, no couplant, non-contact possible, easily automated, sensitive to small surface cracks, immediate results.

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CONDUCTIVE materials only, shallow penetration, sensitive to lift-off and geometry, ferromagnetic materials difficult, reference standards required, skilled interpretation.

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Comparison of NDT Methods Method Defects detected Materials Key advantage Key limitation Visual Surface All Simple, cheap Surface only, subjective Dye penetrant Surface-breaking Non-porous (any) Cheap, portable Surface-open defects only Magnetic particle Surface & near-surface Ferromagnetic only Fast, sensitive Ferromagnetic only Ultrasonic Internal; thickness Most (not coarse-grain) Deep, one-side access Couplant, skill Radiography Internal volumetric Most Permanent record Radiation, two-side access Eddy current Surface/near-surface; conductivity, coating Conductive only Fast, no couplant Shallow depth • Other methods: acoustic emission (detects active crack growth under load — pressure-vessel proof tests), infrared thermography, leak testing (bubble, pressure decay, helium mass spectrometer), magnetic flux leakage, holography/shearography (composites).