Pumps & Compressors Troubleshooting

Pumps and compressors are in almost every plant, and the interview questions on them follow the faults that fill maintenance logs: pumps that lose prime, run noisy, leak at the seal or overheat, and compressors that run loaded all day, cannot build pressure or trip on temperature. The 70 questions below cover centrifugal and positive displacement pumps, reciprocating and screw compressors, seals, valves and air treatment. On this page you will find the usual interview rounds, a revision list, three worked examples (an NPSH check for cavitation, the affinity laws for a pump slowed down, and how long a compressor takes to fill a receiver) and the errors that are easy to avoid.

Original EngineerX flowchart: Pump or compressor: low flow or noise. Follow the numbered checks in order; a “No” sends you back to widen the search.

How pump and compressor troubleshooting interviews usually run

Basics and selection — pump and compressor types, curves, and where each one is used.
Fault finding — a pump will not deliver, a seal leaks, or a compressor will not reach pressure, and you give the sequence of checks.
Calculations — NPSH, affinity laws, power and receiver fill time.
Safety and care — isolation, stored pressure, hot surfaces and oil and filter changes.

Concepts and readings to revise first

Pump curve: head against flow, the duty point and what shifts it.
NPSH: available against required, and the margin that stops cavitation.
Seals: mechanical seal faces, flush plans and leakage from dry running.
Affinity laws: how flow, head and power change with speed.
Positive displacement pumps: relief valves and why they must never run against a closed valve.
Compressor types: reciprocating, rotary screw and their loading and unloading control.
Air system: receiver, dryer, filters, drains and leaks.
Temperature and oil: discharge temperature, oil carry-over and cooler fouling.

Showing the first 70 questions free. The complete question bank is available as a PDF below.

CENTRIFUGAL PUMP CAVITATION & NPSH PROBLEMS

Q1A centrifugal pump makes a loud gravel-cracking noise, vibrates, and head collapses. What is occurring?

AnswerClassical cavitation. Net Positive Suction Head Available (NPSHA) fell below NPSH Required (NPSHR). Fluid pressure dropped below vapor pressure at the impeller eye, forming vapor bubbles that collapse violently against the metal, eroding the impeller.

Q2How do you distinguish between classic suction cavitation and discharge recirculation cavitation?

AnswerSuction cavitation occurs under high flow and low suction head, pitting the visible front suction side of impeller vanes. Discharge recirculation occurs under low flow (throttled discharge), eroding the pressure side tips of vanes and cutwater.

Q3Pumping hot condensate at 95 deg C causes continuous pump rattling, while 40 deg C water pumps silently. Why?

AnswerVapor pressure increases exponentially with temperature. At 95 deg C, water vapor pressure is 84 kPa, leaving virtually zero margin above atmospheric pressure. Lowering temperature increases liquid subcooling and NPSHA.

Q4What piping design error on the pump suction nozzle triggers localized flow separation and cavitation?

AnswerInstalling a concentric reducer with flat side up, or placing a 90-degree elbow directly on the horizontal suction nozzle. Air pockets accumulate in concentric reducers, and elbows induce asymmetric velocity profiles into the impeller eye.

Q5How does throttling the pump discharge valve temporarily suppress active suction cavitation?

AnswerThrottling the discharge valve increases system backpressure, shifting the pump operating point to the left along its H-Q curve to lower flow, which significantly reduces the pump NPSH Required (NPSHR).

Q6An intake suction strainer differential pressure rises to 0.6 bar. What consequence immediately threatens the pump?

AnswerThe 0.6 bar drop directly subtracts from NPSHA (NPSHA = P_atm + P_static - h_f - P_v). As friction loss (h_f) spikes, suction pressure plunges below vapor pressure, inducing catastrophic cavitation.

Q7What metallurgical upgrade protects pump impellers operating in unavoidable borderline cavitation environments?

AnswerUpgrade standard cast iron or bronze impellers to CA6NM martensitic stainless steel, duplex stainless steel (2205), or nickel-aluminum bronze (NAB), which offer superior cavitation fatigue and impact resistance.

Q8How does an inducer installed ahead of a centrifugal pump impeller lower the overall NPSHR of the machine?

AnswerAn inducer is an axial-flow booster screw mounted on the same shaft directly ahead of the main impeller eye. It raises static fluid pressure by 0.5–2.0 bar before fluid enters the centrifugal vanes, eliminating bubble inception.

Q9Inspection of an eroded stainless steel impeller reveals spongy, honeycombed craters on the vane inlet tips. What is this?

AnswerClassical micro-jet cavitation erosion. As vapor bubbles enter the high-pressure zone, asymmetrical bubble collapse fires microscopic liquid micro-jets at velocities exceeding 1000 m/s, fatiguing and pitting the crystal lattice.

Q10Why must a long horizontal suction pipe slope continuously upwards toward the pump suction flange?

AnswerTo prevent the formation of high-point trapped air pockets. Any trapped air pocket gradually restricts pipe cross-sectional area, creating a severe localized pressure drop that triggers continuous vortex cavitation.

MECHANICAL SEALS, GLAND PACKING & LEAKAGE FAILURES

Q11A cartridge mechanical seal puffs white vapor and fails within 10 minutes of initial pump startup. What was missed?

AnswerDry running. The pump was not vented / primed before energizing the motor. Operating mechanical seal faces (silicon carbide vs carbon) without liquid lubrication causes instantaneous thermal shock (>300 deg C), blistering the carbon face.

Q12What is the standard acceptable leakage rate for traditional braided gland packing versus a mechanical seal?

AnswerBraided gland packing MUST drip continuously at 40 to 60 drops per minute to cool and lubricate the packing-to-shaft interface. A healthy single mechanical seal exhibits zero visible leakage (<1–3 drops/hour in vapor form).

Q13Severe heat checking (fine concentric radial cracks) is visible on a silicon carbide mechanical seal face. Cause?

AnswerThermal shock and intermittent lubrication starvation. Flashing of fluid between the seal faces creates rapid cyclical heating and cooling, inducing extreme thermal gradient stress that fractures brittle ceramic/SiC materials.

Q14How does API Plan 11 seal flush piping protect a single mechanical seal from overheating?

AnswerPlan 11 routes a small pressurized stream of clean product fluid from pump discharge through an orifice directly onto the seal faces inside the seal chamber, carrying away frictional heat and venting vapor pockets continuously.

Q15Why does dynamic shaft deflection exceeding 0.05 mm destroy mechanical seal faces prematurely?

AnswerMechanical seal faces maintain an operational fluid gap of only 1 micron. Excessive shaft radial deflection or whip tilts the dynamic seal ring, causing edge chipping, erratic gap opening, and massive product leakage.

Q16What causes an elastomeric dynamic secondary O-ring on a mechanical seal to develop 'hang-up'?

AnswerChemical swelling from aggressive solvents, thermal hardening/embrittlement, or scale deposition on the shaft sleeve. The hardened O-ring locks against the shaft, preventing springs from keeping the faces closed.

Q17A dual pressurized mechanical seal (API Plan 53A) loses barrier fluid pressure mysteriously. How to isolate?

AnswerBarrier pressure higher than seal chamber pressure forces fluid across faces. Check for barrier fluid leaking into process through inner seal faces, leaking to atmosphere across outer faces, or an external piping fitting leak.

Q18What assembly mistake causes traditional PTFE gland packing to smoke and groove the stainless steel shaft sleeve?

AnswerOvertightening the gland follower nuts during startup to stop all dripping, or failing to stagger the packing ring scarf joints by 90 to 120 degrees. Over-compression cuts off lubrication, burning packing and scoring the sleeve.

Q19How does phase change / flashing of hydrocarbons across seal faces lead to dry-face explosive failure?

AnswerLight hydrocarbons (LPG, propane) enter seal faces as liquid; pressure drop across the face width causes liquid to flash into vapor. The vapor expands violently, destroying fluid film support and causing face clatter and chipping.

Q20Why are stationary metal bellows mechanical seals chosen over spring seals for extreme high-temperature refinery pumps (>300 deg C)?

AnswerElastomeric O-rings decompose above 200–250 deg C. Metal bellows utilize welded Inconel or Hastelloy flexible diaphragms, completely eliminating dynamic elastomeric O-rings while providing continuous spring closing force.

CENTRIFUGAL PUMP HEAD, FLOW & SYSTEM CURVE MISMATCH

Q21A centrifugal pump operates at full motor RPM, but delivers only 60% of its rated flow and discharge pressure. First checks?

AnswerMotor rotating in reverse (3-phase wiring swapped; a reverse impeller still pumps but at ~50% head and efficiency), clogged suction strainer, partially closed suction valve, or worn impeller wear ring clearances.

Q22Two identical centrifugal pumps operate in parallel into a common discharge header, but total combined flow barely increases by 10%. Why?

AnswerThe system curve is steep (high friction head). Parallel pumps increase flow significantly only in flat, low-friction systems. In steep friction systems, adding a parallel pump simply increases discharge head with negligible flow gain.

Q23What physical symptom occurs when a centrifugal pump operates at the extreme right-hand end of its performance curve (runout flow)?

AnswerMotor thermal overload trip and cavitation. At runout (near zero discharge head), flow is maximum, driving motor brake horsepower (BHP = Q · H · SG / η) to peak overload while pump NPSHR spikes sharply.

Q24Internal inspection of a multi-stage pump reveals radial wear ring clearance has widened from 0.3 mm to 1.4 mm. What consequence ensues?

AnswerSevere internal recirculation leakage from high-pressure stage back to suction eye (Q_leak ∝ √(Δ P)). Discharge head drops by 15–25%, pump efficiency collapses, and power consumption spikes for identical output.

Q25A pump discharges into an overhead reservoir; when the motor shuts down, the pump spins backward at destructive supersonic speed. Cause?

AnswerDischarge non-return check valve (NRV) failed open, stuck clapper, or broken spring. The static head of water in the vertical delivery pipe rushes backward through the pump, driving the impeller as an un-governed water turbine.

Q26How do the Pump Affinity Laws predict flow, head, and power changes when an impeller diameter is trimmed from 250 mm to 225 mm?

AnswerQ₂/Q₁ = D₂/D₁ (flow drops by 10%). H₂/H₁ = (D₂/D₁)² (head drops by 19%). P₂/P₁ = (D₂/D₁)³ (power consumption drops by 27%). Trimming permanently cuts excess head without throttling losses.

Q27A slurry pump discharge pipe wears completely through at bottom elbows every 4 months. What hydraulic flow regime exists?

AnswerOperating below critical settling velocity with coarse solids, causing particles to drag along the pipe bottom (saltation / sliding bed wear). Increase line velocity or install rubber/polyurethane-lined abrasive piping.

Q28What causes an axial-flow propeller pump motor to overload when the discharge valve is throttled towards shutoff?

AnswerInverse power curve characteristic. Unlike radial centrifugal pumps where power drops at shutoff, axial-flow propeller pumps have power curves that spike steeply to maximum at shutoff head. Axial pumps must NEVER be started against a closed valve.

Q29A pump creates expected shutoff head on gauge, but flow remains near zero when discharge valve is opened fully. What is wrong?

AnswerSystem static head is higher than the pump shutoff head (the pump cannot generate enough pressure to overcome pipe elevation), or the delivery line has an unvented syphon air lock at the high point.

Q30Why does pumping viscous heavy oil (500 cSt) through a standard centrifugal pump cause massive head and efficiency collapse?

AnswerViscous fluid shear drag. High viscosity increases internal disc friction and boundary shear losses, flattening the H-Q curve, collapsing head by 40%+, and tripling required brake horsepower. Positive displacement pumps are required.

POSITIVE DISPLACEMENT PUMPS (GEAR, DIAPHRAGM, PROGRESSIVE CAVITY)

Q31An external gear pump emits a loud metallic screaming noise and its drive shaft seal blows out when started against a closed valve. Why?

AnswerPositive displacement physics. Positive displacement pumps deliver fixed volume per revolution regardless of pressure. Closing the discharge creates infinite hydraulic pressure until piping bursts, shaft shears, or motor stalls. An external relief valve is MANDATORY.

Q32An Air-Operated Double Diaphragm (AODD) pump stalls in mid-stroke and continuously hisses compressed air out of the exhaust muffler. What failed?

AnswerIced or gummed air distribution spool valve, worn pilot spool O-rings, or a ruptured elastomeric diaphragm allowing pressurized compressed air to blow directly into the liquid fluid chamber.

Q33A progressive cavity pump (helical rotor / stator pump) runs dry for 45 seconds; the motor stalls and cannot be restarted. What happened?

AnswerSevere stator burnout. The chrome-plated steel rotor turns inside an interference-fit elastomeric rubber stator. Operating dry generates instantaneous frictional temperatures (>180 deg C), melting and vulcanizing the rubber stator onto the rotor.

Q34What causes erratic, pulsating flow and loud check ball rattling in an AODD air diaphragm pump?

AnswerWorn, ovalized check balls or scarred check valve ball seats. A damaged ball fails to seal on the return stroke, allowing pumped fluid to backflow into the suction chamber on every alternating diaphragm stroke.

Q35An internal gear pump (crescent pump) loses suction prime and delivery volume drops by 50%. Where is the internal wear?

AnswerWear across the crescent seal divider or widened axial clearance between the rotor/idler gear faces and the front casing cover plate. Liquid slips continuously across the crescent back to the inlet.

Q36Why must a pulsation dampener installed on a reciprocating metering pump discharge line be pre-charged with dry nitrogen, NOT compressed air?

AnswerSafety against diesel explosion. Compressing hydrocarbon vapors or flammable chemical vapors against high-pressure compressed air (containing 21% Oxygen) inside a flexible bladder risks auto-ignition under pressure shockwaves.

Q37A peristaltic hose pump ruptures its reinforced rubber squeeze hose every 150 operating hours. What adjustment was wrong?

AnswerExcessive rotor shoe / roller shimming (over-occlusion). Squeezing the reinforced hose tighter than 100% pinch thickness crushes the internal fabric reinforcement braids under dynamic rolling friction, causing rapid fatigue blowout.

Q38What causes high-pressure hydraulic oil foaming and shudder in an axial piston pump swashplate assembly?

AnswerAeration / air ingress along the pump drive shaft oil seal, or suction line vacuum exceeding 0.2 bar below atmospheric pressure pulling air out of solution (micro-cavitation).

Q39An AODD pump's exhaust muffler freezes solid into a block of white ice, stalling the pump in humid summer weather. Cause?

AnswerJoule-Thomson adiabatic cooling. Compressed air expanding rapidly from 7 bar down to atmospheric pressure drops in temperature by up to 40 deg C. Ambient humidity condenses and freezes in the exhaust port.

Q40How do you calibrate the stroke length mechanism of a mechanical diaphragm chemical dosing pump without stopping flow?

AnswerRotate the micrometric adjustment knob while the pump is running under loaded pressure. Adjusting while stopped binds the internal eccentric cam linkage; dynamic adjustment changes the displacement radius smoothly.

ROTARY SCREW COMPRESSORS & AIR END DIAGNOSTICS

Q41A rotary screw compressor trips on 'High Air End Discharge Temperature (>110 deg C)'. What is the troubleshooting checklist?

AnswerLow synthetic oil level, clogged oil filter element, thermostatic mixing valve stuck in bypass, fouled air/oil heat exchanger core, or operating in an unventilated compressor room with high ambient air (>40 deg C).

Q42Thick milky oil emulsion pools inside the oil separator tank of a screw compressor running light loads. What caused moisture condensation?

AnswerThe compressor is operating below the water dew point. If the thermostatic valve is defective or load cycle is too short, oil temperature stays below 75–80 deg C; atmospheric humidity condenses directly into the oil sump.

Q43Compressed air downstream of a rotary screw compressor contains massive oil carryover, consuming 5 liters of oil per day. Root cause?

AnswerRuptured spin-on air/oil separator filter element, clogged internal oil scavenge line orifice / check valve, or oil sump overfilled past the top sight glass level.

Q44What causes an air compressor intake unloader valve to fail to close, preventing the compressor from building pressure (stuck in unload)?

AnswerBlown solenoid valve coil, severed 24V signal, torn unloader control piston diaphragm, or unloader spring broken, holding the air intake butterfly valve mechanically shut.

Q45A rotary screw compressor emits a continuous metallic rattling / chattering growl from the air end drive bearings. Immediate action?

AnswerSHUT DOWN IMMEDIATELY. The paired angular contact thrust bearings on the male/female screw rotors are failing. Continued operation allows rotors to shift axially, contacting the cast iron end-casing and destroying the entire air end.

Q46Why does a screw compressor minimum pressure check valve (MPV) chatter and leak air backward into the separator tank upon shutdown?

AnswerThe internal MPV check valve spring is broken or the rubber poppet seal is pitted. The MPV must maintain 4.0–4.5 bar internal pressure during operation for oil circulation and seal tightly on shutdown to prevent air backflow.

Q47What failure mode occurs if a screw compressor blowdown unloader valve fails to open when the machine unloads?

AnswerThe oil separator tank remains trapped at full 7–8 bar pressure during unload. The motor runs under high unloaded power consumption (consuming 70% power instead of 25%), overheating the oil and wasting thousands in electricity.

Q48How do you measure male and female rotor axial clearances during a precision rotary screw air end rebuild?

AnswerUse depth micrometers and dial indicators to measure axial clearance between the rotor discharge end faces and the discharge housing wall while bearing locknuts are fully torqued. Correct clearance (typically 0.04–0.08 mm) is set by precision ground shims.

Q49Why does an air intake filter with a 0.05 bar differential pressure increase power consumption by 2–3% on a 100 kW compressor?

AnswerA restricted intake filter drops air intake density. The compressor must work across a higher overall compression ratio (P₂ / P₁) to reach terminal discharge pressure, driving up specific power consumption (kW / m³/min).

Q50A synthetic polyolester (POE) compressor oil turns pitch black with an acidic acrid smell after only 1,000 hours (rated for 8,000 hrs). Cause?

AnswerIngestion of acidic atmospheric chemical fumes (chlorine, ammonia, or sulfur from adjacent plating or chemical plants), thermal hotspot in the air end, or mixing incompatible mineral oil during topping up.

RECIPROCATING COMPRESSORS, VALVES & PISTON RINGS

Q51A reciprocating air compressor cylinder head runs burning hot and the intercooler pressure relief valve pops open repeatedly. What failed?

AnswerFailed second-stage suction or discharge reed/plate valve. A broken or leaking second-stage valve allows compressed air to churn back and forth, spiking intercooler pressure until the safety valve lifts.

Q52What causes premature fracture of stainless steel finger/plate valves in reciprocating gas compressors?

AnswerLiquid slugging or heavy condensation carryover. Gas compressor valves are designed strictly for vapor compression; slamming incompressible liquid droplets against thin spring plates shatters valve fingers instantly.

Q53A heavy horizontal reciprocating compressor develops a sharp, metallic knocking sound inside the crosshead guide. Root cause?

AnswerExcessive crosshead shoe-to-guide running clearance, loose crosshead pin bushing (wrist pin wear), or crosshead nut backed off on the piston rod threads.

Q54What is the consequence of oil carryover in a non-lubricated (oil-free) reciprocating oxygen compressor?

AnswerCATASTROPHIC EXPLOSION HAZARD. Pure high-pressure oxygen reacts explosively with lubricating hydrocarbons under compression heat. Oil-free oxygen compressors require strict PTFE/carbon rider rings and zero oil ingress.

Q55Why do PTFE piston rings on dry reciprocating compressors require an initial running-in transfer film to prevent rapid wear?

AnswerPTFE rings do not slide directly against raw steel. During running-in, microscopic PTFE particles transfer onto the cast iron cylinder bore, creating a mirror-smooth self-lubricating polymer transfer film that cuts friction by 80%.

Q56What causes a piston rod packing gland to leak large volumes of toxic or flammable gas into the distance piece?

AnswerWorn segmental bronze/carbon packing rings, broken garter springs, scored piston rod surface (>0.2 micron Ra), or inadequate nitrogen purge pressure on the packing gland barrier.

Q57A single-stage compressor cylinder produces only 60% of normal CFM, and the air intake filter hisses loudly on every downstroke. Diagnosis?

AnswerIntake suction valve leaking / broken. During the compression upstroke, air escapes backward through the failed suction valve into the air filter, blowing air backward out of the intake.

Q58How do you measure piston end-clearance (bump clearance / head clearance) during reciprocating compressor assembly?

AnswerPlace pieces of lead solder wire on top of the piston crown. Bolt the cylinder head down with gasket fully torqued. Bar the compressor over top dead center (TDC) by hand. Remove head and measure squished solder thickness with a micrometer.

Q59What causes carbon buildup and valve fires inside reciprocating air compressor discharge manifolds?

AnswerUsing low-flashpoint mineral oils that oxidize at high discharge temperatures (>160 deg C). Oil vapors crack into hard carbon deposits on discharge valves; glowing carbon embers ignite oil mists, triggering an internal manifold explosion.

Q60How does a pulsation bottle (dampener volume bottle) protect reciprocating compressor piping from fatigue cracks?

AnswerReciprocating pistons generate high-amplitude acoustic pressure pulsations with every stroke. Sized volume bottles attenuate pressure waves through acoustic damping, preventing standing waves that vibrate and crack pipe welds.

CENTRIFUGAL COMPRESSORS & SURGE/CHOKE DYNAMICS

Q61A centrifugal gas compressor emits a violent cyclical bang like a cannon shot, and flow reverses backward through the impeller. What is this?

AnswerCompressor surge. The aerodynamic limit where discharge pressure exceeds the pressure-generating capability of the impeller vanes. Flow separates, collapses, and violently reverses backward into the suction in milliseconds.

Q62What is the fundamental operational difference between 'Surge' and 'Stonewall / Choke' in a centrifugal compressor?

AnswerSurge occurs at low flow / high pressure ratio when boundary layers stall and flow reverses. Choke occurs at maximum flow when gas velocity reaches the speed of sound (Mach 1.0) at the impeller throat, physically limiting maximum flow.

Q63How does an automated anti-surge control valve protect a multi-stage centrifugal compressor from destroying its thrust bearings?

AnswerHigh-speed anti-surge controllers monitor operating point proximity to the Surge Line. Before surge is reached, the fast-acting pneumatic bypass valve opens in <1.0 second, recycling discharge gas back to suction to keep flow above the surge threshold.

Q64What diagnostic symptom confirms that a Dry Gas Seal (DGS) on a centrifugal process gas compressor has suffered face contact?

AnswerHigh primary seal vent leakage flow, differential pressure spike across the vent, and high temperature on the primary vent gas. Nitrogen barrier pressure must strictly maintain >3.5 bar above process gas to keep DGS faces flying.

Q65A centrifugal compressor exhibits high vibration (>30 microns) strictly at Sub-Synchronous frequencies (0.42X to 0.48X running speed). What instability is present?

AnswerOil whirl / oil whip instability in the hydrodynamic journal bearings, or aerodynamic cross-coupling rotordynamic instability caused by gas swirl across balance piston labyrinth seals.

Q66Why do balance piston labyrinth seals wear, causing massive thrust bearing temperature spikes on a multi-stage compressor?

AnswerThe balance piston opposes the cumulative aerodynamic thrust of all impellers. If dirt or rubbing widens labyrinth clearances, balance pressure drops; net axial thrust spikes, overloading and wiping the Tilting-Pad thrust bearing.

Q67How do adjustable Inlet Guide Vanes (IGVs) regulate centrifugal compressor throughput efficiently compared to suction throttling?

AnswerIGVs pre-swirl incoming gas in the direction of impeller rotation, altering the aerodynamic velocity triangle at the blade inlet. This shifts the pressure-flow curve cleanly with minimal thermodynamic throttling losses.

Q68What failure occurs when the interstage gas cooler on a wet-gas compressor allows condensed hydrocarbon liquids into the next impeller?

AnswerSevere liquid droplet erosion on impeller vane leading edges, heavy aerodynamic imbalance vibration, and catastrophic thrust bearing overload caused by liquid mass density spikes.

Q69How does an active magnetic bearing (AMB) system protect high-speed hermetic gas compressors from lubrication contamination?

AnswerAMBs suspend the compressor rotor purely in electromagnetic fields with zero physical contact and ZERO oil. This completely eliminates oil systems, mechanical seals, and process gas contamination risks.

Q70What is the allowable vibration shutdown limit on an API 617 centrifugal compressor monitored by non-contact eddy-current proximity probes?

AnswerAccording to API 617, peak-to-peak vibration limit is: A = √(12,000 / N) mils (typically 20–35 microns peak-to-peak). Trips are set strictly below 50 microns to prevent internal labyrinth rub and seal contact.

Three worked fault examples

Example 1: “A pump draws 40 °C water from a tank 1.5 m below the pump centreline. Suction friction loss is 0.75 m and the pump needs 4 m NPSH. Will it cavitate?”

Atmospheric pressure = 101.3 kPa. Vapour pressure of water at 40 °C is about 7.4 kPa. Density is about 992 kg/m³.
Pressure head = (101.3 − 7.4) × 1000 ÷ (992 × 9.81) = about 9.65 m.
Subtract the 1.5 m suction lift and the 0.75 m friction: NPSHa = 9.65 − 1.5 − 0.75 = about 7.4 m.
NPSHa 7.4 m against NPSHr 4 m gives a margin of 3.4 m, so it will not cavitate as designed.
Now let the strainer choke so that the friction loss rises to 4 m. NPSHa becomes 9.65 − 1.5 − 4 = 4.15 m, and with a normal safety margin the pump will cavitate.

A noisy pump, falling flow and a rattling sound like gravel are the signs. Clean the strainer, shorten or enlarge the suction line, or lower the liquid temperature before touching the impeller.

Example 2: “A pump delivers 100 m³/h at 36 m head and takes 30 kW. A drive slows it to 80% speed. What are the new values?”

Flow is proportional to speed: 100 × 0.8 = 80 m³/h.
Head goes with the square of speed: 36 × 0.8² = 36 × 0.64 = 23.0 m.
Power goes with the cube: 30 × 0.8³ = 30 × 0.512 = 15.4 kW.

A 20% cut in speed gives about a 49% saving in power, which is why speed control beats throttling for varying flow. The laws apply when the system has little static head. If most of the head is static lift, the pump may deliver nothing at 80% speed, so check the system curve before promising the saving.

Example 3: “A 2 m³ air receiver is empty at atmospheric pressure. A compressor with 1.0 m³/min free air delivery fills it to 7 bar gauge. How long does it take, and what if it takes much longer?”

Use t = V × (P2 − P1) ÷ (FAD × P_atm), with pressures in bar. Here P2 − P1 = 7 bar gauge rise and P_atm = 1.013 bar.
t = 2 × 7 ÷ (1.0 × 1.013) = about 13.8 minutes.
If the real time is 20 minutes, the delivery is only about 0.69 m³/min, roughly 31% below the rating.
Check the inlet filter, the inlet valve and unloader operation, belt slip, valve or ring wear on a reciprocating machine, and leaks in the line.
Do a leak-down test: shut the receiver outlet, stop the compressor and watch how fast the pressure falls.

The fill-time formula gives a quick benchmark. Recording it when the machine is healthy gives you a reference, and a longer time later signals lost capacity before the pressure complaint arrives.

Common mistakes that cost candidates the job

Replacing the impeller when the real problem is a blocked suction strainer or an air leak on the suction line.
Running a pump dry, even for a short time, which damages the mechanical seal faces.
Throttling the suction valve to control flow instead of the discharge valve or a drive.
Running a positive displacement pump against a closed valve or a stuck relief valve.
Ignoring condensate drains and air leaks, which waste more compressor capacity than most worn parts do.

Frequently asked questions

How do I tell cavitation from air in the suction line?

Both make noise and lose flow. Cavitation sounds like gravel and follows suction conditions and temperature, while air ingress shows froth in the liquid and leaks at joints and the shaft seal.

Are formulas given in the interview?

Often not. Know the NPSH relation, the affinity laws and the receiver fill time formula, with units.

Is compressor oil change interval fixed?

It is set by the maker by running hours, and is shortened by heat, dust and poor air quality.

Keep preparing

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