1. Probleembeschrijving en reikwijdte

Rotary screw air compressors depend heavily on oil injection for lubrication, sealing, and thermal management. When the discharge temperature exceeds operational thresholds, the unit typically triggers a shutdown to protect against severe mechanical degradation. This diagnostic guide addresses high discharge temperature events (trips occurring above 105 degrees Celsius / 221 degrees Fahrenheit, with warning alarms triggering at 98 degrees Celsius / 208 degrees Fahrenheit) across oil-injected rotary screw compressors operating in automotive, aerospace, food processing, chemical, and general manufacturing facilities. Severity is classified as Critical; continued operation above 110 degrees Celsius / 230 degrees Fahrenheit leads rapidly to varnish formation, oil breakdown, thermal distortion of airend rotors, and catastrophic bearing seizure.

2. Veiligheidsmaatregelen

DANGER: HAZARDOUS ENERGY AND STORED PRESSURE

Before initiating any diagnostic or corrective procedure on an industrial screw compressor, execute strict Lockout/Tagout (LOTO) protocols in compliance with OSHA 29 CFR 1910.147 (US) or BS EN 1037 / PUWER (UK). Verify zero mechanical, electrical, and pneumatic energy state.

• Pneumatic Stored Energy: Isolate the compressor from the plant air header via a lockable ball valve. Verify zero pressure on the air-oil receiver tank pressure gauge before opening any lines. Residual pressure causes sudden fluid expulsion.
• Thermal Hazards: Discharge piping, oil coolers, airends, and separator vessels operate at extreme temperatures (80 degrees Celsius to 115 degrees Celsius / 176 degrees Fahrenheit to 239 degrees Fahrenheit). Wear heavy insulating leather gloves and safety glasses with side shields (ANSI Z87.1 / EN 166).
• Electrical Hazards: Voltage up to 690V AC may be present in the starter cabinet. Use a calibrated multimeter to verify zero voltage before inspecting control wiring or temperature RTDs.
• Personal Protective Equipment (PPE): Flame-retardant coveralls, steel-toe footwear, and hearing protection if operating adjacent to running machinery.

3. Diagnostische hulpmiddelen vereist

4. Initiële beoordelingschecklist

5. Systematisch diagnosestroomdiagram

Follow this decision tree systematically from top to bottom. Do not skip steps.

1. Initial Symptom Verification
   • IF compressor tripped on high discharge temperature (> 105 degrees Celsius):
   • Allow unit to cool to < 40 degrees Celsius before opening panels.
   • Check oil level via receiver sight glass.
     • IF oil level is low: Proceed to Root Cause 1 (Low Oil Level).
     • IF oil level is normal/correct: Proceed to step 2.
2. Thermal Differential & Cooler Inspection
   • Start compressor in unloaded state or monitor immediately upon startup (if safe).
     • Using thermal imager, scan the oil cooler inlet and outlet lines.
     • IF temperature drop across the oil cooler is less than 8 degrees Celsius (Delta-T < 8 degrees Celsius): Proceed to Root Cause 2 (Cooler Fouling / Blockage).
     • IF temperature drop across the cooler is normal (> 15 degrees Celsius) but oil entering the airend remains hot: Proceed to step 3.
3. Thermal Bypass Valve Verification
   • Measure temperature of the oil line returning from the cooler versus the bypass line entering the thermostatic valve housing using an infrared thermometer.
     • IF the bypass line remains hot when discharge temperature exceeds opening threshold (typically 60-75 degrees Celsius): Proceed to Root Cause 3 (Thermostatic Valve Failure).
     • IF thermostatic valve operates correctly: Proceed to step 4.
4. Environmental & Ventilation Assessment
   • Measure ambient temperature at compressor intake and room exhaust.
     • IF ambient temperature > 40 degrees Celsius or ventilation airflow is restricted: Proceed to Root Cause 4 (Adverse Ambient Conditions / Ventilation Failure).
     • IF ambient conditions are within spec: Proceed to Root Cause 5 (Sensor Calibration / Electrical Fault).

6. Fout-oorzaakmatrix

7. Analyse van de hoofdoorzaak voor elke fout

Root Cause 1: Low Oil Level

Explanation: Compressor oil serves a triple function: lubrication, sealing of rotor clearances, and direct heat absorption from the compression chamber. When the oil charge drops below the minimum operating threshold, the smaller volume of oil absorbs the same mechanical and thermodynamic heat load, resulting in a rapid temperature spike. Low oil levels also reduce the oil flow rate through the thermostatic valve and oil cooler, accelerating thermal degradation.

Confirmation: Check the oil receiver sight glass with the compressor shut down and depressurized. If the level is below the minimum mark, or if oil consumption exceeds 0.05% of total mass flow rate, check for external leaks at shaft seals, oil filter gaskets, separator elements, and oil cooler headers.

Consequences: Severe oil starvation leads to direct metal-to-metal contact between male and female rotor lobes, scoring of airend housing bores, rapid bearing cage failure, and total airend seizure requiring complete replacement.

Root Cause 2: Oil Cooler Fouling (External & Internal)

Explanation: Rotary screw compressors utilize air-cooled or water-cooled heat exchangers. External fouling occurs when airborne particulates (dust, lint, paper, chemical mist) are drawn across the aluminum or copper fin pack by the cooling fan, forming an insulating blanket that blocks convective heat transfer. Internal fouling occurs due to carbonization, varnish, and oil oxidation products lining the inner tube walls, restricting thermal conductivity.

Confirmation: Use a thermal imager across the cooler matrix. A fouled cooler exhibits a high temperature differential (Delta-T) across the core face without a corresponding drop in oil temperature, or shows localized cold/hot spots indicating blocked tubes.

Consequences: Reduced heat rejection forces the circulating oil to enter the airend at elevated temperatures (e.g., 70 degrees Celsius instead of 55 degrees Celsius), compounding the compression heat and triggering high-temperature shutdowns.

Root Cause 3: Thermostatic Mixing Valve Failure

Explanation: The thermostatic valve directs compressor oil either directly back to the airend (during cold startup to accelerate warm-up) or through the oil cooler (once normal operating temperature is reached). The internal wax element degrades over time, losing expansion capability, or the sliding spool sticks due to varnish accumulation.

Confirmation: Monitor the oil lines connected to the thermostatic valve body using an infrared thermometer. If the line leading to the oil cooler remains cool while the discharge temperature exceeds 90 degrees Celsius, the valve element is stuck closed, recirculating hot oil directly back into the airend.

Consequences: Total bypass of the cooling circuit prevents heat rejection, causing the oil temperature to escalate exponentially within minutes of loading.

Root Cause 4: Adverse Ambient Conditions & Ventilation Failure

Explanation: Air-cooled compressors reject total electrical and mechanical input energy as heat into the equipment room. If room ventilation capacity (exhaust fan CFM) is inadequate, or if ambient temperatures exceed 40 degrees Celsius (104 degrees Fahrenheit), the temperature differential between the cooling medium (ambient air) and the hot oil diminishes, reducing heat transfer efficiency.

Confirmation: Measure ambient temperature within 1 meter of the compressor air intake using a calibrated thermocouple. Check auxiliary exhaust fan rotation, drive belt tension, and ductwork backpressure.

Consequences: Chronic high operating temperatures shorten compressor lubricant life by 50% for every 10 degrees Celsius rise above 80 degrees Celsius, accelerating additive depletion and varnish formation.

Root Cause 5: Temperature Sensor (RTD / Thermocouple) Calibration Drift

Explanation: Platinum resistance temperature detectors (Pt100 or Pt1000) or thermocouple probes degrade due to vibration, thermal cycling, or moisture ingress, causing them to report artificially high temperature readings to the PLC/controller.

Confirmation: Disconnect the RTD from the controller and measure resistance directly with a DMM. Compare the measured resistance to the standard ITS-90 RTD resistance table (e.g., 100 ohms at 0 degrees Celsius, 138.51 ohms at 100 degrees Celsius). Alternatively, verify sensor tip temperature using a calibrated surface probe pyrometer.

Consequences: Nuisance tripping interrupts production even when actual internal temperatures are normal, leading to operator workarounds or unnecessary hardware replacement.

8. Stapsgewijze oplossingsprocedures

Resolution 1: Correcting Low Oil Level & Replenishing Fluid

1. Execute full LOTO on the compressor electrical supply and bleed all air pressure from the receiver tank to 0.0 bar / 0 psi.
2. Inspect all joints, hoses, shaft seals, and oil filter spin-on gaskets for signs of leakage. Repair or replace compromised fittings.
3. Remove the oil fill plug slowly to ensure no residual trapped pressure exists.
4. Pump or pour OEM-specified synthetic compressor lubricant (ISO VG 46 or 68 depending on ambient spec) into the receiver tank until the fluid level reaches the 3/4 mark on the sight glass with the system settled.
5. Secure the fill plug with a calibrated torque wrench to the manufacturer’s specified torque (typically 45-60 Nm).
6. Clear LOTO, restart the compressor in unload mode for 5 minutes, check for leaks, then load the unit and monitor discharge temperature stabilization (Target: 78 – 85 degrees Celsius).

Resolution 2: Cleaning and Flushing the Oil Cooler

1. Execute LOTO on electrical and pneumatic energy sources.
2. Allow the cooler core to cool below 40 degrees Celsius (104 degrees Fahrenheit).
3. External Cleaning: Using compressed air (regulated to < 2.0 bar / 30 psi) or a soft-bristle brush, blow out dust and debris from the cooling fins in the reverse direction of normal airflow. For greasy residue, apply an industrial alkaline biodegradable coil cleaner, allow 10 minutes dwell time, and rinse gently with low-pressure water, ensuring electrical components are fully shrouded.
4. Internal Flushing (if oil-side fouled): Disconnect oil lines from the cooler inlet and outlet manifolds. Circulate a specialized varnish-removal solvent or flushing oil through the core using an external pump rig for 2 to 4 hours in accordance with ASTM standards. Flush thoroughly with clean compressor oil before reassembly.
5. Re-torque all hydraulic line fittings to correct standards (e.g., JIC flare or DIN fittings per ISO 8434-1).
6. Restart and verify Delta-T across the cooler exceeds 12 degrees Celsius under full load.

Resolution 3: Replacing the Thermostatic Mixing Valve Element

1. Execute LOTO and depressurize the compressor system completely.
2. Drain oil from the thermostatic valve housing down to a level below the valve cover.
3. Remove the retaining bolts on the thermostatic valve cover plate and lift off the cover. Extract the O-ring seal and the wax element assembly.
4. Inspect the internal bore for varnish or scoring. Clean the housing bore with fine emery cloth (600 grit) and solvent cleaner.
5. Install a new thermostatic element kit (ensuring the correct nominal temperature rating, e.g., 71 degrees Celsius / 160 degrees Fahrenheit nominal start-to-open). Fit a new O-ring seal lubricated with clean compressor oil.
6. Bolt down the cover evenly in a cross-pattern to 25 Nm torque.
7. Top off the oil level, clear LOTO, and test compressor warm-up cycle. Verify that oil flows to the cooler once discharge temperature reaches the valve setpoint.

Resolution 4: Remediating Ventilation and Ambient Conditions

1. Verify compressor room exhaust fan electrical supply, rotation direction, and belt tension. Clean intake louvers and removable pre-filters.
2. If room ambient temperature exceeds 40 degrees Celsius, install auxiliary ducted fresh air supply fans drawing from outside the building envelope, sized to meet total heat rejection requirements (typically 3.5 m3/min per kW of compressor motor power per CAGI/Pneurop guidelines).
3. Ensure hot air discharge ducting from the compressor enclosure has zero static pressure restriction and discharges directly outdoors without recirculation.
4. Verify motor current draw with a clamp meter to ensure cooling fans are not overloaded.

Resolution 5: Replacing and Calibrating Temperature Sensors

1. Execute LOTO on the electrical supply. Open the control panel.
2. Disconnect the Pt100/Pt1000 RTD sensor leads from the analog input module.
3. Unscrew the sensor probe from the discharge manifold thermowell.
4. Test the removed sensor in a calibrated dry-block calibrator or ice bath (0 degrees Celsius) and boiling water (100 degrees Celsius) using a DMM. If resistance deviates more than +/- 0.5% from the ITS-90 reference table, discard the sensor.
5. Install a new replacement RTD sensor with thread sealant suitable for high-temperature service (rated to 150 degrees Celsius). Torque to 15 Nm.
6. Reconnect wiring to the PLC terminal block, verify loop integrity, and perform a live read check via the compressor human-machine interface (HMI).

9. Preventieve maatregelen

10. Reserveonderdelen en componenten

To source OEM-equivalent replacement parts, thermostatic kits, sensors, and lubricants engineered for industrial rotary screw compressors, explore the UNITEC-D E-Catalog.

11. Referenties

• ANSI / CAGI – Performance Standard for Rotary Screw Compressors.
• ASME Boiler and Pressure Vessel Code (BPVC) – Section VIII, Division 1 (Compressed Air Receivers).
• OSHA 29 CFR 1910.147 – The Control of Hazardous Energy (Lockout/Tagout).
• BS EN 1037 / PUWER 1998 – Safety of Machinery – Prevention of Unexpected Start-up (UK).
• IEEE 43 – Recommended Practice for Testing Insulation Resistance of Rotating Machinery.
• UNITEC-D Maintenance Engineering Bulletin: Rotary Screw Thermal Management Best Practices (Doc REF: UDM-TR-049).

1. Probleembeschrijving en reikwijdte

Hoge perstemperatuur (HDT) in schroefcompressoren is een kritische indicator voor systeeminefficiëntie, degradatie van componenten en mogelijk catastrofale storingen. Deze gids heeft betrekking op luchtgekoelde en watergekoelde schroefcompressoren die in industriële omgevingen werken. HDT wordt gedefinieerd als bedrijfstemperaturen die de door de fabrikant aanbevolen afvoerdrempel overschrijden, wat doorgaans resulteert in automatische uitschakeling van de machine om het compressorblok te beschermen. Als deze situatie niet wordt aangepakt, leidt dit tot snelle oxidatie van de olie, defecte lagers en voortijdige mechanische slijtage.

Classificatie van ernst: Kritisch. Voortdurend gebruik boven 105 °C (221 °F) brengt de olie-integriteit in gevaar en verkort de levensduur van de componenten met 50% voor elke stijging van 10 °C boven de normale bedrijfslimieten.

2. Veiligheidsmaatregelen

WAARSCHUWING: COMPRESSORSYSTEMEN BEVATTEN OPGESLAGEN ENERGIE. VOORDAT U EEN DIAGNOSTISCHE OF ONDERHOUDSTAAK UITVOERT, MOET U DE VOLLEDIGE LOCKOUT/TAGOUT-PROCEDURES (LOTO) UITVOEREN. ISOLEER DE COMPRESSOR VAN ELEKTRISCHE STROOM, LAAT ALLE LUCHTDRUK AF NAAR DE ATMOSFEER EN LAAT HET SYSTEEM AFKOELEN TOT ONDER 40°C (104°F) OM THERMISCHE BRANDWONDEN TE VOORKOMEN. GEBRUIK GESCHIKTE PBM'S, INCLUSIEF THERMAL GEÏSOLEERDE HANDSCHOENEN, VEILIGHEIDSBRIL EN LAARZEN MET STALEN NEUZEN.

3. Diagnostische hulpmiddelen vereist

4. Initiële beoordelingschecklist

5. Systematisch diagnosestroomdiagram

1. Symptoom: alarm voor hoge perstemperatuur
   • Stap 1: Controleer de werkelijke temperatuur. Gebruik een gekalibreerd digitaal thermokoppel bij de persleiding. Als de sensorwaarde aanzienlijk afwijkt van de meterwaarde, is de temperatuursensor defect.
   • Stap 2: Controleer de omgevingstemperatuur en ventilatie. Controleer de kamertemperatuur. Is de inlaatlucht beperkt? Als de omgevingstemperatuur > 40°C is, zal het systeem oververhit raken.
   • Stap 3: Analyseer de oliekoeler. Gebruik de infraroodcamera om te controleren op koude plekken op de radiatorvinnen.
     • Als er koude plekken zijn, is de koeler vervuild (intern of extern).
     • Als de gehele koeler heet is, loopt de olie niet rond of zit de thermostaat vast.
   • Stap 4: Controleer de werking van de oliethermostaat. Controleer de temperatuur van de olieleiding die naar de koeler leidt. Als de lijn ondanks een hoge perstemperatuur koel blijft, is de thermostaat defect in de bypass-positie.

6. Fout-oorzaakmatrix

7. Analyse van de hoofdoorzaak voor elke fout

7.1 Oliepeil en -kwaliteit

Olie dient zowel als smeermiddel als koelmiddel in schroefcompressoren. Lage oliepeilen verminderen het volume van de warmteoverdrachtsvloeistof, wat leidt tot thermische overbelasting. Bovendien verhoogt olieafbraak (slib/vernis) de wrijving en beperkt het de warmteoverdracht.

7.2 Koelervervuiling

Externe vervuiling treedt op wanneer inlaatfilters defect raken, waardoor stof en vuil de radiatorvinnen kunnen verstoppen, waardoor de efficiëntie van de warmtewisseling afneemt. Interne vervuiling ontstaat wanneer olieafbraakproducten zich in de radiateurbuizen afzetten, waardoor een thermische barrière ontstaat.

7.3 Thermostaatstoring

De oliethermostaat regelt de oliestroom naar de koeler. Als het defect raakt in de gesloten (bypass) positie, wordt de hete olie rechtstreeks teruggestuurd naar het compressorblok zonder door de koeler te gaan. Dit resulteert in een onmiddellijke temperatuurpiek onder belasting.

7.4 Omgevingsomstandigheden

Schroefcompressoren zijn ontworpen om binnen specifieke omgevingstemperatuurgrenzen te werken. Hoge omgevingstemperaturen verminderen de ΔT die beschikbaar is voor warmteoverdracht, waardoor de compressor gedwongen wordt heter te draaien.

8. Stapsgewijze oplossingsprocedures

8.1 Een laag oliepeil oplossen

1. Voer LOTOTO uit.
2. Laat het systeem volledig drukloos worden.
3. Inspecteer op lekkages (fittingen, slangen, oliekeerring).
4. Voeg door de fabrikant goedgekeurd smeermiddel toe tot het juiste niveau.
5. Controleer de staat van het oliefilter (indien geblokkeerd, wordt de oliestroom belemmerd).

8.2 Koelervervuiling oplossen

1. Voer LOTOTO uit.
2. Extern: Gebruik perslucht (minder dan 2 bar) of een mild schoonmaakmiddel om het vinnenpakket schoon te maken. Gebruik geen hogedrukreinigers omdat deze de vinnen kunnen laten inzakken.
3. Intern: Spoel bij vermoeden het koelsysteem door met een aanbevolen oliespoelmiddel volgens het technische bulletin van de fabrikant.

8.3 Thermostaatstoring oplossen

1. Voer LOTOTO uit.
2. Zoek het thermostaathuis (meestal in de buurt van het compressorblok).
3. Verwijder het behuizingsdeksel en verwijder het thermostatische element.
4. Test het element in een warmwaterbad (het moet openen bij de nominale temperatuur, doorgaans 70-80°C).
5. Vervangen door een nieuw element en een nieuwe pakking. Draai vast volgens de aanhaalspecificaties.

9. Preventieve maatregelen

10. Reserveonderdelen en componenten

Raadpleeg voor alle vervangende onderdelen de UNITEC-D e-catalogus: https://www.unitecd.com/e-catalog/

11. Referenties

• ASME PTC 9: Prestatietestcode voor compressoren
• ANSI/CAGI B19.1: Veiligheidsnorm voor persluchtsystemen
• OEM technische servicehandleidingen voor smering en thermische controle