Screw Compressor Troubleshooting: High Discharge Temperature

1. Description of the Problem and Scope of Application

This manual is intended for systematic diagnosis and troubleshooting of abnormally high discharge temperatures in oil-filled screw compressors. High discharge temperature is a critical indicator of potential component failure and can lead to significant equipment damage, unplanned downtime and reduced operational efficiency. The manual covers diagnosing common causes such as low oil, coolant contamination, thermostatic valve failure, and adverse environmental conditions.

Equipment Types Covered:

• Industrial rotary screw compressors with oil injection.

Classification of Severity:

• Critical: The discharge temperature exceeds the set maximum threshold (eg +110°C) or an emergency shutdown is triggered. Requires immediate intervention.
• Significant: Discharge temperature is consistently above normal (e.g. +95°C - +105°C) but below the emergency shutdown threshold. Indicates a progressive malfunction.
• Minor: Discharge temperature is slightly higher than normal (for example, +90°C - +94°C). It can be a sign of deteriorating conditions or the initial stage of a problem.

Relevant Standards: EN ISO 12100 (Machine Safety), DSTU EN 1012-1 (Compressors and Vacuum Pumps - Safety Requirements), EN ISO 14001 (Environmental Management Systems).

2. Precautions

WARNING! Before performing any diagnostic or repair work on the compressor, the following critical safety precautions must be taken to prevent personal injury and equipment damage:

• LOCKOUT/TAGOUT (LOTO): Always turn off and lock out power to the compressor. Ensure that all energy sources (electrical, pneumatic) are isolated and tagged according to your facility's LOTO procedures.
• RELIEF OF PRESSURE: Completely depressurize all parts of the compressed air system. Open the drain valves and ensure that the system is atmospherically vented.
• HOT SURFACES: Compressor equipment can be very hot during operation and after shutdown. Allow it to cool down to a safe temperature (+40°C or below) before touching any components.
• ROTATING PARTS: Always beware of rotating parts such as fans and couplings. Never operate the equipment until all rotating parts have come to a complete stop.
• STORED ENERGY: Be careful with springs, pressure accumulators, and other components that can release stored energy.
• PERSONAL PROTECTIVE EQUIPMENT (PPE): Use appropriate PPE, including safety glasses, heat resistant gloves, protective clothing and protective footwear.
• CHEMICAL SAFETY: When working with compressor oil or cleaning agents, follow the Material Safety Data Sheet (MSDS) recommendations.

3. Necessary Diagnostic Tools

The following tools are required for accurate diagnosis of high injection temperature:

4. Initial Evaluation Checklist

Before starting a detailed diagnosis, you should conduct an initial examination and collect information:

5. Systematic Diagnostic Algorithm

Perform the diagnosis step by step, following the branching logic to determine the root cause:

1. Symptom Detection: High Discharge Temperature
   1. Check the reading of the discharge temperature sensor on the control panel.
   2. Measure the outlet air/oil temperature after the separator with an IR thermometer.
   3. Expected Result: The temperature is above +90°C, or higher than the norm set by the manufacturer.
   4. ACTION: If the temperature > +105°C or the emergency shutdown is activated, stop the compressor immediately and go to section “2. Precautions".
2. Initial Condition Assessment
   1. Step 1: Ambient Temperature Check
      1. Measure the air temperature at the compressor inlet.
      2. Result:
         • IF Ambient temperature > +35°C: Go to 2.b.
         • IF Ambient temperature ≤ +35°C: Go to 3.
   2. Step 2: Room Ventilation Assessment
      1. Check for obstructions to air flow. Does the hot air not recirculate?
      2. Result:
         • IF Poor ventilation/recirculation: PROBLEM: Insufficient heat dissipation. PRIMARY CAUSE: Unfavorable environmental conditions. Go to Chapter 7 (Condition Analysis).
         • IF Ventilation is adequate: Go to 3.
3. Compressor Oil Level Check
   1. CAUTION: The oil level check must be performed with the compressor stopped and de-energized, according to the manufacturer's instructions (usually 5-10 minutes after stopping).
   2. Visually check the oil level through the sight glass or with a dipstick.
   3. Expected Result: The oil level should be within the MIN/MAX marks.
   4. Result:
      • IF Oil Level LOW: PROBLEM: Insufficient cooling and lubrication. PRIMARY CAUSE: Low oil level. Go to Section 7 (Low Oil).
      • IF Oil Level NORMAL: Go to 4.
4. Diagnostics of the Oil Cooling System
   1. Step 1: Check for Oil Cooler Contamination (External)
      1. Visually inspect the outer fins of the oil cooler for dust, dirt, lint, and oil deposits.
      2. Result:
         • IF Cooler outside is CONTAMINATED: PROBLEM: Reduction of heat dissipation efficiency. ORIGINAL CAUSE: Contamination of the cooler. Go to Section 7 (Contaminated Coolant).
         • IF Cooler Outside CLEAN: Go to 4.b.
   2. Step 2: Cooler Temperature Drop Check (Internal Contamination)
      1. CAUTION: This test is performed with the compressor running. Follow all safety precautions.
      2. Measure the oil temperature at the cooler inlet and cooler outlet using a contact thermometer.
      3. Expected Result: The temperature difference should be in the range of 10-15°C (depending on the model, see the manufacturer's manual).
      4. Result:
         • IF Temperature drop is WAY LOWER than normal (< 8°C): PROBLEM: Poor heat transfer efficiency. PRIMARY CAUSE: Cooler internal contamination or insufficient oil flow. Go to Section 7 (Contaminated Cooler).
         • IF Temperature drop NORMAL: Go to 4.c.
   3. Step 3: Check Cooler Fan Operation
      1. Visually check that the cooler fan is working.
      2. Check the power supply to the fan with a multimeter.
      3. Measure the air flow rate through the cooler with an anemometer.
      4. Expected Result: The fan is working, the air flow rate is normal.
      5. Result:
         • IF Fan NOT WORKING or airflow POOR: PROBLEM: Insufficient heat dissipation. ROOT CAUSE: Fan motor, wiring, relay or controller failure. Go to Section 7 (Fan Malfunction).
         • IF The fan is working correctly: Go to 5.
5. Diagnostic Thermostatic Valve
   1. NOTE: This test requires measuring the temperature with the compressor running. Follow all safety precautions.
   2. Measure the temperature of the oil before and after the thermostatic valve, as well as the temperature of the oil going directly to the compression elements (bypassing the cooler).
   3. Expected Result:
      • At low oil temperature, the valve should direct most of the oil to bypass the cooler.
      • When the operating temperature is reached (e.g., +70°C – +80°C), the valve should open, directing the oil through the cooler.
      • The temperature difference between the inlet and outlet of the valve (through the cooler) must be significant.
   4. Result:
      • IF Valve ALWAYS OPEN (oil constantly goes through the cooler, even cold): The oil does not reach operating temperature quickly, but this is not the cause of the high discharge temperature.
      • IF Valve ALWAYS CLOSED (oil BYPASSES cooler, even hot): PROBLEM: No oil cooling. PRIMARY CAUSE: Thermostatic valve sticking in the closed position. Go to Section 7 (Thermostatic Valve Malfunction).
      • IF VALVE NOT FULLY OPENING (restricts flow through cooler): PROBLEM: Insufficient cooled oil flow. ORIGINAL CAUSE: Partial sticking or failure of the thermostatic valve. Go to Section 7 (Thermostatic Valve Malfunction).
      • IF The valve is working correctly: If all previous checks have failed, consider other, less common causes (eg compressor overload, compressor motor problems, internal air leaks). This requires a deeper diagnosis and consultation with a UNITEC specialist.

6. Malfunction-Cause matrix

This table presents the likely causes of high discharge temperature, ranked by likelihood, diagnostic tests, and expected results.

7. Root Cause Analysis for Each Malfunction

7.1. Low Compressor Oil Level

• Detailed Description: Compressor oil performs two key functions: lubrication of moving parts and removal of heat generated during compression. An insufficient volume of oil leads to a decrease in the heat capacity of the system and an increase in friction.
• How to Confirm: Low oil level is easily confirmed by visual inspection through the sight glass or with the dipstick located on the compressor oil tank.
• Reasons: Oil leakage from the system (seals, pipelines, filters), excessive removal of oil with air (defective oil separator), insufficient refueling during maintenance.
• Damage If Left Unresolved: Low oil level quickly leads to overheating of the compression elements (screws), accelerates the wear of bearings, seals and rotors, can cause the screw unit to seize. This is a critical fault.

7.2. Contaminated Oil Cooler

• Detailed Description: The oil cooler (radiator) is responsible for removing heat from the compressor oil to the surrounding air. Contamination can be external (dust, dirt, fluff, sediment that settles on the radiator fins) or internal (sediment, scale, carbon deposits inside the tubular elements).
• How to Confirm: External contamination is visualized. Internal contamination is confirmed by measuring a reduced oil temperature difference between the inlet and outlet of the cooler, as well as an increased oil pressure drop across the cooler (if there are appropriate sensors). The normal oil temperature difference on the cooler is 10-15°C.
• Reasons: Insufficient regular cleaning, operation in dusty conditions, use of low-quality oil that forms deposits.
• Damage if Unresolved: Reduced cooling efficiency leads to constant overheating of the oil, accelerating its degradation, the formation of varnish deposits and shortening the service life of all components in contact with the oil.

7.3. Faulty Thermostatic Valve (Oil Temperature Control Valve)

• Detailed Description: A thermostatic valve regulates the flow of oil through the cooler, maintaining the optimal operating temperature of the oil. If the valve sticks in the closed position, the oil completely or partially bypasses the cooler, causing it to overheat.
• How to Confirm: This is confirmed by measuring the oil temperature. If the temperature of the oil returning to the screw block is high and the cooler remains relatively cool (or the temperature difference across it is small), this indicates that the valve is not directing oil through the cooler. You can also try to measure the resistance of the temperature element (if available).
• Causes: Wear of internal components, accumulation of deposits, mechanical jamming of a spring or heat-sensitive element.
• Unresolved damage: Continuous overheating of oil and compression elements as described above. It can also lead to improper condensation in the system due to the lack of a stable operating oil temperature.

7.4. High Environmental Conditions / Poor Ventilation

• Detailed Description: The efficiency of air cooling of the compressor directly depends on the temperature and quality of the surrounding air. If the temperature in the compressor room is excessively high (> +35°C) or the hot air discharged from the compressor is recirculated back into the inlet, the cooling system cannot dissipate the heat effectively.
• How to Confirm: Measure the air temperature at the compressor inlet (compared to room temperature) and estimate the total air flow in the compressor room.
• Reasons: Insufficient size of ventilation holes, blocked ventilation ducts, malfunction of exhaust fans, location of the compressor near heat sources, lack of air ducts to remove hot air.
• Unresolved damage: The compressor is constantly operating in the mode of increased heat load, which shortens the service life of the oil, seals, electrical components and the screw unit.

8. Step-by-Step Troubleshooting Procedures

CAUTION: Before performing any procedures, ensure that the compressor is completely de-energized and immobilized per LOTO procedures and all systems are depressurized.

8.1. Correction of Low Oil Level

1. Isolation and Security: Follow LOTO procedures. Depressurize the compressor.
2. Cooling: Let the compressor cool down to a safe temperature (+40°C).
3. Topping Up Oil: Use only compressor oil that meets the manufacturer's specifications (eg ISO VG 46 for most). Unscrew the filler cap. Add oil slowly, constantly monitoring the level through the sight glass or with the dipstick, until the level reaches the upper MAX mark.
4. Leak Check: Carefully inspect all piping, fittings, seals, and filters for signs of leaks. Tighten loose connections or replace damaged seals.
5. Verification: Start the compressor, bring to operating temperature. Stop, let it stand for 5-10 minutes, check the oil level again. Control the injection temperature.

8.2. Cleaning the Contaminated Oil Cooler

1. Isolation and Security: Follow LOTO procedures. Release the pressure.
2. External Cleaning:
   1. With compressed air (pressure no more than 2 bar), blow the fins of the cooler from the inside to the outside to remove dust and dirt. Use safety glasses.
   2. If the contamination is oily, use a special solution for cleaning radiators or a small amount of mild detergent, rinse with water under low pressure. Make sure all electrical components are protected from moisture.
   3. Dry the cooler thoroughly before starting.
3. Internal Cleaning (For Heavy Pollution):
   1. CAUTION: This procedure requires disassembly of the cooler and specialized chemical solutions. Performed by qualified personnel.
   2. Dismantle the cooler. Flush the internal channels with a special solution to remove deposits (for example, an alkaline solution or a mild acid, according to the recommendations of the solution manufacturer).
   3. Rinse thoroughly with clean water until all chemicals are removed.
   4. Dry the cooler. Reinstall, replacing all seals. Tightening torque of the cooler fastening bolts: 20-25 Nm (depends on the model, see the manual).
4. Verification: Start the compressor, monitor the temperature difference on the cooler and the general discharge temperature.

8.3. Replacing a Faulty Thermostatic Valve

1. Isolation and Security: Follow LOTO procedures. Release the pressure.
2. Draining the Oil: Partially drain the oil from the oil tank to the level below the thermostatic valve (if the valve is low).
3. Dismantling: Disconnect the pipelines from the valve. Carefully remove the thermostatic valve. Be prepared for a small amount of oil to leak out.
4. Installing a New Valve: Install a new thermostatic valve (make sure the part number matches the original) replacing all seals and gaskets. Tightening torque of fittings: 30-45 Nm (depending on size, see manual).
5. Oil filling: Add oil if necessary.
6. Verification: Start the compressor. Monitor the temperature of the oil entering the cooler and the temperature of the oil after it. Make sure that the thermostatic valve responds adequately to temperature changes.

8.4. Optimization of Environmental Conditions

1. Isolation and Safety: Perform LOTO procedures (if work involves electrical ventilation systems).
2. Room Assessment: Measure the temperature and air flow at various points in the compressor room.
3. Ventilation Improvements:
   1. Provide adequate inlet and outlet ventilation openings according to the compressor manufacturer's recommendations (usually a minimum of 2-3 m² per 1 m³/min of compressor output).
   2. Install air ducts to remove hot air outside the room.
   3. Make sure exhaust fans are working properly and have sufficient performance.
   4. Eliminate heat sources near the compressor.
4. Verification: Monitor the ambient temperature around the compressor and the discharge temperature during the operating cycle. The goal is to keep the room temperature below +30°C.

9. Precautions

Regular maintenance is key to preventing high discharge temperatures.

10. Spare Parts and Components

For quick and effective troubleshooting, it is recommended to have the following spare parts available:

To order original spare parts and components, visit our UNITEC Electronic Catalog.

11. Links

• DSTU EN 1012-1:2014 Compressors and vacuum pumps. Security requirements. Part 1: Compressors.
• EN ISO 12100:2010 Safety of machinery – General principles for design – Risk assessment and risk reduction.
• ISO 8573-1:2010 Compressed air – Part 1: Contaminants and purity classes.
• Compressor Manufacturer's (OEM) Operation and Maintenance Manuals.
• Other UNITEC maintenance manuals (e.g. "Diagnostics of vibration in compressor units").