Oil-Injected Screw Air Compressor Loading and Unloading Failure: Causes, Diagnosis, and Fixes

Oil-injected screw air compressors are widely used across industries due to their efficiency, stability, and durability. However, one of the most common issues operators encounter is loading and unloading failure. When this happens, compressors may start and stop frequently, experience pressure fluctuations, or even break down—ultimately affecting productivity and equipment lifespan.

In this article, we explore the working principle, common failure causes, and practical solutions to ensure your oil-injected screw compressor runs reliably at all times.

 Understanding the Loading & Unloading System

The loading and unloading system in an oil-injected screw compressor is responsible for maintaining system pressure by controlling the air intake. Here’s how it works:

• Loading Phase: When system pressure drops below a preset threshold, the pressure switch or sensor signals the intake valve to open. The compressor starts producing air and loading the system.

• Unloading Phase: When the system reaches the target pressure, a signal is sent to close the intake valve, putting the compressor into an unloading state. It continues to run, but no air is compressed, reducing power consumption.

• Pressure Control: Operators can regulate the compressor’s performance by adjusting the loading/unloading pressure settings via the pressure switch or sensor.

Common Causes of Loading and Unloading Failure

1. 🔧 Faulty Pressure Switch or Sensor

• Cause: Aging, wear, miscalibration, or complete failure.

• Effect: Incorrect pressure readings cause premature loading/unloading or fail to trigger changes at all.

2. 🚪 Intake Valve Malfunction

• Cause: Sticky movement, seal failure, or solenoid coil damage.

• Effect: The intake valve can’t open/close properly, causing inconsistent airflow and pressure control.

3. 💻 Control System Failure (PLC or Wiring)

• Cause: Faulty PLC logic, wiring short circuits, or software glitches.

• Effect: Miscommunication between components leads to failure in initiating correct actions.

4. 🛠️ Pipeline or Valve Leakage

• Cause: Cracks, loose fittings, or worn-out valves.

• Effect: Constant air loss keeps the compressor in loading mode, increasing wear and energy use.

5. ⚙️ Worn Mechanical Components

• Cause: Long-term use, poor maintenance.

• Effect: Intake valve, solenoids, or pressure switches respond slowly or fail entirely, reducing system efficiency.

How to Fix Loading and Unloading Failures

✔️ 1. Calibrate or Replace Pressure Switches/Sensors

• Regularly calibrate switches to ensure accuracy.

• Replace worn or malfunctioning sensors immediately.

• Clean dust and oil from sensor surfaces.

✔️ 2. Maintain the Intake Valve

• Clean and lubricate moving parts.

• Replace faulty seals for better air-tightness.

• Test solenoid coils for electrical continuity and replace if faulty.

✔️ 3. Diagnose the Control System

• Use diagnostic tools to check the PLC and circuitry.

• Update and reset control software.

• Reprogram setpoints if loading logic is incorrect.

✔️ 4. Detect and Seal Air Leaks

• Use soapy water or ultrasonic detectors to find leaks.

• Repair damaged sections and tighten loose fittings.

✔️ 5. Maintain the Oil Circuit

• Check oil level and condition regularly.

• Clean or replace the oil filter to prevent clogs.

• Ensure proper lubrication of moving parts.

✔️ 6. Inspect and Replace Worn Parts

• Schedule inspections for valves and sensors.

• Replace components that show signs of aging or reduced performance.

Real-World Example

A manufacturing facility experienced frequent compressor pressure drops and productivity issues. A structured approach helped restore normal operations:

1. Pressure switch was recalibrated—still failed.

2. Intake valve was removed, cleaned, and resealed.

3. A small pipeline leak was identified and repaired.

4. Lubricant was topped up and filter cleaned.

5. PLC control system was debugged and updated.

The result? A stable system with consistent pressure and no more frequent shutdowns.

Final Thoughts

Loading and unloading failures in oil-injected screw air compressors are not just a technical inconvenience—they can directly impact energy consumption, equipment longevity, and production timelines. Regular inspections, proactive maintenance, and prompt troubleshooting are essential.

By understanding the core issues and implementing the right solutions, businesses can significantly reduce downtime, save costs, and extend the life of their compressors.

Need help choosing or maintaining your air compressor system? Contact our expert team for support or explore our range of industrial-grade screw air compressors.

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