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Rotary Screw Compressor Won't Load

Rotary Screw Compressor Won't Load - Troubleshooting Guide

Over 20+ years working with rotary screw compressors, I've seen "won't load" problems more times than I can count. The compressor starts, motor runs, but the inlet valve won't open and no compressed air is produced.

Symptom: Motor runs fine, but compressor never transitions from unload to load mode. Internal pressure stays low, no compressed air is being produced.

This problem can be several different things, but I'll show you a systematic way to diagnose it—starting with the easiest and most common checks first.

How Loading/Unloading Works (Quick Review)

Before we troubleshoot, here's what you need to know:

Key Components:

  1. Inlet Valve - Controls airflow into the compressor (normally closed without control air)
  2. Solenoid Valve - Electrically controlled valve that directs control air to the inlet valve
  3. Control Air System - Uses compressed air from inside the separator vessel to operate the inlet valve
  4. Minimum Pressure Valve (MPV) - Maintains minimum pressure (2.5-4 bar / 35-60 PSI) inside the separator

How it works:

Unload Mode (Startup):

  • Compressor starts with inlet valve CLOSED
  • A small bypass hole in the inlet valve allows a little air to enter
  • This builds up unload pressure inside the separator (2.5-3.5 bar / 35-50 PSI)
  • This pressure becomes your control air supply

Load Mode (Producing Air):

  • Controller signals the solenoid valve to open
  • Control air flows: Separator → Solenoid → Inlet Valve Actuator
  • Inlet valve opens
  • Compressor produces full airflow

When this system fails, the compressor won't load.

Troubleshooting Tree (Start Here)

Here's the practical diagnostic approach, starting with the most common problem area.

Step 1: Is Control Air Reaching the Inlet Valve?

This is the first check. It tells you where the problem is.

How to check:

  1. Make sure compressor is set to LOAD mode (via controller - force it to load, or set pressure low enough that it should load)

  2. Locate the control air line at the inlet valve

    • This is a small tube/hose (usually 6-10mm diameter)
    • Connects to the inlet valve actuator (pneumatic cylinder on the inlet valve)

  3. Carefully disconnect this line at the inlet valve end

    • ⚠️ Be careful - if control air is present, it will blow out (2-4 bar / 30-60 PSI)

  4. Is air blowing out of the disconnected line?

Result A: NO - No Air (or Very Little Air) - MOST COMMON (80%)

Diagnosis: Control air is NOT reaching the inlet valve. The problem is upstream of the inlet valve.

But we need to narrow it down further: Is there control air available at all, or is the problem in the supply chain?

Next check: Is control air available at the source?

Step 2: Is There Control Air at the Separator Vessel?

How to check:

  1. Locate the control air supply line - Connects from the top of the separator vessel to the solenoid valve

  2. Carefully disconnect this line at the separator vessel (or at the solenoid inlet, whichever is easier)

    • ⚠️ Be careful - if control air is present, it will blow out

  3. With compressor running in UNLOAD mode, is air blowing out?

    • Should be 2.5-3.5 bar (35-50 PSI)

Result A1: YES - Control Air IS Available - 70% of cases

Diagnosis: There IS control air at the source, but it's not reaching the inlet valve. The problem is in the control air supply chain.

Next step: Go to → Control Air Supply Chain Problems

You'll troubleshoot:

  • Solenoid valve (most common - stuck, burned coil, no power)
  • Electrical issues (controller, wiring, safety interlocks)
  • Control air lines plugged or disconnected
  • This is the most common problem area!

Result A2: NO - No Control Air Available - 10% of cases

Diagnosis: There's no control air supply at all. The problem is either:

  • MPV (Minimum Pressure Valve) leaking
  • Bypass hole in inlet valve plugged
  • Intake air filter completely blocked

Next step: Go to → No Control Air Supply

You'll troubleshoot:

  • MPV leakage (hardest to check when hardwired to piping)
  • Bypass hole plugged (may require inlet valve access)
  • Intake air filter condition
  • This is the least common but hardest to diagnose

Result B: YES - Air IS Blowing Out - 20% of cases

Diagnosis: Control air IS reaching the inlet valve. The problem is the inlet valve itself.

Next step: Go to → Inlet Valve Problems

You'll troubleshoot:

  • Stuck or seized inlet valve actuator
  • Worn actuator seals (especially temperature-related)
  • Seized valve mechanism (butterfly shaft, linkage)
  • Broken return spring

Summary: The Three Paths (In Order of Frequency)

Path 1 - Control Air Supply Chain (70%):

Path 2 - Inlet Valve Problems (20%):

Path 3 - No Control Air Supply (10%):

Why This Order?

We check the most common problems first:

  1. Supply chain (70%) - Usually the solenoid valve. Easy to check, easy to fix (replace solenoid: $80-$300)

  2. Inlet valve (20%) - Usually worn seals or stuck actuator. Moderate difficulty (rebuild kit: $100-$400)

  3. No control air (10%) - MPV or bypass hole. Hardest to diagnose and fix, especially on hardwired compressors

This saves you time: You'll find the problem in the first two checks 90% of the time.

Why Start with This Check?

It's practical:

  • Easy to do (just disconnect a hose)
  • Doesn't require special tools or gauges
  • Works on compressors hardwired to piping systems
  • Gives you a clear YES or NO answer

It's fast:

  • Takes 2 minutes
  • Immediately tells you where the problem is
  • Avoids unnecessary disassembly

It follows real-world frequency:

  • 70% solenoid/supply chain (easy fix)
  • 20% inlet valve (moderate fix)
  • 10% no control air (hard fix)

Quick Reference: Common Causes Ranked by Frequency

Based on hundreds of troubleshooting cases:

Control Air Supply Chain (70% total):

  1. Solenoid valve failed - 50% (stuck, burned coil, no power)
  2. Electrical/controller issues - 10% (wiring, safety interlock)
  3. Control air lines plugged - 10% (dirt, moisture)

Inlet Valve Problems (20% total):

  1. Worn actuator seals - 10% (especially temperature-related)
  2. Stuck/seized actuator or valve - 10%

No Control Air Supply (10% total):

  1. MPV leaking - 6%
  2. Bypass hole plugged - 4%

Notice: The solenoid valve alone accounts for 50% of all "won't load" problems. That's why we check the supply chain first.

Recommended Resources

Want to understand rotary screw compressor control systems better?

In-Depth Training:

  • Industrial Compressed Air Systems Course - comprehensive training covering rotary screw compressor operation, control systems, loading/unloading systems, and troubleshooting. Includes detailed modules on control air operation and inlet valve diagnostics.

Related Troubleshooting:

Equipment Information:

Next Steps

Follow this diagnostic tree:

  1. Check control air at inlet valve

  2. Check control air at separator vessel

Work systematically and you'll find the problem.

Good luck! If you found the solution (or have questions), post in the Q&A forum and let me know what you found.