Rotary Screw Compressor - No Control Air Supply

You're here because you discovered there's no control air available at the separator vessel outlet (or at the solenoid inlet).

This means one of two things:

Minimum Pressure Valve (MPV) is leaking - Air escapes faster than it can build up
Bypass hole in inlet valve is plugged - No air enters the compressor to build unload pressure

These are the harder diagnostics because:

Checking internal pressure requires gauge installation (not always easy)
MPV leakage is hard to detect when compressor is connected to piping
Bypass hole may require inlet valve disassembly

But let's work through it systematically.

Background: How Control Air Is Generated

Before we troubleshoot, here's how control air is created:

Step 1: Small Bypass Hole

Inlet valve has a small bypass hole (1-3mm diameter)
Even when inlet valve is closed, this hole allows a tiny amount of air to enter
This is intentional

Step 2: Build Unload Pressure

That small airflow gradually builds up pressure inside the separator vessel
Target unload pressure: 2.5-3.5 bar (35-50 PSI)
This takes 10-30 seconds after startup

Step 3: Minimum Pressure Valve Holds It

MPV only opens when pressure reaches 2.5-4 bar (depends on model)
Below that, MPV stays closed
This traps the unload pressure inside the separator

Step 4: Use It as Control Air

This trapped unload pressure becomes your control air supply
Feeds the solenoid, which directs it to the inlet valve when loading

When this fails:

Either bypass hole is plugged (no air enters) → can't build pressure
Or MPV is leaking (air escapes) → can't maintain pressure

Diagnostic Approach

Here's the practical way to narrow it down:

Check 1: Is There ANY Pressure Inside the Separator?

If you can easily check internal pressure (gauge on compressor, or Elektronikon display), this tells you a lot:

Internal pressure during unload running:

Normal: 2.5-3.5 bar (35-50 PSI)
Too low: 0-1 bar (0-15 PSI)

If pressure is low or zero:

Either bypass hole plugged OR MPV leaking
Go to Check 2

If pressure is normal (2.5-3.5 bar):

Wait... that doesn't make sense. If internal pressure is normal, control air should be available.
Double-check: Is the line from separator to solenoid actually connected? Plugged?

Check 2: Listen/Feel at the MPV

Note: This is easier if you can isolate the compressor from the piping system (close downstream isolation valve). If you can't do that, this check is much harder.

If you CAN isolate the compressor:

Close downstream isolation valve (so system pressure = 0 bar)
Run compressor in unload mode (it should build unload pressure if bypass hole is working)
Listen/feel at the MPV outlet
- Is air leaking out?
- Should be NONE or very minimal leakage

If air is leaking:

MPV is leaking
Internal pressure can't build because air escapes
Solution: Replace MPV valve kit ($50-$300)

If NO air leaking:

MPV is holding pressure
Problem is likely bypass hole plugged → Go to Check 3

If you CANNOT isolate the compressor (hardwired to piping with no isolation valve):

This is much harder. Piping pressure may be higher than internal pressure, so you won't detect MPV leakage (backflow prevents it).

Options:

Temporarily disconnect compressor from piping (if feasible) - big job, often not practical
Skip MPV test and go straight to checking bypass hole
Try a "force test": Close vent valve on separator top (if equipped) - does internal pressure build now?
- ⚠️ Warning: Don't run long like this - you're pressurizing the separator without proper venting

Check 3: Check Bypass Hole in Inlet Valve

The bypass hole is critical but often overlooked.

What it is:

Small hole or passage in the inlet valve (1-3mm diameter)
Allows a small amount of air to enter even when inlet valve is closed
Sometimes it's a drilled hole, sometimes an adjustable needle valve, sometimes an internal bypass channel

Problem:

Gets plugged with dirt from intake air
Especially common if intake air filter is dirty or damaged

How to check:

This varies by compressor model. Here are the common scenarios:

Option A: Visible Bypass Hole

Remove intake air filter housing
Look at the inlet valve from the intake side
Look for a small hole drilled through the valve body or piston
Try passing a thin wire through it (carefully - don't enlarge it!)
Shine a light through it

Option B: Adjustable Needle Valve

Some compressors have a small adjustable needle valve as the bypass
Usually mounted on the inlet valve housing
Check if it's closed (should be slightly open)

Option C: Internal Bypass Channel

Some inlet valves have an internal bypass passage
Not visible from outside
May require partial disassembly to inspect

If bypass hole is plugged:

Clean it carefully (don't enlarge the hole!)
Use compressed air to blow through it (reverse direction)
Check/replace intake air filter (that's usually why it plugged)

Check 4: Intake Air Filter

Often overlooked: A completely clogged intake air filter can also prevent air from entering.

How to check:

Remove intake air filter
Try to blow through it (should have airflow)
Look at cleanliness (should be clean, not black/brown/caked with dirt)

If filter is clogged:

Replace it immediately
This may have also plugged the bypass hole → check bypass hole too

Typical replacement interval: Every 2,000-4,000 hours (sooner in dusty environments)

Real-World Cases

Case 1: Atlas Copco GA200 - Delta-P Reading Indicates Problem

Symptoms:

Internal pressure 0.1 bar (essentially zero)
Solenoid replaced, MPV serviced - no change
Key clue: Delta-P air filter reading was 0.025 bar BEFORE start and didn't change after starting

Diagnosis:

Delta-P across air filter should be near zero when compressor isn't running
Reading 0.025 bar before start = something is wrong with sensor OR filter is completely clogged
Reading doesn't change when running = no air is flowing

Root cause: Clogged intake air filter → bypass hole couldn't let air in → no unload pressure

Solution: Replace air filter, check bypass hole

Case 2: Atlas Copco (Unknown Model) - No Pilot Air

Symptoms:

Inlet valve won't open
Solenoid works OK
No pilot air (control air) anywhere

Diagnosis: No control air supply

Troubleshooting approach:

Check if there's any pressure in separator during unload running
- If YES: MPV leaking
- If NO: Bypass hole plugged or intake filter completely blocked

Common outcome: Usually bypass hole plugged due to neglected air filter maintenance

The Challenge: Diagnosing MPV vs Bypass Hole

Here's the honest truth: It's often hard to tell which one without some disassembly or testing.

Clues that point to MPV leaking:

You CAN hear air venting somewhere (but shouldn't be)
Previous MPV problems or old MPV
Compressor has high hours (MPV valve seat wears out)

Clues that point to bypass hole plugged:

Dirty or neglected intake air filter
Compressor operated in dusty environment
Recent maintenance where air filter was disturbed

Practical approach if you can't isolate the compressor:

Start with the easier fix: Replace intake air filter first (it's due anyway)
Check bypass hole if accessible
If those don't fix it: Plan to replace MPV during next maintenance shutdown

Why this order?

Air filter is cheap ($20-$80) and easy
Bypass hole check depends on compressor design (may or may not be easy)
MPV is more expensive ($50-$300) and you want to be sure before replacing

Temporary Workarounds (Not Permanent Solutions!)

If you need to keep the compressor running while waiting for parts or maintenance shutdown:

Workaround 1: Supply External Control Air

Use shop air or another compressor
Connect to the control air line (at solenoid inlet)
Regulate to 3-4 bar (40-60 PSI)
⚠️ This works but is NOT a permanent solution - fix the root cause

Workaround 2: Force MPV Open

If MPV is stuck closed (not leaking, just stuck)
Some MPVs can be manually opened or adjusted
⚠️ Dangerous if system pressure is high - consult service manual

These are emergency measures only. Fix the actual problem as soon as possible.

Preventive Maintenance

Here's how to prevent "no control air" problems:

Every 2,000-4,000 Hours:

Replace intake air filter - This is critical!
Dirty filter is the #1 cause of plugged bypass holes

Every 8,000-16,000 Hours:

Service or replace MPV
Inspect valve seat, replace valve kit
Cost: $50-$300 depending on size

Every 16,000-24,000 Hours:

Service inlet valve
Clean bypass hole (if accessible)
Replace seals and actuator parts

Immediately If:

Compressor operates in dusty environment → more frequent air filter changes (every 1,000-2,000 hours)

When to Call for Help

These diagnostics require mechanical knowledge and sometimes compressor disassembly.

Call a technician if:

You can't access bypass hole without major disassembly
You're not sure how to safely isolate compressor from piping
MPV replacement requires special tools or procedures
You're not comfortable working with compressed air systems

Typical service call: $300-$800 (includes diagnostics and parts)

Recommended Resources

In-Depth Training:

Industrial Compressed Air Systems Course - comprehensive training covering rotary screw internal components, MPV function, inlet valve operation, and maintenance procedures

Related Troubleshooting:

Main "Won't Load" Troubleshooting - start here
Control Air Supply Chain Problems - solenoid and electrical issues

Summary Checklist

Check intake air filter - Is it clogged? Replace if dirty
Check bypass hole (if accessible) - Is it plugged? Clean carefully
Check MPV for leakage (if you can isolate compressor) - Is air escaping? Replace valve kit
Check internal pressure (if gauge available) - Should be 2.5-3.5 bar during unload running

Bottom Line

No control air supply comes down to two causes:

1. Bypass hole plugged (50%)

Usually due to dirty intake air filter
Fix: Replace filter, clean bypass hole

2. MPV leaking (50%)

Worn valve seat, weak spring
Fix: Replace MPV valve kit

Start with the easy stuff:

Replace air filter
Check bypass hole (if accessible)
If those don't fix it → MPV is probably leaking

The challenge is that checking MPV is hard when compressor is connected to piping. If you can't isolate it, you may need to schedule a maintenance shutdown to properly diagnose and replace MPV.

Good luck! Let me know in the Q&A forum if you found the problem.