Screw Compressor Won't Load - Inlet Valve Problems

You're here because you checked for control air at the inlet valve and air WAS blowing out - which means control air IS reaching the inlet valve.

But the compressor still won't load.

Diagnosis: The problem is the inlet valve itself. Control air is present, but the inlet valve isn't opening (or not opening fully).

Let's troubleshoot the inlet valve systematically.

Background: How Inlet Valves Work

The inlet valve controls airflow into the compressor:

Unload Mode (Closed):

Inlet valve is closed (or nearly closed)
Only a small bypass hole allows minimal air in
This is the default state (normally closed without control air)

Load Mode (Open):

Control air pressure applied to actuator
Actuator piston moves, opening the inlet valve
Full airflow enters compressor
Compressor produces compressed air

Types of inlet valves:

Butterfly valve - Rotates open/closed (most common)
Piston valve - Slides open/closed
Combination valve - Piston with butterfly disc

All use a pneumatic actuator (air cylinder) to operate the valve.

Step 1: Verify Control Air Is Actually Reaching Actuator

Before assuming it's the inlet valve mechanism, double-check that control air is actually making it to the actuator port.

How to check:

Disconnect control air line at the inlet valve actuator
Set compressor to LOAD mode
Is air blowing out of the disconnected line?
- Should be 2.5-4 bar (35-60 PSI)

If YES: Control air is definitely reaching the actuator → Go to Step 2

If NO: Wait, that's strange - you said air WAS blowing out earlier. Either:

Line disconnected somewhere between where you checked and the actuator
Go back and re-check → Control Air Supply Chain Problems

Step 2: Apply Control Air Manually to Actuator

This test bypasses all upstream components and directly tests if the inlet valve actuator and mechanism will operate.

How to test:

Disconnect control air line from inlet valve actuator (already done in Step 1)
Apply shop air directly to actuator port
- Use 3-6 bar (40-85 PSI) compressed air
- Connect via hose or blow gun
Does the inlet valve open?
- You should see/hear the actuator piston move
- You should see the inlet valve butterfly/piston open
- If compressor is running, airflow will increase dramatically (you'll hear it)

Result A: YES - Valve Opens with Manual Air

Diagnosis: Inlet valve mechanism is OK mechanically. Problem is upstream:

Control air pressure too low (but you measured it - should be 2.5-4 bar)
Control air line restriction (partial blockage reducing pressure)
Actuator seals worn (needs higher pressure than available)

Next steps:

Measure control air pressure accurately at actuator port
Check for line restrictions
Consider actuator seal wear → May need rebuild kit

Result B: NO - Valve Doesn't Open (or Opens Very Little)

Diagnosis: Inlet valve or actuator has a mechanical problem.

Possible causes:

Stuck or seized actuator piston
Worn actuator seals
Seized valve mechanism (butterfly shaft, piston guides)
Broken return spring
Linkage binding or broken

Go to Step 3

Step 3: Diagnose Inlet Valve Mechanical Problems

If the inlet valve won't open even with shop air applied directly, here are the common problems:

Problem A: Stuck or Seized Actuator Piston

Symptoms:

Actuator piston won't move at all
Or moves very stiffly, doesn't travel full stroke

Causes:

Dirt or corrosion in actuator cylinder
Dried/hardened grease or oil
Damaged piston seals causing binding

How to check:

Remove actuator from inlet valve (if possible on your model)
Try moving piston manually
Inspect cylinder bore and piston for dirt, corrosion, scoring

Solution:

Disassemble actuator
Clean thoroughly
Replace seals (actuator rebuild kit: $50-$150)
Lubricate with appropriate grease
Reassemble

Problem B: Worn Actuator Seals

Symptoms:

Actuator piston moves but doesn't generate enough force
Or piston moves slowly/weakly
Temperature-related: Works when warm, doesn't work when cold

Causes:

Rubber seals worn, torn, or hardened
Seals don't seal properly (air leaks past piston)
Cold weather makes seals shrink (won't seal until they warm up and expand)

Classic case - Temperature-Related:
I've seen this multiple times: Compressor won't load first thing in the morning (cold), but after running for 10-20 minutes and warming up to 165°F (75°C), it suddenly starts loading fine.

Cause: Rubber seals in actuator don't seal when cold. They expand when warm and seal properly.

Test: Try closing the sump vent valve on top of separator (increases internal pressure). If compressor loads with vent closed, your seals are worn.

⚠️ Warning: Don't run long with vent closed - you're pressurizing separator without proper venting (safety relief may lift)

Solution:

Replace actuator seals (rebuild kit)
Or replace entire actuator if badly worn
Cost: $100-$400 depending on compressor size

Problem C: Seized Valve Mechanism

Symptoms:

Actuator moves but inlet valve doesn't open (or opens partially)
Linkage or shaft is stuck

Causes:

Butterfly shaft seized in bearings (rust, carbon buildup, lack of lubrication)
Piston-type valve stuck in guides (dirt, corrosion)
Linkage binding (bent, damaged, misaligned)

How to check:

Disconnect actuator from valve linkage (if possible)
Try moving inlet valve manually (by hand or with tool)
Should move smoothly through full range of motion

If stuck/binding:

Disassemble inlet valve
Clean shaft, bearings, guides
Replace damaged bearings or bushings
Lubricate with high-temperature grease
Check for bent/damaged parts

Solution:

Clean and lubricate (if just dirty/dry)
Replace bearings or bushings if worn
Replace inlet valve assembly if badly damaged ($300-$1,000+ depending on compressor)

Problem D: Broken Return Spring

Symptoms:

Inlet valve opens OK but doesn't close properly
Or valve opens but doesn't return to closed position

Causes:

Spring broken or missing
Spring weak or collapsed

How to check:

Visual inspection (look for broken spring)
Remove control air - valve should return to closed position by spring force

Solution:

Replace return spring
Check linkage for proper spring mounting

Problem E: Linkage Binding or Broken

Symptoms:

Actuator moves but motion doesn't transfer to inlet valve
Or motion is partial/incomplete

Causes:

Linkage pin broken or missing
Linkage bent or damaged
Connection loose or disconnected

How to check:

Watch linkage movement while applying air to actuator
Everything should move together smoothly

Solution:

Replace broken pins, linkage arms
Straighten bent parts (or replace if badly damaged)
Tighten loose connections

Real-World Case: Kaeser AS30 Temperature-Related

Symptoms:

Sump pressure only 25 PSI (should be 35-50 PSI)
Won't load when cold (morning startup in Pennsylvania winter, 20-30°F)
BUT loads fine once compressor reaches 165°F

Diagnosis: Worn actuator seals (don't seal when cold)

Testing: Closing sump vent valve → compressor loads → confirms seal wear

Solution: Rebuild inlet valve actuator with new seals

Lesson: Temperature-related loading problems are almost always worn actuator seals. If it works when warm but not when cold, replace the seals.

Preventive Maintenance

Here's how to prevent inlet valve problems:

Every 8,000-16,000 Hours:

Inspect inlet valve operation
Check for smooth movement
Listen for unusual noises
Check for air leaks around valve/actuator

Every 16,000-24,000 Hours:

Service inlet valve and actuator
Replace actuator seals (rebuild kit)
Clean and lubricate valve mechanism
Inspect bearings, bushings, linkage

Every Major Overhaul (40,000+ Hours):

Replace inlet valve assembly if worn
Or complete rebuild with all new parts

Immediately If:

Temperature-related loading problems → replace seals ASAP
Inlet valve sticking or binding → clean and lubricate
Unusual noises from inlet valve → inspect immediately

When to Call for Help

Inlet valve service can be straightforward or complex depending on compressor design.

Call a technician if:

Inlet valve removal requires special tools or procedures
You're not familiar with pneumatic actuators
Compressor warranty concerns (don't void warranty by DIY disassembly)
Safety concerns (working on large compressors with high forces)

Typical service call: $300-$800 (includes disassembly, rebuild, parts)

Recommended Resources

In-Depth Training:

Industrial Compressed Air Systems Course - comprehensive training covering inlet valve operation, actuator principles, maintenance procedures, and troubleshooting. Includes detailed modules on inlet valve types and rebuild procedures.

Related Troubleshooting:

Main "Won't Load" Troubleshooting - start here
Control Air Supply Chain Problems - if problem is upstream of inlet valve

Summary Checklist

Verify control air reaches actuator - Disconnect line, is air blowing out?
Apply shop air manually to actuator - Does valve open?
- YES → Control air pressure too low or actuator seals worn
- NO → Mechanical problem, go to next checks
Check actuator piston movement - Stuck? Binding?
Check actuator seals - Temperature-related? Worn?
Check valve mechanism - Shaft seized? Linkage broken?
Check return spring - Broken? Weak?
Check linkage - Binding? Broken pins?

Bottom Line

If control air IS reaching the inlet valve but compressor won't load, it's a mechanical problem with the inlet valve or actuator.

Most common causes:

Worn actuator seals (30%) - especially if temperature-related
Stuck/seized actuator piston (25%) - dirt, corrosion, lack of maintenance
Seized valve mechanism (20%) - butterfly shaft stuck, lack of lubrication
Broken linkage or spring (15%)
Valve out of adjustment (10%)

Diagnostic shortcut:

Apply shop air directly to actuator
Does valve open?
- NO → Mechanical problem (clean, rebuild, or replace)
- YES → Actuator seals worn or control air pressure too low

Most inlet valve problems can be fixed with cleaning, lubrication, and seal replacement. Full inlet valve replacement is rarely needed unless badly damaged.

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