Deposits and corrosion on the centrifugal impellers of air compressors are among the most critical issues affecting reliability, efficiency, and mechanical integrity. Because centrifugal compressors operate at extremely high rotational speeds (often exceeding 10,000 to 30,000 RPM), even a few milligrams of imbalance or a minor surface defect can lead to catastrophic failure.
Here is a detailed breakdown of the causes, consequences, and mitigation strategies.
1. Types of Deposits
Deposits alter the aerodynamic profile of the blades and cause imbalance. The type of deposit depends heavily on the intake location and the quality of the filtration system.
Atmospheric Fouling: In industrial or coastal environments, the intake air carries sub-micron particles. Even if the main filter captures 99% of dust, the remaining 1% (silica, clay, soot) adheres to the blades due to "oil mist" or humidity.
Oil Carryover: In oil-flooded screw compressors feeding the centrifugal (or if there is seal oil leakage), hydrocarbons polymerize under the high temperatures of compression (interstage temperatures). This creates a varnish-like lacquer that is difficult to remove and acts as a "glue" for other particulates.
Moisture and Salts: In humid environments, moisture condenses during intercooling. If the inlet air contains salt (marine environments) or acidic gases (SOx, NOx), the deposits are often hygroscopic, forming mud-like accumulations that harden when the compressor stops and dries.
2. Types of Corrosion
Centrifugal impellers are typically made of high-strength alloys (such as 17-4 PH stainless steel, Inconel, or Titanium) to withstand the centrifugal stress. However, they are susceptible to specific corrosion mechanisms:
Stress Corrosion Cracking (SCC): This is the most dangerous form. It occurs when the impeller material (especially 17-4 PH stainless steel) is subjected to tensile stress (from rotation) and a corrosive environment (chlorides, moisture). SCC occurs rapidly without significant material loss and results in sudden impeller fragmentation.
Pitting Corrosion: Typically occurs in the intercooler passages or on the blade leading edges. Pits act as stress concentrators, leading to fatigue crack initiation.
Erosion-Corrosion: Common in wet compression scenarios. Water droplets striking the impeller at supersonic speeds strip away the protective oxide layer, exposing fresh metal to corrosive agents.
3. Consequences
Efficiency Loss: Deposits increase surface roughness. For a centrifugal compressor, a roughness increase of just a few microns can reduce efficiency by 2–5% and significantly reduce flow capacity (surge margin shifts).
Rotor Imbalance: Uneven deposits cause unbalance. Since centrifugal impellers operate above their first critical speed, unbalance leads to high vibration, bearing wear, and seal rubs.
Catastrophic Failure: If corrosion (SCC) or severe erosion goes undetected, the impeller can burst. Due to the high kinetic energy stored, this usually results in the destruction of the compressor casing and poses severe safety risks.
4. Mitigation and Solutions
A. Inlet Air Quality (Prevention)
High-Efficiency Filtration: Use of high-efficiency particulate air (HEPA) or ePTFE membrane filters (pulse-cleaned) is essential. For coastal installations, coalescing filters are required to remove salt moisture before it reaches the impeller.
Inlet Duct Design: The intake must be located away from cooling tower plumes, exhaust stacks, and chemical vent lines.
B. Operational Strategies
Online Water Wash: Many large centrifugal compressors (e.g., in refineries) are equipped with online washing systems. Demineralized water (or a specific solvent) is injected while the machine is running to dissolve water-soluble deposits. Caution: Improper water wash (using untreated water) can cause severe erosion and corrosion.
Liquid Knockout: Ensuring intercoolers have high-efficiency moisture separators to prevent liquid water from entering the high-pressure (HP) stage impeller.
C. Material Selection and Coatings
High-Grade Materials: For sour gas or highly corrosive environments, upgrading from 17-4 PH to Inconel 718 or Titanium is common, as these materials are highly resistant to chloride SCC.
Protective Coatings: Applying aluminum-filled ceramic coatings or HVOF (High-Velocity Oxy-Fuel) tungsten carbide coatings to the impeller. These provide a sacrificial barrier against corrosion and improve erosion resistance. However, coatings must be applied perfectly balanced; uneven coating can cause imbalance.
D. Inspection and Maintenance
Borescope Inspections: Regular borescope inspections are critical. Look for:
Leading edge erosion: Indicates liquid ingestion.
Varnish/lacquer: Indicates oil contamination.
Rust streaks: Indicates moisture and potential SCC risk.
Balancing: If deposits are found, the impeller must be chemically cleaned (using approved solvents that do not attack the base metal) and re-balanced.
5. Troubleshooting Summary
| Symptom | Likely Cause | Immediate Action |
|---|---|---|
| Gradual vibration increase | Even deposit buildup or slight imbalance | Perform online wash; inspect filters for bypass. |
| Sudden vibration spike | Piece of deposit shed (fouling) or foreign object damage (FOD) | Trip compressor; borescope inspection required. |
| High interstage temperature | Fouling reducing cooling efficiency or flow | Check intercooler and inspect impeller for deposits reducing flow coefficient. |
| Visible rust/cracking | SCC initiation (requires immediate action) | Do not restart. Impeller requires NDT (dye penetrant) and likely replacement. |
If you are observing deposits or corrosion on a specific compressor, it is often advisable to perform a vibration analysis to determine if the imbalance is due to uniform fouling or a discrete mechanical change, and to send a lube oil sample to the lab to check for silicon (dirt ingress) or glycol (coolant leak) contamination.
