Fault Diagnosis and Repair Techniques for Compressor Core Components

I. Causes and Inspection Methods for Cracks in the Compressor Body

Common Causes of Cracks in the Compressor Body

    1.  Winter Freezing: Failure to promptly drain cooling water from the body or cylinder head after shutdown in winter, leading to expansion and cracking upon freezing.

    2.  Casting Internal Stress: Internal stresses generated during the casting process of the component gradually propagate under long-term operational vibrations, eventually forming visible cracks.

    3.  Mechanical Accident Induced:

        Piston rupture.

        Connecting rod screw fracture, causing the connecting rod to break and detach.

        Flyweight from the crankshaft flying out and impacting the body.

        Valve component detachment impacting and damaging the cylinder head.

Crack Inspection Methods

     1. Kerosene Penetration Method

        Procedure:

          1.  Wipe the suspected area with a cotton swab soaked in kerosene.

          2.  Wipe off the surface kerosene with a dry cotton swab.

          3.  Immediately apply a layer of white powder (e.g., chalk dust) to the wiped area.

        Principle: Kerosene trapped in the crack will seep out and discolor the white powder, clearly revealing the crack's path and length.

    2. Hydraulic Pressure Method

    Detects cracks by pressurizing the cooling water system.

        Well-equipped repair shops: Use a dedicated hydraulic pressure tester.

        Simplified method (for limited equipment):

          1.  Plug all water pipe fittings on the body/head, leaving only one connected to the outlet of a hand-operated water pump.

          2.  Install a dedicated cover plate on the body's upper surface to prevent cooling water overflow.

          3.  Start the pump to fill the water jacket. Close the outlet valve once water flows from it.

          4.  Continue pressurizing until the gauge reads 3-4 atmospheres (approx. 0.3-0.4 MPa), then stop the pump.

          5.  Thoroughly inspect all internal and external surfaces of the body and head for water leakage or seepage.

II. Welding Repair Methods for Cracks in Body and Cylinder Head

Welding is typically used for repairing cracks located internally or in areas with high strength requirements.

Oxy-Acetylene Welding (Gas Welding) Repair Process (Preheating Required)

    1.  Pre-treatment:

         Drill Φ6-8 mm stop holes at both ends of the crack to prevent propagation.

         Gouge a V-groove of 80°-90° along the crack. Depth should not exceed 2/3 of the cylinder wall thickness.

    2.  Preheating:

         Slowly heat the workpiece in a furnace to a dull red heat (approx. 600-650°C). This avoids generating new internal stresses or brittle white iron structures due to localized rapid heating and cooling.

    3.  Welding Environment:

         Place the workpiece on an iron pan containing red-hot coke after removal from the furnace.

         Insulate non-welding areas with asbestos plates.

         Maintain the welding area in a horizontal position to prevent molten filler metal from running.

    4.  Welding Materials:

         Filler Rod: High-silicon gray cast iron rod is preferred, diameter 3-4 mm.

         Flux: Use borax. Apply by dipping the rod or sprinkling directly onto the weld zone to prevent oxidation of the molten pool.

    5.  Post-Weld Heat Treatment:

         Reheat the welded workpiece to 450-550°C and hold for approximately half an hour.

         Place in a box of hot sand or the original heating furnace for slow cooling over 8-10 hours to thoroughly relieve welding stresses.

Arc Welding (Electric Welding) Repair Process (Usually No Preheating Required)

    Electrode: Copper-iron composite electrodes (copper core with iron sheath, iron core with copper sheath, or bundled copper/iron wires) with coating are recommended.

    Key Operational Points:

         1.  After welding each segment, immediately lightly peen the weld bead from both sides towards the center using a small hammer.

         2.  While the weld bead is still red-hot, lightly tap with a chipping hammer to remove slag.

         3.  This promotes denser metal structure and prevents porosity.

    Long Cracks: Employ segmented interval welding. Segment length depends on workpiece thickness, typically 20-30 mm. Weld the next segment only after the area about 70 mm from the previous weld has cooled to touch.

    Deep Cracks: Use multi-layer build-up welding. Subsequent layers provide a tempering effect on prior layers.

Final Inspection: A hydraulic pressure test is mandatory after repair. The repair is considered acceptable only if no leakage is detected.

III. Analysis of Causes for Premature Cylinder Wear

Premature cylinder wear is abnormal wear, while "cylinder scoring" is severe localized wear or seizure, classified as accident wear. Causes are categorized into manufacturing and usage/maintenance aspects.

A. Manufacturing Aspects

    1.  Poor manufacturing quality or unacceptable surface roughness of the cylinder (liner).

    2.  Non-perpendicularity of connecting rod to crankshaft (bent connecting rod or crankshaft).

    3.  Non-perpendicularity of piston centerline to its end face.

    4.  Skewed piston ring grooves.

    5.  Excessive piston ring tension or excessively high surface hardness (e.g., containing ternary phosphide eutectic).

    6.  Non-perpendicularity of piston pin boss center to piston centerline.

    7.  Excessive crankshaft axial clearance.

    8.  Insufficient working gap (end gap) of piston rings.

    9.  Improper piston pin assembly causing eccentric cylinder wear.

    10. Excessive clearance between piston and cylinder.

    11. Non-compliant metallographic structure of the cylinder (requires fine lamellar or sorbitic pearlite; free cementite is not allowed).

B. Usage and Maintenance Aspects

    1.  Insufficient oil pump pressure leading to inadequate cylinder lubrication.

    2.  Incorrect lubricant grade (too viscous or too thin).

    3.  Failure to change lubricant contaminated with mechanical impurities after prolonged use.

    4.  Missing or defective air filter on the crankcase filler opening, allowing dust into the lubricant.

    5.  Broken splash rod or excessively low oil level in splash-lubricated compressors.

    6.  Poor cylinder cooling, excessive temperature, severe carbon buildup.

    7.  Defective air filter allowing significant dust ingress into the cylinder.

IV. Common Defects in Connecting Rods

    1.  Bending or Torsional Deformation:

        Occurs in planes parallel or perpendicular to the crankshaft axis.

        Consequences: Disrupts normal bearing fit, causing eccentric wear of bearings and journals leading to rapid failure; causes piston misalignment in the cylinder, resulting in localized contact, scoring, or seizure.

    2.  Ovality and Taper Wear in Small and Big End Bores:

        Formation of ovality and taper.

        Consequences: Results in loose fit with the crankshaft journal or piston pin (crosshead pin), excessive clearance, severely impairs heat conduction from friction, accelerating wear of the bushing or bearing lining alloy.

V. Causes and Inspection of Connecting Rod Bolt Damage

Causes of Damage

    1.  Poor bolt manufacturing quality (material, machining, heat treatment issues).

    2.  Failure to replace bolts and nuts as a complete set.

    3.  Poor fit/excessive clearance between the bolt and its bore in the connecting rod big end.

    4.  Excessive torque applied when tightening nuts, or inconsistent torque between the two nuts on the same rod.

    5.  Poor contact between bolt head/nut and the connecting rod bearing surface, causing misalignment after tightening.

    6.  Excessive connecting rod bearing clearance or excessive ovality of the crankpin.

         Note: Connecting rod bolt failure is usually progressive fatigue failure due to prolonged exposure to the above factors, not instantaneous.

Inspection Methods

    1.  Visual Inspection: Carefully examine bolt fillets and areas near threads using a 5x or 10x magnifying glass for any signs of damage.

    2.  Non-Destructive Testing: Use a magnetic particle inspection unit to check for cracks.

    3.  Gauge Inspection:

        Use gauges to check for bolt elongation.

        Use thread gauges to check for thread damage.

VI. Repair Methods for Metallic Packing Glands

Packing gland faults mainly fall into two categories: gas leakage and piston rod wear. If piston rod wear is due to excessive cylinder/piston wear or misalignment between cylinder and frame centerlines, these root causes must be addressed first.

(A) Piston Rod Repair Procedure

    1.  Disassemble and clean oil/grease from the packing gland.

    2.  Inspect the inner surface of packing rings that directly contact the piston rod. If scratches, scoring, or roughness are present, dressing is required based on the piston rod condition. A serviceable ring surface should be smooth and polished.

    3.  If wear on the piston rod working section exceeds 0.5 mm, turning and grinding repair is necessary.

    4.  For minor surface defects like scratches or scoring, dress with a file followed by hand lapping.

    5.  Use the spotting/bluing and scraping method to fit the packing rings to the piston rod working surface.

    6.  Perform a pre-assembly of the packing gland on the non-working end of the piston rod or a dedicated mandrel to check the fit of all parts. Contacting end faces between rings and packing boxes require lapping for fit (lapping steel rings on a surface plate with coarse abrasive paste, checked by spotting).

(B) Method for Scraping Packing Rings

    1.  Apply a thin layer of spotting paste (e.g., Prussian blue) to the piston rod's working section.

    2.  Slide the packing ring back and forth on the rod several times for marking.

    3.  Remove the ring and scrape the high spots marked by the paste.

    4.  Repeat the process of applying paste, marking, and scraping.

    5.  Scraping is considered complete when the entire inner surface of the ring shows a uniform, fine distribution of spotting marks.

Important Note: If the rings cannot be fitted to a severely worn piston rod, the piston rod must be repaired via metal spraying or hard chrome plating. The mandrel used for pre-assembly must match the actual diameter of the repaired piston rod. Therefore, final assembly must be performed directly on the piston rod itself.