Compressor Failures: Liquid Slugging

Liquid Slugging is where extreme cases of Flooded Start or Liquid Flood Back occur. When large quantities of liquid refrigerant enter the compressor and then mix with the lubricating oil in the compressors sump. The resulting liquid/oil mixing and diluting, then creates major amounts of liquid and oil droplets in a frothy foam mixture.

During extreme cases, this foam/oil/liquid mixture can be carried into the suction gallery and then through the suction valves and into the cylinder. This “Wet” mix will wash the cylinder lubricant away and can also cause the valve reeds to break. Due to this liquid/oil mixture carrying non compressible particles this can cause the valve plate to deflect to the point where it breaks. The Suction valve plate will show small dents or even holes through the plate. The valve plate then falls into the cylinder and on to the piston crown. As the piston travels up to the top of the cylinder during the compression cycle the valve plate is then smashed up into the discharge valve plate. This can cause a hole to be punched through the piston crown and damage to the discharge valve assembly occurs as there is almost no clearance between the top of the piston crown and the discharge valve, which gives the compressor its high efficiency rating. The discharge valve assembly can also be completely smashed to pieces in certain instances.

In very extreme cases of slugging, the cylinder can be virtually filled with liquid and can cause con rod breakages and even broken crankshafts have been the result of hydraulic lock in the cylinder. This is more in Flooded Start/Slugging instances.


Compressors are designed to pump refrigerant in its vapour form only, NOT LIQUID!

The action of the compressor is:

  • The compressors piston as it travels down the cylinder pulls low pressure suction vapour from the evaporator, through the suction valve reed and into the cylinder
  • The piston as its travels back up the cylinder will compress this low pressure suction vapour, as this LP vapour is compressed, its pressure and temperature rises
  • This compressed vapour is now pumped out through the discharge valves as a High pressure discharge vapour, which then passes to the condenser
  • The compressor works fine

When compressors try to pump liquid:

  • If liquid refrigerant returns to the compressor, as it gets drawn into the suction gallery, it then passes through the suction valve
  • The liquid being a solid mass will make the suction valve bend and sometimes break
  • Liquid will then enter into the cylinder which will then have its lubrication washed away
  • This will cause scoring to the cylinder walls and pistons
  • The piston as it travels down the cylinder will draw this liquid and vapour into it chamber
  • As the piston travels up the cylinder, the piston hits the solid liquid mass as it cannot compress it
  • This solid mass is then driven up against the suction valve and also driven up into the discharge valve
  • Again the solid mass of liquid will open and deflect the discharge valve plate against its limit stop and again the valve plate can get smashed to pieces
  • Pieces of valve plate will then fall down into the cylinder, and the piston as it travels up the cylinder will try to compress the bits of valve plate
  • This then can smash the piston
  • Under worse case scenario the cylinder can fill with liquid and hydraulic lock the piston which can break the con rod or even break the crankshaft

Typical damage to compressor parts:

  • Damaged Suction and/or Discharge Valve Reeds and springs or Valve assembly
  • Damaged Piston Crowns
  • Scoring to cylinder bores, pistons and rings
  • Broken con rods (no scoring to big end bearings)
  • Broken crankshaft
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