6.4 Compressor Troubleshooting and Slugging

Liquid Slugging: Physics, Causes, and Damage

Liquid slugging is one of the most severe and destructive occurrences in an industrial ammonia refrigeration system. It refers to the entry of liquid refrigerant, or a large volume of compressor oil, into the compressor cylinders.

The Physics of Incompressibility

Unlike ammonia vapor, which is highly compressible, liquid ammonia is virtually incompressible.

• Massive Hydrostatic Force: When a piston travels upward during the compression stroke, it reduces the volume inside the cylinder. If the cylinder is filled with vapor, the vapor compresses smoothly. However, if the cylinder contains liquid, the liquid cannot compress. As the piston approaches Top Dead Center (TDC), the volume of liquid is larger than the clearance volume. The kinetic energy of the heavy motor, crankshaft, and flywheel is forced against this liquid barrier, generating instantaneous hydrostatic pressures exceeding several thousand psi.
• Catastrophic Mechanical Damage: This immense force immediately damages the weakest mechanical links. The consequences include:
  • Blown Valve Plates: The suction and discharge valves are shattered.
  • Bent or Broken Connecting Rods: The rods buckle under the compressive load.
  • Broken Pistons: Piston crowns are crushed or cracked.
  • Cracked Cylinder Heads: The cylinder head can be blown off, resulting in an immediate and dangerous release of toxic anhydrous ammonia (R-717) into the engine room.

Primary Causes of Liquid Slugging

Operators must identify the conditions that lead to liquid carryover:

1. Evaporator Overfeed: In flooded or liquid recirculation systems, failed liquid feed solenoid valves or incorrect level control settings can overfill the evaporator or surge drum, sending liquid back down the suction line.
2. Sudden Load Changes: A sudden drop in suction pressure (e.g., when a large compressor starts up) causes the liquid refrigerant in a nearby accumulator or evaporator to boil violently. This rapid boiling creates a surge of liquid droplets that carry over into the suction line.
3. Improper Startup SOPs: Opening the compressor suction stop valve too quickly after a shutdown can draw liquid that has pooled in the suction line directly into the compressor.
4. Poor Defrost Management: During hot gas defrost, liquid condensate must be completely drained from the evaporator before cold liquid feed and fan operation resume. If not, the sudden thermal shock and gas velocity can push a slug of liquid back into the suction line.

Preventing Liquid Slugging

• Suction Accumulators: Installing a suction accumulator (liquid separator) in the suction line upstream of the compressor. The accumulator separates liquid droplets from the vapor, allowing only dry vapor to enter the compressor.
• Liquid Level Safety Cutouts: Accumulators must be equipped with high-level float switches or probes that automatically shut down the compressors if the liquid level rises too high.
• Superheat Monitoring: Regularly monitoring suction superheat. A drop in superheat below 10°F is an early warning sign of liquid carryover.

Compressor Troubleshooting Procedures

When a compressor operates outside its normal limits, the operator must systematically identify the root cause and execute corrective actions.

1. Abnormal Noises

Any unusual noise is an immediate indicator of mechanical distress:

• Metallic Clattering or Slapping: Usually indicates valve damage (broken valve springs or cracked valve plates) or that a slug of liquid is hitting the cylinder. The compressor must be shut down immediately.
• Rhythmic Heavy Knocking: A low-pitched, rhythmic knock that matches crankshaft RPM indicates worn main bearings or worn connecting rod bearings. A higher-pitched, sharper tap indicates worn wrist pins or bushings.
• High-Pitched Squealing: Indicates oil starvation in the shaft seal or bearings, causing severe friction.

2. High Discharge Temperatures

Leaking discharge valves are a major cause of high discharge temperatures.

• The Re-compression Loop: When a discharge valve plate is cracked, pitted, or has carbon buildup, it does not seal tightly. During the suction stroke, hot compressed gas from the discharge manifold leaks back into the cylinder. On the next compression stroke, this already-hot gas is compressed again, absorbing even more heat. This re-compression loop causes a rapid, localized rise in discharge temperature.
• Other Causes: Dirty condenser coils (raising discharge pressure), presence of non-condensable gases (air) in the system, high suction superheat, or failed cooling water jackets.

3. Oil Carryover and Oil Level Loss

Oil carryover occurs when excessive amounts of lubricating oil escape the compressor crankcase and enter the system piping.

• Oil Foaming: Rapid pressure drops in the crankcase during startup boil dissolved ammonia out of the oil. The resulting foam is drawn into the cylinder and discharged into the system.
• Worn Piston Rings: Worn oil scraper rings allow oil to bypass the piston and enter the compression chamber.
• Oil Separator Return Malfunctions: The oil separator downstream of the compressor filters oil from the discharge gas and returns it to the crankcase. If the float valve in the oil separator is stuck open, high-pressure gas will bypass back into the crankcase, causing foaming and oil loss. If the float is stuck closed, oil will fill the separator and overflow into the condenser, leading to oil logging of low-side evaporators.

4. Failure to Load or Unload

• Solenoid Failures: Burned solenoid coils or broken wiring prevent electric unloaders from shifting.
• Plugged Oil Passages: Carbon or debris in the compressor oil can plug the small passages feeding the unloader pistons, preventing hydraulic pressure from retracting the depressor pins.
• Mechanical Jamming: Broken unloader springs or binding lifter pins can mechanically lock the unloader in the loaded or unloaded position.

Troubleshooting Matrix Table

Safety Precautions during Compressor Maintenance

1. Lockout/Tagout (LOTO): Always isolate the motor electrical starter and tag it out before removing any covers.
2. Isolation and Pumpdown: Close suction, discharge, and oil return valves. Evacuate the compressor down to 0 psig (using a pumpdown compressor or water-absorption hose).
3. Nitrogen Purging: Once pressure is at 0 psig, purge the compressor crankcase with dry nitrogen to clear any residual ammonia vapor. Never open a compressor that is under a vacuum, as air and moisture will be drawn in.
4. PPE Requirements: Technicians must wear a full-face respirator with ammonia-specific (green) canisters, heavy neoprene gloves, and safety goggles. An emergency eye wash station must be nearby.