What is the most probable cause of knocking in a reciprocating compressor?

Knocking in a reciprocating compressor is primarily caused by liquid slugging. This phenomenon occurs when liquid refrigerant enters the cylinders instead of gas. Since compressors are designed to compress gases, the presence of liquid can lead to a catastrophic situation where the liquid does not compress and can result in severe mechanical stresses within the compressor. This can cause knocking sounds as the liquid slug crashes against the compressor components, disrupting the normal operation.

Liquid slugging can be particularly damaging; it may cause bent connecting rods, damaged crankshafts, and other mechanical failures. To prevent this, systems are designed to ensure that only vapor enters the compressor, and proper maintenance is essential to monitor refrigerant levels and operational conditions to avoid slugs of liquid refrigerant from entering the cylinders.

While worn bearings, insufficient lubrication, and excessive pressure can also cause various types of mechanical noises or failures, they do not specifically lead to the unique situation of knocking that is associated with liquid slugging.