Liquid shock is a common and very destructive failure in refrigeration systems. When the liquid refrigerant or lubricating oil is sucked into the compressor cylinder with the gas, it will damage the suction valve disc, and at the same time, if these liquids are not quickly discharged during the exhaust process, and are compressed when the piston is close to the top dead center, it will produce an instant high hydraulic phenomenon, we usually refer to this phenomenon as liquid stroke. Liquid stroke can be in a very short time on the compressor exhaust valve plate, valve plate, valve pad, piston top, piston pin, connecting rod, crankshaft and bearing shell, which can be called the fatal killer of reciprocating compressor.

The process and phenomenon of liquid hammering

When a large amount of liquid enters the compressor cylinder with the gas, its high momentum and high frequency impact will cause great damage to the suction valve plate, resulting in the valve plate fracture. This is not only one of the typical characteristics of hydraulic percussion, but also indicates that the compressor is about to face serious failure.

If the liquid does not evaporate and discharge the cylinder in time, when the piston approaches TDC, it will compress these liquids instantaneously, creating a huge pressure. This pressure is enough to bend or even break the connecting rod, and the way it breaks is very different from failures such as holding shaft and piston bite cylinder. The fracture of connecting rod caused by hydraulic stroke usually occurs in a short period of time, the piston and crankshaft at both ends of the connecting rod move freely, and there is generally no holding shaft or biting cylinder phenomenon caused by serious wear. At the same time, the extrusion characteristics of the connecting rod and the stubble also show that the hydraulic hammer is the main cause of the fault.

Suction valve plate breakage: Because the density of the liquid is much higher than that of the gas, the momentum and impact force generated when it flows are also greater. When a large number of liquid droplets enter the cylinder, the formed two-phase flow will produce a high intensity and frequent impact on the suction valve plate, similar to the effect of a typhoon with pebbles hitting the window, and eventually cause the valve plate to break.

Crankshaft connecting rod fracture: The liquid that does not evaporate in time will be violently compressed when the piston approaches TDC, producing a phenomenon similar to impact, and accompanied by a metal knocking sound. This compression can directly cause the connecting rod to bend or even break.

The root causes of hydraulic percussion are varied, but they boil down to the following:

Return liquid: that is, the liquid refrigerant or lubricating oil flowing back from the evaporator to the compressor. This may be caused by improper selection of the expansion valve, too small superheat setting, incorrect or damaged installation of the temperature sensing package, and failure of the expansion valve. In addition, in the case of hot gas defrosting system, evaporator frost serious or fan failure, cold storage temperature frequent fluctuations, etc., liquid return phenomenon is also easy to occur.

Start with liquid: When the return air cooled compressor starts, the lubricating oil in the crankcase bubbles violently, which is usually due to a large amount of refrigerant dissolved in the lubricating oil or refrigerant migration to the crankcase after the compressor is shut down. The liquid shock caused by liquid starting only occurs during the starting process, but the harm is equally great.

Excessive lubricating oil: semi-enclosed compressors are usually equipped with oil sight glasses to observe the oil level. If the oil level is higher than the oil scope, it indicates that there is too much oil. Too much lubricating oil will increase the possibility of splashing, once the splashed lubricating oil into the inlet into the cylinder, it may cause liquid shock.

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