Liquid Slugging is damage from hydrostatic pressure when a compressor attempts to compress a liquid–oil or a refrigerate or both.
(Figure 2) The teardown of this Model M compressor revealed suction valve damage resulting from the attempt to compress Liquid refrigerant or oil, or both.
Since a liquid is virtually not compressible, the resulting hammering characteristically damages the suction valves of this machine. In this example, pieces of broken suction valve were wedged against the discharge valve and the side of the discharge gas passage.
This is a severe case. More often, the suction valves remain intact but crack radically or chip when subjected to liquid slugging.
This is the piston and rod assembly that was removed from the same compressor. The piston damage occurred when it contacted the broken valve parts.
When this type of damage Is found, the cylinders are generally damaged to the point where they must be repaired. Similarly, other compressors may show suction valve and spring damage when subjected to severe liquid slugging. The top of the piston will generally have marks caused by contact with spring or valve fragments.
Anytime valve or spring damage is observed, remove the motor and Inspect it and the motor barrel carefully. It Is possible that valve or spring fragments have lodged them- selves In the motor barrel or stator windings where they can cause future spot burns.
CAUSES OF LIQUID SLUGGING
(REFRIGERANT FLOOD-BACK FROM AN IMPROPER EXPANSION VALVE)
An improperly oversized expansion valve selection is one of the principal causes of liquid flood-back and the resultant slugging. While an oversized valve may perform well at full load, at part load it may lose control.
The reason is, at part load the valve attempts to control at its superheat setting but its oversized port passes more liquid than is needed. This overfeeds the evaporator, causing a rapid reduction in the superheat of the leaving gas. In response, the valve closes until normal superheat is re-established.
At this point, the valve again opens to pass another slug of liquid. This “hunting” condition will allow liquid to flood through the evaporator and into the suction line from where it can enter the compressor to cause damage.
It is important to note that some manufactured packaged products are intentionally designed with expansion valves rated for larger tonnage. In this case, the valve has been carefully selected and tested to insure that it meets specific design objectives. Do not confuse this type of valve selection with a “field” selected valve discussed above.
In many cases. field installed valves have been selected by inexperienced personnel. An experienced serviceman should always be suspicious of field selected and installed valves.