Page 321 - Reciprocating Compressors Operation Maintenance

3O6 Reciprocating Compressors: Operation and Maintenance

Cylinder Knocks

Most knocks will be found in the compressor cylinder. The common
causes of knocking in a cylinder include:

• Loose pistons
• Insufficient head clearance
• Too great a piston-to-cylinder bore clearance
• Broken piston rings
«Loose rider bands
• Loose or broken valves
• Moisture carryover or "liquid slugging"

For cylinder knocks, simple steps can be taken to analyze the problem:

1. If the unit knocks only when idling, check piston end clearance.
2. If the unit knocks when it is loaded, check for cylinder wear.
• The piston or piston rings may have worn a ridge in the bore.
• Check for a loose piston. Remove a suction and discharge valve
and bar the unit over. Using wood to restrain the piston, watch for
movement.
• Check end clearances with a lead wire and notice variations in
readings.

Frame or Running Gear Knocks

Possible causes of knocking in compressor running gear include the
following:

1. Loose flywheel or sheave
2. Loose or worn bearings
3. Crosshead pin-to-bushing clearance too large
4. Mechanical packing loose in gland
5. Excessive crosshead-to-guide clearance
6. Connecting rod hitting end of piston rod in crosshead
7. Belts misaligned, causing motor rotor to "weave" and bump
8. Unit not level, causing motor rotor to "weave" axially and bump in
some manner. This problem is often related to belt alignment inac-
curacies.
9. Incorrect rotation of compressor.