Mechanical Problemsbearings

77. Excessive radial thrust in singlevolute pumps

78. Excessive axial thrust caused by excessive wear at internal clearances or, if used, failure or excessive wear of balancing drive

79. Wrong grade of grease or oil

80. Excessive grease or oil in rolling element bearing housings

81. Lack of lubrication

82. Improper installation of rolling element bearings such as damage during installation, incorrect assembly of stacked bearings, use of unmatched bearings as a pair, and so on

83. Dirt getting into bearings

84. Moisture contaminating lubricant

85. Excessive cooling of water-cooled bearings

TABLE 2C Diagnosis from appearance of stuffing box packing in centrifugal pumps

Symptom

Cause

Wear on one or two rings next to packing gland; other rings OK Wear on O.D. of packing rings

Charring or glazing of inner circumference of rings I.D. of rings excessively increased or heavily worn on part of inner circumference

Improper packing installation

Packing rings rotating with shaft sleeve or leakage between rings and I.D. of box. Wrong packing size or incorrectly cut rings Excessive heating. Insufficient leakage to lubricate packing or unsuitable packing Rotation eccentric

TABLE 2D Vibration symptoms and causes in centrifugal pumps

Vibration frequency

Cause

Several times pump RPM Twice pump RPM

Running Speed

Running speed times number of impeller vanes

One-half running speed Random low frequency Random high frequency Subsynchronous frequency at 70% to 90% of running speed

Bad rolling element bearings

Loose parts on rotor, axial misalignment of coupling, influence of twin-volute when gap is insufficient Imbalance of rotor, clogged impeller, coupling misaligned or loose Vane passing syndrome—insufficient gap between impeller vanes and collector vanes. This is also sometimes seen during operating with suction recirculation. Oil whirl in bearing

Internal circulation in impeller or cavitation

Usually resonance

Hydraulic excitation of resonance

TABLE 3 Check chart for rotary pump problems

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