Page 349 - Reciprocating Compressors Operation Maintenance
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334- Reciprocating Compressors: Operation and Maintenance
Ultrasonic
TOG BDC TDC
FIGURE 5-8. Vibration and ultrasonic trace show discharge valve leaking,
ULTRASONIC vs CRANKANGLE PATTERNS
Ultrasonic is inaudible vibration above the frequency range of the
human ear. The frequency range of the ultrasonic pickup is 35,000 to
45,000 cycles per second. In this frequency range, leaking gases, certain
types of abrasion, and clicking-type vibrations are easily detected. In a
manner similar to the lower frequency vibration pickup (100-10,000
cps), the ultrasonic trace will show valve malfunctions, blow-by, scuff-
ing, leaks, and inappropriate valve action.
One might question using both vibration and ultrasonic transducers.
Neither device alone will detect all compressor malfunctions. Lower fre-
quency knocks will only be seen by the accelerometer. The "hissing" of
leaking gases is better seen using ultrasonics.
As with the vibration pickup, valve problems are diagnosed by moving
the ultrasonic probe from valve cover to valve cover. Ring blow-by can
best be seen next to a lubrication port at mid cylinder. Packing problems
on double-acting cylinders are seen at the joint between the cylinder and
distance piece, where the packing case is located (see Figures 5-5
through 5-8).