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).
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