Page 259 - Reciprocating Compressors Operation Maintenance

244 Reciprocating Compressors: Operation and Maintenance

When setting the traveling pin in the desired position, depress the
dial indicator so the needle makes at least one revolution, lock the
indicator neck, and set the dial face so that the needle reads zero.
• Reading the runout. Bar the unit over 360° (one complete revolu-
tion). The dial will go to a limit when the unit has been barred 180°
(the piston should be at its maximum inboard stroke). This reading,
whether plus or minus, is the total runout. The unit is then barred
over the remaining 180° (piston is now at its maximum outboard
stroke) to ensure that the needle returns to the zero setting. If the nee-
dle does not return to zero, redo the measurement after checking the
indicator arrangement for tightness and making sure it is well
secured at its base.
• Vertical runout. This reading generally indicates how well the com-
pressor piston and rod are in line with the cylinder and frame. We
like to see a cold runout of a minus on the piston end. The expansion
of the piston while in service is thus anticipated.
»A positive reading means that the piston is high in relation to the
crosshead, and a minus means that the piston is low, as desired, in
relation to the crosshead. If the piston is high, or if it is lower than
the desired limit, adjustment must be made in accordance with the
compressor manufacturer's instruction manual.

Figure 4-10 shows allowable piston rod runout (vertical and horizon-
tal) as a function of cylinder running clearance (Cooper-Bessemer).
Cylinders may be operated if they fall within these limits. If, after a peri-
od of time, ranout exceeds allowable, this is an indication that wear has
occurred and maintenance is required.

PROXIMITY PROBE METHOD OF MONITORING

A more efficient method of measuring rider band wear is to use a prox-
imity probe, mounted vertically to the packing case, to measure the posi-
tion of the piston rod. Probe gap voltage can then be read with an elec-
tronic rod drop readout device. Monitors such as Bently-Nevada's
Six-Channel Rod Drop Monitor use a Keyphasor®, probe to provide a
once-per-revolution crankshaft pulse. This pulse is used as a reference, so
an instantaneous rod position can be displayed. There are three main
advantages to this method of rod drop measurement.
First, by taking readings at only one point in the stroke, the effects of
scratches, wear, or rod coatings are minimized. Second, the most effec-