Page 76 - Machinery Component Maintenance

Maintenance Organization and Control for Multi-Plant Corporations 61

undesirable manufacturing technique which has been practiced for many
years. The magnetic particle and subsequent dye penetrant inspections
showed several cracks around the eye of the fifth stage impeller in a
multistage barrel compressor rotor installed in relatively clean hydrogen
service. Up to this point the overhaul had been a routine matter, but now
took on far more serious implications. It seems that this particular com-
pressor manufacturer had been in the habit of overspeed testing impellers
and then trimming the eye labyrinth area to size, thus weakening the most
critical structural area of the impeller. The explanation given (to compen-
sate for bore stretch during overspeed) is, of course, unacceptable. Most
manufacturers will now readily guarantee maximum allowable expan-
sion in the diameter of the eye of an impeller as a function of the diameter
before the overspeed test. This is the only acceptable way to buy com-
pressor impellers, either as part of a new machine or as a replacement
part.

Reassembly

Once the machine has been opened and all parts cleaned and inspected,
the reassembly procedure can begin. There are many critical phases in-
volved with this operation, one of the most important being care in han-
dling the rotor. Large heavy rotors (over approximately 2,500 Ibs) re-
quire special handling and, in some cases, special guide fixtures should
be fabricated to avoid damaging components. This is particularly neces-
sary with gas turbines which have many exposed, fragile parts. A solid
rotor cradle is also a very necessary item. Do not jeopardize your most
valuable spare part by failing to protect it during the course of an over-
haul or during transit to or from the storage warehouse.
When fitting housings and other components with multiple O-rings in
blind arcas, wc havc found that it is usually beneficial to first removc the
O-rings and fit the housing by hand to check the alignment of the assem-
bly. Blind dowels or concealed shims can be located in this manner with
pencil marks. The O-ring fits should be touched lightly with Grade 600
wet or dry emery paper to remove any burrs, and then checked carefully
by hand. Lubricate the O-rings with a suitable grease or oil. A cut 0-
ring, worth very little in itself, can bear heavily on the success of an
overhaul.
Bearing clearance is one of the most important checks during reassem-
bly. We have found that after several years of operation, the pads of a
tilting pad journal bearing will wear small depressions in the support ring
or housing which can open the clearance beyond specifications. Also, re-
placement pads may not be within tolerance. The only proper way to