Page 138 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 125

manufactured, installed, and operated. However, many centrifugal compressors
operate in less than ideal services with less than optimal installation and oper-
ation. In such cases, more frequent turnarounds are required. Some causes of a
shortened turnaround cycle may involve corrosion, fouling, and the mainte-
nance of other machinery or systems. As previously discussed, maintenance that
is conducted prior to a planned turnaround is typically centered on predictive
maintenance based on condition monitoring.
During a planned turnaround, the centrifugal compressor unit should
undergo a general and thorough inspection. At a minimum, bearings, couplings,
and compressor shaft endseals are inspected during the turnaround. Auxiliary
lube and seal systems also require maintenance of miscellaneous items includ-
ing filters, coolers, pumps, etc.
Bearing inspection starts with removing the journal and thrust bearings from
the compressor. In almost all compressor designs, these are externally accessi-
ble through their own covers. During visual inspection, the bearings are exam-
ined for signs of wear or distress. The journal and thrust areas of the rotor will be
accessible and also need to be examined for signs of distress such as scoring.
The coupling should be inspected for wear and cleanliness. Coupling inspec-
tion is typically centered on the flexible elements. Any broken parts, yielding,
buckling, corrosion, or scratches should lead to replacement. Couplings can be
returned to the manufacturer for recertification.
Inspection of the compressor internals requires further disassembly. During
a major outage, the rotor should be taken out of the casing to have all deposits
removed as well as to determine the degree and rate of corrosion or erosion. The
casing should be examined and cleaned if necessary. Labyrinth seals should be
examined and replaced if necessary.
The auxiliary equipment frequently requires as much or more maintenance
work than the main compressor and driver. Maintenance of oil pumps, oil
coolers, and filters are often required. Oil pumps are typically arranged to be
serviced. Oil cooler tube bundles are designed to be pulled in one direction
to facilitate cleaning. And, oil filters are arranged such that they are easily
drained and replaced.

Sparing and Availability
Critical equipment is defined as one that would shutdown a process unit in case
of failure. For critical pieces, it is common that entire units, or possibly part
spares be dedicated for that unit to reduce downtime. As such, it is obvious that
a cost-benefit analysis be performed in to weigh the risk of equipment failure on
lost process time to determine the level of sparing or redundancy. Of course, this
analysis is performed more readily in light of operational experience on a given
piece of equipment to evaluate the likelihood of a failure. In some circum-
stances, critical units may be placed in parallel such that they can be brought
offline without any disruption to the process.