Page 137 - Compression Machinery for Oil and Gas

124 SECTION II Types of Equipment

area on the shaft. Increasing vibration can be indicative of a number of prob-
lems. Rotor unbalance and rotor misalignment are two of the most common
causes of vibration. However, there are many other causes of high vibration.
Excessive radial vibration can lead to bearing distress and may cause babbitt
surfaces to fatigue or internal seal rubs to occur.
Axial position is measured using noncontacting eddy-current displacement
probes. In this case, the probes are arranged to monitor the rotor shaft end.
Changes to axial position can occur over a compressor’s normal operating range
and thrust is even known to switch directions. Increased movement beyond a
thrust bearing’s clearance envelope and into the alarm region is indicative of
a highly loaded thrust bearing where some elastic deflection is occurring. Fur-
ther movement could result in wiping of babbitted surfaces.
Bearing pads are instrumented for the purpose of health monitoring. Tem-
perature transducers are embedded in the bearing backing metal near the babbitt
bond line and bearing surface as specified in API 670. High bearing metal tem-
perature can be indicative of potential bearing failure. Bearing metal tempera-
ture that rises in an upward trend without corresponding change to load or speed
is also indicative of potential bearing failure. Bearing metal temperature at the
instrumented location may range from less than 328K for an unloaded inactive
thrust bearing up to 403K for a bearing operating near its design limits. There is
varying opinion with respect to metal temperature limitation. In general, the
manufacturer’s recommendation should be followed, especially for new equip-
ment lacking in historical data.
Monitoring of auxiliaries is also performed such as lube oil system pressures
and temperatures, seal gas differential pressure, and suction scrubber liquid
level. Lube oil supply pressure must be maintained within allowable levels
to ensure the required oil delivery to bearings. This supply pressure is not a stan-
dard value and will differ between vendors.
Performance monitoring can also be used for surge control, but can also be
used for predictive maintenance as well. Polytropic efficiency deviation is often
associated with worn labyrinth seals. Polytropic head deviation can be due to
changes in molecular weight or fouling of the compressor rotor. Power devia-
tion can be due to worn seals especially a worn balance piston seal or due to
molecular weight changes. Discharge temperature deviation can be due to worn
seals especially a worn balance piston seal. Such seal wear as described here can
occur when the compressor experiences high vibration excursions from process
upsets or operation in a surge condition.

Preventive Maintenance
Most maintenance and inspections on the actual compressor are carried out at
regular maintenance intervals during planned turnarounds also known as
planned outages. In clean services such as closed-loop refrigeration cycles, a
centrifugal compressor may operate continuously for 7–14years between turn-
arounds provided the compressor and related systems are well designed,