Page 33 - Compression Machinery for Oil and Gas

Equipment Overview Chapter 2 23

engine, gas turbine, or electric motor). It can be used to identify best practices
and opportunities for improvement.
Reliability is a relative percentage-based measure of planned versus
unplanned downtime for a given compressor it measures how often the com-
pressor is down unplanned for repairs, or planned to be preventatively main-
tained. Although each company defines it differently, annual reliability is
most simply calculated as:
Annual reliability ¼ 8760h unplanned down hours + planned down hoursÞ=
ð
ð 8760 planned down hoursÞ:
Availability is a slightly different measure of unscheduled downtime for a
given compressor. It measures whether the compressor is ready for use when
needed. Although each company defines it differently, annual availability is
simply calculated as:
Annual availability ¼ 8760h unplanned down hoursÞ=8760:
ð
Generally, and as supported by industry studies, gas turbine or electrical
driven centrifugal compressors have proven to be lower cost in overall mainte-
nance and have higher reliability/availability factors than internal combustion
engine driven reciprocating compressors.
The following are typical observational comparisons often used in operating
gas turbines and centrifugal compressors:
1. output power versus control temperatures,
2. output power versus fuel flow rate or fuel pressure (increases in fuel flow
rate or fuel pressure can be an indication of compressor fuel nozzle or meter
calibration issues),
3. compressor discharge pressure versus percent gas producer speed
(decreases in compressor discharge pressure at a given gas producer
speed can indicate fouling of the compressor or an increase in turbine nozzle
area),
4. vibration level (increases in vibration at constant operating conditions can
indicate a number of problems such as compressor internals fouling, bearing
damage, mechanical damage, and loose connections), and
5. exhaust temperature variation (variation across the exhaust collector can
indicate: fuel distribution problems, combustor problems, or air/fuel mixing
problems).
Factors that may adversely affect maintenance intervals on gas turbines include
these factors:

1. starting/Stopping frequency (rule of thumb says one start/stop¼30 run
hours equivalence),
2. frequent rapid load changes, and
3. condition of inlet air to the turbine