070.550-IOM (JUN 2016)
Page 20
NGC GAS ROTARY SCREW COMPRESSOR
MAINTENANCE
VIBRATION ANALYSIS
Periodic vibration analysis can be useful in detecting bearing
wear and other mechanical failures. If vibration analysis is
used as a part of your preventive maintenance program,
take the following guidelines into consideration.
1. Always take vibration readings from exactly the same
places and at exactly the same percentage of load.
2. Use vibration readings taken from the new unit at start-
up as the base line reference.
3. Evaluate vibration readings carefully as the instrument
range and function used can vary. Findings can be easily
misinterpreted.
4. Vibration readings can be influenced by other equipment
operating in the vicinity or connected to the same piping as
the unit.
EARLY FAILURE DETECTION
Rotary Screw Compressors, like all rotating machinery, will
eventually require service. A regular, disciplined, periodic or
onboard (continuous) vibration analysis program can detect
most bearing failures in the very early stages.
In rotary screw compressors, early stages of fatigue are
relatively easy to detect when using rolling element bearings
instead of sleeve bearings. This allows a scheduled shutdown
for compressor repair before damage progresses to the point
that requires rotor or housing replacement.
When bearing fatigue is detected in time, the compressor
rebuild will be much less expensive. This is because only the
bearings will need to be replaced and not the expensive ro-
tors and housings. Failure detection in the early stages also
allows the repair to be scheduled to avoid site disruption to
the plant’s operation.
Normally, compressors can continue to run for months after
the first signs of bearing damage are detected (if vibration
monitoring is utilized to track the progression of the damage)
without risking catastrophic failure.
OIL QUALITY AND ANALYSIS
High quality and suitable oil is necessary to ensure
compressor longevity and reliability. Oil quality will rapidly
deteriorate in systems containing moisture and air or other
contaminants. In order to ensure the quality of the oil in the
compressor unit:
1. Only use Frick oil or high quality oils approved by Johnson
Controls - Frick for your application.
2. Only use Frick filter elements. Substitutions must be
approved in writing by Johnson Controls - Frick engineering
or warranty claim may be denied.
3. Participate in a regular, periodic oil analysis program to
maintain oil and system integrity.
OPERATING LOG
The use of an operating log as included in this manual
permits thorough analysis of the operation of a system by
those responsible for its maintenance and servicing.
Continual recording of gauge pressures, temperatures, and
other pertinent information, enables the observer and
serviceman to be constantly familiar with the operation of
the system and to recognize immediately any deviations
from normal operating conditions. It is recommended that
readings be taken at least daily.
TROUBLESHOOTING GUIDE
Successful problem solving requires an organized approach
to define the problem, identify the cause, and make the
proper correction. Sometimes it is possible that two
relatively obvious problems combine to provide a set of
symptoms that can mislead the troubleshooter. Be aware of
this possibility and avoid solving the “wrong problem”.
ABNORMAL OPERATION ANALYSIS & CORRECTION
Four logical steps are required to analyze an operational
problem effectively and make the necessary corrections:
1. Define the problem and its limits.
2. Identify all possible causes.
3. Test each cause until the source of the problem is found.
4. Make the necessary corrections.
The first step in effective problem solving is to define the
limits of the problem. The following list of abnormal system
conditions can cause abnormal operation of the NGC
compressor:
1. Insufficient or excessive
gas load.
2. Excessively high suction pressure.
3. Excessively high discharge pressure.
4. Excessively high or low temperature coolant to the oil
cooler.
5. Excessive liquid entering the compressor (slugging).
6. Insufficient oil cooling.
7. Excessive oil cooling
8. Incorrect gas line sizing.
9. Improper system piping.
10. Wrong operation of hydraulic operated slide valve.
11. Problems in electrical service to compressor.
12. Moisture present in the system.
Make a list of all deviations from normal compressor
operation. Delete any items, which do not relate to the
symptom and separately list those items that might relate
to the symptom. Use the list as a guide to further investigate
the problem.
The second step in problem solving is to decide which
items on the list are possible causes and which items are
additional symptoms. High discharge temperature and high
oil temperature readings on a display may both be symptoms
of a problem and not causally related.
The third step is to identify the most likely cause and take
action to correct the problem. If the symptoms are not
relieved, move on to the next item on the list and repeat the
procedure until you have identified the cause of the
problem. Once the cause has been identified and confirmed
make the necessary corrections.
