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Chemical Process Industries are prime
candidates for use of oil analysis as an integral part of the
equipment reliability programs, primarily because of the many pieces
of rotating equipment employed to produce product. Examples of such
equipment includes complex extruder gearboxes, reciprocating
compressors, steam turbines and process pumps. Most of the large
critical pieces central to the process have large circulating
lubricant systems, well suited to comprehensive oil analysis with
emphasis on wear debris analysis.
Despite the criticality of this type of equipment in chemical
plants, their oil systems routinely go unchecked for condition. In
other situations, some lubricant vendor oil analysis is performed,
but this analysis for the most part is not adequate for the type of
equipment being tested. It is important that equipment specific
testing regimen be employed. Table 1 is the minimum suggested tests
that need to be performed.
There are other
considerations when working with equipment employed by the refinery
and petrochemical businesses. The major processes (gas compression,
fluids pumping, etc.) are performed by units designed under API 611
3rd Edition guidelines, where the machinery train is lubricated by
one circulating system. This poses a challenge to the equipment
specific oil analysis approach. The best way to handle this is to
consider an important underpinning of reliability theory- " The
chain is only as strong as its weakest link". Applying this to the
API guideline machinery train will determine the most sensitive
component to wear, oil condition and contamination. The weakest
component in many cases is the driver unit, typically a single stage
turbine, or gas fired engine. If oil analysis testing for condition
m monitoring is performed, it should be as if the entire train was a
turbine, or an engine.
The following case studies have been plucked
from our extensive files on critical equipment oil analysis. A
common theme in all three of these case studies is that not one of
the units chosen failed catastrophically, without warning, ie. There
all turned out to be "non-events". This is the hallmark of a
successful oil analysis program. The drawback of this of course is
that "non-events" tend to be taken for granted, and these "saves"
tend to be ignored by management who are looking for concrete cost
savings Real savings can be found by improving lubrication programs
and going to condition based oil change outs rather than preventive
maintenance schedules, but the real savings are always where
failures are avoided.
Case Study 1:
Location: Pasadena, TX
Plant: Plastics Resin Complex
Equipment: Extruder Drive
Motor
Make/Model: GE 291R471 (6000 HP)
Oil analysis Recommendation:
Suspect slinger ring /bearing wear, due to sand/dirt
contamination
A variation on the journal bearing found on high
horsepower industrial motors is the ring oiled sleeve. The rings
(slingers) sit on the shaft and rotate with it, splashing oil to the
top of the shaft. In forced lubrication systems, these slingers
become redundant. However, in the event of an emergency trip, the
force feed lubrication is lost, and these slingers act as a safety
device during the coastdown to prevent lubrication
starvation.
The slingers are generally made of clock brass
(61-5-35.5-3 Cu-Zn-Pb), or general duty bearing bronze (80-10-10
Cu-Zn-Pb), and are considered ‘soft’ materials compared to the
carbon steel shaft. The sleeve bearing is babbitt lined (generally
83-8-8 Sn-Sb-Cu). The slinger rings can ‘stick’ occasionally,
causing them to wear abnormally against the rotating shaft,
releasing a large amount of severe nonferrous debris into the
circulating oil.
Abnormal slinger wear was suspected in an
extruder main drive motor by the PdM engineers at a large
petrochemical complex in Pasadena, Texas. The 6000 HP motor has a
forced-feed lubrication first sent the lab an oil sample in February
1993, and the RFS technique detected serious wear coming from the
slingers. Ferrography and MilliporeÔ Patch Test confirmed the
presence of large nonferrous particles.
It was decided to monitor the lubricant on a
monthly basis. (Fig 2) displays the wear trend over the year. The
RFS results adequately reflected the serious wear that was
occurring. Severe brass wear particles up to 150 m m in diameter
were found on the ferrograms throughout the condition monitoring
period (Fig 1).
Figure 1.
Micrograph X500 showing brass wear particles ³ 50 m m
diameter
fig. 2
Oil analysis
results prompted an oil change in May 1993. It was hoped that the
concentration of debris within the system would be reduced, thereby
minimizing the risk of damage to other components in the lubrication
cycle, until a detailed inspection of the bearing could be carried
out at the next scheduled maintenance overhaul. When this occurred
in October, the slingers were found to have substantial wear, and
debris from the abnormal wear on the rings (Fig 3) contributed
substantially to scoring of the babbitt liner on the sleeve section
(Fig 4).
Figure 3. Slinger Ring
Assembly
Figure 4. Scoring of Babbitt Sleeve
Due to Wear Particles
It was decided to reinstall the
bearing, change the oil and continue monitoring on a regular basis.
Replacement slingers were machined in-house during the October
overhaul for about $2,500, a fraction of their normal cost. An
unscheduled shutdown and resulting disruption to the process line
was avoided. The PdM engineers have approached the motor
manufacturer regarding design changes to reduce abnormal slinger
wear.
Case Study
2:
Equipment: Extruder Gear Box
Make &
Model: Werner Pfeiderer W/P Z58
Lubricant: Texaco Meropa
320
Sampling method & Location: Valve, Reservoir
Reservoir
capacity: 100 gallons
Cost of New unit: $120,000
Lead time to
delivery: 3 months
Estimated lost production time:
$432,000
Oil analysis cost for 1996 for this unit:
$315
A Tennessee plastics
facility began using an oil analysis program on their critical
extruder gearboxes in 1994. Product demand was high, and the
reliability team was concerned about unscheduled work stoppages.
Vibration analysis was also part of the program. Samples were taken
on a monthly schedule, and the oil analysis test package performed,
to determine wear, and if further analysis was needed. Oil analysis
(and vibration analysis) showed normal operation and a baseline
trend was established. In Feb 1996, spectrometric analysis showed
abnormal increases in iron and aluminum with an increase in the
level of silicon. Recommendations were to filter the oil to remove
sand/dirt contaminants, and inspect the gearing for signs of
abrasive wear. The oil was changed, and the next sample revealed a
dramatically reduced level of wear and contaminant debris. The same
problems returned in April (see Fig 5) and similar recommendations
issued. New sampling valves were installed at this time, to ensure
that poor sampling concerns were minimized. Results for oil analysis
wear were so high in July, that the Reliability engineers took the
unit out of service.
fig. 5
It is worth noting that vibration analysis did
not indicate problems. As part of the root cause analysis
investigation as to why the wear was high, ferrographic analysis
revealed severe sliding ferrous gear wear particles up to 50µm
diameter, and large amounts of aluminum particles(Fig 6). The oil
filter debris was also analyzed, and it revealed aluminum particles
up to 200 µm diameter present. Visual inspection of the gear housing
correlated with what was found in the oil i.e. that the thrust
bearings on the output shaft were worn away well beyond operational
tolerances.
fig. 6
It was suspected that there was movement of the
shaft in the axial direction. There was no redundancy for this unit,
and so after a quick check to see that product was not affected
directly by the damage, it was placed back in service and a new box
ordered. A new unit was quoted at $70,000, with a three-month lead
time. In summary, the oil analysis and combination wear debris
analysis pointed to serious problems, well enough in advance that
the company could prepare for an expected failure. This is borne out
by the potential losses in production time that would have arisen of
such a problem did occur. The example also illustrates the very high
benefit to cost ratio of oil and wear particle
analysis.
Case Study
3:
Location: Framingham, MA
Plant:
Pharmaceutical Chemicals
Equipment: Gas Compressor
Make &
Model: Dunham & Bush 300 ton
Oil samples where sent to
NTS on a routine program from a Dunham-Bush screw compressor, which
is used to compress process gas. The compressor is a wet-screw
compressor in which the lubricant is in contact with the process gas
and it is used to lubricate the shaft, rotors and thrust bearings.
The lubricant also provides sealing and absorbs heat generated
during operation. In wet-screw compressors gas/oil separators are
used to separate the process gas from the lubricant before it is
cooled down in the heat exchanger and circulated back to the
compressor.
Using oil analysis, condition of the lubricant
as well as the condition of the machine was monitored.
Tests performed were:
Atomic Emission
Spectroscopy
Provide information on fine and
dissolved particles, for monitoring fine wear metals, contaminants
and additives in the oil. This test provides information on the
condition of wear in the machine as well as condition of the
lubricant. Depletion or concentrations of the additive package
elements of the lubricant are monitored.
Rotrode Filter
Spectroscopy
This test method detects large, coarse
wear metals and contaminants in the lubricant. Very fine dissolved
metals and additive elements are not detected by this method. Large
wear particles and contaminants, usually with particle diameter
greater than six microns are detected.
Viscosity
The
Kinematic viscosity (ASTM D 445) at 40ºC is performed. Viscosity is
very important as it determine the load caring capacity of the
lubricant. Dissolved process gas may decrease lubricant viscosity.
Viscosity also provides information on the condition of the
lubricant. Highly oxidized and acidic oil tend to thicken.
FT-IR Infrared
analysis
FT-IR is used for detecting organic
contaminants, water and oil degradation products. The high
temperature generated during compression of the gas can cause
increased oxidation of the lubricant. Highly oxidized lubricants
become acidic and corrode important machine elements. Therefore
FT-IR detects oxidation, nitration, sulfation and
contaminants.
Total Acid
Number
The total acid number (TAN) is a titration
method used to determine the acidity of the lubricant.
Alarm
Limits
The Original Equipment Manufacturer (OEM) data
sheet provided by the customer was used as the alarming limit in the
condition monitoring of the compressor.
Monitoring the condition of the thrust bearing,
rotor and the shaft using oil analysis is very important. Any damage
in the bearing will cause misalignment in rotors and cause severe
wear of the rotors and the shaft and also any damage of the rotors
or the shaft will cause severe damage of the bearing. Improper
installations of the rotors or insufficient clearance can cause
damage of the rotors, which will lead to severe damage of the
bearing. Using oil analysis it is possible to monitor the condition
of the different components of a compressor. High iron reading will
indicate shaft and rotor wear. If the metallurgy of the shaft and
rotor is known it is possible to identify specifically which part is
wearing. Different alloys of steel are differentiable using
Ferrographic analysis and temper color changes observed by heating
the ferrogram to different temperatures. The plain bearing will
generate nonferrous wear particles, usually copper and/or babbitt
alloy particles.
Table 2 shows the spectrometric wear trend for
the compressor. Note from the wear trend, the fine copper reading is
well outside the acceptable OEM limit. This was found to be due to
the dissolved copper from the heat exchanger tube.
Fine and Coarse Spectrometric
Analysis Result (ppm)
| Date Tested |
10/11/96 |
5/23/97 |
9/8/97 |
11/25/97 |
3/13/98 |
OEM
Acceptable |
OEM
Unacceptable |
| Fine Fe |
14 |
* 112 |
8 |
11 |
12 |
6-15 |
>15 |
| Fine Pb |
2 |
2 |
0 |
1 |
0 |
2-5 |
>5 |
| Fine Cu |
* 24 |
* 27 |
* 20 |
* 27 |
* 19 |
6-15 |
>15 |
| Fine Sn |
2 |
1 |
0 |
0 |
0 |
2-5 |
>5 |
| Fiine Al |
5 |
9 |
0 |
0 |
0 |
6-10 |
>10 |
| Fine Si |
7 |
5 |
1 |
1 |
1 |
10-15 |
>15 |
| Fine Ca |
1 |
0 |
0 |
0 |
0 |
0-40 |
>40 |
| Fine Ba |
0 |
4 |
0 |
4 |
2 |
0-40 |
>40 |
| Fine P |
4 |
21 |
0 |
0 |
0 |
0-40 |
>40 |
| Fine Zn |
3 |
8 |
0 |
0 |
1 |
0-40 |
>40 |
| Coarse Fe |
5 |
19 |
1 |
1 |
2 |
6-15 |
>15 |
| Coarse Pb |
0 |
0 |
0 |
0 |
0 |
2-5 |
>5 |
| Coarse Cu |
2 |
* 31 |
1 |
0 |
3 |
6-15 |
>15 |
| Coarse Sn |
0 |
0 |
0 |
0 |
0 |
2-5 |
>5 |
| Coarse Al |
2 |
22 |
3 |
0 |
3 |
6-10 |
>10 |
| Coarse Si |
1 |
1 |
1 |
0 |
0 |
10-15 |
>15 |
(Table
2)
The coarse spectrometric
analysis (R.F.S) trend indicates low copper in parts-per-million
concentration in the oil, except for sample tested on 5/23/97. The
fact that the copper concentration in the coarse reading is low
indicate the particles detected by the fine atomic emission
spectroscopy are very fine in size and dissolved particles. Sample
tested on 5/23/98 clearly indicates the presence of coarse copper
particles in the oil, indicating wearing of copper alloy component.
A dramatic increase in fine iron concentration from 14ppm in the
previous sample to 112ppm confirms abnormal operation of the unit.
Based on the result indicated by the coarse (RFS) reading, the
customer was given a recommendation not only to inspect the shaft
and rotor but also the bearing of the compressor due to the coarse
copper reading.
Conclusion
Three case histories show how significant cost
savings were acheived by utiliizing a standard oil condition
monitoring program. In all cases the combination of traditional
spectroscopy (SPECTRO) and the new Rotrode Filter Spctroscopy (RFS)
was sufficient to detect severe wear in both ferrous and nonferrous
parts.
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