Oil Analysis Avoids Failures in the Chemical Process Industry

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 particles3 50 m m diameter

Figure 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.

Figure 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.

Figure 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 BTS 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)

Figure 5

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.