PRACTICAL APPLICATION OF LUBRICANT OIL ANALYSIS FOR ELECTRIC POWER PLANT H. ODAHARA, Y. SASAKI, M. KAWABATA TRIBOTEX Co. Ltd., 45-7 Yamaguchi Nagakusa-cho, Ohbu-shi, 474-0052 JAPAN; e-mail: h-odahara@tribo.co.jp T. TAGUCHI TOKYO ELECTRIC POWER COMPANY 1-3 Uchisaiwai-cho, 1-chome Chiyoda-ku, Tokyo,100-0011 JAPAN SUMMARY Total lubricant oil analysis using wear particle analysis of ferrography combined with nominal oil analysis are commercially served for electric power plant maintenance of oil wetted component. This utility of TRIBO-DIAGONOSIS is presented on a hydraulic power plant and a simulated leak current test using actual machine.

Keywords: Wear analysis, Power Plant Maintenance, Lubricant Analysis, Maintenance tribology, Ferrography

1 INTRODUCTION Monitoring of wear of sliding mechanical components has been empirically relied on the detection of sound, vibration, and temperature abnormally. One of much earlier failure detection can be made in the early 1970's by direct observation of wear debris containing in lubricant oil with their shape, size and population by the use of ferrography analyzer[1] by sampling of oil at some frequency, associated with element analysis technique like spectrometric analysis program analysis (SOAP) and X-ray micro-analysis.

A series of intensive work supported by the U. S. Navy has been summarized in well known "Wear Particle Atlas"[2] which facilitates to determine the operating mode within machine, allowing the prediction of imminent behavior of the machine mode.

However, it is necessary to establish a series of technical standard, i.e. sampling period and base line to alarm for the prevention of the machine failure.

We have established these criteria by achieving several experiments using actual hydraulic power plants before scrap by running under such severe condition as overload and poor lubrication, where wear particles are analyzed through ferrography and SOAP[3], of which results show that they are very specifically dependent to the operation condition ,especially for the lubricant oil of shaft supporting bearing like journal and thrust bearing.

These experience has now been expanded to thermal power, atomic power plant and auxiliary power equipment like support bearing of air-heater system of a thermal power plant, coal mill roll, shaft support bearing of condensate pump of atomic plant and so on.

Here, it is shown two examples of lubricant oil analysis of journal bearing of white metal and steel shaft connecting between hydraulic generator and turbine in which special attention is paid to presence of severe wear and sphere particles as serious signal of machine failure.

Finally after these destruction experiment the journals bearing surface was inspected and confirmed the metal contact estimated from the ferrography particle analysis.

Empirically it is widely well known that the spherical particles of which generating mechanism is suspected from cavitation erosion, welding, grinding process and fatigue crack process.

In many industrial systems monitored so far, rolling bearing fatigue spalling has been preceded by the generation of quantities of steel spheres having diameters ranging between one and five micrometers. Fatiguing bearings have been estimated to generate several million spheres in the course of a failure[4]. Here, also, it is shown that even sliding surface of journal bearing produces the sphere particles of the same few micrometers during severe wear condition, but their origin of mechanism is unknown to be sought in future.

Thus, the process to establish TRIBO-DIAGONOSIS system is introduced.

2 TRIBO-DIAGONOSIS Tribotex Co. has been engaged in wear particle analysis in the past fifteen years through the analysis tools as shown in Table 1. For the diagnosis of machine condition it is convenient including healthy check of lubricant oil for the general diagnosis of machine. The total amounts of samples have been dealt more than 50,000 cases with comment of machine diagnosis and oil or filter replacement and filtering. These total analyze process is called as TRIBO-DIAGONOSIS and its precision is acquiring more accurate each year by accumulation of many diagnoses.

Table 1. TRIBO-DIAGONOSIS Oil Analysis Wear diagnoses Oil analysis Ferrography Viscosity

SOAP Water Content

SEM Total Acid Number

Optical Microscope Contaminant

Bicromatic " NAS class/particle counting

Polarized " NAS weight class

IR Absorption Spectroscopy

RBOT*

XMA Insoluble Content *Rotary bomb oxidation stability testing

3 TYPICAL CASE HISTORY OF OIL

ANALYSIS FROM HYDRAULIC GENERATOR PLANT

An example of the TRIBO-DIAGNOSIS for a hydraulic generator plant is presented, where the oil analysis do not show any anomaly except slight higher contaminants by gravity weight method, but within control region, as shown in Table 2. However, the ferrograph in Fig. 1 shows such many abnormal severe particles, which generated at severe wearing mode with normal rubbing particles. As many sphere particles are found aligning along strings, they are surely strong magnetic iron particles.

From the SOAP analysis tin content is determined as 77 ppm which is three times higher than the similar model of the machine. These results suggest that some failure has occurred at the journal bearing and shaft components and advised for inspection by disassembly of the bearing.

The inspection showed trace of eccentric wear on the lower surface of the journal bearing of white metal which confirmed the analysis result, but many sphere particles within few microns diameter, found at the ferrogram, unknown in their origin, but considered indicating serious signal of imminent failure.

Finally, the bearing was amended at the site and we could prevent a serious accident.

Table 2 . Oil Analysis Results Kinematic Water Content Total Acid Contaminant Viscosity (ppm) Number by Weight(mm2/s,40oC) ( mgKOH/g) (mg/100 ml) 55.8 58.0 0.03 4.95 * SOAP (ppm)Na P Fe Sn Zn B Al Si …….. Ba Pb2 1 5 77 2 none*NAS 108 class

Fig. 1 Ferrogram of Entry Region indicating iron sphere

strings, where mark (1) as Rubbing wear particle, (2) Severe particles and (3) Cutting wear particle

4 GENERATOR EXPERIMENT SIMULATED FOR LEAKAGE CURRENT

The shaft of generator is insulated with ground, but insufficient insulation causes leak current which often gives harm effect by electric pitting or corrosion to the bearing soft metal surface, subsequently inducing to break the oil film, i.e., potential to freeze the machine.

In this case it was found many traces looking like caused by electric corrosion on the white metal surface of the bearing. Also, abundant sphere particles were found by ferrography.

4.1 Experimental Procedure Shaft leakage current simulation experiment is conducted as illustrated in Fig. 2, where A. C. voltage is applied between the connecting shaft and the journal bearing cover through an insulated sliding transformer.

The main parameters of the bearing are shown in Table 3. Table 4 gives applied voltage in three steps from zero to 20 volt. In Fig. 3-1 the current and voltage change are shown in relation with time. During the test the oil samples are continuously picked up and analyzed by ferrography together with element analysis by Dissolution-SOAP method, in some case treated by XMA.

G eneratorFly w heel

Sa m pling bottle

Brush

Insulated

transform er

Journal

bearing

Insulated

layer

100 V

Slide

transform er

VA

P

Fig. 2 Block Diagram of Journal Bearing Test of

Leakage Current Simulation

Table 3 . Parameters of Journal Bearing Bearing and shaft diameter (mm) 155Bearing material White metal; Sb8-10,Cu5-6%with Sn Shaft material Low carbon steel; C<0.06, Mn 0.3-1.2%Mean shaft radial velocity (m/sec) 4.9Lubricant oil ISO VG68 turbine oil with additive

Table 4 . Step of Applied Voltage Applied Voltage (V) Step 1 5, 7, 10, 13 Step 2 15, 17, 18 Step 3 20

4.2 Experimental Result (1) STEP 1 case As shown in Fig. 3-1, the shaft current is not observed. Ferrograph at entry region shows few normal rubbing wear particles together with such abnormal particles as severe sliding wear, cutting wear, red oxide and dark-metallo oxide which aligned along strings, but their population is judged within normal region.

Severity index; Is in Fig. 3-2 is normal, where Is is defined as square optical density difference between entry point and 5 mm down the stream of the ferrogram and used as an indication of abnormal wear mode. Also, SOAP analysis indicates no trace of metal content lower than 0.1 ppm.

In Fig. 3-3 particle count measurement is plotted at several particle range from 5 to 100 µm (>100 µm).

0

5

10 15

20

25

30

35

40

45 50

55

60

0:00 0:30 1:00 1:30

Tim e (m in)

Shaft voltage (V)

-10

-8

-6 -4

-2

0

2

4

6

8 10

12

14 Shaft current (A)

　0 30 60 90

Current

V oltage

STEP 1 STEP 2 S TEP 3

Fig. 3-1 Shaft current and voltage

0

5

10

15

20

25

0:00 0:30 1:00 1:30

Tim e (m in)

Shaft current (A)

0

50

100

150

200

Severity index (Is)

Fe

Sn

Current

Ｉ s

STEP 1 STEP 2 STEP 3

　0 30 60 90

SOAP (ppm

)

Fig. 3-2 Severity Index (Is) and shaft current

0:00 0:30 1:00 1:30

Tim e (m in)

Num

ber of particle

(count/100ml)

　0 30 60 90

STEP 1 S TEP 2 STEP 3

5～15μm

15～25μm

25～50μm

50～100μm

100～μm10

2

101

106

105

104

103

108

107

Fig. 3-3 NAS class/ contaminant particle count/100ml

Fig. 4 Ferrogram of STEP 1 at entry region applied lower than 13 volt, where mark (1) as Normal rubbing

wear particle (2) Severe sliding wear particle (3) Cutting wear debris.

(2) STEP 2 case In this case the current is measured as abrupt flow, but no significant change is observed at severity index, Is and population of density shown in Fig. 3 and ferrogram in Fig. 5.

Also, SOAP indicates no content of metal element.

Fig. 5 Ferrogram of step 2 applied 15 to 18 volt,

where mark (1) as Rubbing wear particle (2) Cutting wear debris (6) Sand dust.

(3) STEP 3 case By applying 18 volt the current starts to flow and the color of sampling oil turns dark which looked from generation of many sphere particles as shown in the ferrogram of Fig. 6-1. Also, severity index and population of particle in Fig. 3-2 and 3-3, respectively, increase very drastically even at the larger size of particle even more than 50 µm. Further, SOAP analysis shows iron and tin content, which are main materials of shaft and bearing component, increase gradually and finally reached until 2.2 and 19 ppm for iron and tin, respectively, at the end of the experiment.

As illustrated in Fig. 6-2 SEM photograph of the same ferrography indicates that some sphere particle shows the apparent trace of partial melting with the range of particle diameter around from 0.6 to 10 µm.

Among some sphere particles like P2 in the SEM photograph, which are estimated that it is aggregated from several sphere particles to oval shape by welding.

By the element analysis in Table 5 the sphere particle of P1, P2 and P3 in Fig. 6-1 are constituted from mixing of iron and tin together with copper and lead which are component of the white metal of bearing.

Meas. point Fe Sn Cu PbP1 22 75.2 2.9 noneP2 10.1 83.3 3.6 3.0P3 4.4 93.6 1.5 0.6

Table 5. Elementary analysis of sphere particle through XMA (wt. %)

Fig. 6-1 Ferrogram of step 3 applied 20 volt, where mark

(4) as Red oxide (5) Dark-metallo oxide (6) Sand dust

Fig. 6-2 SEM photograph of Ferrogram with 100 µm

and 10 µm scale of magnification

4.3 Concluding of Experiment Analyzing the sphere particles, which look generated during the leak current process;

(1) The increment of metal particles is found very few and wearing of the journal bearing is not expected, when current flows abruptly (STEP 2), i.e., from inclusion of the contaminants on the sliding surface, but not proceeds to break the oil film. (2) If the current flows directly through the oil film, which induces sparking, where current and voltage fluctuate as in step 3 case of the experiment; the bearing surface erodes very quickly which enhances the wear condition. As this case the oil color is found very dark due to the generation of vast amount of the sphere particles.

(3) The sphere particle size ranges from one to ten micron meter order, though almost of the particles remain below 5 µm, but few particles are observed at the size of 10 to 15 µm. (4) The sphere particle is found by XMA analysis composed from the base materials of shaft and bearing, but their composition is not homogeneous. Also sphere particles increased drastically at step 3 case, but one can not observe both the severe sliding particles and cutting wear particles as found in step 1 and 2 case. Therefore it is reasonably concluded that the electric arcing fuses the white metal and the iron sphere particles are closed within them by plating like phenomena.

5 CONCLUSION The TRIBO-DIAGONOSIS, herein presented, would contribute a strong support to "Condition Based Maintenance (CBM)" by analyzing the equipment of status quo by general oil analysis based on the accumulated data including destruction experiment of actual machine.

This system contributes for extension of presently used "Time Based Maintenance (TBM)" period and has proven resulting in great reduction of cost at some equipment and plant. Some electric company has estimated cutback of EUR 800, 000 per unit during total machine life period by adopting this system.

Further application fields are expected for distributed small power system of gas turbine and Diesel engines.

In near future, this TRIBO-DIAGONOSIS system would be applicable to many public transportation systems like airplane and ship. Its applicability is, also, discussed together.

6 ACKNOWLEDGEMENT Mr. Shuichi Tazawa is gratefully acknowledged for reading this manuscript with fruitful discussions. 7 REFERENCES [1] W. W. Seifert and V. C. Westscot; A Method for the Study of Wear Particles in Lubricating Oil, Wear, 21(1972)22-42 [2] E. R. Bowen and V. C. Westscot, WEAR PARTICLE ATLAS, Naval Engineering Center(1976) [3] T. Akagaki, K. Kato and M. Kawabata; Condition Monitoring of Journal Bearing Using Ferrography and SOAP, TRIBOLOGIST, 39, (1994) 11, 979-986(in Japanese) [4] D. Scott and G. H. Mills; Spherical Particles in Rolling Contact Fatigue, Nature (London), 241(1973)115-116