ML20140D787
| ML20140D787 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 01/17/1986 |
| From: | Warembourg D PUBLIC SERVICE CO. OF COLORADO |
| To: | Johnson E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| References | |
| P-85490, NUDOCS 8602030114 | |
| Download: ML20140D787 (6) | |
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Company of Colorado 16805 WCR 19 1/2, Platteville, Colorado 80651 January 17, 1986 Fort St. Vrain Unit No. 1 P-85490 Regional Administrator r egulatory Commission m22E 611 Ryan Plaza Drive, Suite 1000 Arlington, Texas 76011
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Attention:
Mr. Eric H. Johnson Docket No. 50-267
SUBJECT:
CRDOA Bearing Test Results
REFERENCE:
PSC Letter, Warembourg to Johnson, Dated 6/13/85 (P-85201)
Dear Mr. Johnson:
Enclosed is a summary of the physical test programs conducted to determine operability of replacement bearings in the control rod drive and orifice assemblies.
This letter supplements the
" Engineering Evaluation of CRD0A Bearings" Report, Issued June 11, 1985.
The results of the testing described herein support the pervious conclusion that the replacement bearings' performance is acceptable.
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-2 The bearing test programs, conducted at GA Technoiogies, Inc. SKF Industries, and at Fort St. Vrain, involved cycling CRD shim motor bearings of original and replacement design under actual and simulated loading conditions and observing the mechanisms and extent of bearing degradation.
Shim motor bearings were selected for testing because cyclic requirements and the potential for a f fecting CRD0A performance exceed that of any other bearing by a factor of 10 or more.
The stated objective of comparative testing of original and replacement designs was impeded by unsatisfactory bearing performance during rig testing, later attributed to extraneous vibrations in the test rigs not present in the actual application.
This situation was resolved by demonstrating the successful completion of test goals, in two tests at Fort St. Vrain, using replacement bearings in actual shim motor installations.
In one test, replacement bearings in a shim motor were driven for the equivalent of six (6) test cycles (a test cycle being defined as the estimated cyclic duty of the regulating rod for one fuel cycle, or 8.1 million revolutions of shim motor bearings) on a test rig under back-EMF induced loading.
In the other test, a full set of replacement bearings, mounted in a CRDOA, completed 1000 successive scrams or 1.3 test cycles without failure.
All testing was performed in a helium atmosphere at ambient pressures and temperatures.
Individual test results are presented in Attachments I, II and III.
Conclusions From Testing The physical testing at GA Technologies, Inc. and at Fort St. Vrain demonstrates that the cyclic lifetimes of the replacement CR0 shim motor bearings exceed any credible requirements of service duty.
The direct comparison cf original and replacement design performance conducted at SKF Industries indicates the lubricating properties of both bearings are comparable and that the presence of a dry film lubricant enhances the performance of the bearing.
The wide variation in total cyclic lifetimes exhibited in the total test program does identify a certain susceptability of the bearing design to extraneous vibration in mounting configurations.
It is significant,
- however, that the most satisfactory results were obtained using test configurations most resembling the actual application.
We conclude therefore, that the replacement design is acceptable for use in the control rod drive and orifice assembly application.
Very truly yours, A0. lY /Wtw (o.a D.W.Warembourg,kanager, Nuclear Engineering Division DWW/BEB/ksc
ATTACHMENT I GA TECHNOLOGIES TEST PROGRAM In the test program conducted a GA Technologies, Inc., pairs of test bearings were housed in weighted pendulums and mounted on a rotating shaft.
Rotational displacements of the pendulums, continously monitored by proximity probes, were equated to drag torques transferred between bearing races.
A shaft speed of 1730 RPM was used for the test runs.
In the rig checkout phase, replacement bearings completed.85 test cycles during intermittent operation with an applied radial load of 65 lbs.
No significant bearing degradation was noted in this phase, however, foam damping installed in the rig to eliminate harmonic oscillation of the pendulums affected the free vibratory characteristics of the pendulums.
In Phase II
- testing, undamped vibrations of the pendulums induced dynamic forces which caused seizure of a replacement bearing and displacement of the Bearite rings in an original bearing after.35 test cycles.
Following an extensive dynamic analysis of the test rig, modifications were performed which reduced axial vibrations from 1.4 to 0.6 G's.
Harmonic oscillations were eliminated by attaching the pendulums to strain gage force transducers to monitor torque.
Subsequent test runs were marked by a significant improvement in bearing performance.
Replacement bearings successfully completed 6 test cycles under a calculated radial operating load of 15.3 lb. per bearing pair.
The test bearings turned freely after the test and suffered no apparent damage.
In an additional test, original bearings completed slightly more than one test cycle under a 15.3 lb.
per pair radial load, with no apparent degradation in performance.
ATTACHMENT II SKF INDUSTRIES TEST PROGRAM The engineering and research division of SKF Industries, a major manufacturer of anti-friction bearings, was contracted to perform experimental and analytical evaluations of the original and replacement bearing designs.
The experimental evaluations included rig testing of two pairs of replacement bearings and one pair of original bearings, as well as metrological and metallurgical examinations of one replacement bearing (from the first rig test) and one original bearing (removed from a control rod drive with questionable performance).
The bearing test rig incorporated a rotating shaft with a free floating housing, slung with weights to apply the specified radial load.
Bearings were mounted in opposed pairs, with a spring washer from a shim motor providing axial loading.
Torques transferred from inner-to-outer races were measured with a strain gage force transducer coupled to the bearing housing.
Temperatures of the environment and at the bearing outer races were sensed using thermocouples. A computer monitured torque and temperature values intermittently and was programmed to stop the test should a specified value of drag torque be reached. A drive shaft speed of 2450 r.p.m.
was used, with the direction of rotation reversed every 30 minutes.
The test rig was enclosed in a housing filled with helium at atmospheric pressure and temperature.
A total of three test runs were performed on the rig.
In the first test, a pair of replacement bearings were loaded with 66 lb.
of radial load, corresponding to maximum startup loads in a control rod drive mechanism.
The test ran for.47 test cycles before the cutoff drag torque of 30 oz-in, was reached.
The cutoff drag torque was arbitrarily set 25% greater than the calculated stall torque of the CR00A to explore bearing degradation rates.
Rig tests indicated that bcaring degradation increased sharply from the stall torque to the cutoff torque, usually within ten minutes of continuous operation. A second test was performed with replacement bearings loaded to the calculated operating load of 15.3 lb.
per pair.
After.12 test cycles the test rig tripped on indication of excessive torque.
A third test with unused original bearings under a 15.3 lb. radial load also tripped after a run time corresponding tu
.10 test cycles.
After the test runs, an additional test was performed using standard 203 bearings with no lubrication at all.
The test ran for 45 minutes or 1.5% of a test cycle before excessive drag torque tripped the test rig offline.
9
ATTACHMENT II (cont.)
SKF INDUSTRIES TEST PROGRAM The results of the examination of the failed bearings identified a lack of replenishment of the dry film lubricant as the primary failure mechanism in the test bearings. As indicated in the test run with unlubricated bearings, the presence of the dry film lubricant does extend bearing life.
The relatively short cyclic lives experienced in these tests compared to the GA Technologies tests (Attachment I) and the PSC tests (Attachment III) and the actual application suggest that the test rig did not adequately represent the application.
In the SKF test report, it was noted that the use of a free floating hcusing allowed a much greater vibratory response than a shim motor installation. An increase in radial loading may have damped the resulting vibrations somewhat, accounting for the disparity between radial load and cyclic life witnessed in test runs 1 and 2.
Because of the successful tests that had been conducted by both PSC and GA, the decision was made that refinement of the SKF test rig to better represent the application was not warranted.
e ATTACHMENT III PSC TEST PROGRAM In response to questions raised about the representation of a shim motor application using a test rig with calculated loads, Public Service Company initiated a two part test program to cycle bearings installed in an actual shim motor.
In one test, a shim motor was mounted on a test stand and driven by a three stage gear train coupled to an A-C motor.
Rotational resistance to the motor shaft was applied by attaching a Y-connected capacitor bank across the shim motor phases, braking the motor via the developed back electro-motive force.
This system is identical to the system used to retard the descent of control rods into core in the CRDOA.
By driving the shim motor at the approximate speed of scram operation, similar loads are induced.
To monitor the shim motor bearings, thermocouples were installed at the outer race of the brake end bearings.
The test was performed in a helium atmosphere.
Two tests were run on the rig.
In the first test, a set of replacement bearings logged 5.85 test cycles under continuous operation before the test was terminated.
Maximum static drag torques on the motor shaft registered 4-6 oz-in. before the test and 10-14 oz-in, afterward. An equivalent static torque of 24 oz-in, at the motor shaft is required to stall a control rod drive.
Temperatures at the outer race of the brake end bearing average 120 degrees fahrenheit, increasing slightly throughout the test to a maximum of 125 degrees Fahrenheit.
In the second test, a set of original bearings completed.96 test cycles before a failure of one of the gear train components caused termination of the test.
Temperatures at the brake end bearing outer race ranged from 134 to 138 degrees fahrenheit.
The second part of the PSC test program involved the repeated scramming and withdrawal of control rods in a refurbished control rod drive and orifice assembly with a full set of replacement bearings.
The test was conducted in an equipment storage well, enclosed to provide a helium atmosphere at 2-3 psig.
The acceptance criteria for the test was the ability of the CR00A to scram after 1000 scram / withdrawal cycles, which was demonstrated successfully.
The estimated cyclic duty of a control rod drive in regulating rod service for one refueling cycle corresponds to 761 scrams, w
.