IR 05000321/2008008
| ML082550033 | |
| Person / Time | |
|---|---|
| Site: | Hatch  |
| Issue date: | 09/06/2008 |
| From: | Wert L D Division Reactor Projects II |
| To: | Madison D R Southern Nuclear Operating Co |
| References | |
| IR-08-008 | |
| Download: ML082550033 (28) | |
Text
September 6, 2008
Mr. Dennis Vice President Southern Nuclear Operating Company, Inc.
Edwin I. Hatch Nuclear Plant 11028 Hatch Parkway North Baxley, GA 31513
SUBJECT: EDWIN I. HATCH NUCLEAR PLANT - NRC SPECIAL INSPECTION REPORT 05000321/2008008, 05000366/2008008
Dear Mr. Madison:
On July 24, 2008, the Nuclear Regulatory Commission (NRC) completed a special inspection at your Edwin I. Hatch Nuclear Plant, Units 1 and 2. The enclosed report documents the inspection findings which were discussed on July 24 with you and other members of your staff.
On July 18, 2008, NRC Region II management established a Special Inspection Team using the guidance contained in Management Directive 8.3, NRC Incident Investigation Program. The Special Inspection Team was chartered to identify the circumstances surrounding the degradation of an emergency diesel generator (EDG) coupling on July 12, 2008, resulting in the 1B EDG being declared inoperable.
This inspection was performed in accordance with Inspection Procedure 93812, "Special Inspection," and focused on the areas discussed in the inspection charter described in the report. The inspection examined activities conducted under your licenses as they relate to safety and compliance with the Commission's rules and regulations and with the conditions of your licenses. The inspectors reviewed selected procedures and records, conducted field walkdowns, observed activities, and interviewed personnel. Based on the results of this inspection, no findings of significance were identified.
In accordance with 10 CFR 2.390 of the NRC's "Rules of Practice," a copy of this letter and its enclosure will be available electronically for public inspection in the NRC Public Document SNC, Inc. 2 Room or from the Publicly Available Records (PARS) component of NRC's document system (ADAMS). ADAMS is accessible from the NRC Web site at http://www.nrc.gov/reading-rm/adams.html (The Public Electronic Reading Room).
Sincerely,/RA/ Leonard D. Wert, Jr., Director Division of Reactor Projects Docket Nos.: 50-321, 50-366 License Nos.: DPR-57, NPF-5
Enclosure:
Inspection Report 05000321/2008008, 05000366/2008008 w/Attachments 1: Supplemental Information 2: Event Timeline 3: EDG Inspection Results Summary 4: Coupling Photographs 5. Background Information
cc w/encl.: (See page 3)
_________________________
G SUNSI REVIEW COMPLETE OFFICE Region II Region II Region II Region II Region II SIGNATURE Per Email Per Email Per Telecon Per Telecon NAME J. Brady P. Niebaum S. Crane C. Rapp S. Shaeffer DATE 9/5/08 9/6/08 9/5/08 9/5/08 9/6/08 9/ /2008 E-MAIL COPY? YES NO YES NO YES NO YES NO YES NO YES NO YES NO SNC, Inc. 3 cc w/encl: Jeffrey T. Gasser Executive Vice President Southern Nuclear Operating Company, Inc.
Electronic Mail Distribution
Raymond D. Baker Licensing Manager Licensing-Hatch Southern Nuclear Operating Company, Inc. Electronic Mail Distribution
L. Mike Stinson Vice President Fleet Operations Support Southern Nuclear Operating Company, Inc.
Electronic Mail Distribution
David H. Jones Vice President Engineering Southern Nuclear Operating Company, Inc. Electronic Mail Distribution
Moanica Caston Vice President and General Counsel Southern Nuclear Operating Company, Inc.
Electronic Mail Distribution
Steven B. Tipps Hatch Principal Engineer - Licensing Edwin I. Hatch Nuclear Plant Electronic Mail Distribution
Mr. K. Rosanski Resident Manager Oglethorpe Power Corporation Electronic Mail Distribution
Laurence Bergen Oglethorpe Power Corporation Electronic Mail Distribution
Bob Masse Resident Manager Vogtle Electric Generating Plant Oglethorpe Power Corporation Electronic Mail Distribution Arthur H. Domby, Esq.
Troutman Sanders Electronic Mail Distribution Dr. Carol Couch Director Environmental Protection Department of Natural Resources Electronic Mail Distribution
Cynthia Sanders Program Manager Radioactive Materials Program Department of Natural Resources Electronic Mail Distribution Jim Sommerville (Acting) Chief Environmental Protection Division Department of Natural Resources Electronic Mail Distribution
Mr. Steven M. Jackson Senior Engineer - Power Supply Municipal Electric Authority of Georgia Electronic Mail Distribution
Mr. Reece McAlister Executive Secretary Georgia Public Service Commission Electronic Mail Distribution
Chairman Appling County Commissioners County Courthouse 69 Tippins Street, Suite 201 Baxley, GA 31513
SNC, Inc. 4 Letter to Dennis from Leonard D. Wert, Jr. dated September 6, 2008
SUBJECT: EDWIN I. HATCH NUCLEAR PLANT - NRC SPECIAL INSPECTION REPORT 05000321/2008008, 05000366/2008008 Distribution w/encl:
C. Evans, RII L. Slack, RII OE Mail RIDSNRRDIRS PUBLIC R. Martin, NRR
Enclosure U. S. NUCLEAR REGULATORY COMMISSION REGION II
Docket Nos: 50-321, 50-366
License Nos: DPR-57 and NPF-5 Report No: 05000321/2008008, 05000366/2008008
Licensee: Southern Nuclear Operating Company, Inc.
Facility: Edwin I. Hatch Nuclear Plant Location: Baxley, Georgia 31513
Dates: July 22-24, 2008
Inspectors: J. Brady, McGuire Senior Resident Inspector, Team Leader P. Niebaum, Hatch Resident Inspector S. Crane, Materials Engineer, New Reactor Office Approved by: Scott M. Shaeffer, Chief Reactor Projects Branch 2 Division of Reactor Projects
Enclosure
SUMMARY OF FINDINGS
IR 05000321/2008-008, 05000366/2008-008; 07/22/2008-07/24/2008; Edwin I. Hatch Nuclear Plant, Units 1 and 2; Special Inspection The special inspection team inspection was conducted by a senior resident inspector, a materials engineer and a resident inspector. A. Inspection Results
The licensee's final operability determination (OD) and compensatory measures taken provided reasonable assurance of operability until the emergency diesel generator (EDG)couplings were replaced. The licensee revised the first two versions of the OD to incorporate newly identified information, to address inspector questions and to provide reasonable assurance of operability. Based on interviews with the licensee, coupling gland cracks were observed by licensee maintenance technicians as early as 1994 but were not documented in licensee inspection records. As a result, these observations of EDG coupling cracking were not evaluated. The inspectors found that the licensee did not have any documented prior indication of problems with the couplings; nothing had been communicated through operating experience about these couplings; and there were not any vendor bulletins that would have specifically alerted the licensee to the degradation of the 1B EDG coupling. The potential for unexpected or catastrophic failure of the EDG couplings is a significant problem which could result in the EDG being incapable of performing its intended safety function. These couplings were used by at least four other utilities on their safety-related EDGs. This problem demonstrated the importance of proper periodic inspections to determine if the component will continue to perform the associated safety function. These inspections are of particular necessity where the shelf life and service life have not been specified. As seen in this application, storage of spare couplings in a warehouse actually reduced the shelf life. This reduced shelf life resulted in the spare couplings, purchased in 1988 and stored in the warehouse, being considered unusable by the licensee.
REPORT DETAILS
Event Description On July 12, 2008, the licensee was conducting a 24-hour surveillance run on the 1B EDG. During this surveillance, the licensee observed high vibration on the EDG. The licensee stopped the EDG and subsequently determined that the coupling between the engine and the generator had significantly degraded. The licensee performed troubleshooting, which included additional vibration monitoring and received vibration acceptance criteria from the EDG vendor. The licensee then ran the EDG as part of a trouble shooting plan. The vibration during this subsequent run exceeded the established acceptance criteria resulting in the 1B EDG being declared inoperable. The licensee isolated the problem to a degraded flex coupling which exhibited cracking on the 1B EDG. Evaluation of the other EDGs for common problems identified cracking of the 1C EDG flex coupling and it was declared inoperable. All diesels exhibited some coupling degradation. The 1B EDG coupling gland (see Photograph 1) had separate circumferential cracks that extended approximately 180 degrees on both sides of the coupling. This coupling had been furnished with the EDG when the EDG was installed in 1971.
The licensee replaced the coupling on July 16, 2008, and subsequently declared the 1B EDG operable. As of August 11, 2008, all five EDG couplings had been replaced.
Special Inspection Team (SIT) Charter
Based on the criteria specified in Management Directive 8.3, NRC Incident Investigation Program, and Inspection Procedure 71153, Event Follow-up, a special inspection was initiated in accordance with Inspection Procedure 93812, Special Inspection. The objectives of the inspection listed below are addressed in the identified sections:
1. Assess the available common cause analysis for the 1B Emergency Diesel Generator (EDG) coupling and operability evaluation for the remaining EDGs. (Section 4OA5.1)
2. Develop a sequence of events, including applicable management decision points from the time of the coupling failure. (Section 4OA5.2)3. Assess any reviews, preventative maintenance, or evaluations developed to support continued operation of the coupling prior to failure. (Section 4OA5.3)4. Review licensee documents to assess if the licensee knew or should have known that a coupling failure was about to occur. Specifically, assess the following areas. (Section 4OA5.4) a. Operational decision making b. Operational experience (internal and external) c. Vendor information on expected service life, recommended preventative maintenance, and if any bulletins or addendums were issued d. Impact on Maintenance Rule implementation
5. Review and assess the previous 12 months of surveillance test data for all EDGs. (Section 4OA5.5) 6. Collect data necessary to support completion of the significance determination process, if applicable. (Section 4OA5.6)
7. Identify any potential for generic safety implications. (Section 4OA5.7)
OTHER ACTIVITIES
4OA5 Other Activities -Special Inspection
.1 Review of the Licensee's Common Cause Analysis for the 1B EDG Coupling and Operability Determination for the Remaining EDGs
a. Inspection Scope
The inspectors reviewed the licensee's three revisions of the operability determination (OD) and common cause analysis to determine if the licensee had adequately considered all available information concerning the degraded coupling and if their efforts to gather information pertaining to the couplings on the other EDGs were sufficiently comprehensive to support making an operability determination for the remaining diesel generators. The inspectors interviewed licensee staff and a representative of the coupling supplier; observed the degraded condition of the 1B and 1C EDG couplings which were removed from the EDG, inspected a replacement coupling, viewed an unused coupling that had been stored in the warehouse since 1988; and reviewed the inspection and testing/results of the installed couplings for the 1A, 1C, 2A, and 2C EDGs. Documents reviewed are listed in the Attachment.
b. Findings and Observations
At Hatch, there are five EDGs identified as 1A, 1B, 1C, 2A, and 2C. The engine of each EDG is a Fairbanks Morse turbo-charged two-cycle engine that develops 4500-horse power at 900 RPM. This engine has 12 cylinders using an opposed piston design. Each EDG had a Falk size 62A Airflex coupling (see Photograph 2) installed between the engine and the generator. The coupling on the generator side was exposed and allowed for limited visual inspection. The coupling was designed to cushion shock and limit torque fluctuations, accommodate radial, angular, and axial misalignment, dampen vibration, and operate without lubrication. This coupling consisted of steel inner and outer rims that were permanently bonded to a polyester corded natural rubber gland using a rubber-to-metal adhesive. The size 62A gland, which resembles an automobile tire, is comprised of eight plies of polyester fiber corded natural rubber which is layered on a 45 degree alternating bias around a membrane for sizing and shaping. This coupling is mounted to the flywheel on the engine side and to the generator shaft on the generator side. These EDGs and associated couplings were installed during original plant construction. All the installed EDG couplings were manufactured around 1970.
The licensee conducted detailed inspections of the 1A, 1C, 2A and 2C EDG coupling glands. The inspections required the licensee to observe the location, length, and depth of all cracks on the generator side of the coupling. If the depths of the cracks were less than or equal to 1/4 inch ("), the coupling was considered acceptable. Following completion of the EDG operation, an additional inspection was performed on the previously identified cracks to observe any changes to the cracks' length or depth. The results of these inspections are summarized in Attachment 3. The licensee observed cracking on the 1A, and 2A coupling glands which did not exceed the 1/4" crack depth acceptance criterion. The 1C EDG coupling gland had several cracks that exceeded the 1/4" crack depth acceptance criterion in multiple locations. The 1C EDG was declared inoperable until the coupling was replaced. An additional inspection criterion required a sample of the rubber to qualitatively assess the elasticity/pliability of the rubber when compared to a rubber sample from the degraded 1B EDG coupling. This criterion was later determined not to provide any meaningful information and was not used to support the OD. Vibration readings on the 1C EDG engine block and both generator bearings were also recorded during these operations. Vibration levels did not exceed the vendor limits for continued operation. Also, the licensee observed the cracks during EDG operation, both loaded and unloaded, to determine if the cracks experienced any growth or expansion under these conditions. The crack was observed to expand on the degraded 1B EDG coupling, which the licensee used as an indicator for potential failure.
Based on the common coupling design and similar component age, the inspectors concluded that the couplings represented a potential common mode failure mechanism. Based on the manufacturer-supplied 1/4" crack depth limit, combined with other acceptance criteria (no crack growth, normal vibration, etc), the inspectors concluded that the final OD provided reasonable assurance of operability until the existing couplings on the 1A, 2A, and 2C EDGs were replaced. Additionally, the inspectors concluded that the intermittent service of the EDG couplings contributed to their extended service life (approximately 39 years). This is further discussed in Section 4OA5.7and Attachment 5.
Some cracking of the EDG couplings had been observed since 1994 (see Section 4OA5.4).
These cracks did not result in significant coupling degradation until 2008. This information, along with the OD and vendor input supported the licensee's near term replacement schedule of the couplings. The licensee's OD also included three compensatory measures and one corrective action. The compensatory measures were to perform crack depth measurements after each EDG run to determine if crack depth had exceeded the 1/4" crack depth acceptance criterion, to perform vibration monitoring during each EDG run, to evaluate the trend to determine if additional degradation was occurring and to inspect the engine side of the remaining EDG couplings expeditiously. The corrective action was to expedite replacement of the couplings for the 2C, 1A, and 2A EDGs. The order of the replacement was based on crack depth measurement. The 1B and the 1C EDG couplings were replaced on July 16 and July 23 respectively.
Although the licensee's final OD was adequate, the inspectors identified the following weaknesses which resulted in the licensee having to revise the OD. The previous versions did not fully support operability of the remaining EDGs.
- The initial coupling inspections were performed with the coupling guards installed which allowed for limited visual inspection of portions of the generator side of the coupling. These visual inspections lacked sufficient rigor for the licensee to assess the condition of the entire generator side of the coupling. A subsequent OD revision required inspections of the generator side of the coupling with the coupling guards removed.
- The licensee did not have plans to inspect the engine side of the couplings to support operability of the other EDGs. This was based on the licensee's conclusion that there were no significant differences in the condition of the generator side and engine side of the degraded 1B EDG coupling. However, the engine side of the 1B EDG coupling gland did have more degradation. Therefore, the decision not to inspect the engine side of the remaining coupling glands lacked sufficient justification. Further, the spare couplings in the warehouse, purchased in 1988, had age related cracking on both the generator side and engine side. This condition was similar to that observed for the degraded 1B EDG coupling and reinforced that the licensee's decision not to inspect the engine side of the couplings lacked sufficient justification. The licensee did include inspection of the engine side of the coupling in the final OD.
- As a result of the licensee's inspection of the 2C diesel generator coupling, cracking was observed around a 60 degree radius which the licensee described as "de-lamination" cracking. An acceptability criterion was established by the licensee that, after each EDG run test, the crack would be measured, and if the crack had not grown to greater than 120 degrees circumferentially, the EDG would be considered operable. There was no basis to support that this crack growth criterion would ensure EDG operability. However, once the coupling supplier provided the 1/4" crack depth acceptance criterion, this criterion was replaced in the final OD.
- No analytical calculation for the 1/4" crack depth acceptance criterion was available. However, based on discussions with the coupling supplier, the 1/4" crack depth would not have significantly affected the strength of the coupling gland. Although this criterion lacked an analytical basis, the inspectors concluded that, combined with the other acceptance criteria (no crack growth, normal vibration, etc), the final OD was acceptable. The licensee sent the degraded 1B and 1C EDGs couplings offsite for analysis and materials testing to obtain additional materials information on the rubber portion of the coupling. The results were not available prior to the end of the inspection.
.2 Sequence of Events, Including Applicable Management Decision Points from the Time of the Coupling Failure
The inspectors created a sequence of events starting on July 12, 2008. Included in this sequence of events are the applicable licensee management decisions as they pertain to troubleshooting the 1B EDG coupling failure and the OD for the remaining EDGs. A detailed timeline of the relevant sequence of events is provided in Attachment 2.
.3 Assessment of Reviews, Preventative Maintenance, and Evaluations Developed to Support Continued Operation of the Coupling Prior to Failure
a. Inspection Scope
The inspectors reviewed maintenance work order history and the inspection records for the last two performances of licensee procedure, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection. For the 1B EDG, the inspectors reviewed the inspection records for the previous three performances of 52SV-R43-001-0. Additionally, the inspectors interviewed licensee personnel that had participated in several previous EDG maintenance activities and were knowledgeable of the condition of the couplings. A summary of EDG coupling inspections that show the scope and results of the coupling inspections, performed before and after the 1B EDG coupling failure, is contained in Attachment 3.
b. Findings and Observations
Introduction:
An unresolved item was identified pertaining to the biennial preventative maintenance inspections performed on the EDG couplings. Cracks in the coupling gland that were identified during routine maintenance inspections were not documented in the associated plant records since 1994.
Description:
Based on information provided by the licensee and interviews with licensee personnel, it was concluded that preventive maintenance inspections had observed cracks in the couplings as early as 1994. The inspectors' review of the coupling inspection records for 52SV-R43-001-0 indicated that no damage existed, as shown in Attachment 3. The procedure directed the coupling be visually examined for damage, deterioration, and loose bolts. The procedure did not contain any specific acceptance criteria pertaining to coupling damage or deterioration. Because the earlier observation of coupling cracks were not documented, no actions were taken to determine if the observed cracks were acceptable or required further evaluation to determine operability. The degradation of the EDG coupling subsequently resulted in the inoperability of the 1B and 1C EDGs. The licensee is performing a root cause investigation for the EDG coupling problems and the contributing causes surrounding this event. This issue is unresolved pending NRC review of the licensee's completed root cause investigation, and is designated as unresolved item (URI) 05000321,366/2008008001, Review of EDG Coupling Root Cause Evaluation.
Additional information relevant to this charter item is discussed in other report sections.
.4 Review Licensee Documents to Assess If the Licensee Knew or Should Have Known That a Coupling Failure Was About to Occur
a. Inspection Scope
The inspectors reviewed the documents in Attachment 1 and internal and external operational experience that was available to the licensee prior to the failure of the 1B EDG coupling to assess if the licensee knew or should have known that a coupling failure was about to occur. The inspectors reviewed the prior two-year preventative maintenance (PM) documentation for the each of the diesel generator couplings, reviewed the surveillance tests to determine if there were any observations relating to vibration or the performance of the EDG coupling, and discussed with the licensee the results of a licensee performed search for industry related experience with the same coupling.
b. Findings and Observations
The inspectors found that the licensee had no documented prior indication of problems with the couplings; nothing had been communicated through external operating experience (OE); and no vendor bulletins would have alerted the licensee to the degradation of the EDG couplings. However, as discussed in Section 4OA5.3, the lack of documentation and evaluation of previous EDG coupling gland cracks was identified as a URI.
Internal OE. Based on a review of licensee documents, the licensee was aware that the 1B EDG's bearings had shown a change in vibration levels since June of 2007 and were listed in the Alert range in the monthly maintenance engineering predictive maintenance status report. The licensee review of this condition did not consider the vibration change as a precursor to any operability problem. As such, the licensee continued with routine vibration monitoring. In addition, the licensee determined cracking of the coupling gland has been observed as early as 1994. These observations were not communicated or documented in the preventative maintenance procedure or in a condition report.
External OE. The inspectors discussed the results of a licensee performed search for industry related experience with this coupling. No relevant industry operating experience was identified. The licensee also contacted the Fairbanks Morse Diesel Generator Owners' Group and found that the following licensees had similar experience with Falk Airflex couplings installed on their EDGs. As discussed below, each case of degradation was identified by visual observations or while diagnosing high EDG vibration and bearing wear. None of these licensees identified the degradation through vendor disseminated information.
- In 2002, Millstone identified minor cracks in a coupling. No established inspection criteria existed at that time. When Falk was contacted about the cracks, the cracks were deemed insignificant and Falk informed Millstone that the coupling could be used for another two years. The coupling was replaced in 2004. Originally, Millstone ordered two replacement couplings. However, the licensee determined that the second coupling was acceptable for use. Therefore, the other replacement coupling was kept as a spare until Hatch acquired it as a replacement for the degraded 1B EDG coupling. This coupling was manufactured in 2003. Hatch personnel performed a visual inspection prior to installation. The inspectors also did not observe any surface cracks and it appeared like relatively new rubber compared to the 1B EDG coupling.
- Arkansas Nuclear One (ANO) discovered a cracked coupling gland in 1991. Because no vendor data was available, ANO contacted Falk. Falk informed ANO that the cracking was from the rubber drying (an aging mechanism) and advised ANO that the coupling should be replaced. ANO now replaces their couplings when cracks are observed. ANO replaced couplings in 1991 and again recently in 2008 due to identified cracks.
- In May 2002, Fermi was troubleshooting failed generator bearings and high vibration issues on two EDGs. A contractor identified the coupling as a possible contributor to the high vibration. The contractor noted that, as the rubber in the coupling ages, its stiffness increases and this may have shifted the axial resonance of the rotor/coupling assembly closer to the running speed of the EDG. Falk informed Fermi that the in service life of a coupling gland was approximately 5 to 10 years based on their experience. Falk informed Fermi that the Shore A Hardness, a measure of relative resistance to indentation (or flexibility), of a new gland was 60+/-5. Since the flexibility of rubber changes over time, the indentation reading may also change over time. Fermi performed a Shore A hardness test of the couplings for the two EDGs which was measured at 74 and 75. The results indicated a decrease in the flexibility of the rubber. Also, because there was evidence of "checking" and surface distress, Fermi decided to replace all four couplings regardless of their contribution to the vibration levels. Fermi has since implemented a program to inspect, test, and replace the couplings periodically.
Coupling Supplier Information: The inspectors discussed with both the licensee and coupling supplier the availability of information on expected service life, recommended preventative maintenance, and vendor issued bulletins and addendums. The inspectors also reviewed the information in the licensee's EDG owner's manual, as well as information provided by the vendor after the 1B EDG coupling failure.
The inspectors reviewed the licensee's EDG owner's manual and found that the only information that was included for the EDG coupling was the original 1960 Falk Guide on the Installation of Airflex couplings and the 1960 Falk installation guide for Airflex Couplings with Floating Shaft. With respect to maintenance, the guidance instructs that the rubber glands be protected from oil and other substances which deteriorate rubber by cleaning the gland before installation. In addition, it recommends periodic rechecking of the alignment due to the settling of foundations. The inspectors observed the EDGs were mounted to seismically qualified concrete pad and concluded that foundation settling was not a factor. Furthermore, by procedure, the coupling was aligned following coupling removal.
The licensee did not receive any information on expected service life, recommended preventative maintenance, or vendor issued bulletins and addendums from the EDG vendor, Fairbanks Morse.
.5 Review and Assess the Previous 12 Months of Surveillance Test Data for All EDGs
a. Inspection Scope
The inspectors reviewed the surveillance tests listed in the attachment to determine if there were any observations relating to vibration or the performance of the EDG coupling. The inspectors reviewed this vibration data and interviewed the licensee vibration engineers to determine if there was any indication of an adverse trend.
b. Findings and Observations
There were no comments in the surveillance procedures that related to the performance of the EDG couplings. As discussed Section 4OA5.4, a change in vibration (decreasing) was noted; however, the licensee determined the change did not impact operability and continued with routine vibration monitoring. Based on the inspectors' review of the available test data, the inspectors did not identify any adverse trend that would indicate a precursor to degradation of the 1B EDG coupling.
.6 Collect Data Necessary To Support Completion Of The Significance Determination Process (SDP), If Applicable
The inspectors collected relevant information and data on past operability of the EDGs.
The licensees root cause analysis will be reviewed for additional information to support the SDP analysis.
.7 Generic Safety Implications
a. Inspection Scope
The inspectors performed a literature search in order to fully assess the generic safety implications from the EDG coupling degradation. The results from the literature search are contained in Attachment 5 to this report, with some key points applicable to this application being summarized below.
b. Findings and Observations
Similar EDG couplings could represent a generic safety implication for other utilities if precursor cracking was not identified and evaluated. The potential for unexpected or catastrophic failure of the EDG couplings is a significant problem which would result in the EDG being incapable of performing its intended safety function. These couplings are used by at least four other utilities on their safety-related EDGs. A periodic inspection to determine if the component will continue to perform the associated safety function is of particular necessity where the shelf life and useful life have not been specified. In addition, the storage of the coupling as a spare part in the warehouse actually reduces the service life so shelf life and useful life requirements will likely be different.
The shelf life and useful life of natural rubber are dependant not only on mechanical loading, but also on temperature, thermal dissipation, and the environment. The shelf life can be greatly degraded by exposure to oxygen, ozone, light, heat, humidity or oils and solvents. According to Falk, the shelf life of the coupling was approximately five to seven years, but under ideal conditions the shelf life can be as long as ten years without an appreciable affect on the component's properties. However, adverse conditions such as those previously mentioned, could shorten the coupling's shelf life to less than the stated five to seven years. In addition, there are other applications, such as gaskets in safety-related systems, will be subject to the same aging and fatigue phenomenon that were observed with the couplings.
The Falk representative indicated that the expected service life for a coupling in continuous use is approximately 10-20 years. However, several factors may increase the useful life of the coupling. As mentioned in Attachment 5, periodic cycling of the coupling will release antiozonants to the surface of the rubber and increase the rubber's resistance to ozone attack. In addition, for natural rubber that exhibits strain crystallization, the unrelieved static strain on the assembled coupling has a beneficial effect on fatigue properties, and rest periods between loading dissipate stored energy in the form of heat that increases the fatigue life of the coupling.
4OA6 Meetings, Including Exit
On July 24 , 2008, the inspectors presented the inspection results to Mr. Dennis Madison and other members of the plant staff. The inspectors reviewed information that could be considered proprietary information and was not included in this report.
ATTACHMENT 1:
SUPPLEMENTAL INFORMATION
KEY POINTS OF CONTACT
Licensee personnel
- S. Tipps, Principal Licensing Engineer
- D. Madison, Site Vice President
- W. Bargeron, Plant Manager
- G. Johnson, Engineering Director
- S. Crosby, Maintenance Supertinendent
- H. Mirzakhani, Engineering Support (Diesel Engineer)
- M. Torrance, Maintenance Engineer
- K. Underwood, Performance Improvement
- S. Soper, Engineering Support Manager
- G. Brinson, Operations Manager
- M. Crosby, Engineering Supervisor
- J. Lonting, Corporate Licensing
Other Personnel
NRC personnel
- J. Moorman, Deputy Director, DRP, RII
- S. Shaeffer, Branch Chief, DRP, RII
- K. Kennedy, Director, Division of Reactor Safety, RII
- R. Bernhard, Senior Reactor Analyst, RII
- J. Hickey, Senior Resident Inspector
LIST OF ITEMS OPENED, CLOSED, AND DISCUSSED
Opened
LIST OF DOCUMENTS REVIEWED
Work Orders
- 1030309701, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (1B), 8/7/04
- 1040391701, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (1A), 4/22/05
- 1040391901, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (1C), 12/17/04
- 1050107001, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (1B), 6/8/06
- 1050109101, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (1A), 6/4/07
- 1050111201, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (1C), 5/31/07
- Attachment 1
- 2040443101, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (2C), 12/02/04
- 2041913301, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (1A), 6/19/06
- 2050884601, 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (2C), 5/14/07
Condition Reports
- 2008107432
- 2008107562, Operability Determination #1-08-006, Revision 1
- 2008107562, Operability Determination #1-08-006, Revision 2
- 2008107562, Operability Determination #1-08-006, Revision 3 (PRB Approved)
- 2008107701
- 2008107708
Procedures
- 2SV-R43-001-0, Diesel, Alternator, and Accessories Inspection 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (2A EDG), 6/12/08 52SV-R43-001-0, Diesel, Alternator, and Accessories Inspection (1B EDG), 6/19/08 53PM-MON-001-0, Vibration Monitoring 34SV-R43-001-1, Diesel Generator 1A Monthly Test, 7/3/07
- 34SV-R43-001-1, Diesel Generator 1A Monthly Test, 1/29/08 34SV-R43-001-1, Diesel Generator 1A Monthly Test, 12/5/07 34SV-R43-001-1, Diesel Generator 1A Monthly Test, 10/3/07 34SV-R43-001-1, Diesel Generator 1A Monthly Test, 2/6/08 34SV-R43-004-1, Diesel Generator 1A Semi-Annual Test, 3/10/08
- 34SV-R43-004-1, Diesel Generator 1A Semi-Annual Test, 9/3/07 34SV-R43-021-1S, Diesel Generator 1A LOCA/LOOP LSFT, 3/4/08 34SV-R43-011-1, Diesel Generator 1A 24 Month Operability Test 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 12/13/07 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 1/18/08
- 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 2/11/08 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 3/1/08 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 8/16/07 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 7/9/07 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 9/17/07
- 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 1/16/08 34SV-R43-002-1, Diesel Generator 1B Monthly Test, 11/5/07 34SV-R43-024-1S, Diesel Generator 1B LOCA/LOOP LSFT, 3/4/08 34SV-R43-001-2, Diesel Generator 2A Monthly Test, 9/14/07 34SV-R43-001-2, Diesel Generator 2A Monthly Test, 1/13/08 34SV-R43-001-2, Diesel Generator 2A Monthly Test, 8/17/07 34SV-R43-001-2, Diesel Generator 2A Monthly Test, 11/5/07
- 34SV-R43-001-2, Diesel Generator 2A Monthly Test, 7/12/07 34SV-R43-004-2, Diesel Generator 2A Semi-Annual Test, 10/7/07 34SV-R43-004-2, Diesel Generator 2A Semi-Annual Test, 12/21/07 34SV-R43-011-2, Diesel Generator 2A 24 Month Operability Test Attachment 1 34SV-R43-003-2, Diesel Generator 2C Monthly Test, 2/10/08 34SV-R43-003-2, Diesel Generator 2C Monthly Test, 1/29/08 34SV-R43-003-2, Diesel Generator 2C Monthly Test, 11/26/07 34SV-R43-003-2, Diesel Generator 2C Monthly Test, 9/25/07
- 34SV-R43-003-2, Diesel Generator 2C Monthly Test, 7/26/07 34SV-R43-003-2, Diesel Generator 2C Monthly Test, 8/21/07 34SV-R43-006-2, Diesel Generator 2C Semi-Annual Test, 10/23/07 34SV-R43-013-2, Diesel Generator 2C 24 Month Operability Test 34SC-R43-003-1, Diesel Generator 1C Monthly Test, 7/16/07
- 34SC-R43-003-1, Diesel Generator 1C Monthly Test, 12/17/07 34SC-R43-003-1, Diesel Generator 1C Monthly Test, 11/16/07 34SV-R43-013-1, Diesel Generator 1C 24 Month Operability Test
Other Documents
- Final Safety Analysis Report Section 8
- Technical Specification 3.8.1 and associated basis Engineering Evaluation # 1697 Engineering Evaluation #1697, Attachment 1: 1B EDG Vibration Fault Tree - Index of Elimination Methods, 7/12/2008 Vibration Data for 1A, 1B, 1C, 2A and 2C EDGs, 2004 to 2008 Event Timeline
- 1B Vibration Event Timeline 1B Initial Visual Inspection 1B EDG Run Summaries Falk Installation of Airflex Couplings, 8800 (1960)
- Falk Airflex Coupling Installation and Maintenance, 488-110 (2007) Falk Installation of Airflex Couplings with Floating Shaft, 8800 (1960) Falk Airflex Coupling Installation and Maintenance, 488-112 (2007) Falk Failure Analysis, 488-910 (1985) Emergency Diesel Generators 1R43 and 2R43 4
th Quarter 2007 System Health Report W.V. Mars, A. Fatemi, RUBBER CHEM. TECHNOL.
- 77, 391 (2004) R. W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials (Fourth Edition), John Wiley & Sons,
- NY 1996
- Vogtle Electric Generating Plant License Renewal Revised RAI Responses, July 17, 2008 (ML0819904580)
- P.C. Painter, M. M. Coleman, Fundamentals of Polymer Science (Second Edition), Technomic Publishing Co., Inc., 1997 Letter from Henry "Skip" Lynn, III, Rexnord, to Steve Tipps, 7/18/2008 Letter from Henry "Skip" Lynn, III, Rexnord, to Steve Tipps, 7/24/2008 Robert P. Lattimer, C. K. Rhee, Robert W. Layer, "Antiozonants," Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., NY, 2001 Bathias, C., LeGorju, K., Lu, C., and Menabeuf, L., "Fatigue Crack Growth Damage in Elastomeric Materials," Fatigue and Fracture Mechanics: 27 Volume, ASTM
- STP 1296, R.S.
- Attachment 1 Piascick, J. C. Newman, and N. E. Dowling, Eds., American Society for Testing and Materials, 1997, pp 505-513.
- Event Timeline Saturday 7/12/08
- 09:58:
- 1B DG started and loaded to 2800 kW for 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> surveillance test per 34SV-R43-012-2 13:25:
- Minor vibration observed on the EDG; load maintained at 2800 kW
- 14:25:
- Excessive vibration observed, entire engine shaking, generator & turbo charger end vibration worst.
- When load was reduced from 2800kW to 2000kW, observed vibration decreases 14:38:
- 1B DG declared inoperable but available 14:56:
- Output breaker opened; vibration reduced further
- 15:03:
- 1B DG secured
- 17:00:
- Management decision to not operate 1B DG until inspections of the diesel and generator were complete 19:00:
- Management decision to perform operability runs on remaining DGs to support a common cause analysis. 21:42: 1B DG declared unavailable Sunday 7/13
- 03:00:
- Visual inspections completed satisfactory on the 1B DG
- 03:00:
- Walkdown with Maintenance and Engineering to inspect the 1B coupling cracking.
- Additional information from DG vendor requested due to limited data in manual, noted to watch coupling and cracking during maintenance run 11:57:
- DG common cause operability runs completed satisfactory per Tech. Spec. action statement 18:00:
- Generator side inspections completed satisfactorily 19:00:
- Fairbanks Morse representative onsite, plan to review work completed and perform maintenance run on 1B EDG with representative concurrence.
- Fairbanks representative requested additional vibration monitoring points during proposed test run Monday 7/14
- 03:35:
- 1B EDG Maintenance test run began
- 04:26:
- 1B EDG test run stopped prior to reaching rated speed due to vibration action limit on Diesel Engine being reached at 816 RPM, A crack on inner hub found to widen to ~1/4" gap during run 07:00:
- Management decision made to replace 1B coupling regardless of potential additional causes due to its degraded condition 07:00:
- Management decision to not utilize spares in warehouse and procure a new coupling 08:00:
- Coupling removal began 12:00:
- Management decision to support running diesel uncoupled to eliminate additional causes. 19:00:
- Uncoupling 1B EDG work complete, alignment "as found" data was
- SAT 21:03:
- 1B
- EDG 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> AS expanded to 14 day
- AS 21:57:
- Attachment 2 Tuesday 7/15
- 00:26: 1B EDG vibration analysis SAT, within vendor specs, Diesel Engine eliminated as possible cause 07:00: 1B EDG Coupling replacement began using coupling obtained from Millstone
- 18:00: 1B EDG Coupling replacement complete, alignment as left data completed SAT per 51GM-MNT-032-0 20:00: 1B EDG maintenance run procedure start, No abnormal data noted during unloaded portion, Vibration data lower than previous unloaded maintenance run and within vendor specifications 20:46: 1B EDG loaded, No abnormal data noted during loaded portion, Visual vibration noted as lower than initial condition run and within vendor specifications 04:49: 1B EDG available but inoperable Wednesday 7/16
- 07:30:
- Management decision made to further evaluate operability of other four EDGs based on "extent of condition" review of cause 09:30:
- Engineering Evaluation 1697 completed concluded that the coupling was the direct cause of the initial condition 12:32:
- 1B EDG completed operability run 14:15:
- 1B EDG started 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> surveillance per licensee procedure 34SV-R43-012-2.
- Thursday 7/17
- Following conference call with NRC RII management, management decided to perform detailed coupling inspections with coupling guards removed.
- 1A, 1C, 2A, 2C EDGs operability determination approved.
- Completed 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> surveillance run of 1B EDG including hot restart of the EDG.
- Friday 7/18
- 2C EDG detailed visual inspection with cover removed.
- Operability determination revision started due to results of more detailed visual inspections. Vendor contacted with detailed results of 2C inspection. Falk vendor provided additional inspection criteria.
- Management decision to call a Plant Review Board (PRB) to review the revised operability determination.
- Saturday 7/19
- Operability determination revision 2 approved.
- Monday 7/21
- Following the coupling inspection, 2A EDG was started and loaded and declared operable after satisfactory completion of surveillance.
- Attachment 2 Tuesday 7/22
- Following the coupling inspection, 1A EDG was started, loaded and declared operable after satisfactory completion of the surveillance.
- Wednesday 7/23
- Maintenance personnel inspected 1C EDG coupling, Based on Falk vendor criteria coupling found unacceptable.
- Two locations measured approx. 5/16" in depth.
- The 1C EDG declared inoperable in accordance with operability determination.
- Thursday 7/24
- Management Decision to inspect the diesel side of the EDG couplings and revise operability determination.
- Following coupling replacement, 1C EDG was aligned, started, loaded and declared operable after satisfactory completion of surveillance.
- Friday 7/25
- Two small cracks were noted around the inner hub and one measured 1/8" depth and the other was not measurable.
- 1A EDG was started, loaded and declared operable following satisfactory completion of the surveillance
- Monday 7/28
- No cracks of measurable depth were observed.
- 2C EDG was started, loaded and declared operable following satisfactory completion of the surveillance.
- Tuesday 7/29
- No cracks of measurable depth were observed.
- Wednesday 7/30
- EDG INSPECTION RESULTS SUMMARY EDG RUNTIME (hours) Pre 1B EDG Coupling Failure Post 1B EDG Coupling Failure Date Criteria per 52SV-R43-001-0 ConditionDate Criteria per work order Condition 04/22/2005 SAT 1/4" Crack Depth 1 SAT Elasticity/Pliability of rubber sample Good elasticity/pliability;
no cracking or crumbling
- No crack growth loaded/unloaded SAT Vibration while loaded/unloaded SAT; < vendor limits; no change from baseline 1A 1830 06/04/2007 Visual examination for damage, degradation, loose bolts
- SAT 07/22/2008No post-run crack growth observed
- SAT 08/07/2004
- SAT 06/08/2006 Left blank 1B 2754 06/19/2008 Visual examination for damage, degradation, loose bolts Left blank Coupling replaced with Millstone spare coupling on 7/16/2008 1/4" Crack Depth 1 UNSAT 12/17/2004 SAT 1C 1746 05/31/2007 Visual examination for damage, degradation, loose bolts Left blank 07/23/2008On 7/23/2008, the 1C EDG was declared INOP.
- Declared operable on 7/24/2008 1/4" Crack Depth 1
- SAT 06/19/2006 Left blank Elasticity/Pliability of rubber sample Good elasticity/pliability;
no cracking or crumbling
- No Crack growth loaded/unloaded SAT Vibration while loaded/unloaded SAT; < vendor limits; no change from baseline 2A 1702 06/12/2008 Visual examination for damage, degradation, loose bolts Left blank 07/21/2008No post-run crack growth observed SAT 1/4" Crack Depth 1
no cracking or crumbling No crack growth loaded/unloaded SAT Vibration while loaded/unloaded SAT; < vendor limits; no change from baseline 2C 2260 05/14/2007 Visual examination for damage, degradation, loose bolts Left blank 07/18/2008No post-run crack growth observed SAT
- Notes:
- 1.
- 1/4" crack depth acceptance criterion was supplied coupling vendor
- 2.
- This criterion was not used in the final operability determination Coupling Photographs Crack around inner rim on 1B EDG coupling viewed from generator (Photo 1)
- Background Information To assess the technical validity of the licensee's operability determination, the inspectors reviewed additional information provided by the coupling supplier, the EPRI Aging Assessment Field Guide, and several books and journal articles on degradation of natural rubber.
- The following is an overview of rubber failure mechanisms and factors that affect both the shelf life and useful life of rubber.
- The shelf life and useful life of natural rubber are dependant not only on mechanical loading, but also on temperature, thermal dissipation, and the environment.
- The shelf life can be greatly degraded by exposure to oxygen, ozone, light, heat, humidity or oils and solvents, which lead to excessive hardening, softening, cracking, crazing (which is a void forming process) or other surface degradation (EPRI 1007931).
- According to Falk, the shelf life of the coupling is approximately 5-7 years, but under ideal conditions the coupling can last as long as 10 years without an appreciable affect on the component's properties.
- However, adverse conditions, such as those previously mentioned, could shorten the couplings shelf life to less than the stated 5-7 years.
- In 1985, Falk updated its service manual with a section on failure analysis.
- This section covers how to identify various types of gland failures including peak torque overload (low cycle fatigue), torsional fatigue, misalignment failures, environmental effects, and manufacturing defects.
- Each subsection gives the conditions under which failure would occur and describes the characteristic markings or cracking of the failed gland (Falk 488-910).
- It is manifested as cracks or tears that start at the inner rim and spread radially toward the outer rim.
- Since the failure propagates through the plies, it has the appearance of torn fabric (Falk 488-910).
- Torsional Fatigue is a high cycle fatigue type failure and occurs from operating the coupling near the torsional critical frequency, which is the particular resonant frequency at which damage or degradation in performance is likely.
- The resonance amplifies the loading on the coupling.
- Torsional fatigue may be caused by coupling misalignment or by an increase in stiffness due to aging.
- Torsional fatigue appears as cracks between the inner rim and the gland and propagates along the inner rim (Falk 488-910).
- Its appearance is similar to that of the 1B EDG (Figure A).
- Misalignment failures appear similar to torsional fatigue failures.
- As mentioned above, coupling misalignment can be a factor in torsional fatigue.
- However, misalignment may also manifest in tears along the outer rim in addition to the inner rim (Falk 488-910).
- Falk's updated installation and maintenance instructions provide a parallel and angular alignment limit of 0.030 total indicated runout (TIR) and cautions that exceeding the limit reduces coupling life (Falk 488-110).
- Natural rubber is sensitive to several environmental factors including oxygen, ozone, ultra violet light (UV), temperature, oils, and solvents.
- The conditions under which the rubber is stored and used will greatly affect the rubber's material properties, shelf life, and useful life.
- Attachment 5 Ozone is particularly damaging to rubbers.
- It is typically present in the areas that contain ozone generating equipment such as high voltage electrical equipment, mercury vapor lamps, or any other equipment capable of generating electric sparks or electrical discharges (EPRI 1007933).
- Ozone cracking will manifest as hairline cracks oriented perpendicular to the direction of strain (Falk 488-910).
- UV radiation from strong sunlight or artificial light with a high UV content can also degrade the rubber as it drastically increases the oxygen absorption rate of the rubber and accelerate its oxidation.
- Increased oxidation embrittles the rubber and leads to increased moisture absorption.
- It can also lead to crazing and chain scission (the breaking of a molecular bond in the polymer chain), which will reduce the rubber's strength and may lead to spontaneous fracture. (EPRI 1007943) Similar to ozone, UV degradation will manifest as hairline cracks oriented perpendicular to the direction of strain (Falk 488-910).
- The temperature at which the rubber is stored and used can affect its storage life and useful life.
- The fatigue life, or cycles to failure, for a rubber component is partly dependant on temperature.
- The crack growth rate of natural rubber is increased with temperature due to the associated increase in oxidation rate and decrease in energy dissipation.
- In addition, increased temperature can lead to embrittlement, a reduced elongation limit, a change in hardness, and crazing (EPRI 1007933).
- Low temperatures can increase the rubber's stiffness and, as mentioned in the discussion of torsional fatigue, can move the rubber's resonance frequency closer to the critical frequency, which will amplify the loading on the coupling.
- Oils and solvents can also degrade natural rubber.
- Exposure to oils can lead to plasticization which manifests as a gross softening of the rubber.
- Solvents can lead to the selective dissolution or leaching of additives that results in embrittlement and crazing, dissolution of the natural rubber that leads to gross distortion, and stress cracking that can lead to spontaneous cracking and severe embrittlement (EPRI 1007931).
- To increase the life of rubber components, anitiozonants are typically added during rubber formation.
- These substances help prevent or slow the effects of oxidation due to ozone and limit the embrittlement of the rubber.
- Since ozone attack is a surface phenomenon, the antiozonant must migrate to the surface of the rubber to provide protection.
- The repeated flexing of the rubber releases the anitozonants to the surface and helps prevent cracking.
- If the rubber is not periodically flexed, it is more likely to experience cracking due to a breakdown of the antiozonant film and subsequent exposure to ozone (Lattimer 1992).
- Therefore if we exclude the effect of mechanical loading, a component that is used, even sparingly, may have a longer life than one that has not.
- In addition to environmental effects, the coupling's useful life can be affected by the mechanical loading of the coupling.
- The fatigue life of the coupling is related to the mechanical loading (both frequency and load ratio) and the rubber's ability to dissipate energy (Bathias 1997).
- Natural rubber undergoes a process called strain crystallization.
- This is the process in which strain energy is dissipated through crystallization.
- Strain crystallization can increase the rubbers fatigue life and ultimate tensile strength under the proper conditions.
- However, strain crystallization is dependant on the materials load history.
- It is most effective when the load is Attachment 5 not completely relieved.
- This may have contributed to the extended life of the coupling (Mars 2004).
- The fatigue in natural rubber is also dependant on energy dissipation.
- The energy can be dissipated in the form of heat, fatigue crack propagation, or in the case of natural rubbers through strain crystallization (Hertzberg 1996).
- Through cyclic loading, rubber accumulates hysteretic energy.
- Hysteresis is the process in which the force necessary to strain, or elongate, the rubber is greater than that necessary for recovery.
- Therefore more energy was required during the loading than the unloading of the material.
- That strain energy may be dissipated as heat and will cause an associated temperature rise with each cycle.
- That energy may also be dissipated through strain crystallization (Hertzberg 1996).
- The energy dissipation and associated rate of temperature rise is in part dependant on loss compliance, a measure of energy lost as heat, which in turn is dependant on temperature.
- At lower temperatures, there is a small change in the loss compliance with temperature; however, as the temperature increases near the glass transition temperature, the loss compliance increases rapidly.
- Therefore, the temperature rise is more pronounced near the end of the fatigue life of the component (Hertzberg 1996).
- In the case of natural rubber that can undergo strain crystallization, some of the energy will be dissipated through crystallization and limit the temperature increase; however, as the temperature rises less of the energy will be dissipated through strain crystallization.
- This can be seen in the decreased fatigue resistance of rubbers at elevated temperatures.
- However, studies have shown that intermittent rest periods between cycling allow for the dissipation of heat to the surroundings (Hertzberg 1996).
- This may extend the fatigue life of the rubber and partially explain the prolonged life of the intermittently used EDG couplings.
- In summary, both the shelf life and useful life of natural rubber can be degraded due to environmental effects such as exposure to oxygen, ozone, light, heat, humidity or oils and solvents.
- The failure of flexible couplings such as the Falk Airflex Coupling may be due to several mechanical failure mechanisms such as peak torque overload (low cycle fatigue), torsional fatigue, and misalignment failures.
- The useful life of the coupling may vary with the use of anitiozonants, which are released to the rubber's surface upon flexing and prevent ozone cracking; with intermittent rest periods between cycling to allow for the dissipation of heat to the surroundings, and through strain crystallization when a static load is applied.