ML20082E368
| ML20082E368 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 11/17/1983 |
| From: | Kemper J PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC |
| To: | Murley T NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| References | |
| 10CFR-050.55E, 10CFR-50.55E, SDR-95, NUDOCS 8311280172 | |
| Download: ML20082E368 (9) | |
Text
PHILADELPHIA ELECTRIC COMPANY 2301 MARKET STREET P.O. BOX 8699 PHILADELPHIA. PA.19101 JOHN S. MEMPER
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NOV 171983 0
g-'-}9 Mr. Thomas E. Murley, Director Office of Inspection and Enforcement Region I United States Nuclear Regulatory Commission 631 Park Avenue King of Prussia, PA 19406
SUBJECT:
Limerick Generating Station, Units 1 & 2 Significant Deficiency Report No. 95 Metal Framing System Strut Clip Cracking NRC Construction Permit Nos. CPPR-106 & 107 FILE:
QUAL 2-10-2 (SDR No. 95)
REFERENCE:
Telecon between M. Conner NRC I&E and J. P. Evans, PECo dated 10/18/83
Dear Mr. Murley:
Patented metal framing systems are used at Limerick for supporting Class I electrical raceways, conduit, HVAC ductwork, instrument tubing gutters and instrument supports. These systems consist of prefortaed channel shaped members or struts which are bolted together using various configurations of metal clips.
Inspections of these connection clips found that certain types of clips had cracks which developed during installation.
This condition was reported as a potential significant deficiency in the Referenced telecon. We have completed our evaluation and consider this a significant deficiency per 10CFR50.55e.
8311280172 831117 PDR ADOCK 05000352 5
PDR N
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The details of our evaluation are contained in the attached report.
Please do not hesitate to contact us should you desire further discussion or clarification of any aspect of this item.
Very truly yours, IM[
Copy to: Director of Inspection and Enforcement United States Nuclear Regulatory Commission Washington, DC 20555
'S. K. Chaudhary, Resident NRC Inspector (Limerick)
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Limerick Generating Station, Units 1 and 2 i
Significant Deficiency Report - SDR No. 95 Metal Framing System Strut Clip Cracking NRC Construction Permit Nos. CPPR-106 and 107
1.0 INTRODUCTION
In compliance with 10CFR50.55 (e), this is issued as the, final i.
report on the strut clip connection cracking problem.
This
- report describes the problem, gives results of investigative programs, discussed corrective measures, and gives conclusions for the strut clip problem for Limerick Generating Station.
2.0 DESCRIPTION
OF DEFICIENCY 2.1 Deficiency Patented metal framing systems, supplied by Unistrut, Powerstrut and Superstrut are used at Limerick for supporting Class I electrical raceway, conduit, HVAC duct work, instrument tubing gutter and instrument
- supports.
These framing systems consist of preformed steel channel shapes or struts bolted together using various configurations of metal clips.
l Certain of these strut connection clips with interior angles less than 90 degrees used on these systems were found to have cracks.
Connection clips are fabricated by bending flat plates to a predetermined angle; the cracks occurred on the inside radius of the angle and range in depth from barely detectable to about half the 1/4" fitting. thickness.
2.2 Sequence of Events 2.2.1 In July 1983, a Bechtel QC inspector
- identified two fittings with possible cracks.
As a result, 260 additional samples were visually inspected of which 23 were considered cracked and documented on nonconformance i
report N&D 1634.
After dye penetrant testing, 19 of the 23 were confirmed to have cracks.
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2-2.2.2 Additional tests were performed to determine the extent of the problem.
2 2.2.2.1 Sixty new fittings were examined by liquid penetrant examination (PT) to determine if cracks were limited to installed fittings or if cracks exist on new fittings.
One fitting had an indication; the remaining 59 fittings showed no cracking, indicating the cracking occurs during or after installation..
2.2.2.2 Since the original cracks and the above examination.were discovered on one i
particular type of fitting, a sampling of i
other ' types of fittings from the same manufacturer was performed.
Using PT, other fittings were examined; 15 of 40 fittings examined had cracks to some degree.
2.2.2.3 Since an apparent cause of the cracking is reverse bending of the. fittings during installation, testing of three different types of samples (the fittings with i
interior angle of 45 degrees, 90 degrees and 135 degrees) was performed to determine the fittings' sensitivity to reverse bending.
Reverse bending occurs during installation when the clips are
" opened" to match the actual angular
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alignment of the strut members at the connection.
For the 135 degree angle fittings, there were no cracks observed throughout the test.
The 90. degree angle fittings, after less than 2 degrees of reverse bending, showed visual surface indications.
Further investigation performed indicated that the cracks formed at 90-degree fittings were relatively shallow and did not propagate even after being reverse bent to greater than 135 degrees; therefore, the fittings r
are not considered fracture sensitive in i
' service.
The 45 degree angle fittings l
developed a much deeper crack during the test and this crack propagated when it l
was bent open to 90 degrees.
It is i
concluded from the test results that the more severe the angle of bending during fabrication, the more susceptible the 1
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fittings become to crack development.
Therefore, based on the test results, 4
significant cracking is limited to those fittings bent.through more than 90 degrees.
2.2.2.4 The'above investigations were performed on fittings used on, or-purchased for HVAC hangers. -All fittings were from one i
supplier, Unistrut.
However, similar cracking. problems were also observed on the fittings manufactured by the other suppliers. 'A sample visual examination of electrical raceway supports was 4
performed; five fittings out of approximately 320 inspected were determined to have cracks.
Since. visual i
examination is generally not conclusive, the number with actual cracks might be higher or lower.
(Based on para. 2.2.1 above).
l 3.0 CORRECTIVE MEASURES 3.1 Cause Based on testing and metalurgical examination, the causes of cracking appear to be:
1)
Forming the angles with an inside bending radius of about 1/16", whereas 3/8" minimum would be recommended by AISC code for this application.
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.This operation caused severe cold working of the material.which is the predominant reason for the resulting low ductility of the fittings in the vicinity of the bend.
2)
Use of material that has not been killed, and one which has not been annealed or normalized.
The. kill process for steel addes a deoxidizer such as aluminum or silicon to reduce the possibility of L
strain age embrittlement.
The anneal or normalize i
process is the heat treatment of steel plate to remove any residual stress resulting from manufacturing of plate (e.g., cold rolling, etc.)
3)
Hot-dip galvanizing of material, af ter the severe cold working.
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+j The highly stressed material.is susceptible to strain age embrittlement.
This could be further accelerated by heat of hot-dip galvanizing.
In addition, the acid cleaning to prepare the. surface for hot-dip galvanizing of highly stressed material increases susceptibility to hydrogen embrittlement.
4)
Reverse bending during installation.
Reverse bending introduces further stress to a highly stressed area.
3.2 Resolution Resolution of the cracked strut clip angle problem is divided into two parts:
- 1) those yet to be installed and 2) those installed prior to the discovery of the problem.
3.1.1 For future installations, design details in drawing E1406-1, M830-G40, Cl360 and Cl390 series have~been revised to preclude the use of clips susceptible to cracking during installation.
Instead, ungalvanized, field-fabricated plates with larger bend radii are to be used.
Alternatively, welded i
connections, eliminating the use of clips, may be used.
3.2.2 For previously installed double angle fittings, a combination of analysis and connection testing was performed.
The analysis was to determine the connection loads and the test was to determine the capacities for both new and previously installed double angle fitting connections.
In the test, the clip angle connection was tested in two assembly configurations (Figure 3.2.2).
The first assembly had double angle fittings with the one bent through an angle greater than 90 degrees having a known crack l
to simulate the most severe as-built hanger condition.
The second assembly had only one i
uncracked ' fitting that was bent less than 90 degrees simulating the case of a completely broken fitting.
Both static and fatigue capacities were determined for the above assembly configurations.
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HVAC ' duct supports, cable tray supports and i
most conduit / gutter supports have connections l
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, similar to Figure 3.2.2 Configuration 1, using the-fitting susceptible to cracks in conjunction with fittings that have no cracks.
The resolution essentially consists of comparing the tested capacities (with an appropriate safety factor) against computed connection loads..All hangers which do not meet design requirements will be repaired.
For instrumentation supports, only one support had used a single crack-susceptible angle connection which requires repair.
Details for the repair have been issued for implementation.
A significant number (approx. 300) of conduit / gutter supports exist where the suspect clips are used alone.
This is considered significant because in the testing and deriving of allowable loads, only those fittings not susceptible to cracks are relied upon to carry load since the load-carrying capacity of clips with cracks is indeterminate.
These supports using single clip connection will require rework; details for this rework have been issued to the field for implementation.
4.0 ANALYSIS' OF SAFETY IMPLICATIONS
-4.1 Strut _ angle fittings with interior angles less than 90 degrees potentially affect the structural integrity of
. hangers supporting - Category I HVAC ducts,
' instrumentation and electrical raceways.
Based on the previously described testing and analysis, the following provides the conclusion on reportability:
1)
For HVAC duct supports, out of approximately 750 total hangers in Unit 1 and Common, all have been analyzed and only four fixes were required.
These conclusions are documented in the engineering disposition to NED 1634.
2)
Of all instrumentation supports, one rack required a fix.
(Drawing M830-G40, Sheet 5).
3)
For cable tray supports, there are no single angle fittings'used for~ support connections.
All supports have been analyzed with capacity reduction
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. of the clip connection and found to be structurally adequate.
4)
For conduit / gutter supports using double angle fitting connections, a small number of supports require rework.
For those supports using single less-than-90 degree angle connections (approximately 300) will be repaired or replaced since there is no reliable method of developing the load capacity of a less-than 90 degree connection with a single potentially cracked clip.
The potentially deficient strut fittings are not considered a significant deficiency for HVAC duct, instrumentation, and cable tray supports.
However, single angle fittings are used in a significant number of conduit / gutter supports.
Due to the uncertainty of the extent of the cracking, the connection capacity is indeterminate and fittings will have to be repaired or replaced.
Because of the number of conduit / gutter supports requiring rework, the deficiency is considered significant.
Due to the indeterminacy of the single-angle connection capacity, it can be stated that had the condition gone uncorrected, there could have been an adverse effect on the safe operation of the plant and thus this condition is considered reportable under the provisions of 10CFR50.55 (e).
5.0 CONCLUSION
5.1 Engineering analysis in conjunction with testing has determined the adequacy or need to repair of existing affected supports.
5.2 Positive measures have been taken to prevent the recurrence of the strutclip connection cracking for future installations.
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