ML20215C306
| ML20215C306 | |
| Person / Time | |
|---|---|
| Site: | Satsop |
| Issue date: | 06/11/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20215C291 | List: |
| References | |
| TAC-64852, NUDOCS 8706180068 | |
| Download: ML20215C306 (7) | |
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Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION APPLICATION OF ITT/GRINNELL STIFF CLAMPS TO SAFETY-REl.ATED
""7TFILYSTEMS AT WNF-3 DOCKET NO. 50-508
1.0 INTRODUCTION
On July 31, 1985, WPPSS submitted a report which presents a detailed evalua-tion and. resolution of the safety concerns regarding the applicatien of the l
ITT/Grinnell FSgure 215 type stiff clamps to safety-related piping systems at WNP-3. This evaluation was performed by EBASCO Services, Inc. :.s a con-tractor to WPPSS.
Standard Review Plan Section 3.9.3, II-3-b(f), Revision 1 of July 1981, states the concern thet " snubber response characteristics may be washed out by the added flexibility in the support structure including the clamp."
s This concern is also addressed in WPPSS Technical Specification 3240-4 for Piping and Piping Supports, which requires that " clamps used as the non-integral attachment to the piping component in a snubber-strut assembly shall have as a minimum a spring rate greater than five times the spring rate of the snubbing device." In response to these requirements, ITT/Grinrell designed in 1981 a stiff clamp configuration labeled " Figure 215 Stiff Clamp." After the construction start of WNP-3, the NRC was notified of certain deficiencies in the design pursuant to the requirements of 10 CFR 50.55(e). After review of the ITT/Grinnell study performed to l
review these deficiencies, additional concerns were expressed by WPPSS l
and the NRC staff which included, among others, the effects of l
differential thermal expansion and pre-load on the overall clamp stiffness, toughness properties of the materials at elevated temperatures, localized pipe wall stresses, and code case applicability. To resolve these concerns, ITT/Grinnell modified the original clamp design. This modified design is shown in Figure 1.
New requalification testing was performed 1
on this design, and new torque and pre-load levels were determined to achieve the necessary pre-load and stiffness requirements.
In addition,
'm a detailed engineering assessment was made of the materials, design,
.fng fabrication, inspection, qualification testing, and installation q c.
procedures. This assessment was issued as a report on December 8, 1981, di5 and addressed and closed all except two open items. These were the QQ acceptance of ASME Code Case N-249-2 and the NRR review and evaluation of the clamp design methodology and qualification.
On June 18, 1982,
$6 representatives of WPPSS, EBASCO, and ITT/Grinnell met with NRC staff, at 88 which time 13 questions were raised regarding the application of the m<
stiff clamps. On April 4, 1983, the staff issued seven additional 5
follow-up questions. WPPSS has addressed these concerns in extensive Rg correspondence. This report represents a summary of the concerns and I
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their proposed resolutions. However, additional concerns were also 1
raised and addressed during its review.
This SER addresses the resolution of all outstanding concerns.
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2.0 ' EVALUATION-The' major.NRC concerns falliinto'three general groups, each of which is discussed separately.
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1..
Materials'of ConstructionL During the investigations of the origina11y' identified deficien-cies, concerns were expressed regarding the materials-'and the code ' acceptability of the straps and the heat-treatment 'of the i
rivets. The specific concerns were the. susceptibility of high-
-strength' straps to brittleness and stress corrosion, and the compliance with ASME Code. Case N249-2 for high strength materials.
This: Code' Case lists additional materials and certain requirements for ASME III,; Subsection.NF, component supports which are fabri-cated without welding. The straps'are manufactured from ASTM A693 G630 high-strength stainless steel strip material in the H1075.-
condition, which are listed in Table 5.of the-Code Case with a ~
cspecified minimum ultimate tensile-strength (UTS) of 145 ksi.
The Code Case-also specifies that,the. maximum measured UTS should not exceed 170 ksi in view of the susceptibility of high-strength materials to brittleness and stress corrosion cracking. For certain applications,.e UTS of up to 190 ksi is.also acceptable, provided that impact testing of the material was specified, and that' effects of sustained loads, environment, and heat treatment on'the susceptibility to stress corrosion cracking.of these-materials were considered. WPPSS stated that a sample.of four straps, which were tested for UTS,thad an average UTS of 173 ksi and the highest measured UTS of 178'ksi.: However, the maximum.
thickness of the straps'is:less than:1/8 inch.. According to ASME Code Paragraph NF-2311, materials with a thickness.less than 5/8 inch are not required to be, impact. tested. Therefore,
'in this case the strap materials were' stated:to be exempted from the Code Case impact testing requirement. We find this acceptable..
ASME Code Tables I-7.1 and I-13.1 also indicate that the strap material experiences a reductica in toughness at roorn temperature after exposure.to 600'F for about.5000 hours0.0579 days <br />1.389 hours <br />0.00827 weeks <br />0.0019 months <br />. WPPSS has. indicated that.98% of the' applications of stiff clamps at WNP-3 are below temperatures of 500*F No other degradation,of material properties is indicated,by:the Code. Therefore, for these applications no long term effects are expected due to the sustained tensile loading in the straps.- In those instances where temperatures exceed 500'F, WPPSS has' stated that all applications will be reviewed on a case-by-case' basis and, where the reduction in toughness is unacceptable, the support will be redesigned to eliminate the stiff clamp. We find this ccceptable.
Studies of.the straps ander high tensile loading have also been conducted which indicate that the strap material.is moderately susceptible to stress corrosion cracking depending on the environ-ment. WPPSS'has therefore performed an. investigation of the
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environment at WNP-3 that would exist during long-term and acci-3 dent conditions, and has determined that no design basis con-ditions will exist under which stress corrcs' ion cracking could occur. For the rivets, WPPSS has stated that the heat treatment j
for their material is in accordance with that prescribed in i
Footnote No. 31 of the Code Case.
We f3nd this acceptable.
Based on the discussion above, we find that the requirements of Code Case N-249-2 for the strap and rivet muerials have been satisfied, and consider this issue resolved.'*
l 2.
Pipe Wall Induced Stresses In accordance with a prior commitment, WPPSS provided the results of analytical calculations and tests performed on clamp-tube com-binations of various sizes. The purpose of these tests and cal-culations was to determine the magnitude of the stresses induced in j
the pipe walls because of pre-loading, the tensile and compressive loads from the support action, and the stresses induced due to the clamp restraint by the internal pressure and the pipe thermal expansion. These tests and calculations were performed in l
response to concerns that the interaction between pipe and clamp i
had the potential to induce significant stresses in the pipe wall which were not included in the design calculations. These con-cerns were also raised generically in IE Information Notice 83-80.
The test programs extended over a period of four years (1981-1985) and were performed by ITT/Grinnell. The purpose of these tests was to determine the actual pipe wall stress magnitudes and distributions due to pre-load alone, and pre-load with ex-ternal tensile and compressive loads. The earlier set of tests indicated that the induced stresses depended on the amount of contact between the yoke and the pipe. This contact, in turn, depends on the pre-load, yoke radius, and pipe and clamp fabrica-tion tolerances, which vary with the particular application.
This amount of contact was therefore determined to be a signifi-cant parameter in any code stress evaluation. To further eval-unte the stresses under various load conditions and to obtain cuitable data for design, additional tests were performed in con-junction with a concurrent analytical effort.
The specific objectives of this combined effort were to: (a) develop an ana-lytical relation between pre-torque and lift-off loads, (b) estab-lish experimentally the worst condition of contact between yoke and pipe, (c) develop an analytical model using the finite
- The latest revision of this Code Case is No. 8.
The requirements of this revision are similar to those of Revision 2, and are also considered to be
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L. q element method to simulate these conditions for all combinations of pipe / pipe schedule / cit.mp sizes, (d) perform tests for several pipe /clarap combinations to verify the analysis, (e) develop corrective factors for conditions where the contact between yoke and pipe are other than the worst. condition, and (f) use the analytical results to determine the maximum magnitude of the induced stresses as a function of pipe diameter, pipe wall thickness, clamp size, pre-torque, applied load and pipe tem-perature.
The worst condition of contact was determined to be single point contact between the yoke and the pipe. To simu-late thic condition during testing, a hardened wire was inserted between the various sized yokes and the pipes, which were then subjected to various corrbinations of pre-load (bc1t pre-torque) and tension and compression t3pe applied loads. Similar tests were also performed under thermal expansion. Finally, tests were performed nnder pre-torque and compressive loading where the contact conditions between yoke and pipe were controlled such that the contact load was distributed over a finite portion of the pipe. From these last tests, stress reduction factors -
were determined to assess the stresses in specified contact conditions based on the stresses for the single point condition.
Based on these tests, simplified analytical and finite element models were developed to achieve the objectives stated above.
A linear relationship was developed between lift-off force and bolt torque for each pipe / clamp application. This lift-off force is the limiting load which can be applied to a given pipe / clamp combination for a given pre-torque. Thermally-induced stresses, stress reduction factors, and pipe / clamp i
stiffness were also determined analytically. A comparison of
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the tests and analytical results indicated that the latter were generally conservative and in good agreement with the test results, except for the 2-1/2, 3 and 4 inch pipe sizes.
For these pipe sizes the analytical liftoff loads as functions of pre-load torque have been reduced by 17%, based on the largest difference between analytical and test results found for the 2-1/2 inch pipe.
We have reviewed the stated summaries and comparisons of the
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test and analytical results and found them acceptable as a basis for the application to the local design of piping with attached clamps. The piping acceptance criteria which govern the design of the pipe at the clamp locations are based on ASME i
Code Cases N-391 and N-392, accepted by the NRC ctaff as stated i
in Regulatory Guide 1.84, Revision 24. As there are no industry-wide established acceptance criteria for clamp-induced stresses, WPPSS selected to evaluate the acceptability of the local pipe stresses based on the methodology of these code cases.
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which pertain to local stresses induced in a pipe by welded l
attachments. The clamp-induced stresses are evaluated on an
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s elastic ~ basis'and,added to the other pip ng system stresses.at the~ stiff clamp locations.- The ASME allowables are the saine onetas those specified in these code cases-In: addition, the stresses; induced by the pre-torque condition'are' limited by;the' yield strength of_the material. The staff finds this acceptable.'
3.
. Installation Requirements and Control WPPSS has also addressed a concern regarding the. possibility n
of torque relaxation and its effect on the clamp stiffness.
Tests
. sample;were conducted by ITT/Grinnell on a number of pipe clamp d
s' subjected to cyclic thermal loading. Some of.the I
samples exhibited a loss of pre-load torque. The lift-off4 force.
required.after this cycling was found to be greater than during-the initial-pre-torquing. This phenomenon was attributed tc the.
redistribution of the tension forces ^and contact areas.in the i
! straps. Thus, :the relaxation ~of torque..does not appear,to have
-an adverse effect on the stiffness of,the' clamp, norfdoes the justification. ppear-to diminish.. This appears to be a reasonable I
lift-off load a l
TheLselection-of the pre-torque to be applied to each stiff clamp will be based on the maximum _ load acting on the stiff clamp 3
derived from the appropriate design load combinations for-piping support design, as specified in WPPSS Technical Specifica-tion 3240-4; Section 4.02C. This pre-torque will be increased
.by 10% to' provide extra-. margin of safety. However, these torques will not exceed the values of the ITT/Grinne'11 load capacity.
sheets for this type clamp' ITT/Grinnell has developed. load.
rating values.for these stiff clamps which are stated to be in; accordance with the requirements of ASME III,.' Subsection NF, 1980.
The load rating is based on the imposition of a total load on one-or more duplicate full size samples, equai to or less than the load under which the support fails to perform its required function. The load retings for various pipe / clamp sizes for ASME Service Levels A, B, C, D were determined ny following the pro-cedures in Subsection NF for standard component supports.
ljj WPPSS has also stated that this stiff clamp will.be installed on l
safety-related piping systems with maximum operating conditions as
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follows:
Pipe Material Pressure Temperature
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('F)
Stainless Steel 2310 350 Carbon Steel 1800 110 Carbon Steel 1170 454 j
Carbon Steel 1143 563 y
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.The. installation program will include procedures listing the appropriate torque settings. After the final torque setting the: studs and nuts (double nutted) will be marked which wil1 j
a? low for guide. visual inspection as required by the Technical a
Specifications.. No physical modifications will be performed j
without engineering approval. We find this acceptable, i
3.0 CCNCLUSION Based on the review of the report and additional documer.tation provided by WPPSS,~all outstanding issues concerning the specification and insta11ation' of Irr/Grinnell Figure 215 Stiff Clamps at WNP-3 are considered to be
-resolved;-
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NRC Contributor: Mark Hartzman l
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.Date: Jene 12, 1967 I,
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