ML20058C782
| ML20058C782 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 06/26/1982 |
| From: | Rodabaugh E E.C. RODABAUGH ASSOCIATES, INC. |
| To: | Terao D Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20058C714 | List: |
| References | |
| NUDOCS 8207260445 | |
| Download: ML20058C782 (4) | |
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Attachment G E. G. cRodaGaug& dsacciates, Dac.
4S25 CEMETERY ROAD e HILLIARD, OHIO 43026
.614/876-5719 June 26,1982 Mr. Davo Terao
!! ail Stop P-522 Mechanical Engincoring Branch l
U. S. liuclear Regulatory Commission Vashington, DC 20555 Subjects llecting at LaSalle Plant, June 25, 1982 en Tc::hnical Aspects of Torque Wrench Calibration Dear Daves A copy of the Agenda is in:iluded as Enclosure 1.
My cencents con-cerning each Agenda item follow.
1.
Motor Operator / Yoke Cennection Tightness Check Roger Lanksbury (!!RC, Region III) found (6/18/82) that two nuts on a motor operator / yoke connection vore'" finger tight".
( A possible reaeon is that it was necessary to renove the operator for clearance during con-struction and all of the nuts were not re-tightened during replacement of the operator.) The Applicant has comnitted to checkin'g all safety-related motor operated valves inside containment (24 valves) immediately an,d all safety-related motor operated valves outside containment within 6 months. is a listing of motor operated valves inside contain=ent.
It includes a column headed " Torque Spec." Apparently, this information' vas obtained in the last week or so and, prior to then and presu::abl~y during the time of the alleged lack of accurate torque vrench calibration, there was no specified torque for these particular bolts when, or if, they were loosened during plant construction and then re-tightened. A discussion of the technical significance of the torque wrench accuracy, if in fact a torque wrench was used in the re-tightening, is irrevelant.
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2 Attachment G Considerable discussion occurred at the meeting as to what was meant by "Wrenchtight". In =y opinion, "'ilrenchtight" is an adequate instruction to any competent workman. However, the Applicant agreed to put "A value" in place of "Urenchtight". Once that specified torque is established (as it is for two of the four valve canufacturers as shown in Enclosure 2),
the technical significance of the torque vrench accuracy becomes relevant.
Fro:a a technical standpoint, torque during bolt tightoning is a method of trying to achieve an appropriate clamping force. As brought out by Mr. Hood of SPS Technologies, the relationship between torque and clamping force, for a given nominal bolt geo ctry (size, threads per inch, thread tollerances, etc.) can vary videly depending upon the coefficient of friction.
My own experience (on flanged bolt bolting) indicates that if one first calibrates torque versus clanping force using a reasonably good thread and other cating surfaces lubricant, than if one continues to use that sa:ae lubricant and nominal bolt geometry, the clamping force can be predicted from the torque within a scatter of 330% vith about 907, confidence.
However, if the lubricant is not controlled, then froc vhat I have read it appears that the clanping force, for a given torque, could easily vary by a factor of 4 or more.
Fortunately, the safe operation of bolted connections of the type under consideration are not very sensitive to preload clamping forces.
To the extent that loads are cyclic, it is desirable to have the preload highor than those loads so that the bolt stresses vill remain essentially constant and fatigue of the bolts vill not occur.
In addition, even if no loads are anticipated in paper calculations, most power plant equip-cent operate in an environment of at least low-level vibration and bolts should be tightened sufficiently so that the nuts do not vibrate loose in service. These considerations establish einimum desired proloads.
The naxinum preload may be significant if, by any chance, prying effects are not properly ovaluated in calculating loads. However, if l
Ioods are appropriately calculated and' bolt sines are colected to acoor.no.
date those loads, then naxinum preload is not significant in a leading 1
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Attachment G 3
sense. Some bolting caterials such as SA 193 Grado B7 can be susceptible to stresa corrosion cracking and excessive preload :::ay contribute to such cracking. ( A perhaps.necessary co-condition is lack of adequate heat treat.-
ment control in manufacturing the bolts.)
In su.:cary, a preload not less than calculated loads is desirable and, recardless of calculated loads, the bolts should be tight enough to pro-vent loosening due to vibration. Ovortightening could be a problem if stress corrosion cracking is a possibility. With controlled lubrication, a specified torque could be useful. Without controlled lubrication, "ilrenchtight" is probably better than specified torque. In view of the large variations of clamping force for a given torque, torque vrench accuracy of +203 is adequate.
2.
LaSalle County Expansion Anchor Installation and Inspection Values Sargent and Lundy's description or their basis for setting installa-tion and inspection specified torques for concrete anchors ( spocifically, Hilti Wedge Anchors) indicates to me that torque vrench accuracy of 20%
is adequate.
3 Torque Urench Calibration Accuracy (*2% + d6%)
The t2% is the calibration fixture accuracy and the 46% is the limits within which a torque wrench must be. accurate as determined by use of the calibration fixture. In principle, the torque vrench accuracy is 58%.
Based on personal use of tcrque wrenches some years ago, I suspect that in " tight" placos where the torgde wrench can not be pulled on in the cc. e manner as in the calibration, additional inaccuracles could occur.
However, in view of the vide scatter in torque versus clanping force without controlled lubricadon, whether the torque vrench is accurate to aS% or *20% ( as used in tight places) is a relatively trivial consideration.
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Attachment G Incidentally, there is a good discussion of preloading of bolts in flanged joints in the AS1E B&PV Code,Section VIII, Division 1, Appendix S.
That Appendix gives an equation that.gives an indication of what "vrenchtight" a:oans in terns of clanping stress in the bolts.
S = 4500d/ >T (1) where S = probable bolt stress developed nanually when using standard vrenches d5 nominal diaceter of bolt.
Equation (1) was de/cloped by E. C. Patrie in 1937. It is linited to a 2:1/2" and probably to d d 2n; y,11 lubricated threads and other mating surfaces; IIC threads below d el"., the 8 thread per inch series above d=1".
Yours Very Truly h&Y E. C. Rodabaugh Enclosures 1 and 2 cc S. E. Moore 6
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