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Appendix J: Bolting Preload
Pages 217-225

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From page 217...
... The procedure for tightening flange bolts must conform to various industry and OEM specifications to ensure bolts are tightened as evenly as possible.1,2 Flange bolting procedures include the following: • A criss-cross bolt tightening pattern, • A bolt tightening sequence (e.g., all bolts are hand tightened, then all bolts are tightened to 25 percent of desired preload, then 50 percent, then 75 percent, then 100 percent) ; • Determination of required bolt preloading, as discussed below; and • Method of achieving and verifying bolt preload, also discussed below.
From page 218...
... API Spec 17D specifies the following tensile preloading of flange bolts: • All 6BX and 17SS (types of API flanges) flange bolts be preloaded to between 67 percent and 73 percent of a bolt's yield stress.3 • A maximum allowable tensile loading on flange bolts, including forces from rated internal pressure, based on the root area of the thread, of 83 percent of yield strength (through API Spec 6A and ISO 10432)
From page 219...
... Although the RCSC technique references ASTM standards rather than API specifications, and was developed for bolted structural connections on bridges and buildings, it may have some applicability to flange connectors. Some interesting aspects of the RCSC technique are -- addresses both LRFD (probabilistic)
From page 220...
... Looram, Good Bolting Practices, A Reference Manual for Nuclear Power J Plant Maintenance Personnel, Volume 1: Large Bolt Manual, Electric Power Research Institute, Palo Alto, Calif., 1987.
From page 221...
... Looram, Good Bolting Practices, A Reference Manual for Nuclear Power Plant Maintenance Personnel, Volume 1: Large Bolt Manual, Electric Power Research Institute, Washington, D.C., 1987. estimated that 50 percent of the torque was consumed by underhead friction and 40 percent of the torque is consumed by thread friction.
From page 222...
... Shoberg, PE, Engineering Fundamentals of Threaded Fastener Design and Analysis, PCB Load & Torque, Inc., Farmington Hills, Mich. FIGURE J.3  Torque vs.
From page 223...
... API Specification 17D, "Specification for Subsea Wellhead and Christmas Tree Equipment," specifies bolt loading limits as a percentage of yield stress (not ultimate tensile strength) , because plastic deformation of bolts is as undesirable an event as an actual bolt failure if it leads to failure of the connector to maintain the pressure boundary of the well.
From page 224...
... It is useful to compare the safety margins for subsea wellhead and Christmas tree equipment, as specified in API Spec 17D, to the safety margins for surface wellhead and Christmas tree equipment, as specified in API Spec 6A.29 For surface wellheads bolt preloading is specified at only 50 percent of yield; the operational bolt loading remains at 83 percent of yield. Consider that the operational loading on the onshore Christmas tree bolts would be mainly from internal pressure on the connector, which is known to a high degree of certainty.
From page 225...
... As discussed above in the section on Bolt Preloading, torqueing is a very inaccurate method for achieving bolt preload. The ±25 to 30 percent accuracy range of using torque to preload bolts and nuts should be considered when determining the suitability of 20.5 to 50 percent (preload and operating)


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