Comparison on Mechanical Behavior of Bolted Flanges Per Old and New Section VIII Division 2

The allowable stress for 2.25Cr 1Mo 0.25V steel per ASME Section VIII Div.2 2007 edition (hereinafter called New Div2) becomes higher than that per old Div.2. Therefore the thickness of flange can be thinner than that per old Div.2. The internal fluid leakage is main concern as well as stress level especially for large size flanges per New Div.2. In this paper the analysis method using axi-symmetric model are investigated and the numerical analyses using axi-symmetric models for two types of flange design are performed in order to compare the flange rotation, gasket contact stress and stress level of flange and bolts. The contact stress of New Div.2 flange was lower than that of Old Div.2 flange totally. However the maximum gasket contact stress of New Div.2 flange which occurred at outer edge of the gasket is almost same as that of Old Div.2. Therefore it is expected that the sealing performance of flange connections per both Old Div.2 and New Div.2 is almost same.

Investigation on Different Tightening Sequences on Several Bolted Flange Types, Dimensions and Their Associated Gasket Types

In the last years, several projects have been performed on alternative tightening sequences in the US and in Japan. This work has led to the publication of documents introducing alternative tightening procedures as JIS B 2251:2008 / HPIS Z 103 TR (JAPAN) and the new revision of ASME PCC-1:2010. In this study, the required bolt torque is calculated according EN1591-1 [1] (EN standard for bolted flange calculation according to a given tightness class) to achieve a given tightness class for each investigated gasket (6 types) and flange type (CLASS/PN)/dimension (from 4″/DN100 to 16″/DN400). The calculated bolt torque is then applied using a hydraulic torque wrench following three tightening sequences based on Legacy cross-pattern of ASME PCC-1:2010, alternative pattern #1 of ASME PCC-1:2010 and HPIS Z 103 TR. The bolted flange connection is then pressurized with Helium. The load for each bolt is continuously monitored using strain gauges, through the bolt-up phase and during pressurization. The average obtained bolt load, the bolt load scatter, the evolution of bolt load versus passes and elapsed time are compared for all the tested configurations (tightening sequence/ bolted flange type & dimension/ gasket type). A “Quality factor” is defined enabling comparison between the tightening quality (according to several criteria) of the different investigated tightening sequences in the tested configurations.

Effects of Post-Tightening on the Residual Bolt Force and the Sealing Performance of Flanged Connections With PTFE Gaskets (Long Term Effects at Room Temperature)

It is known that bolt forces reduce significantly after tightening bolted flanged connections in which expanded PTFE gaskets are used. Bolts are often post-tightened in practice after a while to compensate for the reduction of bolt forces. The viscoelastic characteristic of expanded PTFE gaskets is the main cause for the phenomenon. However, the long term effects of the post-tightening on the residual bolt forces and the sealing performance of flanged connections have not been clarified yet. In this study, two sets of flanged connections (2 inch in nominal size), in which PTFE gaskets were used, were prepared and tightened. One of the flanges was post-tightened after a designated time. The reduction of bolt forces and the change in the sealing performance were measured for about two months. The results were compared with those of flanges without post-tightening. Based on the test results, the effectiveness of post-tightening was discussed from the viewpoint of the residual bolt force and the sealing performance. It has been clarified that the effectiveness of post-tightening depends on gasket material and that post-tightening is effective for expanded PTFE gaskets.

Optimization of Re-Torque and Relaxation Parameters of the GUCP

Self-loosening of bolts in non-permanent connections has been the subject of a myriad of investigations; however, because the vast majority of these studies focused on applications with long-term service conditions involving creep and/or cyclic (working or vibratory) loading, there are no analytical techniques that have been developed to address the load decay behavior of bolted assemblies with gaskets under a primary followed up by a secondary torque. The Ground Umbilical Carrier Plate (GUCP) includes a gasketed ASME-type flange assembly that transfers pressurized, cryogenic hydrogen from the Space Transportation System (STS). Although the threaded stud material is nominally elastic under service conditions, the gasket material, a polytetrafluorethylene (PTFE) matrix filled with 25% chopped glass fibers, undergoes viscous, strain hardening deformation. The consequence of over-torquing the assembly is yielding the stud. Alternatively, the consequence of under-torquing is premature loosening and subsequent fuel leakage. As such, identifying the interactions between assembly configuration, initial torque, etc. to relaxation behavior of the assembly has been identified as a means to reduce the dwell period (the time between initial torque and re-torque). Research is carried out to identify the optimal torque parameters that confer a minimal dwell period. This article documents the Gasket Relaxation and Re-Torque Optimization (GRRO) program used to modify procedures employed when connecting the flanges to the fuel tank of the Space Transportation System prior to a launch.

Study of Pullout Performance of Self-Tapping Screws for Human Bone

The study investigates the pullout strength of self-tapping pedicle screws using analytical, finite element, and experimental methodologies with focus on medical device applications. The stress distribution and failure propagation around implant threads in the synthetic bone during the pullout process, as well as the pullout strength of pedicle screws, are explored. Based on the FEA results, an analytical model for the pullout strength of the pedicle screw is constructed in terms of the synthetic bone material properties, screw size, and implant depth. The characteristics of pullout behavior of self-tapping pedicle screws are discussed. Both the analytical model and finite element results are validated using experimental techniques.

Effects of Nut Thinning due to Corrosion on the Strength Characteristics and the Sealing Performance of Bolted Flange Joints Under Internal Pressure

The effects of nut thinning due to corrosion on the strength characteristic and the sealing performance in 3B bolted flange joints under internal pressure are examined from both FEM calculations and experiments. The following results are obtained. When bolts and nuts in a bolted flange joint are tightened with 50% of yield stress, no yield region at the engaged screw thread occurs, however, when they arc tightened with 90% of the yield stress, a small scale yield region occurs at the roots of the engaged three screw threads from the bearing surface of the nut. While verifying this phenomenon using FEM, it was newly discovered that when the height of the nut was reduced to 1/3 (equivalent to approximately two threads) of the original dimensions according to the standards, the sealing performance could no longer be assured.

Ratchet Limits for a Crack in a Welded Pipe Subjected to a Cyclic Temperature Load and a Constant Mechanical Load

This paper presents the ratchet limit analysis of a pipe with a symmetric crack in a mismatched weld by using the extended Linear Matching Method (LMM). Two loading conditions are considered: i) a cyclic temperature load and a constant internal pressure; and ii) a cyclic temperature load and a constant axial tension. Individual effects of i) the geometry of the Weld Metal (WM), ii) the size of the crack, iii) the location of the crack and iv) the yield stress of WM on the ratchet limits, maximum temperature ranges to avoid ratchetting and limit loads are investigated. Influence functions of the yield stress of WM on the maximum temperature ranges and limit loads are generated. The results confirm the applicability of the extended LMM to the cracked welded pipe.

Optimal Shape Design Under Elastic-Plastic Behavior Based on Reference Volume Method

The main objective of this paper is to determine the regions in a component or structure that directly participate in inelastic action (reference volume) using a new robust simplified method, namely the Elastic Modulus Adjustment Procedure (EMAP). The proposed method is based on iterative linear elastic finite element analysis that is implemented by modifying the local elastic modulus of the material at each subsequent iteration. The application of reference volume on optimum shape design is demonstrated through some practical examples including thick-walled cylinder, shank-head component and overlap joint weld. The results show that the reference volume concept can be used to optimize the shape of a body with respect to load carrying capacity and fatigue strength.

Stress Concentration at the Root of Bolt Thread Under Twist With Deformable Flanges

Bolts and Nuts are required to fasten objects, such as flanges in various types of plants. It is known that stress concentration at the root is relevant for its strength for both static integrity and fatigue. A lot of investigations were carried out to understand the behavior of the bolt using experimental and numerical methods. Recently, finite element analyses have been used to examine the detailed behavior of the bolt root successfully. The authors reported such example in a recent study [13] using submodelling (or so-called zooming) technique that handled only bolt tension with rigid fastened surface; some factors were missing. In this paper, the twist of the bolt was taken into account and the fastened object is replaced by deformable plates. The stress concentration factor has been reconsidered.

Finite Element Stress Analysis for Leak Tests of Pipe Flange Connections Subject to Internal Pressure and Bending Moment

This paper reports the results of finite element analysis representing the sealing performance tests on the slip-on type pipe flange connections for 8 inch and 16 inch. The flange connections are subjected to internal pressure and bending moment. Internal pressure is applied by helium gas and the bending moment is loaded through 4 points bending equipment. Gas leak rates are measured by pressure decay method. During the test, the variations in the axial bolt force are monitored for all the bolts by strain gages. The pipe stress at the junction of pipe and flange is also measured. Finite element analysis simulates the tests and the simulated results are compared with the measured data. Then the behaviors of the slip-on type flange connections under internal pressure and bending moment as well as the sealing performance are clarified by the experiment and the finite element simulation.