Pressure Vessel Research Council Research Articles

Combined Effect of Temper and Hydrogen Embrittlement on Threshold for Hydrogen-Induced Fracture in Cr-Mo Steels

This paper presents the results of the research activities of the subcommittee on hydrogen embrittlement of the Japan Pressure Vessel Research Council (JPVRC). The combined effect of temper embrittlement and hydrogen embrittlement in Cr-Mo steels is discussed. It has been recognized that Cr-Mo steels used widely in the refining and petrochemical industry are quite susceptible to temper embrittlement. Although the synergistic relation between temper embrittlement and hydrogen embrittlement is a matter of major concern, studies regarding this subject are rarely encountered. Task group VIII (TG8) of the JPVRC conducted fracture toughness tests for three kinds of 2.25Cr-1Mo steels and 2.25Cr-1Mo-0.3 V steel. These steels were prepared by subjecting them to normalizing, tempering, and postweld heat treatments (PWHTs) which simulated actual conditions. Some specimens were embrittled by step cooling (Socal-1 treatment). It was found that the threshold for hydrogen-induced fracture was lowered when the specimen was exposed to pressurized hydrogen gas (15 MPa) for 48 h at 450 °C and there was no marked indication of the synergistic action concerning this embrittlement.

Stress Analysis and Sealing Performance Evaluation in Rectangular Box-Shape Bolted Flange Connection With Gasket Subjected to Internal Pressure

The contact gasket stress distributions in a rectangular box-shape bolted flange connection with a compressed sheet gasket subjected to internal pressure were analyzed using the finite element method. Leakage tests were also conducted for an actual rectangular box-shape bolted flange connection with a compressed sheet gasket under internal pressure. Using the obtained contact gasket stress distributions and the results of the leakage tests, the new gasket constants were calculated. The difference in the new gasket constants between the values obtained from the present study and those from the Pressure Vessel Research Council (PVRC) procedure was substantial. In addition, a method to determine the bolt preload for a given tightness parameter was demonstrated and the difference in the determined bolt preload was shown between the present study and the PVRC procedure. The characteristics of the rectangular box-shape bolted flange connection were demonstrated by comparing those of circular bolted flange connections.

Proposal of Sealing Performance Evaluation Method Using Gasket Strain in Various Sealing Tests at Room/Elevated Temperature

The relationship between the gasket stress and the tightness parameter has been used in the sealing performance evaluation of gaskets using the ROom Temperature Tightness / Hot Operational Tightness Test (ROTT/HOTT) proposed by the Pressure Vessel Research Council (PVRC). A new evaluation method using the gasket strain rather than the gasket stress is proposed in this paper. In the present study, a new test sequence, namely, the Preheated Hot Operational Tightness Test (PHOTT) is proposed in order to cancel the thermal strain of the experimental setup. A testing machine for ROTT, HOTT and PHOTT that continuously controls the gasket temperature up to 450 °C is constructed. The proposed test can be conducted under either stress controlled or strain controlled conditions, whereas conventional tests are conducted under mixed stress and strain conditions with fluctuating temperature. Test results obtained at room and elevated temperature show that the relationship between the gasket strain and the tightness parameter become a unique relationship on the log-log chart. Therefore, calculation of the bolt load for the assembly process and the operational process is expected to be simplified by evaluating the leakage based on the gasket strain. In addition, the proposed evaluation method is applicable to both asbestos and non-asbestos gaskets (compressed sheet gaskets and spiral wound gaskets).

On the Effect of External Bending Loads in Bolted Flange Joints

Most current flange design methods use an equivalent pressure to treat bolted flange connections subjected to external bending loads. This oversimplified approach together with the lack of a proper assessment of the actual affected tightness make these methods inadequate for modern flange design. The substitution of the external applied moment by an equivalent pressure is excessively conservative and not realistic since it assumes that the achieved tightness is that of a gasket unloaded entirely to a minimum stress whereas in reality only a small section of it is, the rest of it is actually at a much higher stress. The successfulness of a valid analytical approach in yielding to an acceptable solution resides in its ability to account for the circumferential distribution of the gasket contact stress and its effect on leakage. This paper presents an analytical model based on the flexibility of the flange to treat flanges subjected to bending loads such as those produced by external moments and misalignments and capable of integrating leakage around the gasket circumference. The bolted joint sealing performance in the presence of such loads is evaluated using the new Pressure Vessel Research Council (PVRC) gasket constants Gb, a, and Gs, obtained from room temperature tightness (ROTT) tests. The analytical results including leakage predictions are validated by comparison to those obtained numerically by finite element analysis and experimentally on different size flanges. The overconservatism of the equivalent pressure is demonstrated.

On the Residual Stress Profiles in New API 579/ASME FFS-1 Appendix E

In this paper, the mechanics basis underlying the parametric through-thickness residual stress profiles proposed for the new joint fitness for service document referred to as API 579/ASME FFS-1 Appendix E is presented. The proposed residual stress profiles are described to a large extent by a unified parametric function form valid for a broad spectrum of pipe and vessel welds. The functional relationship is established based on the comprehensive knowledge base developed within a recent major international joint industry project (JIP) under the auspice of Pressure Vessel Research Council (PVRC) and a large amount of residual stress measurement data from recent literature. One of the most important features associated with the proposed revision is that residual stress profile is uniquely determined by two important sets of governing parameters: (1) parameters relevant to pipe geometry, i.e., r/t and t; (2) a parameter related to welding linear heat input Q (J/mm), referred to as the characteristic heat input \(\hat Q\)which has a dimension of J/mm3. As a result, the corresponding through-wall residual stress distribution exhibits a continuous change as a function of r/t, t, and \(\hat Q\), instead of falling into a few discrete and unrelated profiles, as seen in the current Codes and Standards.

Design for Leakage in Flange Joints Under External Loads

This paper uses an analytical model based on the Pressure Vessel Research Council (PVRC) ROTT test gasket constants to compute leakage in gasketed flange joints subjected to internal pressure and external bending moments. The model results are compared to test data, and design recommendations are made, consistent with the ASME∕PVRC tightness-based methodology.

Evaluation of uni-axially expanded PTFE as a gasket material for fluid sealing applications

A new process was invented for preparing an improved polytetrafluoroethylene (PTFE) sealant material from a paste formed of PTFE resin with special plasticizer. This expanded PTFE sealant material consisted of a special node and fibrous microstructure. The preferred orientation for mechanical strength of the fibrous structure was parallel to the direction of calendering. The expanded PTFE material was investigated for its mechanical properties, including creep relaxation, compressibility, recovery, torque retention, and gas permeability, as defined by the ASTM and DIN standards. The sealing performance of the material was further characterized by the room temperature tightness test established by the Pressure Vessel Research Council (PVRC). The test results suggest that uni-axially expanded PTFE is soft and compressible, with excellent sealability. It is strong, possessing good resistance against creep and cold-flow. These favorable properties of expanded PTFE, as compared to traditional PTFE-based gasket types, make it a promising gasket material for fluid sealing applications.

Sealing and mechanical behaviors of expanded PTFE gasket sheets characterized by PVRC room temperature tightness tests

This paper presents sealing and mechanical properties of isotropically expanded PTFE gasket sheets, characterized by the room temperature tightness test (ROTT), developed by the Pressure Vessel Research Council (PVRC). The expanded PTFE sheets are highly fibrillated and porous. Three types of sheets were tested, including two regular 1/16 and 1/8-in. thick sheets of density 0.85 g cm−3, and one 1/8-in. thick rigid type gasket sheet of density 1.2 g cm−3. Helium mass leak rates and gasket deflection values were measured. The regular 1/16 in. sheet leaks less than the 1/8 in. sheet by 2–4 times, and the regular and rigid 1/8 in. sheets show comparable leakage. All the tests reveal a drastic improvement in sealability between gasket stress of 1025 and 5390 psi; above that, the leakage still steadily decreases, although improvement occurs at a lower rate. The deflection results, when adjusted with initial thickness, are similar between the regular 1/16 and 1/8 in. sheets, but lower in value for the rigid sheet due to its higher density, and hence lower compressibility. Both leakage and gasket deflection results indicate very similar sealing and mechanical behaviors for all types of expanded PTFE products. The results of expanded PTFE sheets are further compared with those of a skived, virgin PTFE sheet and a filled PTFE sheet. The deflection results confirm the high compressibility, low recovery and excellent creep resistance for the expanded PTFE. These property differences are attributed to the differences in micro-structures of the materials. The comparison shows that the expanded PTFE sheet seals the tightest and creeps the least up to 6500 psis. Tightness hardening effect has greatly limited the sealing performance of the skived and filled PTFE at such high gasket stresses. In addition, the sealing and mechanical behaviors of virgin and filled PTFE are less consistent than expanded PTFE in terms of test-to-test and thickness variations. Gasket selection criteria based on the ROTT test data are discussed. Further, the tightness concept, tightness data, and PVRC gasket constants associated with the test results are presented.

Elastic-Plastic Analysis of the PVRC Burst Disk Tests With Comparison to the ASME Code Primary Stress Limits

This paper provides a comparison between finite element analysis results and test data from the Pressure Vessel Research Council (PVRC) burst disk program. Testing sponsored by the PVRC over 20 yr ago was done by pressurizing circular flat disks made from three different materials until failure by bursting. The purpose of this reanalysis is to investigate the use of finite element analysis (FEA) to assess the primary stress limits of the ASME Boiler and Pressure Vessel Code (hereafter the Code), and to qualify the use of elastic-plastic (EP-FEA) for limit-load calculations. The three materials tested represent the range of strength and ductility found in modern pressure vessel construction and include a low-strength, high-ductility material, a medium-strength, medium-ductility material, and a high-strength, low-ductility, low-alloy material. Results of elastic and EP-FEA are compared to test data. Stresses from the elastic analyses are linearized for comparison of Code primary stress limits to test results. Elastic-plastic analyses are done using both best-estimate and elastic-perfectly plastic (EPP) stress-strain curves. Both large strain-large displacement (LSLD) and small strain-small displacement (SSSD) assumptions are used with the EP-FEA. Analysis results are compared to test results to evaluate the various analysis methods, models, and assumptions as applied to the bursting of thin disks. The test results show that low-strength, high-ductility materials have a higher burst capacity than do high-strength, low-ductility materials. Linearized elastic FEA stresses and ASME Code primary stress limits provide excessive margins to failure for the burst disks for all three materials. The results of these studies show that LSLD EP-FEA can provide a best-estimate analysis of the disks, but the accuracy depends on the material stress-strain curve. This work concludes that SSSD EPP analysis methods provide a robust and viable alternative to the current elastic linearization method of satisfying the primary stress limits of the Code. [S0094-9930(00)01602-4]

Effect of Loading Rate on Fracture Toughness of Pressure Vessel Steels

The master curve method has recently been developed to determine fracture toughness in the brittle-to-ductile transition range. This method was successfully applied to numerous fracture toughness data sets of pressure vessel steels. Joyce (Joyce, J. A., 1997, “On the Utilization of High Rate Charpy Test Results and the Master Curve to Obtain Accurate Lower Bound Toughness Predictions in the Ductile-to-Brittle Transition, Small Specimen Test Techniques,” Small Specimens Test Technique, ASTM STP 1329, W. R. Corwin, S. T. Rosinski, and E. Van Walle, eds., ASTM, West Conshohocken, PA) applied this method to high loading rate fracture toughness data for SA-515 steel and showed the applicability of this approach to dynamic fracture toughness data. In order to investigate the shift in fracture toughness from static to dynamic data, B&W Owners Group tested five weld materials typically used in reactor vessel fabrication in both static and dynamic loading. The results were analyzed using ASTM Standard E 1921 (ASTM, 1998, Standard E 1921-97, “Standard Test Method for the Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range,” 1998 Annual Book of ASTM Standards, 03.01, American Society for Testing and Materials, West Conshohocken, PA). This paper presents the data and the resulting reference temperature shifts in the master curves from static to high loading rate fracture toughness data. This shift in the toughness curve with the loading rate selected in this test program and from the literature is compared with the shift between KIc and KIa curves in ASME Boiler and Pressure Vessel Code. In addition, data from the B&W Owners Group test of IAEA JRQ material and dynamic fracture toughness data from the Pressure Vessel Research Council (PVRC) database (Van Der Sluys, W. A., Yoon, K. K., Killian, D. E., and Hall, J. B., 1998, “Fracture Toughness of Ferritic Steels and ASTM Reference Temperature T0,” BAW-2318, Framatome Technologies. Lynchburg, VA) are also presented. It is concluded that the master curve shift due to loading rate can be addressed with the shift between the current ASME Code KIc and KIa curves. [S0094-9930(00)01302-0]

The Pressure Vessel Research Council of the Welding Research Council, Inc.

[S0094-9930(00)02101-6]

Three-Dimensional Stress Criteria—Summary of the PVRC Project

This paper summarizes work done by the Pressure Vessel Research Council (PVRC) on three-dimensional stress criteria, using primarily elastic two and three-dimensional analytical techniques, finite element analysis. The focus of the work was to recommend guidelines on evaluation of elastic stresses relative to the ASME Boiler and Pressure Vessel Code (Code) defined failure modes as they relate to stress limits. The guidelines are developed such that they may be used to update and expand the procedures for evaluating the required stress limits in Code Section VIII, Division 2 and Code Section III, Subsection NB. This paper summarizes the recommendation using ten guidelines. The project addresses eleven example problems, nine of which include finite element analyses. These example problems validate the recommendations. The detailed information is published in the Welding Research Council Bulletin 429, “3-D Stress Criteria Guidelines for Applications.” [S0094-9930(00)01601-2]

Design for optimum gasket assembly stress and leak rate control in bolted gasketed joints

The behavioural factors affecting the design of a bolted gasketed joint for a prescribed leak rate are described. The traditional approach is now known to be ill founded while the proposed alternative has disadvantages. These approaches are outlined and their problems described. An improved design method is presented, on the basis of a design chart. In this the optimum gasket assembly stress to achieve a given level of leak tightness is plotted against the known gasket stress change as the fluid pressure is applied, giving curves for a series of leak-tightness values. The design chart can be derived graphically from a Pressure Vessel Research Council (PVRC) ‘ROTT’ test-data plot or by calculation from the PVRC gasket coefficients Gb, a and Gs; the procedures are described. The approach presented has the advantages of simplicity and accuracy. Examples of the new design chart are given for ten types of gasket and the effects of varying individual PVRC gasket coefficients are demonstrated.

Fatigue Strength Reduction Factors for Welds Based on Nondestructive Examination

Based on the author’s hypothesis that nondestructive examination (NDE) has a major role in predicting the fatigue life of pressure vessels, a project was initiated to develop a defined relationship between NDE and fatigue strength reduction factors (FSRF). Even though a relationship should apply to both base metal and weld metal, the project was limited to weld metal because NDE for base metal is reasonably well established, whereas NDE for weld metal is more variable, depending on application. A matrix of FSRF was developed based on weld type (full penetration, partial penetration, and fillet weld) versus the NDE that is applied. The NDE methods that are included are radiographic testing (RT), ultrasonic testing (UT), magnetic particle testing (MT), dye penetrant testing (PT), and visual testing (VT). The first two methods (RT and UT) are volumetric examinations, and the remaining three are surface examinations. Seven combinations of volumetric and surface examinations were defined; thus, seven levels of FSRF are defined. Following the initial development of the project, a PVRC (Pressure Vessel Research Council) grant was obtained for the purpose of having a broad review. The report (Hechmer, 1998) has been accepted by PVRC. This paper presents the final matrix, the basis for the FSRF, and key definitions for accurate application of the FSRF matrix. A substantial amount of additional information is presented in the PVRC report (Hechmer, 1998).

Japanese developments in the ultrasonic examination of pressure vessels

In Japan, nuclear power plant has grown to such an extent that it now provides a fundamental part of total power generation. In consequence, the assurance of structural integrity is increasingly important. The activities of three representative organizations, the Nuclear Power Engineering Test Center (NUPEC), the Japan Pressure Vessel Research Council (JPVRC) and the Japan Power Engineering and Inspection Corporation (JAPEIC) are briefly overviewed in this paper. NUPEC is carrying out the proving test program of in-service inspection. Manual ultrasonic inspection testing was completed in early 1984; factors affecting defect detection and sizing were examined and some of these test results are introduced here. Mechanized automatic inspection testing with devices now in use in recent operating plants is underway. JPVRC is participating in the international round robin test, PISC-II, and in the test between US-PVRC to investigate the reliability of ultrasonic examination, and some test results are introduced. JAPEIC recently expanded its activities into research and development in the area of structural integrity, robotics, mechanized inspection and non-destructive inspection. Some topical items for these programs are overviewed.