Analysis of Stresses and Strains in Packed Stuffing-Boxes

Packed stuffing-boxes are mechanical sealing systems that are extensively used in pressurized valves and pumps. Yet there is no standard design procedure that could be used to verify their mechanical integrity and leak tightness. It is only recently that standard test procedures to qualify the packing material have been suggested for adoption in both North America and Europe. While the packing contact stress with the side walls is predictable using existing models there is no analytical methodology to verify the stresses and strains in the stuffing-box housing. This paper presents an analytical model that analyzes the stresses and strains of all the stuffing box components including the packing rings. The developed model will be validated both numerically using FEM and experimentally on an instrumented packed stuffing box rig that is specially designed to test the mechanical and leakage performance of different packing materials.

Selecting Material Properties for Maximizing Gun Firing Power

The design of gun barrels aims at maximizing its firing power determined by its SMP — the maximal allowed firing pressure, which is considerably enhanced by inducing a favorable residual stress field through the barrel’s wall commonly by the autofrettage process. Presently, there are two distinct processes: hydrostatic and swage autofrettage. In both processes the barrel’s material is fully or partially plastically deformed. Recently, a 3-D code was developed, which finally enables a realistic simulation of both swage and hydraulic autofrettage, using the experimentally measured stress-strain curve, and incorporating the Bauschinger effect. This code enables a detailed analysis of all the factors involving the final SMP of a barrel, and it can be used to establish the optimal process for any gun barrel design. A major outcome of this analysis was the fact that the SMP of an autofrettaged barrel is dictated by the detailed plastic characteristics on the barrel’s material. The main five plastic parameters of the material that have been identified are: the exact (zero offset) value of the yield stress, the universal plastic curve in tension and in compression, the Bauschinger Effect Factor (BEF) curve, and the Elastic-Plastic Transition Range (EPTR). A detailed analysis of these three materials points to the fact that the major parameter determining the barrel’s SMP is the yield stress of the material and that the best way to determine it is by the newly developed “zero offset” method. All these four parameters have a greater influence on the SMP of an hydraulically autofrettaged barrel than on a swaged one.

Leakage Analysis of a High Pressure Flange Connection Under Bending Moments: An Analytic Procedure

High pressure flange connections for LDPE plants are designed using lens ring gaskets and bolted flange connections. The sealing principle is to achieve a high contact pressure between the spherical lens shape and the conical tube end. This contact pressure must exceed the fluid pressure by a safety margin under all operating conditions. Bending moments acting on the flange connection will create an uneven distribution of contact pressure over the lens ring surface. In this paper a procedure is shown to analyse the influence of the bending moment under consideration of internal pressure on a flange connection using 3-dimensional finite element models. The variations in bolt stress and distribution of contact pressures due to the bending moment are of specific interest. A first approach will be provided to derive some simplified design rules regarding leakage assessment of such flange connections.

Performance-Based NDE Personnel Qualifications

This paper establishes an alternative approach for nondestructive examination (NDE) personnel qualification for the boiler and pressure vessel (B&PV) industry. This is the “white paper” developed by the Section XI, Division 2, Task Team, Performance-based NDE Personnel Qualifications. It is anticipated to be the basis for the NDE personnel qualification criteria for the revised Section XI, Division 2, REQUIREMENTS FOR RELIABILITY AND INTEGRITY MANAGEMENT (RIM) PROGRAM FOR NUCLEAR POWER PLANTS that is currently being developed. Based on review of a number of quantitative NDE reliability studies conducted to-date the current deterministic approach to NDE personnel qualification based on such schemes as ASNT SNT-TC-1A, ANSI/ASNT CP-189, EN-473, ISO-9712 and other similar approaches are not as effective as desired. The goal of this document is to present an alternative approach to the deterministic NDE personnel qualification schemes. This paper presents a systematic approach to training together with performance-based tests, psychometrically validated evaluation of knowledge and skills that will improve a NDE candidate’s performance. A majority of traditional employer-based written examinations are not developed or validated psychometrically. The use of third-party psychometrically validated examinations would replace the current practice of employer developed and administered examinations. It improves upon traditional ASNT SNT-TC-1A, ASNT/ANSI CP-189, ISO-9712, etc., requirements by including more comprehensive hands-on practical examinations on a statistically valid set of samples containing flaws representative of those expected to be encounter in shop and field conditions. The sample sets will be designed for either a “general” NDE method, or “limited” technique(s) of a method, or for industry specific sector needs, as applicable.

Experimental and Numerical Analysis for Fluid-Structure Interaction for an Enclosed Hexagonal Assembly

This paper is related to fluid-structure interaction analysis of sodium cooled fast reactors core (Na-FBR). Sudden liquid evacuation between assemblies could lead to overall core movements (flowering and compaction) causing variations of core reactivity. The comprehension of the structure behavior during the evacuation could improve the knowledge about some SCRAMs for negative reactivity occurred in PHÉNIX reactor and could contribute on the study of the dynamic behavior of a FBR core. An experimental facility (PISE-2c) is designed composed by a Poly-methyl methacrylate hexagonal rods (2D-plan similitude with PHÉNIX assembly) with a very thin gap between assemblies. Another experimental device (PISE-1a) is designed and composed by a single hexagonal rod for testing the dynamic characteristics. Different experiments are envisaged: free vibrations and oscillations during water injection. A phenomenological analysis is reported showing the flow behavior in the gap and the structure response. Also computational simulations are presented in this paper. An efficient numerical method is used to solve Navier-Stokes equations coupled with structure dynamic equation. The numerical method is verified by the comparison of analytic models and experiments.

Simplified Methodology for Analysis of Reflected Detonation and DDT: Part 1 — Elastic Analysis

This paper introduces a simplified approach for analyzing local elastic hoop response of piping to reflected gaseous detonations and deflagration-to-detonation transitions (DDT). A consequence associated with gaseous explosions is the potential for DDT to occur near the end of a closed pipe or gas pocket. As an accelerating deflagration flame approaches a closed end, the unburned fuel ahead of the flame front is compressed to an elevated initial pressure. This process is often referred to as pressure piling or pre-compression, and the combination of detonation reflection with the elevated initial pressure, can produce extremely high peak pressures and large values of impulse. In this paper, the event where DDT occurs immediately ahead of the reflecting surface is referred to as a reflected-DDT (R-DDT). In addition to gas mixture and initial conditions, the peak pressure and shape of the pressure time-history associated with a reflected detonation or DDT is a function of the detonation propagation distance prior to reflection and the relative distance between the ignition location, the DDT location, and the reflecting surface. An empirical pressure time-history has been developed using pressure data from approximately 190 explosion tests using hydrogen and nitrous oxide mixtures in 2-inch and 4-inch pipe to describe events ranging from fully-developed reflected detonations to R-DDTs. The empirical model of the pressure time-history was validated by comparing measured elastic hoop strains to finite-element model predictions using the pressure time-history model. Part 2 of this paper compares the methodology to plastic response data.

Potential Difference Distribution on Flat Plate Around Semi-Ellipsoidal Thinning on the Back Surface

Local wall thinning is one of serious problems in aged power generating plants and is known to occur mainly by Flow Accelerated Corrosion (FAC) and Liquid Droplet Impingement (LDI) erosion. As the thinning grows inside the pipes, it is difficult to detect and evaluate it from the outer surface of pipe. The direct-current potential difference method (DC-PDM) is thought to be a suitable non-destructive technique to monitor the initiation and growth of these damages. In this study, an approximate analysis method of the potential difference around a semi-ellipsoidal thinning on the back surface of flat plate is presented and the accuracy is discussed based on the results obtained by the finite element analyses.

Study on Simplified Prediction Method of Thermal Ratchet Deformation Based on Parallel Bar Model

For elevated temperature structures such as fast breeder reactor components, inelastic deformation is likely to occur because of reduction of yield stress and occurrence of creep deformation. The typical failure modes for elevated temperature structures are excessive deformation caused by the accumulation of inelastic deformation and creep fatigue caused by inelastic strain concentration at structure discontinuities. In order to prevent such failures, it is necessary to evaluate inelastic deformation adequately. Thermal ratchet deformation, namely the progressive plastic deformation induced by cyclic thermal stress with uniform primary stress, has some possibility resulting in excessive deformation. ASME boiler & pressure vessel code provides elastic evaluation methods for thermal ratchet. However, these methods are so focused on preventing thermal ratchet deformation, that it could be too conservative under some conditions. Therefore, a simplified elastic evaluation method to quantify thermal ratchet deformation is desired. In this paper, the simplified prediction method for thermal ratchet deformation is proposed using parallel bar model, which represents stress redistribution mechanism of cylindrical vessels. The solution of thermal ratchet deformation of parallel bar model was derived and compared with FEM calculation results of cylindrical vessels. This theoretical solution is proposed as a prediction method for thermal ratchet deformation of cylindrical vessels. The applicable area of the proposed prediction method is the cylindrical vessel under linear and parabolic temperature distribution through the wall thickness.

Managing Transient Operating Conditions of Hyper Compressors

Special reciprocating compressors are required to reach the very high pressures necessary to feed the reactors used in LDPE plants. The working conditions of the compressors are determined by the reactor operating pressures and are considered for evaluation of the loads, torsional, pulsation and vibration conditions. Performance and safety are key considerations and operator experience is essential for optimal and safe plant operation. Plant automation contributes to safety and optimizes process conditions. The compressor start-up and shutdown procedures, commonly detailed in maintenance manuals, are analyzed studying the operating parameters to draw up guidelines for avoiding damage to plunger seals and keeping loads within design limits, without compromising the design performance. The multiple gas compositions of co-polymers and the fact that gas expansion can cause a reduction in temperature, need adequate consideration to avoid damage caused by polymer inside the compressor cylinders. The paper will also give recommendations on the necessary procedures to reach the operating pressures with correct distribution of the total pressure ratio between the two stages. These will also include the requirement of minimum pressure on the plunger head to maintain contact with the crosshead and avoid failure of such a brittle component.

A Unified Constitutive Model for High Temperature Multiaxial Creep-Fatigue and Ratcheting Response Simulation of Alloy 617

The study is developing a unified constitutive model for Alloy 617 which is the prime candidate material considered for intermediate heat exchanger (IHX) of next generation nuclear power plant. Alloy 617 can experience long term exposure to elevated temperatures as high as 950°C, however, the ASME design code (Subsection NH) does not include design provisions for this temperature range. In addition, the Draft Alloy 617 Code Case specifies that the inelastic design analysis for temperatures above 649°C must be based on unified constitutive model. Therefore, this study focuses on developing a unified constitutive model to simulate high temperature uniaxial and multiaxial creep-fatigue and creep-ratcheting responses of Alloy 617. As multiaxial response simulation is a key factor for design-by-analysis of IHX, a set of biaxial tests with varying degrees of loading non-proportionality has been performed at different steady temperatures within 25°C–950°C, and with different strain rates and strain ranges. From the tests, it has been observed that temperature, strain rate, strain ranges and non-proportionality of loading path greatly influences the creep-fatigue and creep-ratcheting responses of Alloy 617. Thus, development of a unified constitutive model considering dependence of these parameters is required. The current Chaboche viscoplasticity model with static recovery term can simulate uniaxial responses very well but it overpredicts biaxial ratcheting responses. Hence, a modified Chaboche model has been developed to improve biaxial ratcheting simulations. Multiaxial modeling features of non-proportionality and ratcheting are investigated. These modeling features and improved response simulations are presented in the paper.