Strength and leakage finite element analysis of a GFRP flange joint

Abstract

The strength and leakage analysis of a glass fiber reinforced plastic (GFRP) modified stub flanged joint is presented. The analysis is carried out using three-dimensional and axisymmetric finite elements. To model the GFRP material, we use an orthotropic material model. To model the loss of contact between the flange and the gasket, we use a contact condition between these mating parts. In the axisymmetric case, we allow fluid to penetrate the area where contact is lost; the three-dimensional contact formulation, however, does not support this fluid penetration option. We use a commercial finite element package (Abaqus) to perform the analysis. The primary emphasis is on assessing the strength of the flange and the leak tightness of the gasket. Bolt interaction is not considered, therefore we assume a constant bolt load over the history of the analyses.

Introduction

Glass fiber reinforced plastic (GFRP) flanged joints are widely used in the piping and pressure vessel industries. The flanged joint is used because systems in these industries usually include pumps, valves and other fittings that require periodic removal for maintenance. GFRP materials are used as a result of their corrosion resistant properties. Flanged joints are susceptible to two failure mechanisms regardless of their material composition: material failure and leakage. Therefore, a joint must be properly designed to prevent these two failures. However, current joint design procedures to address both these failures are primarily empirical due to the complexity of the stress and leakage analyses. Previous investigations of GFRP joints have generally focused on design for strength, whereas leakage has received little attention. All except two of the previous investigations regarding strength are primarily concerned with the transfer of metallic design calculations, without accounting for the orthotropy of the composite material properties [1], [2]. Sun [3] presents stress calculations for a GFRP flange joint using lamination theory and finite element analysis; however, he did not study leakage. Godwin et al. [4] studied GFRP joint leakage by investigating the relationship between clamping force and joint sealing for stub, and full-face GFRP flanged joints using experiments and axisymmetric finite element analysis. The normal stress on the gasket was found to be the principal factor affecting the leak tightness of these joints. Finite element analysis was conducted to predict the internal pressure at which leakage occurs and the pressure distribution on the gasket. Although tensile normal stresses developed over a part of the gasket face, no flange–gasket separation was allowed, because their finite element analysis code did not support contact formulations. In this paper, we describe detailed strength and leakage three-dimensional and axisymmetric finite element analyses, in which we use a contact formulation to allow loss of contact between the mating flange and the gasket as the internal pressure increases. Also, in the axisymmetric model, we allow fluid penetration into the space where this contact loss occurs; however, this type of loading is not supported in a three-dimensional analysis. These finite element analyses are performed using the commercial finite element package Abaqus.

The analyses were performed on a modified stub flanged joint (Fig. 1). This GFRP joint is a modified version of a typical GFRP joint, the stub flanged joint, and was developed by Estrada and Parsons [5] to address some problems particular to GFRP joint geometries currently in use. The pipe and hub are filament wound as an integral unit. The metallic backing ring is used to connect the joint to other members. The joint was proportioned using the design guidelines presented in Ref. [6], and the ASME gasket design criteria; the dimensions are shown in Fig. 2.

We also attempted to validate the ASME gasket (leakage) design formulation using detailed finite element analysis for a particular joint and gasket. The formulation can easily be extended to other joint configurations and gaskets. The ASME code leakage design guidelines are based on two gasket constants, m and y factors; these are determined empirically. The yield factor y is defined as the minimum gasket stress required to cause the gasket material to deform into the flange face irregularities. The product m×p is defined as the minimum gasket stress needed to hold the joint sealed under the internal pressure p. These factors are used to determine the bolt load to be applied for an effective seal. That is, the y factor is used to determine the initial bolt load to be applied to the joint, and the m factor is used to determine the gasket pressure needed to prevent leakage while the joint is in operation. The implementation of these factors is through the calculation of the design bolt load: each factor is used to compute a bolt load, and the larger of the two is used to design the joint for strength.

The paper is organized as follows. First, we describe the evolution of leakage over the loading history of the joint and give a description of the gasket used in the analyses. We then discuss the detailed finite element analyses: we describe the three-dimensional analysis, and then discuss the analysis for the axisymmetric model. In the three-dimensional model, we make use of the cyclic symmetry of the system and only consider a 1/64 segment of the total circumference of the joint (see Fig. 2). Since Abaqus does not support pressure penetration in the three-dimensional contact analysis, we also conducted an axisymmetric analysis to properly model joint leakage. In the last part of the paper we present a summary of the results.