Achieving Leak-Free Flanged Joints Through Proper Application of Design Codes

Introduction

Leaks from bolted joints are frequently reported in such a way that many facilities consider flanged joint leaks a normal occurrence, not reporting them as a loss of containment incident, although they are potentially hazardous. Many published investigations and literature focus on areas of ambiguity, including assembly quality issues, external loads applied to flanges, and the quality of bolting materials. While these are valid concerns and need attention during either design or assembly, inadequacies in the use of the design calculations of flanged joints–especially custom flanges–also contribute to many reported flange leaks. 

This article examines a trial conducted to develop a procedure for the correct use of design and supplementary rules for the identification of bolt tightening requirements in order to standardize a comprehensive design output. Such a procedure should help eliminate the ambiguity in the industry, which has led to multiple cases of reported leaks.

The rules of the design of custom (non-standard) flanges under Appendix 2 in ASME BPVC Sec. VIII Div. 1 have been used in the industry for a long time, with successful results. Commercial software and manual calculations are well-established and commonly used for this purpose. However, the correlation between the flange design results and the bolt load specifications is poorly understood despite the development of ASME PCC-1. In many cases, the design output ends by calculating the flange and gasket dimensions and does not determine the recommended bolt load.

It should be the designer’s role to include the bolt tightening load/torque requirements during the design and not make it the responsibility of the field assembly crew. In addition, correct application of the hydrostatic test after fabrication using the same bolting and gasket specifications as those used in the design as proof of the full-flanged joint integrity is a necessary step in the joint design process.

Case Histories

The following case histories address areas of ambiguity. They include explanations for the proposed design phase method, which have been compiled using the requirements of Appendix 2 along with ASME PCC-1 and WRC Bulletin 538.

Case History 1

A custom flange with dimensions modified from 2.5-inch class 600 (per ASME B16.5) to match a pipe size of 3 inches (Figure 1), to be used at the end of stacked fired heater tubes at intermediate pressure and temperature (40 bar or 580 psi and 400° C or 752° F). Standard 3-inch flanges could not be used due to limited space in this area.