Convection Section Failure Analysis and Fitness-for-Service Assessment

Introduction

In-service equipment failures present a considerable challenge to reliability engineers. Since these events are unplanned, the spare parts and resources to quickly and efficiently make repairs may not be readily available. Facility management is faced with decisions to perform short-term repairs to equipment to safely operate until repair parts, equipment, manpower and product supply permit an extended outage and long-term repair. In these instances, the reliability engineer will need to answer many questions, but in the case of a failure of convection tubes in a furnace (as presented in this article), they will need to answer whether the facility can restart the furnace and continue to operate reliably until the next planned shutdown.

Tube Failure Analysis

A large-scale natural gas manufacturing plant in the United States suffered a tube failure in the convection section of a fired heater. The failure occurred in one of the eight passes of finned tubes near the top of the convection section (see Figure 1).

Results of the failure analysis indicated that the finned convection tube failed by tensile overload from a through-wall crack near the “hot face” of the tube. A brittle fracture occurred through the circumferential tube weld at the center of the tube located at the middle tube support (see Figure 2). The weld was embrittled by sigma phase (σ) formation in the material of construction that results in a loss of fracture toughness in heat resistant castings from elevated temperature exposure.

The tube bundle was supported by three tube sheets (tube supports); two on the ends and one in the middle. Subsequent inspection identified that both end tube supports had dropped down due to failure of the four support bolts (two for each end support). The loss of support at the end tube sheets resulted in the tube coil having a high tensile stress, with a bending and possibly a torsional component, on the top of the tubes at the failure site(s) along the middle support.