Qualitative RBI – Simplified Screening Tool for Asset Management

Introduction

Inspections are the primary, ongoing source of data acquisition for asset integrity management (AIM) programs and are key to the effectiveness and success of these programs. Over the years, inspection methods and techniques have been refined with the aim of improving the accuracy of damage mechanism diagnoses, reliability, and equipment remnant life predictions. Risk-based inspection (RBI), which emerged as a new inspection planning and prioritization methodology roughly two decades ago, calculates the probability and consequence of failure in order to optimize inspection intervals and related activities based on relative risk [1]. Many facilities and inspection personnel are reluctant or slow to implement RBI programs due to numerous challenges, such as timeline interruptions, budget limitations, data gaps or inaccuracies, uncertain or intangible benefits, etc.

Even today, the effectiveness of RBI is a matter of question for many users as they haven’t reaped the benefits of RBI due to various reasons that can be classified as “known unknowns” and “unknown unknowns.” Among the known unknowns are the existence and quality of inspection data, as well as the design of assets. Inadequate consideration of corrosion and material degradation during the design and operation phases of many older facilities has complicated the implementation of modern-day RBI programs on those assets today.

This article presents a case study of a heat exchanger which had an inconsiderate design for modern-day RBI programs. This study also discusses a simplified qualitative RBI-based screening matrix to address the risks in a typical cryogenic shell and tube heat exchanger with welded dish end.

Case Study

A shell and tube heat exchanger in cryogenic service (liquid oxygen) on the tube side and seawater on the shell side was leaking at the flanged connection between the tube bundle and the shell. The exchanger was decommissioned, followed by dismantling, mothballing, and further inspection. During inspection, pitting was found on the flanged faces. An inspection and repair plan was formulated for the flanged connections, shell, and tube bundles in line with industry practices and original equipment manufacturer (OEM) guidelines. The uniqueness of the heat exchanger was that it had a dish-shaped channel head that was welded to the tube sheet, unlike common shell and tube heat exchangers where the channel heads are generally bolted to the tube sheets. Due to the welded configuration, the proposed work sequence involved the removal of the dish end via cutting, followed by tube inspections, repair, pressure testing (for tubes only without welded dish end), re-welding, and final pressure testing at the maximum allowable working pressure (MAWP) for the entire bundle. The dish head’s material was type 304L stainless steel (UNS S30403), and the tubes were made from duplex stainless steel grade SMO 254 (UNS 31254). The tubes also comprised helical spiro-vanes (material: brass) to improve the heat transfer between shell and tube side fluids. Considering the high flammability risks associated with oxygen service, the tubes required extreme cleanliness followed by inspections using a black light. The flange faces were inspected visually and dial gauges were used to inspect for out-of-roundness. It was found that the flanged faces had some out-of-roundness which was attributed to thermal distortion from welding of the flanged connections during the original fabrication. Areas with pitting were ground and built-up using ER 308L filler wire, followed by in-situ machining (i.e., flange facing) to achieve a smooth flange face and compensate for the out-of-roundness of the flange.

The dish end was dismantled via cutting, followed by pneumatic pressure testing of the tubes at 0.9 MPa. The leaking tubes were plugged with pre-machined plugs and seal welded. In the meantime, the edges of the dish ends were built-up using an E308L electrode. After black light inspections and a successful pressure test (at 0.9 MPa), the dish end was re-welded to the tube sheet per conventional welding procedure specification (WPS) using an E308L electrode. During the re-welding, cracks were found in the weldment, which appeared immediately upon cooling of the weldment. Also, high hardness (240 HB) was found in the heat-affected zone (HAZ). Another WPS was proposed using ENiCrFe-3, which eliminated the issue of weldment cracks. Upon weld completion using the new WPS, the equipment was inspected, and the tube side of the exchanger was filled with water in preparation for pressure testing. The heat exchanger’s tube side was set to be pressurized incrementally, namely 25% of MAWP, 50% MAWP, 75% MAWP, and 100% MAWP. Upon reaching ~50% of MAWP (i.e., 2.0 MPa), leakage was observed at the expansion portions (i.e., tube-to-tube sheet junctions) from almost every tube. The heat exchanger was immediately de-pressurized, drained, and further inspected. It was found that nearly every tube ruptured, leaving the heat exchanger’s tube bundle beyond repair.