Inspection and Assessment Methodologies for Estimating Remaining Creep Life of Outlet Pigtails in a Steam Methane Reformer

Introduction

A methanol plant had undergone a planned shutdown in July 2021 to replace burners in the radiant section of the steam methane reformer (SMR). Based on the inspection observations in the 2018 turnaround and 2019 planned shutdown, an emphasis was placed on the inspection and assessment of the outlet pigtails carrying the reformed gas from the catalyst tubes to the outlet manifold headers. This work provided information about the condition to ensure the reliable operation of the outlet pigtails until the next major turnaround planned in October 2023.

This article will discuss a prominent damage mechanism found in the outlet pigtails of steam methane reformers – creep deformation. It will also cover the causes of damage, appropriate inspection methods, and assessment methodologies adopted for estimating creep remaining life. All of this is presented in an effort help others ensure the reliability and safety of their outlet pigtail systems.

Based on the authors’ experience, inferences from steam methane reformer inspection assessments, and a review of relevant published literature, this article provides guidelines on the importance of the following:

• Effective management of reformer outlet cross header temperatures.
• Quality control procedures when procuring new outlet pigtails during the pigtail tube manufacturing and fabrication process.
• Future retirement limits of outlet pigtails in terms of creep growth and measures undertaken for safe operation and procurement of new pigtails upon observing creep growth.

Overview

The methanol plant’s reforming furnace is heated by burning natural gas as fuel. The 768 catalyst tubes within the furnace contain the process fluid that transforms methane (CH_⁴) and steam (H₂O) into intermediate reactants of hydrogen (H₂), carbon dioxide (CO₂), and carbon monoxide (CO).

The outlet pigtail tubes transport the process fluid from the catalyst tubes to 24 collection headers. The stresses due to relative thermal expansions between the catalyst tubes and the outlet manifolds are accommodated by tubes known as pigtails. Failure of these outlet pigtails is a major cause of downtime and a potential safety risk in the event of a through-wall crack and release.

Plant History

The methanol plant was commissioned in April 2010 and designed to produce 3000 MTPD. Planned major turnarounds occurred in April 2014 and April 2018. Planned shutdowns occurred in August 2019 and July 2021. Creep growth observed on the outlet pigtails during the planned shutdowns prompted an inspection of 100% of pigtails in July 2021. Based on creep growth and microstructure analysis, 10 pigtails were replaced. After the July 2021 shutdown, a procurement process to replace all pigtails was initiated for the next planned turnaround in October 2023. In parallel, a detailed analysis was performed on the 10 pigtails that were removed. Based on the results of the detailed analysis, a planned shutdown was undertaken in March 2023, during which 15 additional pigtails were replaced due to accelerated creep growth and microstructural degradation. Despite these efforts, a pigtail failure occurred in May 2023 forcing an unplanned shutdown of the facility.