The steam reformer is a major and critical component in the production of ammonia. Much attention is paid to the reliability and inspection of catalyst tubes but equally important in ensuring reliable operation are the pigtails carrying the reformed gas from the catalyst tubes to the collection manifold. The pigtail design is based on protecting against creep due to exposure to high temperature and pressure in the same way as the catalyst tubes, using a code such as API 530. There are however, a number of mechanisms by which pigtails may ultimately fail and these include: Creep rupture (the basis of design); thermal movement of the manifold relative to the catalyst and/or manifold tube causing bending at the terminal weld; mid-wall cracking at bends; and environmental attack e.g. nitriding. This paper describes case studies that illustrate the range of damage mechanisms which are active in outlet pigtails and attempts to identify common factors that have contributed to the failures associated with such systems. For each case, appropriate inspection methods and reliability strategies are presented. In particular, the way in which material is processed at the time of pigtail manufacture has been found to result in material properties that contributes to some of these failures. The behaviour of Alloy 800H/HT, by far the most common material of construction, may be directly influenced by these processing methods. Methods for avoiding such pitfalls are presented and alternative material procurement requirements are described.
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