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Inspectioneering Journal

Laser Profilometry Applied To Catalyst Tubes In Reformers For Ongoing Condition Monitoring of Creep Damage

By Richard D. Roberts at Quest Integrity Group. This article appears in the November/December 1999 issue of Inspectioneering Journal

Steam reformers are an integral part of ammonia, methanol, hydrogen, and gas process plants around the world. They are one of the highest cost, both in capital and maintenance, pieces of equipment in the plant. Typically, reformers contain several hundred vertically oriented straight tubes, referred to as catalyst tubes. These tubes represent a significant cost for replacement and can be a major source of plant unavailability if unplanned failures occur. The plant operator is faced with balancing production needs against tube life and risk of tube failure.

The Inner Diameter (ID) of these reformer tubes is generally between 76 mm (3.0 inches) and 127 mm (5.0 inches). During plant operation the catalyst filled tubes are externally heated to allow the reforming reaction to occur. One of the major concerns in plant operation is that the reformer tubes operate at an elevated temperature such that they are susceptible to a failure mechanism referred to as “creep”. This condition exists due to the elevated temperatures and stresses imposed by internal pressure, thermal gradients, and mechanical loading cycles. Being able to identify and locate such damage in its early stages is essential for optimizing plant operation.

Conventional Nondestructive Examination (NDE) inspection techniques currently applied to reformer tubes are geared to finding creep damage in the form of internal cracking. However, with the trend towards larger tube diameters and longer intervals between turnarounds, the detection of such defects may not allow for sufficient time for forward planning of tube replacements. Also, such ‘end of life’ techniques do not allow any differentiation between the ‘good’ tubes. Identification of underutilized tube life can prevent the lost opportunity on both unrealized production through running them too cool and tube life ‘giveaway’ if good tubes are discarded prematurely.

Typically, destructive testing is used on a small number of tubes removed from the reformer to try and determine the absolute life remaining. Whatever the method that is used to do this the results are from a sample size that is statistically not valid. It is imperative therefore that all the tubes are surveyed with a NDE technique to characterize their relative condition in order to make sense of the absolute condition assessment provided by the destructive metallurgical testing.

Reformer tubes undergo creep strain, in the form of diametrical growth, from the first day that they are fired. The ability to accurately measure and record this growth means that the tubes’ condition should be monitored from day one. Therefore, not only can individual tubes be retired from service at an appropriate time, but also the reformer as a whole can be assessed for performance. The use of the internal laser mapping technique is not only useful in preventing tube failures but has huge potential in optimizing production from the whole tube set without sacrificing reliability.

Of course, external diameter measurements can be used but they are limited as the automated devices only measure across one diameter and are often access restricted by tube bowing. Manual measurements are too time consuming to provide more than a few readings per tube. Furthermore, neither way can provide diametral growth data at or through the reformer refractory.

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