A Closer Inspection - How Highly Developed Inspection Tools are Used to Identify Tubular Damage
By: Charles Thomas and Dan Drabble, Quest Integrity
As seen in the March/April 2017 issue of World Fertilizer Magazine. Download the PDF version.
Unscheduled failures of primary reformer
catalyst tubes are one of the most common and costly causes of syngas plant
shutdowns. A single tube failure may
result in an outage costing many millions of dollars in asset repairs and loss
of production. The risk of online failure can be significantly reduced by
effective inspection and life assessment. Quest Integrity pioneered the
measurement of creep strain using their proprietary inspection techniques to
follow the progress of creep damage in catalyst tubes. These inspection tools
have been supplemented by specialized life assessment methodology, and have
resulted in a deeper understanding of damage accumulation and the confident
development of tube replacement strategies. Given the trend to longer run
times, a particularly significant advantage has been the identification of
tubes in danger of failure prior to the next scheduled outage. These methods
have been applied for many years and there are a growing number of examples for
which it is clear that online failures have been avoided. This includes:
- Tubes predicted to fail and left in the
furnace subsequently failed. This has occurred on a number of occasions.
- Onsite life assessment at the time of the
inspection predicted that a tube was close to failure requiring immediate
- A life assessment based on a previous
inspection indicated a number of heavily damaged tubes. The next inspection
confirmed this finding revealing a number of tubes were unlikely to survive to
the next outage.
This article describes the capabilities of
highly developed inspection tools, specialized life assessments and their
application. This is illustrated using case studies of inspection and life
assessment that have successfully identified damaged tubes that either lead to
online failure or would not have survived until the following scheduled
The importance of plant reliability cannot
be overstated. A typical steam reformer
catalyst tube failure will likely involve a plant outage of the order of one
week or more, and the cost associated with such an outage is measurable in
millions of dollars. Steam reformer tubes suffer progressive damage due to
creep. Understanding the rate and extent
of creep damage is an essential input to reliable operation. Inspection
of catalyst tubes is an essential aspect of any asset integrity management
Inspection of steam reformer tubes is
problematic using traditional inspection methodologies:
- Ultrasonic inspection is difficult due to
the high attenuation typical of the spun cast, coarse grained alloys
universally used for catalyst tubes. Ultrasonic techniques require a couplant
and have required flooding of the tubes during inspection.
- Radiographic inspection is not a feasible technique
due to the large areas to be inspected and the significant health and safety
- Eddy current signals are affected by a
large number of factors and interpretation is difficult. Nonetheless,
significant experience using eddy current as a base technology has been
established and success using this technique has been achieved.
Figure 1. The relevance of inspection technologies at times throughout the life cycle of a catalyst tube.
Figure 2. Internal steam reformer (left) and deployment (right).
All of these techniques, certainly in their
generic form, primarily target cracking and flaw detection. Creep, however, is a deformation process and
cracking may not occur until late in life. Figure 1 is a schematic illustration of a creep curve indicating at
which part of the life process the various inspection technologies will
generate meaningful data. As a
deformation process, the progress of creep involves the development of strain.
In the case of internally pressurised tubes, the consequence of creep is an
increase in diameter. Some specially developed inspection tool technologies, including
Quest Integrity’s LOTIS® and MANTIS technologies, are now well established and
have one fundamental objective - the measurement of tube diameter. This
increase in diameter is a creep strain and is a direct and simple measure of
the progress of creep damage. The data obtained from such an inspection not
only provides qualitative information about the extent of creep damage but also
is directly amenable to input to life assessment procedures.
Quest Integrity also developed a
methodology and software package for that purpose, known as LifeQuest Reformer™.
This has involved:
- The development of a comprehensive database
of reformer tube alloy creep properties to understand and quantify the change
in creep strength through the life of a tube and define the creep history in
terms of strain development.
- Performing comprehensive finite element
modelling of the tube to account for changing stress distributions at startup
and shutdown and during steady state operation.
- The application of a well-established creep
model to generate remaining life information from the inspection data, creep
property database and calculated stress distribution.
- Using the results of the output from this
model to generate comprehensive information and recommendations about tube
replacement, tube procurement and furnace process operation and temperature
This customized software life assessment application
is consistent with the requirements of API 579 Level 3 fitness for service assessment.
The combination of inspection and
specialized life assessment has led to a growing body of experience. This article
briefly describes the technologies behind these techniques and describes case
studies in which their application has prevented online failure or, as in one
case, predicted failure that subsequently did occur. The primary objective of the specialized
assessment software is remaining life. In addition, however, a wealth of
diagnostic information is developed.
Quest Integrity’s advanced inspection
technology, LOTIS, has been deployed for approximately 20 years and has
undergone a number of upgrades improving accuracy and precision. This spinning laser-based
tool very accurately maps the internal surface of a tube as it passes from top
to bottom. This requires the tube to be free of catalyst. The opportunities for
advanced inspection have fallen significantly over its lifetime as the average
run time between catalyst replacements has increased. This led to the development
of an external crawler known as MANTIS (Figure 3), that has the same primary
objective as LOTIS, i.e. the measurement of tube diameter and strain. The
combination of LOTIS and MANTIS allow inspection at all convenient outages. Both
systems generate similar comprehensive information. Figure 4 shows an example
of a profile for a single tube and Figure 5 shows tube data over an entire
furnace allowing the identification of local hotspots. The dataset used in the
creation of these graphical representations of strain can be analysed as
required and formatted for input to the specialized life assessment
Figure 3. External crawler tool technology.
Figure 4. Tube profile data.
Figure 5. Tube data in furnace layout format.
The specialised life assessment technology,
known as LifeQuest, life assessment flow sheet is illustrated in Figure 6.
Conventional life assessment such as the
Larson Miller approach  requires knowledge of:
Figure 6. The LifeQuest Refomer flowsheet
Creep strength of most materials is well
represented by standard data [1,2]. Reformer tube materials, however, are subject to in-service aging which
means the creep strength continuously changes relative to the as-cast strength
provided by tube manufacturers. The creep properties and the changes that occur
due to temperature and time have been extensively studied and a comprehensive
database developed .
The hoop stress is known accurately from
operating pressure. LifeQuest enhances the stress information by finite element
analysis to account for through-wall temperature gradients and startup/shutdown
Creep life is strongly dependent on
temperature. This is an input around which a large degree of uncertainty
exists. Measurement of tube metal temperatures in reformer furnaces is
problematic with potentially large errors. Typical measurement uncertainty of
the order of 30°C will introduce errors
in remaining life calculations from -75% to +300% . LifeQuest Reformer does
not require input of temperature data. The strain based creep property database
allows the process to be reversed by calculating the temperature at which the
tube had operated in order to generate the amount of strain measured at the
time of inspection.
The LifeQuest model takes the inspection
data, the creep model database and service process information to calculate the
progress of strain into the future. The model is similar to the Omega approach
which has found wide acceptance as a life assessment methodology . The
essential output from the model is:
- Predicted failure date
- Recommended replacement date
- Calculated tube skin temperature profile
This information leads to:
- Significant reduction in the risk of unscheduled
- Targeted tube replacement planning
- Justification for tube procurement
- Balancing of furnace process output against
tube life consumption
These inspection and life assessment
methodologies combine to represent a powerful tool in providing the important
information required to ensure the reliable operation of reformer
furnaces. In the following section, a
number of case studies are presented. These case studies show the flexibility
and problem solving capabilities. Case
study: recovery from overheating incident
An ammonia producer operating a furnace
containing 72 HP50Nb modified reformer tubes suffered an overheating incident
in which 7 tubes were ruptured. Additional damage occurred to the outlet
manifold bull tees and there was significant refractory damage. Quest Integrity
was invited to assist with the investigation into, and recovery from this
incident. The damaged tubes were
identified and replaced. A significant question was the extent of damage to the
intact tubes and whether or not these tubes would survive to the next scheduled
outage. The LifeQuest Reformer assessment methodology was employed to define a
"critical strain” above which the risk of failure prior to the scheduled outage
was considered unacceptable. This
investigation took into account both the strain that had occurred previously
and the short term strain that had occurred during the overheating incident.
The ongoing commitment to inspection by this operator meant that the data
differentiating between progressive in-service creep strain and short term
strain was available for analysis.
This analysis identified 14 tubes to be
changed within one year and a further 22 tubes to be replaced at the following
shutdown in 2016. Unfortunately, the furnace suffered a tube failure in 2014
implying that the life assessment, in the case of one tube at least, had been
incorrect. On investigation however, it was found that the tube that failed was
one of the 14 that had been recommended for replacement; it had inadvertently
not been replaced. While this was a highly undesirable occurrence, it did
illustrate the accuracy of the assessment predictions; a tube had been
identified as at risk of imminent failure and that tube did indeed fail within
the next operating period.
study: detailed analysis
Part of the LifeQuest Reformer offering
involves attendance on site of a highly qualified assessment engineer at the
time of the inspection. The objective of this service is to provide
sufficiently detailed analysis of the inspection data as it is obtained to
identify those tubes that are at risk of failure prior to the next scheduled
outage. The speed of this analysis permits replacement of those tubes prior to
restart of the plant. In the event that insufficient replacement tubes are
available or the replacement would adversely affect the outage schedule and the
restart, the information permits informed decision making as to the risk.
A LifeQuest Rapid assessment was undertaken
for inspection data obtained from a reformer furnace, that for very sound
process reasons, was operated at well above average operating temperatures and
consequently tube life was typically of the order of 10 years or less. Three
tubes were identified as at high risk of failure and recommended for
replacement prior to restart of the plant.
An output of the LifeQuest Reformer process
is a calculated failure date for each tube. In line with API 
recommendations however, a recommended replacement date is provided and this is
set at 80% of the life from commissioning to calculated failure date. It is
considered that tubes operating at greater than 80% consumed life are at
significant risk of failure. Because of scheduling and related issues, the
plant owners opted to accept the risk of tube failure and chose not to replace
these three tubes. One tube subsequently failed on-line, necessitating a plant
outage at which time all three of the at-risk tubes were replaced.
Subsequently, a full LifeQuest Reformer assessment was undertaken on the
inspection data and identified a further 15 tubes to be replaced at the next
scheduled outage. Given the past experience, the plant owners scheduled
replacement of those tubes.
study: burner management
A LifeQuest Reformer assessment was
undertaken on inspection data obtained from a side fired reformer on an ammonia
plant. The inspection noted that a few
tubes appeared to be significantly bulged. An example of this is shown in
Figure 7. The data illustrated in Figure 7 shows two consecutive inspections.
It can be seen that diameter growth (creep) is occurring rapidly in the area of
the bulge and the separation between the two datasets is significant.
Elsewhere, the rate of growth is more modest. It was determined that this bulge was directly opposite a radiant wall
A particular value of the inspection was
the identification of undesirable heat distribution. The LifeQuest Reformer
assessment put the level of risk into perspective. For the tube with the bulge, the failure time
and the recommended replacement date were 3 years and less than 1 year in the
future, respectively. In the case of other tubes with very similar inspection
data other than the presence of the bulge, the equivalent failure and
replacement dates were 10 and 7 years in the future. A clear outcome of the
inspection and assessment is the identification of opportunity to undertake
improvement in furnace operational control that would have negligible impact on
output while significantly improving reliability.
Figure 7. Example of tube profile showing localised growth.
study: problem solving
A reformer on a large manufacturing complex
had operated reliably and conservatively for 10 years. Inspection and life
assessment of the tubes was undertaken with some specific questions in mind
over and above the fundamental issue of tube life. The plant was undergoing a
significant debottlenecking exercise during the turnaround. This had an impact
on the furnace, and was expected to result in an increase of approximately 20°C
in operating temperatures during subsequent operating periods. A critical issue
therefore was to understand the impact of this temperature increase on
remaining life of the tubes. LifeQuest Reformer has the capability of assuming
changes to operating conditions in the future. As noted previously, the
analysis does not require plant temperature data as an input to the assessment.
The software will calculate an effective temperature at which the tube has
operated in order to generate the level of creep strain that is measured during
the inspection. In most assessments, that same creep effective temperature is
used for future operation. In this case, the plant owners specifically
requested that the effect of a 20°C increase be assessed. This was undertaken
and no tube replacements were recommended prior to 2021. This result provided
confidence that the plant debottlenecking exercise being undertaken would not
impact adversely on the safe and reliable operation of the reformer.
The plant inspectors had noted hot spots
near the top of a few tubes leading up to the outage. It was reported that the
hotspots had been present for approximately 18 months. The inspection data did
indeed show the consequences of these hotspots (Figure 8). The LifeQuest
predictive facility assumes that previous damage has accumulated since the tube
was commissioned. However, in this case the plant operators were of the opinion
that these hot spots had only been present for approximately 18 months. The assessment was therefore undertaken by
assuming all the creep damage at this location had occurred in the previous 18
months. This assessment indicated that replacement would not be recommended
prior to 2024. If the hot spots were eliminated, longer lives, at least based on
damage at this location could be expected.
The calculated temperature data from this
assessment is shown in Figure 9 and compared with plant pyrometer IR
measurements. The calculated tube metal temperatures at this location show good
agreement with the maximum tube temperature exceeding the average by
approximately 20°C. The measured temperatures however show a tendency to suggest
that the central tubes are running hotter. It is well known that Infra-Red measurement technologies may suffer
errors due to issues such as path length and reflectivity . It is suspected
that the IR measurements techniques on this site would benefit from the
application of correction software to improve accuracy.
This case study illustrates the abundance
of useful information that can be derived and tailored to address specific
issues and problems.
Figure 8. Tube profile showing unusual growth at the top of the tube. Case
study: timely decision making
An operator had performed reformer tube
inspection repeatedly over a number of shutdowns. Despite the evidence from these inspections
showing steady and consistent growth in the tube diameters, the owners had not
previously elected to quantify the impact of the inspection results in terms of
remaining life assessment. A rapid
assessment was undertaken at the time of the latest turnaround at which time
the inspection data revealed that significant creep strain had occurred. This
assessment indicated that 17 tubes would not survive until the next outage.
Since this information was not previously available, the plant had not made
time available within the turnaround window to replace this number of tubes and
only 8 tubes were replaced. These findings have required the development of a
management plan to closely monitor and control process and tube temperatures.
LifeQuest Reformer since has been used to undertake a "what if” sensitivity
analysis to understand the risk associated with temperature control allowing
the plant operators to set in place, integrity operating windows and action
plans to maintain operation with those limits.
This case study illustrates the power of
the assessment and inspection package to assess risk and develop operation
plans to guard against unscheduled tube failure. It also illustrates the
importance of understanding tube condition and risk of failure throughout the
life of the furnace.
The case studies described above clearly illustrate
the advantage that a combined package of strain based tube inspection and
technically sound life assessment technology brings to the operators of steam
reformer furnaces. In the absence of the knowledge that these technologies
provide, plant owners are effectively operating in the dark running risk of
tube failure and the high associated costs. Conversely, risk averse owners will
operate under conditions where optimum production is not achieved which may be
equally costly. These technologies allow
informed decision making and represent an essential component of any asset
integrity management plan for syngas plants.
- API 579-1/ASME FFS-1, June, 2016. Fitness for
- API 530 April 2015, Calculation of Heater-tube Thickness
in Petroleum Refineries
- C W Thomas, T Anderson, T.
Hill, Life Assessment of Reformer Tubes from Strain Measurement, Paper 11155,
NACE Corrosion 2011, Houston.
- P. Saunders, Radiation
Thermometry- Fundamentals and Application in the Petrochemical Industry, SPIE