Heater Tube Inspection and Remnant Life Evaluation
Understanding the uncertainties associated with heater tube
remaining life analysis is critical to the quality and accuracy of your
predictions. Heaters are vital pieces of
equipment in most plants. A heater tube failure can result in the loss of
life, millions of dollars in lost production, and equipment replacement costs.
Generally, inspection is performed to gather information used to reduce
the uncertainties in the engineering analysis when assessing the current damage
state and remaining life of heaters, so that appropriate action can be taken
well in advance of a tube failure. Typical
types of inspection include UT thickness readings, strapping (diameter
measurements), field metallography and replication, etc. Sometimes, heater tube inspection can be impeded
by access limitations. The following
interview with Rich Roberts provides answers to some of the questions our readers
have about small, specially designed pigs carrying NDE technologies that can
inspect nearly every area of a tube from the ID. We hope you find the interview valuable. Please drop us an e-mail with any additional
questions on this topic or any others related to asset integrity management. We will do our best to get you the answers!
IJ: What are some
relatively new NDE techniques (and how are they) helping to supply critical data
and information necessary for engineering assessments of coils in fired heater?
RR: Inspection of serpentine coils in fired heaters has historically
been a challenge for the owner/operator due to limited physical access,
especially within the convection section. Most conventional NDE approaches (i.e. manual ultrasonics) attempt to
inspect from the coils exterior surface; however, access is typically limited
to only the coils radiant section, leaving the convection portion of the coil
uninspected. More importantly,
conventional NDE is time consuming and the inspection coverage typically only
encompasses 2-3% of the coil’s surface. Engineers attempting to carry out Fitness for Service (FFS) applying manually
obtained inspection data results in a lot of guess work needed to fill in the information
voids. Use of ultrasonic based
intelligent pigging technology has eliminated these challenges. 100% inspection coverage of both the
Convection and Radiant sections now provide engineers with a much higher lever
confidence in the FFS analysis, ultimately allowing plants to operate the heaters
longer and often at higher throughput.
IJ: How many
ultrasonic (UT) sensors are necessary on the Intelligent Pig to ensure adequate
inspection coverage when inspecting coils in fired heaters?
RR: The diameter of the heater coil should ultimately dictate
the number of ultrasonic (UT) sensors the intelligent pig should have designed
into it. For example, a 3-inch diameter
heater coil should have a minimum of forty-eight (48) ultrasonic readings acquired
around the coil circumference, while a 12-inch diameter should have one hundred
sixty-eight (168) ultrasonic readings taken around the coil circumference. Additionally, the intelligent pig should be
acquiring a minimum of four (4) samples for every one inch (25mm) of axial
IJ: What is the
maximum diameter of the ultrasonic (UT) sensor on the Intelligent Pig
instrument to ensure both adequate inspection resolution and sensitivity?
RR: The physical dimension of damage mechanisms such as
pitting, isolated corrosion and mechanical fretting can be small and in
locations where it is impossible to access externally using manual NDE. Intelligent pigs applied from the interior
surface should have a maximum of ¼” diameter (or smaller) ultrasonic (UT)
sensors in order to ensure adequate resolution and sensitivity to reliably
detect these flaws. Both of these key
elements are compromised if the UT sensor is larger in diameter.
IJ: What better
information/data do we need from advanced NDE to improve our engineering
assessments of heater coils?
RR: It’s the critical, delicate balance of maintaining both
accuracy and inspection coverage. If one
or both of these two critical elements is missing, the engineering assessment output
is compromised. Damage mechanisms in
heater coils can often be localized to a specific region within a coil and the
physical dimension of the flaws (i.e. pitting, fretting, etc.) can be very
small. Without 100% inspection coverage,
a flaw can be missed resulting in premature failure of the coil.
IJ: What further
improvements in NDE techniques are needed to provide more and better data for
engineering assessments of heaters?
RR: Intelligent pigging technology designed specifically
for the fired heater coil inspection application has rapidly advanced over the
last few years and continues to do so. Many specialized heaters in chemical and upstream industries contain
coils which are 2” diameter. Ultrasonic
base intelligent pigging for these small diameter coils is currently in
development and expected to be available in a matter of months.
IJ: What typical types of industrial
heaters justify complex remaining life analyses and specialized inspection? And
RR: There are many different types of
process heaters throughout refineries and chemical plants. Their designs are specific to the process for
which they are intended. Many heaters
operate at elevated temperatures and pressures, making the internal coils
susceptible to both corrosion and creep strain. Ultrasonic based intelligent
pigging is capable of detecting and quantifying damage caused by both. Having the ability to import all inspection
data captured by the intelligent pigging technology into one software platform
and perform an engineering assessment is critical in understanding the coils’
degradation and remaining life.
IJ: Is engineering input provided in
advance to give the direction needed to create the inspection strategy?
RR: Prior to starting any inspection
it’s critical for the plant operators who manage the fired heaters to share information
relating to heater upsets or abnormal operating patterns (i.e. elevated temperatures,
plant trips, etc.). This ensures the
data analyst and engineer who are working with the inspection data are paying
particular focus to damage that may have been caused as a result of the asset’s
operating history. Additionally, it’s
critical the service provider conducting the intelligent pigging inspection
service has a strong engineering and heater background to ensure all elements are
combined and leveraged to gain maximum output for the dollars spent.
IJ: What dictates the degree of accuracy
needed for the inspection and results?
RR: As the old adage goes "garbage in,
garbage out”. If the accuracy of the
inspection results is in question, so is the engineering assessment
output. Selecting an inspection method
with a proven history of accurate and repeatable results is critical in
ensuring reliable engineering recommendations. Many clients today proactively conduct internal live performance
evaluations of any vendor supplying intelligent pigging which I feel is
essential in qualifying both the technology and vendor’s capabilities prior to
arrival on site.
IJ: What dictates the amount of area to be
RR: Coils in fired heaters are
constantly being exposed to harsh high temperature and corrosive environments
on both the interior (process side) and exterior (environmental side) surfaces. Damage mechanisms can vary broadly over the
entire coil length and surfaces. Without
100% inspection coverage it’s nearly impossible to ensure the owner/operator
has good control on what damage is present and how long the heater can continue
to be operated without the risk of failure.
IJ: What type of access is needed for the
various types of heater tube inspection technologies?
RR: Standard heater coil designs
typically are flanged at both ends, allowing mechanical cleaning of their
interior surfaces. Use of intelligent
pigging to inspect this type of coil design is fairly straight forward,
especially since the intelligent pig is sent through the coil directly after
the mechanical decoking pigs. More
complex heaters that contain Common Headers used to be a challenge to both
clean and inspect; however, with Header Deliver Systems (HDS) readily available
on the market today, cleaning and inspection for these types of heater coils
has also become routine.
IJ: How much time does it take to perform
RR: The intelligent pigging
instruments typically travel through the coil at 2 feet (0.6 meters) per second
while providing 100% inspection coverage. A typical coil length of 1,200 feet (366 meters) would require 10
minutes, running from one end to the other.
IJ: What are surface preparation
RR: The interior of heater coils are
routinely cleaned (decoked) to ensure heat transfer efficiency. This cleaning is also necessary prior to
applying the intelligent pigging technology. The intelligent pigging technology is also capable of confirming that
all foreign materials (i.e. scale, coke, etc.) have been removed from the coil’s
IJ: What is the total cost of engineering +
NDE implementation? What is it worth to
the operator (i.e. versus a potential loss)?
RR: Cost is driven by many variables
such as coil complexity, length, pipe diameter(s), etc. An operator interested in obtaining a cost
for the services should provide detailed coil drawings and complete a Pre-Job
Questionnaire (PJQ) form, which will then allow a Technical Advisor to
calculate a total fixed cost for both engineering and inspection. Feedback from several clients indicates that the
cost of applying intelligent pigging is a small fraction of cost compared with
an unexpected heater shut down or a catastrophic failure in the worst case.
IJ: Because intelligent pigging technologies are complex instruments, what type of formal procedures and technical training does a company need to provide their field teams in order to ensure accurate test results within the inspection reports?
question, intelligent pigs are advanced technologies which require specialized
training in order to operate them during the data collection exercise. Detailed formal procedures are a natural
requirement of the NDE community; however, the importance of a well-documented
process is essential when applying these complex instruments. More importantly, the training of each data
analyst is critical to ensure that they understand both how to process data and
the unique characteristics associated with damage mechanisms commonly found in
process heater coils. We’ve concluded
that there is a tight connection between the internal service companies NDT Level-III
personnel and Fired Heater Reliability Engineers when preparing the training
curriculum for all field personnel. We
also encourage clients to audit their vendors to ensure they have robust procedures,
training and certification programs in place.
IJ: Where do you see the future of NDE & Engineering service companies who provide fired heater inspection and engineering services headed?
RR: I see a much tighter partnership between customers and NDE & Engineering service providers. Building relationships with the heater’s owner/user elevates the service provider familiarity with the asset they are evaluating, ultimately providing an outcome customized to that particular unit. The relationship ensures client access to historical inspection and engineering results the service provider commonly archives.