Achieving a Comprehensive Fired Heater Health Monitoring Program
By: Tim Hill, Principal Consulting Engineer, Quest Integrity Group
As seen in the September/October 2014 edition of Inspectioneering Journal. Download the PDF version.
For the past 30 years, infrared (IR) thermometry
has been used to monitor tube metal temperatures in refining and chemical
furnaces. Tracking temperature levels and variations determine performance capability limits and reliable
tube life. However, the application of IR thermometry has often been
characterized as highly operator dependent, which can result in less-than-optimal
data accuracy as a consequence of poorly applied and interpreted results.
IR thermometry is an excellent diagnostic tool for
detecting tube hot spots from internal fouling or non-uniform heat distribution
in fired heaters, but to ensure the full capability of IR thermometry, operators
should employ the right instruments for the job and implement a proven
methodology to measure accurate temperatures in a repeatable process. With an
effective IR thermometry health monitoring program, operators can manage the
mechanical integrity of fired heaters and optimize production rates.
IR thermometry is primarily accomplished with two
instrument types: thermal imaging cameras and pyrometers. A thermal imaging
camera forms a two-dimensional thermal image of the target surface, while a
pyrometer provides only a single target point temperature. Because each
instrument has its own inherent advantages and disadvantages, an effective
inspection program should incorporate both types of instrumentation. For
The imaging camera should be used to provide
meaningful images and measurements for a historical record that can be used to
assess tube creep damage rates and long-term performance changes.
The pyrometer should be used for accurate field
measurements to compare specific tubes and troubleshoot real-time performance
Figure 2. Pyrometer
All infrared measurements, whether made by an
imaging camera or pyrometer, are subject to measurement factors which can
affect the accuracy and repeatability of the measurement. The fired heater’s
environmental measurement factors are the target tube’s emissivity, target
reflectance and the flue gas effect on the measured temperature. The
instrument factors affecting the temperature measurement are the instrument
infrared wavelength, calibration, size of source effect, vignetting, and the
emissivity setting. Each of these factors must be understood to achieve an
effective infrared inspection program. Without an adequate understanding,
measurement errors as much as 180°F can occur, which also affects the
repeatability of the measurements.
Fortunately, a comprehensive and effective infrared
health monitoring program designed for fired heaters (or reformers) can account
for these measurement factors. By following simple field data collection
practices and then applying rigorous correction calculations, the tube’s
surface temperature can be accurately measured. This process allows any operator
using either IR instrument to collect repeatable tube temperature measurements.
The correction calculations employ algorithms based on well-established
physical principles of blackbody infrared radiation and radiation exchange,
including a specific geometrical model of the subject fired heater and
characteristics of the measurement instruments. Software is now commercially available
that can automate the rigorous correction calculations.
Infrared measurement factors that should be
- True tube temperature–desired outcome
- Environment factors–tube emissivity (including angle
of incidence), target reflectance (including fired heater geometry) and flue
gas absorption and emission
- Instrument factors–wavelength, calibration and
size of source effect, vignetting and instrument emissivity setting
Emissivity (Îµ). As an environmental factor, emissivity refers to the
ratio of radiation flux emitted by the target tube to that emitted by a blackbody
at the same temperature as the target. For example, an Îµ of 0.85 absorbs
and emits 85% of a blackbody radiation amount at same temperature and reflects
15% of the surrounding radiation. Emissivity is a surface phenomenon and is affected
by radiation wavelength. Target tube Îµ is typically 0.85 (@ 1µm), 0.82 (@ 3.9µm), but it can vary
depending upon the condition of the tube’s surface.
Reflection. Reflection errors occur inherently due to the emissivity of the target
tube. The reflected radiation from the tube is
captured by the instrument and must be removed from the measured radiation to
achieve the desired outcome. Imaging cameras that include reflection error
correction assign one number to describe all of the surrounding objects. Reflection
error cannot accurately
be represented by one number. The effective background temperature depends on the
geometry and position of the target tube and is a weighted average of the sum
of all of the surrounding surfaces like walls, the floor, roof and tubes.
gas effect. Absorption and emission errors can be introduced
via flue gas (atmospheric) as the target radiation travels from the tube to the
instrument. Specifically, spectral emission lines, at which radiation is
absorbed and emitted by flue gas, must be taken into consideration. By
selecting the appropriate instrument, the flue gas effect can be minimized, but
not eliminated. The magnitude of flue gas absorption and emission errors is
affected by the flue gas temperature and the travel path length. Operators who
measure the same tube over two different path lengths should be able to identify
Wavelength.The wavelength of the instrument is chosen based on the expected target tube
temperature and to minimize the flue-gas emission errors. For fired heater
applications, either a 1µm or 3.9µm wavelength instrument should be used.
setting. Most instruments have the ability to set an emissivity value. Since
the instrument is calibrated to a blackbody temperature, the emissivity value
of the target tube must be applied to correct the indication. As discussed,
reflection errors significantly affect the radiation from a target tube’s
surface, causing the target tube’s apparent (or effective) emissivity to be
higher than its inherent surface value. Setting the instrument’s emissivity
value to the inherent target value will not adequately correct the indication
from a blackbody value to the target’s value. For this reason, it is
recommended that the instrument emissivity setting be set to 1.0 (assumes the
target is a blackbody) and then apply correction calculations outside of the
instrument for target emissivity and reflection error. The tube’s radiance
temperature (i.e. total emitted and reflected radiation) is measured when instrument ÆÉª = 1.0.
Size of source effect. Ideally, the instrument should detect only the
radiant flux within its well-defined field of view. Yet, the reality is that
some of the flux that is within the field of view will miss the detector, and
some of the flux from outside the field of view will be detected. This phenomenon
is called size of source effect (SSE). Some factors of SSE
correction to consider are:
- Imaging cameras have a large SSE correction,
primarily due to the large surface area covered.
- Pyrometers usually have a small SSE correction
(i.e., can be ignored).
- SSE correction for each instrument must be
laboratory measured and then applied to radiance temperature measurements.
- The SSE typically causes the radiance
temperature to be higher than the actual radiance value.
- To minimize SSE error, operators should keep
lens dust and scratch free and ensure that the field of view is well overfilled
with neighboring objects at the same temperature as target area.
Vignetting. Vignetting refers to the obscuring of the lens’ field of view,
resulting in a reduction in radiation falling on the detector (i.e.,
temperature reading will be low). This is a common problem for operators when
they are working with furnaces and are looking through a sight door. Capturing
portions of the sight door wall in the image will lead to vignetting.
To correct for common problems and ensure reliable
and repeatable results, the following field data collection procedures should
the instrument emissivity to 1.00 and the background to ambient.
the effect of flue gas absorption or emission on the thermometer readings.
the target tube that is viewable from two different sight doors.
doors should have different path lengths to target tube and similar background.
sight doors and wait for the furnace to reach equilibrium; then take a series
- Measure short-term target temperature
fluctuations by selecting one tube and record the temperatures.
be the same tube used to measure flue gas effect.
radiance temperature of target tubes.
readings quickly to avoid target influence from open sight door.
that the tube is in focus and avoid viewing through flames.
that the edge of the sight door does not overlap the field of view.
tubes should overfill the focus circle and avoid capturing non-uniform
temperature objects in field of view.
the radiance temperatures of each surrounding object.
image-sighting guidelines of the target tube.
the surrounding object into sampling parts increases the accuracy of the target
After the above field procedures, the collected
data should then be corrected with rigorous calculations. For example,
operators should correct the radiance measurements for emissivity and
reflection error, SSE, flue gas emissions and other instrument and
environmental errors. They should calculate the uncertainty associated with these
factors. And they should calculate the effective background temperature taking
into account the geometry for each target tube.
TEMPERATURE CORRECTION CASE STUDIES
The following two case studies show operational
improvements using an IR temperature correction program to manage the health of
reformers and fired heaters. Software is used to automate correction
calculations in order to remove common errors from IR thermometry tube
study. The first case study focuses on a complex refinery with more than 40
fired heaters. The refinery’s hydrogen reformer was challenged with tube metal
temperatures that were limiting hydrogen production. In addition, poor heat distribution constrained
output. The operator was also concerned with the equipment’s creep damage rate.
- In 1998, the
refinery implemented a full-time heater health monitoring program with an on-site
- In 2003, the
operator shifted the program to part-time monitoring, serviced by an on-site
- In 2011, the
refinery implemented the above-described IR temperature correction program to
improve accuracy and repeatability of IR measurements.
As part of the program implementation, the on-site NDE
contractor was trained in proper IR data collection procedures and software
applications. The IR camera SSE error was measured and corrections were
applied. The corrected temperatures were well below operating limits, therefore
the reformer operation was continued at the same production rate, and the creep
damage rate concerns were alleviated (see Table 1).
1. Uncorrected temperatures versus
the corrected temperatures for the hydrogen reformer tubes.
case study. The second case study focuses on a complex refinery with more
than 20 fired heaters in practically all possible services. The refinery
operator wanted to increase the plant’s run length between decokes of the coker
heater. The run length of the coker heater was limited by skin thermocouple indications
(TI). The IR monitoring activities at the thermocouple locations indicated that
the thermocouple readings were too high.
In 2010, the refinery began a heater health
monitoring program with an off-site contractor performing periodic routine
monitoring. In 2013, the refinery shifted the program to in-house NDE staff performing
the routine monitoring and implemented the above-described IR temperature
correction program to improve the accuracy and repeatability of IR
As a result, a significant difference between
actual tube metal temperatures and skin thermocouple readings were documented. Specifically,
the skin TI readings were reading higher than actual. The IR temperature correction
program confirmed the reading difference, thus the operator was able to gain
confidence in the plant’s tube integrity program by having accurate and
repeatable data upon which to base decisions (see Figure 3).
Figure 3. Tube metal temperature trend for coker
heater allowing optimum de-coke planning to occur.
Clearly, an effective infrared health monitoring program
is an absolute necessity to monitor the integrity of the fired heater tubes, as
well as provide a wealth of diagnostic information that may be used to evaluate
the performance and reliability of major fired heater parts (e.g., tubes, tube
supports, burners, refractory and structural systems). By fully understanding
the IR measurement factors and employing field collection practices and IR
temperature correction calculations, accurate and repeatable infrared
temperature measurements are achievable.
For further information, see "Radiation Thermometry. Fundamentals and Applications in the
Petrochemical Industry” by Dr. Peter Saunders, or email
Tim Hill, P.E., is a Mechanical Engineer and
Principal Consulting Engineer for Quest Integrity Group with 30-plus years of
experience, including the evaluation of thermal processes, fired heater
operations and maintenance, risk assessment, root cause failure analysis and
life assessment for fired heater equipment in the petrochemical, refining and
power generation sectors. For 10 years, Tim was responsible for the operation
and reliability of all furnaces in a major refinery in the USA. During this
period, he developed and implemented effective integrity management tools
(including infrared inspection) covering operation, maintenance and risk