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 example:
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.
Figure 1. Imaging camera
The pyrometer should be used for accurate field measurements to compare specific tubes and troubleshoot real-time performance issues.
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 considered are:
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.
Emissivity 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:
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 be followed:
- Select the target tube that is viewable from two different sight doors.
- Sight doors should have different path lengths to target tube and similar background.
- Open sight doors and wait for the furnace to reach equilibrium; then take a series of measurements.
- Measure short-term target temperature fluctuations by selecting one tube and record the temperatures.
- Can be the same tube used to measure flue gas effect.
- Take readings quickly to avoid target influence from open sight door.
- Ensure that the tube is in focus and avoid viewing through flames.
- Ensure that the edge of the sight door does not overlap the field of view.
- Target tubes should overfill the focus circle and avoid capturing non-uniform temperature objects in field of view.
- Follow image-sighting guidelines of the target tube.
- Dividing the surrounding object into sampling parts increases the accuracy of the target tube temperature.
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.
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 temperature measurements.
Reformer case 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.
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).
Table 1. Uncorrected temperatures versus the corrected temperatures for the hydrogen reformer tubes.
Fired heater 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 measurements.
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 firstname.lastname@example.org.
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 assessment.