Process heaters are critical pieces of equipment in the refining and petrochemical industries, and monitoring the working condition of these heaters is essential. The traditional method of heater monitoring is visual, using viewing windows and peepholes, to ensure proper flame pattern and burner management, and to make sure that no flames impinge on the tubes.
During visual observation, any bowing, bulging, scaling, discoloration, etc. of the tubes is noted. In parallel, the skin temperature of the tubes also is monitored by skin thermocouples and sent as real-time data to the control panel. Tube metal temperature always must be maintained within design limits to avoid overheating.
Over and above all these age-old methods, infrared thermography is gaining acceptance in the oil industries as an effective diagnostic tool for online monitoring of process heaters. Refiners around the world are adopting infrared technology as part of their reliability and predictive maintenance plan. While using infrared imaging for condition monitoring of fired process heaters presents some challenges, and it can help improve reliability in many process heating applications.
Where to Use ItInfrared thermography can be used for condition monitoring of fired heaters in a range of applications.
Overall Temperature Profile Monitoring of the Heater Tubes and Validation of the Skin Thermocouple Readings.The tube skin and box temperatures of some of the most important furnaces in petroleum refineries are listed in table 1.
Infrared cameras can be used to monitor the overall temperature profile of the heater tubes. Skin temperature indication by thermocouple provides information about the heater tubes only where the thermocouple is welded. Infrared imaging gives information about the entire heating surface of the tubes.
Thermography is the best tool for analyzing the relative temperature distribution on the tubes. In absolute scale, this method can be used to find out the exact temperature on the tube and also to validate the thermocouple readings. This can be done quite accurately in gas-fired furnaces and reformer heaters. In oil-fired furnaces, however, the images sometimes show higher temperature than the actual one due to flame effect and tube external surface condition.
Detection of Scaling and Hot Spots.Scaling is a high temperature phenomenon caused by oxidation or sulfidation of heater tubes. Normally, it occurs in the fire-side of the heater tubes. Scale is a poor conductor of heat and thus increases the skin temperature of the tubes.
An infrared camera can be used to monitor the relative temperature profiles of the scaled tubes and identify hot spots (figure 1). By regularly conducting thermography scans, condition monitoring of the tubes can be done effectively.
Infrared thermography can be used effectively to detect early formation of coke layers by analyzing the temperature distribution on the tubes via image analysis. Figure 2 illustrates the presence of coking. In the image on the right in figure 2, the area of suspected coking is outlined. (Note that the color scale is reversed in these images, so black is hot.)
Based on the performance history of a particular heater and correlation with the pressure drop and visual appearance of the tube, infrared thermograms can be used to predict the extent of coking in a particular tube. In visbreaker and coker heaters, which operate in heavy oil systems, such practices are common; however, such applications require a lot of experience. When left undetected, coking will cause tube bowing or sagging, and even premature ruptures.
Assessment of Refractory and Ceramic Blanket Condition Inside the Furnace.This is one of the most important and widely accepted applications of infrared imaging of heater tubes. For this application, long wavelength (8 to 14 micron) cameras are used as they are the most effective for outdoor applications.
Refractory or ceramic blankets inside the heater shell or stack get dislodged due to spalling and other issues, causing an increase in the heater casing temperature at various locations. These high temperature zones are captured in infrared images as hot spots. Infrared thermography can be used to monitor the extent of refractory damage on the heater shell (figures 3). From an energy efficiency point of view, these types of applications are most useful.
How to Interpret What You SeeWhile infrared imaging is useful as a process heater condition-monitoring tool, image interpretation requires a good understanding of the operating conditions of the heater as well as infrared science. Based on my experience, the factors that govern the accuracy of infrared readings are summarized. These points should be kept in mind during thermal imaging of the heater tubes:
Camera Selection. Selecting a suitable infrared camera for a heater application is most important. Note that cameras with band pass filters in the range 3.6 to 3.9 micron are best to use through flames for heaters inside. Some refiners just use infrared cameras that operate in the mid-wave (2 to 5 micron) range for such applications. However, readings are to be interpreted with caution. Some studies show that a long wavelength camera with a filter at 10.4 to 10.6 micron also can be used when the thermogram is taken through the flames.
Ordinary infrared cameras should not to be used in this type of application. These cameras will show erroneous abnormally high readings if focused through the flame. However, these cameras can be used when assessing the refractory damages inside the heater shell as this is an outdoor application where atmospheric transmission is required.
In order to assess the coking/hot spots in heater tubes, a detector with high resolution (320x240 pixels or higher) should be used. These detectors may be of cooled or uncooled focal plane array type. For outdoor applications in heaters, cameras with lower resolutions may be used. For both of these applications, some refiners use two separate infrared imagers -- a mid-wave (with flame filter) and a long-wave type.
Effect of Flame Interference or Background Reflection.When heater tube thermography is done, the most difficult challenge is avoiding the reflection of flame on the tubes, i.e., reflected temperature problems. This phenomenon occurs when the surrounding temperature of the tubes is greater than the tube metal temperature. It causes the image to show a skin temperature higher than the actual temperature.
A temperature correction can be made to avoid the effect of reflection. Ambient temperature correction is also made during image processing. The reflected temperature correction is done as per the box temperature of the heater (table 1).
This problem is quite significant in oil-fired furnaces. During image analysis, flame effect and reflection on tubes need to be judged properly before inferences about the temperature profile can be made.
Emissivity Factor.Temperature distribution obtained through infrared images also is a function of emissivity of the tube. Normally, emissivity is set as 0.85 to 0.92 while taking images through flames. However, emissivity also varies with the tube's external surface texture, temperature, etc. Tube emissivity needs to be judiciously selected to minimize errors. At times, emissivity is calculated on the temperature obtained by the skin thermocouples, and the same value is used for infrared image analysis. Verification is done with sample coupons or with the help of contact type thermocouples or other contact sensors.
Condition of the Tube External Surface.In most oil-fired heaters with carbon steel or alloy steel tubes, thin layers of external scaling and soot deposition on tubes are common occurrences. Therefore, while capturing infrared images, it is possible to obtain much higher temperature than the actual skin temperature of the tubes. This is because the camera does not see the actual tube metal temperature -- as is recorded by skin thermocouples, which are welded to the tube and thus form an integral part.
To avoid this problem, the camera should be calibrated with the skin thermocouples, and then inferences can be drawn based on the temperature distribution. Repeated thermal images should be taken at regular intervals to make accurate predictions about the tube condition/tube temperature. The heaters with multiple skin thermocouples help correlate the temperature with thermal images in a much better way.
To avoid this problem, during turnaround periods, scales are removed completely from the external surface and bare metal is exposed in a few tubes near the thermocouples. Just after the heater is lit up, the temperature at that point is recorded and analyzed based on infrared images and then compared with skin thermocouples. This way, proper calibration is achieved and can be used for future analysis of images during the run.
Changes in the Gas Composition Inside the Heaters.Gas composition often changes inside the heater depending on process parameters. The percentage transmission of radiation of a certain wavelength or band through the gaseous environment also varies with the composition, which affects the recorded temperature. In fuel gas-fired furnaces, the flame effect is almost completely eliminated; by contrast, in oil-fired furnaces, the flame effect remains in the captured images of mid-wave cameras or even cameras with band pass filters.
Fewer numbers of viewing ports and sight areas obtained through the ports often pose limitations to image coverage. The viewing angle, change of firing rate, and burner changes during transient phases of operation also pose difficulties in capturing images. Imaging should be done in close liaison with the operations people.
Infrared imaging of heaters inside refineries is a critical application but it is done in extremely hazardous working environments. Therefore, it is important that safety measures for people be in place. Protective equipment such as safety helmets, safety shoes, goggles, gloves, and gas masks must be worn while taking images.
In summary, infrared imaging can be used as an effective nondestructive tool for condition monitoring and optimizing the operation of fired heaters in refineries and petrochemical industries.
- Infrared Training Center, "Furnace and Heater Tube Inspection."
- Robert K. Weigle, "Infrared Thermography to Detect Refractory Damage in a Fluid Catalytic Cracking Unit."
- Robert K. Weigle, "Applications of Infrared Thermography for Petrochemical Process Heaters."
- Albert Amedee Ohliger and Gabriel E.L. Alvardo, "Texaco's Use of Infrared on Fired Process Heaters," Proceedings of InfraMation, Vol. 2, October 2001, p. 49-53.
- Ron Lucier, "General Procedure for Thermographic Heater Tube Inspection."
- A.J. Leclercq, "The Art of Furnace Tube Skin Temperature Analysis," Proceedings of InfraMation, Vol. 4, October 2003, p. 125-128.