When it comes to the industrial furnaces, heaters and boilers used in the chemical, petrochemical and utility industries, the actual operational designs are as varied as the many applications. Some plants have only two or three heaters while larger plants may have more than 50. Some heaters simply deliver the feed at a predetermined temperature to the next stage of the reaction process while others perform reactions on the feed while it travels through the tubes.
Whatever the application, accurate, cost-effective inspection of these process heaters, furnaces or fired heaters in action presents unique challenges. Ever more, industrial predictive maintenance (PdM) programs and external inspection companies turn to thermal cameras fitted with a spectral waveband filter designed specifically to see through flame, at temperatures from -40 to 1500°F (-40 to 816°C) and more.
Infrared Locates Coking, Provides Temperature Validation
Tube metal temperatures are particularly critical to distillation furnace operations. When operating a furnace close to the maximum allowable tube metal temperature, changes of less than 200°F (111°C) can dramatically reduce tube life on tubes rated for 100,000 hours to only a few hundred hours.
In distillation furnaces, a primary service consideration is the determination of carbon scale buildup, or coke formation. Areas with coke buildup preclude the product from uniformly absorbing the tube’s heat and can result in higher furnace firing rates. In some cases, overfiring can cause temperatures that exceed the tube metal design criteria, and this, coupled with pressure inside a plugged tube, may cause a rupture and leak.
Unfortunately, viewing buildup on the inside of a tube (coking) from a set number of access ports can only supply a limited amount of information that is somewhat compromised by an oblique viewing angle. A through-flame thermal camera, however, fitted with an extender, at that same port, can provide an improved angle of vision that safely allows for a far more comprehensive scan. Thermal data and imagery can clearly reveal areas of coking, usually due to excessive temperatures, as well as locations where there are tube restrictions or outright plugging.
An expert examination of furnaces with a through-flame thermal camera can reveal whether refractory is damaged, if flames have the right shape and even dust deposits on the tubes, which cause poor heat transfer and typically lower the product temperature. Unlit burners are revealed as are burners causing flame impingement on the tubes. Oxidation development also shows up on the thermal image — valuable knowledge as this area will eventually become a weak spot.
Infrared cameras also can provide insight into process equipment by validating traditional temperature-measurement devices. In furnaces, thermocouples are installed at several points to provide accurate tube temperature readings. However, when coking occurs around a thermocouple, the sensor is liable to detach or provide inaccurate data. An infrared scan can quickly validate the accuracy of a furnace tube temperature reading provided by a thermocouple. With thermal imaging cameras to validate the thermocouple readings, refineries can safely increase production without safety concerns.
The data provided by thermocouples is limited, however. Typically, only one to three thermocouples per pass are installed that supply data for a specific point on the tube.
Thermal imaging cameras allow temperatures to be read from every pixel in the radiometric thermal image or video taken during internal and external furnace and maintenance inspections. Coking can be detected over the extent of the tube with suitable high-temperature infrared equipment because areas with coke buildup show up as warmer than other areas of the tube surface.
Potentially catastrophic problems that could be easily missed when relying solely on thermocouple data or visual inspection are caught in the early stages, preventing unscheduled shutdowns and creating safer working conditions. With the implementation of an ongoing infrared inspection program, comparative and trending analysis is possible, permitting changes in firing conditions so that run times can be extended. As an added benefit, the cameras can do double duty in thermal inspections of mechanical or electrical components.
How Through-Flame Imaging Works
Designed specifically to monitor all types of furnaces, the flame filter on cooled mid-wave thermal cameras allows only thermal radiation with certain wavelength — 3.8 to 4.05 µm — to pass through to the detector. Flames emit much more thermal radiation at some wavelengths than others, and at certain points in the spectrum. A flame emits hardly any thermal radiation at all. Because everything but that specific range of the spectrum is filtered out, the thermal imaging camera can see through the flame and make temperature measurements even through exceptionally hot flames.
Thermodynamically and hydraulically complex, process heaters incorporate turbulent convective gas flow in addition to radiant heat from the flame, refractory and other tubes, with temperatures fluctuating widely over time. The highly sensitive cooled detectors (NETD of less than 15 mK) utilized in through-flame cameras permit these heat transfer patterns to be visualized — even minute temperature differences.
The real-time thermal video and still image captures produced by these compact, handheld infrared cameras can reveal dangerous buildup on both the interior (coking) and exterior (slag) of furnace and boiler components that otherwise would be obscured by flame, combustion gases and dust. Images, generally 320 x 240 pixels in resolution, are viewed through a high-resolution viewfinder and on a color LCD display. Both radiometric and nonradiometric infrared video can be recorded directly to an internal memory card along with visual digital camera video and images. The traditional visual images then can be associated with the matching infrared footage.
The stored data can be transferred to a computer via several output options and undergo further processing in proprietary infrared software programs to yield the most complete picture of heat-related conditions in the plant. Detachable heat-shields can be added to reflect heat away from the camera and camera operator, providing increased protection.
3 Action Items for Reliable Through-Flame Imaging
Now that you understand how through-flame imaging can help optimize plant and process equipment operation, a review of the profiling process will help you begin to plan your predictive maintenance plan.
1. Gather All the Necessary Information
Before beginning any furnace inspection, it is important to acquire as much process information as possible, including:
- Type of service (coking or fouling).
- Type of fuel (natural gas or refinery fuel).
- Location of the heater unit in the run.
- Inlet and outlet temperatures and pressures.
- Whether the unit is prone to fouling.
Obtaining information on the target to be imaged also is imperative. What is its material composition? Tubing material may vary from section to section, and the acceptable temperature of coked areas may vary accordingly. Ignorance of that fact could result in a call for emergency service when, in fact, the decoking could have been performed in the regular reduced production window.
2. Perform Baseline Scans and Implement Continuous Inspection Program
Thermal anomalies on furnace tubes can be a function of actual hot tubes, surface conditions such as scale and oxidation, or a combination of several factors. The thermographer must be able to accurately evaluate the thermal profile data and make a determination on what is valid tube-metal temperature data and what should be avoided. A series of infrared scans over time can help determine whether an anomalous surface temperature is the result of scaling or is actual coking on the tube interior. Remember that consistency in procedures always will facilitate comparative analysis and result in a better understanding of the furnace.
Coked areas change in appearance over time, usually increasing in temperature and size. If coke formation is suspected in a furnace, it is much easier to make a positive determination if previous baseline data exists. A regular infrared inspection program allows the extent of the problem to be determined and pre-emptive changes — firing configuration changes or charge-rate reductions, for example — to be made to mitigate the issue. If decoking is necessary, post-infrared inspection can document effectiveness. And if the decoking is a steam/air process, infrared can be used to monitor and control the actual procedure.
3. Understand Input Parameters, Particularly Emissivity
Determining the actual tube-metal temperature in some furnaces can be extremely difficult. Be certain that the area being measured is a valid target — that is, tube metal and not scale or other surface anomaly — and know the emissivity of that target. Be certain of background temperature based on field measurement or furnace data. Correct for interference from furnace atmosphere. In very adverse atmospheres, this is valid for a relatively limited area and is based on a snapshot in time.
If possible, include a known valid thermocouple or external reference thermocouple in the image. Take multiple images of the subject area and attempt to minimize the interference. Minimize the temperature span to determine the extent of the flame interference. This will be a factor of fuel type (natural gas vs. refinery gas), burner type, firing rate, draft, and infrared camera and filter.
To calculate temperatures correctly, you need to know the emissivity of both the material under regard and the background. The emissivity of the tube surface can be difficult to determine and typically is derived from two sources correlated to thermocouple measurements. To achieve the highest degree of accuracy, some companies have designed probes fitted with thermocouples known to be accurate that can be positioned in the furnace next to the tube under regard.
In conclusion, furnace and boiler equipment is prone to failure — coking that plugs the inside of tubes and impedes product flow, slag buildup on the outside of tubes, clinker damage, under- and overheating, flame impingement on tubes due to burner misalignment, and product leaks that ignite and cause serious damage to the equipment. These failures cause more than quality problems: They also can shut down an entire process line.
Through-flame thermal imaging cameras can detect most of these equipment problems during operation — and at an early stage — so failures can be prevented. This allows an orderly shutdown and component replacement, thereby reducing maintenance costs and production losses.