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Current inspection practices utilized by refiners to inspect furnace tubing are very time consuming and costly. The total cost of lost production, scaffold erection, gaining tube access, special safety considerations and inspection labor varies widely. Furnace down times of seven working days can also be expected using current inspection procedures. Many areas of the furnaces complex tubing network are inaccessible by conventional inspection methods. Additionally, these procedures only provide either limited coverage or non-quantitative measurements.
Typical NDT inspections performed on furnace tubing consist of manually collected ultrasonic thickness (UT) readings at given increments (i.e., 2, 4, 6 ft) along the tube and at 90 degree intervals around the circumference, if access permits. Typically access to the tube near the wall is inaccessible for spot UT thickness readings and therefore skipped over. The general strategy is to return to these exact locations year after year to determine if interior/exterior deterioration has taken place. In addition, this limited amount of data is used to calculate a very crude corrosion rate for tube remaining life. However, in most cases, ultrasonic inspection from the tubing exterior can only be performed in the radiant portion of the tube. In the convection section, where extended surfaces in the form of studs or fins are attached, the ultrasonic method cannot be used. Access is extremely limited in the convection section which also restricts inspection ability. At best, this results in only partial inspection of a typical furnace.
Less frequently, a furnace will be inspected with X-ray. X-ray inspection can provide 100 percent coverage of a section of tubing if two sides of the tube allow access; however, it provides only qualitative information. Like current ultrasonic methods, X-ray inspection is labor intensive and requires direct access to the heater tubing coil.
The development goal of the FTIS was a tool capable of rapid in situ measurement of an entire furnace, without entering the heater, in less than a day. This is expected to significantly reduce the cost and time associated with periodic furnace inspections.
Figure 1. Optical Triangulation
4.0" Furnace Tube Inspection System (FTIS) tool.
As you can see in the picture the eight (8) ultrasonic, 0 degree, compression wave transducers are fixed at 45 degree intervals in the third module from the end. The OT module (first module) houses the laser which rotates at approximately 6000 rpm during the inspection.
Figure 2. FTIS Components
The modules shown in Figure 2 provide specific functionality to the system. The OT and UT sensor modules contain the primary sensor components as described previously. Each of these modules has an adjacent processor module that contains a dedicated high-speed digital signal processing (DSP) unit, which digitizes, preprocesses, and stores the recorded measurements in nonvolatile memory. The processor modules each have a data capacity equivalent to a 2000 ft tubing scan. An additional module records axial position via encoder wheels that roll along the inner tube surface and record the linear travel of the tool. System power is provided by a final module, which contains a rechargeable NiCad battery. The modules are connected using custom flexible connectors to allow the tool to navigate the close radius bends found in furnace systems. The flexible connectors provide mechanical coupling and a waterproof conduit for the signals passing between modules.
The tool operates autonomously and is propelled through the furnace with ordinary clean water. It is designed to negotiate one diameter (1D) return bends or larger traveling at 2 ft/sec, which enables a 2000 ft. inspection to be completed in about 17 minutes. Once a scan has been completed and the tool recovered from the furnace system, the battery module cover is removed and the tool is connected to the Interface Unit to recharge the battery and download the recorded measurement data. Software running on the Data Station is utilized to download and archive the recorded data. An interface unit is used to connect the computer to the tool during the download and for charging the battery pack.
This data capture image displays the Furnace
Tube in a contour view or C-Scan along with both axis views at the top
and bottom of the C-Scan. As the cursor is moved within the C-Scan the
"X" & "Y" views are automatically updated to show ID and OD information
in the same areas.
As stated before the "red dots" are the Inner Diameter and the "blue dots" are the information from the eight individual 0 degree compression wave transducers. The "red line" that connects the "red dots" is actual data obtained with the Laser portion of the tool and overlaid for conformation of each other.
You will also noted in the data capture image we can display the various defect types in numerous formats depending upon the failure mechanism sought after.
NOTE: OT=Optical Triangulation of LASER DATA and UT of course = Ultrasonics.
Figure 5. Details Of Attached Image: This data capture image displays the Furnace Tube in a cross sectional view or B & B'-Scan. The "red dots" are the Inner Diameter and the "blue dots" are the information from the eight individual 0 degree compression wave transducers. The "red line" that connects the "red dots" is actual data obtained with the Laser portion of the tool and overlaid for conformation of each other. This information is available every 0.020" through an entire 2000' - 3000' furnace network system. As you can see this offers a substantial amount of more information than the conventional methods utilized today to inspect a furnace system.
Utilization of this system will enable refiners to determine corrosion rates more accurately than ever before. The data is easily archived for direct comparison with future scans. Accurate corrosion rate information is expected to improve long-range maintenance scheduling, which translates into increased plant efficiency and reliability.
The major benefit to the refiner is increased production because the downtime associated with current inspection methods is greatly reduced. Additionally, the FTIS does not require erection of scaffolding, entry into the heater, or removal of return bends. The service only requires a single entry point into furnace, a single exit point from the loop and an available supply of clean water. |Top|