Introduction

Fired Heater (Furnace) tubes in petrochemical industries present a significant challenge for the NDE inspector. Their significant length sometimes exceeding 2,000', multiple serpentine bends, extended surfaces in the convection section and difficult access, make furnace tubes extremely difficult to inspect using conventional NDE methods. An autonomous inspection tool has been developed by QUEST Integrated, Inc. that employs the combined capabilities of high-speed laser and advanced ultrasonic wall thickness measurements. This tool is transported through the piping via a column of clean water, which provides propulsion, a path for the laser beam, and coupling for the ultrasonic signals. The data is processed and stored in the tool using high-speed digital signal processing modules. At the conclusion of an inspection run, the operator downloads the data from the tool to a portable data station. Features such as internal pitting, corrosion, erosion, swelling, and deformation are located and quantitatively mapped. Custom software provides the ability to view the data with contour, cross-sectional, and tabular displays. The Furnace Tube Inspection System (FTIS) is expected to significantly improve both the quality of inspection data and overall safety in the operation of refinery furnaces.

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.

THEORY OF FTIS TECHNOLOGY OPERATION

The FTIS relies on two complementary technologies to measure the inner radius and wall thickness of the furnace tubing. Radial measurements are made using laser-based optical triangulation, while concurrent wall thickness measurements are made using ultrasonic methods. The following paragraphs discuss the application of these two measurement methods.

Optical Triangulation

Figure 1. Optical Triangulation

The basic geometry of optical triangulation is demonstrated in Figure 1. A laser beam is focused on the interior of the furnace tube. The scattered light from the tube surface is collected with a lens and focused onto a position sensitive photodetector. As the radius of the tube changes, the location of the image spot on the photodetector also changes. The electrical signals from the photodetector are processed using high-speed digital signal processing techniques to provide an accurate radial measurement. Tube wall thickness is achieved by using ultrasonic methods. The following paragraphs discuss the application of these two measurement methods. Rotation of the optical train at a high rate combined with a high sampling rate and axial motion of the tool through the tubing system results in a high density helical sampling pattern as the tool moves through the furnace.

ULTRASONIC APPLICATION

Ultrasonic measurement of the furnace tube wall thickness is based on conventional ultrasonic techniques. The UT module utilizes multiple transducers to transmit ultra-high frequency acoustic energy into the water between the tool housing and the inner surface of the tubing. The flush water serves as the couplant between the transducer and the tubing wall. On arrival at the tubing wall a portion of the pulse energy reflects back towards the transducer while a fraction of the pulse energy propagates onto the steel tube wall. At the outer tube surface a similar reflection occurs, sending energy back in the direction of the inner wall and transducer. On-board digital signal processing of the returned echoes determines the time of flight in the tubing wall. Time of flight is then used to compute the tubing wall thickness based on the known acoustic propagation properties of the tubing material.

FTIS SYSTEM DESCRIPTION

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.

The FTIS was developed to detect and measure general wall thinning and inner surface anomalies such as pitting, corrosion, and deformation. The tool is comprised of multiple modules (see Figure 2), and is propelled with clean water throughout the length of a furnace tubing system. The use of complementary measurement technologies combined with a graphical data analysis package results in high resolution, digital, and quantitative inspection data for the tubing system. This data can be obtained in a matter of minutes without removing return bends or entering the furnace firebox.

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.

DATA ANALYSIS

Once the data is downloaded to the data station, the custom application software can be used to process and view the data. The software is a robust Windows NT(tm) application that takes full advantage of the Windows graphical user interface. The software automatically identifies straight sections of tubing and categorizes the flaws within each section based on user definable limits. A table of sections provides the user instant tabular information on the most severe flaws within a section. From the table of sections, the user may choose to view the data in either a cross-sectional format or a C-scan view. Multiple views may be displayed simultaneously, with linked navigation among the views. The cross-sectional view is often useful for examining the ovality of the tube at a given axial position, while the contour view is very useful for viewing generalized wall thinning and corrosion. As data is reviewed, an automatically generated tubing inspection report can be edited with the addition of comments and data snapshots to produce a detailed hard copy report of the tube's condition.

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 4.

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.

BENEFITS TO THE CHEMICAL/PETROLEUM INDUSTRY

As part of Furnace Tube Inspection Service (FTIS(), the FTIS is the key to a complete and comprehensive inspection data collection service. Inspections can be completed quickly, minimizing furnace down time. Preliminary results can be available within minutes after the data are downloaded from the tool enabling engineers to make real-time decisions concerning the return of the furnace to service.

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.