Development of an Automated - Ultrasonic Gauging System for Tubes and Pipes

Transcription

1 Indian Society for Non-Destructive Testing Hyderabad Chapter Proc. National Seminar on Non-Destructive Evaluation Dec. 7-9, 2006, Hyderabad Development of an Automated - Ultrasonic Gauging System for Tubes and Pipes V.H. Patankar 1, V.M. Joshi 1 and B.K. Lande 2 1 Electronics Division, Bhabha Atomic Research Centre, Mumbai Electrical Engineering Department, V.J.T.I., Matunga, Mumbai vhpatankar@yahoo.co.in Abstract Tubes/Pipes employed in critical applications such as Power Plants, Petrochemical industry and others, demand very stringent quality control. Various Non-Destructive Testing (NDT) techniques are used for inspection of such objects both during fabrication as well as in-service stages. Ultrasonic testing (UT) is a very popular NDT technique adopted in such applications. Flaw detection and thickness gauging are the main UT methodologies adopted in such inspections. Today, ultrasonic imaging systems are also available and such a system can be utilized for complete volumetric inspection of the desired tubes/pipes. In flaw detection, the prime objective is detection of internal flaws, if any, where as gauging enables high resolution thickness and dimension measurements. Imaging systems basically provide both the operations, though gauging using imaging technique may not provide very high resolution. An advanced system capable of both, B/C-Scan imaging and high resolution gauging of tubes/pipes has been developed at Electronics Division, B.A.R.C. The system is based around an Industrial Personal Computer and all the electronics hardware needed for imaging and gauging has been developed at B.A.R.C. Extensive application software based on Visual Basic and Visual C++, running under Windows98 has been developed to provide realize an advanced system suitable for tube/pipe inspection and gauging. A two axes motorized mechanical scanner has been fabricated and it provides automated inspection of tubes from inside using water immersion technique, under the control of host PC. Initial trials have been carried out using Zircaloy and Stainless Steel sample tubes with artificial defects. This paper highlights the hardware and software features of the ultrasonic system and includes a discussion on the trial inspections carried out with the system. Keywords: Ultrasonic Imaging, Ultrasonic gauging, A-Scan, B-Scan, C-Scan, NDT/NDE, Wall Thickness, Tube/Pipe 1. Introduction In industry, Ultrasonic Inspection/ Imaging (UI) of tubes and pipes is carried out to meet stringent Quality Assurance (QA) demands particularly when the inspected tubes/pipes are constituents of strategic equipments or critical plants. To perform quality checks on tubes, UI and gauging are preferred techniques. The UI technique enables the NDT experts to perform flaw detection and sizing. Full volumetric inspection of the tube needs to be carried out in order to follow the zero error concept for tubes. The UI method is complex, time consuming, costly but is fully suitable for volumetric inspection of tubes. On the other hand, the ultrasonic gauging method

2 V.H. Patankar, V.M. Joshi and B.K. Lande provides information regarding the Wall Thickness (WT), Inner Diameter (ID) and Outer Diameter (OD) of the tube and the variation therein due to stresses and corrosion. The gauging method is precise, fast, reliable and economical but internal defects cannot be detected with it. Generally gauging technique is used to confirm the dimensional integrity of the tube and the UI technique is employed at suspected locations. Gauging of tubes can be performed using contact or water immersion method. Pulse-echo technique employs the same transducer as a transmitter and receiver of ultrasound and using direct contact method, tubes with smooth surface and with access either from OD or ID side can be inspected. Here, a couplant (oil/water/grease) is essential for good coupling of ultrasound to the tube under inspection. Water immersion method is mostly preferred for inspection and gauging of tubes. The tube to be inspected is immersed into the water and using the immersion transducers, the desired inspection is carried out either manually or by automation, without wear and tear of the transducer face. 2. Ultrasonic Gauging / Dimension Measurement Thorough inspection of tubes/pipes, employed in Nuclear, Petrochemical Industries, Defence and Aviation, is a vital task. This may be carried out routinely during Pre-Service, In-Service and Post- Service Inspection schedules. Immersion based Pulse-Echo technique is widely used to carry out wall thickness gauging as well as Dimension measurement for tubes and pipes. Generally, high frequency, focused - immersion transducers are needed for this purpose. The tubes can be gauged using water immersion technique in two ways as indicated in Fig. 1 and Fig. 2. In method I, the tube to be inspected is filled with water with an access from inside and the immersion transducers are placed apart, each facing the ID of the tube. In method II, tube is placed into a pool of water with an access from outside only and the immersion transducers are placed apart, each facing towards the OD of the tube. Using automation, the entire transducer assembly or the tube itself can be rotated and indexed linearly to gauge the entire tube under inspection. S S Tube is filled with water Transducer TR1 ID = WP1 + WP2 + S OD= ID + WT1 + WT2 Transducer TR2 Fig. 1: Inspection of Tube from Inside TR1 TR2 OD= S - (WP1 + WP2) ID= OD - (WT1 + WT2) Tube is placed inside a water pool Fig. 2: Inspection of Tube from Outside For Fig. 1 and Fig. 2, WT1= Wall Thickness measured by TR1 or a transit time measured between the echo signals corresponding to ID and OD of tube. WT2= Wall Thickness measured by TR2 or a transit time measured between the echo signals corresponding to ID and OD of tube. WP1= Water Path measured on TR1 side or a transit time between the transmit pulse and the interface echo reflected either from ID of tube for Fig. 1 or from OD of tube for Fig. 2. WP2= Water Path measured on TR2 side or a transit time between the transmit pulse and the interface echo reflected either from ID of tube for Fig. 1 or from OD of tube for Fig. 2. S= Face to face separation between TR1 and TR2. [1,2]. 220

3 Development of an Automated - Ultrasonic Gauging System R X 1 R X 8 T R G 1 T R G 8 T R G 8 - C h a n n e l U ltr a s o n ic P u ls e r - R e c e iv e r U n it T R 1 T R 8 M u ltic h a n n e l S e q u e n c e r B o a r d V m u x A m p lifie r B o a r d V O M S P S D ig it iz e r B o a r d T o p T a n k T U B E S t e p p e r M o to r C o n tr o l a n d D r i v e U n i t R S L I N K P C B U S In d u s t r ia l P C B o t t o m T a n k Z - T h e t a S c a n n e r L im it S w it c h e s F o r Z A x is a n d S h a f t E n c o d e r s & P r o x im it y D e t e c t o r s F o r Z a n d T h e t a A- x e s Fig. 3: Block Diagram of Multichannel Ultrasonic Imaging System ULTIMA 89C51 RD2 Microcontroller Board RS232 - RS485 Converter TG2 RS 485 Link Industrial PC COM 3 COM 4 TG1 (100MHz) High Speed Counter Board (CH1) (100MHz) High Speed Counter Board (CH2) Controller & Drive Unit For Z - Theta Scanner TRG1 RX - 1 TRG2 RX - 2 TX - 2 TR2 W ater Ultrasonic Spike Pulser - Receiver Board (CH1) TR1 Ultrasonic Spike Pulser - Receiver Board (CH2) TX - 1 Sample Tube/Pipe Fig. 4: Block Diagram of an Ultrasonic Tube Dimension Measurement System - UTDMS 221

4 V.H. Patankar, V.M. Joshi and B.K. Lande Fig. 5: B-Scan Images (a) to (C) are acquired by UTDMS using Gauging Technique. And BScan Images and corresponding A-Scan waveforms (d) to (f) are acquired by ULTIMA 100M8, using ultrasonic imaging technique 3. Ultrasonic Systems for Inspection and Gauging of Tubes The metallic tubes are inspected under automation, by ultrasonic imaging techniques using A-Scan waveform and B or C-scan image mode for detection of internal flaws/defects and sizing of defects, if any. On the other hand, the integrity of tubes can be confirmed by ultrasonic gauging technique. Two kinds of ultrasonic systems are available for testing of tubes where, the first kind of system can carry out ultrasonic imaging for flaw detection and the second kind of system can perform ultrasonic gauging for confirmation of dimensional integrity. It has been observed that, there is a need of the industry to develop an integrated system, which can perform both ultrasonic inspection and gauging. In order to perform an automated ultrasonic imaging and gauging operation for tubes/pipes, Electronics Division, BARC has developed an advanced system, which 222 integrates the imaging and gauging requirements. The gauging system is linked to the imaging system by RS485 interface. Both the systems are capable of operating in standalone mode also. The overall system configuration is 1. Multichannel Ultrasonic Imaging System ULTIMA 100M8 and 2. Ultrasonic Tube Dimension Measurement System UTDMS The block diagram of ULTIMA 100M8 system has been shown in Fig. 3. The ULTIMA 100M8 system consists of PC Addon Boards - 1) 100MSPS, 8 Bits Digitizer, 2) Multichannel Sequencer, 3) Low noise Programmable gain Amplifier and 4) 8channel ultrasonic spike pulser-receiver unit. The system is based on the Industrial PC for shop-floor inspection activities. The Win98 compatible, Ultrasonic Tube Imaging Software (UTIS) has specifically been developed for this system using Visual Basic and VC++ (6.0). The system captures the RF

5 Development of an Automated - Ultrasonic Gauging System echo signal in B/C-Scan imaging mode, in synchronization with the water-metal interface echo, for immersion mode. The B and C-Scan cross-sectional images respectively, are acquired and reconstructed under automation and they permit visualization and sizing of internal defects. The ULTIMA 100M8 system s host PC communicates with the controllers of a Z-Theta mechanical scanner, via RS485 serial link. This scanner is designed to inspect tubes from ID side. With the help of eight immersion transducers, mounted (45 o apart) on the inspection head, imaging of the selected region of the tube can be performed, by imparting linear and rotational movements to the inspection head simultaneously. [Ref. 4] The tube is held at both the ends with the help of sealed jackets and the tube and the jackets are filled with water during testing. The UTDMS is a two-channel tube dimension measurement system, which is based on 89C51RD2 micro-controller. The block diagram of the UTDMS is shown in Fig. 4. UTDMS consists of two PCBs containing of High frequency ultrasonic spike pulser receiver and High-speed counter for each transducer. The unit is linked to the host PC via RS485 interface. The firmware for the micro-controller is developed using standard assemblers. Win98 compatible data acquisition software has been developed using VB 6.0 for the host PC. The UTDMS is capable of measuring the wall thickness with 25 microns precision for Zircaloy tubes. However, the ID and OD can be measured with a precision of 100 microns. In gauging also, the B-Scan view of the selected region of the tube can be synthesized by computing together the values of WT, ID and OD and plotting them over a full length of the tube. Under automation, the ultrasonic tube inspection and gauging is carried out by ULTIMA 100M8 and UTDMS respectively. For this purpose, a dedicated Z- Theta mechanical scanner has been developed. The scanner enables the user to test a metallic tube from inside, by ultrasonic water immersion technique. The tube to be inspected is clamped in the top and bottom jackets. O-rings are used for obtaining a fully watertight environment. Using a pump, the tube is filled with degassed water. An inspection Head carrying eight, line focused, 20MHz, immersion transducers are employed to move the transducers in both rotary (Theta) and linear (Z) directions with the help of two stepper motors. The stepper motor controllers are interfaced to host PC using RS485 protocol. Using +/- JOG, +/- Absolute MOVE and +/- Immediate MOVE commands, the inspection head can be positioned at desired location in the tube. In both the inspection, and gauging modes, the data acquisition is always carried out when the transducer is stationary. The B-Scan imaging is carried out, by moving the inspection head along a single axis (either in Z or Theta direction). The C- Scan image acquisition is performed by a combination of Z and Theta movements, for covering the entire volume of the tube. 4. System Overview for UTDMS Ultrasonic Tube Dimension Measurement System has been configured in 4U x 19 rack mounting format. This embedded system is based on the Philips 89C51RD2 microcontroller, and a multidrop interface for stepper motor controllers and the host computer, for automated data logging and post acquisition analysis. As the UTDMS is intended for precise dimension measurement of Tubes, suitable automation has been incorporated into the system. The hardware constituents of the UTDMS are 1) 89C51RD2 micro-controller board with RS232 interfaces, 2) Ultrasonic Spike Pulser and Receiver board (2 channels), 3) High Speed counter board (2 channels), 4) RS232 To RS485 converter (commercially available) module, 5) Host Industrial computer with two ports of RS485, 6) RS485 based 2-axes stepper motor controller and drive unit; Z- Theta mechanical scanner suitable for 223

6 V.H. Patankar, V.M. Joshi and B.K. Lande inspection of Tubes from ID side, under immersion scanning mode; Ultrasonic Inspection Head with facility to mount 8 nos. of immersion transducers; overhead and bottom tanks with water pump for closed loop water circulation. The firmware for micro-controller has been developed using standard assembly tools of 89C51. The system software, which runs on the Windows98 platform, has been designed with an extensive GUI for user-friendly operation, using VB (Ver. 6.0) to perform data logging and analysis. The UTDMS acquires and transmits the data with appropriate headers, in response to the command given by the host PC. The stepper motor controllers have been configured in closed loop mode with associated feedback and safety elements. Design details of the UTDMS are given in the following sub sections C51RD2 Micro Controller Board Philips 89C51RD2 serves as the core controller and controls the entire operation of UTDMS. On the power on, the microcontroller detects the presence of stepper motor controller and then polls all the keys, located on the system front panel. The front panel has in all eight switches for- +JOG1, STOP1, -JOG1 for Z-axis and +JOG2, STOP2 and JOG2 for Theta axis, in order to position the transducer for gauging and two keys for initiation of the B-Scan and C-Scan image acquisition modes. Pressing `B-scan push button switch, the B-Scan mode of acquisition is initiated. The data acquisition is always performed in synchronization with the movement of the transducer. The B-scan image data can be acquired either advancing the transducer in linear (Z) or in rotary (Theta) direction. The `C-scan mode is initiated by pressing relevant push button switch and the UTDMS moves the transducers in a sequence of Z and Theta axis increments in order to cover the user specific region of the tube. When the transducers are stationary, the data for wall thickness and water path is acquired and transmitted to COM3 of PC, for data storage. The COM4 port of PC is utilized for post acquisition analysis i.e. for bringing the transducer back to the required location, for confirmation of located defects in B and C- Scan mode of acquisition. This feature is called as Return-On-Defect mode. 4.2 Ultrasonic Spike Pulser and Receiver Boards To achieve a high axial resolution, use of high frequency and highly damped transducers is imperative. To energise such transducers, a spike pulse is suitable. As the tubes under inspection have smaller wall thicknesses (1-10 mm), a suitable wideband Ultrasonic Spike Pulser and Receiver (UPR) board has been developed. The UPR consists of 1) Trigger Section; 2) High Voltage Spike Pulse Generator; 3) Limiting network and Wideband Amplifier. For precise measurement of WT, ID and OD of the tube, two ultrasonic immersion transducers are used and they are placed 180 o apart. Instead of multiplexers, two independent channels of ultrasonic Pulser Receivers are employed for excitation of these transducers. 4.3 High Speed Synchronous Counter Boards The high-speed counter boards have been developed to perform simultaneous precision counting of WT & WP for two channels. The functions of a counter board are broadly classified into Digital Control Section; Highspeed counter section; Analog signal conditioning and comparator section for generation of pulses equivalent to WP&WT. The Digital control section is further divided into three subsections namely 1) Trigger synchronization; 2) generation of a pulse, whose width is equivalent to the water path and 3) generation of one more pulse, whose width is equivalent to the wall thickness of the tube. The received RF echo signal has three components viz. i) Initial ringing of the transducer corresponding to transmit pulse, ii) An interface echo corresponding to ID of the 224

7 Development of an Automated - Ultrasonic Gauging System tube and iii) The phase inverted RF Echo signal corresponding to the back surface of the tube (i.e. reflection from OD), as shown in the Fig. 1. Using a transmit trigger signal and the ID-echo signal, the ID-pulse is generated whose width is equivalent to the water path. The water path is computed using a 10 MHz clock, derived from the on board 100 MHz clock. The 8 bits, high-speed counter measures the water path delay precisely, with a resolution of 100nsec. An OD-pulse, whose width is equivalent to the wall thickness, is generated using the ID-echo signal and OD-echo signal. The wall thickness is measured with a precision of 10nsec, by using 100 MHz clock and 8 Bits counter. Both the counters for WP and WT are interfaced to the microcontroller in memory mapped I/O manner. The count values corresponding to WP and WT of each channel are transmitted to PC for storage purpose. The WP and WT values along with the known separation between the transducers are utilized to precisely compute ID, OD and WT of the tube at any location. 4.4 RS232 to RS485 Converter Module The 89C51RD2 microcontroller has a RS232 port for serial communication. The data and commands are transmitted / received between the two axes stepper motor controllers and the host PC. To translate the single ended RS232 signal of the microcontroller into a differential RS485 signal (for better noise immunity over large distance), a commercially available converter module has been used. 4.5 Host Computer with Two RS485 Ports The microcontroller transmits the WT and WP data to the COM3 port of the host - Industrial PC along with suitable header, in order to distinguish data from commands. Scanner movement can either be controlled by UTDMS or through the COM4 port of the host PC. COM4 of the PC is used to activate Return on Defect mode, which provides confirmation of any anomaly found during the gauging of a tube. This feature can be invoked during the retrieval of B and C-Scan images Axes Mechanical Scanner, Transducers and Water Tanks and Pump Assembly Suitable for Tube Inspection from ID Side by Immersion Mode A specialized Z-Theta mechanical scanner has been designed exclusively for this work and it has been used for ultrasonic imaging as well as for gauging of tubes. The scanner is capable of inspecting tubes and pipes from ID side, under immersion mode. The multichannel approach for inspection and gauging of tube, results in fetching substantial saving of inspection time. In essence, the Z-Theta mechanical scanner developed for tube inspection from ID side enables to carry out: (1) Volumetric inspection (i.e. flaw detection) of tubes with the help of ULTIMA 100M8 and (2) On-line gauging (i.e. dimension measurement) of tubes with the help of UTDMS. 5. Experimental Results For the evaluation of the ULTIMA 100M8 and UTDMS, sample tubes of Zircaloy and Stainless Steel (SS) have been fabricated. Inspection results are as follows: The sample tube made of Zircaloy has typical dimensions such as: ID = /- 0 mm, OD = /- 0.3 mm and WT = 4.3 +/- 0.4 mm. On this tube, artificial defects in the form of a groove and a Flat Bottom Hole (FBH) are machined on the outer surface. The groove is 2.75mm deep and 10 mm wide and the hole is 2.75 mm deep and 4 mm in diameter. The B-scan image data of a tube covering these two defects has been acquired using UTDMS. A B-Scan image of the same zone of the tube has been acquired using ULTIMA 100M8 system. The results are shown in Fig. 5. It has been observed from the Fig. 5 that, the results acquired using Ultrasonic Imaging technique and Gauging method matched very close with an accuracy 225

8 V.H. Patankar, V.M. Joshi and B.K. Lande of +/- 100 microns. The B-Scan image acquired by ULTIMA 100M8 has provided information regarding the interiors of the tube along the wallthickness and about the orientation of these defects. The gauging results also provide a profile of ID, OD, and WT of the tube under testing, over the same length. [The groove is not machined, exactly perpendicular, to the tube length and the effect can be seen in the B-Scan images, acquired using ultrasonic imaging system and also in the form of an offset in the B- Scan images acquired by UTDMS.] The sample tube of SS has dimensions: ID = 103.6mm, OD = 112.7mm and WT = 4.5mm. Two grooves have been machined on the OD, as artificial defects. The first groove has a depth of 1mm and a width of 20mm; the second groove is 10 mm away from the first groove with 2mm depth and 15mm width. Experiments similar to section 5.1 have been carried out by acquiring the B-scan images using a gauging and an ultrasonic imaging technique. The WT, ID and OD values for Zircaloy tubes have been confirmed by the digital vernier caliper with 10 microns accuracy and the results derived from inspection and gauging technique have been found within 50 microns accuracy for WT and within 100 microns accuracy for ID and OD. 6. Discussion The ultrasonic tube dimension measurement system UTDMS has been tested for gauging of various samples of metallic tubes and the results have been supplemented by the cross-sectional B- Scan images acquired using a multichannel ultrasonic imaging system ULTIMA 100M8. For the UTDMS, the wall thickness measurement has a precision of 10 nanoseconds, which is equivalent to 24 microns in Zircaloy. (Noting that, the acoustic velocity of Zircaloy is 4800m/s). The water path is measured with a precision of 100 nsec., which is equivalent to 75 microns in water, considering the acoustic velocity of water as 1500m/s. In conclusion, the UTDMS provides highly precise and accurate measurements of dimensions for metallic tubes/pipes. The total system is extremely useful for establishing the procedures for inspection and gauging of metallic tubes and pipes. Tubes/Pipes of longer length can be tested by using a modified scanner capable of handling such tubes, with a provision to compensate for temperature variation. 7. Acknowledgements Authors are extremely grateful to Shri. M.D. Ghodgaonkar, Head, Electronics Division, BARC, Dr. S.K.Kataria, Ex- Associate Director (E), E&I Group, BARC and Dr. Mrs. Dixit, Ex-Director, VJTI, Mumbai for their encouragement and support for this work. Authors wish to express their thanks to Shri. S.P. Srivastava of CDM, BARC for his valuable guidance in the mechanical design of the scanner. Very useful suggestions made by Shri. P.P. Nanekar and Shri. M.D. Mangasulikar of AFD, BARC regarding the procedures for ultrasonic NDT of tubes, are also gratefully acknowledged. Authors are also thankful to Mrs. Shewta Rane, Mrs. P. Jyothi and Shri. L.V. Muralikrishna of Electronics Division, BARC for their technical assistance in the fabrication and testing of system hardware and making useful phantoms for system calibration/checking. 8. References 1. Non Destructive Testing Hand Book, second edition, Volume 7 Ultrasonic Testing, American Society for Non Destructive Testing, 1991, ISBN Kazys R., Mazeika L. et al. from Kaunas University of Technology, Lithuania Ultrasonic Measurement of Zirconium Tubes used in Channel Type Nuclear Reactors, NDT&E International, Vol. 29, No. 1, pp , Feb

9 Development of an Automated - Ultrasonic Gauging System 3. Patankar V.H., Agashe A.A., P. Jyothi, Joshi V.M. ULTIMA 100M8 Multichannel Ultrasonic Imaging System for NDE of Tubes/Pipes, X th National Conference On Ultrasonics, Osmania University, Hyderabad, India, March 15-16, Patankar V.H., Joshi V.M., Srivastava S.P. Development of a 4-Channel Ultrasonic Imaging System for Volumetric Inspection of Solid Cylindrical Forgings, Journal For Non Destructive Evaluation (JNDE) Vol.2, Issue 3, pp 35-40, December, Anish Kumar, Rajkumar K.V., Palanichamy P.P., Jayakumar T., Patankar V.H., Joshi V.M., Lande B.K., Chellapandian R., Kasiviswanathan K.V., Baldev Raj Development and Applications of C-Scan Ultrasonic Facility, Journal For Non Destructive Evaluation (JNDE) Vol.4, Issue 1, pp 11-16, June, Ph.D. Thesis of Patankar V.H., University of Mumbai,