New generation phased-array technology aims to cut vessel inspection costs28 February 1998
Improved economics is increasingly required of reactor pressure vessel in-service inspections, in addition to the usual demands for high standards of technology and quality. There is a growing need to cut down on the time required for inspections thereby shortening nuclear power plant outages and reducing the cost of performing such inspections. The SAPHIR (Siemens ALOK PHased-array Integrated Reliable) ultrasonic examination system aims to achieve these goals.
Non-destructive inspections on reactor pressure vessels typically have the following characteristics:
• Heavy-walled components and extremely large examination volume.
• Complex geometries and materials which are difficult to examine.
• Fast processing of measured data and precise analysis of indications.
The SAPHIR inspection system in conjunction with a new data acquisition and evaluation system (pictured above), is the first system of its kind to combine the conventional inspection technology of standard search units with the advantages of the phased-array technique. It provides a considerable number of search unit channels (at least 96) and, when used with phased-array search units – which can carry out several different inspection tasks through the alteration of transmission and receiving angles – has allowed compact search unit systems to be built which are capable of covering all the necessary functions for simultaneous inspection of longitudinal and transverse defects in heavy-walled components, requiring only a single scanning pass in each area.
Compared with previous technologies, SAPHIR requires fewer scanning passes and fewer changes of search unit systems – reducing the time for which other activities are blocked out of the reactor well, which impacts the critical path of the outage. This time has been reduced by more than half in German PWR plants.
One example is that in 1997 the vessel occupation time at unit 2 of the Neckarwes-theim nuclear power plant was reduced to 3.75 days for a 100% inspection, as compared with 8.33 days four years previously.
SAPHIR is characterised by an extremely high degree of reliability, which can largely be attributed to the use of advanced technologies and to the automatic internal self-testing capability.
A dozen complete systems have been produced to date and in 1997 SAPHIR superseded previous Siemens ultrasonic inspection systems. The technology has since been used successfully for numerous examinations in Germany and abroad. This was made possible through qualification and approval by the German inspection agency, TüV, which functions as the authorised inspector for in-service inspections. A licence has also been issued by SVTI in Switzerland, while licence applications for the USA and Sweden are also presently being processed by the relevant authorities. In addition the system has been deployed successfully in Hungary, Slovakia, Ukraine, and Russia.
ANATOMY OF SAPHIR
The SAPHIR ultrasonic systems are linked via Ethernet LAN (Local Area Network) to the controls of the inspection manipulator as well as to the workstation for data acquisition and evaluation. This allows setting of parameters for subsystems, operation of the equipment and transmission of inspection data and manipulator position data to be done while scanning is going on. These functions can also be controlled by a superordinate inspection data management system which has access not only to data from the inspection in progress but also to historical data from previous inspections, allowing rapid direct comparison of results.
SAPHIR can accommodate simultaneous operation of up to six phased-array search units each with 16 elements, or 96 conventional search units. Any combination of the two types of search units is also possible.
ACQUIRING THE DATA
To allow the measured data, which are supplied at a high-frequency following digitisation, to be stored on standard data storage devices, the data are compressed using one of the approved and conventional methods, the pixel mode or the “i/k algorithm” (ALOK) mode. The possibility of parameter adjustment with these data compression methods in conjunction with adjustable gates (FEB) and amplitude thresholds makes it possible for the complex signals from the inspection to be stored as a simple sequence of compressed data in a form which simplifies data analysis and the presentation of results.
SAPHIR is designed to be useable as mobile equipment in problematic environments. The electronics are housed in a rugged portable casing which protect them from impacts, high ambient temperatures, dust, contamination and electromagnetic interference.
Proper functioning of all channels, as well as of the processing of all analog and digital signals, can be checked quickly before and during an inspection, using an automated process. The result is output and recorded as a good/bad statement if desired, together with all the measured values and corresponding tolerances, and for quality assurance purposes is stored together with the measured data on the original data storage device.
The specification for the equipment self-testing routine was prepared together with an authorised calibration agency and qualified by the German authorised inspection agency, TüV. It is therefore recognised as providing verification of reproducibility in line with the applicable German industrial standard, DIN 25450.
The software package for data acquisition and evaluation runs on high-performance HP workstations under UNIX. The system can use both rugged industrial computers for mobile applications and more economic desktop variations for office applications. Hardcopies can be output on a fast colour printer. A modem is integrated in the system to allow remote diagnosis and software maintenance.
The workstation initially saves onto a magneto-optical disk all the parameter settings for the inspection equipment system in a header together with the ultrasonic measured data.
The measured data are simultaneously sorted and pre-processed ready for evaluation. This includes depth– amplitude correction unless this has already been performed in the ultrasonic device. During data acquisition, coupling images are computed for selected inspection functions, which can be seen immediately as a screen image.
Processing and evaluation of the test results is performed directly upon completion of data acquisition and all outputs can be documented on colour hardcopies.
The standard version offers the following possibilities for data evaluation:
• Output coupling images with statistical data.
• Geometrically-correct top-, side- and end-projections of amplitudes corresponding to times of flight, incident angles and scanning directions.
• Interactive modification of colour scale as an aid to evaluation.
• Definition of indications in the scan presentations using search functions for peaks and defect lengths.
• Display of amplitude and time histories for specific indications.
• Storage of specific indications and documentation of the results in list form.
• Display of amplitudes as TD image.
• Circle reconstruction of indications for analysis purposes.
The software package also contains numerous tools for preparing scans, for securing original data as well as a database for parameter settings and measured data.