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Application of NDT for In-service Inspection of pressure vessels and heat exchangersN Rajeshwar Rao, C Phani Babu, D S Sastry, K N S Gupta, S V Swamy, N Saratchandran
Nuclear Fuel Complex, Hyderabad, INDIA
Critical process equipment such as pressure vessels and heat exchangers need to be periodically subjected to In-Service Inspection, to monitor their condition and to assess their life. Non-Destructive Testing techniques such as Eddy Current Test, In-situ Metallography, Magnetic Particle Test, Radiography, Ultrasonic Test for flaws / corrosion monitoring, Visual Test etc., play a vital role in this respect. Using the appropriate techniques, several equipment, including SS 310S reduction retorts, High Pressure Accumulators, H2S / LPG Storage Vessels, Heat Exchangers, pipe lines and piping welds were inspected to assess their condition and fitness for continued service.
This paper brings out the considerable experience and expertise gained in the judicious use of various NDT techniques for In-Service Inspection.
Key Words: In-Service Inspection, Heat Exchangers, Pressure Vessels, LPG Bullets, NDT Techniques
The next few pages present the considerable experience gained at Nuclear Fuel Complex (NFC), Hyderabad, India, in In-Service Inspection (ISI) of
NFC manufactures Zirconium alloy Core Structurals and Uranium Dioxide Fuel For Pressurized Heavy Water and Boiling Water Reactors. Zirconium sponge is produced from Zirconium Tetrachloride using magnesio-thermic reduction, followed by vacuum distillation of the sponge. The reduction retort is critical equipment, subjected to a moderate pressure but high temperature and an extremely corrosive environment. The retort is fabricated from SS 310S plates by welding, to meet ASME Boiler and Pressure Vessel Code. Since the retort sees an extremely corrosive environment resulting in material and weld degradation, regular ISI is essential. The SS 310S plates are subjected to Visual and Ultrasonic Tests and the welds are subjected to Visual, Dye Penetrant and X-ray radiographic examination. Radiography has been preferred over Ultrasonic testing for qualification of the butt welds since, after use, due to corrosion, ultrasonic attenuation increases enormously, making UT of welds impossible. ISI makes a judicious use of X-ray radiography, Visual and Ultrasonic Thickness monitoring.
We have one interesting observation about thickness monitoring, to be shared. While the fresh SS 310 S plates could be checked for thickness using a normal digital ultrasonic thickness gauge, after some use, the thickness readings were increasing or due to increased attenuation, the digital thickness gauge was unable to read at all. The problem was traced to the formation of a semi-metallic corroded layer whose ultrasonic velocity is less. Monitoring of good metal thickness was achieved by the use of Ultrasonic Flaw Detectors employing A-scan presentation which could clearly delineate the echoes from the interface between the corroded metal and the back wall echo from the relatively sound metal.
In-situ metallography, ferrite measurement and chemical analysis of the weld metal were used in addition to the above NDT techniques to successfully analyze the premature failures of these critical equipment.
Titanium is a critical metal in aerospace and chemical process industries. While the present needs of Titanium Sponge in India are being met by imports, efforts were on to develop a viable indigenous technology for the production of Titanium Sponge. A 4t combined Reduction-Distillation cycle has been developed. The reactor is made of SS 310S or SS 304, lined in either case inside with SS 430 to protect the nickel bearing Austenitic stainless steel from liquid magnesium. The reactor is made by welding of SS 310S / 304 plates clad with 430 to ASME code. Other internal structures are protected by weld overlaying of SS 430. The retort is qualified by use of VT, UT, PT and RT at the time of fabrication. After each cycle, the reactor is thoroughly inspected for the integrity of 430 layer, all butt and fillet welds etc., by using VT, and PT. Ferrite measurement, and in-situ chemical analysis of the 310 S plates to check for nickel-leaching by liquid magnesium were effectively employed during Failure Analysis of one experimental retort.
NFC employs a 3150 t horizontal hydraulically operated extrusion press for extrusion of Zirconium alloy Ingots and Various Steel billets into different shapes. The force is developed through high-pressure hydraulic accumulator bottles, which need periodic ISI. Ultrasonic thickness monitoring of the bottles was taken up using conventional digital thickness gauges. A number of spots in some areas of the bottles were showing abnormally low thickness indications. Visual inspection of the outside of the bottles (which are made from carbon steel plates and welded) did not reveal anything abnormal. Visual inspection of inside of the bottles was not feasible in the limited time available. The problem was again successfully resolved using a regular ultrasonic flaw detector using normal as well as angle beam probes. The abnormal reflectors inside the steel plates were found to be inclusions.
LPG tanks were taken up for ISI to certify their fitness for use as per statutory requirements. Visual Inspection and Ultrasonic Thickness gauging was carried out which established the soundness of the tank all over, except at the support rings, which were found to be severely corroded. A question, which arose whether the corrosion has affected the LPG vessel at the region of the support, needed to be answered. The same was successfully accomplished by a visual inspection and Ultrasonic thickness gauging from inside the tank, which clearly established that the corrosion of the support ring was limited to outside.
Periodic inspection of storage vessels, which hold toxic chemicals like H2S is a statutory requirement. H2S is an important ingredient in the production of heavy water through exchange process. The various weld joints of the storage vessels were inspected by visual, magnetic particle, ultrasonic, penetrant and wall thickness tests. The parent material of the vessel was subjected to ultrasonic thickness gauging.
Heavy Water Plants, which produce the Heavy Water requirements of the Indian Pressurized Heavy Water Reactors, are located at various places in India. Several Heat Exchangers are needed in the Heavy Water Plants to take care of the H2S - H2O streams. Multi-frequency Eddy current testing is employed with optimally in-house designed and developed Differential type I.D. Bobbin probes, to clearly differentiate the signals from the tubes and the baffle plates.
The main fertilizer used in India is Urea and a number of plants based on Natural Gas, or Naphtha have been established to meet the ever increasing demand for this nitrogenous fertilizer. One important process step is the synthesis of Ammonia from Synthesis Gas, which is a mixture of CH4 (6.6%), H2 (51%), N2 (17%) and NH3 (21%) - all % figures are approximate. This is admitted into the tube side of the boiler at an inlet temperature and pressure of 450° and 190 kg/cm2 respectively. The shell side fluid is boiler feed water, at a temperature of 324° and pressure of 120 kg/cm2.
Iron in the form of pellets / granules is used as catalyst and fine particles impinge on the Incoloy 800 tubes of the Boiler causing erosion and in some cases leakage. Multi-frequency Eddy Current Testing is the preferred method of In-Service Inspection of the Incoloy tubes. However, not only the straight portion but the U-bend portion of the tubes needs to be inspected in some cases. Specially designed Eddy Current bobbin-type probes which could go around the U-bend and at the same time do not sacrifice the sensitivity have been developed in-house and Synloop Boilers as they are called were inspected at 2 fertilizer plants. The Eddy current Testing was carried out by calibrating the equipment with artificially produced defects. Through holes, flat-bottomed holes of different depths from O.D. side, O.D. and I.D. grooves were made on a tube similar to the ones used in the boiler. These defect standards were mostly as specified by ASME Section - V, Art. 8 and Appendices I & II.
NFC inspection team also carried out Single-frequency Eddy Current Test of a Chiller at a company producing Hydrogen Peroxide.
NFC inspection team also carried out In-situ metallography of welds and pipes at a fertilizer plant and Angle Beam Ultrasonic Testing of Pipe-line welds at one of the Heavy Water Plants. Normal beam and angle beam ultrasonic testing of the undercarriage struts of a helicopter was carried out as a part of failure analysis and similar in-situ testing was carried out on other helicopters.
In-Service Inspection (ISI) of any critical equipment requires a good understanding of Chemistry, Metallurgy, NDT etc., and best results are obtained when a multi-disciplinary approach is used. Ability to adopt suitable NDT techniques followed by Failure Analysis as needed is critical for the successful timely completion of a ISI project. Ability to make suitable eddy current probes of differential / absolute coil type will be an asset to quickly cater to the varying needs in a developing country where ready availability of such probes cannot be taken for granted.
NFC has acquired the necessary expertise over these years. A strong multi-disciplinary team consisting of Metallurgists, NDT Specialists, Corrosion Engineers, Physicists etc. is available for carrying out ISI, Failure Analysis and other elements of Quality Assurance (QA) functions for other organizations.
The authors are grateful to Dr. C. Ganguly, Chief Executive, Nuclear Fuel Complex for his keen encouragement and permission to present this paper at the 15th WCNDT, October 2000, Rome, Italy.
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