World first on safety support system for power industry3 August 2002
A metal fatigue problem in the recirculation cooling water line at Cernavoda in Romania was discovered. A permanent solution was urgently required. By Jim Cuthill
Application of new technology within the nuclear industry is stringently monitored and validation is often a long process. Yet new technology is at the same time the life blood of any
forward-looking industry - and after considerable auditing of the methods and materials, a world first has been achieved on a safety critical system using the latest composites technology for a critical path repair.
Material fatigue on a 90° tee connection on a 24 inch recirculation cooling water line in Cernavoda was identified following an inspection. The plant produces 11% of the power for Romania, and shutdown would have had a major impact on the national power supply. The line had been subject to vibration and cyclical loading and it was probable that fatigue damage had been suffered. A permanent, reliable, on-line strengthening solution was urgently required.
The solution came in the form of advanced composites technology from the FD Alliance - a partnership formed between DML Composites and Furmanite International. After weeks of preparation, in-depth assessment, validation, and complex specifications, approval was given for the new technology (already being widely used in the oil and gas and chemical industries) to be put into practice for the first time on a safety support system line in the nuclear industry.
Conventional methods of permanently solving this position were out of the question. Under different circumstances, the line would either have been drained and replaced - something that could not be done given the critical function of this line - or a bypass would have been created around the damaged section using hot-tapping, to enable removal and replacement, but vibration in the pipe due to its proximity to the pump would have precluded any degree of accuracy.
FDA's composites solution, using carbon fibre and epoxy resin, allowed the necessary work to be undertaken on-line. The materials used can be applied while the plant continues normal operation, require no hotwork, and could accommodate the complex geometries and overcome the potential difficulties of restricted access.
Assessment followed by calculation and design of the repair - surface preparation required, repair thickness, resin formulation - are carried out in accordance with AEA composites specifications. Prior to the installation being approved for use, the FD Alliance was subject to full QA, including commercial and technical audits covering materials data, design data, installation, testing, quality and safety.
Although extensive validation testing has been completed to gain approval from the auditors, further testing was specified, involving 'repair' of two six inch diameter pressure test spools under the same conditions and using the same materials as would be used on-site for the tee repair - one for a pressure test, the other as an NDE trial piece. An initial pressure of 12.5 barg (just over the design pressure of the line) was applied to the 'repaired' spool and held for ten minutes, with the pressure subsequently increased to 60 barg and held for a further ten minutes, thus easily passing the hydro pressure test. The test spool was not taken any further than this due to the pressure ratings on the flanges. The repair was built up in a layering and wrapping process using carbon fibre and epoxy resin.
Any deviation during installaion from the required stages and application techniques could compromise performance or longevity of the repair. Preparation of the pipe surface (usually using grit blasting, to a specified SA2.5 (75 µm) finish), for instance, is critical, to remove oxidation from the metal and ensure a good bond for the fibre and resin - and must be done no more than four hours before the repair is applied to avoid re-oxidation.
A glass fibre tie-coat is then applied, providing a high quality interface and a degree of electrical insulation to guard against galvanic reaction between the substrate and the carbon fibre - something that has never actually occurred but is a possibility that must be avoided.
Layers of epoxy resin-impregnated carbon fibre are wrapped around the pipe, and built up according to the pre-calculated design thickness - commonly as little as 5mm and in this instance 26mm. Another of the advantages of the composites repairs is that they are up to ten times the strength of steel and twice as stiff, yet less than a quarter the density, with the repair often stronger than the original structure. Finally, a sacrificial peel ply layer is applied to remove excess resin and provide a surface finish ready to accept paint or any other finish that may be required, and is removed once the resin has cured.
Given the original likely cause of the fracturing, the repair was designed to limit the cyclical stress to the steel to below 48MPa (fatigue stress limit set out in ASME NB-3222.4 (c). Once cured at ambient temperatures (25ƒC) the repair was fully validated and restored full structural strength and pressure containment to the pipeline for a 25 year lifetime, providing a permanent solution. Shutdown had been avoided, with no loss of production.
Nicolae Beldea, technical superintendent at Cernavoda, said: "Now that we have validated composites for use in the nuclear power industry, I'm sure it will be something we will refer to again. It is an ideal solution, since it requires no hotwork, can be carried out on-line and can be classed as permanent.