Manipulators to inspect pipes28 July 2000
Increasing regulatory safety demands and plant aging have required the development of sophisticated remote inspection techniques. Siemens now has over 20 years’ experience of developing remote controlled manipulators to inspect and repair pipes.
Piping within nuclear power plants links components within the reactor cooling system (RCS) and the RCS to auxiliary systems. The pipes must meet stringent safety regulations, so plant operators must ensure that their maintenance and safety systems are state of the art. Technological developments drive regulatory demands, which in turn impact technology.
As part of the regulatory regime, it is often necessary to inspect, test and repair the inside of pipes. Siemens has developed manipulators which can carry out video inspection, grinding, milling and welding under remote control. The manipulators can operate in pipes with diameters ranging from 100mm to 1,200mm.
Typically, an in-pipe manipulator will comprise a unit carrying a camera and equipment to carry out the required task, and three or four drive units, connected by a flexible coupling mechanism. Siemens’ manipulators can turn very tight corners, down to one and half times the pipe diameter; they can also cope with negotiating four or five corners before reaching the area where the work is to take place.
Inspection cameras use a variety of lenses depending on the specific task. Fish-eye lenses with a short focal length are useful for examining mechanical integrity and locating foreign bodies. Zoom lenses offer enlargement of surfaces, necessary in identifying and assessing corrosion and other flaws in pipe integrity. A camera with a fish-eye lens requires little manipulation, but zoom cameras are mounted on a pan/tilt head.
Power is supplied to the manipulator through a cable dragged behind the vehicle. For pipes with diameters over 700mm, Siemens developed a cable-less vehicle powered by batteries.
One of the tasks Siemens has had most experience in is grinding down pipe welds on the interior surface to ensure that they are flush to the pipe. In Germany in 1978, the regulatory authority demanded that this be done in the country’s nuclear power plants so that ultrasonic inspection of the welds could be carried out to examine their integrity. If the weld is not ground flush to the pipe, ultrasonic waves can behave in a similar fashion to the way they would were there a crack in the pipe or weld. Siemens has now ground flat over 6,000 weld roots and developed more than 300 designs of in-pipe manipulators. Other benefits of grinding welds flat is that it improves flow conditions, and can pre-empt corrosion attack and eliminate stress peaks.
The grinding module uses a turbine- driven grinding disk as its cutting tool.
A motor-driven tool slide provides both circumferential and disk feed motion. Operators use a camera to get the tool to its working position and monitor progress during the work.
Milling machines to remove flaws in the pipe wall and a welding head to perform local weld repairs are also available. The milling machine is similar in design to the grinding machine, but the tool carrier has to be more robust due to the higher reaction forces it must absorb. A collector sucks cuttings up like a vacuum cleaner, ensuring no debris are left within the pipe.
Welding modules carry an automatic welding head that uses the pulsed tungsten arc orbital welding process. The filler metal is carried by the manipulator. Welding parameters are stored in a welding power source programme and the process is monitored using two video cameras. One of the cameras is directed towards the molten weld pool and monitors the entry of the filler metal wire into the pool and its fusion with the weld edges. The other camera monitors the finished bead.
Siemens has developed a variety of driving mechanisms to address different situations. Pneumatic or hydraulic wheels can contract, making insertion into the pipe through an open valve easier. Once inside the pipe, the wheel expands to exert the required pressure on the inner pipe surface to move the manipulator to the work site. Once in position, the pneumatic or hydraulic pressure can be increased to ensure the equipment remains stable during operation. During an emergency situation, if the cable were to be cut for example, the pneumatic or hydraulically loaded wheels automatically retract so that the manipulator can be pulled back to the exit by the steel cable within the cable package.
Other forms of driving mechanism include the stepping mechanism, often necessary if high accuracy or strong clamping is required. A variation on the stepping mechanism is the spider legs, or curved clamping assemblies, a design on which Seimens has a patent. This design is particularly useful if the manipulator has to negotiate obstacles on its way to the work site. Based on the same principle as the stepping mechanism, the driving unit comprises two clamping assemblies made of highly elastic fibre glass or carbon fibre rods, tensioned to form a curve. When the tensioning pin is activated, the two ends of the clamping assemblies are drawn together, causing the curvature to increase so that the assemblies are pressed against the pipe wall. This allows the manipulator to move and also to be secured at the work location. The spider leg design is also very light, weighing only 18-20kg.
Yet another design is the vacuum unit, which creates lower pressure on one side of the manipulator, which is then pushed by the pressure difference. This design is very useful in carrying out surface crack inspection, particularly with very narrow insertion conditions relative to the pipe diameter. This is because it is possible to insert the manipulator components individually and connect them up within the pipe.
All these techniques involve operating in a dry pipe, but Siemens has also developed a manipulator which can operate inside pipes still full of water. Movement is created using a hydraulic system, and the manipulator is designed to be able to cope with vertical pipe sections, pipe elbows, 70% reductions in the pipe diameter, gate valves, pump casings and lateral nozzles. It is also capable of negotiating small obstacles within the pipe. The device carries ultrasonic and eddy current inspection equipment and video cameras provide operator visuals.
An important repair and maintenance technique for working under water is electric discharge machining. The technique can be used for cutting and shaping parts, and offers significant advantages over conventional mechanical cutting processes. Electric discharge machining can remove material as long as it is conductive. The benefits of this technique include: no interference with material properties; no cutting difficulties no matter how hard the material is; the only residues are tiny particles which can be filtered out; the process is fully automatic; and complex geometric shapes can be made by suitable shaping of the electrodes.
Siemens has used electric discharge machining since 1976, and the technique has been used for a wide range of applications, including: • Refurbishment work on thermal shield attachment elements involving making cuts in parts up to 190mm thick with a cut width of up to 250mm.
• Cutting up the thermal shield of a research reactor.
• Making bypass holes in highly radioactive fuel assemblies.
• Removing brackets that were causing interference on the walls of spent fuel storage pools.
• Making attachment holes in reactor internals.
• Removing lock welds and lock beads for replacement of core shroud bolts.
• Removing broken cutting tools.
Manipulators have been developed which can perform eddy current and ultrasonic inspections within pipes. Eddy current inspection is useful for examining the surface of pipes, penetrating to a depth of 4mm within the pipe material. Ultrasonic waves penetrate the pipe material and can indicate flaws or cracks within the entire depth of the material.
Another inspection technique Siemens has developed uses dyes. Dye-penetrant examination involves applying a dye to a surface for 20-30 minutes. The surface is then cleaned with demineralised water. If there are any cracks in the pipe, the dye will penetrate them and remain once the surface has been cleaned. After the surface is cleaned a developer is applied which makes the dye visible, hence exposing cracks.
Siemens has carried out remote in-pipe maintenance and repair work at nuclear power plants throughout Europe and North America. The company is planning to expand its services to plants in Eastern Europe and the Far East as well as further penetrating the North American market. Its expansion plans are based partly on the opportunities the planned merger with Framatome will present and partly on the fact that it has over 20 years of experience, which puts it at the leading edge of the technology.