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·Methods and Instrumentation
Automatic Ultrasonic Inspection at 170°C - The TRAKIT SystemJ.F.Quirk
Phoenix Inspection Systems Ltd
In order to carry out the repairs it was necessary first to excavate the whole of each weld and then re-weld completely. The whole operation involved experts in many fields who brought their considerable knowledge to bear on the problem.
In order to satisfy the Safety authorities each weld and its adjacent H.A.Z had to be inspected before repair and after re-welding to the very high standards called for by the Inspection Qualification requirements. The task of designing and producing an automated inspection manipulator and ultrasonic probes capable of operating at elevated temperature with high accuracy and repeatability fell to Phoenix Inspection System Ltd.
After welding, but before post weld heat treatment, a full inspection then had to be carried out with the boiler surface at 170ºC. This was to be followed by a further full inspection after post weld heat treatment, again carried out at 170ºC. A final inspection was then to be completed at ambient before the station returned to full power.
Following the successful repair it was intended to use the equipment to monitor the condition of the welds during the remaining life of the plant. The equipment, which was designed and manufactured by Phoenix ISL to carry out these inspections is now known as "TRAKIT".
The design and construction of the complete system had to be completed for a fixed price and produced within the very short time scale of fourteen weeks.
For hot scanning the couplant used was a silicone oil compound whilst for ambient scanning a water based gel was used. The couplant supply system had different couplant cylinders and umbilicals for the oil and gel. The scanner was designed to fit within a small envelope, which was defined by the positions of horizontal support beams and vertical pipes running close to the boilers. In order to keep the amount of insulation removed from each side of the weld to a minimum the scanner had to fit partly over the insulation. In addition, to reduce heat loss during the hot scans, only a limited area of the boiler was exposed at any one time yet the system had to inspect the maximum possible length of the exposed weld.
The system was designed to operate with any multi channel flaw detection system with a DC servo controller.
It can be configured to perform axial scans with circumferential increments and vice versa.
Cable management was particularly difficult to achieve under the prevailing conditions and which was further complicated by the high temperatures involved.
Finally, it was necessary to protect from heat the more delicate parts of the manipulator, in particular the electronic circuitry, encoders and motors. This was achieved by the use of air-cooling and skilfully designed heat breaks. Stainless steel was used widely and knife-edge contact between parts was used wherever possible. Plastic bearings and silicone seals were used in strategic areas. These and other design features resulted in a temperature gradient of 150ºC between the probes on the boiler surface and the outside of the scanner, a total distance of only one hundred millimetres.
In order to be able to carry out probe changes whilst at high temperatures, the probe pans could be removed without disturbing the rest of the scanner thus avoiding loss of positioning if this operation had to take place during a scan.
The probes were kept in contact with the undulating surface of the boiler shell by means of constant force springs used in conjunction with tool posts which could pivot in two planes.
The time for a complete scan of the weld in pulse/echo was ninety-six hours in the hot state and thirty-six hours at ambient. The TOFD scans took thirty-one hours hot.
The total distance scanned during the operation was twenty-seven kilometres of weld at ambient and thirteen kilometres at high temperature.
The probes, which were designed to operate at 270ºC, experienced no difficulty operating at 170ºC even with out cooling.
There were no equipment failures and no personal injuries during the inspection and all programmes and cost targets were met.
As a result of this exercise a modular, high temperature general purpose weld inspection system was produced capable of inspecting general fabrication welds. Because the scanner uses the same track as the welding head and the system can operates at high temperature, welding and inspection can be carried out at the same time.
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