·Table of Contents
·Methods and Instrumentation
Ultrasonic inspection of high strained component parts in hazardous areas with intrinsically safe flaw detectorAuthor: Ralf Dix, DMT GmbH, D - Bochum
Co-Author: Johannes Büchler, KRAUTKRÄMER GmbH & Co. oHG, D-Hürth
The safety of operation of these highly-stressed parts of hoisting devices is of utmost importance to the mine operators also from the economic point of view. For this reason, "Technische Anforderungen für Schacht- und Schrägförderanlagen" ("Technical requirements for shaft hoisting installations and inclined hoisting installations"), under the abbreviation TAS, and legal requirements, such as the "Bergverordnung für Schacht- und Schrägförderanlagen" ("Mining decree regarding shaft hoisting installation and inclined hoisting installations") (BVOS) of 1977, were issued for such systems. Both requirements refer, among other things, to suspension gears and balance rope suspensions. According to the definition, the suspension gear parts include, among other things, clamp eyes and thimbles. The balance rope suspensions include, according to TAS, all connecting parts between the balance ropes and the joint plates or supports rigidly attached to the conveying device or counterweight.
The BVOS requires the manufacturer's or supplier's certificates of conformity on the quality of the material used prior to the commissioning of suspension gears and balance rope suspensions. After the commissioning, an annual visual inspection of the mounted equipment, to be carried out by expert controllers, is specified among other things. During this inspection, individual parts shall be subjected to a visual inspection in order to detect any corrosion, changes in shape and incipient cracks. Every two years of operation, the supporting members of intermediate cage suspensions subject to tensile or bending stresses must be recurrently tested by suitable methods of nondestructive materials testing for any incipient cracks. This is done by experts in certain cycles. This arrangement applies to all suspension gears and balance rope suspensions in operation.
Since some time around 1994, the possibility has existed to replace the visual inspection of suspension gears in selected open shafts with nondestructive tests of the mounted equipment on agreement with the competent mining authorities and the DMT's Export Body for Safety - Test Centre.
The ultrasonic flaw detector, weighing about 3.5 kg, consists of a sealed and solid plastic housing, and meets the high requirements of class EEx ib I (mining industry) and EEx ib IIA T4 (generally potentially explosive atmospheres) according to the standard specifications EN 50014 and EN 50020. In addition, it has all the important ultrasonic testing functions, such as DAC evaluation and trigonometric functions, digital display of up to 3 measured values, four variable frequency ranges (between 0.1 MHz and 15 MHz), and a data memory for 100 A-scans and instrument settings, just to mention a few. The test range has accordingly been adapted to the fields of application within the underground mining industry, and is between 2.5 mm and 10m. This enables the use of this instrument both for wall thickness measurements on pipework and for crack detection on long roof bolts for mine support, with only their faces being accessible. A wide variety of probes has been approved for the areas subject to explosion hazards (angle-, straight-beam and TR probes), having different rated frequencies and element diameters.
|Fig 1: Intrinsically safe ultrasonic flaw detector USM 23Ex including probes|
Due to extreme cost and manpower savings within the German coal mining industry, possibilities were looked for and found to reduce the maintenance costs for the operation of hoisting gear and slope conveyor systems to a minimum, without neglecting the officially specified and operationally required safety objectives.
Using the approved ultrasonic flaw detector USM 23Ex and a test configuration which is especially designed for critical cross sections of suspension gears and bolts, these specified safety objectives can now be achieved with a yet improved information value even in mines susceptible to firedamps.
3.1 Preliminary test or qualification test
Suspension gears that should be tested when mounted, must first be subjected to a single, non-recurring ultrasonic volume test. For this purpose, the measurement ranges and probes shall be selected so that a reflector size evaluation according to the DGS method can be carried out. Sound path and transfer corrections shall be taken into consideration. Scanning is carried out directly or via an angle consisting of at least two scan directions which are offset by 90°. Registration and acceptance levels were defined on the basis of a large number of recurrent tests on suspension gears and balance rope suspensions showing variably sized real and artificial damages.
On the basis of the number of hoists per year of the individual conveyor systems, a difference can be made between conveyor systems subject to "high", "medium" or "low" stresses.
3.3 Inspection cycles
The inspection intervals for suspension gears are determined according to their loads, i.e. the number of hoists per day or per year. Moreover, the climatic and mechanical operating conditions, e.g. the cage guides in the pits, are taken into consideration. The service lives of the hoisting ropes and the accessibility of the critical cross sections to be covered with regard to the ultrasonic test also play an important role in view of the test intervals.
For example, the application intervals of the suspension gears on a man-riding and material hauling system can in this way be doubled if the mounted suspension gears are ultrasonically tested in the areas subject to tensile and bending stresses (critical areas). In another example of a system subject to low stresses, with a maximum of 20 hoists per day, the application interval of the suspension gears is even quadruplicated. In this case, the ultrasonic test of the mounted equipment is only required after an operating time of 24 months.
3.4 Inspection of mounted equipment
In pits containing explosive media, the test of the mounted suspension gears is exclusively carried out using ultrasonics. The calibration and sensitivity setting is directly determined on the suspension gears using a known side-drilled hole. A sensitivity of 2 mm ERS + 6 dB is used in the test, with the registration and acceptance levels being identical with those of the preliminary and qualification tests (volume test) described above.
Universal joint parts
|Universal joint parts||Concealed devices|
|Fig 2: Ultrasonic testing of highly-stressed cross-section areas of suspension gears|
The areas of the clamp eyes, clips and universal joint pieces which are relevant from the point of view of test techniques are the areas of the drilled holes and material transition points in which the highest stresses occur in the component due to the reduced cross section. Figure 2 indicates the scanning positions, starting from the faces of suspension gear parts, in order to test the entire critical area for any incipient material separations. As all suspension gear parts which should be tested in this mode have previously been subjected to a volume test using ultrasonics, all types of inclusions inside the component can be excluded. Damages can therefore only be caused on highly-stressed cross sections which are reliably detected by this method because, as a rule, a corner reflector for the ultrasonics is given here.
|Fig 3: Ultrasonic Flaw Detector USM 23 Ex in field use below ground|
Figure 3 shows the field application of the new intrinsically safe ultrasonic flaw detector in an upcast ventilating hoisting shaft. The mounted suspension gears and balance rope suspensions are tested here in the potentially explosive atmosphere for any signs of material fatigue. This test configuration helps the mine operator to save considerable costs for maintenance and repair of the above-mentioned components. The information value regarding the safety of operation can even be improved by observing the test specifications because even the smallest material separations can already be reliably detected.
The fans suck out the used current of air (air currents in motion), which may be contained in an explosive concentration in the firedamp developed by the mining of coal seams, from the pits and drifts. Due to the corrosive constituents of the used currents of air, these fan blades are cast of high-tensile and corrosion-resistant steel G 4313 G-X5 Cr Ni 13 4, having an apparent yield point of 650 N/mm² and a tensile strength of 800 N/mm².
The fan blades to be tested when mounted were made and mounted in the summer of 1994. A 100% nondestructive test for any surface cracks was previously carried out using the magnetic particle test method. The highly-stressed fan part, the transition point from the bearing journal to the blade, was additionally checked for any internal material separations by means of ultrasonic testing.
After some 10,000 operating hours, temperature-dependent vibrations, which intensified in the course of further operation, were observed on the fan and measured. Corrective measures by a repeated balancing of the individual fan blades did not lead to any measurable success.
During a stop and active repair period, the mounted fan blades were then tested for the first time by means of the intrinsically safe ultrasonic flaw detector USM 23Ex for any material separations in the highly-stressed areas. This time, the test was not only limited to the transition points between bearing journal and blade, but was also extended to the bearing journals concealed by the mounting situation.
Fig 4: Representation of reflectors of bearing journal
The test configuration for such tests, with alternating thin-walled and curved geometries, was worked out by means of reserve blades and the technical documentation. Because of the curved surface and the varying material thicknesses due to the casting process, the calibration and sensitivity setting is carried out on every single bearing journal to be tested. Drilled holes, the circumferential grooves, and the recesses are used as reference reflectors. The figures below show the corresponding ultrasonic screen displays of the above-mentioned reflectors.
A clear distinction between reflections from the geometry and reflections from the material can be made on the basis of the known distances of the reflectors which are partly even overlapping.
During a test of a mine fan having a total of 24 integrated fan blades, a reflector was detected on a bearing journal between the reference surface (curved surface) and the first groove. The reflector could be traced almost circumferentially on the entire curved coupling surface, with its maximum amplitude being somewhere around the middle of the bearing journal. All in all, an ultrasonically determined weakening of the bearing journal by approx. 50 % to 50 % could be assumed. This fan blade was immediately exchanged and released for further nondestructive and destructive tests.
|Fig 5: Circumferential incipient crack||Fig 6: Macroscopic cross sectional diagrams of bearing neck|
In the subsequent surface crack inspection of the dismounted equipment, a circumferential crack became visible at the bearing journal end. Any further use of this fan blade was excluded. A destructive material test should find out the cause for the occurrence of this crack in the bearing journal which was not located in the highly-stressed area on the basis of the known loads on the fan blade. The bearing journal was separated twice in lengthwise direction for macroscopic micrographs according to the ultrasonic test results.
The flaws detected are clearly defects of fabrication which were already produced during casting or during the subsequent solidification of the melt. The increased vibrations in the mine fan in the course of its running time are likewise due to these flaws and the resulting crack propagation. After the defective fan blade had been exchanged, no more balance errors or vibrations were measured on the mine fan.
The operation of systems and plants will thus become more economic and safer as the costs for maintenance and repair are reduced by the reduced amount of tests required on the mounted equipment.
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