WIRE ROPE CONDITION ASSESSMENT

Tad S. Golosinski

Mining Engineering, University of Missouri-Rolla, MO65409-0450, USA. Fax: 0011-1- 573- 341 6934, Email:

SUMMARY

The paper outlines current status of the wire rope condition assessment technology and provides detailed brief on rope magnetic inspections, the most important tool in rope condition assessment. It then proceeds to discuss typical applications of the magnetic rope inspections, including its unique application to condition assessment of the guy ropes at a 120 m high flare stack in an Australian oil refinery.

Keywords: wire ropes, condition assessment, magnetic rope inspections.

1. WIRE ROPE WEAR

Wire ropes are used in a variety of industrial applications. Perhaps the most demanding is their application in mine winding where safety of personnel and mine ability to deliver the mineral or coal to the surface are directly related to wire rope performance. Criticality of this application has resulted in large volume of research intended to develop reliable rope condition assessment techniques and methods. Consequently, this presentation emphasizes mining applications of wire ropes.

Depending on the specifics of an application the ropes wear in several distinct ways. The wear may have a form of:

  • Fatigue breaks of wires caused by fluctuating tensile stress and repeated rope bending. This is a predominant form of wear for ropes which frequently bend over pulleys, sheaves and drums, in applications such as head ropes of friction winders or hoist ropes of draglines.
  • Abrasion and plastic wear of rope exterior as evidenced by flattening of crown wires and reduction of the rope diameter. This wear is a result of rope interaction with drums and sheaves or its rubbing against hard surfaces. Typically this form of wear is present in head ropes of drum winders, in drag ropes of draglines, and in similar applications. A sample of heavily worn drum winder rope is shown in figure 1.
  • Internal wire nicking and breaks caused by relative movement of rope strands and wires as the rope bends or twists during work. This form of wear often occurs in applications where multi-strand, non-rotating ropes are used. It may also be a main reason for wear of the ordinary, cross lay ropes that are used in many industrial applications.
  • Corrosion wear. Corrosion of rope wires may be severe in some rope applications such as stationary, poorly lubricated ropes with long service periods. This form of wear is common in track ropes of aerial ropeways, tail and head ropes of mine winders which operate in wet shafts or in upcast ventilation shafts, and the like.
  • Plastic deformation and mechanical damage. It may result from interaction of moving rope with stationary objects, as is the case for drag ropes of draglines. It may also be caused by mechanical impacts, such as the damage to tail ropes on friction winders caused by debris falling down the shaft.

Other forms of wear may also be faced in particular rope applications, such as rope crushing on small diameter pulleys, heat damage, sometimes a result of lightning strikes, electric arcs or other heat sources, rope damage in attachments, etc. Severity of wear increases if it is concentrated in certain sections of the rope, as opposed to uniform wear along rope length and circumference.


Figure 1. Worn out winder rope discarded from a double drum winder. Note heavy abrasion wear of the crown wires.

2. ROPE CONDITION ASSESSMENT

Rope condition assessment requires that all forms of wear are quantified and their cumulative impact on rope weakening is assessed. This needs to be done at rope weakest point, identification of which is often difficult. Typically the rope condition assessment involves a combination of visual and non-destructive inspections. The destructive rope testing is also used in some applications but for most ropes in service its use is not feasible.

2. 1. Visual Inspections.

Visual rope inspection involves both visual inspection of a rope and measurement of its basis construction parameters. Visual inspection is done while rope moves at a slow speed, approximately 1 m/s. It allows for identification of external wire breaks and identification and quantification of mechanical damage and deformation.

Several rope construction parameters are of concern including rope diameter and its lay length. As a mater of routine both the rope diameter and lay of length are measured and the measured values compared to the original values as recorded at rope installation. Change of both quantifies the severity of rope deterioration. In addition external, and if feasible also internal rope corrosion is quantified.

2. 2. Non-destructive inspections.

A variety of non-destructive rope inspections techniques were proposed and tried, including Eddy currents, acoustic emissions, X raying, etc. However, today only the magnetic inspections are sufficiently developed to meet the demands of an industrial application. Magnetic inspection apparatus is portable, inspection of long ropes can be done relatively quickly, the method can be used to inspect a wide range of rope sizes and constructions, and methods for sophisticated processing of inspection results are defined (Geller and Udd, 1990). In several countries with strong underground mining industry, including Australia, the conduct of magnetic inspections of ropes is governed by national standards (ASTM, 1993; Draft Australian Standard, 1997; South African Standard, 1996; also various German and Polish standards).

3. MAGNETIC ROPE INSPECTIONS

Magnetic inspections of ropes involve magnetization of the inspected rope and subsequent measurement of either magnetic flux in the rope, the leakage magnetic flux, or both. The first is assumed to be directly proportional to the metallic cross-section area of the rope, while the second indicates existence and sometimes allows quantification of local faults (LF).

A fragment of the typical record of rope inspection that has identified one broken wire is shown in figure 1. It includes two Local Faults (LF) signals (the two topmost traces in figure 1) that indicate presence of broken wires or other local faults. Availability of two LF signals recorded by coils of different diameter and at different signal sensitivity setting for each allows to locate the position of the fault within the rope (Golosinski, 1996).

The Loss of Metallic Area (LMA) signal, shown as the second lowest trace in figure 1, allows quantification of the overall rope weakening. For the rope length shown in figure 1 the measured LMA is insignificant.

The lowest trace in figure 1 shows the sum of LF over a pre-set rope length. Integration of the LF signals over the rope length equal to that of length of influence of a broken wire simplifies assessment of rope weakening. It also allows for easy quantification of the rope weakening resulting from internal rope wear, and most notably from the phenomenon known as wire nicking.

The scale on the top of the recording indicates rope length and its speed in the head of the apparatus. It allows for easy relation of the indicated damage to the specific portions of the rope.


Figure 1. Typical record of rope condition done during magnetic rope inspection.

The type of information gathered during magnetic rope inspections depends on the type of apparatus used and may differ from case to case. The minimum requirements determined as such in the Draft Australian Standard (1997) include the record of local faults (LF) and the loss of magnetic area (LMA). These two need to be related to the location of the LF, or LMA on the rope so that this portion of the rope can undergo a follow-up visual inspection. Results of both the magnetic and visual inspections have to be considered to allow for quantification of rope weakening.

Magnetic rope inspections have several limitations (Hecker, 1997). These relate to the type of ropes that can be reliably inspected and to accuracy of rope condition assessment. The method can not be applied to assess condition of a rope in its attachments, as the method requires unimpeded access to the inspected part of the rope. Where rope condition in the attachments needs to be inspected, such as in bridge suspension ropes or at dragline boom suspension ropes, the more expensive and time-consuming X-Ray inspections are usually employed. Furthermore the magnetic inspection is not easily adaptable to stationary ropes without easy access to the rope. This limitation has been largely overcome by development of a movable magnetic apparatus as described in the following part of this paper.

To define rope weakening (loss of breaking strength or LBS) the changes of the measured magnetic area of the rope are assumed to be directly proportional to the loss of metallic area. The latter, in turn, is assumed to be directly proportional to the loss of the rope breaking strength (LBS) which is of interest and concern to the rope users. Both assumptions may not be correct under certain circumstances what complicates the evaluation of inspection results and may render its results inconclusive (Geller and Udd, 1990). The relations in question depend on rope construction, its wear pattern and working condition. As such they are application specific and can not be generalized.

4. RECENT ADVANCES IN ROPE CONDITION ASSESSMENT

Magnetic rope inspection methods have been subject to extensive research and development during the past decade. A large study of the electromagnetic and magnetic inspection methods of winder ropes was conducted jointly by CANMET in Canada and the now defunct USBM (U.S. Bureau of Mines) in the U.S.A. This study provided a wealth of related information that can be summarized as follows (Geller and Udd, 1990)

  1. Introduction of compulsory magnetic rope inspections in the mining industry has lead to significant improvement of mine safety. However, the accuracy of the method is not always satisfactory.
  2. Accuracy of the magnetic rope inspection is determined, to a large degree, by qualifications of the rope inspectors and their familiarity with the method. Standardization of the method may significantly improve its accuracy.
  3. Digitization of the data acquisition process during the magnetic rope inspections offers the potential to significantly improve the accuracy of the method.
  4. The relation between the loss of magnetic area of a rope measured during its magnetic inspections and the rope strength is difficult to quantify and depends on a number of factors specific to rope application.
  5. A number of enhancements to the method and to the conduct of the rope inspections are possible, including periodic calibration of the apparatus, making it less sensitive to the external influences and eliminating signal drift.

This study triggered a major worldwide effort to improve and further develop the method. The recent developments can be grouped into three broad areas. These are: improvements to the magnetic apparatus, improvements to and standardization of the examination methods, and in some situations development of the quantitative relations between results of magnetic examination and the loss of breaking strength of a rope (Golosinski, 1998).

The improvements to the magnetic apparatus, and in particular the digitization of data acquisition and processing, are the two most significant advances. Availability of digitized data facilitates use of sophisticated analysis techniques that, in turn, provide a wealth of additional information on the condition of the rope (Geller et al, 1995; Golosinski, 1998). As a result rope condition assessment of today is more accurate and, in most cases, allows providing the rope operator with reliable quantitative information on rope wear and its remaining life.

Standardization of the method and its conditions has significantly reduced the possibility of errors of human nature that had resulted from incorrect apparatus settings, operation or lack of calibration. Relevant standards are now available in Australia (Draft Australian Standard, 1997), in South Africa (South African standard, 1996) and in the USA (ASTM, 1993). The proposed Australian standard is the most advanced and addresses the specifics of the wire rope use in this country.

Significant progress has also been made in defining quantitative relations between the remaining rope strength, or its loss, and the results of magnetic rope examination expressed as the loss of magnetic area (LMA) and local faults (LF) of the rope. This relation is site specific and depends, between other factors, on rope size, construction and wear pattern. Furthermore, its definition requires quantification of several other phenomena, such as the degree of wire hardening which takes place as it wears.

In another development the magnetic rope inspections can now be used to inspect various stationary ropes. This was made possible by provision of rope mounts that allow all the magnetic apparatus to be mounted directly on the rope and equipping it with a self-propelling device which provides for its relative movement against the inspected rope. One application of this technology is described below.

5. GUY ROPE EXAMINATIONS

As pointed out above, until recently conduct of magnetic inspection of wire ropes required unrestricted access to the inspected length of the rope. It also required that the inspected rope has the ability to move relatively to the magnetizing head. To overcome this limitation the NDT Unit of the W. A. School of Mines has modified the MERASTER apparatus that it uses. The modification included design of a customized apparatus mount which allows it to be attached to the inspected rope and provision of the apparatus propel device which allows for its relative movement along the inspected rope.

In addition the modification required provision of independent power supply to facilitate conduct of the inspection and modification of the data acquisition system. To conduct the inspection of a rope, the apparatus is installed in the mobile mounting and pulled along the inspected rope by a winch installed at the opposite end of the inspected rope.

The modified apparatus was recently used to inspect guy ropes on a 120 m high flare stack in an Australian oil refinery. The inspected ropes are in service for a number of years and heavy corrosion of their top parts was of concern to the plant management. The apparatus set-up on one of the inspected ropes and the conduct of the inspection are illustrated in figure 3. Overall nine guy ropes were inspected providing the refinery operator with the accurate information on condition of the ropes thus facilitating planning of their timely replacement.



Figure 3.Magnetic inspection of guy ropes on a 120 m high flare stack. Rope inspection on the left, apparatus installation on the right.

6. CONCLUSIONS

Sophisticated wire rope condition monitoring technology is available. In most rope applications it allows for quantitative assessment of rope weakening. While developed primarily for inspection of mine winder ropes, this technology is available for use of industry at large.

Basic tools in condition monitoring or wire ropes are their magnetic inspections. These need to be followed by visual inspections of weakened parts of the rope to allow for accurate condition assessment. Recently a number of standards were developed to assist with examinations of wire ropes and to set the conditions that each examination has to meet.

Modifications to the magnetic apparatus done at WASM allow for conduct of magnetic examinations of stationary ropes with no direct access to the inspected rope.

7. REFERENCES

1. ASTM Standard: E 1571-93. 1993. Standard Practice for Electromagnetic Examinations of Ferromagnetic Steel Wire Rope

2. Draft Australian Standard DR 97350. 1997. Non-destructive examination and discard of wire rope.

3. Geller L. B. and Udd J.E. 1990. How accurate are non-destructive-testing based estimates of mine shaft rope breaking-strength losses. CIM Bulletin, vol. 83, no. 944, pp. 47-50.

4. Geller L. B., Leung K. and Kitzinger F. 1995. Computerized operational control of an electromagnetic wire rope tester. Materials Evaluation, September, pp. 1002-1006.

5. Golosinski T. S. 1996. New method and apparatus for quantitative assessment of wire rope wear. Non Destructive Testing Australia, vol. 33, no. 4, pp. 207-210.

6. Golosinski T. S. 1998. On magnetic examinations of wire ropes. Annual General Meeting of the Canadian Institute of Mining and Metallurgy, Montreal, Canada, 1998.

7. Hecker G. F. K. 1996. The Safe Use of Mine Winding Ropes. Report GAP054. Mine Hoisting Technology CSIR, South Africa.

8. South African Standard SABS 0293:1996. Code of Practice: Condition assessment of steel wire ropes on mine winders.

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