Underground » Maintenance
This project was the first stage of a research program which has as its primary aim the introduction and development of condition based monitoring (CBM) maintenance practices into underground coal mining operations. The project was conducted by BHP Research in collaboration with BHP Steel, Appin Colliery and BHP Engineering. Condition based monitoring is an activity in which selected parameters associated with machinery operation are observed, so that the machinery can be more effectively maintained, to provide increased machine reliability and a reduction in the loss of production due to unscheduled machinery breakdown. It has been widely accepted by maintenance personnel as the most effective maintenance strategy and is being increasingly utilised in many industries worldwide.
Introducing CBM to the coal face is seen to be an extension of existing CBM practices into a difficult environment. Unfortunately, the lack of intrinsically safe (IS) sensors, systems and data communications networks has meant that CBM has been applied to a limited extent in hazardous zones in underground coal mines.
This situation is beginning to change with the certification of data collectors and other condition monitoring equipment. Demonstration of the effectiveness of CBM at the face is one way of ensuring the continuing development of equipment for IS applications.
Project Objectives
Broadly, the aim of this first stage project was to examine the ways in which monitoring the condition of underground mining equipment can lead to more effective maintenance practices and to increased machine reliability and availability. Machine reliability/availability is one of the major limits to increasing the productivity of underground coal mines. Improved maintenance practices should therefore lead to improved productivity.
The aim of this project was covered by a set of specific objectives. These objectives were to:
- Provide the scope and evaluate the requirements for effective on-line CBM of underground mining equipment.
- Identify and prioritise mining equipment for which CBM will provide a major benefit, and outline the resources required to facilitate such monitoring.
- Demonstrate the viability of CBM by adapting existing IS instrumentation to the monitoring of mining equipment located in hazardous areas underground.
- Determine the system specifications of a dedicated IS monitoring system and data communications protocol, which could be developed at a later stage of the research program.
Conclusions
The results of the survey indicated that there was an awareness of condition monitoring and that it was generally viewed in a positive light. By way of example, the average (unweighted) response of those responding to whether there was sufficient industry support for CBM was on the scale of 1 to 5 (a 5 indicating that the respondees believe that there is very good support available). A figure of 3.5 suggests that there is potential for suppliers of CBM equipment and expertise to expand their services to the industry.
Maintenance engineers confidence / expectation in CBM rated 4.0 (5 being high confidence). Maintenance personnel rated the cost effectiveness of CBM as 4.1, the safety effectiveness as 3.6 and other contributions of CBM as 3.9. Ratings of management in these three categories were 3.5, 3.6 and 4.1 respectively.
The level of implementation of CBM ranged from virtually non-existent to fully net-worked on-line monitoring systems. The most widely use CBM method was oil analysis, with 25 out of 29 mines indicating that oil analysis was used. Sixteen respondees indicated that they used vibration analysis (four respondees gave no reply), twelve respondees that they monitored the mechanical condition/wear of major components (six respondees giving no reply) and six respondees that electronic equipment was monitored (ten respondees giving no reply).
Other CBM techniques used by individual mines included motor current signature analysis and face equipment monitoring. Some of the reasons given for not using one or more CBM techniques were the high cost and the poor availability of intrinsically safe sensors and systems.
In Longwall mines, 14 out of 20 respondees indicated that shearers were considered to be highly critical (a rating of 5 out of 5). For all mines surveyed, 21 out of 29 rated continuous miners as highly critical and 20 out of 27 rated belt drives as highly critical. In addition, individual respondees identified a wide range of equipment which caused continuing maintenance problems at their mines, the most numerous of which was various problems with conveyor belts.
Unscheduled maintenance was experienced by 26 of the respondees (the other three did not reply). Seventeen of the 26 provided a figure for unscheduled maintenance as a percentage of all maintenance costs.
These figures ranged from 2% to 50%, with an average of 16% (production weighted). Five of these mines also provided the cost of their maintenance budget as a percentage of total costs. For those five mines, unscheduled maintenance accounted for approximately 6.2% (production weighted) of the total costs.
The wide range in the percentage of unscheduled maintenance suggests that different mines use different criteria for estimating unscheduled maintenance. One possible reason for this wide range could be that the cost of lost production resulting from in-service equipment failure has not been included in the cost of unscheduled maintenance. In some mines this can amount to almost $90,000 per shift. The inclusion of the cost of lost production in unscheduled maintenance costs would significantly increase the 6.2% estimated above.
Overall, the results of the survey show that CBM has been adopted by some within the industry who believe that CBM can improve mine productivity. However, there are others who believe that the benefits of CBM are not sufficient to warrant the necessary expenditure. This belief is less prevalent in mines where a high rate of production is supported by a high degree of mechanisation. For example, all longwall mines carry out some form of CBM.
Trial Results
Three successful trials were held at Appin Colliery on 25 November 1993 and on 8 June and 5 July 1994. The results of the first trial showed that the AFC maingate drive gearbox had suspected bearing faults and that the crusher had suspected alignment problems. The alignment of the crusher was a known fault and was due to the loss of some pick boxes.
A change-out of the Appin longwall took place in December / January 1994. Crusher pick boxes and coupling rubbers were replaced. As a consequence of the monitoring results, the maingate drive gearbox was overhauled. A second trial indicated that the bearing faults were still evident and that there was evidence of 'knocking' in the gearbox, possibly due to wear of the first gearset.
Other results from this trial showed that the left and right hand cutter heads of the shearer may be developing faults, possibly related to gear meshing. The levels of the suspected faults were moderate and low, respectively. Of the five longwall components monitored, only the shearer was monitored in the final trial.
In this trial, conducted with an IS data collector, the fault in the left hand cutter head was confirmed but the suspected low level fault in the right hand cutter head was no longer evident. The possible fault in the AFC maingate drive gearbox remains to be confirmed.
The project objective of conducting monitoring trials of CBM was completed with the final trial on 5 July 1994. However further monitoring at Appin Colliery has been scheduled for 250,000 tonnes of coal to be extracted from the longwall. In addition other pieces of equipment such as auxiliary fans are to be monitored on a monthly basis. The benefit of monitored face equipment in the hazardous zone has been clearly demonstrated.