Global Processing

How to reduce MTBF by 400 percent in a mineral processing application

August 22, 2006

If the mineral processing industry had an Academy that handed out awards for excellence like the film industry’s Academy Awards, then one of the strong contenders for an award would be the maintenance team at Israel Chemical’s Dead Sea Works. Improvements made by the team, together with SKF engineers, increased mean time between failure (MTBF) by 400 percent and reduced meantime to repair by almost half.

The main plant is located next to the Dead Sea, the lowest point on the earth’s surface where summer temperatures reach 45 degrees C in the shade. The average annual temperature is 35 degrees and even in winter, daytime temperatures rarely fall below 20 degrees C. A major part of the plant is associated with the extraction of potash, by evaporation, from the waters of the Dead Sea. Three different grades of potash are produced at the plant, each for a different segment of the market. The standard form is in large crystals and intended for agricultural applications in developing countries where it is spread by hand. A thinner crystal type is produced for use in downstream products while a third grade called “granulated” is produced by compacting for use in bulk blending. The granulating plant has six compactors each of which presses powdered potash between two rolls, making it into a “marble-like” form that is then granulated by a horizontal impact crusher.

Four threats to bearing performance

The shaft supporting each compactor roll rotates slowly at about 18 r/min in an ambient temperature that exceeds 50 degrees C. In addition to this, the roll shaft bearings carry a heavy loading (C/P<2-3) in an atmosphere thick with abrasive potash particles. Even though cooling water passes through the roll shafts and through each bearing housing this is still a demanding application in a harsh environment and is made even more demanding by the pressure of continuous production where any unplanned downtime is extremely costly. To combat the problem of bearing breakdown, senior maintenance personnel from DSW have for some years been working closely with SKF in what has turned out to be a highly successful two-phase approach.

Phase One

One of the earliest steps was to try to increase the time between breakdowns by focusing mainly on bearing maintenance, cleanliness, lubrication and mounting techniques. Because of the slow rotational speed the bearings were first lubricated with high viscosity grease to prevent deterioration due to metal contact of the rotating elements within the rings. Further steps included greasing of the labyrinths, the use of positive pressure greasing to push out any contamination and the fitting of high quality seals. Whenever bearing replacement became necessary, the original bearing was replaced by an SKF SensorMount bearing. SensorMount bearings make mounting of the large taper bore bearings onto the compactor shaft extremely easy. Driving the bearing up the hollow water-cooled shaft becomes much more accurate and faster because there is no need for complex calculations, feeler gauges and specially trained personnel. There is also the added reassurance of knowing that any possible mounting errors have been eliminated and the service life of the bearing increased.

Predictive Maintenance Program

Another major step in preventing frequent and unplanned downtime was taken with the introduction of a Predictive Maintenance Program based on equipment from SKF and software from Prism. Several types of monitoring techniques were tried with the compactors but the most successful for dealing with the very low shaft speed was an ultrasonic detector coupled to an SKF Microlog. The signal was enhanced by passing it via an SKF Microlog Acceleration Enveloping Filter.

SKF Microlog is a spectrum analyzer that allows a trained analyst to compare a signal from a compactor (say a dynamic vibration measurement) to the compactor base line vibration signature (or any other earlier collected data). The analyst can then predict any development that might lead to a failure. It replaces the random response of dealing with one parameter at a time by making an examination of all parameters involved in the life cycle of the machine. The result is a more comprehensive approach to detecting the cause of failure

Results of Phase One

At the conclusion of Phase One Mean Time Between Failures (MTBF) had been tripled. At the same time Mean Time To Repair (MTTR) was reduced by half.

Phase Two

The aim of Phase Two was to build upon the improvements brought about by Phase One. DSW, together with the compactor manufacturer and SKF application engineers, succeeded in producing a new design to provide the compactor with oil lubrication. This gave better control of bearing temperature and reduced contamination by filtering the oil. This was followed by thorough assessment of the compactor drawings, and all the available bearing records and reports. Senior DSW maintenance personnel and SKF industrial bearing specialists held several meetings and inputs were invited from bearing experts in various SKF European factories. These discussions, using the 6 Sigma process, resulted in a decision to introduce several changes.

Fixed procedures for certain aspects of maintenance and repair associated with the roll shaft and bearings were drawn up, agreed, and made available to maintenance personnel. This action supported maintenance staff training that was introduced in Phase One. Closely associated with this was the introduction of a bearing refurbishment program for the compactor roll shaft bearings. This service is to be provided by SKF and includes the provision of SensorMount on all refurbished bearings.

On the predictive maintenance side, a decision was made by DSW to replace the CMVA 10 SKF Microlog introduced in Phase One with a new CMVA 65 SKF Microlog and because of the slow shaft rotation to introduce a special ultrasonic technique. Another new addition was an SKF Copperhead Fault Detection System. This is a plug and play system that monitors the complete machine rather than just the rotating components. It incorporates a new line of rugged vibration and temperature sensors that were installed on the four bearing housings for one of the compactors. Data from the system is fed to the DSW Plant Information System that collects and displays all operation parameters.

Results of phase two

Phase Two produced further benefits with an MTTF increase to about 30 percent together with an MTTR reduction of about 40 percent.

Continuing to strive for excellence

Although well pleased with such excellent results the DSW maintenance team and the SKF specialists are (unlike the film industry people) not interested in recognition and applause. Instead they are eager to see what further steps can be taken to overcome the four threats to bearing performance and increase plant efficiency. Already they are planning a third phase that aims to provide oil lubrication for all four compactors, install SKF Copperhead detectors on all compactors and test a new type of SKF bearing for the compactor shafts.