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Mining and metals refining
Dec 19, 2017

Mill maintenance – 3 simple ways to keep your mill at its best

With the grinding mill critical to a plant’s throughput, it pays to ensure mills receive the necessary care and maintenance. Too often mill maintenance is reactive, occurring due to a breakdown. The result is higher repair costs as resources are rushed to site, as well as site maintenance staff under unnecessary stress, battling to get the mill operational again.
Mill maintenance
Figure 1. A fundamental part of mill care is cleaning the mill.

In an ideal world, mill maintenance would only ever take place over planned shutdowns. Whilst this ideal may never be achieved, with good planning, you can identify potential problems early, saving thousands of dollars and wasted hours in the process. Here are three simple ways to keep your mill operating at its best.

Step 1 – keep your mill clean

This may sound obvious – but a fundamental part of mill care is cleaning the mill – and this step is often neglected. If a mill is kept clean, then ‘leaks and creaks’ (such as grease leaking from bearing seals) will be identified earlier, cost less to repair, and fixed before major equipment failure.

A faulty bearing seal, when identified early, can be quick and inexpensive when compared to a complete bearing overhaul from a significant bearing failure. Weeping from shell flanges, hard to identify unless the mill is clean, is a critical indicator in diagnosing loss of fastener tension or failure issues. In addition, damage to equipment, protective coatings and cracking of grout will be more visible. Early repair can prevent more extensive damage.

P.S. clean is not just a hose down!!

However straightforward cleaning a mill may sound, it is not as simple as hosing equipment down periodically. Hosing down a hot casting may cause it to crack, or shrink it onto its contained bearing, reducing the bearing clearance and spinning the bearing on the shaft. A thoughtlessly directed high-pressure-spray could lead to water ingress behind a bearing seal, causing premature bearing failure. It is important, therefore, to ensure that staff are trained and competent for the cleaning task; this is where it definitely pays to consult with your mill supplier, who should be able to talk you through important steps and offer useful hints and tips. So, in summary,

Clean the mill regularly

  • Do not clean hot components (especially castings)
  • Do not clean seals with water sprays or jets
  • Ensure staff are trained
  • Get advice from the mill supplier

Step 2 – Inspect your mill regularly

After ensuring that the mill is kept clean, the next logical step is to inspect it regularly, ensuring early detection of any leaks or damage. Most mill control panels show sensor readings from mill critical parts, and this can be reliably used to inspect these parts during operation.

Spare pinion
Figure 2. A spare pinion - ready for stripping and rebuilding.

Some areas, such as gear tooth temperatures and gearbox bearing vibrations, may need specialist instruments to take readings, whilst other parts will simply need visual inspections (e.g. leaks from seals). Some inspections will not be possible during operation, so scheduling minor shutdowns to inspect areas like the ring gears, bearings and shaft alignments might be necessary.

Step 3 – Keep regular records on your mill

Most mill control panels will show transducer readouts from various inspection points, and even give alarms at ‘high’ and ‘low’ warning set points before the mill is tripped on ‘high-high’ or ‘low-low’ alarm settings. This is a useful function to protect the mill, but the ‘high’ and ‘low’ alarms are reactive, rather than proactive, and sometimes can only warn of the inevitable. The monitoring process is therefore paramount to identify trends of any developing problems. Good monitoring will include: being methodical with inspections, keeping good records of inspections and tracking records and values over time.

Perhaps the best way to demonstrate this is with an example... Suppose the oil temperature for the mill bearings was slowly and steadily increasing, but had not yet tripped a high alarm. Following further investigation in the oil tank heaters and oil cooling system, the only irregularity was with the cooling water on the discharge of the heat exchanger which was steadily lowering in temperature. This would indicate that the source of the oil temperature increase is probably due to the heat exchanger, and this could be flagged for close monitoring until a detailed inspection at the next shutdown.

 Seals with excessive contamination and seals cleaned
Figure 3. Before (left) - seals with excessive contamination. After (right) - seals cleaned.

How to plan for your maintenance shutdown

When planning maintenance shutdowns, first identify outstanding maintenance and inspection tasks. These will include regular tasks (shaft alignments, journal inspections, fastener tensioning, slip ring chamber cleaning, inch brake adjustments, etc) with various ad hoc tasks flagged from inspections and monitoring. These tasks should be grouped on their importance (low, medium, critical) to the continued mill operation and used to prioritise objectives, balancing this with the usual budgetary and time constraints. All too often a mill shutdown is extended through one task taking more time and resources than expected, but if properly understood, non-critical tasks can be postponed until a later shutdown, or critical tasks assigned more resources.

Once the maintenance tasks, man-hours and resources are planned, the following need to be identified and secured - specialist tools and equipment, personnel and additional manpower, and spare parts for repairs. The plan should also consider unnecessary conflicts. For example, some tasks require the mill to be inched periodically. Unless completely unavoidable, it is not good planning to schedule high voltage motor inspections at the same time (unless the coupling has been disconnected) as the motor shafts will rotate during inching. Your mill supplier can be key in such a planning process as they are aware of issues such as interrelated tasks, which deliver best results when completed in sequence, and the required resources for each.

Lubrication failure damage  and pitting damage
Figure 4. Lubrication failure damage (left) and pitting damage (right).

Successful maintenance shutdowns

Apart from operational errors, an unscheduled mill stop represents a failure of the last planned maintenance shutdown. A successful mill shutdown should not just be measured by the usual ‘on time, on budget’, but also by how long the mill operated continuously after the previous shutdown. This is where mill maintenance requires an intimate knowledge of the workings of your mill, through a planned care, inspection and monitoring programme.

The mill is one of the more complex pieces of process equipment and, as such, it will always pay to involve industry expertise in the inspecting, planning and execution of mill maintenance. Outotec’s mill service team, for example, offers services including shutdown assistance, pre-shutdown inspections, training and information on current best practices.

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