Estimating liner wear life
In order to maintain the power draw, liner wear is compensated for by adding grinding media; therefore liner wear is estimated by measuring the ball charge increase.
Static ball charge measurement
The mill must be stopped for the static ball charge to be measured. Drop the weighted rope in the ball port at the same reference point on the ball port as the initial ball charge reading. The distance between the first mark on the rope and this second mark is the depth of the ball charge.
NOTE: This measurement can only be taken with the mill stopped.
Record the static ball charge on the Commissioning Data Sheet once every month for new mills and every other month for others to improve the accuracy of the wear life estimate.
Operating parameters to be recorded at the same time as the static ball charge:
- Power draw
- Operating hours/tonnage
- Percent solids (by weight)
When the static ball charge level has risen by 300 mm (1 foot), it is an indication of a worn screw liner and a liner inspection/change is recommended. This is applicable only for mills operated with a constant power draw.
After any liner change, a new initial ball charge should be measured/recorded, followed by a repeat of the above procedure.
Measuring liner wear
VTM liners have a relatively complex geometry, but they can be measured using standard measurement tools with the following procedure.
Aim for the center of the screw shaft and go through the bolt holes when picking a point to:
- Measure the thickness (T1, T2, T3); do this at the same place every time.
- Measure the distance from the bolt holes to the outer edge (D1, D2, D3).
- Measure the shortest distance to the lowest bolt (X).
Maximum end liner wear
Sites commonly change out their liners too soon. The example below shows how the end liners and related screw flights can be worn. The red line shows the maximum wear, meaning that it is okay to wear into the rubber-covered screw flight. The screw flight and end liner can be worn up to 2” from the nearest bolt hole, depending on the VTM model, and it will still give sufficient structural support for new end liners.
Safer, faster and easier
The Vertimill product range has grown over time and so has the overall size and mass of the equipment and its components. VTM liners are inherently difficult wear components to handle due to their complex shape and installed orientation. For years, screw liners have been installed and removed on smaller model sizes with traditional lifting tools: chains, slings and shackles, but this was less than ideal for the larger VTM-3000/4500 due to the increased size and weight of the liners.
Since the practice relied upon the tools on hand and the experience (tribal knowledge) of the workers, the risks involved in safely and efficiently performing the task were inherently high. Customers in many regions began approaching Metso to come up with a safe and effective solution to this problem.
New VTM liner handler tool
The VTM liner handler fulfills a long list of requirements, including center of gravity for both new and used liners, common attachment points, minimal headroom and practicality for manufacturing, while still being simple and functional.
The new attachment design greatly improves safety, reduces liner replacement time and is available for all existing and future VTM customers.