Vertical roller mill grinding roller and disc liner

Vertical roller mill grinding roller and disc liner

Discussion on maintenance and energy saving of vertical roller mill grinding roller and grinding disc liner In the cement industry, vertical roller mills can be divided into four categories: coal mill, raw mill, cement mill (grinding), slag mill. Due to the different functions and different abrasives, there are considerable differences in the timing and methods that must be repaired when wear causes a drop in production.

1 repair timing

When the grinding rolls and the disc liner wear out too much, the output will drop, and the drop in production means waste of energy. Taking the FLATOX50 vertical roller mill as an example, the power of the ATOX50 vertical roller mill motor is about 4000 kW/h (including the motor power of the vertical roller mill main motor, the classifier, the feed pump, etc.), that is, about 4 per hour. 0 (30 kWh, equivalent to about 2,000 yuan, the annual electricity consumption is about 15 million yuan. If the efficiency is reduced by 10%, the annual power consumption loss is about 1.5 million yuan, resulting in the extreme energy Great waste. So if you only consider energy saving, the less the wear, the earlier the maintenance will be more energy-efficient. However, the replacement of the grinding roller and the disc liner is a time-consuming and labor-intensive project, and involves the loss of production due to downtime. In fact, the two are equally important, and the former is invisible (electricity is the total of the whole plant). Although this power consumption can be accounted for separately, most of the owners only pay attention to the cost of replacement, and often ignore the waste of energy. The cost, so I will prove this in the form of numbers.

2 replacement costs

The cost of the replacement includes three items: replacement of spare parts; replacement of labor costs; loss of production.

2.1 Replacement of spare parts costs

There are two options for replacement: 1 new product; 2 old hard surface regenerative surfacing repair. The cost of the former increases with the number of replacements, while the latter's hard surface surfacing is based on the amount of weld overlay, and the service life after surfacing is longer than the new one, so the cost will decrease with the number of replacements.

2.2 New labor costs

This fee will still increase with the number of replacements, but there is no such cost if online overlay welding is used.

2.3 Loss of production loss

The loss of production depends on the time required for the replacement or the time required for online repair. Since each production will lose hundreds of thousands or even millions (depending on the capacity of the production line), if it is to repair the grinding roller or the disc liner separately. The cost of the downtime is quite considerable, and it is usually necessary to cooperate with the whole plant overhaul to stop the machine. It is better to replace or repair the grinding roller and the disc liner during the overhaul period. And this is the key point to determine whether the grinding roller or disc liner can be repaired by online surfacing. In general, raw mills and cement mills require dozens of days to complete the surfacing repair due to their large amount of weld-up. If online construction is used, the cost of production loss will be quite large.

3.1 new products (high chromium cast iron)

Casting new products from high-chromium cast iron is a commonly used method. It usually has a carbon content of about 3% and a chromium content of about 21%. Both of them form Cr7C3 at high temperatures and are embedded in iron-based (austenitic or martensite) to form a very wear-resistant structural structure having a Rockwell hardness HRC of about 56. The high risk of high chromium cast iron is a problem of fracture. There are two main reasons for the fracture: First, the hardness is too high. When the content of carbon and chromium is higher, the hardness will be higher and wear-resistant, but the possibility of fracture is also increased. Second, the hardness is uniform. When chrome cast iron is cast and heat-treated to increase its hardness, if the difference in hardness uniformity is too large (usually the Vickers hardness HV difference cannot be greater than 30), cracking may occur. Therefore, as the volume of the object increases, the uniformity is more difficult to control and the possibility of fracture is greater. This is also the reason why the new products such as HLM56.2+2 or UM50.4 are broken at the moment.

3.2 new products (casting + surfacing)

The medium-carbon steel is used as the material to cast the wear part, and then the hard surface welding is used to carry out the hard surface surfacing. This method has no risk of breaking. Moreover, because the hardness of the wear layer of hard surface surfacing is higher, the HRC is about 60 (generally, the carbon content is about 5%, the chromium content is about 27%), and the CrTC, the metallographic structure is better, so compared with the new product. High chromium cast iron is more resistant to wear. The big risk of this method is the peeling of the wear layer, and there are two kinds of peeling: one is that the wear layer is peeled off from the carbon steel in the base material; the other is the peeling between the wear layers. Generally, the thicker the wear layer is, the higher the possibility of peeling off, and the peeling will inevitably cause downtime and the loss is huge. To be wear-resistant and not to peel off depends on: 1 the choice of welding wire; 2 construction process; 3 employee professionalism. These three are equally important, and neglecting one of them can have serious consequences.

3.3 hard surface regenerative welding

The worn grinding roller and the disc liner are repaired to the original size by hard surface welding wire (or welding rod). This method has been widely used by manufacturers because of its advantages of wear resistance and low cost. . Since the tensile strength of the wear layer is almost zero, it is said that the wear layer only has the function of wear resistance, so the strength of the base material after the wear is insufficient to support the pressure during the grinding operation, although the weld is adjusted to the original size, but it is used. There is also a risk of breakage. And local heating occurs during surfacing, and local heating increases the possibility of fracture of the high chromium cast iron base metal.

4.1 Online construction

In this method, the worn roller or the disc liner is not required to be removed, and the surfacing equipment is erected on site to perform surfacing repair.

The advantage of this method is that it does not need to be replaced, so it can save part of the cost of replacement.

The disadvantage of this method is that if the workload is too large, the downtime will be lengthened, resulting in increased production losses. The online construction is not ideal due to the working conditions, and the maternal material after the wear can not be fully inspected, and the possibility of breakage (including during construction and operation) is greater, and the risk is very high, so the author thinks that it should not be used. Generally used in cases where it is not allowed, such as slag grinding with surface peeling, because it wears faster, and the efficiency of wear increases is fast, the welding repair cycle is short, and the workload is not large.

4.2 offline construction

This method is to remove the worn grinding roller or disc liner and transport it to the surfacing factory for surfacing repair.

The disadvantage of this method is that the grinding roller or the disc liner must be removed, which increases the cost of replacement.

There are three advantages to this method:

(1) Regardless of the amount of surfacing work, the off-line construction will not affect the downtime and will not cause any unnecessary losses;

(2) After the worn grinding roller or disc liner is removed, the base material can be inspected for cracks in detail to reduce the risk of fracture;

(3) The quality of off-line construction can be guaranteed. Based on the original drawing size and the overall construction technology and technology, the roundness and balance of the grinding roller or the disc liner can be ensured during construction to avoid vibration during use. If it is too large, it will cause damage to the mill, and it can also save power.

The maintenance and energy saving of the grinding roller and the disc liner in the above vertical roller mill are only the shallow experience of the author. No matter which method is adopted, it mainly depends on the timing, risk and cost of the enterprise. The enterprise must record the running hours, output and power consumption of the mill. The above analysis makes the ideal operation repair cycle and method. , take regular maintenance, to save energy and reduce consumption as a prerequisite.