Eight things to consider with mill pumps – How to increase plant production 3/3

1. Ensure motor power availability

A well-designed plant has enough power allocated to mill pumps. Pumps typically operate on variable speed drives, meaning that there are many process variables affecting speed and, finally, the power draw. It is a good idea to look at SCADA data on historical power drawn to better estimate the amount of power that would be available for tonnage increases. Rather than using engineering data sheets that are somewhat oversimplified, it is beneficial to use a point cloud type plot showing flow and pump pressure as a function of time. This information makes it possible to determine the optimal size of all the pumps and cyclones for the plant.

2. Minimize impeller wear when reducing inlet velocity

High inlet velocities combined with large, sharp objects is a significant cause of impeller wear. As a guideline, 4.5–5 m/s (meters per second) is considered the maximum limit for abrasive duties, but it is useful to check the inlet velocity with the supplier before increasing the pump flow. Typically, pump suppliers offer a variety of impellers and inlet liners for the same casing that can help reduce inlet velocity without sacrificing efficiency or requiring an expensive pump upgrade.

3. Consider gearbox cooling at higher power

As pump duty is increased, it usually also increases the power transmitted through the gearbox. This means that the amount of heat increases as well: a gearbox that is sized marginally for air to air cooling may overheat with higher continuous duty. Consideration must be given to the cooling capacity of the lubrication system, particularly at higher ambient temperatures and altitudes.

4. Gland seal water pressure at higher heads

The pump gland seal water system should be sized so as to be able to deliver a constant flow of gland water under all operational conditions. This applies to the pump duty, including any increase in head due to tonnage increases. It should also be checked that the gland seal water system is adequate when other demands are placed on it, such as hose downs or flushing.

5. Consider larger pipes with friction and wear in bends

If you double the speed, the rate of material loss increases 16-fold and the rate of abrasive wear on the surface is approximately proportional to the fourth power of velocity. If there is a significant increase in input, it is necessary to consider whether the pipe sizing is optimal. The right size allows friction losses and wear to be minimized. Of course, if there is a large variation in flow, then minimum velocity to prevent settling should be examined.

6. Increase capacity in sumps. Live volume reductions.

To allow stable process operation, the volume of sumps needs to be reconsidered when there are large increases in throughput. Typical residence times for a mill sump are in the order of 30–60 seconds. If the residence time is too short, then level control may be difficult.

7. Prepare for crash stops by calculating floor sumps

Should there be a plant crash stop, prepare for the maximum inflow based on calculations on the live volume of floor sumps. This may include the mill static overflow and any dump valves to empty pipes and sumps. If sump size is increased or the mill volume changed, then the sumps may be undersized. In this case, the existing sumps can be deepened or enlarged, to deal with the volume, or then additional sumps created. Typically, mill sumps should be separated from the other sumps in the plant due to the possibility of mill balls entering the sump.

8. Consider automatic control of cyclones to ensure availability

A well-designed plant should have enough redundant cyclone capacity to cope with routine maintenance of at least one cyclone, even at maximum recirculating load. If the tonnage or recirculating load increases, then this will reduce the number of available cyclones. It is tempting to use the “maintenance” cyclone for additional capacity, but this can result in a frustrating lack of availability. If the cyclone clusters are currently under manual control, then any upgrade is an ideal time to consider the benefits of automatic control.

This blog post is part of a three-piece series on how to increase plant production when it is already running at full capacity. The full series include:

1. Assessing processes for best value

2. Comminution circuit bottlenecks 

3. 8 things to consider with mill pumps