When the grinding mill is one of the most critical components in a plant’s throughput, it makes sound economic sense to protect the mill from unnecessary, preventable and costly damage.
Larger mills today are subjected to higher loads and stresses requiring them to have sophisticated bearings and lubrication systems. Given that these mills have higher throughputs, any unscheduled shutdown can result in lost production and opportunity losses.
Modern control systems
Modern mills need to integrate an up-to-date control system with a control philosophy. Transducers will sense in real time, the condition of each given part. This is then fed back to the control system which, as part of the control philosophy will analyse the information and make decisions based upon pre-set interlocks.
In general, there are three types of interlocks:
- Permissive interlocks: Conditions that must be met before a mill is allowed to start
- Alarm interlocks: Conditions that will cause the mill to raise an alarm, but not prevent the mill from operating
- Trip interlocks: Conditions that will cause the mill to cease operation
Set points for these interlocks are selected to prevent the mill from operating under conditions that have the potential to cause significant damage.
It is not advisable to change the set points or bypass the interlocks to keep a mill operational. Instead, the conditions must be assessed to find out the reason for the tripping. This may save an extended shutdown in future as well as thousands of dollars in repair costs.
Standalone mill control systems
Standalone mill control systems will not allow interlocks to be bypassed easily. They usually have password-protected PLC programs, and require specific knowledge or highly skilled personnel to bypass or change the interlocks.
Standalone mill control systems are always recommended for existing system upgrades or when purchasing a new mill.
Control system upgrades
It is important to speak regularly to manufacturers and discuss if the existing control system can be upgraded to meet current best practices.
Firmware upgrades are very important, as they are generally released by the PLC manufactures to update the functionality of a CPU or to prevent a re-occurrence of a recently discovered fault.
The need for a firmware upgrade should be reviewed every six months and will require the services of skilled PLC programmer.
Tips on maintenance of control systems
Site maintenance personnel also need to be aware that the control systems protecting the mill require regular maintenance.
- Ensure that the control system is installed in a protected area, away from the presence of extreme heat, moisture and dust
- Ensure the panel exhaust fans are operating correctly and filters cleaned regularly
- Ensure the panel doors are sealed sufficiently to prevent the ingress of dust and water
- Ensure that any PLC forces and bridges are recorded and action put in place to remove as soon as possible
- Review the system set points regularly to ensure they are correct
- Ensure the PLC firmware is regularly reviewed and updates downloaded to the PLC
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