Are Your Process Variables Under Control?
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KEY PROCESS VARIABLES
- What would it mean to your process if the variation in your key process variables could be cut in half? For example, consider greatly reduced variation in:
- Leach circuit pH.
- Thickener flocculant concentration.
- CIL circuit cyanide concentration.
- Cyclone feed density.
- Do your operators know if variation is random or non-random?
- If your operators were more familiar with the process control methodology, would it make a difference?
- What would be the result if your operators could troubleshoot and optimize each process control loop?
There is no doubt that minimizing variation in key process variables improves recovery. Optimizing the control of any process variable requires knowledge of:
- The physical layout of the process including the associated equipment, piping, sensors, and control devices.
- Physical, chemical, and/or thermodynamic changes occurring in the process.
- The process control logic including the sensitivity of the controller output to changes in the error signal.
- The sensitivity of the control device to changes in controller output.
With a first-class training program, your operators can become expert on each of these requirements!
Once the operators have a good grasp of the process and the particular control loop, trainees would be taught to troubleshoot process variable problems logically, by drawing on their knowledge of the process equipment, flows, and control methodology. Operators must often check to see if the process controller output makes sense based on the deviation between the variable and set point. For example, addressing an issue with a sump level controlled by a discharge control valve, may involve the following steps:
- If the operator notices that the sump level is too high, possibly because an alarm activated, his or her first step is to check the output from the level controller. We would expect to see the output high and increasing in order to open the discharge control valve (assuming the controller output is direct acting).
- If the controller output is not doing what the operator expects, check to see that the controller is in Automatic mode.
- If it is not in Automatic mode, the controller will do nothing more than maintain its output signal. If the controller is in Automatic mode and is not acting correctly, check the set point.
- If the set point is not correct, adjust it as necessary. If the set point is correct, the operator can switch to Manual In Manual, the operator can increase the output by hand. This will modulate the discharge control valve to a more open position to control the level. If the controller output is not working as it should, the instrument department is then notified.
- If the operator determines that the controller is acting correctly, the next step is to check the final control device in the field to determine whether it is working correctly.
- In the example, the final control device is a control valve operated by an air actuator. For the control valve to operate correctly, it requires compressed instrument air. First, check to see whether the valve (or other device) has moved as requested by the controller output.
- In the example, it should be fully open or moving in that direction. If it is not, check that there is enough instrument air pressure.
- The valve will not work if there is insufficient air pressure at the transducer. The transducer converts the Delta V or electronic signal from the controller to an air signal (3 to 15 psi). The pressure can be checked at the pressure gauge located at the transducer.
- If the controller and final control device both seem to be operating correctly, check the process that the final control device is acting on. In our example, the high level in the sump may be caused by a physical blockage in the discharge box upstream of the controlling valve. The blockage may be blocking the flow, preventing the control valve from doing its job.
- Finally, check the sensor in the field. The sensor in the example provides the level measurement to the controller. This check can normally be accomplished by going out to the sensor and checking an independent measuring device such as a level gauge glass or by physically inspecting the sump level.
Once the details of the process and control methodology, as well as control loop troubleshooting techniques are covered in the classroom, simulation drills can be conducted by the instructor with the operator trainees.
For example, the instructor might say that the field operator in the ball mill area has just reported that the mill discharge slurry appears to be very thick (high slurry density). The target slurry density is 60 percent solids, but the calculated discharge is running at about 70 percent solids and seems to be increasing. The trainee group would then discuss the symptom of the problem and, given their classroom knowledge of the process and control methodology, tell the instructor what they would do. For example, they will know from the training that there is a ratio control loop that modulates process water flow rate into the mill in proportion to the solids entering the mill. The trainee team might say that they would check the ratio controller and the output to the process water control valve. The instructor would then explain what they would see regarding the flow controller set point and output to the flow control valve. This back-and-forth process goes on until the trainee group finds the answer. The instructor can then critique the troubleshooting methods used by the trainees.
With proper training on the layout of the plant equipment, the process flow, process control methodologies, and troubleshooting techniques, variation in key process variables can be significantly reduced, optimizing the process and increasing mineral recovery and profits.