Process Control is defined as all the activities involved in ensuring a process is predictable, stable, and consistently operating at the target level of performance with only normal variation. It is a technique which takes the guesswork out of quality control. It is a scientific, data-driven methodology for quality analysis and improvement.
Process Control can also be defined as an industry-standard methodology for measuring and controlling quality during the manufacturing process. Quality data in the form of Product or Process measurements are obtained in real-time during manufacturing. This data is then plotted on a graph with pre-determined control limits.
Before attempting to control a process it is necessary to understand how the process works and what it does. Once the process is well understood, operating parameters such as temperatures, pressures, flow rates, and other variables specific to the process must be identified for its control. In order to maintain a safe and hazard-free facility, variables that may cause safety concerns must be identified and may require additional control. It is important to identify the measurables that correspond with the operating parameters in order to control the process. Measurables for process systems include: temperature, pressure, Flow rate, pH, Humidity, Level, Concentration, Viscosity, Conductivity, Turbidity, Redox/potential, Electrical behavior, Flammability.
Once the measurables are identified, it is equally important to locate where they will be measured so that the system can be accurately controlled. Selecting the proper type of measurement device specific to the process will ensure that the most accurate, stable, and cost-effective method is chosen. There are several different signal types that can detect different things. These signal types include: Electric, Pneumatic, Light, Radio waves, Infrared (IR) etc.
In order to control the operating parameters, the proper control method is vital to control the process effectively. On/off is one control method and the other is continuous control. Continuous control involves Proportional (P), Integral (I), and Derivative (D) methods or some combination of those three. Choosing between a local or distributed control system that fits well with the process affects both the cost and efficacy of the overall control. Understanding the operating parameters allows the ability to define the limits of the measurable parameters in the control system. Choosing between feed-forward, feed-backward, cascade, ratio, or other control logic is a necessary decision based on the specific design and safety parameters of the system. Even the best control system will have failure points; therefore it is important as well to design a redundancy system to avoid catastrophic failures by having back-up controls in place. Fail-safes allow a system to return to a safe state after a breakdown of the control. This fail-safe allows the process to avoid hazardous conditions that may otherwise occur.
Depending on the control logic used in the process, there may be lag times associated with the measurement of the operating parameters. Setting lead/lag times compensates for this effect and allows for accurate control. By investigating changes made by implementing the control system, unforeseen problems can be identified and corrected before they create hazardous conditions in the facility. The proper integration of a new control system with existing process systems avoids conflicts between multiple systems.
Control limits are determined by the capability of the process, whereas specification limits are determined by the client's needs. Data that falls within the control limits indicates that all equipment in the system are operating as expected. Any variation within the control limits is likely due to a common cause—the natural variation that is expected as part of the process. If data falls outside of the control limits, this indicates that an assignable cause is likely the source of the product variation, and something within the process should be changed to fix the issue before defects occur. This is where maintenance comes in.
In general, the term maintenance is described as any activity – such as tests, measurements, replacements, adjustments and repairs — intended to retain or restore a functional unit in or to a specified state in which the unit can perform its required functions. It involves fixing any sort of mechanical, plumbing or electrical device should it become out of order or broken (known as repair, unscheduled, or casualty maintenance). It also includes performing routine actions which keep the device in working order (known as scheduled maintenance) or prevent trouble from arising (preventive maintenance).
Having read all the above, you can agree with me that process control cannot be overemphasized when it comes to maintenance. Because prevention will always be better than cure, maintenance must rely on variables generated by process control mechanisms in order to accurately predict when a system failure is most likely to occur. Generation of data by process control has to be followed up with diligent analysis of such data. If this is not done then the whole aim of process control had been defeated as data should not be generated and stored without analysis.
Every manufacturing facility has to have an Equipment Maintenance Plan, EMP, which has a bedrock on process control. The smooth running of the operations of such a facility will depend on how good the EMP is. QA personnel, Production personnel including supervisors and machine operators are to be trained adequately on process control. They need to know what to monitor, when to monitor and how often. There must be appropriate documentation in writing or in electronic form to support such monitoring otherwise nothing has been done.
Generally speaking, there are four types of maintenance in use:
Preventive maintenance, where equipment is maintained before break down occurs.
Operational maintenance, where equipment is maintained in use
Corrective maintenance, where equipment is maintained after break down. This maintenance is often most expensive because worn equipment can damage other parts and cause multiple damages.
Adaptive maintenance, where equipment is maintained by letting it adapt to new environment.
Process Control best suits preventive maintenance. Preventive maintenance (PM) has the following meanings:
The care and servicing by personnel for the purpose of maintaining equipment and facilities in satisfactory operating condition by providing for systematic inspection, detection, and correction of incipient failures either before they occur or before they develop into major defects.
According to Bamiro, Nzediegwu, Oladejo, Rahaman and Adebayo (2011) , preventive maintenance is the work carried out on equipment in order to avoid its breakdown or malfunction. It is a regular and routine action taken on equipment in order to prevent its breakdown.
BENEFITS OF PROCESS CONTROL TO MAINTENANCE
- Preventive maintenance reduces cost of maintenance and enhances productivity which is affected if process control is not done and equipment are allowed to break down
- Process Control gives room for adequate planning of production activities
- It allows for proper Inventory of equipment spares to be kept and prevent the stocking of unnecessary spares which tie finances down
- It makes employees up and doing as they know their work will be monitored and the data they generate properly analyzed
- It improves safety of operations
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