JUL-AUG 2019

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INTECH JULY/AUGUST 2019 39 AUTOMATION BASICS In many instances, yearly calibrations are not necessary. For example, some flowmeters re quire calibration only once every three or four years depending on the process fluid, operation, and criticality. In other cases, instrumentation may require calibration much more frequently, possibly monthly, to maintain a safe, efficient, or regulatory-compliant operation. It is also impor - tant to realize that calibration intervals are not always fixed and may fluctuate based on usage and other factors. The first step to addressing these and other is sues is an assessment. A plantwide assessment of all instrumen- tation to identify and make a list of all instru- ments is the first step in a calibration plan. This list should also include details such as descrip- tion, location information, operating condi- tions, working range and history, and any other items that provide a better understanding of the instrument and system function. Users should then identify which instruments are critical to the application, the envi ronment, and operator safety. The quality, me trology, maintenance, and automation groups should pool their knowledge of the process environ- ment, the condition of the installed instru- ments, the type of maintenance work carried out, and any limitations imposed by the plant in terms of servicing. The group should assess each instrument and ask: "Does this instrument have an impact on the quality of the product, on process functionality, or on operator safety?" The next step is to create a calibration plan, taking all the above factors into account. Assigning calibration criticality All instruments should be assigned to one of four categories, ranging from critical to noncrit- ical. The first category—instruments critical to the product—are those affecting product qual - ity or regulatory compliance. These instruments have a direct link to company profits, whether regarding measurement of ingredients in food processing, mixing of chemicals, custody- trans fer, or other critical applications. The next category—instruments critical to the process—are those that can upset the overall plant or other processes, such as shutting down the entire process. The failure of an instrument in this category could cause inefficiencies and production losses but have no direct effect on product quality or safety. Instrumentation critical for safety has a direct impact on operator safety, equipment protec- tion, and/or the environment. These instruments do not necessarily have to be extremely accurate, which lessens their criticality with respect to cali- bration, but they certainly have to work properly and reliably. Finally, noncritical instruments have no im- pact on product quality, the overall process, safety, or the environment. These types of in- struments are typically only used for local or remote monitoring, or when manual operations are performed. After all instrumentation has been identified and classified into these four categories, a maxi- mum permissible error (MPE) is assigned to each device. MPEs define the tolerance for each function being measured. A critical instrument will usually have a more stringent MPE than a noncritical one. Over time, most instruments will experi- ence slight accuracy degradation due to aging and simple wear and tear on mechanical com - ponents. This needs to be considered when establishing the MPE. Modern instrumentation management software keeps track of all the informa - tion about each instrument, including when it needs to be calibrated. The first step in a calibration plan is to identify all the instru- ments in the plant and enter data about them into asset management software.

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