Chemical Processing - February 2008 - (Page 31)

Use of the alarm system for notification messages. At many plants, operator information overload is made worse by having the alarm system handle routine notifications (information messages). In fact, this has been blamed as a contributing factor to the severity of certain well-publicized plant disasters. Using the alarm system for routine notifications sometimes is pursued as a convenience for the automation engineers and sometimes represents a limitation of a vendor’s automation software. Regardless, notifications shouldn’t appear as alarms to operators. Nuisance alarms. Operators at many plants are frustrated by a large number of alarms that don’t represent abnormal situations or don’t require a response. In cases where operators receive frequent nuisance alarms, they may lose respect for the alarm system and then sometimes miss real abnormal situations while responding to (or ignoring) nuisances. Batch processing adds complexity Plants that rely upon batch operations face additional challenges in configuring and managing alarms. These include: • organization of processes as a sequence of steps/ phases, often with transients involved in moving from step to step; • the non-steady-state nature of batch processes, with time-varying set points and alarm limits; • increased use of notifications; • need for different time reference, i.e., relative time rather than calendar time; • necessity to query, sort, and report alarms by lot number and other batch parameters; and • adhering to the ISA S-88 Batch Standard [4]. Let’s look at each of these particular challenges in a bit more detail: A sequence of steps. While all chemical processes have multiple steps/phases (even continuous processes have startup and shut-down steps), batch processes typically are characterized by a relatively short total cycle time (e.g., days), numerous steps, and significant activity (automated or manual) involved in transitioning between steps. For instance, an overall batch sequence of unit operations might include: preparation of raw materials (thawing, milling, screening, putting into supply tanks, etc.), chemical reaction, filtration, chromatography purification and then crystallization. In addition, a sequence of steps might take place within a single unit operation. As an example, a chemical-reaction unit operation might consist of: automated cleaning of the vessel, raw material filling (including weighing operations), heat-up, reaction, cool-down and finally harvesting. Most alarms are relevant to one or more of the batch steps but not others and, therefore, need to be configured www.chemicalprocessing.com as a function of process step/phase. In addition, whenever possible, the alarm record tag should note the applicable process step or phase to facilitate obtaining relevant historized information specific to a step. Non-steady-state operations. In contrast to continuous processes, batch processes typically have few, if any, steady-state characteristics. For example, control of the chemical reaction step of the process described above might involve monitoring or control of time-varying reactant feed rates, temperature, pH, etc. The control of time-varying processes can result in nuisance alarms if alarms aren’t appropriately configured, such as immediately following a set-point step change. Some plants pursue ramping of set points and use of “deviation from set point” alarm tolerances (rather than absolute values) as one technique to help avoid nuisance alarms. Sometimes alarm limits must be calculated time-varying values. So check whether vendor software can accommodate this need. A very popular feature, developed, implemented and published by one manufacturing company, recognizes the difficulty operators have in remembering what the typical values of time-varying process parameters should be at particular points in time. Therefore, its plant’s trend plots of current production runs include a backdrop showing the calculated timevarying normal range of successful historical runs (Figure 3). The process variable (PV) of a current run crossing over these historical backdrop lines can be the basis of an alarm (i.e., an indication that the current run is not normal). Notifications. Batch processes frequently require the generation of information messages because they typi- >> Sources of alarm S95 Level MES data warehouses, etc. Batch historian Alarm loggers HMI PC, PLC, DCS, SIS LAN Alarm ﬁeld horns, lights Field elements (Sensors, transmitters, valves) Figure 2. Many levels of plant automation can generate alarms. 3 Expert systems 2 0,1 February 2008 • 31 http://www.chemicalprocessing.com

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Chemical Processing - February 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Nanoparticle Safety Raises Questions Take the Pressure Off Vacuum Systems Achieve Optimum Centrifugal Pump Performance Rethink Batch-Manufacturing Alarm Systems Dr. Gooddata Orlando Plant Pioneers HMI Migration Strategy Process Puzzler Plant InSites Equipment & Services Ad Index Product Spotlight/Classifieds End Point

Chemical Processing - February 2008

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