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 .
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 start-up 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 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 time-varying 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).
Figure 3. Comparing a current batch run to the historical range of satisfactory runs can identify an abnormal situation.
Notifications. Batch processes often require the generation of information messages because they typically have a large number of sequential steps and the transition from one step to the next may involve prompts to operators to pursue specific manual operations.