Most sites typically rely on multiple types of process control applications, each of which can be used to independently enter and control respective targets, constraints or limits. Although these applications may relate to the same process measurements, they sometimes may use inconsistent or conflicting limits (Figure 2). This situation results in inefficient operation, costly plant incidents and frequent process shutdowns.
Various groups within the plant are responsible for maintaining safe operating limit information. As these variables often are system configuration parameters entered by people, it's possible values may fall outside of the safety and compliance envelope. Additionally, some processes have dynamic, continually changing safe operating limits — a situation that's challenging for operators to manage.
Plant owners must understand the operating envelope encompasses both operating and alarm limits. However, without linkage between these limits, it's impossible to keep them consistent. In principle, operating and alarm limits should match — this allows alarm limits and their rationalization to benefit from the economic understanding of operating limits that exists in most plants. The common disparity between alarm and operating limits stems from the inability to see, compare and work within the operating envelope.
Managing alarms largely is a matter of correctly monitoring and managing operating envelopes. It involves having operators and process engineers ensure alarms, operations monitoring, operating instructions and alerts are consistent with process and equipment limits imposed by the equipment and process design, as well as environmental and safety constraints.
When embarking on a program to capture the plant operating envelope and then monitor and validate activities against the associated limits, it's imperative the alarm system be well rationalized. Trying to monitor against an operating envelope with an unhealthy alarm system just won't work. Instead, plants must strive to implement a solution that changes the culture of console operators from operating to alarms to operating to the operating envelope (i.e., "secondary alarms"). This results in operators paying attention to notifications of operating envelope deviations rather than constantly reacting to alarms. They immediately can review cause, consequence and action information, and plan the appropriate response.
Many plant operations departments are rethinking their approach to operational excellence to gain the maximum benefit from ongoing technology developments. Instead of simply managing the effects of operating outside established boundaries, they're striving to expose the operating envelope to all appropriate stakeholders and ensure it's well understood across operations and related groups.
Today's operations management tools, coupled with well-designed work processes, provide the proper visibility and communication to allow operators, planners, engineers and others to accurately steer an operation within true operating envelope boundaries. This is key to ensuring safety, reliability and profitability.
The effective management of boundary limit information and its dissemination through uniform work processes constitute best practices for optimizing asset and people effectiveness. Accurate boundary data should be available to operators via operating instructions to establish proper limits for production processes, embodied in the control system as a managed set of alarms and alerts consistent with unit constraints and limits, and presented as unified results that can serve as a yardstick for learning and continuous improvement efforts.