Many process plants devote considerable resources to rationalizing their alarm systems — to allow operators to effectively manage the process instead of merely responding to alarms throughout the shift. A properly designed and well-functioning alarm system is crucial to plant safety, but simply staying within alarm boundaries isn't enough. Managers must know if units are running in a range that will satisfy production plans as well as critical limits (equipment- and control-related, economic, environmental, etc.).
Success in the quest to maximize asset uptime, minimize maintenance costs and avoid unplanned outages requires a clear grasp of process variables, operational constraints and production targets. By understanding the operations management landscape and using the latest technologies available, managers can help promote plant safety, reliability and profitability without jeopardizing operations.
THE OPERATING ENVELOPE
Studies by the Abnormal Situation Management (ASM) Consortium (www.asmconsortium.net) have shown that worker actions cause 42% of abnormal situations or upsets in processing operations. Equipment problems lead to 36% of upsets, with half of these attributable to equipment or process units functioning outside of their "operating envelope."
An operating envelope is a collection of boundary limits that, when exceeded, put the integrity of assets at risk. These limits typically are based on combinations of factors such as unit capacity, equipment constraints and safety concerns. They can be implemented in alarm systems and serve as operating targets.
Managing an operating envelope once consisted of counting the number of alarm breaches for a given variable. This frequently resulted in hundreds, if not thousands, of operating envelope "excursions" per month. Most weren't legitimate breaches, but rather indications of the control system or operator quickly moving the process back within established constraints.
It's critical for control room personnel to distinguish between false and genuine boundary excursions, and ensure deviations are relevant. Operating envelope deviations aren't real-time alarms like those on a control console; operators only want to react or respond to real deviations. A variable that briefly strays out-of-bounds once per hour isn't necessarily a meaningful deviation.
Unfortunately, using a generic notification tool or trying to use the alarm system to monitor the operating envelope likely will lead to many false positives. That's why technology has been purposely designed to monitor the operating envelope, to identify only real deviations. By accurately identifying genuine excursions, it makes more manageable retrospective analyses that can prompt both short- and long-term steps to minimize the occurrence of process upsets or, worse yet, loss of containment.
To maximize the life of an asset, it must be operated according to design parameters, not simply within process alarm ranges (Figure 1). However, operating strategies must extend beyond operator visibility to the entire operations team and all those interacting with the process. Without a comprehensive limit-management solution, operators simply lack the insight needed to run the plant within operating envelope boundaries.
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.