Know your safety limits
24 July 2012
Suzanne Gill reports on a new module for an alarm management software system which aggregates, validates, and displays physical, design, and safe operating limits, allowing a process to work more closely to its safe operating limits.
Traditionally, safe operating limit information has been maintained by various groups within a plant and is not readily available to operators. Additionally, some processes have dynamic safe operating limits that are continually changing, which poses a challenge for plant operators.
Mark Tibbitts, director of operations effectiveness, business development for PAS, explains further. “Information relating to safe operating limits can be maintained in a number of different places and operators may not have direct access to it. It is rarely displayed in a consistent format in one place. ”Offering an example of this, he said: “Take a distillation column, where there are a number of different safety boundaries involved. You would have the design conditions – the maximum pressures that the columns are built for – and you have relief valve settings that are set to automatically relieve pressure in an overpressure situation. All these different limits are related and have to be matched. Because they are not all housed in one place and cannot be visualised in one place, it can take a lot of leg work to ensure that they are set consistently.”
PAS, a provider of software and services designed to improve human reliability in the industrial environment, has introduced inBound, a module within its PlantState Suite alarm management software that aggregates, validates, and displays physical, design, and safe operating limits. These values may be manually entered, calculated, or imported from other applications and databases to provide maximum flexibility. "There are standards that relate to how many alarms an operator can receive – EEMUA in Europe, and ISA 18.2 in the US. PlantState Suite allows operators to aggregate alarm information so they can do metric analysis and achieve a master alarm database," said Tibbitts.
InBound can display a plant’s safe operating limits and proximity of the current operating point to those limits to improve safety and compliance. It helps develop a boundary hierarchy, which allows the application to automatically detect and report deviations, such as an alarm setting that is higher than a safety instrumented system trip point. This provides assurance that modifications to configuration parameters, such as alarm limits and instrument ranges, remain within the safe operating envelope of the plant.
InBound uses the plant’s Master Alarm Database as the source of its information and applications such as inferential analysers and simulators that are capable of writing to a SQL database can provide dynamic boundary data.
Presenting safe limits in context
InBound displays safe operating data in a high-performance graphic display that reveals critical information all within context. The graphic display uses pattern recognition to enable quick interpretation of the plant’s current state. Compatible versions of these objects are available for most of the major automation systems.
Figure 1 gives an example of this. The light blue colour indicates optimal operating ranges with uncoloured areas above and below it indicating the full normal range of operability. When the value moves beyond the normal operating range and near an alarm limit boundary, a colour change to yellow occurs along with a diamond icon, which indicates a lower priority alarm. If the value continues to stray further from normal and the next alarm limit boundary is crossed, a colour change to red occurs along with a square red icon indicating it is of higher priority. By selecting a specific alarm, the table in the lower right corner of the graphic provides a list of causes and actions for it.
In conclusion, Tibbitts said: “Many DCS’s will have been rolled out over a number of years, resulting in many 1,000s of alarms having being added. The operator is often unable to respond to them all, resulting in alarm flooding, which has contributed to a number of industrial accidents over the years, where operators have been flooded with alarms and missing which results in appropriate action not being taken.”
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