Recently at the International Health, Safety, Environment, and Loss Prevention Conference in Kuwait, Emerson’s David Walker had the opportunity to present on the topic of process safety. His presentation, Using a Smart Safety Instrumented System (SIS) to Make Better Operational Decisions, described the role of diagnostics in monitoring the health of the devices and surrounding process in safety instrumented functions.
David noted how accidents still manage to occur even with the safety standards and methodologies, which build layers of protection that guard against these incidents. The majority of these events occur because of the failure of plants to implement best practices in process safety and instill a pervasive safety culture. Also, safety implementations and standards are having difficulty keeping pace with the automation system alarm and display technologies. A few examples of these advances are alarm prioritization and narrowcasting by location and/or function. These capabilities often were not in existence when the plant process safety standards were developed.
David shared the data I mentioned in an earlier post where more that 90% of the failures in safety instrumented functions occur outside the logic solver–in the sensor or final control element (FCE). Traditionally, the focus of safety engineers has been on maintaining and supporting the logic solver, mainly because it could not provide diagnostics from the sensors and final control elements to identify potential problems. These missing diagnostics are even more critical as the components of the safety loop age and become less reliable. This equipment is more likely to cause spurious trips that unnecessarily shut down the process–decreasing overall availability.
Newer safety instrumented systems like DeltaV SIS communicate via HART digital communications to safety-certified or proven-in-use sensors and smart final control elements. The diagnostics from the sensors and FCEs help detect, report, and often give operators and maintenance technicians time to respond to the abnormal situation before a shutdown sequence is initiated. David listed some examples of these device diagnostics:
- Partial stroke testing of FCEs
- Low supply pressure to FCEs
- Temperature sensor failure and hot-backup capabilities
- Earth leakage detection
- Pressure transmitter impulse line plugging
- Flow transmitter two-phase flow
- Degraded voting logic upon transmitter failure
- Remote from hazardous location device testing by operations and maintenance staff.
David closed his presentation with the thought that the trend is to integrate this information from the SIS with the basic process control system (BPCS) instead of interfacing through a gateway because of the value of this diagnostic information. This helps operators and maintenance personnel make better operational decisions to avoid process shut downs and identify and address abnormal situations as quickly as possible.
