Why wireless sensors benefit process safety

Key highlights

• The article explains how wireless systems can be implemented in line with ISA-TR84.00.08 and IEC 61511.
• It clarifies that wireless sensors enhance safety through diagnostics and monitoring without being part of the safety loop, giving engineers more tools without altering critical SIF design.

Published in 2017, ISA-TR84.00.08 technical report addresses how to install wireless for safety. It was discussed in my September 2022 column. Wireless sensors benefit process safety without directly being part of the safety function/loop itself. 

Functional safety and its associated safety instrumented function (SIF) can be continuous demand, meaning the safety function is always active, such as closed doors on a passenger train. They can also be on-demand, meaning they’re called on to act in unusual circumstances, such as an emergency shutdown valve in a process. Diagnostics can help in both scenarios, but they’ll have a larger impact in on-demand scenarios.

Wireless sensors and networks built in compliance with ISA-TR84.00.08 and good industry practices use sensors’ embedded self-diagnostics to monitor the integrity of measurements. Likewise, Industrial Internet of Things (IIoT) sensors that monitor equipment health provide additional information on the condition of critical equipment.

Access to continuous health and status information has key benefits:

• Reduced dangerous, undetected failure rate (λDU), which is a key parameter in probability of failure on-demand (PFD) calculations. Continuous diagnostics actively monitor the health and functionality of SIF and its components, alerting users to dangerous failures that would otherwise remain undetected until a proof test, effectively reducing the average rate of dangerous undetected failures over time.
• Extended proof test intervals. Continuous diagnostics provide high confidence in the operational status of SIF. Because online monitoring provides a degree of failure detection, it reduces the need for frequent manual intervention. The required frequency of manual proof tests can often be extended as a result. Assuming other factors remain constant, longer proof test intervals generally lead to a higher PFD, but the significant reduction in λDU due to continuous diagnostics usually outweighs this effect, resulting in a lower overall PFD.
• Diagnostic coverage. Continuous diagnostics offer more diagnostic coverage because they’re constantly monitoring themselves with embedded diagnostics and the equipment or process to which they’re connected. Higher diagnostic coverage values directly translate to lower PFD.
• Insights into system behavior. Data provided by continuous diagnostic systems offers valuable insights into failure modes and operational and equipment health trends of the safety equipment, which can be used to improve design, maintenance strategies, and future equipment selection. Integrating diagnostic data to provide feedback into the design process is the foundation of the digital thread/digital twin concept.
• Increased confidence in safety system performance. Continuous monitoring provides operators and safety personnel greater confidence that safety systems will function as intended when required.

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ISA 84 and the IEC 61511 team keep seeking the best ways to quantify and consider the effect of continuous diagnostics for on-demand SIFs. Combined with an expected, combined, annual growth rate of 5-8% for the process industries,  demand for SIFs continues to grow, according to Global Industry Analytics Inc. They’re driven by:

• Increasing awareness and enforcement of safety standards to meet mandatory risk reduction targets;
• Strong emphasis on avoiding costly accidents, minimizing downtime and reducing liability; 
• Growing complexity of industrial processes, such as oil and gas, chemicals, power generation, pharmaceuticals, and food and beverage, which require advanced safety systems; and  
• Rapid industrialization and urbanization in Asia-Pacific, particularly in China and India, which creates growth opportunities for the safety instrumented systems (SIS) market used to implement SIFs.

You can see why wireless enables integrating technologies such as IIoT and artificial intelligence (AI) to assist with associated diagnostic information and analyses to support SIFs. You can also see why it’s enhancing predictive maintenance and real-time risk assessment capabilities, further driving their effectiveness and adoption.