Emerson’s Dale Perry alerted me to a great article on safety-certified sensors in the November 2008 issue of Control Engineering magazine. Dale manages the Rosemount pressure measurement line of products.
The article, Practice Safe Sensing; Safety-certified sensors promise to cut costs and boost performance. But the tradeoffs must be carefully considered., described the advances in both numbers and intelligence of sensor devices used in process safety applications. The article defined these devices as:
…sensors can be certified by third parties to meet safety integrity levels [note: I’ve added hyperlinks for additional reference], or SIL, designations found in IEC 61508. One positive result of this is the potential to use fewer sensors without compromising safety, leading to a decrease in wiring and installation costs. Another positive effect is the potential for improved process control, largely due to increasingly intelligent sensors.
Exida‘s principal partner, Bill Goble, shared how the number of safety certified transmitters from automation suppliers has increased from five in 2003 to 24 in 2007–with more in testing and certification as automation suppliers improve the design and testing processes required to achieve certification to the safety integrity levels.
To mitigate risk for higher SIL applications, often you need multiple sensors (if not safety-certified sensors) connected in a one-out-of-two (1oo2) or two-out-of-three (2oo3) voting arrangement. Dale is quoted in the article and he discussed and amplified on the fewer sensors tradeoff:
Fewer sensors increase the possibility of a false alarm, which carries a cost since it might shut down a process needlessly.
The economic tradeoff is capital cost savings of fewer sensors and the associated installation and maintenance costs versus the probability of lost production from unplanned shutdowns caused by spurious trips.
Dale described how incorporating the features necessary for certification became part of research & development best practices. The R&D team incorporates these best practices as new devices are developed and existing ones are enhanced. These safety-certified sensors still carry extra expenses like order checks of options for the required SIL application, failure modes, effects, and diagnostics analysis (FMEDA) documentation, serial numbers and failure data shipped with each sensor.
On the increasing level of intelligence, Dale noted:
The same intelligence that makes sensors safer increasingly supplies other capabilities… Users demand predictive diagnostics beyond the sensor. They want this functionality because more insight into a process helps prevent abnormal, and potentially unprofitable or dangerous, situations.
Dale also gave a peek at future of Rosemount safety certified sensors when he stated:
We see these advanced process diagnostics, as well as loop diagnostics, being included in future safety certified products.
When developing and executing your IEC 61511 safety lifecycle programs, the intelligence in these sensors and throughout your safety instrumented functions (SIF) can help improve the diagnostic coverage and reduce manual testing.
