Those involved with safety instrumented systems (SIS) know that the performance of the system to perform on demand requires all elements of the safety loop: logic solver, sensor, and final control element to correctly perform their role.
Data from the Offshore Reliability Database (OREDA) points out that 92% of the failures come from problems in the sensors or final control elements.
The IEC 61508 international safety standard stipulates requirements for equipment to be used in safety applications. It must be suitable for the safety application, that is, for the appropriate safety integrity level (SIL). SIL is defined:
Safety Integrity Levels (SILs) are a safety-measurement standard defined by several bodies including the International Electrotechnical Commission in IEC 61508 to quantify the chance of dangerous failures in electrical or electronic safety devices, that is, the probability of the device to fail in performing its safety function.
Process manufacturers typically seek products certified to this standard by a reputable independent agency like one of the TÜV certification agencies to achieve compliance with the IEC 61511 international safety standard.
Suppliers like Emerson can seek certification either by designing new products to achieve certification based upon the safety requirements or by being “proven in use” as defined by the IEC 61508 standard. The folks at Exida have an excellent write up describing the latter method entitled, What does Proven In Use imply?
I caught up with Al Samson, Director of Product Support for our Micro Motion Coriolis flow meter products. Earlier this year Micro Motion flow meters became the first flow meter to be TÜV-certified to the IEC 61508 safety standards.
The Micro Motion 1700/2700 transmitter family has had more than 5 years in service with high reliability so the team used the proven in use method to achieve certification for use in SIL 1-3 applications. These transmitters are used with the Micro Motion Elite, F, T, and DT sensor families.
The Micro Motion team worked with Exida to develop the Failure Modes, Effects and Diagnostics Analysis (FMEDA) and other essential documentation necessary for the TÜV process audit required to receive the proven in use certification.
Al pointed out that the combination of an inherently redundant Coriolis sensor along with a high level of internal diagnostics provided a safe failure fraction of 92% which is the best among this class of sensors.