PLC architecture can provide high safety integrity

There's no safety like nuclear safety. Consequently, process safety instrumented systems (SIS) requiring a high safety integrity level (SIL) can benefit from a programmable logic controller (PLC) architecture used in nuclear safety systems. (See this issue's cover articles for related safety topics.


There's no safety like nuclear safety. Consequently, process safety instrumented systems (SIS) requiring a high safety integrity level (SIL) can benefit from a programmable logic controller (PLC) architecture used in nuclear safety systems. (See this issue's cover articles for related safety topics.)

For instance, a PLC module developed by Framatome Technologies is among those using redundancy and diversity to enhance reliability. If an unsafe condition is detected, the module's two safety-function microprocessors can cause any of the relay outputs to open. Watch-dog timers will open the relay outputs if either processor stops running. Also, an OR gate can be substituted for the AND gate for devices requiring a contact closure (non-failsafe output) to actuate.

Using microprocessors that differ in design, microcode, and software compiler manufacturer minimizes common mode failures that could defeat safety interlocks. Using a common functional design document, diverse software is developed by two software teams working independently. Software is then tested and validated by a third team, independent of the developers.

Different microprocessors with different software ensure the SIS will achieve its safety mission, even if a hardware and/or software fault disables one microprocessor.

Diversity aids reliability

The PLC's self-contained redundancy and diversity complements other redundant and diverse elements to provide an SIS with SIL 3 integrity. SIL 3 is quantified in the ANSI/ISA-S84.01-1996 standard, "Application of Safety Instrumented Systems for the Process Industry," as a probability of failure on demand average range (PFD avg) of 10-3to 10-4.

Specifying diversity in sensor type, manufacturer, and activation methods reduce common mode failures. For example, using one RTD (resistance thermal detector) and one thermocouple, and pressure sensors and ventvalves from different manufacturers, reduces common mode failures. Adding a hardwired, manually operated emergency shutdown circuit also provides diversity. When a high-high pressure, or high-high temperature input signal is detected, the PLC's fail-safe outputs open redundant emergency vent valves to depressurize the reactor.

PLC testing is done on line using continuous diagnostic routines. Off-line testing uses a test computer that injects simulated process signals into the PLC module. Input signals are varied by the test computer and PLC output responses are monitored. Both microprocessors are tested at the same time, and hard copy test records are developed.

Shared memory and a separate microprocessor in the PLC module handle communications with external systems. This architecture makes SIS data available, yet prevents communication interrupts from interfering with safety requirements. This PLC architecture provides the same high reliability and high availability of two PLCs.

For more information about Framatome Technologies, visit .

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Raising the standard: What's new with NFPA 70E; A global view of manufacturing; Maintenance data; Fit bearings properly
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Upgrading secondary control systems; Keeping enclosures conditioned; Diagnostics increase equipment uptime; Mechatronics simplifies machine design
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.