Hardware configuration of redundant safety integrated systems

Control system architects apply redundancy selectively to ensure reliability while minimizing false alarms. Here are some of the strategy options.

09/06/2013


Major SIS (safety integrated system) suppliers provide redundant hardware configurations based on 1oo2 (1 out of 2), 2oo3, and 2oo4 configurations. Applications of these configurations are designed to provide a higher level of reliability while simultaneously reducing the probability of a false trip shutting down a process unnecessarily. This discussion reviews how these strategies developed along with a more detailed overview of two redundant architectures commonly applied in large-scale SIS SIL3 applications, namely 2oo3 and 2oo4 as applied by Triconex and Honeywell, respectively. These two major companies are only examples as similar hardware configurations are available from other SIS suppliers.

This article is intended to stimulate discussion among users, SIS suppliers, and other SIS specialists to elaborate on the benefits, advantages, and disadvantages of implementing redundant hardware configurations according to relevant SIS industry standards from ISA and IEC. You can comment directly via the online version of this article, or by sending an email to the address listed at the end.

Basic SIS architectures

1oo1—This simplex output circuit opens the switch to de-energize a device and shut down the process safely. A safe failure would be when the contacts open without an associated cause, which would be classified as a nuisance trip, although such events are not without their associated negative economic impact to the overall process facility. A dangerous failure would be if there was indeed a safety shutdown cause and the contacts failed to open. This could be caused by the contacts overheating and becoming welded closed over time. Such events are classified as a failure to operate on demand.

1oo2—This approach has two outputs (1oo1) in series for a normally closed and energized safety shutdown circuit. Only one SIS has to function to initiate a shutdown. Of course, having two 1oo1 circuits presents twice the potential for nuisance failures, which can be costly due to the loss of revenue for the overall process. However, it is a safer circuit since only one contact is required to operate to achieve a shutdown and the probability of a dangerous failure to operate on demand is much lower. Neither 1oo1 nor 1oo2 has any ability to reduce the potential for nuisance trips.

2oo2—These systems have the outputs wired in parallel, requiring both contacts to operate to initiate a process shutdown. Since the contacts are in parallel, nuisance trips by one contact are reduced but the obvious drawback that a dangerous failure scenario with a failure to operate on demand is doubled, making the system less safe.

As shown, 1oo2 and 2oo2 systems are not effective for both safety and nuisance trips. However, with SIS diagnostics it is possible to achieve higher availability, referred to as 1oo2D (1oo2 with diagnostics).

Advanced SIS architectures

2oo3 or triple modular redundancy (TMR) safety shutdown systems are commonly used for applications such as gas turbines, compressors, and heaters, and for individual process units within a refinery such as coker units.

Each SIS architecture offers its own characteristics for maintaining a high level of safety while avoiding nuisance trips. Getting a system that can improve both at the same time requires a relatively high level of complexity. Courtesy: CFE Media

As the switching diagram indicates, the 2oo3 configuration requires two out of three channels to agree as to the output even though the third does not. If only one SIS trips its pair of contacts, one of the legs still remains closed so the process continues operating. Real-world systems use a voting scheme to maintain the output when 2oo3 are OK but the third signal is ignored, allowing for a fault tolerant configuration.

Industrial implementations

Industrial installations built by major vendors use more sophisticated versions of these basic concepts. The examples that follow describe how two major SIS suppliers provide diagnostics to achieve their 2oo3 and 2oo4 configurations. These companies and other suppliers that use similar approaches can provide the necessary data for MTBF (mean time between failures), failure probabilities, and failure to operate on demand, which serve as the basis for a complete SIS implementation evaluation.

2oo3 as triple modular redundancy

Every Trident system contains three main processors (MPs), A, B, and C. Each MP controls a separate channel and operates in parallel with the other two. A dedicated I/O control processor on each MP manages the data exchanged between the MP and the I/O modules. A triple I/O bus, located on the base plate, extends from one column of I/O modules to the next using I/O bus cables.

2oo3 as applied commercially by Triconex. Courtesy: Invensys Operations Management

The I/O control processor polls the input modules and transmits the new input data to the MPs. The MPs then assemble the input data into tables, which are stored in memory for use in the voting process. The input table in each MP is transferred to its neighboring MP by the TriBus. After this transfer, voting takes place. The TriBus uses a programmable device with direct memory access to synchronize, transmit, and compare data among the three MPs.

If a disagreement occurs, the signal value found in two out of three tables prevails, and the MPs correct the third table accordingly. One-time differences which result from sample timing variations are distinguished from a pattern of differing data. The MPs maintain data about necessary corrections in local memory. Built-in fault analyzer routines flag any disparity and use it at the end of each scan to determine whether a fault exists on a particular module.

Three good, four better?

One question to consider is whether the double redundancy concept, 2oo4, is considered safer or less safe due to the additional hardware and software involved.

Quadruple modular redundant (QMR) architecture is based on 2oo4D (D refers to inherent diagnostics) voting, dual-processor technology in each QPP (quad processor pack, the processing module of the system). This means that it is characterized by an ultimate level of self-diagnostics and fault tolerance.

The QMR architecture is realized with a redundant controller. This redundant architecture contains two QPPs, which results in quadruple redundancy making it dual fault tolerant for safety.

The 2oo4D voting is realized by combining 1oo2 voting of both CPUs and memory in each QPP, and 1oo2D voting between the two QPPs. Voting takes place on two levels: on a module level and between the QPPs.

2oo4D adds hardware to build redundancy. Courtesy: Honeywell Process Solutions

Process safety practitioners have debated the pros and cons of various redundant configurations for many years. Have you been part of these conversations? Send us your thoughts on maintaining the delicate balance of overall safety vs. avoiding nuisance trips. Comment online or send me an email.

Robert I. Williams, PE, is a systems consultant specializing in DCS, SIS, and SCADA. Reach him at riwilliams1(at)cox.net

Key concepts:

  • An effective safety system must shut down a process in an emergency, but it must also avoid causing false alarms.
  • Safety system architects have created a variety of strategies, some simple and some complex, to accomplish these two key tasks. 

ONLINE

http://iom.invensys.com

www.honeywellprocess.com

See related stories below.



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 Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

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.