The Great Safety Debate
April 10, 2007
For one thing, the tight coupling of complex safety and process control systems—if not done correctly—could allow engineers, operators or maintenance personnel who are making control system changes or repairs to inadvertently make dangerous modifications to the safety system, these individuals suggest. For another, the approach could risk “common mode failures” that could affect both safety and control systems, they contend. A system that combines safety and control in the same hardware platform risks a failure “that could basically take out both your control and safety, and if that happens, then you must have a secondary means of shutting that process down in a safe and orderly fashion,” warns Lawrence Beckman, TÜV functional safety expert at SafePlex Systems Inc., a Houston-based safety systems integrator.
The Great Safety Debate
Some process controls vendors are touting a new class of integrated safety and control system architectures that promise lower costs for end-users. But critics contend that the approach may compromise safety.
A war of words is raging in the process control industry over the “integration” of safety and control systems. It’s a debate that has been ongoing for years, but the recent introduction of new integrated systems by several process controls vendors has lately added fuel to the fire. “This is a terribly controversial issue right now,” observes Bill Goble, principal partner at exida.com, a Sellersville,
Too far?
On one side are those who warn that some controls vendors and their customers may be compromising safety by going too far with the concept of integrated process control and safety instrumented systems (SIS). This group says that the traditional requirement for separate and independent operation of safety and control systems may be violated by some of the newer integrated architectures that rely upon “functional” or “logical” separation of safety and control, instead of actual physical separation.
The traditional approach to separation is “like the belt and suspenders approach,” agrees Luis Duran, brand director for Triconex, a vendor of stand-alone safety systems that is an Irvine, Calif.-based unit of Invensys Process Systems. “But with what’s been happening lately, the problem is not so much the integration, but embedding the safety system into the controls, and making that fine [separation] line disappear,” Duran says. “Then you don’t have your belt and suspenders any more, so if you lose one, you lose them both.”
FUD spreading?
On the other side of the argument are those who tout the potential end-user benefits of integrated safety/control approaches; these include savings in engineering, training, maintenance and service costs, as well as reduced spare parts requirements. In all, these could add up to savings of 30 percent or more in total cost of ownership for an integrated safety/control system, compared to a separate systems approach, proponents say. This group agrees that appropriate separation of safety and control must be maintained, in order to avoid safety issues. But they contend that such separation can be accomplished through careful design of systems that integrate safety with control.
The safety separation traditionalists are spreading “fear, uncertainty and doubt,” or FUD, some in this group contend. And they are quick to point out that international safety standards including IEC 61508 and 61511, promulgated by the International Electrotechnical Commission, do not prohibit such control/safety integration.
“The standard does actually recognize that safety and non-safety functions can reside in the same system if ‘…it can be shown that the implementation of the safety and non-safety functions is sufficiently independent (i.e. that failure of a non-safety related function does not cause a dangerous failure of the safety related functions) –IEC61508-2 clause 7.4.2.3,’ ” says an opinion-editorial piece authored by Switzerland-based controls vendor ABB, which offers an integrated system.
Various integrated safety/control systems are on the market today that have met this requirement, say their vendors, as evidenced by certifications received from TÜV, an independent international certification organization. And once a system is TÜV-certified as meeting international standards for use at a specific safety integrity level, or SIL, that should end any debate, these vendors contend.
In the ABB op-ed piece, titled, “The Truth about Integrated Control and Safety,” ABB says that much of the public debate about the safety/control integration issue “is between suppliers with vested interests, defending their current product or technology, rather than between end-users on the merits and drawbacks of different acceptable approaches. There is a lot of disinformation being communicated on this topic,” ABB says. “Unfortunately, it only makes the issue more confusing to end-users.”
Who’s driving?
Putting it another way is Buddy Creef, sales vice president at RTP Corp., a Pompano Beach, Fla., vendor that received TÜV certification in October last year for its RTP 2500 integrated safety/control system. ...
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