If all you need is a garden hose...

InTech, Aug 2000 by Aldridge, Roy G

When I started to think about this article, my mind immediately launched into attributes of safety systems. I wanted to discuss the history of safety systems, how we have progressed from relays to solid state to programmable safety systems. I wanted to discuss the roots of the technology so everyone would understand safety systems and their use and say, "Wow, I want to go and buy a safety system right now." Then reality, as well as the length of this column, set in, and I decided to limit myself to looking at where the safety market is today.

Today there is a lot of marketing hype over safety systems--so much so that it is sometimes difficult to understand how a company should apply what type of technology. to a particular critical process control system. The underlying question to all this hype is, how safe is safe? Moreover, do we need safety systems because we all assume we are safe?

We install smoke detectors in our homes for safety. There might even be a local building code that requires their installation. In some instances, we receive reduced insurance premiums for fire protection systems and equipment. With all this technology, we might assume our homes are safe from fire damage. Carrying this line of thought further, we could even consider removing all the fire hydrants on our streets and decide we have no need for the fire department. We could save a lot of money in our community because we know our homes are safe and a simple garden hose fits our budget, but realistically, who would go that far?

There are many critical process control technologies available-- from simplex systems to dual and quad systems up to triple modular redundant systems-and each can provide plant protection that is acceptable for a specific application. A host of international standards and certifying agencies such as ISA, the IEC, and TOV publish standards to assist users in selecting and implementing safety shutdown systems. But rarely is the intent of the full standard applied, despite the fact that in the U.S., the ISA standard is federally mandated.

How does this happen? As an example, there are two TUV certifying agencies in Germany that operate independently within the same standards organization. One agency requires manufacturers to publish restrictions for their safety systems, while the other agency does not. These restrictions specify how to apply a certain technology in order for it to gain TUV certification. A project's specifications might state that the safety system must be TUV compliant, when in reality they should state that the safety system must be TUV compliant incorporating restrictions.

In any project development, cost is always a consideration, and a customer can compromise the safety system if he looks only at the initial cost of buying a system. The true cost of a safety system has four components:

Initial design and installation

Acceptable nuisance trip rate

Ease of maintenance

Ease of upgrade

A safety system should perform flawlessly for 10 to 15 years, while the processes around it can change dramatically every couple of years. The safety system must be able to grow to meet plant requirements.

Too often at the project implementation level, the accept nuisance trip rate for a safety system is not considered, and vendors do not readily volunteer their calculated nuisance trip rate. As an extreme example, consider a loo20 relay system with all the relays wired serially. Mathematically, this system will prove to be the safest technology in the world; however, in the real world you could never keep this type of system running. Thus you would end up with an extremely safe system but be disappointed with the system availability. Remember that a safe system is not always an available system, so consider a blend of safety and availability when designing critical process control systems.

Safety systems must be a serious consideration in the overall plant control strategy, and there are many technologies available that can provide safe plant operation for years to come.

You can rely on simple garden hoses or place fire hydrants on your street or park a fire truck in your garage. All are acceptable to meet a need, but remember to consider the full life cycle cost of a system. Safety and availability should be your choice.

Behind the byline

Roy G. Aldridge is vice president of marketing and sales for Triconex Corp., a member of the Invensys Group. His address is ratdridge@ triconex.com.