Simplified safety: fail-safe functionality for hazardous environments

Incorporating fail-safe functionality into intrinsically safe technology simplifies system setup and maintenance reducing control costs.

06/14/2011


Installing fail-safe machine control systems in areas that have explosive atmospheres has been a problem because the products normally used in most machine-control equipment may cause sparks when disconnected. Any such sparks in an explosive atmosphere are dangerous to life, limb, and equipment.

The ultimate solution is to redesign the equipment to limit voltages and currents to levels where generation of sparks is impossible. That is, of course, good news for everyone, from the fire department in the surrounding community to the maintenance worker at the potential point of ignition. No disaster is a good disaster!

It sounds easy, but putting this concept into practice requires completely redesigning the control electronics from the semiconductors up. This makes the equipment intrinsically safe, meaning that explosion proofing is not an add-on. It’s not something like a shield that could be defeated or fail. It’s part of the equipment’s basic design.

In intrinsically safe equipment, all voltages and currents are limited to values that ensure that sparks cannot be generated. In addition, all electronics contain circuit elements, such as capacitors or MOVs in parallel with switch contacts to absorb kickback energy when interrupting currents in inductive circuits, and to bleed static charges that may build up anywhere within the equipment.

Limiting voltages and currents, of course, limits the energy available to form kickbacks to levels where the atmosphere itself is able to snuff sparks before they can appear. This is what is meant by “intrinsically safe.”

Intrinsically safe: No failure mode

The advantage of intrinsically safe control components is that there is no failure mode. You can’t get a spark because there is nothing there capable of making a spark. The probability of an explosion collapses to zero for all conditions.

Previously, engineers designing installations where explosive atmospheres could exist have had to set up a safe barrier to protect the dangerous area. While the term “safe barrier” calls to mind images of heavy concrete walls, it’s not like that at all.

Of course, there are walls around areas where explosive atmospheres gather. That’s what makes combustible gases, vapors or dust build up to explosive levels in the first place. Those walls, however, are the problem, not the solution.

The safe barrier solution is, instead, a set of electronic system components that reduce signals to safe, spark-free levels before they enter the hazardous area. Figure [safe barrier] illustrates the concept. Control-system components outside of the barrier all operate at standard voltage, current and power levels.

Incorporating fail-safe components into intrinsically safe control systems eliminates the need for safe barriers. That drastically reduces wiring, cabling, and engineering costs. The reason combining intrinsically safe technology with fail-safe technology is so important is that fail-safe devices, especially, can be located as close to the processes they control as possible. That means putting fail-safe devices within the hazardous atmosphere. That is impossible without including intrinsically safe technology as well.

Explosive atmosphere applications

Hazardous-atmosphere issues arise in many industries, such as chemical, petrochemical, oil and gas, even farming, where organic dust is famous for exploding with no warning. Wherever you have the manufacture, processing, transport or storage of combustible materials, you typically have creation or release of gases, vapors or mist into the environment. There are other processes that create combustible dust, such as grinding and milling, where you’re creating granular solids or dust.

Today, there are intrinsically safe fail-safe devices manufactured for use in explosive-atmosphere environments. These are the first generation of modular solutions for I/O in machine safety loops in hazardous areas, such as the modular distributed SIMATIC ET200 iSP, which is an I/O solution designed for installation in hazardous areas. The ET 200iSPO is programmed in a similar way to the standard SIMATIC PLC and distributed I/O modules with STEP 7 software over PROFIBUS. All these modules can be replaced during operation even under explosive conditions. If technicians need to replace something, they can just go pull out a module, and put a similar module in there, and continue with the operation thus reducing downtimes and further increasing productivity.

Overall, the benefits of combining intrinsically safe technology with machine safety include: no safe barriers required, reduced wiring, one cable carries both standard and safe communications, in-built HART communications, hot swapping, reconfiguration in run mode, integrated diagnostics. No barriers, less wiring, less cabling, less engineering translate directly into less installation and operation costs. Now, hazardous area application designs can be further simplified and made more efficient with the use of a fail-safe intrinsically safe control system solution.

 

 

John D’Silva is marketing manager for Safety Integrated solutions at Siemens Industry. Contact him at john.dsilva@siemens.com

For more information on safety automation related products, visit the Siemens Industry website at www.siemens.com.

www.usa.siemens.com/safety

This article was submitted for the Siemens Simplified Safety custom newsletter. See other articles in the Siemens Simplified Safety newsletter.

www.controleng.com/newsletter/siemens-simplified-safety.html

../newsletter/siemens-simplified-safety.html



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Virtualization advice: 4 ways splitting servers can help manufacturing; Efficient motion controls; Fill the brain drain; Learn from the HART Plant of the Year
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
Detecting security breaches: Forensic invenstigations depend on knowing your networks inside and out; Wireless workers; Opening robotic control; Product exclusive: Robust encoders
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.
Integrator Guide

Integrator Guide

Search the online Automation Integrator Guide
 

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control 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.