by Randy Frank, Contributing Editor
It’s a dangerous world out there—especially for sensors
If you’re designing in sensors in a hazardous application, you may want to get some tips from the experts. We’ve consolidated tips from several sensor experts for designers encountering hazardous environments for the first time.
There certainly is not a lack of information available to those seeking more information about applying sensors in hazardous areas. However, that does not prevent confusion about hazardous sensor applications.
“When I talk to someone, I can usually figure out pretty quickly whether they are familiar with it or not,” said Renard Klubnik, an applications engineer for Meggitt (Maryland) Inc. (formerly Wilcoxon Research). “The questions are just different.”
Perhaps one of the first things to understand is the meaning of hazardous environment.
Nick Tolley, from FUTEK Advanced Sensor Technology, explains, “As it applies in test and measurement, the term hazardous environment is most often used in particular reference to chemical hazards. For example, a hazardous environment is often an area in which a volatile (easily turns to gas) substance that is either corrosive or combustible is being used or processed.”
1. Identify the standards and certifying agencies that impact you.
In many instances, insurance companies and local government dictate requirements for hazardous locations. They rely on criteria established by testing laboratories, standard institutes and certifying agencies. In North America, organizations that deal with standards and testing of materials used in hazardous areas include:
• American Bureau of Shipping (ABS)
• American National Standards Institute (ANSI)
• Canadian Standards Association (CSA)
• Factory Mutual Research Corporation (FM)
• Instrument Systems and Automation (ISA)
• National Electric Code (NEC)
• National Electrical Manufacturers Association (NEMA)
• National Fire Protection Association (NFPA)
• Underwriters Laboratories (UL)
Other regions have their own standards and certification requirements. Europe, for example, has ATmosphères Explosives (ATEX), Committee for Electrotechnical Standardization (CENELEC), International Trade of Electrical Equipment used in Explosive Atmospheres (IECEx) and GOST R certification for Russia.
All of these agencies’ websites have a wealth of information about their requirements for hazardous areas.
2. Understand the difference between the standards/certifications, terminology and acronyms.
As shown in Table 1, hazardous environments are classified by classes and divisions for North America and groups and zones in Europe. In North America, the classes are divided into groups. The group terminology is used for rating the explosive potential of the material in the environment, such as Group A ( for example acetylene), Group B (hydrogen), Group C (ethylene), Group D (propane), Group E (metal dust), Group F (coal), an Group G (grain).
One of the essential terms for hazardous locations is intrinsically safe (I.S.)—equipment that is incapable of releasing sufficient electrical or thermal energy to cause ignition of a hazardous environment mixture.
“If you look at the terminology out there, intrinsically safe is an indication of a Class 1, Division 1 environment,” said Klubnik. “A lot of times people say ‘I’m looking for an intrinsically safe for Class 1, Division 2’ and those two terminologies are incompatible with one another.”
Class 1 Division 2 is for less severe environments and is not intrinsically safe but it does involve non-incendive devices, those with insufficient energy to ignite a flammable mixture.
“Even though a sensor is classified Class 1 Division 1, you can use it in a Class 1 Division 2 environment but you must follow the rules for Class 1 Division 1 when installing it,” said Klubnik. “If you install one of our sensors in a Class 1 Division 1 environment, it requires the use of a safety barrier. If you put this sensor in a Class 1 Division 2 environment, you still need the safety barrier if you want it to be considered a certified installation.”
Explosion proof is another key hazardous area term. Also known as flameproof in Europe, these types of devices must contain any type of explosion within the sensors itself.
“For our sensors, there are three main categories,” said Greg Montrose from American Sensor Technologies. (AST). “There’s the explosion-proof rating, intrinsically safe and non-incendive or non-sparking.”
Measurement applications with combustible substances typically use either sealed or intrinsically safe sensors.
“Sealed is a term more common to force measurement while Intrinsically Safe is more common to pneumatic and hydraulic pressure measurement,” said Tolley. “Both terms basically amount to isolating the sensor’s electrical components from the surrounding environment so that electricity flowing through the sensor cannot serve as activation energy to an exothermic (gives off lots of energy) reaction.”
3. Know the unique aspects of the specific transducer you are working with and the impact of a hazardous environment.
Piezoelectric vibration transducers provide an example of a sensor with unique requirements. These sensors convert mechanical energy into electrical energy by stressing a piezoelectric crystal. The electrical energy output is a direct function of the transducer design and the mechanical energy input. Therefore, when a piezoelectric sensor is installed in a hazardous environment, it may need to be protected from shocks exceeding a set limit.
4. Know where the sensor is being installed.
A sensor offered for sale in one country may wind up in a hazardous location in other countries. While approval agencies are working to harmonize standards, there are still subtle differences between agencies, countries and continents. In addition, terminology is not always interchangeable and there is no perfect pyramid describing the flow of highest ranking approvals. Also, onshore and offshore approvals can make a big difference in how a product is used.
“AST recently upgraded from ABS Design approval to ABS Type Approval,” said Montrose. “This involved a quality audit of our facility and quality management system to ensure that products were made to specifications.”
The new approval helps marine industry customers expedite their documentation and qualification for offshore and ship use.
5. Realize that the hazardous environment involves more than the sensor.
In addition to certified sensors, there are certified Class 1 Division 1 cables, where a metal sheath provides the barrier, and Class 1 Division 2 cables as well as connectors for these environments, too. How do the pieces come together?
“Usually our response to that is that decision is made at the site by the plant engineer,” said Klubnik. “He is the guy who has to sign off on the installation.”
Montrose reinforces the concern for cables and connectors.
“The electrical configuration is probably the most crucial in terms of the UL and CSA certifications, in terms of how can I connect to the sensor,” he said.
For explosion-proof certification, connectors at the sensor are not allowed, so leads that are potted into the housing provide a solution.
Switches are frequently used as threshold sensing devices. Maria Cancino, senior global product marketing manager at Honeywell acknowledges one of the approval problems for switches and other hazardous location products.
“Due to the complexity of achieving and maintaining worldwide approvals, many manufacturers limited safety approvals to their home region or country,” she said. “For customers and OEMs that supply end products requiring explosion-proof safety switching components to worldwide customers, meeting all applicable regional approvals could create a major drain of engineering and purchasing resources to locate, procure, and verify the regional components required.”
Honeywell’s solution to alleviate the need for multiple explosion-proof safety switch components by region is its MICRO SWITCH GSX series switch. Designed for use in hazardous applications requiring a positive-break safety mechanism, GSX series explosion-proof safety switch meets universal approvals certifications (UL/CSA, ATEX and IECEx). The GSX safety switch may be applied in all countries requiring IECEx certification, which supersedes most other regional requirements.
6. Know the latest specification and qualification status.
Keeping up to date on specifications and product qualifications is the penultimate tip. For example, North American approvals are changing from division to zone classifications. This has not been finalized, but it is definitely something that needs to be taken into account.
“The fact that things are changing and moving constantly, keeping up with these types of certifications is a challenge—not only to the sensor manufacturers but also to the integrators as well to ensure they are meeting the latest codes,” said Montrose.
Another aspect to keep up to date on is the approval status of a supplier’s or competitor’s products. For example, when Balluff achieved IECEx hazardous area approval rating for its Micropulse TA12 explosion-proof linear position transducer, it quickly added that to its product’s advantages.
7. Seek the help of a knowledgeable/experienced partner.
With all of the potentially unfamiliar and constantly changing requirements for hazardous sensor applications, perhaps the most beneficial tip is engaging the help of an expert or at least a more familiar partner to navigate the unfamiliar hazardous territory. This can involve working closely with the customer with the hazardous application for the sensor supplier or working more closely with the sensor supplier with hazardous environment experience for the sensor integrator.
American Sensor Technologies, Inc. (AST)
www.astsensors.com
FUTEK Advanced Sensor Technology Inc.
www.futek.com
Honeywell Sensing & Control
www.honeywell.com
Meggitt (Maryland) Inc.
www.wilcoxon.com