In hazardous areas, sensor safety rating is crucial
Appropriate selection is critical to proper position-sensing-system deployment.
A significant proportion of industrial equipment will be deployed in hazardous areas, or possibly applications that require a functional safety assessment. Hazardous areas include locations where there is the risk of exposure to flammable or even explosive substances. In such circumstances, ensuring ongoing operation can prove to be challenging. It is, therefore, critical that the component parts that make up such items of equipment are suitably constructed to function in these uncompromising environments.
Functional safety applications are ones in which safe operation must be ensured due to the risk of equipment damage or personal injury. Linear-position sensors are widely used in all manners of industrial systems. In many cases, the potential for hazardous events occurring needs to be taken into account; otherwise, the consequences could be serious—with expensive equipment being severely damaged or even lives being put in danger.
In hazardous areas, the presence of flammable gas, vapor, liquid, or dust can bring major risk to the activity of an electrical device, as there is the prospect that the device could ignite the flammable substance. For equipment incorporating position-sensing technology, the constituent sensor device (or devices) should have appropriate features to address these issues.
Before performing an in-depth study, it must be made clear that hazardous-area-rated devices are designed to operate in a potentially explosive environment while functional safety products are designed for applications where there is risk to equipment or people. These two parameters are not necessarily interrelated. An application might require either a functional safety or a hazardous-area rating without necessarily requiring the other.
Defining the hazardous area
There is a multitude of hazardous areas. Among the most commonplace are oil-exploration rigs, gas-utility plants, chemical/pharmaceutical-fabrication facilities, sewage-treatment sites, and large-scale dry-cleaning operations, as well as places where industrial chemicals are processed/produced, powders (such as magnesium and aluminum) are stored, or chemically active products (such as fertilizers) are manufactured.
For any electrical device that will be situated in potentially flammable or explosive surroundings, compliance with stringent industry standards is mandated. The most prominent of these standards are the International Electrotechnical Commission's IECEx, the European Union's ATEX (as described in directives 99/92/EC and 94/9/EC), and the NFPA 70: National Electrical Code (NEC) used in the United States. For devices being used anywhere in North America, testing should be carried out by a nationally recognized testing laboratory and labeling prerequisites should be followed.
Ratings are categorized by class and division to denote the type of hazard. These can run from the unlikely presence of an explosive substance right up to a continuous presence. Different protection methodologies are employed to attain the necessary approval rating. These are dependent on the type, condition, and nature of the environment involved.
IEC 61508 functional safety
The concept of functional safety is that, having detected a potentially hazardous situation, measures can be taken to avert a hazardous event from occurring or to ensure that, if it does occur, its effect is mitigated to an acceptable degree—so that the welfare of operatives are not put at risk and damage is not done to valuable pieces of equipment.
IEC 61508 provides a standard to assess that a safety function performs to the required level, including failure modes. Adherence to the IEC 61508 standard makes it possible to lower the risk of failure for a particular hazard via safety functions that enable its detection. In addition, it allows an assessment of the probability of failure.
Devices are categorized in accordance with a specific safety integrity level (SIL), which relates to the probability of failure occurring. A SIL 1-rated device has a probability of failure between 0.01 and 0.1 for low-demand operation, which translates into a probability of failure lower than 0.00001/hour for high-demand operation.
A SIL 2-rated device has a failure probability between 0.001 and 0.01 for low-demand operation, equating a probability of failure that is within the confines of 0.000001/hour for high-demand operation. Failures are classified as either safe or dangerous and they can either be detected or undetected. The safe-failure fraction defines the ratio of failures that are either safe or detectable over the total number of failures. From this, it is possible to determine the likelihood of dangerous, undetectable failures.
Even in the best-case scenario, failure of equipment and/or instrumentation will lead to operational downtime. This will reduce productivity and result in repair costs and the expense of replacing component parts.
Through functional safety, it is possible to quantify the probability of a hazardous event taking place and what the consequences would be. When developing industrial systems that will be situated in hazardous areas, engineers need to include functional safety aspects into their thought process. This cannot be left until that last minute; it should be a key consideration throughout the development cycle.
Key protection methods
There is a wide array of methods that can be employed to mitigate the impact of hazardous environments. Shielding can be put in place to offer basic protection. The system design takes into account the need to keep delicate components away from places where the conditions are most severe. There is great value in system redundancy, so that if a failure happens there is provision for the required function to continue.
When there is a constant threat from explosive substances, housing electronic/electrical equipment inside an explosion-proof enclosure may be deemed necessary. Through this, it will be possible to contain an internal explosion so that the external environment is not affected.
Position sensing in hazardous areas
Sensing equipment is deployed in many applications so that positioning feedback can be delivered. This often has to be done in hazardous scenarios. Examples include the gas/steam turbines found inside power generation plants, oil/gas-drilling apparatus, steel/wood presses, the equipment in fuel-servicing depots, and oil-drilling rigs, but the list goes way beyond this. A variety of sensing mechanisms can be used to accomplish position measurement in such settings. Among these are:
- Potentiometers: These act as voltage dividers with position measurement being established via a voltage signal that is proportional to the point in which a wiper resides on a linear transducer element. This method has been very popular in the past, but it does have shortfalls that are becoming more apparent in modern-day implementations. The most notable of these is that potentiometers are prone to mechanical wear and tear. This is due to the contact between the transducer and the wiper.
- Encoders: These employ a reader head to scan a marked scale and, thereby, indicate incremental changes in position. Long-term operation of this kind of mechanism is subject to failure because of the presence of vibrational movement and high temperatures. Furthermore, oil, grease, and other substances often found in heavy industrial environments mean that they will require cleaning and maintenance work on a regular basis (all of which adds to the running costs).
- Linear variable differential transformer (LVDT) devices: LVDTs rely on the movement of a ferromagnetic core that varies the magnetic coupling between primary and secondary coils. Thanks to their high temperature rating, these devices have seen fairly widespread deployment in hazardous areas. However, they exhibit an intrinsically poor linearity. Also, they need to be recalibrated periodically.
- Magnetostrictive linear sensor devices: Using the principles of magnetostriction, where a magnetic field can alter the physical properties of a ferromagnetic material, magnetostrictive sensors have shown themselves to be highly effective at delivering accurate position measurement in hazardous applications. Since these devices give an absolute position figure, rather than a relative one, they eliminate the need for recalibration work. In addition, as they dispense with the need for reader heads, the time allocated and costs relating to cleaning and maintenance work can be taken out of the equation.
It is also worth mentioning that they have much stronger resilience to shock and vibration, as well as far greater immunity to electromagnetic interference than other measurement options. Finally (and most important, in this context), it is relatively straightforward to integrate functional safety mechanisms into these devices.
Magnetostriction and magnetostrictive sensing
When a ferromagnetic material is placed within a magnetic field, microscopic changes to its structure are brought about. As a result of this, its dimensions will be altered—this is known as magnetostriction. The magnitude of the dimensional change correlates directly with the strength of the magnetic field applied. This phenomenon furnishes a highly effective noncontact sensing method. As there are no moving parts involved, magnetostrictive sensors offer heightened reliability and a prolonged operational lifespan.
The increasing need for functional safety to be addressed in hazardous applications is bringing about a steady migration away from traditional LVDT position sensing and toward a more sophisticated approach based on magnetostriction. As well as disadvantages in terms of performance and operational longevity, LVDTs and the other position sensors mentioned above are not able to offer the same degree of functional safety that magnetostrictive devices can. Appropriate sensor selection is critical to position-sensing-system deployment. There are many different factors that must be given serious contemplation if a suitable device is to be specified. It is clearly worthwhile to engage with a sensor manufacturer that offers a comprehensive sensor portfolio and has a strong understanding of functional safety issues.
Based in Cary, N.C., Matt Hankinson is global business intelligence manager for MTS Sensors.
- Linear-position sensors are widely used in all manners of industrial systems. In many cases, the potential for hazardous events occurring needs to be taken into account.
- For any electrical device that will be situated in potentially flammable or explosive surroundings, compliance with stringent international industry standards is mandated.
- A variety of sensing mechanisms can be used to accomplish position measurement in such hazardous settings.
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