Manufacturers and bulk processors are well aware of the dangers of powders, dust, and flyings in their manufacturing areas. The ever-present risk of dust combustion requires safety precautions and mitigation procedures that are critical to safe operations. These dangers are classified into classes. Of particular interest to bulk powder handling are Class II and Class III locations (Class I is gases or liquids rather than powders, dust and flyings).
In North America, the most widely used hazardous location classification system is the National Fire Protection Association (NFPA Publication 70, National Electric Code in Articles 500 to 506). These regulations specify the type of hazardous substances that are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures.
Almost all developed countries are members of the International Electrotechnical Commission (IEC). Positive pressure enclosures or “welding habitats” work on the principle of overpressure. This protection principle is regulated by IEC standard 60079-13:2017. The IECEx certification is compulsory to operate electrical equipment in explosive atmosphere in most countries outside Europe. Australia, New Zealand, UAE, Malaysia, and the Philippines accept IECEx certification directly. ATEX certification is the national certification standard of the European Union, and mandatory to operate equipment in explosive atmospheres in Europe.
Materials ignite at different temperatures. Mitigation of risk depends on the hazardous material and the manufacturing process that is taking place in the hazardous zone. It is important to understand area classifications and associated zones to determine the level of protection required to reduce the appropriate level of risk. These zones are dependent on whether the flammable materials are gases, vapors, or airborne dust or some other type of flammable material.
Class II Locations
Class II hazardous locations are areas where combustible dust, rather than gases or liquids, may be present in varying hazardous concentrations. Class II locations are further subdivided into two divisions.
Class II, Division 1
One of the following three situations must exist:
1. Where combustible dust is present in the air under normal operating conditions in such a quantity as to produce explosive or ignitable mixtures. This could be on a continuous, intermittent, or periodic basis.
2. Where an ignitable and/or explosive mixture could be produced if a mechanical failure or abnormal machinery operation occurs
3. Where electrically conductive dusts in hazardous concentrations are present
Class II, Division 2
One of following two situations must exist:
1. Combustible dust will not normally be in suspension in the air in quantities sufficient to produce explosive or ignitable mixtures, and dust accumulations will normally be insufficient to interfere with the normal operation of electric equipment or other apparatus, but combustible dust may be in suspension in the air as a result of infrequent malfunctioning of handling or processing equipment.
2. Resulting combustible dust accumulations on, in, or in the vicinity of the electric equipment may be sufficient to interfere with the safe dissipation of heat from electric equipment or may be ignitable by abnormal operation or failure of electric equipment.
Some manufacturing or processing sites can eliminate either oxygen or flammable material, but in most cases it is easiest to eliminate an ignition source. Common ignition sources can be electrical sparks, hot surfaces, or electromagnetic fields, all generated by electronics.
Intrinsic safety (IS) is a protection technique for safe operation of electrical equipment in hazardous areas that limits the energy--both electrical and thermal. Using electrical equipment in hazardous areas imposes different regulations that govern equipment manufacturing and safety to reduce the ignition source risk. Many of these regulations are local to specific countries. Manufacturers must adhere to these regulations and provide the appropriate level of protection depending on the classification of the hazardous area.
Reducing the Risk of Using Weighing Equipment in Hazardous Areas
Weighing vessels and silos with legs in compression or suspended in tension is generally accepted as the most accurate and repeatable method to measure their contents, especially for inventory or batch manufacturing purposes.
Weighing equipment used in areas where there may be explosive concentrations of vapors or dust must be equipped with special wiring and other intrinsically safe electrical components. Hazardous (classified) locations might exist in any manufacturing location with vapors, dust, or “flyings,” but are common in large bakeries (flour), plastics manufacturing plants (vapors or dust), chemical plants (vapors), and grain silos (dust or “flyings”), to name a few. Other examples include locations where vehicles are fueled or there are any transfer mechanisms for inherently hazardous materials.
Designing Safety into a Weighing System
For potentially hazardous Class II and Class III environments, bulk powder manufacturers and processors must design safety into their manufacturing systems by using electronic components with an appropriate level of spark protection for different applications.
One way to do this is by using intrinsically safe, low-energy weighing components in hazardous areas (including load cells, scales, and summing boxes) and limiting power and current to them through intrinsic barriers that prevent energy from crossing over from the safe area (see Figure 1).
Intrinsic safety barriers are essential components of an intrinsically safe application. When used in areas with hazardous dust, flyings, or other ignitable atmospheres, limiting the energy helps prevent any possible fires and explosions. Manufacturers of weight processors, weight controllers, rate controllers, or weight modules often feature proprietary technologies that have great value for hazardous area applications and confined spaces. For example, electronic calibration allows calibration of the weighing system without ever entering the environment. This reduces the risks and costs of bringing test weights into the hazardous area. Integrated diagnostics used for system diagnostics and troubleshooting can be performed from the safety of the safe area reducing risk to operators. Other technologies can eliminate the effects of vibration on the weight signal so only stable weight readings are communicated to the control system.
Programmable logic controller (PLC) and programmable automation controller (PAC) weight modules read and condition data from strain gage load cells in the hazardous area and communicate the data through intrinsic barriers over the I/O chassis backplane to the weight processor. Weight modules provide basic weight data or are loaded with sophisticated algorithms to perform application-specific industrial weighing processes from simple batch weighing to loss-in-weight control, filling, or dispensing.
Using Intrinsically Safe Weighing Instruments in the Hazardous Zone
A different solution is to use intrinsically safe weighing instruments in the hazardous area. These instruments are designed with the low power and spark protection that allows them to reside in Class I, II, and III areas.
These instruments are generally connected with fiberoptic cable due to the low power conduction. In addition, many of these weighing instruments are powered by batteries. Generally, a site will need three batteries: one to use, one to recharge, and one as a backup. Other intrinsically safe instruments use fiberoptic to AC converters. These are generally connected through conduits from the safe area so they still eliminate the risk of sparks in the hazardous areas.
Figure 2: Intrinsically safe instruments work in the hazardous area, allowing operators to use weight readings directly. These devices limit current, voltage, and total energy to eliminate spark risk. To operate in the hazardous area, they must use batteries or fiberoptics converters with power conduits to draw AC or DC power from the safe area.
No matter what your weighing application, look for weighing equipment that is designed for use in hazardous areas not only with the integrated features you need, but also enable safe and easy operation.
Tim Norman is senior product manager and Janice Kallis marketing manager, Hardy Process Solutions (San Diego, CA). For more information, call 858-278-2900 or visit www.hardysolutions.com.