Weighing systems have a wide range of commercial and industrial applications. In industrial applications, weighing scales are used for counting parts, batching, drum filling, mixing, dispensing, chemical formulation, and so on. Owing to their diverse applications, weighing systems are used in a broad array of environments. In laboratories, weighing scales are used in near-ideal conditions. In comparison, weighing equipment used for outdoor weighing applications can be subjected to extreme temperatures, vibrations, air currents, electrical interference, etc. When selecting a weighing system for your weighing application, it is critical to consider the environment in which it will be used. Load cells and weighing equipment for use in hazardous environments are required to meet stringent requirements.
Weighing in Hazardous Areas
There are many industrial processes that require weighing equipment to be positioned in hazardous areas. For instance, scales are commonly used in factories for weighing explosive, corrosive, or flammable materials, such as specialty chemicals, agrochemicals, polymers, petrochemicals, soaps, detergents, inks, paints, fragrances, and inorganic chemicals. Other industrial applications that may require weighing equipment to be positioned in a hazardous environment include formulation, filling, dispensing, and blending processes. Load cells and weighing scales used for such processes are required to meet certain specifications.
Weighing systems that are used in hazardous environments should have suitable protection to prevent fire or explosion. For an explosion to happen, the following elements are required: sufficient amount of combustible gas, vapor, mist, or dust; sufficient amount of air or oxygen; and source of ignition. If a weighing scale is not properly protected, it can provide the energy required to trigger an explosion. Load cells and weighing scales for use in hazardous locations are specially designed to ensure that they do not produce the energy required to ignite explosive fumes/gases or flammable liquids.
Load cells and weighing equipment for use in hazardous environments are required to meet international standards. These sets of legislation define the technical as well as operational requirements for weighing systems installed in places where explosion is highly probable. Most countries in Europe use European ATEX (ATmosphere EXplosive) regulations, while U.S. uses codes and standards set by National Fire Protection Agency (NFPA). In the U.S., the regulations are defined in the National Electrical Code (NEC), and electrical products are tested for certification by Underwriters Laboratories Inc. (UL) or Factory Mutual Research Corp. (FM).
Hazardous areas are categorized according to the properties of the hazardous materials and the possibility of their presence. Electrical equipment should not be installed in hazardous places unless when it is impossible to have them in designated safe areas. If weighing systems have to be installed in a hazardous location, they should be made safe for that area by protecting them.
Methods of Protecting Weighing Systems
Most weighing systems consist of load cells, controls, displays, junction boxes, and cabling. These components can be situated in a safe or a hazardous location depending on the weighing application. To reduce the risk of fire or explosion, each component that is situated within the hazardous area should be protected. The following are some of the methods used to prevent explosions in hazardous environments:
Use of intrinsically safe systems
The electrical energy available in the circuits is limited to ensure that it is below what is required to trigger an explosion. The limited energy is incapable of producing arcs and/or temperatures that can cause fire or explosion under normal or fault conditions.
Use of explosion-proof enclosures
This protection method involves housing electrical equipment in enclosures that are capable of containing internal explosions. Such enclosures can withstand internal explosions to prevent ignition of combustible gases or vapors that are external to the system.
This method involves surrounding the potential source of ignition with fine grained sand. This prevents arcs from inside the enclosure of the equipment from igniting the external combustible atmosphere.
Purged and pressurized enclosure
This protection method entails filling a sealed enclosure with an inert gas to prevent entry of combustible fumes, vapor, or dust. The inert gas is filled under pressure.
Use of nonincendive equipment
This method involves use of equipment that is incapable of releasing thermal or electrical energy that is sufficient to ignite flammable combustible vapors or gases under normal conditions.
When selecting load cells, cables, displays, or controls for your weighing system, it is critical to consider the environment in which they will be used. Only protected equipment should be used for weighing applications in hazardous environments. You can prevent fires or explosions in hazardous areas by using intrinsically safe equipment, nonincendive equipment, explosion-proof enclosures, or purged enclosures.
Joe Flanagan is a senior electronic engineer at Tacuna Systems, Golden, CO, specialists in load cells. The company offers tools for sensing, data acquisition, and controls for a variety of industries. For more information, visit tacunasystems.com.