Safely running motors and low voltage AC drives in dusty environments

08 February 2010

To ensure safety, the use of electric motors and low voltage AC drives in areas with combustible dust is regulated by the ATEX directives; these environments can be just as hazardous as atmospheres with gas.

Safely running motors and low voltage AC drives in dusty environments

Low voltage AC drives used with electric motors can offer significant process improvements and energy savings for pumps, fans and other equipment used in dusty environments. These benefits are the same as those achieved in applications in ordinary, non-hazardous areas. There are several ways to secure safe running of motors and low voltage AC drives in dusty environments.

Combustible dust is an insidious hazard. Sometimes its presence may even be known, but ignored because we don’t think of ordinary substances like sugar, plastic or wood as being hazardous materials. However, when reduced to dust, such materials have a very large surface area and can burn extremely rapidly, causing an explosion with very high energy.

The main reason that dust hazards are frequently overlooked is a simple lack of understanding about the materials. Some of the materials that can form combustible dust – and there are many of them – include coal, food products like flour and sugar, pharmaceuticals, many chemicals and even metals, such as aluminum and aluminum alloys.

Like all explosions, dust explosions require fuel, oxygen and an ignition source to get started. In addition, dust explosions need two more components, dispersion and confinement. Dispersed in the air, dust can create a rapidly burning fireball. When the dust is confined inside a building or a piece of equipment, a powerful explosion can occur and propagate.

A dust fire can have devastating effect. In addition to injuries and loss of life, it can destroy a plant as the explosions rip through ducting and ceiling voids. Yet, dust fires are completely preventable.

Since 2006, hazardous areas with dust come under the ATEX regulations that control installations in hazardous areas. Areas with dust are classified in the same way as hazardous areas with gas and equipment is selected on the same basis.

Prior to ATEX, the use of hazardous area motors in industries other than oil and gas was very limited. The explosion and subsequent collapse of a 40 meters high grain silo in France, in 1997, woke the industry up to the hazards posed by combustible dust. The investigation concluded that the most likely source of ignition was either a malfunctioning fan on the centralized dust collection system, or auto-ignition caused by overheating of the collected dust. The explosion traveled from the handling tower along a gallery, penetrating the open silos and producing a further violent explosion. In the wake of this accident, regulations began to be tightened up.

Motors and low voltage AC drives

Safely running motors and low voltage AC drives in dusty environments

Ex tD motors are used in atmospheres where explosive dust surrounds the motor, or where dust settles under its own weight on the motor. Dust is measured either as a cloud of dust or a layer of dust. The ignition temperatures for various types of dust can be obtained from commercially available reference tables. The ignition temperature for a cloud of dust must be at least 50% above the motor’s marking temperature. The ignition temperature of a 5 mm layer of dust must be 75°C above the marking temperature of the motor. It is the responsibility of the user to stage maintenance periods so that the dust layer does not build up above 5 mm.

Motors with low voltage AC drives can be installed in hazardous areas, offering safe, economical power combined with effective control. However, the drive must always be installed in a safe area and it is important to recognize the effects the drive has on the motor, mainly to prevent motor surface over-temperature. The motor temperature can be controlled by using a temperature measurement signal from the motor and use this signal to initiate shut-down at high temperature. Alternatively, it is possible to monitor the amount of energy transferred to the motor and control the temperature this way; the energy flow can be easily monitored if using an ABB industrial low voltage AC drive with the motor control platform DTC (direct torque control). Otherwise, a combined test with the motor and low voltage AC drive is necessary to ensure that the motor surface temperature does not exceed the limits. It is also possible to purchase an ATEX approved package with motor and low voltage AC drive.

End-user responsibilities

The ATEX regulations requires users to draw up an Explosion Protection Document to decide whether hazardous area motors are needed, assessing each area of the plant for hazardous gas or dust and dividing the plant into zones. An area can be declared safe only as the result of a risk assessment. Once the plant is correctly divided into zones, the appropriate equipment for each zone can be selected.

Because of the addition of dust within ATEX, the new directives apply to a large number of industrial sites which were not regulated under previous explosive atmospheres legislation. These industrial sites include power plants using coal or organic material, timber industries, food manufacturing and many other sites.

Global impact

Safely running motors and low voltage AC drives in dusty environments

Although the ATEX directives are only applicable within the EU, they are based on European standards which have lead to their principles being demanded by users throughout the world. However, ATEX approval is generally not recognized outside Europe and industries with hazardous areas should investigate which approval schemes apply locally.

The Zone system is used worldwide, except in the United States, where a system of Classes and Divisions is used to denote hazardous locations. The Classes are based on the type of hazard and the Divisions on the risk of explosion that the material poses. However, some US manufacturers have started referring to Zones to enable them to sell equipment to end-users in the EU.

The right protection

It may be tempting to try and simplify the risk assessment by using a blanket zone to cover the entire site but this could be a mistake. More expensive, over-protected equipment will have to be bought, installed and inspected. The use of over-specified equipment can have long-term financial implications, as the maintenance and repair obligations under ATEX depend on the Category of equipment, not the Zone it is installed in. Blanket zoning also raises a suspicion that the risk analysis may not have been carried out in sufficient detail.

Although the ATEX directives have been in force since 2006, many companies still seem unaware of their obligations and of the dangers posed by combustible dust. As the law now requires companies to work with prevention, industrial disasters caused by combustible dust explosions should hopefully become a thing of the past.