Controlling the risk of a dust explosion

Kevin Spiess, EMEA Sales Manager for Explosion Protection at BS&B Safety Systems, looks at how companies that handle, store and process solids and bulk materials can protect themselves and help prevent dust explosions

Any industry which handles, stores and processes solids and bulk materials is at risk of the fine organic dusts from these substances burning and exploding. Every year, premises are severely damaged or destroyed by dust fires that usually start in dust extraction equipment. Just a few burning embers entering a dust collector, bin or silo is enough to trigger a dust explosion which would typically lead to fire.

The effects of accidental fires or explosions can be devastating in terms of lives lost, injuries, damage to property and the environment, and to business continuity.

Typical applications which are associated with dangerous levels of dust

The following processes are likely to produce high dust concentrations:

• Moulders
• Routers
• High speed machining
• Wide belt sanders
• Wood processing of composite board
• Centralised dust extraction systems
• Shredders
• Hammer mills

What does it take to cause an industrial explosion?

A fire or explosion results from ignition of a combustible material (dust, gas or vapour) when mixed with oxygen in the air. When this takes place inside a process or storage enclosure, the rapid rise in pressure could cause a potential explosion in milliseconds, placing personnel and property at risk.

Most materials handling, processing and storage equipment is not designed to resist the pressure of an expanding flame-ball which proceeds below the speed of sound in air – known as a deflagration – as compared to a detonation, which exceeds the speed of sound in air.

Protection measures

An explosion risk assessment will typically recommend a series of protection measures to be implemented. These range from investment in protection equipment to improvements in housekeeping to eliminate the build-up of deposits of combustible dust which may accumulate on beams in the factory. These may be disturbed by a primary explosion in the process equipment and result in a more severe secondary explosion.

Codes and standards are now very clear in requiring isolation of vented equipment to prevent secondary explosions, which typically have much greater potential for damage and destruction.

We have developed a bespoke spark detection and extinguishing system designed to detect sparks, glowing embers and hot particles that might become the ignition source for a fire or explosion if allowed to travel on through pneumatic ductwork and conveyors towards other material handling equipment.

By preventing sparks, embers and hot particles from reaching dust rich downstream process equipment such as dust collectors, bins and silos, both fire and explosion risks can be managed.

Using infra-red detectors, the system detects the radiated light emitted from sparks, glowing embers and hot particles travelling past the detection point and activates control circuits. The greatest sensitivity occurs when these sensors are employed in a dark area such as closed ductwork, although optional daylight detectors are also available for use on open conveyor belts and for applications where light is likely to be visible. Upon detection this system provides several options to manage the ignition risk:

• An electrical signal generated by the sensor activates control circuits typically used to operate an automatic water extinguishing curtain. Sparks can be extinguished without stopping production.
• An automated shut down of the process can prevent the feed of combustible material.
• Alarm and control systems can be activated upon the detection of hot particles for other control devices such as diverter valves.

The system detectors and control unit are ATEX certified for use in a dusty working environment. This means there are no expensive costs for additional wiring to run from detectors to remote mounted control units, and plant personnel have direct access to the status of the equipment. In the event of an alarm condition, the operator can see immediately which process is at risk. A web based monitoring system, which allows plant managers to receive system alarms and faults to their smart phones, as well as monitoring the systems live on the internet, is also available.

In addition to this preventative system, systems for protection are available, should an explosion occur. A chemical isolation system isolates an explosion and prevents it from affecting interconnected processes. If unprotected, the ductwork and piping, as well as all the connected vessels and equipment are at risk. This system is used in combination with explosion protection equipment on each vessel, bin, or dust collector and could include chemical suppression, explosion venting or indoor venting.

Regulations, codes and standards

Because of potential health problems, laws exist to ensure employers in the EU protect their workers from being harmed by dangerous substances in the workplace. Employers must carry out risk assessments, and take steps to ensure they prevent or adequately control exposure. It is important to include in the assessment foreseeable incidents and maintenance work and plan for measures to be taken in these circumstances.

ATEX is the name commonly given to Directive 94/9/EC which provides the technical requirements to be applied and the relevant conformity assessment procedures before placing equipment and protective systems intended for use in potentially explosive atmospheres on the European market.

A four-step approach to risk assessment

• Make an inventory of the materials used in workplace processes and those generated by the process such as welding fumes or organic dust.
• Collect information about these substances, i.e. the harm they can do and how this can happen. Safety data sheets (SDS), which must be provided by the supplier of a chemical, are an important source of information.
• Assess exposure to the identified dangerous substances, looking at the type, intensity, length, frequency and occurrence of exposure to workers, including combined effects of dangerous substances used together and the related risk.
• Rank the severity of the established risks. This list can then be used to draw up an action plan to protect workers.

Business benefits

By investing in explosion protection, organisations can protect themselves economically, follow appropriate health and safety standards, and protect workers from potential risks.

For more information about protection and prevention systems against the risk of explosion please contact BS&B Safety Systems by telephoning +44 (0) 161 955 4202 or email sales@bsb-systems.co.uk or visit www.bsbipd.com

Published in Solids and Bulk Handling – February 2013  Controlling the risk of a dust explosion