Mark Shannon from BS&B Safety Systems explains the importance of having efficient spark detection fitted on foundry dust collectors to prevent fire and dust explosions.
In various parts of the foundry environment, the risk of dust explosion is extremely high. Whether it’s the generation of dust from machining or sanding shell moulds or sedentary metal dusts accumulated from the foundry, there is a real and dangerous risk of stray sparks igniting dust in dust collectors; this compels putting the right safety measures in place to protect staff and equipment.
The most usual suspects to protect against are dusts generated from aluminium, bronze, iron, steel and phenolic resin. Different parts of the foundry require controlled protection to combat the dangers of dust ignition and in the worst case, explosion.
We’ve seen examples around the world, where accidents with igniting dust collectors have seriously injured staff. In 1999 a phenolic resin dust explosion killed three people in an iron foundry in Massachusetts (USA). In 2011, five people lost their lives to a suspected iron dust explosion in a foundry in Tennessee (USA).
With years of experience in providing protective solutions for dust explosion risk, BS&B Safety Systems has visited and surveyed factories in every process industry all over the world. While owners are making the effort to comply with DSEAR and ATEX regulations to ensure their staff’s safety, the execution of preventive measures has not always been correct or complete. In any industry where a large amount of dust is generated, mitigating dust ignition correctly means understanding the nature of what is required.
How combustible is your dust?
Most people think that dust is, for the most part, the same. However, different types of dust have different particle sizes, properties, ignition temperatures and ignition sources.
Dusts are given explosion severity classifications – St1, St2 and St3. ‘Not specified’, means the material is non-explosive and St3 is the most explosive type of material. Returning to the explosive phenolic resin, its dust is measured at a variable St1-2, so any protection measures need to take that into account. Iron dust is measured at St1. Aluminium comes in at the highest at St3. What these figures say is that wherever these dusts are generated and handled, there must be measures taken to detect or suppress spark ignitions that can be fuelled by these dusts.
Laboratory dust testing is designed to identify two key performance characteristics of dust, which in turn influence what level of explosion protection is needed on and around equipment and a facility:
- The first measures maximum pressure of a dust explosion (Pmax in bar)
- The second identifies the speed of the rise in explosive pressure (Kst in m/sec)
Very fine and highly explosive magnesium powder, as an example, has a Kst of approximately 500 bar m/s. In comparison, wheat flour only has a Kst value of around 110 bar m/s – a significantly slower rate of explosion pressure rise. Nonetheless, it could remain a danger as we have seen with levelling explosions in grain and flourmills.
So how does a dust explosion occur?
The originating causes of dust explosions may be varied; a spark, friction on badly maintained machinery, an electrical fault, or grinding and milling friction. However, the fuelling and propagation of a possibly fatal explosion is almost always caused by agitated or suspended dust in the atmosphere.
The most destructive explosions are caused by the five key elements known as the ‘explosion pentagon’; when ignition, rising pressure in a confined area, fuel (suspended dust), dispersion, and oxygen, converge. When these elements come together in an enclosed area, like a dust collector, with rapid increases in temperature, a deflagration can occur.
This primary explosion can cause a pressure wave carrying with it a flame that disturbs any accumulated dust. Nearby personnel and property are put at great risk of harm when this happens.
Once any other suspended dust is agitated, this is where the extremely dangerous secondary explosion risk is created and can spread through pipes and ductwork until no part of the facility is safe from this travelling explosion risk.
Dust collectors – incubators for ignition
Unfortunately, dust collectors are very susceptible to fires started from stray sparks. So, one way of preventing sparks from entering a dust collector is to install a spark detection system. Spark detectors fitted in the ductwork identify and extinguish any sparks before they reach the filter media in a dust collector.
Advanced spark detection and extinguishing systems are designed to detect hot particles and sparks which could become the source of a fire or explosion. It’s not hard to imagine the potential cost and downtime caused by a few hot particles entering a dust collector holding combustible material; a dust explosion could also happen.
Using infrared detectors, spark detectors search for elevated temperature particles. When hot material is detected, the risk is reduced by:
- Activating water spray nozzles placed downstream of the detection point.
- Activating automated shutdown of the process to end the feed of combustible material.
- Providing a signal to activate other control devices including shut-off valves.
Advanced spark detection systems operate in a continuous automatic mode, with critical circuits supervised in readiness to raise an immediate alarm in case of application difficulties. The spark detectors also use accurate type SDN or SDD sensors to detect sparks and hot particles.
These SDN/SDD sensors activate control circuits connected to the automatic water extinguishing module without interrupting the air flow.
Identifiable sources of sparks and other hot particles are extinguished in-situ prior to reaching the dust collector.
The installation of a spark detection system immediately reduces fire and explosion risk, and is a vital part of dust explosion protection.
Poor or no risk management
Having no spark detection allows potential ignition sources to reach the dust collector. It’s important to seek advice from an explosion protection consultant who can provide the right advice on what controls are needed for a facility. For example if installation of safety equipment is not based on measurable and current process requirements, a facility may be under-protected.
One important step prior to the installation of safety equipment is to carry out a combustible dust test and thereafter review combustible dust classifications (Kst and Pmax levels) at regular intervals as process operations and materials change. This provision should be incorporated into management of change procedures.
For more information on BS&B Safety Systems explosion protection solutions visit: www.bsbipd.com