Key points
Mark Shannon from BS&B Safety Systems discusses the importance of dust testing when taking protective measures against dust explosion risk
Why is dust dangerous in process industries?
In any industry that handles or stores organic materials for processing, there is always a risk of dust explosions occurring. A myriad of powdered or ground organic materials ranging from carbon to milk powder are prime constituents for ignition and fuelling of ensuing primary explosions within process facilities.
Taking every precaution to ensure that such explosions don’t happen is of paramount importance for the protection of personnel, equipment and facilities.
Dust explosions occur when an ignition source ignites combustible organic material that is fuelled with oxygen in a closed atmosphere. This can take place inside a grain silo, process or storage enclosure, or even originate in pulverising or grinding process equipment. It takes only milliseconds for a violent explosion to occur after a rapid pressure rise in process equipment.
Depending on the type of equipment or vessel that holds the materials, certain fail-safes are implemented to stop an explosion occurring. This is where a thorough understanding of dust characteristics becomes important.
In the UK and Europe, there is a requirement to identify any potentially explosive substances in the workplace. In the UK this requirement is governed by the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). DSEAR classifies dangerous substances as those used or present at work that could, without adequate controls, cause harm as a result of a fire or explosion.
In Europe, ATEX is the common name given to two European directives that specify what controls employers should use to prevent explosion risks, which are divided into danger ‘zones’.
Both DSEAR and ATEX recognise dusts as explosive risks.
As far as BS&B are concerned we offer various solutions to protect against dust explosions. Before we recommend the right ones to a client, the very first question we ask is “what type of materials are you handling in your facility?”
Different types of dust have different particle sizes, properties, ignition temperatures, and ignition sources, from each other. Dusts are given explosion severity classifications; St1 to St3. ‘Not specified’, means the material is non-explosive and St3 is the most explosive type of material.
Most people think that dust is, for the most part, the same. However, the range and qualities of dangerous dusts is expansive across the process industries and include organic materials that most of us would not even consider as an explosive risk. Nevertheless, for the purposes of manufacturing and processing, a closer analysis of dust characteristics is required to specify operational conditions and protection.
For example, according to the Health & Safety Executive’s guidelines Safe Handling of Combustible Dusts, wheat has an average particle size of 80 µm; wood flour is 65µm, tissue paper is 54µm; magnesium at 28µm, which has a classification of St3 and is highly combustible.
Dust testing is designed to identify two key performance characteristics of dust, which in turn influence explosion protection equipment design and their application as well.
- 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)
Returning to the very fine and highly explosive magnesium powder as an example, the KSt value can be 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.
Testing under laboratory conditions will tell you what St class your dust is, based on its KSt value. It will also reveal the dust’s performance under specific equipment operations and processes; is it being milled, ground, poured or dried?
What is its Minimum Ignition Temperature (MIT) while being subjected to these processes – in other words, what temperature will the dust withstand before it becomes an ignition risk?
All aspects of the dust’s behaviour in relation to its process environment can be determined to inform the correct precautionary measures against a potential explosion.
Match the protection to the type of dust
Because dusts have different explosive properties, these are often handled and stored in different ways and locations, the protective measures in each location will be tailored to meet the associated risk.
Facilities can be protected from the risk of dust explosions by various means depending on your application. An explosion protection consultant will advise on the best solution for your facility:
Spark Detection devices:
Sense hot particles, sparks and flame that could become the ignition source for a fire or explosion. They can include automated shut-down systems to interrupt the feed of combustible material along the process equipment. All processes may be monitored by an operative via a control panel to assess any further risks. Spark detection is particularly useful to manage fire and explosion risks in process equipment such as dust collectors, bins and silos.
Chemical Suppression systems:
Designed to detect the pressure wave at the very start of an explosion and deliver dry, inert chemical extinguishing agents into a developing internal deflagration. These suppression and isolation systems can be activated either by pressure, optical or vent sensors. Any deflagration travelling through interconnected equipment is quickly and efficiently extinguished, preventing any spreading explosion damage.
Flameless vents:
The preferred passive method to relieve explosive pressure in a process or storage vessel containing combustible materials. It is not always practical or safe to vent the pressure and flame to a particular area, therefore these vents intercept, quench and retain all burning materials, preventing them from expelling into the atmosphere. They are particularly useful for dust collectors, bins and bucket elevators.
Isolation:
Designed to prevent any kind of deflagration or explosion from spreading to other parts of process equipment. There are two methods of isolation used. The Passive method does not require detectors, or control and indicating equipment. These may be arrestor mesh, rotary valves, lock valves, rotary screws, flap valves or diversion valves.
The Active method is activated by detectors and a control and indicating equipment, which are parts of the system, such as pinch valves, chemical Isolation or fast acting valves.
Even though chemical suppression is used to reduce the initial building pressure in an enclosure, some hot gasses will attempt to exit to areas of lower pressure. This causes a flow through the inlets and outlets of the enclosure, hence the need for isolation.
Safety comes first
Of course there is no substitute for exercising good housekeeping in factory facilities. Regular removal of dust accumulations in the factory space is also a vital means of mitigating against dust explosions. Dust that accumulates in eaves, ceilings spaces and beams are waiting fuel to propagate a secondary explosion.
Up to one ton of accumulated dust has been recorded as the fuel for a devastating explosion in a pharmaceutical facility in 2003. Investigators of the disaster said it had collected in the ceiling spaces and workers were unaware that the risk was above their heads.
Conducting a factory survey and dust testing service for clients as a matter of course is essential to achieve optimal dust explosion protection. It’s the first step in ensuring that they receive the appropriate explosion protection solutions for their business.
When it comes to the safety of equipment and personnel in dust explosion risk zones, thorough identification of organic dusts is good business practice.
It pays to know your dusts.
