Containing dust explosions in pharmaceutical processing
30 September 2016
During the handling, storage and processing of pharmaceutical raw materials, the explosion risk of bulk powders is always present. Kevin Spiess of BS&B Safety Systems shares insights into explosion containment using different methods of explosion protection suited to pharmaceutical manufacturing.
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 silo, process or storage enclosure, or even in pulverising or grinding process equipment. It takes only milliseconds for a violent explosion to occur and, in a confined space such as a storage vessel or compounder, then a subsequent rise in explosive pressure can have devastating consequences.
In 2003, the West Pharmaceutical Services plant in Kinston, North Carolina, was virtually destroyed by a chain of propagating dust explosions, with six lives lost and 36 employees injured on the site. The shock wave broke windows up to 300 metres away.
Even though the damage was so great that the exact point of ignition could not be determined in the ensuing investigation, there were various theories expounded about how the explosion started. Investigators concluded the most likely scenario involved powdered polyethylene, and the ignition originating in processing equipment that used this substance to coat rubber strips. This equipment was known to have suffered several internal fires previously, including one that generated enough force to blow off a mixer door.
Regardless of the mystery, there was no doubt, that an originating spark was the cause. Dust cloud accumulation in the atmosphere did the rest.
First test your dust
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). Both DSEAR and ATEX recognise dusts as explosive risks.
Before applying any kind of explosion protection, we always recommend to our clients that they must test their dust. In the pharmaceutical industry, it’s not just question of protecting against dusts with high explosibility ratings, (measured as Pmax), even though that is a primary consideration. One must also take into account the nature of the substances being processed and whether their escape into the atmosphere poses a toxic risk, as well as an explosive one. There may be steroids, hormones or narcotics that through direct exposure could cause harmful effects to employees.
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.
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)
For example, according to the UK Health & Safety Executive’s (HSE) guidelines Safe Handling of Combustible Dusts, wheat has an average particle size of 80 µm; wood flour is 65µm, tissue paper is 54µm and magnesium at 28µm, which has a classification of St3 and is highly combustible.
Magnesium powder has a KSt value 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, as we know from high profile grain mill explosions, it is still a significant explosive danger.
In comparison, there are common substances used in pharmaceutical manufacture with relatively similar KSt values, for example:
• Cellulose: 229 bar m/s
• Organic pigments: 73-288 bar m/s
• Dextrin: 168 bar m/s
• Corn starch: 202 bar m/s
The above substances have KSt values significantly higher than wheat flour dust, which we know can be a devastating explosion fuel. From these measurements, it’s not hard to see the obvious risk in the pharmaceutical industry.
It is also necessary to determine what the dust’s Minimum Ignition Temperature (MIT) is, while being subjected to any 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 risk
Assuming that you have carried out dust testing and determined all its key performance characteristics, then it’s time to apply the correct protective solutions. There is a myriad of explosion protection solutions available and they are effective when handling the majority of organic dusts.
As we have established, dusts have different explosive properties, and are handled and stored in different ways and locations. Therefore, the protective measures in each location should be tailored to meet the associated risk. The different options include the following:
Spark detection devices: sense hot particles, sparks and flames 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, such as bicarbonate of soda, into a developing internal deflagration. These suppression 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.
Flame free 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.
One or a combination of these measures may be applied in a factory or facility that processes dust-generating organic materials.
In pharmaceuticals, it’s all about containment
Whichever method of protection is employed, the primary objective for explosion safety in the pharmaceutical industry is achieving containment. The threat of an explosive fire and toxic contamination means that it is safer to keep a lid on things. A combined solution is usually extremely effective.
For example, advanced chemical suppression and isolation systems often work in tandem. They are designed to detect and extinguish an explosion at a very early stage, (the point of ignition), and extinguish it using chemical agents.
Chemical extinguishing systems offer extremely efficient and rapid protection. They can easily prevent a deflagration from travelling through ducts, piping and connected equipment thereby preventing any spreading explosion damage. They can be employed most effectively when there is no risk of compromising the quality of pharmaceutical raw materials when they are exposed to a chemical extinguishing agent during a primary explosion event.
In instances where the use of chemical extinguishing agents may taint expensive, raw materials in the equipment, then mechanical isolation can step in to save the day. It’s just as efficient at preventing any kind of deflagration or explosion from spreading to connected process equipment, just like chemically-driven methods.
However, mechanical isolation only works when a primary explosion is allowed to take place. It may sound counter-productive, but in the long run it could save on substantial material and economic loss. This is how it works:
* Mechanical isolation is a cleaner means of explosion protection. There are no additional agents being fired into the process equipment to stop an explosion and therefore no mess to clean up.
* Toxic dusts are prevented from escaping, in spite of the initial explosion taking place. Explosive pressure is diminished by use of isolation valves. So when an explosion actually occurs, the pressure travels into either into a single or double acting valve (depending on your required specification), which seals the pressure wave inside the valve and prevents it from spreading throughout the equipment.
There are two methods of isolation:
* Passive Isolation: 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.
* Active Isolation: Activated by detectors and electrical control and indicating equipment. Examples would be pinch valves, chemical Isolation or fast acting valves.
Advanced isolation valves are manufactured with pharmaceutical-grade housing which makes it easy to clean any surfaces that come into direct contact with the products being processed.
All valves have an explosive rating that his higher than that of the dust being processed. Hence the importance of dust testing in the first instance to determine what that maximum Pmax would be.
The other vital reason for ensuring primary explosion containment in equipment is the risk of explosion propagation from dust in the atmosphere.
There is no substitute for exercising good housekeeping in factory facilities. Regular removal of dust accumulations in the factory space is a primary safety precaution against explosions but it’s commonly overlooked as a practice.
Dust that accumulates in eaves, ceilings spaces and beams are waiting fuel to propagate a secondary explosion. Up to one ton of accumulated dust was recorded as the fuel for that devastating pharmaceutical site explosion in 2003. Investigations revealed that dust had collected in the ceiling spaces and workers were unaware that the risk was even above their heads. So when the primary explosion occurred, uncontained, there was fuel waiting in the rafters to proliferate it. As we know, the consequences were fatal for many employees.
Whether dealing with valuable or volatile raw materials, or not, there are explosion protection options that tick the boxes relevant to each respective facility. Always carry out necessary due diligence and risk assessments to inform appropriate preventative measures. A crucial supply chain, from manufacturer to end-user, depends on taking every precaution to ensure safety and uninterrupted productivity. Lives literally depend on it.
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