Kevin Spiess from BS&B Safety Systems discusses solutions for protecting pharmaceutical facilities against dust explosions
UK pharmaceutical manufacturing is a substantial contributor to the economy. In 2013 it accounted for 9% of the UK’s total manufacture output, with the total pharmaceutical sector turning over about £60bn. It is an economic asset worth protecting and it’s on the rise.
Yet, in spite of the explosion risk management requirements set out in DSEAR (Dangerous Substances and Explosive Atmospheres Regulations 2002) and ATEX European regulations, the dangers of dust explosions in the pharmaceutical manufacturing sector remain just outside of the safety spotlight. Devastating dust explosions have occurred in pharmaceutical facilities around the world, destroying equipment, injuring or taking lives and halting businesses. It’s not a risk worth taking.
Why is dust dangerous?
During the handling, storage and processing of pharmaceutical raw materials, the explosion risk of bulk powders is always present. Powders can form dust clouds that linger in the atmosphere acting as a fuel for a primary explosion that may be ignited by the smallest of sparks. Suspended dust can ignite and burn rapidly causing a secondary explosion – this is where property and personnel are subject to substantial risk of harm. If such an explosion occurs in a confined space, such as a storage vessel, or a compounder, then a subsequent rise in explosive pressure can literally have devastating consequences.
This was clearly illustrated in 2003 when a major pharmaceutical facility in the US was ripped apart by a chain of propagated dust explosions. This accident claimed six lives and injured 36 members of staff on site. Investigations concluded that powdered polyethylene was the culprit and the ignition originated in processing equipment that used this substance to coat rubber strips. For years, several inches of polyethylene dust had accumulated in the facility’s roof spaces pumped through the ventilation systems, just waiting for the right spark to set it off. It was also surmised that there was an inadequate assessment of the dangers that the dusts posed to make mitigating provisions.
Elements of an explosion
The ‘explosion triangle’ consists of three main elements required to cause a dust explosion: ignition, fuel and oxygen. When one of these elements is interrupted or controlled, an explosion can be prevented. Active pharmaceutical ingredients (APIs) have the potential to release explosive pressure between 9-10 bar and often act as the fuel for an explosion in a pharmaceutical facility. However, the additives used in conjunction with the APIs also have explosive capability, such as starch or lactose. Consequently, the pharmaceutical manufacturing industry needs to manage explosive dust potential of two kinds:
- materials that may cause a deflagration
- materials that have detonation potential (which require much more rigorous controls and fail-safes to prevent ignition and detonation.)
The most common cause of ignition in the pharmaceutical industry is electrostatic discharge; this occurs during mixing, pouring or sieving processes. Coupled with a higher than normal oxygen concentration in the process equipment and the presence of a highly combustible API, the explosive trinity is complete.
How to protect against dust explosions
While some pharmaceutical raw materials have a lower sensitivity or explosivity reaction to an ignition source, others have more sensitive reaction parameters. Dust testing is always recommended to identify three key performance characteristics of organic dusts, which in turn influence explosion protection equipment design as well as their application:
• 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)
• The third measures Minimum Ignition Energy (MIE)
Dusts differ in characteristics and therefore performance. At BS&B we believe that explosion protection solutions depend on good risk assessment and understanding. All dust hazard projects should start with dust testing. In this way an explosion protection expert can make an informed safety recommendation for the facility involved. There are basic explosion safety methodologies which, if correctly applied, effectively mitigate dust explosion risk according to the specialised process needs in the pharmaceutical industry:
Good housekeeping
There’s no substitute for diligently cleaning a process facility. An explosion risk can be caused by the build-up of deposits of combustible dust which may accumulate on surfaces in the processing facility. Accumulated dust may be disturbed by a primary explosion in the process equipment and result in a more severe secondary explosion. The Health and Safety Executive (HSE) in the UK, recommends that as well as a rigorous cleaning regime, the elimination of high-level horizontal surfaces is preferable, e.g. by use of sloping surfaces to minimise dust accumulation.
Particularly in the pharmaceutical industry, the dissipation of any electrostatic ignition sources by bonding and grounding of equipment is recommended.
Chemical Suppression Systems
By stopping a deflagration at the outset, further explosive damage is immediately interrupted and contained. In the pharmaceutical industry where the raw materials are often costly, depend on highly sanitised handling and have a high Kst value, explosion risk becomes a serious business risk. The contamination or destruction of such valuable raw materials by propagating explosion could result in significant economic loss – an undesirable commercial position to be in.
Advanced chemical suppression systems such as the BS&B System V, are designed to detect the start of an explosion (point of ignition) and deliver dry, inert chemical extinguishing agents into a developing internal deflagration. For the handling of pharmaceutical materials that are subject to deflagration ( as opposed to detonation) these suppression and isolation systems can be activated by various means, whether triggered by pressure, optical or vent sensors. A possible deflagration travelling through ducts, piping and connected equipment is quickly and efficiently extinguished, thereby preventing any spreading explosion damage.
Flameless venting
In the worst case, should an explosion actually occur inside a process or storage vessel containing combustible materials, a rapid rise in pressure can result putting staff and equipment at risk. Explosion vents are the preferred passive method to relieve such explosive pressure however it is not always practical or safe to vent the pressure and flame to a particular area.
The most advanced flameless 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. A process shutdown can also be activated to limit the intake of additional fuel to other process areas.
Chemical isolation systems
While pharmaceutical process equipment can be constructed to withstand an explosive pressure of 10 bar, the design isn’t usually designed to isolate an explosion should it occur, hence opening the risk to further explosive propagation. Explosion isolation is designed to prevent any kind of deflagration from spreading to other parts of process 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 a control and indicating equipment, which are parts of the system, such as pinch valves, chemical Isolation or fast acting valves.
All the above are well used methods to safely control dust ignition and explosions. However, any pharmaceutical dust explosion risk would always warrant further attention.
Protecting against dust explosions requires due diligence, which informs appropriate preventative measures. To do anything less would invite disaster because just one spark could destroy lives and livelihoods, not just on site but along a vital supply chain.