How dust can affect people, process and plant

In a series of three articles, Finch looks at the use of powdered ingredients used in the food industry, and how if not controlled effectively, the dust from these can adversely affect people, processes and plant. The first article in the series covered people, and in particular health.

Posted

20.08.2024

Written by

Tristan Pulford

In this article the focus is on process safety, specifically with regards to the explosive risks and hazards associated with the dust handling.

Any dust with fine particles, which is flammable can cause a potentially explosive atmosphere when appropriately mixed with Oxygen and at the correct concentrations. Typically, any organic materials will be potentially explosive as a dust and hence fall under the Dangerous Substances and Explosive Atmosphere Regulations (DSEAR). These regulations primarily deal with managing the risks of explosion through physical, practical and organisational methods.

Taking a step back, most people are familiar with the fire triangle, which says that for a fire to occur you need a fuel source, oxygen and an ignition source. For a dust explosion we consider the dust explosion pentagon:

To prevent the potential for explosions these 5 elements must be controlled.

Fuel: In the first instance for a dust explosion to occur, dust must exist which is flammable, and has a small enough particle size to allow for the propagation of an explosion. Typical examples of this in the food industry are: flour, sugar, grain dust, tea, various spices etc. Using the hierarchy of control the first question is, is the material required or can the material be substituted for a non-flammable substance? In most cases this is not possible as the process requires the substance. There is also a duty to reduce the amount of the substances stored to as little as reasonably practicable.

Oxygen: For an ignition to occur there must be oxygen present. Frequently this cannot be controlled and will be present. In Silos and other containers, there is the potential to use an inert gas (typically Nitrogen) which prevents an explosive atmosphere being present, due to the lack of Oxygen. It should be noted that this can create other hazards i.e. asphyxiation so should be carefully considered as part of a risk assessment.

Ignition Sources: Any explosion requires an ignition source, which comes in a variety of forms such as hot surfaces, flames, hot gases or sparks, Mechanically generated sparks, electrical sparks, static electricity, lightning, electromagnetic waves and ionising radiation.

One of the key controls that DSEAR requires is that equipment located within a potentially explosive atmosphere is specified and designed appropriately so that it can not form an ignition source.

Care must also be taken that portable equipment is not brought into the area to create a possible ignition source in an explosive atmosphere area, especially when considering maintenance activities.

Dispersion: For an explosion to occur the dust cloud must be airborne. This may be as part of a process, for example conveying of dust, or it may be accidental such as bumping a surface with a layer of dust on it. One of the reasons that a dust explosion is so hazardous is because an initial explosion will disperse any other dust into the area, creating further secondary explosions. This is one of the main reasons for ensuring dust deposits are minimised by control at source and good housekeeping.

Confinement: The main difference between a fire and an explosion is confinement. If a cloud of potentially explosive dust was ignited in the middle of a field, there would be a fireball. If the same amount of dust was ignited within a small room, the confinement of the walls would allow the pressure to build causing the potential for an explosion. Typically, any controls in place at this stage are mitigative such as explosion panels/ vents.

In the ideal case, many of the factors discussed above can be suitably controlled at the design phase, by designing equipment appropriately such that these elements are either eliminated or suitably managed. Finch can provide assistance with the verification/ validation of a design, along with documenting the results in a design risk assessment which can be transposed into an explosion risk assessment as part of DSEAR.

In this article, we cover the requirements of DSEAR in more detail, along with the duties on the employee.

One key consideration is that many of the controls required to prevent an explosion are similar to those which are required to protect the health of any person in the area. Finch can provide expert guidance considering both the health and the explosion hazards associated with dusts. If you require advise or assistance with any of your dust related hazards please contact Tristan Pulford at [email protected]