How to prevent dust explosions

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Pavel Jílek reveals what you need to know about dry chemical supression systems

What can be done when explosion venting is not suitable and explosion isolation is not enough? And if that’s not complicated enough, what if your company is in the pharmaceutical, chemical or food industry, so increased sanitary standards are a must? Sometimes, the only option is dry chemical explosion suppression to secure
a production process properly.

Quick Introduction

First of all, the definition of combustible dust varies. The most well-known is likely to be from the American NFPA 652-2019 standard, where combustible dust is defined as “a finely divided combustible particulate solid that presents a flash-fire hazard or explosion hazard when suspended in air or the process-specific oxidising medium over a range of concentrations.” The European standard 60079-10-2 defines it as: “Finely divided solid particles, 500 μm or less in nominal size, which may be suspended in air,.., can burn or glow in the air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures.” Put simply, it is any material that has the ability disperse in the air and combust after exposure to an ignition source.

It is essential to know that the majority of all industrial dusts are combustible. Even if the material is not combustible on its own, if divided into particles of the right size and concentration, explosion may occur. Combustible dust may include materials in the physical states of powders, flakes, fines, fibres, etc. So overall materials that can cause dust explosions include: natural organic material (grain, linen, sugar, etc.); synthetic organic materials (plastic, organic pigments, pesticides, etc.); coal and peat; and metals (aluminium, magnesium, zinc, iron, etc.) Silicates, carbonates, nitrates, and substances such as Portland cement, sand, limestone, and the like can be excluded from the list.

Risk, Hazard, Danger And Fatal Consequences

Combustible dust explosions differ from gas explosions and can be much more devastating. If a mixture of dust and air ignites, the risen pressure wave can swirl up existing dust layers where new dispersed dust ignites. Under specific conditions, this secondary explosion can create a chain reaction, leading to a series of explosions and destroying entire buildings or facilities.

Dust explosions occur directly in technological equipment during milling, drying, pneumatic or mechanical transport, material or product storage, or filtration. Unfortunately, the consequences of such explosions can be fatal. The highest risk of explosion primarily threatens workplaces, where dust is handled as the main product (e.g., production of synthetics) or as a byproduct during production (e.g. waste). There are three major ways to secure a workplace against the consequences of explosions: venting; isolation; and suppression. This article focuses on the advantages of explosion suppression – dry chemical explosion suppression, to be exact.

How It Works: Suppressing An Explosion

A suppression system is designed to detect an explosion in the incipient stage, either by a rise in pressure or the presence of a flame within the protected enclosure. Usually, these systems consist of four basic components: the detector(s); control unit(s); suppressant storage container(s); and suppressant dispersers. The control unit then actuates the suppressant dispersers to envelop and suppress it to prevent the full impact of the deflagration from developing.

High Rate Discharge Saves The Day

Dry chemical suppression systems are an advanced method to secure every type of industry from the danger of explosions. The technique of explosion suppression may be considered for any enclosure in which flammable gases, vapours, mists, or combustible dusts are subject to deflagration in a gas phase oxidant. The extinguishing medium, usually sodium bicarbonate, offers safe solutions in industries such as food or pharmaceuticals, that require higher sanitary standards.

The explosion suppression system is designed to provide an active method to protect process system equipment from a dust explosion hazard. The chemical suppression system consists of a controller, pressure or optical detectors and high-rate discharge (HRD) cylinders. The principle of operation for the HRD system is the detection of the pressure rise during the initial stage of an explosion, followed by fast injection and homogeneous distribution of an extinguishing agent into the protected vessel.

As deflagration develops, the pressure inside the vessel will begin to build rapidly. The explosion pressure detector will detect the rate of pressure rise over a specific time period and activate the HRD. Once activated, the HRD bottles will disperse suppressant into the protected vessel inhibiting the explosion from further developing, thus reducing the pressure and preventing any vessel breach.

It is highly recommended to add extra protective devices on connected pipelines, e.g., mechanical devices such as explosion isolation back flap or HRD barrier. By doing so, the technological process as a whole is protected.

Pavel Jílek is senior project manager at RSBP