The highest risk of explosion is present particularly in production processes that involve dust. Under ideal conditions, literally any dust may explode – grain, flour, starch, sugar, milk powder, cocoa, coffee, coal or aluminium dust and other types of dust. And sometimes, very little is necessary for an explosion to occur – the right concentration of dust, turbulences, a source of ignition or, frequently, spontaneous combustion. The principle behind spray drying lies in the transformation of liquid into dry power in a single manufacturing process during which dust particles prone to spontaneous combustion originate. The series of accidents, fires and explosions that occurred in the past made experts turn their attention towards the development of systems and devices that could protect industrial technology from such incidents.
The Risk of Spontaneous Combustion is Significant
Potential causes of spontaneous combustion include dry bulk. Whether the temperature critical for spontaneous combustion occurs depends upon physical properties of the dry bulk in question, particularly on the heat release kinetics, the thickness of the dry bulk layer, heat exchange conditions with the ambient environment and the time period for which the critical heat conditions last. Fires or explosions may also be caused by mechanical impurities present in poorly filtered air brought into the dryer. All the dryer friction surface areas represent a risk (especially those on nebulizers, ventilators and turnstiles) as well as electrical sparks that originate, e.g., by friction between soiled fan blades and the cover or by static electricity. We also have to be aware of the fact that any individual component of the dryer may be contaminated by an ignition source.
For every substance, base explosion limits have been determined. Explosiveness of a particular substance depends upon the size of its dust particles, their concentration and degree to which they have been churned up in the environment along with the amount of oxygen present in the ambient environment and its temperature. The finer and drier the powder and the higher its temperature, the more combustible the substance and the greater the resulting explosion. Layers of sediment dust present in the piping, cyclone separator or filters that can be churned up in various ways, is not the only danger. Any reduction in the flow rate and any increase of the ambient temperature due to energy outages, poor filtration of air or poorly cleaned dryer, for instance, is also risky.
Quality Prevention Forestalls Explosion
Spray dryer explosions may not only damage the factory but also kill. Companies should not underestimate this fact and should focus on preventative measures as intensively as possible. Under Directive 1999/92/EC of the European Parliament and of the Council (ATEX 137), spray dryer operators must undertake an explosion risk analysis. RSBP offers a comprehensive analysis of both the operational conditions of individual drying technology components and the explosion characteristics of the dust that originates in the manufacturing process. This analysis results in a detailed classification of the environment based upon fire/technical and explosion parameters: zones where explosive atmosphere may occur are classified along with the equipment that may be used there, potential ignition sources are identified and preventative measures proposed.
Some measures are organizational. However, in practice, the risk analysis frequently makes it clear that passive protection must be installed in the dryer. Such structural explosion protection elements can suppress any explosion and eliminate it inside the particular piece of equipment or release it in a safe manner. In addition to its technical parameters, the selection of the explosion protection system also depends on the location of the dryer in question.
How to Secure Spray Dryer against Explosion?
Passive explosion prevention used in spray dryers consists in various forms of structural protection. This means, elements providing for the venting, isolation and suppression of any explosion are installed. Before these explosion protection elements are put in place, the dryer’s structure must be tested to determine whether it can withstand explosion pressure. The degree to which a particular dryer is resilient to the explosion pressure should be indicated in documentation provided by the manufacturer or supplier of the particular drying technology. If these values are missing, the resilience must be tested in practice using a test static strength calculation concerning selected parts of the technology used. After the pressure resilience is verified, the elements of explosion protection are installed.
Such elements may include a comprehensive explosion protection systems for spray dryers offered by RSBP to its clients. The company’s focus lies on the protection of industrial facilities that are subject to the risk of explosion. The system whose technology is based upon the RSBP’s know-how offers two basic options for explosion protection: explosion suppression and explosion venting. In both cases, however, explosion isolation elements are also used.
Explosion Isolation and Suppression is a suitable option for dryers located inside buildings where explosions may not be released outside – into a safety zone – or for locations where toxic material is handled. In these cases, a high-rate discharge system (HRD) that suppresses explosions should be installed along with HRD barriers. These can separate the explosion from the dryer’s drying chamber and the downstream technology: for instance, by means of a cyclone separator and a separating filter. The HRD system is a well tried system used to suppress explosions. It detects the initial explosion within the equipment it is installed on and rapid extinguishes any fire within the protected space. The system’s reaction time counts in milliseconds. It reduces the explosion pressure inside the equipment under its pressure resilience limit thanks to which the equipment is not destroyed. The system consists of three key elements: a pressure detector, a HRD container featuring a valve and a control unit. CONEX, the control unit used, also communicates with the master control system. This allows for an immediate isolation of the technology segment concerned from the rest of the equipment. Not only will explosions leave the technology untouched, there will hardly be any impact on a great amount of the product manufactured. Thus no significant threat to the assets and human lives occurs. Only the dust inside the unit is eliminated. After the unit has been cleaned and checked, it can be used again.
Flameless Explosion Venting is another system that can be installed inside buildings. Under normal conditions, the equipment’s escape hatch is covered by a membrane. If the equipment being protected is located inside a technology unit, an additional device for flameless explosion venting is installed. This ensures that the flame front does not affect the factory hall. Under common explosion venting, temperatures up to 2600 °C may be reached. Using a specially constructed flame filter, the device cools the flame and combustion products to a level safe for both the technology and persons present. This device does not need any operator to function properly.
The second option of explosion protection consists in Explosion Venting. It is suitable particularly for spray dryers which allow for the explosion to be released outside the building using the so-called deflectors or for cyclone separators and filters located outside the equipment, within a safety zone. In both cases, the equipment’s escape hatch is covered by a membrane. When the operational pressure level is exceeded inside the equipment, the membranes on its casing open and release the pressure and flames from the endangered area outside. Thus, the drying technology is subject to pressure levels lower than its pressure resilience limit and no destruction occurs. Explosions may be released only into safety zones.
Effective Protection of People and Assets
Passive explosion protection used with drying technology is a key part of effective safety solutions for both persons and assets in industrial operations. Operators should always discuss any prevention-related issues with specialists that possess extensive experience. RSBP, headquartered in Ostrava, is one of them. It is an expert firm that has been focusing on comprehensive security measures for industrial facilities – specifically, fire and explosion prevention and protection – since 1992.
It has devoted much of its time and energy to its own research and development projects and so it can answer even very specific needs clients from a range of industrial sectors may have.
RSBP offers effective systems for perfect explosion protection and the corresponding modification to the industrial equipment, where necessary. “Safety Comes First” cannot be discarded as an empty cliché.Click here for more information about RSBP Click here for related articles and newsSearch for explosion protection equipment manufacturers in our equipment guide
Sign up for Explosion Protection & Process Safety E-Updates
Recent Explosion Protection and Process Safety e-newsletters
Read our last “Explosion Protection and Process Safety” e-newsletter campaigns here:
RSBP spol. s r. o. provides a wide range of protection products and services in the field of fire and explosion protection and prevention. The company designs and completly solves safety of industrial plants and separate technologies from points of view of fire and explosion prevention, risk analysis, engineering, and documentation according to ATEX ...