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Dust extraction and centralized vacuum cleaning systems vary in their design, performance and costs. Different companies have different approaches to their design, however, there are some basic rules that must be followed if these systems are going to be immediately effective and avoid future problems.
Many of the products used and produced in pharmaceutical production contain hazardous ingredients that can be toxic because of the quantities being processed. Some, in certain circumstances, are also potentially explosive and, therefore, pose hazards to personnel and the plant in which they are being processed.
The capture and safe handling of the ingredients used within the pharmaceutical manufacturing process is essential. Extracting hazardous dusts at their point of generation and conveying them to a correctly designed filtration system with in-built safety features are necessary to protect both personnel and plant.
Because of the size of tablets and the raw material costs associated with their production the amount of dusts generated tends to be low. However, any dusts that are generated from the production process must be extracted to ensure a safe, efficient and clean work area.
Some manufacturers of pharmaceutical production machinery build a dust extraction facility that must be connected to a dust extraction system into the design of their production equipment. Because of the size and complexity of the production machines, the pressure differential caused by the small diameter extraction ducts provided, causes a high pressure differential, which must be included within the design calculations of the resulting dust extraction system.
When the dusts have been captured within the extraction air stream, they can be successfully conveyed to a dust collector under a negative pressure at the correct conveying velocity to keep the dust in suspension.
A CVC system filter separator.
A dust collector filters the dusts from the conveying air and enables the dusts to be dispensed at the base of the unit and the conveying air to be filtered through filtration media before being released into the surrounding atmosphere.
It is imperative that the correct amount of filtration media is provided within the design of a dust collector to ensure the air-to-media face velocity is kept within acceptable limits.
The selection of the correct media, which can be provided with various features built into it, such as antistatic characteristics, is imperative. If pleated cartridge media is employed it is important that the depth of the pleat and the distance between the tips of the pleated media is kept 'open' to ensure that dusts are released from the media. This ensures it will not become blinded with trapped product. If the media becomes partially blinded the face velocity of the useful media will be increased and if this is increased beyond acceptable limits it will cause too high a pressure drop across the media. This leads to a reduction in the efficiency of the extraction capability at the intake/process end of the system as the effective displacement of air by the fan is automatically reduced because of the increase in resistance on the system.
A pharmaceutical dust collection unit with an indoor explosion unit and rotary valve.
Some filtration media can achieve product emission levels of below 1 mg/m3. Media are most commonly cleaned by the reverse jet technique, which employs high-pressure compressed air being injected into the clean side of the media (on a cyclic basis). This produces a 'shockwave' down the media that dislodges the dusts off its surface, allowing the waste to fall into the collection chamber below the bank of filters.
Although there are many types of dust collectors available in the market place, collectors' employing the reverse jet technique have a proven track record in the pharmaceutical industry because this type of cleaning is 'online' when the system is operating, continuously providing the media in optimum condition.
The quality of the compressed air is important and must be clean and dry to ensure the media do not become blinded. If damp compressed air is present, it will cause blinding of the media when the dusts comes in contact with the media surface.
A pulse jet unit showing cartridge installation from explosion vent area.
Pulse jet dust collectors can have their media cleaned continuously. Alternatively, to conserve compressed air — which can be expensive to generate — the reverse jet cleaning can be controlled by a 'pulse-on-demand' system that operates between high and low pressure differential set points monitored across the filtration media.
'Safe change' is the term given to a procedure where personnel are protected during both the operation of filter cell changing, and the handling of extracted materials during discharge of the product from the dust collector hopper for final disposal. It should always be recommended that personnel be provided with the correct personal protection equipment (PPE) when undertaking these operations.
Fortunately explosions are not commonplace within dust filtration plants and with the correct protection systems in place, personnel and production facilities can be designed to be safe environments to work in. When handling a potentially explosive product, protection systems are put in place to protect the dust collector from rupture. If an explosion occurs in a unit that is not provided with a protection system, it is likely that the dust collector casing would rupture.
A special continuous discharge arrangement for a pharmaceutical project.
Although this is a major concern, it is not the most critical problem: the real devastation occurs if the collector casing ruptures or splinters and the explosion fire and shockwave is released into the surrounding environment. It is the secondary combustion caused by the primary explosion being able to vent into a production plant that can completely destroy a facility and potentially cause fatalities.
When designing the explosion protection system certain criteria are required such as the dust collector 'dirty air side volume', 'vessel strength' (Pred), 'operating pressure' of the explosion protection system (Pstat), explosion value of the products being handled (Kst) and the 'maximum pressure value' which occurs during an explosion (Pmax).
Explosion protection systems are available to dust control system manufacturers in several different formats, including rupture panels, indoor explosion protection devices, inert powder suppression systems and containment.
Explosion rupture panel. This is the simplest and cheapest of the options. It requires the panel to be vented to the atmosphere through a correctly sized and stressed vent duct. It can only be provided if the distance from the panel to atmosphere can safely be achieved within a straight distance of 3 m or less. This can also only be undertaken if the dusts being handled by the system can be vented to the atmosphere in this manner. Some agencies and authorities do not allow for certain products and their associated dusts to be vented using this method because of the impact on the local environment and wildlife.
Indoor explosion protection device. This device allows for the explosion to occur within the dust collector and the resulting pressure is vented safely from the dust collector. The fire is extinguished on passing through the device, and the associated hot gases are vented into the surrounding area. This system allows for little or none of the dusts to be emitted into the surrounding area.
Inert powder suppression systems. A detector monitors an increase in pressure within the dust collector indicating an explosion is developing. This information is sent to a highly sensitive and reactive control console, which then instructs the suppressant bottles to activate to release their contents of inert powder. Controlled explosive detonators are set off so that the high pressured insert powder is injected at pressure into the vessel, dampening and extinguishing the explosion.
Containment. This is a method that is rarely employed. Here the dust collector vessel is designed to withstand a developing explosion without any damage to the vessel itself. The filters, however, would need to be replaced.
Some pharmaceutical products are made up of fillers that are nonexplosive. However, tablets are often covered with a sugar-based hard shell, and sugar-based products are potentially explosive. Although the sugar to filler ratio is small, there is always an explosion potential. When an explosion potential exists protection must be incorporated into the system and equipment design.
In the event of an explosion occurring within a dust collector, consideration must be given to whether or not flame propagation might occur within the incoming duct system. A risk assessment should be undertaken and protection provided if it is considered that this situation could realistically occur. Several different methods of protection can be provided including 'slam-shut valves' and 'inert power chemical barriers'. A predetermined length of duct from the collector to the protection device has to be allowed and the material strength of the duct from the collector to the device must be at least the same as the dust collector casing so that it will be strong enough to withstand the pressures associated with a developing explosion and the operational shock of the activation of a protection device.
As stated, consideration and a risk assessment must be undertaken to determine if flame propagation protection is necessary. The costs involved can be more than the dust extraction system itself and can seriously affect the viability of a project.
Products that have been separated from the conveying air stream and held within the dust collector hopper can be dispensed from the unit using numerous methods. The more common methods are manually or pneumatically operated discharge valves. The valves can be fitted with a 'safe change' containment spigot on to which a plastic bag or plastic keg liner can be attached. When a single valve is provided the dispensing of product can only be achieved when the dust extraction system is not operational. Rotary valves can be employed to dispense the product on a continuous basis, allowing them to be discharged while the system is operating. These valves must be designed and certified to ensure they will not allow a flame to pass the rotor tips in the event of an explosion occurring during system operation.
Other methods or interaction with other systems whereby extracted products are centralized for final disposal can be provided to meet the requirements of the specific project.
To protect personnel and the environment that the dusts are to be released into, a high efficiency particulate arresting (HEPA) filter is usually installed. (HEPA absolute filters have an efficiency of 99.997% at 0.3 mm).
When handling toxic materials, to enable changing of the HEPA filter cell is safely undertaken, employing the same 'safe change' technology as that provided for the main dust collector cell changing techniques, again it is recommended that personnel are provided with the correct PPE.
Motive air for any dust extraction system is generated by a fan or similar unit and these must be suitable for the application they are to serve. Noise levels are always an issue and from a Health and Safety perspective these should be kept to a minimum or in line with the specification set for an application.
Dust control is not only limited to the dusts that are generated during the production process and extracted at source. Cleaning of the production area and associated equipment must be undertaken to ensure product integrity if a product line is changed or is a requirement of the production process. For large pharmaceutical plants, the most effective system to undertake this is a centralized vacuum cleaning (CVC) system. This type of system, when designed correctly, provides an 'online' cleaning system with all the features necessary for handling potentially explosive and toxic dusts safely.
The difference between a dust extraction system and a CVC system is that a dust extraction system extracts high amounts of air at low vacuum levels whereas a CVC system displaces low volumes of air at high vacuum.
It is not good practice to mix the requirements of both duties because if dust extraction is taking place when a vacuum cleaning hose connection point is opened, this can unbalance the system and reduce the extraction velocity and effectiveness, enabling dusts to escape out of the dust extraction systems 'capture zone'. When a dust extraction system is used for vacuum cleaning duties, the velocities employed for dust extraction are not sufficient to undertake vacuum cleaning duties and, therefore, the cleaning work achieved will be poor. Relatively dense particles, for example, complete tablets will immediately fall out of the air stream of a dust extraction system as the ductwork velocity are far less than those which is required for vacuum cleaning.
A CVC system comprises a number of hose connection points, interconnected with a correctly sized and graduated pipe work system connected back to a filter separator, absolute HEPA filter and exhauster unit. The features of the filter separator and HEPA filter will be identical to that described previously for a dust extraction system, but in thicker gauge material because of the much higher vacuum levels involved.
When designing this type of cleaning system, the critical factors depend on how many operators are required to use it simultaneously, the distribution of the operators, location of the plant relative to the hose connection points and configuration of the interconnecting pipe work system. With this information the design engineer can do a system resistance calculation and design the system accordingly.
The phase density of product-to-air volume is much higher for a CVC system compared with a dust extraction system. The maximum size of product that can be conveyed is determined by either the size of the product being produced or by the size of the vacuuming hose being used.
Conveying velocities are critical when designing this type of system to ensure that product is kept in suspension from the vacuuming tool to the filter separator. Multioperator capacity systems require a graduated pipe work system that increases in diameter towards the filter separator as more operators connect into the system and the displaced air volume increases.
Other highly critical factors are orientation of the pipe fittings and ensuring the fittings are suitable for the proposed application by providing relatively long radius bends and intersections.
Systems must comply with the Control Of Substances Hazardous to Health (COSHH) and the Environmental Protection Act (EPA) regulations. In addition, since July 2003, the industry has had to meet the requirements of the European Atmospheres Explosive (ATEX) Directive 137 (Directive 1999/92/EC). The ATEX Directive addresses the requirements and responsibilities of handling potentially explosive materials safely.
Some extraction companies have been proactive in working for many years to these requirements prior to the Directive's implementation by asking the correct questions at the time of initial enquiry and thus, setting out the necessary information within their quotations, and supplying compliant and safe equipment and systems.
This is now essential: clients and end users have a responsibility to have their production facilities 'zoned' under this directive and make information on the products they use available.
Under the ATEX Directive 137 manufacturing companies are responsible for having their production facilities classified under the requirements of the Directive. They are also responsible for obtaining all required information regarding potential explosion characteristics of both the raw materials they use and the products they produce. Subsequently, all this information can be made available to suppliers of equipment and systems which might come in contact with these products.
Jon Whitaker is a director at Dust Extraction International Ltd, UK.