Fit for hazardous areas

Author Thomas Ramme Sales Manager, Volkmann Vakuumtechnik

The multiple-stage Multijector vacuum pump creates the required air flow rate for conveying at an optimum working pressure of 5.5 bar. Suction volumes of up to 40,000 l/min and negative suction pressures up to 910 mbar have been recorded in tests. However, with some applications it is difficult to maintain an operating pressure of 5.5 bar throughout the pneumatic conveying process, especially when several discharge locations are being fed at once. Fortunately, an alternative nozzle system that achieves the maximum possible vacuum while operating at a pressure of only 3.5 bar has been developed by Volkmann for this specific purpose.

This system combines the common advantages associated with Multijector pumps such as small size, sturdy but lightweight construction and optimal use of the compressed air consumed, and improves all of them by lowering the necessary compression work to 3.5 bar. What’s more, the number of available models is doubled owing to the modular design of the Multijector vacuum pump. This gives clients a better chance of finding a system to meet any challenge that may arise in connection with their specialised conveying task.

All Multijector vacuum pumps also have the added benefit of operating in cycles. Most bulk materials and powders are conveyed in batches; however conventional suction-air generators must operate continuously, even if no product whatsoever is conveyed. This is partly due to the large amount of time these generators require to build up the vacuum which is needed to convey any product; on top of this, electromechanical engines are not designed for frequent starting and stopping. By allowing cyclic suction, the Multijector technology reduces the amount of air consumed and creates a sufficient vacuum much faster than similar continuous-blower systems.

In the pharmaceutical and food industries it is considered essential for all machines that come into contact with a product to be easily cleanable. Vacuum conveyors built with flexible components comply with this design principle by being simple and straightforward to disassemble and clean. While 316L stainless steel is standard, a variety of other speciality materials can be used for customised applications. In situations where this is not feasible, for instance if a large conveyor is mounted in a fixed or difficult-to-access plant, a CIP solution can easily be developed on request.

One of the main design features that facilitates cleanliness and mobility is an externally operated discharge valve. The entire pneumatic system is installed outside the conveyor body, eliminating any danger of product contact. This also means that the interior of the conveyor has no gaps or crevices where product, or water for that matter, could become lodged: thorough rinsing is guaranteed. In addition, this externally mounted component can be disassembled without tools, as can the remainder of the conveyor. A full sized vacuum conveyor can be dismantled into its constituent parts in seconds. Each of these parts can then be submerged in an ultrasonic bath or an industrial washing machine in instances where simply rinsing is not sufficient.

As a rule, a vacuum conveyor must be mounted directly above the unit to be loaded. Since the remaining space between the existing plant equipment and the roof often proves to be very small, it is extremely important to keep the overall height of the conveyor to a minimum. Thanks to the modular design principle, three height reduction steps are pos-sible and the conveyor can fit into the available space without compromising its suction ability.

This also gives vacuum conveyors the benefit of being mobile. Due to its light weight and modularity a vacuum conveyor can either be coupled to a particular discharge location like a mixer, so that it can be attached by the operator or mounted to a height adjustable trolley. This trolley can then be used to adjust the height until a dust-tight connection is made. Furthermore, the trolley’s height and pivot can be controlled either manually or automatically. The trolley can conveniently house the diverse aux-iliary components necessary for conveying. The control panel, suction lance, suction line, Hepa filter and vacuum pump, for example, can all be secured to the frame. The fact that the trolley is adjustable incidentally enables the vacuum conveyor to be extracted easily for cleaning.

In pneumatic conveying systems, electrostatic fields can lead to sparking and possibly cause dust explosions with disastrous consequences. With conventional, electromechanical conveyors these fields are either generated by the components of the conveyor itself or created by the relative movement between the particles being conveyed. Both these causes must be taken into account when designing a conveyor that is intended for hazardous areas. Vacuum conveying inherently offers more safety than positive pressure conveying because the oxygen concentration required to produce an explosion is lower in the negative pressure region than with overpressure. This concentration can be further reduced by introducing an inert gas into the separator tank during the discharge process.

In addition to analysing the various physical and chemical properties of the conveyed material, it is necessary to examine the individual components of the conveying system in more detail.

Multijector vacuum conveyors are driven exclu-sively by compressed air. The individual conveyor components, such as the air-shock tank and discharge valve, are also controlled completely pneumatically. No electrostatic fields are generated, therefore, by electric motors or coils.

Since they work on the principle of an electrically powered motor, conventional vacuum generators pose another ignition risk. The very engine that is used to create the vacuum also gives off a large amount of radiant heat, which is a further ignition source. Conversely, compressed air-driven Multijector pumps, which generate a vacuum by expanding air streams, actually cool off during operation.

Insulated conductive components in the conveying system represent a third ignition source in the form of spark discharges. These components include, but are not limited to, sealed inspection gates, doors of filter chambers, insulated conveying lines or pipes and couplings. In this case, one important factor influencing electrostatic charge is the gap between two insulated parts in a system. These two parts together act as a capacitor in which the electric charge is stored. The larger the gap, the greater the accumulated energy potential. If the charge reaches a certain value, spark-over is possible.

Once again, Multijector vacuum conveyors avoid this source of sparks. All parts in contact with the product are connected together in such a way that only a single common earthing point, which diverts any residual electric charge that may build up during conveying, is necessary. If suction hoses are used, electrically conductive and anti-static variants are possible.

Another aspect when assessing the risk of sparking is the size of the separator tank. Studies have shown that explosions hardly ever occur in tanks under a certain volume. Thanks to their extremely small size and cyclic conveying (suction – discharge) of compar-a-tively small powder quantities, Multijector vacuum conveying systems have proven to be particularly safe. This is also confirmed by conveying tests carried out in concert with a major chemical company. These tests involved conveying PE granules, which are ideal for generating electrostatic charge, from a metal tank to a plastic barrel over a distance of 25 m. The electrostatic field [kV/m] was measured throughout the entire conveying process: first at the suction lance, then along the conveying pipeline, next at the vacuum conveyor itself and fi-nally at the product discharge. The readings taken were within the safety limits at every stage of conveying.

Conveying speed is likewise important, of course. The speeds at which dilute phase conveying occurs (20 to 35 m/s) should be avoided with critical applications. The Multijector pump offers a solution to this problem as well. Since it is able to create a final vacuum of 90 mbar, plug flow (or dense phase) conveying can be achieved and the air velocity reduced to a much safer 1 to 9 m/s.

If parts made of electrostatically chargeable – but non-conductive – material exist in the conveying system, particular account has to be taken of the possibility of a brush discharge. Recent findings have suggested that for pure dusts, brush discharges are not an effective combustion source. This new data has significantly influenced the details of vacuum conveyor design. Certain materials which are primarily sought after in the pharmaceutical industry can be used in product contacting areas, for example. This is further described in the manufacturer’s Atex certificate. In the case of highly combustible bulk materials or hybrid mixtures, the actual conveying process can be carried out under inert conditions.

With Multijector vacuum conveyors the basic operating principle for safe and economical transfer lies in the prevention of explosions, in other words a pressure rated design is not required. If a conveyor is merely pressure resis-tant but cannot prevent an explosion by design, there is an additional risk that the explosion will propagate to other areas. For instance, if an explosion were to be transmitted farther down the production line to a silo or another storage device, the damage would quickly escalate. Such problems are avoided completely by all standard Multijector vacuum conveyors.

cpp-net.com/0114449