In Switzerland alone – traditionally a country with a great deal of tunnel construction – the ratio changed between 1998 and 2005 from 100% explosives in cartridge form to 95% pumped emulsions. The benefits are huge: the components, which are not dangerous individually, are not mixed until they are on site. The finished mix fully uses the whole borehole, but also goes into small openings. Certainly, high-performance, customised pumps which meet the special challenges of the emulsion matrix or of the explosive are the basis for this. NETZSCH Pumpen & Systeme GmbH has developed its own progressing cavity pumps for this purpose, which enable flow control while in operation and are even resistant to ammonium nitrate solution. With this, the operating principle ensures stable pressure conditions with little pulsation and only minor shear forces.
Tunnel and dam constructions along with deep and open-cast mining for the most part require complicated explosive work. Emulsion explosives have established themselves over recent years in this field due to their easy storage and often better performance. (Image rights: joujou, pixelio.de)
Storing or transporting larger amounts of "live" explosives is always a huge risk. Safety has therefore been a problem for a long time on construction sites where explosives are used. The development of emulsion explosives and mixes, such as ANFO (Ammonium Nitrate Fuel Oil), meant the creation of the possibility of storing the individual components of the explosive material separately. The explosive is not mixed until it is on site, as soon as it is needed. Complicated and expensive storage of explosive which has already been sensitised can thus be reduced to a minimum.
In addition, however, the benefits in terms of performance and handling also make emulsion explosives stand out: the charges can be filled more quickly from the mixer than with explosive in cartridge form and can also be inserted precisely into narrower boreholes. On the other hand, where there are boreholes with a large diameter, the whole volume of the hole is fully utilised through the flow properties of the viscous to watery materials, so that the drill grids can be coarsened. In addition, emulsion explosives, as well as their mixtures with ANFO (heavy ANFO), are insoluble in water and can therefore also be used in damp conditions.
This type of explosive material certainly requires special conveyance conditions. In the German Accident Prevention and Insurance Association’s Regulations (BGV) for solid uniform explosives and for blasting oils, it is for instance stated that pumps should prevent a dangerous strain on the materials or a separation of the emulsion. The pump specialist NETZSCH has therefore developed special equipment for mobile mix loading vehicles to convey emulsified explosives. The proven NEMO® SY progressing cavity pump has therefore been extensively reworked to implement the necessary testing possibilities and safety measures. The two inspection openings which have been incorporated above the joints of the coupling rod are the greatest difference with respect to conventional designs. These mean that it is possible to check whether the joints are still sufficiently lubricated and the seals are still intact while the pump is in operation. Otherwise the metal on metal friction could lead to dangerous heat generation.
The two inspection openings which have been incorporated above the joints of the coupling rod are the greatest difference with respect to conventional systems. These mean that it is possible to check whether the joints are still sufficiently lubricated and the seals are still intact while the pump is in operation.
So-called K-joints are fitted for the connection parts themselves, which transmit the torque from the motor to the pump’s rotor. These geared joints are different from pin joints in that they have a double seal and therefore provide increased safety: if one of the rubber sleeves becomes cracked, there is another one underneath which immediately takes over its tasks. This means that there is no way that oil can leak out of the joint. In addition, the coupling rod is drilled through, so that the joints can undergo a pressure test if necessary. If there is leakage of the pressure applied, this means there is a fault. The mechanical seals have been shifted outwards, specifically for pressure equalisation, so that any possible overpressure can escape. There is also the option of the pump being fitted with a safety stator. With this, grooves reduce the strength of the walls, so that it can open if the pressure increases too much.
These geared joints are different from pin joints in that they have a double seal and therefore provide increased safety: if one of the rubber sleeves becomes cracked, there is another one underneath which immediately takes over its tasks.
With this pumping principle, the stator and rotor together form chambers in which the medium is transported. Firstly, this means pressure-stable conveyance without any pulsation or shear forces. Secondly, the technology also allows the transport of filled materials, such as emulsion already sensitised with hollow microspheres. The length of the rotor-stator area has been specially shortened for this application: normally, longer pump bodies are used to increase the pump pressure, which is exactly what needs to be avoided here.
The pump is set in motion using a hydraulic drive which is connected to a vehicle motor, for instance the one belonging to the transport truck. We have deliberately managed without an electric motor, as the hydraulic system is space-saving and anyway power is not available on most building sites, or only to a limited extent. In addition, in terms of explosion protection, the avoidance of sparking has as far as possible been ensured in the drive.
Moreover, NETZSCH also offers customised pump versions for transporting and combining the explosive elements. These were initially developed at the request of an ANFO producer. In this process, a wax-oil mixture is added to an ammonium nitrate solution at high temperatures in an emulsifying device. The high heat in combination with the salty solution however encourages corrosion and attacks the elastomers in the pump.
The experts therefore chose highly-resistant viton for the seals and stators, so as to achieve the longest possible service life. At temperatures below 100°C and with a wax content of less than 4%, stators made of EP and SBBPF could also be used instead. Stainless steel is used for the metal components. In addition, the pump housing has been equipped with a heating jacket, in order to maintain the required heat during conveyance as well. At the same time, a measuring system monitors the friction heat in the pump with a temperature sensor and therefore protects against dry running. The pump is also driven by a hydraulic motor.
NETZSCH’s progressing cavity pumps can be used both for on-site mixing according to the Site-Mixed System (SMS) and also for introducing prefabricated emulsion matrix according to the Repumpable System (RPS). The explosive pumps are made according to the customer’s individual requirements. The pump manufacturer is completely flexible in terms of the pump capacity at different pressures through the pump design. Pumps for use in potentially explosive atmospheres are ATEX compliant.
Author: Roger Willis, Manager Focus Application Mining.