How screw pumps strip out even highly viscous media – When emptying an oil or fuel tank (a process refferd to as stripping), quite often residual product of the highly viscous media is left behind. But does it have to be that way? Pumps with excellent intake at low NPSHr values can in fact completely empty tanks – even of crude oil and bitumen. And one technology in particular outperforms the rest...
Tanks are ideal for storing media that is volatile, potentially hazardous or has to be protected against contamination. They are therefore commonly used for mineral oils and oil products from diesel and petrol to polyols, as well for plant-based products such as palm oil. All these materials cause problems when it comes to emptying tanks, such as when medium is pumped to the next step in the production chain.
This may be because of comparatively high viscosity or a low vapour pressure. If you use conventional centrifugal pumps, the poor intake often leaves behind residual product that then cannot be used – a significant cost factor with diesel prices of around €1,000 per m³.
The wrong type of pump can also cause cavitation resulting in interruption to flow, which will lead to pump failure in the long term. Screw pumps are therefore being used increasingly as an alternative to centrifugal pumps, because they have lower NPSHr values and therefore significantly better intake behaviour. They also make it possible to strip out a tank completely with nothing more than the pump.
A Plethora of Features for Reliable Tank Stripping
Specialists companies such as Netzsch offer these pumps as two-screw, three-screw or geared twin-screw models (2NS, 3NS or 4NS) with the 4NS installed in about 90 percent of all cases, as it offers the best intake behaviour and high flow rates, the manufacturer states.
Only recently, one of the world’s largest companies handling petroleum products wanted its 4NS pumps at a terminal in England to be fitted with an additional heating jacket to keep the bitumen hot in the pumps. The heating jackets maintains media temperatures between 130 and 200 °C, so that the product can be kept at a comparatively low viscosity of 500 cS.
The performance of the pumps is unaffected by the heat, as the robust units are designed for temperatures up to 300 °C. But the technologoy is also in high demand on the other side of the globe as a Brazilian oil company pumps fuel oil at a stable flow rate of 130 m³/h at 605 cSt, 60 °C and 15 bar pressure with three 4NS screw pumps.
Besides the separation that is required for structural reasons between bearing and pumped medium, this is possible thanks to a range of materials from grey cast iron and chrome-nickel steel to Hastelloy alloys that are used for the pumps. The choice of materials also contributes to pump performance, as spindles made with these materials also have almost no deflection, which allows for fine production tolerances between the dynamic and static parts, thereby reducing backflow. In combination with a pump chamber with geometry designed for optimum flow and to minimize turbulences, it is possible to achieve high rates of efficiency while still gently pumping the medium, Netzsch states.
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