Pumping systems 101: Advantages and pitfalls of VSDs Harry Rosen, UNIDO pump expert and MD of TAS Online, uses pump and system curves from typical pumping systems to explain scenarios where variable speed drives (VSDs) are an excellent choice for realising substantial energy saving and, most importantly, those pump systems where a VSD will not improve energy efficiency or pumping performance.
A multistage mine dewatering pump delivering high pressure water to overcome a very high static head. This pumping system offers minimal friction loss, which results in a flat system curve.
I nstalling a variable speed drive (VSD) will not always improve the energy efficiency or performance of a pumping system. In fact, in some systems, pump performance will be compromised and energy use will increase when the pump speed is reduced. Variable speed drives for pumping systems are not a new concept. They have long been used to control the flow in pumping system, as an alternative to stop-start pumping, throttling using a flow restricting control valve, using a bypass valve to recirculate excess flow back to the inlet or using multiple pumps in parallel. Other strategies like trimming the pump impeller or changing the V-belt pulley size to reduce the input drive speed from the motor, are of a more permanent nature and cannot react to real time changes in the system. Superficially, using some sort of variable speed drive to reduce the flow to match demand seems to be a more efficient way of managing flow requirement from a pump. But this depends on the pumping system, most notably, the balance between the portion of system pressure that is due to friction losses and that portion needed to overcome the static head.
The affinity laws and pump system curves According to the affinity laws for centrifugal system: reducing the pump speed by 20% will reduce the flow by 20%, but will reduce the input power by 50%. This applies to pumps and fans, where the power absorbed by a centrifugal pump varies with the cube of the pump speed. That means that small decreases in speed can significantly reduce energy use, which implies a great opportunity for installing variable speed drives (VSDs) in pumping applications. But the fluid power delivered by a pump is governed by the system’s pressure-flow relationship.The fluid power output of a pump is propor - tional to the head multiplied by the flow at the operating point of the system. On a pump system curve, the fluid power delivered into a system by a pump (H×Q) can be ‘seen’ as the area of the rectangle formed by draw- ing a horizontal line from the pump operating point onto the pressure axis and a vertical one onto the flow axis (blue square on Figure 1). The area of this rectangle and therefore the delivered fluid power, changes with every change of the pump’s operating point.
6 ¦ MechChem Africa • March-April 2022
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