MechChem Africa March-April 2022

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 performancewill be compromised and energy usewill increasewhen 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 alternativetostop-startpumping, throttlingusingaflowrestrictingcontrol valve, using a bypass valve to recirculate excess flow back to the inlet or usingmultiplepumps inparallel.Other strategies like trimming thepump impellerorchangingtheV-beltpulleysizetoreducethe inputdrivespeed fromthemotor, areof amorepermanent nature and cannot react to real time changes in the system. Superficially, using some sort of variable speed drive to reduce the flowtomatchdemandseems tobeamoreefficientwayofmanagingflow requirement fromapump.But thisdependsonthepumpingsystem,most notably, the balance between the portion of systempressure 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 speedby20%will reducetheflowby20%, butwill reducetheinputpower 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.Onapump systemcurve, thefluidpower delivered intoa system byapump(H×Q)canbe ‘seen’ astheareaof therectangleformedbydraw- ingahorizontal linefromthepumpoperatingpointontothepressureaxis andavertical oneontotheflowaxis (bluesquareonFigure1). Theareaof thisrectangleandthereforethedeliveredfluidpower, changeswithevery change of the pump’s operating point.

6 ¦ MechChem Africa • March-April 2022