Introduction to variable speed concept
The use of variable speed drives (VSDs) in pumping systems may be motivated not only by the need to adjust the pump output to the supply or demand, but also by the prospect of economic benefits, primarily related to energy and maintenance savings, and reliability improvements. Other benefits may be a reduction of hydraulic transients, as VSD pumps can be soft started and stopped, a reduction in leakage from a system when it operates at a reduced pressure, and benefits associated with better control of pumping operation. These have to be evaluated against the capital costs associated with the purchase of VSD equipment.
The energy savings are unquestionable in systems dominated by friction losses, such as water circulating systems. In such systems, the pumps operate at a constant efficiency irrespective of the pump speed. However, in systems dominated by static head, the pump efficiency changes with the speed. It is typically reduced at lower speeds and hence energy savings may not always be realised (see Section 4.2.2).
There are also some potential problems with the use of electronic VSDs in pumping systems. One category of problems is related to electromag netic emissions that must be carefully filtered to avoid interference and ensure compatibility.
Another category of problems is associated with pumping liquids containing solids, such as industrial slurries or wastewater. In order to transport solids in liquids, the mixture has to move at a velocity higher than a certain critical velocity below which the solids are deposited. If solids are allowed to deposit in pipes, the friction losses increase progressively as the available flow area is reduced. Also pumps can clog with solids when they operate at a reduced speed. Pumping under such conditions is not only wasteful in energy, but it may cause complete blockage of the system or pumps and disrupt pumping completely.
When considering various options of controlling pump operation, appropriate considerations have to be given to both the technical function ality of the system as a whole and the costs involved.
VSDs offer many advantages, but their potential drawbacks should not be ignored. See Chapter 11 for a more detailed analysis of the benefits, drawbacks and operational issues of using VSDs.