Economical Aspects of Adjustable Speed Drives in Pumping Systems

Abstract

Speed variations of pumps have become increasingly popular as the technology to produce variable frequency drives has progressed. Variable speed drives have many advantages compared to throttle valves when it comes to regulation of flow. They offer large energy savings and, hence, large cost savings if they are applied correctly. Recent experiences have shown that there are many cases where no savings are realized; there are also cases where the cost of pumping actually has been increasing. In addition, pump and motor equipment failures have been more prevalent when used in variable frequency drive systems. Why does this happen? It is important to realize that all of the components of a pumping system are interdependent. Sub-optimization on the component level can easily lead one astray. End users and design engineers should use a systems approach when analyzing pumping systems for optimized energy efficiency and economic considerations. Designers must consider the shape of the system head curve. Any pump system with static head is a potential candidate for trouble if the pump system parameters are not fully analyzed and understood. This paper will point out some of the pitfalls when dealing with pumping applications which contain static head. When comparing different solutions for optimizing a pump system it is helpful to be able to easily identify the system effectiveness and to compare different solutions. In order to make an intelligent choice, some basic facts will need to be established. The first is, what are the Process Demands? The Process Demands will generally steer you in a certain direction. For example, do you need to be able to vary the flow rate and, if so, should it be continuously variable or can you vary your flow rate in steps? Can you use on-off batch pumping? What is your peak flow rate and how is the flow rate distributed over time? The answers to these questions will determine if, and how, to regulate the flow, It will also give some guidance regarding the design of your pumping system.