For the last 10 to 15 years, the wastewater industry has been undergoing a fundamental shift towards variable speed pumping. It’s easy to understand why: the ability to adjust pump speed according to input should, in theory, spell smoother operation, energy savings and lower maintenance costs. However, operators have reported disappointing results and some have seen no benefits at all. A groundbreaking, new white paper aims to explain why and provide a road map out of this mire.
By Steve Roman
“There’s a lot of misunderstanding out there and also a lot of lack of understanding – basically a lack of knowledge,” says Stefan Abelin, co-author of “Variable speed wastewater pumping,” the white paper he presented at Singapore International Water Week in June.
As Director of Project Execution for Xylem’s Transport Wastewater Pumping division, Abelin has been working directly with customers, investigating why their upgrades hadn’t yielded the expected cost and energy savings.
“The cost of adding a variable speed pump control system, or including it with a new purchase, can be on par with the cost of the pumps, so there’s a significant cost associated with it,” he says. “It has the potential to give several positive benefits, but it is really important that the system is understood and engineered correctly throughout the process.”
What causes energy inefficiency?
According to the white paper, when a variable frequency drive (VFD) is installed, there are two factors that typically drag down energy efficiency: partial clogging of the impeller (due to longer run cycles and less back flush) and operation deviating from the pump’s best efficiency point.
The paper gives advice for correcting these problems. But that’s just the beginning of the story. After years of studying customers’ applications and performing lab tests, the engineers at Xylem came to realize that getting the most out of a VFD would require checks and adjustments to a number of facets of a system.
What is groundbreaking about the new white paper is that it is the first to address all of these areas. Though only 12 pages long, it is a comprehensive distillation, three years in the making, covering technical aspects of system curves, pump and motor selection, process control, electrical issues, control strategies and more.
“What’s different here is that we have collected aspects from many different fields, so we discuss pump and motor aspects, both mechanical and electrical, we discuss electrical aspects with the variable speed drive and probably most importantly, what’s new here, we are discussing how pumping wastewater is affected by varying the pump speed ” says Abelin.
“In the white paper, we wanted to collect many of these key aspects and put them in more of a total problem, total solution and total opportunity perspective.”
Using intelligent wastewater controls
One of the more promising improvements suggested in the white paper is the use of application-specific, intelligent wastewater controls. Their software uses advanced algorithms to maximize pump system efficiency and they come preconfigured, making them easy to install.
According to Abelin, this type of advanced control system will not only save energy, but just as importantly, will increase system reliability.
The problem is that only Xylem and a very few other companies offer these controls and they’ve only been on the market for the last two to three years. “Many customers have not yet taken this in and so they are not even aware of the existence of them,” he says. “They don’t yet know what the benefits can be.”
So just as with the other findings in the white paper, Abelin and his colleagues will be working to get the word out. “It’s an educational trip we’re starting here,” he says.