Wastewater Pumps and Variable Speed Drives
I enjoyed the previous Pump Quarterly's article by Dr. Nelik on improving pump reliability and saving energy by improving pump efficiency. While his proposed method is effective, there is an alternative which does not require removing the pump from service, piping modifications or changing impellers.
Install a Variable Speed Pump Controller. This may accomplish the desired goals of matching pump performance to changing job requirements and saving energy. In most wastewater applications with little static head, this may save more energy than trimming the impellers and also provide increased pump and motor life due to the soft-start and soft-stop features of a variable speed system. Reducing pump speed reduces starting torque and thrust as well as eliminating the high starting current created using a standard across the line starter.
VFD versus VSD versus Variable Speed Controller
For clarification of a few common industry terms, people use VSD (variable speed drive) and VFD (variable frequency drive) interchangeably. My electronically proficient co-workers prefer to differentiate. To them a VFD is a hardware item used in a variable speed controller. In a pump application, the VFD requires a “macro” or “pump specific software” to properly control the pump system. Not all VFD’s are created equal as their standard software will not necessarily control a pump system. In the remainder of this article, I will use VSD for consistency.
Operating three-phase wastewater pumps using VSD’s is becoming very popular. They provide benefits not found in a fixed speed application especially in applications with large flow variations and low static head. The Goulds Pumps R&D lab has successfully tested and operated a premium cast iron construction, three-phase wastewater pump between 30 and 60 hertz operation. During R&D testing, we found the system operated best when we programmed the VSD for a minimum speed of 30 hertz, as this prevented continuous operation yet still provided sufficient starting torque. It is recommended that users never operate a wastewater pump above 60 hertz due to increased motor HP loading, higher amperage and the resultant heat rise (see HP ratings in 70 hertz multipliers). We used a 0 – 7.5 psi transducer with an Aquavar™ CPC controller which allows on/off set points between 0’ and 17.3’ (7.5 psi x 2.31’). You can program any range within those points as your on/off points.
I have read many articles in the past few months on the energy savings which can be realized by using a VSD, Dr. Nelik used some good examples in his article. Therefore, I would like to take a different approach for those unfamiliar with pump curves and discuss creating variable speed curves.
If you decide to operate a pump with a variable speed drive and you want to create a variable speed curve use the Affinity Laws or the multipliers that follow to easily make a minimum speed curve. Once you program it, the controller will automatically keep the pump operating between the full speed and minimum speed curve.
A set of formulas called Affinity Laws govern the performance of centrifugal pumps. Because we are concerned with speed changes rather than determining an impeller trim, another use for the Affinity Laws, we’ll concentrate on the laws for speed.
In the Affinity Law formulas the following abbreviations are used to represent needed values. Q = Capacity (gpm), H = Head (feet), BHP = Brake Horsepower and N = Rotational Speed (rpm). Subscript1 represents values for the original data and subscript2 represents the new data. Assuming the impeller diameter is held constant the mathematical relationships between these variables can be expressed as follows:
See Formula 1 (below)
The relationships in Formula 1 mean that an increase in pump speed will produce more flow at a higher head and will require more horsepower to run the pump. The inverse is also true, a decrease in pump speed will create less flow and head and require much less hp and therefore less energy.
Solving these equations provides us with the following Performance Multipliers, which can easily be used to create variable speed range curves. We used the Performance Multipliers for 30 hertz below and data points from Curve 1 to create Chart 1.
Hertz Performance Multipliers
70 - Q2 = Q1 x 1.17 H2 = H1 x 1.37 BHP2 = BHP1 x 1.6
60 - Q2 = Q1 x 1.0 H2 = H1 x 1.0 BHP2 = BHP1 x 1.0
50 - Q2 = Q1 x .83 H2 = H1 x .69 BHP2 = BHP1 x .57
40 - Q2 = Q1 x .67 H2 = H1 x .45 BHP2 = BHP1 x .30
30 - Q2 = Q1 x .50 H2 = H1 x .25 BHP2 = BHP1 x .125
Curve 1 is a 60 hertz, 1750 rpm, 30 hp 4NS wastewater pump curve and a lower curve for the same pump slowed using a VSD to 30 hertz or 875 rpm’s.
See CURVE 1 (below)
Make a chart for calculating curve data
To create a variable speed curve it is best to use several points including the far left (shut-off head) and far right (wide open) points on the curve. The chart below shows the 1,750 rpm curve data and the calculated 875 rpm data that we plotted to create our 4NS, 30 hp curve showing operation at half speed, 875 rpm or 30 hertz.
See Chart 1 (below)
As the VSD slows the motor shaft, bearings and impeller they require less cooling and lubrication. Because we are operating the pump slower it uses less energy, see Law 3a, and creates less heat thus reducing motor cooling requirements. Heat is a major cause of motor failure, therefore, reducing the heat created by the motor increases longevity and reduces pump maintenance and repair expense.
Another major benefit of variable speed technology is reduced thrust on the motor thrust bearing and shaft. The controllers act as a soft-start device and create less start-up torque and much less thrust than a fixed speed pump, this reduces thrust bearing wear.
Depending on the application, a variable speed drive may benefit the operator with more control over a standard, fixed speed system. While not all installations benefit by using a VSD, it is imperative that each application be evaluated by the operator, engineer and factory. The pump’s actual operating point in relation to its best efficiency point, large system inflow variations, cost per kilowatt hour and yearly hours of operation all factor into the potential energy cost savings.
We hope this information helps you and your facility in making informed pump system decisions which benefit your bottom line and our environment.
About the author:
George Strally is a Technical Marketing Manager for ITT Water Technology Inc. - a subsidiary of ITT Industries, Inc.