Every month the electric bill for your pumping stations comes in to the utility office and accounts payable cuts a check to pay it. Yet, in many instances, no one takes the time to understand which pump stations are running efficiently and which ones are wasting energy at a rate that would make you think the pump owns stock in the electric company.
Inefficient pumps are not going to reveal themselves readily. An inefficient pump doesn’t look, sound or smell any different than an efficient pump. A utility manager needs to understand how each pump uses energy to determine if it is a star performer or should be headed to the scrap yard.
Bad pumps can be grouped into two different categories:
1. Those that are behaving as designed but are being used inappropriately; and
2. Those that are no longer behaving as designed.
Tools & data
The utility manager has a variety of tools to study pumps, some of which are already in place. The utility SCADA system provides much of the data needed to analyze the pumps, the utility hydraulic analysis model contains the tools needed for analysis and the power bill is the final check. The information can be supplemented by pump efficiency tests, but these can be reserved for pumps that are already identified as questionable.
Most utilities have a hydraulic analysis model of their system, such as WaterCAD/WaterGEMS. Not only can these models be used for planning or design, but they can also be used by operations personnel for analyzing pumping energy costs.
The first step in separating the good and bad pumps should be to dig up the original pump curves for the pump. Hopefully these curves are available, but if not, the utility should contact the pump manufacturer to get copies (and then store them in a safe place). The pump head curve from the manufacturer should be plotted on the same graph as the pump head and flow reported by the SCADA system. If the curves line up, the pump is apparently as good as new. If not, the detective work begins to determine if the pump was modified since purchased (e.g., trimmed impeller) or if it is simply worn out.
Matching pump & system
Even if the pump is as good as new, it still may not be matched appropriately with the system. Compare the operating point reported by the SCADA system with the pump’s best efficiency point. If the pump is most efficient at 1,200 gal per minute (gpm) but is running at 900 or 2,000 gpm, it is likely wasting a lot of energy.
This is where the hydraulic analysis model comes into play.
First, check the pump head curves in the model to determine if they agree with the operating points reported by the SCADA system. While system head curves can be developed manually for trivial systems, for complex water distribution systems, use the hydraulic model to develop a set of system head curves and overlay them on the pump head curves using Bentley’s WaterCAD water distribution model.
If the pump is operating to the right of the best efficiency point (more flow than desired), then it is encountering less resistance than the pump was designed for. This may be due to the system having more carrying capacity, the discharge-side tank being at a lower water level than the pump expects or the pump having been initially oversized. A pump may have been designed properly, but the distribution system has evolved over the years to make it incompatible. Although it still produces water, it does so at a very expensive rate.
At this point, it is best to modify the pump (trimming the impeller) or replace the pump with one that is compatible with the system. Before purchasing a new pump, get the curve for the replacement pump from the manufacturer and use the model to confirm that it will operate efficiently. Models like WaterCAD have the capability to calculate energy cost before and after the pump switch to document the savings from replacing the pump. In some cases, a new pump may pay for itself in a few years on energy savings alone.
If the pump is operating to the left of its best efficiency point (lower flow), then the pump is facing more flow resistance than it can overcome. This may be due to undersized piping, an undersized pump (in terms or head), a system that has changed over the years (e.g., tank level higher than designed) or a pump that was initially selected incorrectly.
Depending on the expected growth in the area served by the pump, additional piping capacity may be the solution or the pump may simply need to be replaced. Once again, analyzing the system with this model will help determine the energy savings from installing the new pump, or the pipe capacity required to make the system run efficiently.
Variable speed pump traps
Variable speed pumping can sometimes be the answer to pumping system energy waste for pumps discharging into pressure zones with no elevated storage. However, there are some misconceptions about variable speed pumping. Even with variable speed pumping, pumps can still find inefficient operating points. Variable speed pumps have a best efficiency point, and the further the demand drops away from that point, the more energy is wasted.
The calculations for variable speed pumps are not as simple as they are with constant speed pumps and are tedious to perform manually, but they are necessary. The pump speed and efficiency needs to be determined as a function of demand and time of day. These calculations can be automated with a model like WaterCAD, which can perform both the hydraulic and energy calculations needed.
Pumps that must operate over a wide range of flows will usually find some operating points that are energy wasters.
The solution to this problem is often to install a smaller (jockey) pump to run at off-peak hours. Calculating energy costs within the model will indicate if energy savings can justify an additional pump. When the larger pump begins to run inefficiently, it cuts off and the smaller pump picks up the load and saves energy.
Pumping during buildout
Many land development projects take several years to reach full buildout, yet the pumps are often sized for final buildout. Especially in systems with no elevated storage, during the early years, it may be economical to install and run a small pump.
In some instances, such as a resort community for which pumps are sized for busy weekends, this smaller pump or a hydropneumatic tank can pay for itself. Hydropneumatic tanks will enable the pumps to shut down much of the time, and when a pump is shut down, it isn’t wasting energy. Once again, using a hydraulic model with energy-costing capability can quantify the savings from running a smaller pump.
Energy expenses can be a significant operating cost for any utility, and without looking for and eliminating wasteful pumps, a utility may be spending money on energy it can spend better elsewhere. Utility managers may be able to accurately determine energy costs and identify energy-wasting pumps by using a combination of data from a SCADA system and a hydraulic model.