Why be concerned about efficiency if a pump is supplying the necessary flow to maintain plant operations? Many pumps perform well enough for the systems and processes where they are installed, but they may not be the most efficient pumps for the assigned tasks. Inefficient pumps can increase maintenance as well as energy costs.
With the continued pressure to reduce costs for maintenance, plant operations and system downtime, it becomes imperative that we operate in the most efficient and reliable mode possible. Recent studies indicate that pumping systems account for about 20% of the worlds electrical energy demands. In some facilities, the electricity consumed by pumping systems ranges from 25% to 50% of the total electrical energy required for plant operations. Choosing the appropriate pump designs, sizing them properly to the loads, and monitoring their health can improve reliability, as well as materially reduce energy costs.
Whether for initial installation, repair or replacement, the life-cycle cost must be taken into consideration. Life-cycle cost is the cost of the pump during its operational life expectancy. Most life-cycle cost calculations show that the major expenses of a pump over its life span are energy consumed and maintenance of the unit. Overall, energy and maintenance account for roughly 75% of the life-cycle cost.
So, to maximize pump efficiency, two areas must be explored. The first is pump operation: looking at the pump itself and determining its operational efficiency and application within a system or process. The second is how to maintain the pump and systems operating at peak efficiency.
To select the right pump, the design engineer determines the operating parameters needed for the system or process and the type and capacity of the pump needed to satisfy those requirements. The selected pump will be either a roto-dynamic or positive-displacement type based on the system characteristics (Figure a).
PD or not PD
A dynamic pump, typically a centrifugal pump, will be used in applications where a continuous, steady flow of fluid is required. The positive-displacement type will be used for applications where a specific capacity or quantity of fluid is required.
To select the exact pump, you must match the pump operating characteristics with the system operating requirements. There are three basic steps to the selection process. The first is to obtain the pump performance curves for several pumps that meet the operating parameters. The second step is to determine the system resistance coefficient curve and graph it. The third is to superimpose the system resistance coefficient curve onto the pump performance curves of several pumps. The point at which these two curves intersect is the system operating point. The correct pump is the one that has a best efficiency point (BEP) nearest the system operating point. With this information, we should be able to select the most efficient pump for the application every time (Figure b).
Select for efficiency
Pump efficiency (Î·) is a ratio of hydraulic power to brake horsepower. Hydraulic power (Po) is the power delivered to the fluid by the pump, and brake horsepower (Bhp) is the power required to drive the pump. The equation for determining pump efficiency Î· = Po/Bhp.
The BEP is the most efficient operating point for your pump. The pump manufacturer uses a test stand to derive this data with pump performance curves, so the as-built efficiency, hydraulic power and brake horsepower may be obtained from the manufacturer.
Normally, the pump is designed to operate at a fixed speed. The head and capacity of the pump is then plotted for this speed. Pump performance curves depict, for a given pump speed, the relationship between required capacity and the pump efficiency, horsepower required, net positive suction head (NPSH) required (NPSH), and total head developed by the pump.
Pump efficiency can involve additional parameters that must be considered during selection. Depending on the application, you may have to evaluate its thermal efficiency and volumetric efficiency. Other system requirements and the operational environment must also be considered.
Beware pump enlargement
Bigger is not always better, but surveys indicate a majority of pumping systems have oversized pumps installed. Oversized pumps are robbing you of operation and maintenance dollars, so why are they there?
Many pumps are oversized because everyone included in the selection process decides to add his own safety factor. For example, the design engineer determines the required flow as 200 gpm, but adds an extra 5% (210 gpm) to this capacity to cover any calculation inconsistency. The plant owner adds another 10% (231 gpm rounds up to 235 gpm) as a safety factor. You send the specification to the pump supplier, who wants to be sure he sends you a pump with sufficient capacity. He informs you he does not have a stock pump for this flow rate and recommends a stock 250 gpm pump, saying that by using this pump you will not incur the additional expense and delay of modifying a pump to supply a flow rate of 235 gpm. You now have a pump that is 25% larger than needed by the original design.