Improperly sized and selected pumps can cost thousands in annual maintenance, operating and capital costs. However, using the PIPE-FLO® Pump Selection feature from Engineered Software Inc., users can minimize pumping costs when selecting pumps based on specific design criteria.
The Piping System
Engineered Software conducted a study on a piping system that pumps from a supply tank to a pressurized discharge tank. The pump was sized to supply 600 gallons per minute (GPM) to the discharge tank, and a flow control valve (FCV) was installed to control the flow rate. The pump was selected using PIPE-FLO, and an ESP pump running at 3,560 RPM was also selected.
Once in operation, it was discovered that the flow rate through the system would be 400 GPM for the next three years rather than the originally planned 600 GPM. Since that change occurred after pump selection, the system flow rate was also changed, affecting pump operation.
Pump Operation
Operating at a flow rate of 600 GPM, the differential pressure across the FCV is just over 16 psi, or over 37 feet of headloss. Using the pump curve, the pump is operating very close to its best efficiency point (see Fig. 1). However, the differential pressure across the FCV set to 400 GPM is over 68 psi, or a headloss of over 158 feet of fluid. In addition, the pump is running much further up on its curve with lower efficiency (see Fig. 2).
The excess energy is equal to the difference between the total head across the pump at the design flow rate and the total head across the pump at the desired flow rate. Half of the 312 feet of total head is wasted across the control valve. Since this will be a long-term operation, it's important to verify how this would affect the operating cost of the pump.
Annual Operating Costs
The PIPE-FLO Pump Selection feature is used to evaluate the pump. To define an annual operating load profile for the piping system, it's important to specify the flow rate, hours per year operating at the flow rate, and power cost. This system will run 6,000 hours per year at 400 GPM with a power cost of $0.10 per kWh for a total cost of $21,982 per year.
Alternative Options
Reducing the impeller diameter or decreasing the pump speed will produce less excess head, which will cause less wear on the control valve and consume less power, resulting in saving pumping and valve maintenance costs. PIPE-FLO's System Resistance Curve feature calculates this savings and exports the curve into the Pump Selection feature. The impeller diameter or pump speed can then be changed to determine how much it will cost to operate at 400 GPM.
In Figure 3, the black line serves as the pump curve, and the blue line serves as the System Resistance Curve. At a flow rate of 400 GPM, the difference is 68.01 psi, or 157.1 feet.
Variable Frequency Drive
To calculate how much it would cost to run the pump with a variable frequency drive, the cost analysis can be run again with a variable speed instead of a fixed speed using the PIPE-FLO Pump Selection feature. The pumping cost for one year is $10,331, ultimately saving over $11,600 annually in energy costs.
Summary
Using the PIPE-FLO Pump Selection program is ideal to determine actual operating costs under a variety of conditions. Often the payback for system modifications is less than a year.
For more information on Engineered Software and its PIPE-FLO Pump Selection software, visit www.eng-software.com.
