My last column (Pumps & Systems, April 2014) explained why resistance in implementing pump system improvement programs often occurs. Because pump systems touch many disciplines and departments within an operating plant, it is difficult for everyone involved to clearly understand how a piping system operates. In this month’s column, a piping system will be discussed, including the individual items and how they work together as a total system. It will explain the system’s operation so that everyone in the plant can visualize its function. Figure 1 shows a demineralized water supply system consisting of a pump, two tanks, a water treatment unit, pipelines and a control valve. The elements can be grouped into three general categories based on how the energy is used in the piping system. The pump element is the only component that adds hydraulic energy to the fluid in the system. The process elements consist of the equipment required to create the product or provide the service. Hydraulic energy is consumed when the fluid passes through the process element to make demineralized water. The control valve regulates the flow rate through the system to improve the product quality and system efficiency.
05/29/2014
Figure 1. Demineralized water system
The system is designed to produce 600 gallons per minute (gpm) of demineralized water for use throughout the plant. Figure 1 shows the elevations, levels and pressures in the raw water and demineralized water tanks. The distribution piping to provide demineralized water to plant loads is not shown because tanks and vessels make excellent locations to divide complex systems into more easily understandable “subsystems.” The distribution system can be assessed separately. To understand how the system operates, each element needs to be evaluated, starting with the pump.
Figure 2. Pump performance curve for the raw water pump
Table 1. The energy balance for the system—hydrauilc energy usage in the demineralized water system including all associated costs
Notice that the differential pressure across the control valve represents approximately 33 percent of the total energy developed by the pump. That appears to be a high percentage, but it is less than the 66 percent consumed by the process. Those are interesting figures, but demineralized water is required for the plant to make its products. Speaking in abstracts (feet of head), it is difficult to get a handle on what is occurring.
Look what happens when the associated costs are considered. Using data from the pump curve, the head, flow and efficiency of the raw water pump at 600 gpm can be determined. Then the horsepower supplied to the pump shaft can be calculated. Using the efficiency of the electric motor, the amount of electrical power consumed can be determined. With the pump running 8,000 hours per year (hrs/year) and a power cost of $0.10 per kilowatt hour (kWh), the operating cost of the raw water pump is $284,400 per year. Now the energy usage for each item in the system and its associated cost can be seen.
The information in Table 1 looks more like a balance sheet for the energy costs of the process. With this information, everyone in the plant has a much better idea of how the system operates and the true costs involved. With accurate cost data, management can evaluate this system like any other plant expense. Questions can be asked and discussions started about ways to reduce operation costs and improve system efficiency.
In next month’s column, an existing system will be evaluated, considering different improvements, and the results will be explained.