Assessing a System with Multiple Loads

Previous “Pump System Improvement” columns covered the assessment of simple systems with a single load. The system discussed in this column has multiple loads. Figure 1 is a schematic of a piping system under evaluation. This system in Figure 1 has three circuits. Circuit 1 consists of the tank (GS-TK-000), the pump (GS-PU-001), the heat exchanger (GS-HX-001), the process element (GS-PE-001), the control (GS-FCV-001) and the destination tank (GS-TK-001), along with the interconnecting pipelines. The supply tank (GS-TK-000), the pump (GS-PU-001) and the heat exchanger (GS-HX-001) are common elements to all three circuits. Circuit 2 includes the supply tank, the pump, the heat exchanger, GS-PE-002, GS-FCV-002 and the destination tank (GS-TK-002).

Previous columns showed that the total head produced by the pump element equals the head consumed by the process elements, including static head, pipeline friction losses and friction losses in process equipment. In other words, the energy added to the fluid by the pump equals the energy consumed by the process and control elements. Pump Energy = Process Energy + Control Energy This basic concept is more complex in systems with multiple circuits. Because of the law of conservation of energy, the energy added by the pump must equal the energy consumed by the process and control elements for each circuit. Table 1 shows the energy use in each circuit. The 236.5 feet (ft) of pump head is supplied to each circuit. The process elements for each circuit are grouped together. The total head (both pressure and elevation) for each tank is included in the identifier field, and the result is displayed in the left column. The number of pipelines in each circuit is displayed along with the sum of the head losses for each circuit. The control elements include the head loss across the control valves for each circuit. The total pump head equals the head loss of the process and control elements in each circuit (see Table 1). Understanding energy consumed by each element in the system allows annual cost of operation to be calculated. Circuit 1 has a flow rate of 400 gallons per minute (gpm), and Circuits 2 and 3 have flow rates of 200 gpm each. The system operates 8,000 hours per year with a power cost of $0.06 per kilowatt-hour. The pump efficiency is 76.5 percent, and the motor efficiency is 94 percent. Table 2 shows the energy cost balance sheet.

Expanding the System

The plant needed to modify the system to meet increased process requirements—Circuit 1 needed to supply 450 gpm, and Circuits 2 and 3 each needed to supply 250 gpm. The engineering group determined that the existing pump could have the capacity to meet the additional flow requirements by increasing its impeller diameter from 15.5 inches to 17.5 inches. The energy cost balance sheet for this modification is shown in Table 3.

Considering the Cost

Increasing the total system flow rate from 800 to 950 gpm raised the operating cost of the system from $23,801 to $30,454. The annual cost to pump one gallon of water in the original configuration was $29.75. For the increased capacity, the cost is $32.05. Increasing the impeller diameter would not decrease costs. The energy cost balance sheet showed that the annual cost for the piping system in Circuit 3 is much greater than the cost for Circuits 1 and 2. Further investigation revealed that the fluid velocity in Circuit 3 is 13.4 feet per second (ft/sec) when passing 200 gpm and 16.75 ft/sec when passing 250 gpm after the system expansion. The high operating costs for the pipelines in Circuit 3 suggested that other options for increasing the flow rate needed further exploration. In the unmodified system, the fluid velocity in the three pipelines in Circuit 3 was excessive. Increasing the pipe diameter from 2.5 to 4 inches would reduce the fluid velocity to 6.3 ft/sec. After performing a preliminary calculation for the new pipe, the estimated operating cost of the pipelines in Circuit 3 dropped from $4,899 to $1,000 per year. Based on this information and the potential for additional savings, plant personnel decided to increase the pipe diameter in Circuits 2 and 3 from 3- to 4-inch pipe without increasing the impeller diameter. Table 4 is the energy cost balance sheet showing the operating cost after the change. Increasing the pipe diameters reduced the head loss in the pipelines for Circuits 2 and 3. This allowed the existing pump to meet the revised system needs without a larger impeller diameter. Much of the cost was transferred from the pipelines to the control elements. Further savings can be obtained by reducing the head developed by the pump. This can be done by trimming the impeller or installing a variable speed drive (VSD) and operating the pump at a reduced speed.

Conclusion

Using the results in the energy cost balance sheet, plant management decided to have an outside maintenance contractor increase the pipe size to 4 inches in Circuits 2 and 3. This new pipe will be fabricated and installed during plant operation and will be tied into the existing system during a plant outage. The existing pump will continue to meet the increased capacity requirements while greatly reducing the system’s pumping cost. Plant management also considered adding a VSD to reduce the pump head. The team delayed the addition because of the large workload during the scheduled outage. In the future, they plan to perform a system assessment to determine if a VSD would provide sufficient savings to justify its expense. My next series of columns will discuss the components of a piping system and how they affect total system operation. The first column will cover the pump element. I enjoy hearing from readers, so if you have any questions or would like me to look into a specific topic in future columns, send me an email.