Getting greater efficiency with modelling software

Software has been developed with the aim of analyzing and modeling these systems. Using this approach, a study at the University of Matanzas in Cuba came up with a solution saving thousands of dollars a year.

According to the United States Department of Energy, pumping systems consume about 20% of world energy offering major potential to promote a program of energy efficiency while reducing carbon dioxide emissions. As a result, the educational initiative Pump Systems Matter (PSM) was created by the Hydraulic Institute (HI) USA, to help process industries, organizations and municipalities reduce energy costs, optimize pumping systems and take competitive business advantage through strategic management tools based on energy efficiency.

The first tool available in PSM is the pumps system improvement modeling tool (PSIM). This software is freely available and has been created for educational use allowing engineers to model the hydraulics of small pumping systems to evaluate potential savings. The US Department of Energy also made the made pumping system assessment tool (PSAT) available.

The life cycle cost is the best method of identifying the optimal integration of motor driven power systems but is not yet an industry practice. The aim here is to analyze the possibilities of improving the efficiencies that can be made to a pumping system using these tools as well as a life cycle cost analysis tool (LCCA).

In the water pumping system at the University of Matanzas, water is sucked from a 690 m3 tank by a centrifugal pump and raised to 363, 36 m3 <> tank capacity. The piping system consists of a six inch section suction pipe while the others are five inch. The discharge section has pipes of five, four, five, four and 10 inch diameters in that sequence.

A check from the bottom of the tank was removed and the tank was filled until the water level exceeded the pump level. A check was installed in the pump discharge preventing gasket damage if the water system drew back and turned the pump rotor counter-clockwise.  

For calculation, the piping system was divided into seven sections, two at suction and five at discharge with the corresponding accessories. The pumping head could be obtained by a mechanical energy equation for incompressible and steady flow.

For modeling the different performances PSIM (2007) software was used. A first run was made with the data flow measured at the installation and the results were analyzed with the characteristic curves entered to compare results and provide an effective diagnosis of the pumping system. This tool was also used to analyze the installation changes. PSAT (2008) was employed to find the optimal pump to take on the task with the same flow and load requirements, analyzing energy costs.

To achieve low power consumption various modifications to the pump system were made and analyzed. Changes were made to the pump including impeller trimming analysis as well as selecting a new pump. The modifications analyzed where the greatest losses and energy savings occurred following the improvements.

The results of the calculations using PSIM are shown in Fig.1

According to the pump curves, the efficiency vs. flow chart, which has a maximum of 50%, shows a value of up to 45%.

Operating flow was 136.9 m3/h and the measured flow was lower (129.6 m3/h). This could be because of factors such as maintenance, wear and higher than normal clearances so decreasing pump flow, while still staying within the allowable parameters.

The pump efficiency is 45.1% which satisfies the η ≥ 45% condition. It is also noted that the operating point of the system is at 132.6% and the maximum is 133% BEP, so the pump is working in the range even if is closer to the boundary parameters.

The pump is not working under the regime of cavitation because it holds that: NPSHdisp> NPSHreq. An analysis through PSAT reveals values similar to the calculations above. It shows the current pump is operating at 70.1% of optimum efficiency, which may save up to $1,300 a year.

The graph below show results of the costs vs. volumetric flow for the most efficient pump by implementation of PSAT (2008) for conditions of flow and head in accordance with the system curve and based on delivering 260m3 of water.

From Figure 4 it can be appreciated that lower energy apply to pumps handling flows between 70 and 90 m3/h with head less than 27m. Three manufacturers’ pumps are selected and modeled for the existing system with PSIM (2007).

With this program impeller trimming is also modeled, showing results of 88%. The results are shown in Table 4 with comparative savings in Table 5.

The above savings apply to the current pump. Analyzing Table 5 it can be seen that the ideal pump for this facility is from producer C, with an impeller of 127 mm in diameter at 3,500 rev / min. This offers the lowest cost for 15 years of the life cycle.

With regard to accessories, a discharge check for suction was changed at the bottom of the tank. A gate valve only let in the discharge and the 10 inch section gate valve was maintained. These modifications to the system were modeled using PSIM (2007) incorporating the current pump, the trimmed impeller pump and the pump from producer C.

For the current pump and the improved system, the flow modeling results yielded 150 m3/h and shaft power of 29 kW. For the improved system results obtained do not give significant savings, because operating flow is reduced and 92% of the system head is static. These system improvements will not be offered, only proposed to solve the problems of misplaced accessory (check) and the extra accessory (valve).

With these modifications the system with PSIM and the pump from manufacturer C was modeled, showing energy savings estimated at 193 kW-h, saving $53.

Modifications made to the piping system did not provide significant savings because 92% of the system head is static. The proposed solution to improve the working efficiency of the pumping installation UMCC is to install the pump from producer C, followed by removal of the discharge check, putting it in the suction pipe located at the bottom of the cistern. This proposal leads to an annual saving of 8927 kW-h, which represents 2,277 CUP to the college and $2410 a year to the country.