Pump systems are common to plants of all kinds. Their reliable operation is critical to production, and their associated operating costs represent a significant part of plant operating costs. Pump systems account for about 25 percent of electrical energy consumption, and that value rises to more than 50 percent in pumping-intensive industries, according to the U.S. Department of Energy. Fortunately, pump system optimization can reduce these systems' energy consumption by as much as 20 percent in a typical plant.
In spite of their large operating cost, most pump systems are inefficient by design. Among the common design problems are non-optimal pipe sizing or control valve utilization. Perhaps the most common problem is incorrectly sized pumps. The breadth of the problem is demonstrated by the fact that a leading chemical company recently evaluated its internal practices found that it had been sizing 90 percent of its pumps incorrectly. While this may sound like a pump problem, focusing efficiency improvements solely on the pump is misguided. How a pump operates depends on the system, so improvements must focus on the system as a whole.
System ModelingPump system computer modeling is the application of well-accepted principles of fluid mechanics to the calculation of flow through pump and piping systems, replicating the interdependency of the various system components in the digital domain. System modeling allows changing any aspect of system design or operation to see how the system as a whole will respond.
To focus on pumping systems' energy savings, efficiency and economics, the Hydraulic Institute, Parsippany, N.J., developed Pump Systems Matter (PSM), an educational initiative intended to help North American pump users gain a more competitive business advantage through strategic, broad-based energy management and pump system performance optimization. PSM produced the pump system improvement modeling tool (PSIM), a free educational software tool focused on helping pump system engineers better understand the hydraulic behavior of pumping systems and how modeling tools can improve pump system efficiency.
PSIM's capabilities include:
- System hydraulic calculation.
- Pump efficiency and BEP evaluation.
- Variable-speed pumps.
- Flow and pressure control valves.
- Impeller trimming.
- Automatic pump curve viscosity corrections.
- NPSH calculations.
- Pump vs. system curve generation.
- Pump energy use and energy cost.
PSIM users build pump system models in a diagrammatic format within a drag-and-drop interface. Pump, pipe and component data then are added through specification windows for each component. PSIM includes built-in databases of pipe materials and a range of fluids, or users can enter their own pipe and fluid data.
Modeling with PSIM: An ExampleConsider a system consisting of a three-cell cooling tower, two circulating pumps and interconnecting piping. Figure 1 illustrates the simplified system model as it appears in PSIM's workspace. Remember that while it provides useful, quantitative results, PSIM is an educational tool. Comprehensive, detailed system modeling will require one of the commercial modeling tools available.
PSIM models have only two types of objects: junctions and pipes. Junctions are system components such as the cooling tower basin, pumps, valves, cooling load, spray discharges and branch points, all of which are connected by pipes. Producing a model requires dragging the junctions from the toolbox onto the workspace and then connecting them by drawing pipes from junction to junction.
Double-clicking a pipe or junction opens a specification window that displays its data input fields. Pipes require a diameter, length and friction information (roughness, Hazen-Williams factor, etc.). These either may be selected from the built-in pipe database or specified directly. Optionally, you can include loss factors for the fittings and valves within the pipes by selecting from the included fittings and valves list, which contains several hundred items.
While elevation data is common to all junctions, other junction data is as varied as the types of junctions available within PSIM. Reservoir junctions need a liquid level; valves either may be selected from the included list or specified using a K factor, Cv or resistance curve. Pumps can be modeled as fixed flow, fixed head or pressure rise, or by entering performance curve data for flow, head, efficiency and NPSHR. In addition to efficiency data for the pump itself, you can include efficiency data for the drive motor and variable-speed drive, as applicable.
In the example shown in figure 1, a reservoir junction represents the cooling tower basin; pump junctions represent the pumps; a valve junction stands in for a throttling valve used for flow control; a general component junction covers the central plant heat exchangers the system serves; and three spray discharge junctions represent the distribution piping and nozzles in the cooling tower's three cells.
Pressing the ButtonsThe valve in the example demonstrates the flexibility that modeling offers in evaluating systems for which data may be limited. Consider the case in which the valve's operating position is known but resistance data is unavailable. You can determine the valve's effective resistance using data available for the other components and the operating data for flow rate and pressure. Vary the valve's K values -- effectively changing the valve position -- during iterative runs. When the available flow data and the calculated flow rate converge, you've found the operating valve resistance. This simple process works because modeling simulates not just the characteristics of individual components and piping but also the interdependency of the system elements.
The paired pumps in the example are specified using the manufacturer's head, efficiency, NPSHR and motor efficiency data. Similarly, the spray discharge junctions are set to an elevation corresponding to the cooling tower inlets and specified with a resistance curve based on the tower manufacturer's data for design flow and supply pressure.
Other variables specified in the example include an energy cost of $0.06/kW-hr and a one-year time period to evaluate annual energy cost. Any time period can be used, and interest and inflation rates can be specified to calculate the net present value of the energy cost.
Reading the ResultsThe text-based results are shown in PSIM's output window (figure 2), which is arranged in three sections. The general section at the top includes summary reports for the pumps, valves and reservoirs as well as the cost report. The middle section is the pipe output, and the junction output section is at the bottom.
Figure 2 illustrates only a fraction of the fully configurable PSIM output data that can be presented in either English or SI units. In addition to text, PSIM includes two graphical outputs. The visual report displays selected input or output data with the model diagram. The graph results provide graphing capabilities for a range of system variables such as the pump vs. system curve (figure 3).
The pump summary report in figure 2 provides key information related to system efficiency. Overall power for each pump -- that is, power into the motors as a function of pump and motor efficiency -- is 213.3 hp each. Energy consumption for one year is $83,569 for each of two pumps. While according to the manufacturer's data, these pumps have a 76 percent peak efficiency, PSIM shows they are operating at only 65 percent efficiency in this scenario. The reason is shown in the “% of BEP” column, which indicates the pumps are operating at only 72 percent of the best efficiency point (BEP) flow: In other words, these pumps are oversized. The valve at the pump discharge must be throttled to achieve the design flow rate. The last line in the output window says the valve throttling imposes a loss of 39' of head. So, system efficiency takes a hit from two directions: the pumps' lower operating efficiency and the head loss across the throttling valve. Better matching the pumps to the system requirements will address both of these issues.
Tweaking the NumbersTwo potential modifications that would help better match the pumps to the system are trimming the impellers and reducing pump speed. Both assessments can be made within PSIM's modeling capabilities. Likewise, PSIM can handle an alternate operating line when only two cooling tower cells and one pump are operated during cold weather. Table 1 illustrates the PSIM results for the current and modified pumps for both cases.
The first row represents the initial situation, which suffers from reduced pump efficiency and throttling losses as discussed above. The second row shows the results of reducing pump capacity by trimming the pump impellers; the third row shows the results of reducing pump operating speed. Both alternatives eliminate the need for throttling because the pumps are now matched to the system requirements.
Impeller diameter and speed affect pump performance in a similar manner, so the amount of impeller diameter change required is the same as the speed adjustment needed. Note that when optimized, annual energy cost is reduced by $75,236, or 45 percent. Trimming impellers requires heavy lifting, while speed adjustment requires installing a variable-speed drive. Take your pick.
Rows four through six illustrate how the system will operate under cold weather conditions. Throttling the original pumps will reduce the flow. Some throttling also is needed for the trimmed impellers because the trim was based on the original case. Still, there is an annual energy cost reduction of almost 44 percent compared to the existing pumps. Variable-speed drives, however, without throttling, can reduce the annualized energy cost to 47 percent below the original case. Whether this savings justifies installing variable-speed drives will depend on the reduced flow mode duty cycle.
Finally, the last row explores the effect of increasing the size of the main supply and return lines from 20" to 24" along with trimming the impellers. Reducing piping losses and matching the pumps to the system reduces annual energy cost by 53 percent, or about $14,000 more than just trimming the impellers. Whether this additional savings is justified by the cost of changing the piping is doubtful, but it illustrates the potential savings when building a new system, expanding an existing system or replacing corroded piping. The additional cost is only the incremental difference for the larger piping.
In conclusion, if this example has piqued your interest in learning about the benefits of system modeling, try PSIM yourself. The free software can be downloaded from Pump Systems Matter at www.pumpsystemsmatter.org. The PSIM software includes several examples with step-by-step instructions available via its comprehensive help menu.