Pump systems are common to plants of all kinds, their reliable operation critical to production and their associated operating costs representing a significant part of plant operating costs. Pump systems account for about 25% of electrical energy consumption and more than 50% in pumping-intensive industries, according to the U.S. Department of Energy. Further, pump system optimization can reduce energy consumption of these systems by 20% 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, control valve utilization and, perhaps most common, incorrectly sized pumps. Recently, a leading chemical company evaluating its internal practices found that it had been sizing 90% 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.
Pump 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.
Focused on pumping systems’ energy savings, efficiency and economics, Pump Systems Matter (PSM) has made available the Pump System Improvement Modeling Tool (PSIM). PSIM is 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 (Figure 1).
Figure 1. The opening page for the free PSIM software.
Because it’s based on the leading commercial software package, AFT Fathom, 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 familiar diagrammatic format within a user-friendly drag-and-drop interface. Pump, pipe and component data are then 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
Consider a system consisting of a three-cell cooling tower, two circulating pumps and interconnecting piping. Figure 2 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 full-featured commercial modeling tools available.
Figure 2. Drag and drop the elements into place to generate the flow diagram.
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 may either be selected from the built-in pipe database or the diameter and roughness information may be specified directly. Optionally, you can include loss factors for a variety of fittings and valves within the pipes by selecting from the several hundred items available from the included fittings and valves list.
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, while valves may either 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 our example, a reservoir junction represents the cooling tower basin, pump junctions represent the pumps, a valve junction subs 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 buttons
The valve in our 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.