When selecting a pump, there are many criteria. But the most important consideration is selecting the pump to meet system requirements, not the other way around. What is meant by that is “tried and tested” pumps may not be the most effective as the application parameters may have changed – or even the pump. The optimal approach is to assess the entire system to improve performance, efficiency, and reliability.
When choosing a pump, consider the Total Cost of Ownership (TCO). You might think the actual pump purchase is an important element of TCO, but in fact, the initial cost of the pump accounts for only 10% of the TCO, while energy consumption accounts for a whopping 65%. Other TCO components include:
- Installation and commissioning costs (including training)
- Energy costs (predicted cost for system operation, including pump driver, controls, and any auxiliary services)
- Operation costs (labor cost of normal system supervision)
- Maintenance and repair costs (routine and predicted repairs)
- Downtime costs (loss of production)
- Environmental costs (contamination from pumped liquid and auxiliary equipment)
- Removal and disposal costs
So, while initial cost needs to be considered, you see there are many more factors in the TCO. Most importantly for specifiers or purchasing agents is to overlook the initial cost and buy a pump system that will work best for the application, handle the materials or volume, manage the flow of energy, and ensure reliability. An incorrect purchase can result in more money spent over time on repairs, poor energy efficiency, or lost production potential.
The energy consumption required for any system depends on the flow rate of the entire system, including the pressure (head) and how often the pumps are operating. Some pumps run all the time, whether the process needs all that flow or not. When systems divert flow, operators are paying for power they are not using productively.
One way to reduce the energy consumption is by utilizing a Variable Frequency Drive (VFD), which can increase or decrease flow as needed. You can purchase a VFD that has the pump performance programmed in by the factory or retrofit existing pumps. Retrofitting requires that you have the space to install the VFD near the pump. In addition, installing instrumentation and connecting it as well as programming creates opportunities for error.
Another solution to offset energy costs is to use a bank of large and small pumps and stage them to turn on and off to meet demand.
Tip: Pumps that have an HI (Hydraulics Institute) Energy Rating Label will increase the savings.
Consider the pump’s footprint — Frame Mounted vs. Close Coupled vs. Inline
How much available space and the pump’s footprint are factors to consider in the selection process.
Frame mounted pumps include a bearing housing to prolong the life of the bearings and allows for continuous operation with high radial and thrust loads. Typically used for larger applications where power ratings range from 20 hp to 200 hp, frame mounted pumps don’t need custom motors, giving you more choices to select a motor that meets the needs of your application.
Close coupled pumps occupy a smaller footprint and have only one set of bearings inside the motor casing. Because they do not need couplings, as the motor is directly mounted to the pump on a single shaft, they are also typically less expensive than frame mounted. However, the motor bearing must handle the axial and radial loads of the applications, which limits the size and power of the motor used. Close coupled pumps may be limited to 100 hp to 150 hp but offer space savings of 20%.
Inline pumps can dramatically reduce the footprint. Flow enters and exits on a single axis, requiring minimal floor space. For example, inline pumps can occupy a third of the floor space compared to a typical frame-mounted pump. Keep in mind that an inline pump also requires vertical space because they typically have a vertical motor above the pump.
Flow Requirements and Fluid Properties
When choosing a pump, fluid properties must be considered: density, viscosity, solids content, and temperature. Understanding fluid properties are critical to avoid failure or the need for continuous and costly maintenance. Key considerations include:
Acidity/alkalinity and chemical composition. Corrosive and acidic fluids can degrade pumps and should be considered when selecting pump materials.
Operating temperature. Pump materials and expansion, mechanical seal components, and packing materials need to be considered with pumped fluids that are hotter than 200°F.
Solid’s concentrations/particle sizes. When pumping abrasive liquids such as industrial slurries, selecting a pump that will not clog or fail prematurely depends on particle size, hardness, and the volumetric percentage of solids.
Specific gravity. The fluid specific gravity is the ratio of the fluid density to that of water under specified conditions. Specific gravity affects the energy required to lift and move the fluid and must be considered when determining pump power requirements.
Vapor pressure. A fluid’s vapor pressure is the force per unit area that a fluid exerts to change phase from a liquid to a vapor and depends on the fluid’s chemical and physical properties. Proper consideration of the fluid’s vapor pressure will help to minimize the risk of cavitation.
Viscosity. The viscosity of a fluid is a measure of its resistance to motion. Since kinematic viscosity normally varies directly with temperature, the pumping system designer must know the viscosity of the fluid at the lowest anticipated pumping temperature. High viscosity fluids result in reduced centrifugal pump performance and increased power requirements. It is particularly important to consider pump suction-side line losses when pumping viscous fluids.
The Flow Rate Factor
The flow rate is determined by total volume and the time to move the fluid through the casing and out to the desired location or piping system.
For example, with centrifugal pumps, the flow varies with changing pressure. These pumps impart momentum to the fluid by rotating impellers that are immersed in the fluid. The momentum produces an increase in pressure and when pressure is created, flow results.
When a system includes a centrifugal pump, an important design issue is matching the head loss-flow characteristic with the pump so that it operates at or close to the point of its maximum efficiency. When fluids of higher viscosity move through the system, the pump’s efficiency with decrease in head and flow.
On the other hand, a positive displacement pump can handle many applications that include viscous products. They can operate at high pressure and relatively low flow more efficiently. However, they are less able to handle low viscosity rates than centrifugal pumps.
These are many nuances to choosing the correct pump, with these listed being just a few to keep in mind. So, which pump to choose? That’s where the JMI Pump Systems team can help. Our knowledgeable and experienced professionals are available to assist you in finding the right pump for an application. For more information, contact us at 262-253-1353 or firstname.lastname@example.org.
Note: Content for this blog includes information from the following resources: Processing Magazine, Pumps & Systems magazine, and the U.S. Department of Energy.