Editor's Note: Adapted from "Improving Pumping System Performance: A Sourcebook for Industry" developed jointly by the U.S Department of Energy Efficiency and Renewable Energy and the Hydraulic Institute (HI).
Pumps transfer liquids from one point to another by converting mechanical energy from a rotating impeller into pressure energy (head). The pressure applied to the liquid forces the
fluid to flow at the required rate and to overcome friction (or head) losses in piping, valves, fittings, and process equipment.
Pump system designers must consider fluid properties, determine end use requirements, and understand environmental conditions to achieve the optimal results. This includes a thorough understanding of constant or variable flow rate requirements, single or networked loads, and the properties of open loops (nonreturn or liquid delivery) or closed loops (return systems).
The properties of the fluids being pumped can significantly affect the choice of pumps at most plants and commercial institutions. Subsequently, maintenance and engineering professionals should strongly consider the following fluid characteristics when choosing the best pump system for their needs:
Acidity/alkalinity (pH) and chemical composition. Corrosive and acidic fluids can degrade pumps. So, pump materials should be actively researched before their selection.
Operating temperature. Pump materials and expansion, mechanical seal components, and packing materials should be carefully considered with pumped fluids that are hotter than 200°F.
Solids 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 in an effort 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.
End Use Requirements - System Flow Rate and Head
The design pump capacity, or desired pump discharge in gallons per minute (gpm) is needed to accurately size the piping system, determine friction head losses, construct a system curve, and select a pump and drive motor. Process requirements may be met by providing a constant flow rate (with on/off control and storage used to satisfy variable flow rate requirements), or by using a throttling valve or variable speed drive to supply continuously variable flow rates.
The total system head is also designed with three components: static head, elevation (potential energy), and velocity (or dynamic) head. Static head is the pressure of the fluid in the system, and is the quantity measured by conventional pressure gauges. The height of the fluid level can have a substantial impact on system head, while the dynamic head represents the pressure required by the system to overcome head losses caused by flow rate resistance in pipes, valves, fittings, and mechanical equipment. Dynamic head losses are approximately proportional to the square of the fluid flow velocity, or flow rate. If the flow rate doubles, dynamic losses increase fourfold.