Pumps are machines that transfer liquids from suction to discharge by converting mechanical energy from a rotating impeller into what is known as head. The pressure applied to the liquid forces the fluid to flow at the required rate and to overcome frictional losses in piping, valves, fittings, and process equipment.
When it comes to pump selection, reliability and efficiency go hand-in-hand. Generally, a pump that has been selected and controlled properly for its normal operating points will operate near its best efficiency point (BEP) flow, with low forces exerted on the mechanical components and low vibration — all of which result in optimal reliability.
There are several factors like process fluid properties, end use requirements, environmental conditions, pump material, inlet conditions, and others which should be considered while selecting pumps for industrial applications. Selecting the right pump type and sizing it correctly are critical to the success of any pump application. Pumping applications include constant or variable flow rate requirements, serving single or networked loads, and consisting of open loops (nonreturn or liquid delivery) or closed loops (return systems).
Some crucial factors considered while pump selections include:
Fluid Properties: The pumping fluid properties can significantly affect the choice of pump. 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.
- 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: It affects the energy required to lift and move the fluid and must be considered when determining pump power requirements.
- Vapor pressure and Viscosity: Proper consideration of the fluid’s vapor pressure will help to minimize the risk of cavitation. 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.
Materials of Construction: It is always required to check the compatibility of materials of construction with the process liquid or any other liquids the pump might encounter. The initial cost of these materials is normally the first consideration. The operational costs, replacement costs and longevity of service and repair costs will, however, determine the actual cost of the pump during its lifetime. Charts are available to check the chemical compatibility and identify the most appropriate materials of construction for the pump.
The impact of the impeller material on the life of a pump under cavitation conditions is shown in Figure 1. As an example, changing from mild steel (reliability factor of 1.0) to stainless steel (reliability factor of 4.0) would increase the impeller life from cavitation damage by a factor of four. Hard coatings, such as certain ceramics, can also increase the impeller life under cavitating conditions.
Pump Sizing and Performance Specifications: The desired pump discharge 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 can be achieved by providing a constant flow rate, or by using a throttling valve or variable speed drives.
Initial sizing of a pump can be done based on Cordier Diagrams (Figure 2), using other available charts in open literature, or by using pump design programs like AxSTREAM® to create a low fidelity conceptual design, or to create a fully fleshed out design with 3D geometry, performance curves, and optimized in CFD analysis. In pump sizing, the specific speed can be a criterion for which a pump with an unspecified diameter would run to give unit volume flow and pressure. The correlation for specific speed and specific diameter can be seen here:
where, ‘N’ is rotational speed (RPM), ‘Q’ is flow rate (ft3/sec), ‘H’ is head (ft), ‘D’ is diameter (ft)
An incorrectly sized pump can lead to operating problems within the pump fluid transfer system. Process industry estimates indicate that the cost associated with using oversized pumps is staggering, and costs companies billions of dollars each year. In addition to costs, oversizing a pump has additional implications, such as excessive vibration, premature bearing failures, operating temperature increases and cavitation issues.
On the other side, under-sizing an industrial pump can also be a concern, especially with certain pump types, such as progressive cavity pumps. The flow of a progressive cavity pump is determined by the size of the cavity. An undersized pump with a smaller cavity needs to pump faster and work harder, which can strain pump components and lead to reduced suction capabilities, increased wear, and premature failure.
Pump Type: After selecting the proper material of construction to be compatible with the process and initial sizing the pump for the operating parameters, the next step is selecting the pump type. Many different types of pumps are available to select from for the different process industries. A few examples are shown in figure 3.
Environmental Considerations: Important environmental considerations include ambient temperature and humidity, elevation above sea level, and whether the pump is to be installed indoors or outdoors. If outdoors, special construction or installation considerations may need to be made for freezing temperatures. If the environment is hazardous, such as if it contains explosive vapors or dust, special motor features will be required.
These are the necessary factors which should be accounted for while performing the pump selection. There are other factors which can be considered by the designer or end users, like the number of stages, different pump configurations like axial, radial, in-series or parallel etc. to enhance the pump performance along with safe operations. Generally, end users select pumps from the available designs offered by manufacturers, due to which End users have to compromise on aspects like performance, safety, reliability etc. In such a scenario, they can also consider utilizing tools like AxSTREAM, where an engineer can design a basic conceptual design, or a completed design with all the geometry created and accounting for rotor dynamics analysis, bearing supports, and with estimated leakages, secondary flows, and other efficiency losses. They could then take this design to a manufacturer, and create a custom pump that meets all industry standards as well as the requirements for the end-user.
To learn more about AxSTREAM, or request a trial, visit our page software page at https://www.softinway.com/software/.