Selecting the right submersible pump requires both assessing the pump’s technical criteria from the manufacturer, but also a thorough analysis of the intended application. In this month’s SWPA Insight, SWPA member Randy Crawford of Mody Pumps, Inc. answers your questions about pump selection and lays out the key reminders every pump users should follow.
How can pump users assess their application to select the right pump? What are the first items on a checklist every pump user should have?
Submersible pumps come in different designs and sizes, each having specific characteristics and capabilities to meet various operating conditions. They are typically selected after all system data has been gathered and computed. These data include the flowrate and pressure the pump must produce to achieve the desired end result, properties of fluid or the medium being pumped, installation type and environment, service duty cycle and available electrical power.
After the system flow rate and pressure has been determined, one of the first items on a checklist every pump user should know before selecting a pump is understanding the properties of the liquid being moved. This would include PH range, specific gravity, vapor pressure, viscosity, temperature, entrained solids size, type and percent of solids, and the presence of other impurities or gases. Knowing this information will help determine best pump features for the application. These would include materials of construction for component castings, elastomers, mechanical seal, impeller, motor shaft, motor size and rotational speed, plus any optional accessory items that might be recommended by the pump manufacturer.
Pump capacity is linked primarily to impeller performance, but what other factors—such friction, leakage, shock losses, etc.—should pump users be aware of and how to they affect pump capacity?
There are internal hydraulic friction, leakage, and shock losses that occur during pump operation within the impeller and casing passages.
Friction losses are generally proportional to the surface roughness of the wetted areas of the impeller and casing. Leakage losses result from the flow of liquid between the clearance of stationary and rotating parts, such as impeller to stationary wear ring. Shock losses occur as the liquid enters the impeller entrance vanes and as the liquid flows from the impeller into the casing.
These internal losses vary throughout the flow range of the pump and the best way to determine variances or actual pump performance compared to a manufacturer’s published head-capacity curve is by testing.
The clearance ratio between the impeller and the volute suction is critical for solids handling with submersible centrifugal pumps. What questions should pump users be asking?
To preserve optimum pump performance, maintain efficiency and to prevent “ragging” or “clogging” that can take place between wear surfaces, the clearance ratio between the impeller inlet face and volute suction cover (or pending pump design – volute wear ring) should be maintained according to the manufacture’s tolerances.
During pump operation, as wear takes place over time, the clearance ratio between the impeller inlet face and the volute suction cover will increase. Thus, creating a larger gap. The pump user should ask if the clearance ratio between these two components can be adjusted back to the manufacturer’s tolerance without having to replace any components. Are any special tools required other than a standard mechanic’s tool box? What is the procedure and is it covered by the IO&M manual?
Materials of construction and rotational speed of the pump also impacts wear that will affect the clearance ratio. It is always a good idea to discuss options with the manufacturer or the manufacturer’s representative before settling on a specific pump model or design.
How can the wastewater pump user best make use of maximum spherical solids size ratings data? What factors go into different “sphere size” criteria and how does that affect the pump’s use?
The wastewater pump user can best make use of the maximum spherical solids size ratings data by understanding the system in which the pump will be installed, and knowing the maximum entrained solids size that will likely pass through the pump. This really goes back to understanding the properties of the liquid being moved, by both the pump manufacturer and user.
Each pump manufacturer’s literature specifies the maximum solids size that can be handled by a particular pump model. In most cases, this information can found on a particular pump’s head-capacity performance curve.
Factors that influence “sphere size” handling criteria includes the size of the selected pump, and the design and type of impeller. A standard 4-inch submersible wastewater pump will generally handle spherical solid sizes ranging from 2-inches to 3-inches. Of course, there are larger pumps being produced today that can handle much larger diameter solids.
With respect to impeller type, there are a number from which to choose. Two of the most basic types found in wastewater applications are the enclosed channel style and the semi-open vortex style. Each has its advantages and disadvantages. The vortex impeller is best for fiber laden liquids, but may require more horsepower for the pump to operate due to lower hydraulic efficiency. The enclosed channel impeller provides better hydraulic efficiency, so typically less horsepower is required to achieve the same operational duty points, and as long as its passage ways are large enough to accommodate the spherical solids size entrained in the liquid, this impeller is a good choice.
Again, always consult with the pump manufacturer or the manufacturer’s representative for specific application issues, concerns and recommendations.
What is the relationship between the standardized testing and user experience?
Most reputable pump manufacturers test their pumps under strict controls in a test laboratory at the factory in accordance with published industry test standards. Once the pump reaches the field and undergoes start-up testing, variances most always appear when comparing the certified test report from the factory to that in the field. These variances can be due to a number of reasons:
System head-capacity design calculations differ from as built system requirements
- Incoming power anomalies
- Rotational shaft speed variances
- Pressure gauge locations in system and calibration issues
- Flow meter location, compatibility and calibration issues
For More Information:
The Submersible Wastewater Pump Association (SWPA) is a national trade association representing and serving the manufacturers of submersible pumps for municipal and industrial wastewater applications. Founded in 1976, the association’s primary focus is on industry guidelines, education and promotion. For more information, contact SWPA headquarters at 847.681.1868 or visit www.swpa.org.
MODERN PUMPING TODAY, May 2016
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