Considerations for system optimization amid dwindling supply & growing demand
The need to efficiently and effectively locate, treat and distribute clean water for human consumption is a growing one. Last year, a government-backed study explored how the combined forces of population growth and climate change will pose significant challenges to water management in the U.S. This scenario places additional pressure on water treatment plants to improve their process reliability, efficiency and production. Energy requirements to provide water treatment and conveyance activities are substantial. Poor selection and application of the pumps, flow control, and motors, leads to wasted energy and less reliable water treatment processes.
Inspired by the Hydraulic Institute’s (HI) publication “Water Treatment Plant Pumps: Guidelines for Selection, Application and Operation,” which was developed by a committee of subject matter experts, below are a few considerations for water treatment professionals to make when assessing pump and implementing pump solutions.
Know the Range of Applications
Whether potable water originates from streams, oceans or aquifers located below the ground surface, it undergoes many process streams to remove solids and contaminants and support disinfection to kill microorganisms and viruses. These process streams within a treatment plant require consideration of some key properties during pump selection, including solids content, viscosity, corrosiveness and temperature.
As a result, a conventional surface water treatment plant can have around 14 different pumping processes (depicted in Fig. 3), each of which requires specific pump designs to handle the various liquid stream properties that range from clean water, sludges, and water laden with solids to chemicals for disinfecting. The plant designer must understand the demands of each of these processes, the characteristics of each process liquid and an understanding of the appropriate pump designs and materials of construction suited for the application.
For each of the 14 pump applications listed in Fig. 3, there are several suitable pump designs. The designer must consider and evaluate all relevant factors to ensure the selected pump can perform the service as intended.
Consider Flow Capacity & Material
Because the treatment plants are sized for the drinking water demand of the community, pumps must be sized to operate over a wide range of flow and head to ensure that the demand is met while maintaining a consistent system pressure. The high-service pumps, such as horizontal split case (Fig. 2), horizontal end suction, and vertically suspended/vertical turbine (Fig. 1), are required to move high volumes of treated water into distribution systems serving communities. Normally, multiple units operating in parallel are used to adjust to the changes in demand. Multiple smaller pumps or pumps with variable speed drives can also be used to allow the pumps to match the plant’s flow capacity.
The pumps used in this treated water service should also be constructed of material suitable for drinking water application. Materials in contact with treated water should be lead free and comply with NSF 61.
Follow a Thorough Pump-Selection Procedure
Overall, many evaluations must occur during pump selection. Below are some crucial points of procedure that water treatment plant designers can use in coordination with pump manufacturers for pump selection, verification, refinement and final pump selection.
- Identify the treatment plant process and the important characteristics of it.
- Determine the maximum and minimum flow requirements of the process, the system curve(s), and range of net positive suction head available (NPSHA).
- Reference Appendix A of “Water Treatment Plant Pumps,” choose a pump design type from the recommended list for the process application, and identify the orientation of the pump to be installed.
- Based on the NPSHA and the maximum flow rate and the maximum suction specific speed (S) desired, the designer should calculate the maximum rotational speed for the pump. Refer to industry standards ANSI/HI 14.32 and ANSI/HI 9.6.13 for additional details on this topic.
- Select a pump curve that meets the maximum operating points at the speed determined in Step 3. For select pump types that are below 200 hp, the HI Energy Rating Program (http://er.pumps.org) is a resource to choose the most efficient pump that meets the operating points. Determine if the design operating envelope for the pump falls within the preferred operating region (POR) for the pump, and if it does not, review if reliability and energy impacts will be unacceptable due to operation outside the POR. Multiple smaller pumps operating in parallel, variable speed pumping or a combination of both should be considered to achieve operating in the POR. For full information on the achieving operation in the POR, refer to ANSI/HI 9.6.34, and HI’s “Application Guideline for Variable Speed Pumping.”
- Overlay the selected pump curve(s) with the system curves to verify the pump(s) will operate in an efficient and reliable manner, and select a driver that will be non-overloaded throughout the operating envelope.
- Consult Appendix C of the “Water Treatment Plant Pumps” to identify the proper materials for construction for the casing, impeller, shaft, seals and hardware. Review these material selections with the pump manufacturer(s) based on your specific application requirements and make final material selections.
- Determine if the pump installation will require upfront dynamic analysis to ensure separation of the operating speed and other forcing frequencies from structural natural frequencies or rotor critical frequencies. Refer to ANSI/HI 9.6.86 for recommendations.
- Make final pump selection, create equipment data sheet with purchase and commissioning.
A drinking water distribution system serving a residential community is able to save 33% on its energy consumption by changing from bypass to variable speed control when maintaining 80 pounds per square inch (psi) to the community. The system diagram for each is shown in Figures 4 and 5. The return on investment for this indicates that the investment will return approximately $35,000 or internal rate of return of nearly 18%.
Selecting the right pump design for the application is vital to efficient and reliable operation. The process designer, must have a thorough understanding of the treatment process, its liquid composition, industry standards and best practices and work collaboratively with trusted pump manufacturers to ensure a successful outcome. No pump selection procedure can cover every detail that needs to reviewed and considered, but what has been listed covers some of the most important aspects of pump selection.