Computational Fluid Dynamics (CFD) offers significant benefits to the water industry as it can enable engineers to gain insight into performance of existing facilities such as pumping stations and contact tanks. Use of CFD at the design stage is also becoming increasingly widespread, as the detailed insights offered by CFD can be leveraged to optimise design performance and efficiency, while also increasing confidence in design performance across its lifetime and range of operating conditions.
Computational Fluid Dynamics CFD for Pumping Station Performance
Pumping stations are used in water supply and wastewater treatment facilities, and comprise a sump from which water is pumped using either submersible or dry well pumps. Guidelines such as the American National Standard for Pump Intake Design  are used by engineers to design sump dimensions, details, and pump layouts in order to ensure effective operation of the sump under all operating conditions. The aim of effective design is to avoid undesirable hydraulic features such as swirl flow, vortex formation, and unsteady velocity fluctuations local to the pump inlets. These phenomena can have an adverse impact on pump operating efficiency by reducing flowrate, head, and power, and increasing susceptibility to pump cavitation, as well as pump vibration and noise.
Increasingly, CFD analysis is being used at the design stage to demonstrate the efficient operation of pumping stations, and increase confidence in the design. CFD analysis can serve as a cost effective alternative to physical scale modelling, which has traditionally been used to investigate the performance of pumping stations with challenging requirements such as high flowrates, unsymmetrical design features, or details deviating from accepted design standards .
In order to assess a pumping station via CFD, a virtual model of the sump, inlet and extraction pipework, and pumps is developed. To accurately capture the pump operating flowrates and pressure conditions at the pump inlet, a pump characteristic curve is incorporated into the analysis, and resistance coefficients are added to incorporate the effects of system piping not included in the model geometry. Defining the sump inlet flow as a model boundary condition, and selecting the required pump operating speed then allows flow through the sump to be modelled.
Analysis of Pump Intake Conditions:
Swirl flow, pressure, and velocity fluctuations at the pump inlet can be quantified and compared to acceptable values as outlined in design standards , to ensure that conditions likely to affect pump performance are not encountered. CFD also allows vortex flows to be identified, allowing surface and subsurface vortices which may be detrimental to pump performance to be detected.
Analysis of Mixing Performance:
Particularly in the case of wastewater treatment, it is important to mitigate against stagnant flow regions within the sump, as the sewage in these regions can turn septic. The turnover of water in a given region can be assessed by adding a tracer to the water at the inlet location in the CFD model. The simulation incorporating the tracer can be carried out over the filling and emptying cycle, allowing evolution of the concentration with time to be tracked throughout the sump. This analysis allows the turnover of water to be quantified, and highlights regions of low turnover ‘dead zones’ which may be of concern with respect to flow stagnation and septicity.
CFD for Water Treatment Plant Reservoir Baffling Factor Analysis
Chlorine is widely used as a disinfectant for the inactivation of waterborne pathogens in drinking water supplies . Chlorination as part of primary disinfection in a water treatment plant can be achieved within a contact tank, where the chlorine disinfectant can mix with the water in order to achieve the contact time required to eliminate waterborne pathogens.
As described in the Environmental Protection Agency Disinfection Manual , contact time can be determined using a tracer test, which involves introducing a tracer at the tank inlet, and recording the evolution of the tracer concentration at the outlet with time. The time required for the outlet concentration to reach 10%, (t10) is the time value associated with 90% of the water passing the contact tank volume having a greater residence time than t10.
t10 is then used to calculate a baffling factor for the contact tank, which represents the deviation of the tank from ideal conditions for mixing. A value of 1 indicates perfect conditions for mixing such as would exist in pipeline flow, while a value of 0.1 represents an ‘unbaffled’ condition within the contact tank. A low baffling factor indicates that there are flow paths within the tank which result in short-circuiting of the flow and the formation of dead zones within the flow field, which reduce mixing performance compared to the ideal condition.
The baffling factor for the contact tank is calculated using the following formula:
CFD analysis can be used to determine the baffling factor for a contact tank by virtually performing the tracer test as described above. Results of a tracer test conducted either physically or via CFD can be used to refine published estimates of the baffling factor which relate to typical tank geometries and baffling wall arrangements, and thereby relate the baffling factor directly to the tank geometry in question. CFD allows for the opportunity to assess the baffling factor across a wide range of operating conditions and scenarios which may not be practical with physical tracer testing, and can be carried out without the practical operational and health and safety considerations necessary for a physical test.
In addition to providing a means to calculate the baffling factor, CFD analysis offers significant insights into the flow field, which cannot be obtained from physical testing. These insights are gained from observation of flow streamlines and concentration evolution across the tank with time, which are used to identify short circuit paths and dead zones detrimental to mixing performance. These learnings can be utilised to implement design improvements both within existing contact tanks and within future designs.
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- American National Standard for Pump Intake Design ANSI/HI 9.8-1998. 1998, Hydraulic Institute.
- Water Treatment Manual: Disinfection. 2011, Environmental Protection Agency.
SEAM would like to thank Ryan Hanley and Coffey Construction for their permission to publish this content.
About the Author:
Patrick Donnellan is a chartered engineer and a member of the Institution of Mechanical Engineers, with several years’ experience in the assessment of water treatment facilities via Computational Fluid Dynamics (CFD). Within SEAM, Patrick focusses on providing solutions to clients across a wide variety of industries through the use of CFD and Finite Element Analysis (FEA), including include medical device, pharmaceutical, manufacturing, civil and environmental engineering, and HVAC.