Design of a Wastewater Pump

A wastewater pump or sewage pump is used to transfer sewage liquids and solids from homes and businesses through sewer pipes to the wastewater treatment plants. This system is made up of a network of pipes, and by means of gravity the waste flows into the main sewer. Consequently, wastewater pumps are required to lift the wastewater from lower pipes to higher pipes so that it can continue to flow to the treatment plants.

Wastewater pumps have grown in importance with the rise of solid content in water. However, their design can be quite challenging because in addition to meeting acceptable efficiency and suction levels, their impellers are also required to be able to pass massive chunks of waste material (which could include soft solids up to 3″ in diameter) and still be structurally strong. So, this is clearly a complex multidisciplinary problem, and that is why conventional methods often involve large numbers of CFD and FEA runs in order to ensure that the appropriate trade-off is achieved, which consumes significant computational time and resources.

Therefore, the question is whether it is possible to develop a rapid optimization methodology focused at maximizing the solid-handling capacity of the impeller but without adversely impacting its efficiency, cavitation and stress aspects, and this is what we aim to explore through this project.

Pump Design Process
The first step in the design of any pump is to identify the required specific speed regime of the pump. This will dictate the meridional shape and hence the general flow direction through the pump. For example, a low specific speed pump is likely to be a radial or centrifugal type, whereas a high specific speed pump is likely to be of mixed flow or axial type.

From the required specific speed, we can identify the main flow phenomena and loss mechanisms dominant in that particular range as shown in Figure 1. From this information we can use design tools and 3D CFD to investigate our pump designs, and what follows is a set of principal design guidelines based on the fluid dynamics considerations of reducing dominant flow losses for a given impeller / diffuser.

 

Figure 1:  Loss breakdown in centrifugal pumps

Meanline Design of Wastewater Pump
Here are the specifications that are used to design the wastewater pump impeller:

•  

  • Centrifugal pump
    • Rotational Speed: 900 rpm
    • T-T Head: 6.096 m
    • Mass flow rate: 18.89 kg/s
    • Impeller TE Diameter: 257.175 mm
  • Sphere diameter: 76.20 mm (3”)
  • Specific speed ns: 247 [rpm, m³/min, m]
  • Specific speed nq: 32 [rpm, m³/s, m]
  
  

Using the meanline code TURBOdesign Pre, it is very easy to enter the given specs and verify that it sits in the high efficiency centrifugal region of the specific speed diagram, and then it quickly generates the meridional shape of the impeller in less than a second as shown in Figure 2. For comparison, a design with no constraints specified is shown alongside, which would have given us a 5-bladed impeller but with highly insufficient inlet and outlet passage widths. Thus, the inlet and outlet dimensions and axial length are specifically constrained so that a 3” sphere can pass through the impeller passage, and so a 2-bladed impeller is recommended in this case. However, such over-sized channels are prone to flow separations at duty point and so they need to be designed very carefully. The meanline code also generates a detailed report including the estimated efficiency and some important dimensions, which will be used for the 3D inverse design of the impeller blade in the next section.

 

Figure 2: Meanline design of wastewater pump impeller in TURBOdesign Pre

Software Demo - Meanline Design of Wastewater Pump

3D Blade Design of Impeller
Figure 3 presents the setup for the baseline impeller in our 3D inverse design software TURBOdesign1, where the initial settings automatically come from the meanline code TURBOdesign Pre. It is also possible to impose a custom thickness profile, which is very important for stress considerations. The spanwise work distribution is free-vortex, with zero swirl at the leading edge and a constant value from hub to shroud at the trailing edge. The loading distribution is aftloaded for this initial design, and then these inputs result in the 3D geometry of the impeller also shown, along with a smooth relative velocity distribution throughout the blade surface.

 

 
Figure 3: 3D blade design of wastewater pump impeller in TURBOdesign1

Software Demo - Design of Wastewater Pump Impeller

Baseline CFD Analysis
Once the wastewater pump impeller design is ready, a CFD analysis is run on the baseline stage to check the performance. As Figure 4 shows, ANSYS TurboGrid is used for the fully structured grid of the impeller, and CFX for the flow analysis. Here are the different CFD settings where the boundary conditions are chosen to match the TURBOdesign Pre case:

• P₀₁ = 1.0 atm
• m₂ = 18.89 kg/s
• Impeller-diffuser interface = frozen rotor
• Turbulence model = k-omega SST
• Total mesh size ≈ 1.8 M
• Average blade y⁺ < 10

 

Figure 4: Wastewater pump impeller CFD setup

Figure 5 presents the CFD results which indicate that the baseline impeller actually exceeds the target head and there is a margin considering the losses in the downstream volute. Furthermore, the baseline impeller is able to achieve a high peak efficiency of more than 80%.

 

Figure 5: Wastewater pump impeller baseline CFD results

However, as mentioned earlier, there is an additional requirement of non-clogging on the impeller. So when a verification is performed by attempting to pass a 3” sphere through the impeller passage, it is found to get stuck in a couple of locations as reported in Figure 6. Therefore, it is decided that in order to impart the required waste-handling capacity to the impeller, the narrow section between the impeller leading edge and the adjacent blade suction surface needs to be enlarged, and the same needs to be performed near the trailing edge.

 

Figure 6: Wastewater pump impeller solid-handling verification

In the second part of this article, we reveal how the waste-handling capacity of this baseline impeller can be enhanced through a rapid automatic optimization without adversely impacting its efficiency, suction performance or structural integrity.