Tsuyoshi MAEDA*
Takahiro NOJI*
Tetsuya ISHIWATA*
*
Oversea Business Development Department, Development Division, Building Service & Industrial Company
Fig. 1 Model DKEXU sewage submersible pump
Product specifications of the model DKEXU are shown in Table 1.
Table 1 Product specifications of model DKEXU
Conventional models are designed with sufficient internal flow passages to prevent clogging inside the pump, discharging foreign matter through the flow inside the pump. However, in severe environments where a lot of foreign matter, such as non-woven cloth and fibrous materials, flows into the pump, conventional models can become clogged.
To discharge foreign matter more reliably, the model DKEXU (Fig. 2) is equipped with an anti-clogging mechanism that physically removes foreign matter in addition to using the flow inside the pump. The anti-clogging mechanism consists of three parts: a back swept impeller, guide pin, and suction cover as shown in Fig. 3.
Fig. 2 Structural drawing
Fig. 3 Anti-clogging mechanism
Foreign matter sucked in by the pump has the characteristic of entangling with the leading edge of the impeller rotating at high speed. To address this characteristic, adopting a back-swept impeller2） allows the foreign matter to move radially without resisting centrifugal forces, facilitating discharge.
Guide pins are employed to ensure a more reliable discharge of foreign matter caught on the leading edge. These guide pins are placed opposite the impeller, serving the role of physically collecting foreign matter entangled on the leading edge.
The suction cover of this pump features a groove that connects to the casing flow passage. This groove serves to push foreign matter collected by the guide pins into the casing flow passage. It also suppresses the leakage flow occurring in the gap between the back swept impeller, contributing to high efficiency.
These above three parts enable foreign matter caught by the back swept impeller to be collected by the guide pins and discharged through the groove provided in the suction cover as shown in Fig. 4.
Fig. 4 Behaviour of foreign matter with anti-clogging mechanism
A foreign matter passage test was conducted to evaluate the passage of foreign matter. Specimens were continuously sucked in during pump operation and the foreign matter passage rate was calculated from the number of specimens that passed. Non-woven cloth (length 279 mm x width 242 mm) (Fig. 5) and rope (φ 4 mm, length 900 mm) (Fig. 6), which are actual causes of clogging at pump stations, were used as specimens. A comparison of the foreign matter passage rates of the conventional model and model DKEXU is shown in Fig. 7. The foreign matter passage rate of the model DKEXU has been significantly improved compared to that of the conventional model, achieving a 100 % passage rate for all test specimens.
Fig. 5 Foreign matter passage test: Non-woven cloth
Fig. 6 Foreign matter passage test: Rope
Fig. 7 Comparison of foreign matter passage rates
Additionally, tests were conducted for adult diapers (Fig. 8) and tiger ropes (length 1500 mm) (Fig. 9), confirming similar discharge performance.
Fig. 8 Foreign matter passage test: Adult diaper
Fig. 9 Foreign matter passage test: Tiger rope
CFD (Computational Fluid Dynamics) analysis has been carried out in the development of the back swept impeller, which is one of the main features of this pump. The performance was checked with different impeller and casing shapes, and the pump was designed with optimal parameters to achieve the prescribed performance and efficiency. An example of the CFD numerical analysis for a two-blade-shaped impeller is shown in Fig. 10.
Fig. 10 Example of CFD numerical analysis
A comparison of pump efficiency and head between the conventional model and model DKEXU for a typical type with a bore of 100 mm/7.5 kW is shown as an example (Fig. 11). The high-efficiency design increased the head by about 2.5 m near the best efficiency point and improved the best efficiency by about 15 points. As a result, the pump efficiency of the model DKEXU improved by an average of about 10.1 points compared to that of the conventional model across all types. This improvement expanded the selection chart for the model DKEXU compared to the conventional model, enabling the pump to be selected over a wider range (Fig. 12). Additionally, the overall efficiency of the model DKEXU improved by an average of about 11.9 points compared with conventional models as the model is equipped with an IE3 motor.
Fig. 11 Comparison of pump efficiency
Fig. 12 Model DKEXU selection chart
In the back swept impeller, a feature of the model DKEXU, the outer diameter of the impeller decreases in the low output range, significantly reducing the internal flow passage in the two blade-shaped impeller as shown in Fig. 10. Since foreign matter always passes through the pump even when the pump is equipped with an anti-clogging mechanism, there was a concern that such small internal flow passages would worsen the passage of foreign matter. To address this concern, a back swept single blade impeller (Fig. 13) was developed by reducing the number of blades to one while incorporating an anti-clogging mechanism including the back-swept impeller design to ensure sufficient internal flow passage. (Japanese registered design patent No. 1733476)
Fig. 13 Back swept single blade
The model DKEXU uses an Internal Cooling System (ICS) for cooling the motor. A cooling jacket covers the outer periphery of the motor frame and dedicated cooling water is enclosed in an independent space where no foreign matter from the handled liquid can enter, allowing for forced circulation. This enables cooling even when the motor section is exposed to the air, enabling continuous operation in the air. This technology has been applied to the model DSC43), a submersible wastewater, sewage pump with higher output (up to 110 kW) than the model DKEXU, as well as to the low output range (1.5 to 45 kW) of the model DKEXU.
For pump installations in stations where generating explosive atmospheres is a concern, products conforming to explosion-proof standards certified by the Factory Mutual Approvals of the USA can be installed as an option.
A temperature-sensing element (thermal protector) is embedded in the motor winding to prevent burnout due to an abnormal temperature rise in the motor. If the winding temperature rises abnormally, the thermal protector detects it and issues an alert externally. Since thermal protectors are embedded in each phase of the three-phase windings, it is possible to detect overheating even if only a specific phase is overheated due to causes such as open phase.
The pump is equipped with a leakage detector to detect any leakage from the mechanical seal to the motor side, preventing short circuits and bearing damage caused by liquid entering the motor. The leakage detector is installed between the mechanical seal and the motor section and detects any leakage from the mechanical seal and issues an alert externally. A highly reliable float type detector is used.
A thermometer (resistance temperature detector: RTD) can be optionally installed on the lower bearing to detect abnormalities in the bearing at an early stage.
This product meets various demands in the North American market and has increased the product competitiveness of our sewage submersible pumps. Based on the model DKEXU, we will further develop products that meet the market requirements in each region. In addition, a detailed explanatory video of this product is available on YouTube. https://www.youtube.com/watch?v=U_LS1eCuGGk
Lastly, we would like to express our deep appreciation to all the people who have provided their cooperation in this development.
1）Submersible sewage pump model DL3/model DML3，Ebara Engineering Review，No.257，p.28(2019-4)．
2）Shigeru KONDO，A New Submersible Sewage Pump for the Overseas Market，Ebara Engineering Review，No.202，p.19-22(2004-1)．
3）Chikara MAKINO，Development of Internal Cooling System for Sewage/Wastewater Pump，Ebara Engineering Review，No.228，p.3-6(2010-7)．