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Determination of machining plan of open impeller
The foreword: The impeller is the core component of an industrial pump, and its design and configuration significantly influence performance. Open impellers, which lack a cover, expose the blades directly to the static wear plate (fixed to the pump body and cover). When handling media containing fibers, high viscosity substances, crystals, or particles, the small gap between the impeller and the wear plate can cause significant wear due to the relative motion, leading to scraping and material removal. This makes fully open impellers unsuitable for many applications. A fully open impeller centrifugal pump is shown in Figure 1.
Figure 1: Fully open impeller centrifugal pump structure
1. Pump
2. Front wear plate
3. Open impeller
4. Rear wear plate
5. Pump cover
Second, machining and clamping methods for open impellers: Dalian Blue Pump Co., Ltd. produces EAPK series pumps (design pressure 5.0MPa, center-supported) and ESHK series pumps (design pressure 2.5MPa, foot-supported), both featuring fully open impellers without front covers or mouth rings. These pumps are equipped with wear-resistant plates and are suitable for handling media with particles, making them widely used in various industries. As one of the company’s flagship products, the ESHK series is shown in Figure 1.
One of the key components in these pumps is the impeller (as shown in Figure 2). Its machining accuracy directly affects the pump's performance. Poor accuracy can lead to uneven gaps between the blades and the wear plate, resulting in reduced head and efficiency. Therefore, ensuring precise machining is essential.
During the machining process, the original model lacked a non-machining umbilical, requiring workers to manually find a clamping ring and weld it to the blade before grinding. Experienced craftsmen found this time-consuming and inefficient, as it involved welding, power consumption, and the use of expensive dissimilar steel welding rods. To address this, they proposed modifying the model by adding a small umbilical for easier lathe processing.
The technology center collaborated with the machining shop and foundry to analyze and finalize a plan for machining the open impeller. Two main options were considered:
Option 1: Welding a clamping ring on the blade.
(1) The model may not be fully machined by the machine itself.
(2) The model does not move.
(3) The clamping chuck must be manually aligned on the CW6163 lathe to find the outer diameter of the hub and the back of the blade, then turn the hub to the required size. After removing the impeller, the mouth ring is knocked off using an angle grinder. The rear hub is then clamped again, and the front part is turned to meet the drawing specifications.
(4) Advantages: No need to modify the pattern; operators can manually select a spot-welded area on the blade for clamping. Stable clamping ensures consistent quality during intermittent turning.
(5) Disadvantages: Manual selection of the spot-welded area is time-consuming, especially for inexperienced workers. Post-weld polishing is required, and mass production becomes challenging. Additional electrodes and power are needed, and welding may damage the microstructure of high-corrosion-resistant alloys.
(6) This method was previously used.
Option 2: Casting a process boss at the impeller inlet.
(1) A casting process boss is added to the inlet hub, with a diameter of (φd - 5), where φd is the inlet hub diameter. The boss extends 20–40 mm beyond the blade, depending on the impeller size.
(2) The model must be modified accordingly.
(3) The first step is to turn the rear hub and process the cylindrical surface for clamping. The process boss is then turned, followed by final machining of the hole and process boss.
(4) Benefits: Provides a permanent clamping solution.
(5) Disadvantages: Slight increase in blank weight increases material costs. Processing time also increases due to the additional boss length. Larger impellers may experience chatter.
(6) This method had not been used before.
Option 3: Casting a process ring around the impeller.
(1) A casting process ring is added to the outer circumference of the impeller.
(2) The model is modified accordingly.
(3) Machining follows a similar procedure to Option 2.
(4) Benefits: Offers a stable clamping solution with a larger clamping diameter, ensuring consistent quality.
(5) Disadvantages: Significant increase in blank weight and material cost. Processing time increases further.
(6) This method had not been applied prior.
Third, conclusion: After thorough discussion, the Dalian Blue Pump Technology Center and relevant departments reached a consensus on the following principles:
1) For open impellers with a nominal diameter ≤ 250 mm, an imported wheel with a casting process boss is used (Option 1).
2) For open impellers with a nominal diameter > 250 mm, a welding clamp ring is generally adopted (Option 2).
3) In special cases, such as poor welding performance or corrosion resistance issues, an integral process ring can be cast on the outer edge of the impeller.
Options 1 and 2 are the most commonly used. Practical application has proven that the selected clamping program effectively solves the long-standing issue of machining open impellers, significantly improving efficiency and quality.