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Improvement of Pump Impeller and Turbine Wheel Casting

Posted: 2014-12-11 21:58:16  Hits: 1046
Abstract: In pump impellers and turbine wheels production process, the shell is easy to fracture because of their complex structure. Through improving investment casting process of pump impeller and turbine to solve the problem of the mold fracture, and increase the quality casting rate of pump impellers and turbine wheels. Therefore the production requirement for running is satisfied.
 
Keywords: Dewaxing Casting, Pump Impeller, Turbine Wheel, Investment Casting of Pump Impeller and Turbine Wheel
 
Pump impellers and turbine wheels are the main components of key equipments in power plant such as utility boiler feed water pump group, hydraulic couplers etc. Their quality has a great influence on safe running of equipment, so their control requirements of the production are very high. But structures of pump impeller and turbine wheels are complex, the mould of blade groove near shaft end is easy to fracture in the process of pouring using investment casting. As a result, the casting scrap rate is over 40%, which is difficult to meet the requirements of production and technology. For the problems, fully consider the structure characteristics of the pump impeller and turbine wheel castings. After improving the production process of pump impeller and turbine wheel castings, the qualified rate are improved significantly, and the casting quality reaches the standard requirements.
 
1. Problem Analysis of Pump, Turbine Structure Analysis and Investment Casting
In the process of investment casting, because of the particularity structure of the pump, turbine, taking turbine YOT46-550 as an example (YOT46-550 pump wheel is similar to other types of pump and the turbine), its max OD is Φ472mm, height is 83mm, net weight is 70 kg, blade number is 48, blade thickness is 6mm, the blade groove (the gap between the blades) number is 48, and the thickness of blade groove gets larger from nearly shaft end to outward, increasing from 10 mm to 20 mm. Thus, the mould shell thickness is thinner or coating combination is bad when blade groove nearly shaft end are coating, making the strength of shell mould low. It is easy to fracture because the stress is greater than the strength during dewaxing, roasting and pouring. In the production process, 87.6% of broken shells appear in the blade groove near shaft end.
 
Turbine's 3D Picture
   
     Fig.1 Turbine's 3D Picture
 
2. Mould Design
The blade groove near shaft end fractures because the shell stress is greater than its strength. In order to solve this problem, we should reduce the stress and increase the strength of the shell mould. Repair the mould and raise the rounded corners (See arrows in Figure 2. Other blades are at the same location) from R2 to R3, which can effectively prevent the stress concentration problem because of small round corner in casting, and reduce the fracture phenomenon. Fig.2 is the wax pattern after increasing the round corners.
  
Turbine Wax Pattern
             
  Fig.2 Turbine Wax Pattern
 
3. Shell Making Technology Improvement
3.1 Coating Preparation
In the shell process of investment casting, shell mould is made of sizing agent and sand. Sizing agent is binder between shell mould and fire sand, which is made of sodium silicate and powder according to certain proportion. The more content of colloid SiO2 in sodium silicate is, the the number of silica gel separated out will be more. The shell’s wet strength and high temperature strength rises as well. Thus, appropriately increasing the modulus of sodium silicate can improve the strength of the shell mould effectively. On the base of 3.2 ~ 3.2 modulus of raw water glass, using 3.4 ~ 3.65 modulus (excessive increasing the sodium silicate module is bad for collapsibility when cleaning after finishing casting) can effectively guarantee the gel. Improving the modulus of sodium silicate can improve the shell mould strength of the blade grooves nearly the shaft end.
 
3.2 Coating
In the process of coating, the space between of blade grooves gets smaller with the number of coating layer increasing. Under the action of surface tension, it's easy to cause a heap of pulp, which causes difficult sanding and weak combination between each layer. Otherwise, too much grout pile will easily produce hardening halfway and drying halfway in the follow-up process. That may greatly reduce the strength of shell mould nearly shaft end, letting nearly shaft end become a weaknesses part in the coating process. Therefore, strengthening brush in the process, using the combination of artificial centrifuge and swabbing method can effectively prevent the heap of pulp, and ensure the strength of blade grooves nearly the shaft end.
 
3.3 Drying
Residual hardener on the shell mould should continue their homogeneous diffusion and permeation hardening during shell mould drying, to establish further strength. So, drying has important significance for the strength of the shell mould. Because the spacing between blades is small, the shell mould of blade groove is hard to dry. We can increase the natural air drying time to strengthen the drying during the hot air drying time. E.g., previously, we keep the layer drying by hot air for about 45 minutes after coating, and then enter the next process. Now, we change the hot air drying time to 20 minutes, turn off the hot air and then let it naturally dry for 10 minutes. Continue hot air drying for 25 minutes after the water seeping to make the shell mould neither too day nor too wet. Make sure the combination between layers is solid to improve and meet the strength requirements of the shell mould.
 
3.4 Roasting
Because the size of the pump and turbine is bigger, it is necessary to conduct packing and sand filling before pouring, in order to make sure the cooling and contraction deformation of liquid steel after pouring. On the premise that the shell mould is wrapped by sand boxes and sands, make the request for the roasting time and temperature in order to guarantee the dryness and strength of shell mould when pouring. If the roasting temperature is low and the keep warm time is short, the shell mould will be not dry enough and the thermal strength will be low. In order to meet the requirement after roasting, after many experiments and practice, we extended the soaking time for high temperature roasting to 2 hours before pouring and raised roasting temperature to 40in the scope of original process, which ensures that every part of the shell mould is roasted evenly and makes every part of the mould get the best strength.
 
4. The Improvement of Pouring
Because the pump and turbine is special, the number of blade is large and thin, in order to ensure enough pouring, pouring should be done quickly. The impact on casting is very great. To prevent the shell mould fracturing impacted by the molten steel, we can take the following two measures:
 
4.1 Avoid liquid steel flushing into blade shell mould directly when pouring, and reduce the impact of liquid steel on the shell mould of blade grooves. Let the liquid steel flow along the casting head wall into the bottom of the shell mould, making the liquid steel steady rise in the shell mould. That can prevent the blade grooves from breaking under the action of molten steel impact after high temperature liquid steel softening.
 
4.2 Prolong the static solidification time after pouring (usually 30~40minutes), and lift to cleaning area after liquid steel solidification. It can avoid the soften shell mould of blade grooves breaking under the high temperature condition caused liquid steel shaking.
 
5. Summary
Through the above several improvements of technological measures, we can avoid the blade grooves nearly shaft end breaking caused by unreasonable mould design, imperfect shell mould technique and improper pouring operation. The rate of blade grooves fracture can drop to below 5% taking the above measures. We achieved increasing product qualified rate and got good effects.

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