clearance between impeller and casing.docx

clearance between impeller and casing. both side of impeller The front clearance between the impeller impeller and the casing is set to achieve an acceptable pump efficiency. If this clearance is allowed to grow, either through corrosion or erosion, the pump will lose efficiency. More power will be required to achieve the same flow rate from the pump. This is caused by increased recirculation from high pressure zones to lower pressure zones across the front of the casing. While this recirculation may not be great, the flow disturbance also contributes to a loss of efficiency. The back clearance must also be controlled to allow the pump-out vanes to do their job, which is to reduce the pressure at the back of the impeller. If this clearance is allowed to grow outside of tolerance, an increase in axial thrust can lead to early thrust bearing failure. Rear clearance can be measured with feeler gauges, after impeller installation and before the volute is installed. Front clearence can be measured with a special clay used by engine builders to measure piston-to-head clearance. clearance. The down-side is you have to apply the clay to the impeller, assemble the pump, dis-assemble the pump and measure. The quick & dirty method is to install the volute w 0 clearance, and back off the shaft & impeller just a hair. This method ignors rear clearance. Pump is the mechanical device that changes speed energy to pressure energy. Impeller expose the speed energy to the fluid inside i nside while casing changes its energy to pressure term. So gap (clearance) will leak back the discharged fluid to suction side again which will lead to low efficiency and flow loss. Impeller is rotating component and casing is static one. So, Gap and/or clearance is a necessary evil. Reducing clearance will improve internal recirculation susceptibility and improve efficienty also. On the other hand, Exteremely narrow clearance will occur excessive eddy current between impeller and casing and it will cause erosion. (This is the reason why we have to have replaceable impeller and casing wear ring. If it is eroded, you can change and it is new pump.) Due to the configuration of almost impellers, it has two clearance. One is s uction side and the other is discharge side. You can understand the same manner with one difference. Suction side wear ring is exposed to suction pressure and discharge side one is exposed to discharge pressure. You can measure this clearance very easily. I forgot its name. There are many metal plates with proven thickness. If you insert it one by one, you will know the gap's size.

In practical, there is no apparent (or noticeable) efficiency drop when the wear ring is eroded not much. Such efficiency drop frequently misunderstood as efficiency drop by other reason since it is quite small. It is recommended to dismantle the pump while plant turn around. One more tip. Since wear rings are always under eddy (strong or weak), one side (either casing side or impeller side) wear ring must have higher hardness than another one. This will ensure that only weaker side will be eroded and you can save spare part money. The backside of the impeller is not as important to the efficient operation of the pump as the suction side. That is due to the fact that the backside is, in actuallity, another "mini-pump". The backside has what are called "pump-out vanes" which actually act as a small impeller to continuously pump the medium away from the seal area to 1) - reduce the pressure in that area and 2) - remove debris away from the seal so that it doesn't cause harm to the seal or shaft. Therefore, the small amount of leakage that may be experienced in that region is minimal.