Adjusting Packed Centrifugal Pumps with Open Impellers
Torbjörn Idhammar, President & CEO IDCON Inc.
Open impellers are frequently used in centrifugal pumps which pump liquids that contain suspended solid materials (slurries, paper stock, etc). They do not plug as easily as closed impellers and usually have a simple external adjustment to maintain the correct impeller clearance as internal parts wear.
The front, or suction side, of the vanes must be set with a close clearance against the suction wear plate to achieve high efficiency and good performance. As the impeller and suction plate wear, performance will deteriorate over time. Regular clearance checks and adjustments will maintain good performance.
However, as the impeller is adjusted toward the suction wear plate, the clearance behind the impeller increases. Most open impellers have small vanes on the non-pumping side, called “back pump-out vanes”. These vanes act like another centrifugal pump and have the purpose of maintaining a low pressure at the stuffing box. Without these vanes, the stuffing box pressure would be close to the discharge pressure of the pump, which could result in contamination of the packing and low packing life.