One of the most common repairs on centrifugal pumps is replacing worn or damaged wear rings. To restore efficient, reliable operation and prevent catastrophic pump failure, it is critical to restore proper clearances between the stationary casing wear ring and the rotating impeller wear ring. Although many pump manufacturers provide clearances and dimensions, some do not. There are plenty of aging pumps around from now-defunct manufacturers for which dimension data is simply not available.

In such cases, the rule of thumb that follows provides some guidance for acceptable running clearances, or the minimum running clearance chart in American Petroleum Institute (API) Standard 610 can be used as a guide.

One of the most common repairs on centrifugal pumps is replacing worn or damaged wear rings. For pumps with closed style impellers (impellers with a front shroud as described below), there will be a casing wear ring and possibly an impeller wear ring which is fitted to the outside diameter (OD) of the impeller suction eye. Impellers may also have rear wear rings which are important for controlling axial thrust. Pumps with open style impellers generally do not have suction eye wear ring clearance concerns, but they often have rear wear rings. The clearances between the stationary casing wear ring and the rotating impeller wear ring are critical to proper pump operation. Although many pump manufacturers will provide proper clearances and dimensions, some do not; there are plenty of old pumps around from now defunct manufacturers where dimension data is simply not available.

Most centrifugal pumps today have mechanical seals. However, those pumps that still use packing glands instead of mechanical seals require frequent maintenance and are more likely to show up in the service center for repair. The average mean time between failure (MTBF) for mechanical seals is about 7500 hours. Packing glands require packing replacement after about 1,000 hours of operation. And the shaft or shaft sleeve for packing gland pumps is subject to wear, so pump overhaul to repair the shaft or replace the sleeve is inevitable. Improper installation or adjustment of packing glands can greatly accelerate wear, making much more frequent repair necessary. Replacing packing is not a technically demanding task. However, there are some tips that will improve the performance of the packing, limit the energy usage and extend the MTBF. Attention to these details will allow service centers to provide quality repairs for their customers.

For most centrifugal pumps, the shaft has packing or a mechanical seal where the shaft enters the wet end. Fundamentally, a sealless pump substitutes a magnetic drive for the shaft seal. The impeller shaft is fitted with a magnetic rotor which is contained in a thin metal cover. The shaft, impeller, the rotor and the bearings are all "wet" components; that is, they are completely submersed in whatever liquid is being pumped. Over the outside of the magnetic rotor and cover is a magnetic drum which is driven by the power source, usually an electricmotor. The magnetic coupling between the rotor and drum delivers torque to the shaft and impeller. There are a number of variations in design, but all use sleeve type bearings lubricated by the pumpage.

Most maintenance and operations personnel who work with centrifugal pumps have been warned to never start a pump unless it is primed. They have been warned that a pump that is started when loaded with air may cause the seal or packing to be scorched and permanently damaged, and that when the suction liquid level is below the pump (suction lift), the pump would not begin to pump. Then they encounter a pump that they are told is "self-priming," and they begin to question if all that caution is necessary. So, what's the real scoop on self-priming pumps?