Gene Vogel
EASA Pump & Vibration Specialist
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.
Before providing a rule of thumb and reference chart information, it’s important to understand that the wear ring clearances are more critical on certain style impellers and less critical on others. The difference is related to the relationship between the head and flow characteristics of the pump. That relationship is described by the “specific speed” of the pump design.
Impeller speed
Note that the specific speed is not the pump rotating speed; it is simply a ratio of head to flow. See Figure 1. The higher specific speed impellers (to the right of the illustration) produce high flow but low head. Low specific speed impellers (to the left in the illustration) are designed to produce greater head at lower relative flow rate. The purpose of the suction eye wear ring is to limit the flow of pumpage from the high pressure discharge of the impeller to the low pressure suction side. Therefore, the wear ring clearances are more critical for low specific speed impellers since the pressure differential is greater and any leakage across the wear ring represents a greater percentage of the total flow. Conversely, wear ring clearances are less critical for high specific speed impellers.
It will also be helpful to understand that the purpose for rear wear rings is different than for suction side wear rings. Obviously the suction side wear ring clearance limits the flow of pumpage from the higher pressure discharge side to the lower pressure suction side; flow across that wear ring represents volumetric loss of efficiency. The rear wear rings have a completely different purpose. When rear wear rings are present, the impeller will have holes drilled through from the suction eye of the impeller to the back side of the impeller. See Figure 2. These holes conduct the lower suction pressure to the back of the impeller. The resulting low pressure behind the impeller and inside of the rear wear rings reduces the axial thrust force created by the impeller suction. Thus, when rear wear ring clearances are excessive, the result is not lost efficiency, but high axial thrust force that can damage the thrust bearing.
The tolerance guidance provided below applies to both suction side and rear wear ring clearance.
Manufacturers' tolerances are always the best guidance for evaluating as-found wear ring clearances and fitting replacement wear rings. When manufacturer’s tolerances are not available, the following (see Figure 3) guidelines work well for common water service pumps (municipal water, waste water, HVAC applications, etc.).
Further guidance is provided by the API-610 Minimum Internal Running Clearances chart (see Table 1).
These are minimum, not maximum, clearances for pumps used in petroleum processing applications. Minimum clearances are specified since tighter clearances would risk incurring contact between the rotating and stationary components, likely resulting in a catastrophic machine failure.
Wear ring clearances are always specified in diametric measurements. That is, the difference between the inside diameter (ID) of the stationary ring and outside diameter (OD) of the rotating ring. So the actual running clearance is one half the diametric clearance.
Ensuring efficient, reliable pump operation
The guidance provided in this article will be useful for designing replacement wear rings when the manufacturer’s tolerances are not available. They can also be a guide for evaluating the as-received clearances and deciding if a replacement is necessary. Some manufacturers recommend replacing wear rings when clearances have increased by 40% of the original specified clearance. This recommendation is good for pumps in common water service applications, and for impellers in the mixed flow range of specific speed. As described above, pumps with low specific speed impellers should be held to tighter clearances and pumps with high specific speed impellers have more leeway. Proper wear ring clearances are a fundamental concern for ensuring efficient and reliable pump operation. The guidance provided here should help service centers achieve that objective.
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