Circulator pumps are used in central and district heating systems and domestic service systems. They need to be able to run quietly and consistently for long operational periods – often up to 5,000 hours a year. Usually the circulator pumps are designed to work for a maintenance-free 15 year lifetime. However, it is not seldom to see in our daily life that some circulator pumps fail to work in 1 year after installation or even as quickly as in several months. The reasons might be pump seizure, motor burnt, or shaft rapid wear. For the first two, it is easy to see and understand. But for the shaft rapid wear, many users or even maintenance personnel don’t know why. So they just simply say, “ oh, it’s a bad quality circulator”. Bad quality might be one of the reasons. But is that all? Today, let’s talk about the most common reasons for rapid wear of circulator pump shaft.

Shaft Material

Traditionally the shaft/bearing systems are made of steel shafts and carbon bearings. This material combination does not offer optimum abrasive wear resistance to limescale and the most common abrasive wear particle – black iron oxide – which is present in heating plants containing mild steel parts. The gradual wearing of the shafts and bearings increases noise levels, reduces pump efficiency and in some cases causes pump seizure.

Efficiency and longevity have become increasingly critical selling points for pumps – so important that manufacturers are looking at every element of the design and manufacturing process to see where improvements can be made. Some manufacturers investigated how the pump performance could be improved and found that the combination of alumina ceramic shafts and bearings offered superior reliability and increased lifetime of their pumps. Ceramic shafts / bearings rotating in the water, containing abrasive wear particles, result in almost negligible wear, therefore maintaining tight clearance tolerances and virtually no noise increase over time.

Circulator Pump Orientation

Circulator pump should be oriented so that its motor shaft is horizontal. It is preferable to install the circulator pumps in a vertical pipe pumping upwards. This position ensures that the pump shaft is horizontal, which reduces the thrust bearing load and ensures positive air purging from both the rotor chamber and impeller housing.

Pumping downwards in a vertical pipe is not recommended as this may lead to air locking of the pump, with resultant loss of performance. However, pumping downwards is acceptable provided an effective air vent is incorporated in the system, before the pump.

Where pumps can only be installed in horizontal pipework, it is imperative that the pump shaft is horizontal, or slightly higher at the vent plug end.

The shaft must not fall below the horizontal plane, even by a few degrees, as this causes premature wear of the top bearing and shaft.

Pumps should not be installed with the shaft in a vertical plane, as this may lead to dry running of the top bearing, noise and possible pump failure.

To avoid cavitation noise and risk of damage to the bearings the minimum pump inlet pressure should be 1.4m at 82°C (water temperature).

Entrained Air

Increases in air entrainment more than 1.5 to 2 percent will have immediate and deleterious effects on the pump from both an immediate performance aspect and a mechanical aspect in a protracted way.

As air bubbles become trapped at the pump suction, they block the fluid flow, and the pump performance will drop off. The flow rate will decrease, the developed head will drop off, and the efficiency will decline. Air entrainment even at values as low as 2 to 4 percent will cause increased pump vibration, which leads directly to premature bearing failure. Vibration is frequently caused by the unbalanced hydraulic loads on the impeller due to partial air blockage.

The unvented air also collects in the seal chamber (standard stuffing box configurations) to create air pockets that cause dry running of the shafts. Dry/non-lubricated shafts operation contributes to shortened life and ultimate failure.

Air entrainment is one of the major contributors to broken pump shafts because of the hydraulic surging and axial shuttling that occurs from a pump that is stalled one second (no load) and pumping the next (full load) in an endless and unintended “go to” loop of stress fatigue cycling.

Air entrainment also introduces unwanted free oxygen into the system, which is the major component in both general and chloride stress corrosion formulas, just to name two types.

Air entrainment at 2 percent will reduce pump performance by up to 12 percent. At 4 percent, it will reduce pump performance by 40 percent and at 10 percent, it will likely stall the pump altogether.

Conclusion

The potential reason that can cause the rapid wear of circulator pumps is not only one. Basically to get optimum abrasive wear resistance, we should use ceramic shafts & bearings combination. To reduce the wear, we should make sure the shaft is horizontal while installing the pump, and vent the air in pump systems before use the pump.