1. Radial force
Industry statistics show that the biggest reason for centrifugal pumps to stop running is the failure of bearings and/or mechanical seals. Bearings and seals are the "canaries in the mine" - they are early indicators of the health of water pumps and also a precursor to the internal conditions of water pump systems.
Anyone who has been in this industry for a long time may know that one of the best practices is to operate pumps at or near their Best Efficiency Point (BEP). On BEP, the designed pump will withstand the minimum radial force. The resultant vector of all radial forces generated by operation away from BEP forms a 90 degree angle with the rotor, attempting to deflect and bend the shaft.
The large radial force and the resulting shaft deflection are the killers of mechanical seals and important factors in shortening the life of bearings. If it is large enough, the radial force will cause the shaft to deflect or bend. If the pump is stopped and the runout on the shaft is measured, no errors will occur because it is a dynamic condition, not a static condition.
The bending shaft (deflection) running at 3600rpm will deflect twice per revolution, so it actually bends 7200 times per minute. This high cyclic deflection makes it difficult for the sealing surface to maintain contact and to maintain the fluid layer required for proper sealing operation.
2. Oil pollution
For ball bearings, over 85% of bearing failures are caused by the entry of dirt, foreign objects, or water. Only 250 parts per million (250ppm) of water will reduce the lifespan of bearings by four times.
Reasonable use of lubricating oil is crucial for its lifespan.
3. Inhalation pressure
Other key factors that affect bearing life include suction pressure, coupling alignment, and pipeline stress.
For single-stage horizontal cantilever process pumps, the combined axial force acting on the rotor is directed towards the inlet, so to a certain extent, the limited reverse suction pressure actually reduces the axial force, thereby reducing the load on the thrust bearing and extending its service life.
4. Calibration
Misalignment between the pump and motor can cause overload of the radial bearings. When calculating the misalignment, the radial bearing life is an exponential factor.
For example, for a small deviation of only 1.52mm, the end user may encounter some kind of bearing or coupling problem after running for three to five months. However, for a deviation of 0.0254mm, the same pump may operate for more than 90 months.
5. Pipeline stress
Pipeline stress is caused by the misalignment of the suction and/or discharge pipes with the pump flange. Even in robust pump designs, the pipeline stresses generated can easily transfer these potential high forces to the bearings and their respective housings. Force (strain) causes improper fit of bearings and/or inconsistency with other bearings, resulting in the centerline being located on different planes.
6. Fluid characteristics
Fluid characteristics such as pH, viscosity, and specific gravity are key factors. If the medium is acidic or corrosive, the contact parts of the pump, such as the casing and impeller materials, need to maintain their functional state. The quantity, size, shape, and grinding quality of solids present in the fluid will all be influencing factors.
7. Work status
The strictness of the working state is another major factor: how often the pump starts within a given time.
Some pumps start and stop every few seconds. Compared to pumps that operate continuously under the same conditions, these pumps in operation wear out at an exponential rate. In this situation, the system design urgently needs to be changed.
8. Cavitation allowance
The higher the margin of available net positive suction head (NPSHA), the less likely the pump will experience cavitation if it exceeds the required net positive suction head (NPSHR). Cavitation can damage the pump impeller and generate vibrations that can affect the seals and bearings.
9. Pump speed
The operating speed of the pump is another key factor. For example, a 3550 rpm pump wears out 4 to 8 times faster than a 1750 rpm pump.
10. Impeller balance
Unbalanced impellers on cantilever pumps or certain vertical designs can cause shaft deflection, just like the radial force of a pump when operating away from BEP. Radial deviation and deflection may occur simultaneously. If the impeller is trimmed for any reason, it must be rebalanced.
11. Pipe shape
Another important consideration for extending pump life is the geometry of the pipeline or how the fluid is' loaded 'into the pump.
For example, elbows on the vertical suction side of a pump have less harmful effects than horizontal elbows. The hydraulic load on the impeller is more uniform, so the load on the bearings is also more uniform.
12. Working temperature
Whether it is high or low temperature, the working temperature of the pump, especially the temperature change rate, will have a significant impact on the service life and reliability of the pump. The working temperature of the pump is very important, so the pump needs to be designed to operate at this temperature. However, what is more important is the rate of temperature change. Suggest (in a more conservative scenario) to keep the rate of change below 2 degrees Fahrenheit per minute. Different qualities and materials expand and contract at different rates, which may affect gaps and stresses.
