Almost all of the factors that go into a pump's reliable life expectancy are up to the end user, especially how the pump is operated and maintained. What factors can the end user control to extend the life of the pump? The following 13 noteworthy factors are important considerations for extending pump life.

1. Radial Forces

Industry statistics show that the biggest cause of unplanned downtime for centrifugal pumps is bearing and/or mechanical seal failure. Bearings and seals are the "canaries in the coal mine" - they are early indicators of pump health and a precursor to failure within the pumping system. Anyone who has worked in the pump industry for any length of time probably knows that the first best practice is to operate the pump at or near the Best Efficiency Point (BEP). At the BEP, the pump is designed to withstand minimal radial forces. When operating away from the BEP, the resultant force vector of all radial forces is at a 90° angle to the rotor and attempts to deflect and bend the pump shaft. High radial forces and the resulting shaft deflection are a mechanical seal killer and a contributing factor to shortened bearing life. If the radial forces are large enough, they can cause the shaft to deflect or bend. If you stop the pump and measure the shaft runout, you will find nothing wrong because this is a dynamic condition, not a static one. A bent shaft running at 3,600 rpm will deflect twice per revolution, so it will actually bend 7,200 times per minute. This high cycle deflection makes it difficult for the seal faces to maintain contact and maintain the fluid layer (film) required for the seal to function properly.

2. Lubricant Contamination

For ball bearings, more than 85% of bearing failures are caused by contamination, which can be dust and foreign matter or water. Just 250 parts per million (ppm) of water can reduce bearing life by a factor of four. Lubricant life is critical.

3. Suction Pressure

Other key factors that affect bearing life include suction pressure, driver alignment, and to some extent pipe strain. For ANSI B 73.1 single-stage horizontal overhung process pumps, the axial force generated on the rotor is toward the suction port, so to some extent and within certain limits, the reaction suction pressure will actually reduce the axial force, thereby reducing thrust bearing loads and extending the life ofdeep well vertical turbine pumps.

4. Driver Alignment

Misalignment of the pump and driver can overload the radial bearing. The life of the radial bearing is exponentially related to the degree of misalignment. For example, with a small misalignment (misalignment) of only 0.060 inches, the end user may experience bearing or coupling problems after three to five months of operation. However, if the misalignment is 0.001 inches, the same pump may operate for more than 90 months.

5. Pipe Strain

Pipe strain is caused by misalignment of the suction and/or discharge pipes with the pump flanges. Even in a robust pump design, pipe strain can easily transfer these potentially high stresses to the bearings and their corresponding bearing housing fits. The forces (strain) can cause the bearing fit to be out of round and/or out of alignment with other bearings, causing the centerlines to be on different planes.

6. Fluid Properties

Fluid properties such as pH, viscosity, and specific gravity are critical factors. If the fluid is acidic or corrosive, the flow-through parts of a deep well vertical turbine pump such as the pump body and impeller need to be corrosion resistant. The solids content of the fluid and its size, shape, and abrasiveness are all factors.

7. Frequency of Use

Frequency of use is another important factor: How often does the pump start in a given period of time? I have personally witnessed pumps that start and stop every few seconds. The wear rate on these pumps is much higher than when the pump is running continuously under the same conditions. In this case, the system design needs to be changed.

8. Net Positive Suction Head Margin

The greater the margin between the Net Positive Suction Head Available (NPSHA, or NPSH) and the Net Positive Suction Head Required (NPSHR, or NPSH Required), the less likely a deep well vertical turbine pump will cavitate. Cavitation damages the pump impeller, and the resulting vibrations can affect the life of seals and bearings.

9. Pump Speed

The speed at which the pump operates is another critical factor. For example, a pump running at 3,550 rpm will wear four to eight times faster than one running at 1,750 rpm.

10. Impeller Balance

Unbalanced impellers on cantilever pumps or certain vertical designs can cause shaft wobble, a condition that deflects the shaft, much like radial forces when the pump is running away from the BEP. Radial deflection and shaft wobble can occur simultaneously.

11. Piping Arrangement and Inlet Flow Rate

Another important consideration for extending pump life is how the piping is arranged, i.e. how the fluid is "loaded" into the pump. For example, an elbow on the vertical plane on the suction side of the pump will have less detrimental effects than a horizontal elbow - the hydraulic loading of the impeller is more even, and therefore the bearings are loaded more evenly.

12. Pump Operating Temperature

The operating temperature of the pump, whether hot or cold, and especially the rate of temperature change, can have a large impact on the life and reliability of a deep well vertical turbine pump. The operating temperature of the pump is very important and the pump must be designed to meet the operating temperature. But more important is the rate of temperature change.

13. Pump Casing Penetrations

Although not often considered, the reason that pump casing penetrations are an option rather than a standard for ANSI pumps is that the number of pump casing penetrations will have some impact on the life of the pump, as these locations are the primary locations for corrosion and stress gradients (rises). Many end users want the casing to be drilled and tapped for drain, exhaust, instrumentation ports. Every time a hole is drilled and tapped on the shell, a stress gradient is left in the material, which becomes the source of stress cracks and the place where corrosion begins.

The above is only for the user's reference. For specific questions, please contact CREDO PUMP.