Users often ask, "How long will the pump run?" The standard answer is, "It depends." ”
In the pump reliability life expectancy formula, almost every factor depends on the end user, especially how the pump is operated and maintained. For example, if a standard L-structure ANSI pump is operating near the optimal/designed operating point and properly maintained, it can be expected to operate for 15 to 20 years, and in many cases more than 25 years.
What factors can the end user control over a given pump design to extend pump life? While not an exhaustive list, the following 13 notable factors are important considerations for extending pump life.
1. Radial force
Industry statistics show that the biggest reason for the discontinuation of centrifugal pumps is the failure of bearings and/or mechanical seals. Bearings and seals are early indicators of pump health and a harbinger of what's happening inside the pump system.
The first best practice for centrifugal pumps is to run the centrifugal pump at or near its optimal efficiency point (BEP). At the BEP point, the pump is designed to withstand minimal radial forces.
High radial forces and consequent shaft deflection are killers of mechanical seals and factors that contribute to shorter bearing life. If high enough, radial forces can cause the shaft to deflect or bend. If you stop the pump and measure the runout of the shaft, no problems will be found because this occurs dynamically.
For example: a bending shaft running at 3,600 RPM with 7,200 bends per minute. High circumference will make it difficult for the sealing surface to maintain contact and maintain the fluid layers required for proper sealing operations.
2. Lubricate contamination
For bearings, more than 85% of bearing failures are caused by contaminants entering the market, whether dirt, foreign matter or water. Just 250 parts per million (ppm) of water reduces bearing life by a factor of four.
Lubricating oil life is critical. Continuously running a pump is similar to a car running continuously at a speed of 100 kilometers per hour. 7 days a week, 24 hours a day, it doesn't take long to enter some miles on the odometer - 2400 km per day, 870 000 km per year.
3. Inhalation pressure
Other critical factors affecting bearing life include suction pressure, drive equipment alignment, and a certain degree of pipe strain.
For single-stage horizontal cantilever process pumps, the axial force on the rotor is oriented towards the suction, so counteracting the suction pressure (somewhat and limited) actually reduces the axial force and thus the thrust bearing load and thus extends service life.
4. Device alignment
Misalignment of the pump and drive equipment overloads the radial bearings. Radial bearing life is exponentially related to misalignment. For example: due to a misalignment of only 1.5 mm, the end user may experience some kind of bearing or coupling problem after three to five months of operation; However, at a 0.025mm misalignment, the same pump may run for more than 90 months.
5. Pipe strain
Pipe strain is caused by misalignment of the suction/or discharge pipe with the pump flange. Even in robust pump designs, the resulting pipe strain easily transfers potential stresses to the bearings and their respective housings. Forces can cause a poor bearing fit and/or inconsistency with other bearings.
6. Fluid characteristics
Fluid characteristics such as pH, viscosity and specific gravity are key factors. If the fluid is acidic or corrosive, parts that come into contact with the liquid, such as housing and impeller materials, require special design. The amount of solids present in the fluid and its size, shape, and abrasive quality are all factors.
7. Services
The severity of the service is another major factor, how often does the pump start in a given time? For example, a pump that starts and stops every few seconds has a much higher wear rate than a pump that operates continuously under the same conditions.
Under the same conditions, a pump with submerged suction will operate more reliably than a pump with lifting suction. Lifting conditions requires more extra work and provides more opportunities for air intake or worse (drying up).
8. Net suction head
The higher the margin of the available positive suction head (NPSHA) relative to the desired positive suction head (NPSHR), the less likely it is for pump cavitation. Cavitation can cause damage to the pump impeller and the resulting vibrations can affect seals and bearings.
9. Pump speed
The operating speed of the pump is another key factor. For example, a 3,550 rpm pump wears out 4 to 8 times faster than a 1,750 rpm pump.
10. Impeller balancing
Unbalanced impellers on cantilever pumps or certain vertical designs can cause shaft agitation situations, which deflect the pump shaft like the radial force of the pump when operating away from BEP. Radial deflection and agitation can occur simultaneously.
It is recommended that the impeller be balanced according to at least the International Organization for Standardization (ISO) 1940 class 6.3 standard. If the impeller is modified for any reason, it must be rebalanced.
11. Pipe geometry
Another important consideration for extending pump life is the pipe geometry, or how the fluid is "loaded" into the pump. For example, an elbow in the vertical plane on the suction side of the pump causes fewer harmful effects than an elbow on a horizontal elbow. The hydraulic load of the impeller is more uniform, so the load of the bearing is also uniform.
12. Pump operating temperature
Whether it is high or low, the operating temperature of the pump, especially the rate of temperature change, will have a great impact on the life and reliability of the pump. The operating temperature of the pump is important, and the pump needs to be designed to accommodate this problem. More important is the speed of temperature change.
It is recommended to keep the rate of change below 2 °C per minute. Different materials expand and contract at different rates, affecting gaps and stresses.
13. Pump casing penetration
Although not often considered, the number of pump casing penetrations can have some impact on the life of the pump. Many end users want to drill and tap the pump housing, e.g. by installing vibration sensors. Each drill and tapping on the pump casing becomes the origin of stress cracks in the pump casing and the starting point of corrosion.
