Factors that Affects Pump Working Life
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 "canary in the mine"-they are an early indicator of the health of the pump and a harbinger of the internal conditions of the pump system.
Anyone who has been in the industry for a long time may know that one of the best practices is to operate the pump at or near its best efficiency point (BEP). On BEP, the pump is designed to withstand the smallest radial force. The resultant force vector of all radial forces generated by running away from the BEP is at a 90 degree angle to the rotor and will try to deflect and bend the shaft.
The large radial force and the resulting shaft deflection are the killer of the mechanical seal and an important factor in shortening the life of the bearing. If it is large enough, the radial force can cause the shaft to deflect or bend. If user stop the pump and measure the run out on the shaft, no error will occur because it is a dynamic condition, not a static condition.
The bending shaft (deflection) running at 3600 rpm will deflect twice per revolution, so it is actually bent 7200 times per minute. This high cyclic deflection makes it difficult to maintain contact between the sealing surfaces and to maintain the fluid layer required for proper sealing operation.
For ball bearings, more than 85% of bearing failures are caused by the ingress of dirt, foreign matter or water. Only two hundred fifty parts per million (250 ppm) of water will reduce bearing life by four times.
Reasonable use of lubricating oil is essential to pump life. The operating pump is similar to a car running continuously at a speed of 96km per hour. Driving at a speed of 7✖24 hours a week, the mileage on the odometer does not need to be too long:2317km per day, 16222km per week, and 843553km per year.
Other key factors affecting bearing life include suction pressure, coupling alignment, and pipe stress. For single-stage horizontal cantilever process pumps (such as the ANSI B 73.1 model), the resultant axial force acting on the rotor is towards the inlet, so to a certain extent and limited reverse suction pressure will actually reduce the axial force, thereby reduce the thrust bearing load, thereby extending the life. For example, a standard S-type ANSI pump with a suction pressure of 10 psig can usually have an expected bearing life of 6 to 7 years, but at a suction pressure of 200 psig, the expected bearing life will increase by more than 50 years.
Misalignment of the pump and motor can overload 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 three to five months of operation. However, for a deviation of 0.0254mm, the same pump may run for more than 90 months.
Pipeline stress is caused by misalignment of the suction pipe and/or discharge pipe with the pump flange. Even in a sturdy pump design, the resulting pipeline stress can easily transfer these potentially high forces to the bearings and their respective housings. The force (strain) causes the bearing to fit incorrectly and/or to be inconsistent with other bearings, so that the centerline is located on a different plane.
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 housing and impeller materials, need to remain functional. The number of solids present in the fluid and their size, shape and grinding quality will all be influencing factors.
The rigor of the working condition is another major factor: how often does the pump start in a given time?
I have seen the pump start and stop every few seconds. Compared with pumps running continuously under the same conditions, these working pumps wear out at an exponential rate. In this case, the system design urgently needs to be changed.
The higher the available net positive suction head (NPSHA) margin, and the higher the required net positive suction head (NPSHR), the lower the possibility of pump cavitation. Cavitation can damage the pump impeller, and the resulting vibration can affect seals and bearings.
The operating speed of the pump is another key factor. For example, a 3,550 rpm pump wears 4 to 8 times faster than a 1,750 rpm pump.
Unbalanced impellers on cantilever pumps or certain vertical designs can cause shaft deflection, just like the radial force of a pump running away from the BEP. Radial deflection and deflection may occur at the same time. It is recommended that the impeller reach at least the ISO1940 6.3 level standard. If the impeller is trimmed for any reason, it must be rebalanced.
Another important consideration for prolonging the life of the pump is the geometry of the pipe, or how the fluid is "loaded" into the pump.
For example, the elbow on the vertical surface of the suction side of the pump has less harmful effects than the horizontal elbow. The hydraulic load of the impeller is more uniform, so the load of the bearing is also more uniform.
The fluid velocity on the suction side should be kept below 3.048m/s. It is recommended to keep the speed at less than 2.4384m/s and 1.8288 m/s is even better (assuming non-mud fluid is used). Laminar flow instead of turbulent flow will affect the way the impeller is loaded and change the rotor dynamics.
Whether it is high temperature or low temperature, the working temperature of the pump, especially the temperature change rate, will have a significant impact on the 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, more important is the rate of temperature change. It is recommended (in a conservative situation) to keep the rate of change below 2 degrees Fahrenheit per minute. Different masses and materials expand and contract at different rates, which may affect gaps and stress.
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