Operating at low flow places the machine under a great amount of duress. It is always wise to have a mental picture of what is happening within the passages of the machine to understand why this is the case.
The days have long passed where pump vibrations were viewed as a matter of mechanical balance. Now, we recognize that even if the pump had perfect mechanical balance, it would still exhibit vibrations.
The intensity of this remnant vibration turns out to be flow related with its minimum level being at or around best efficiency point (BEP).
Centrifugal pumps are sometimes used in environments where the pumped product contains suspended solids. While some pumps are specifically designed for solid handling or slurry applications, normal centrifugal pumps do not contain features to prevent performance degradation from the impact of solids.
There are a few key signs that a conventional centrifugal pump is suffering from erosive and abrasive wear. Here are assessment and mitigation strategies to be considered and applied when this occurs.
Particles are a problem in a centrifugal pump due to the way the machine adds velocity to the liquid as it passes up the impeller channels. In general, the higher the speed at the tip of the impeller, the more energy that is imparted to any particle that is suspended within the liquid. This energy can then cause damage to anything it impacts.
A major steel plant on the East Coast had been experiencing catastrophic failures with its five-stage descaling pumps. The plant operated using three multistage axially split (BB3) pumps with two spares. All five of the pumps had a mean time between repair (MTBR) of two years. In this case, the plant water quality was considered to be less than ideal, and the entrained abrasives were a factor contributing to the repeated failures.
Poor water quality can lead to a number of pump reliability issues. When pumping fluids with abrasive material, pumps experience erosion and corrosion, and the effects can rapidly degrade both the casings and critical inner elements. While erosion and corrosion alone are not always a difficult problem to solve, it is important to have a firm understanding of the relationship between various types of erosion and corrosion and the metallurgy used in designing the pumps.
Further analysis showcased excessive clearances and inconsistencies with component fits that also contributed to pump performance degradation outside the abrasion. In order to increase the MTBR of the pumps at the plant, the aftermarket pump service provider recommended several engineered upgrades including new impellers to be manufactured using advanced mold technology, specifically addressing the surface finish and dimensional consistency.
As new design and manufacturing technologies are developed, end users can affordably upgrade their systems and verify better performance.
Written by:Bob Jennings & Dr. Gary Dyson (Hydro, Inc.) Publisher:Pumps & Systems / August 2015
The rising cost of electrical power has caused many industrial plants to shift their focus to energy consumption. Plants often run pumping equipment continuously, and much research has pointed to opportunities for cost savings by optimizing pumping equipment.
When evaluating the potential for energy savings, end users cannot consider a pump in isolation. The suitability of the pump for the system within which it operates is vital. Even the best designed and most efficient equipment offers power-saving potential if it is run off its best efficiency point (BEP) in a system for which it is ill-applied.
Image 1. Much research has pointed to opportunities for cost savings by optimizing pumping equipment. (Images and graphics courtesy of Hydro, Inc.)
Many plants have been in operation for more than 40 years, and their operating philosophies have changed over time. Plant improvements have enabled higher throughput, often increasing production by as much as 125-150 percent. Unfortunately, little is done to improve or increase the performance of the support-service pumping equipment, such as cooling water pumps.
As system flow demands increase, the duty point of the pumps is forced to shift far to the right of the BEP, well outside the acceptable operating range (AOR). This causes efficiency and pump reliability to decrease dramatically.
Casting tolerances, surface finishes, and impeller/volute or impeller/diffuser geometry have all dramatically improved during the last 40 years. But because many pumps were installed when the plants were commissioned, the existing pumps were manufactured using techniques that would be considered obsolete today. The result is higher energy costs and reduced reliability and availability, which often cause production delays. Continue reading →