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.
Image 1. A reciprocating pump’s fixed volume. Flow is determined by stroke, area and speed. (Images courtesy of Hydro)
Understanding the differences between these types of pumps can mean avoiding difficulties and reliability problems.
The demand for the duties that fall within the performance range of reciprocating pumps is rising. Process flows are falling while the pressures required are increasing.
Engineers are generally familiar with operating principles, performance curves and selection criteria for centrifugal pumps, but the training and knowledge around the operating principles of reciprocating pumps is not as common.
Unlike centrifugal pumps, reciprocating pumps have a stronger interaction with the system within which they sit. This is due to the pressure pulsations they generate.
If we think about any linear reciprocating motion of a piston, at some point the velocity of the piston is zero as it changes direction at the top and bottom of its stroke. This means that the pressure pulsations are much larger in a reciprocating machine than in a centrifugal machine.
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This pump type’s unique system design requirements are often ignored or misapplied, which affects reliability and operation.
Written by: Gary Dyson and Herb Tackett Jr. (Hydro, Inc.) Publisher: Pumps & Systems / July 2015
Because Centrifugal pumps are widely used, pump and rotating equipment engineers are generally familiar with this equipment’s operating principles, performance curves and selection criteria.
Image 1. While centrifugal pumps are the subject of much training, the dwindling population of reciprocating pumps has led to a loss of understanding of this pump type’s unique system design requirements. (Images and graphics courtesy of Hydro, Inc.)
While Centrifugal pumps are the subject of much training, the dwindling population of reciprocating pumps has led to a loss of understanding of this pump type’s unique system design requirements. Centrifugal pump specifications are now commonly and incorrectly applied to reciprocating pumps, which can lead to significant reliability problems.
End users should consider these 10 key facts about reciprocating pumps that can influence reliability and operation. Continue reading →
Join instructor, Robert Piotrowski of Turvac, Inc., as he discusses the key ingredients to successfully align machinery, the symptoms you will see if machinery is subjected to run under a misalignment condition, and more.