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Wear in Centrifugal Pumps

Centrifugal pumps are sometimes usedin environments where the pumped product contains suspended solids. While slurry pumps are specifically designed for solid handling or slurryapplications, normal centrifugal pumps do not contain features to preventperformance degradation from the impact of solids.

There are a few key signs that aconventional centrifugal pump is suffering from erosive and abrasive wear. Hereare assessment and mitigation strategies to be considered and applied when thisoccurs.

Particles are a problem in acentrifugal pump due to the way the machine adds velocity to the liquid as itpasses up the impeller channels. In general, the higher the speed at the tip ofthe impeller, the more energy that is imparted to any particle that issuspended within the liquid. This energy can then cause damage to anything itimpacts.

 Particle velocity is directly associated to the tip speed of the impeller

Image 1. Particle velocity is directlyassociated to the tip speed of the impeller. (Images courtesy of Hydro, Inc.)

It is important to draw thedistinction between tip speed and rotational speed. A small diameter impellerrunning at high speed could have a lower tip speed than a large diameterimpeller running slowly. Tip speed is the velocity of the impeller at itsoutside diameter.

In general terms, the material lossby erosion is determined by the velocity of the particle cubed (Equation 1).

Equation 1:
Erosion = XC3

C is the velocity of the particle
X is a coefficient based on the liquid
being pumped

The velocity of the particle isdirectly associated to the tip speed of the impeller (Image 1). Lowering thetip speed of a machine has a significant impact on particle velocity and, thus,the erosive energy.

Equation 2:
Tip Speed= πD2n/60

In particular, this affectsparticles exiting the impeller with the high velocity impact onto the pumpvolute lips, causing erosive damage.

As a high-energyparticle passes from the impeller onto the volute lip, a very particularpattern of damage occurs. The damage is generally concentrated at theintersection of the casing volute lip and the side wall of the casing.Particles that impact the center of the volute lip are swept away into the meanstreamline flow, and the time they are in contact with the lip is limited(Image 2).

Image 2. Illustration of wear at the volute lip

Image 2. Illustration of wear at the volute lip

The particles that impact the volute lip closeto the side wall become influenced by the double boundary layer interface thatexists between the lip and side wall. These particles are not quickly passedinto the mean streamline flow and begin to spin(Image 3)

Representation of particle movement illustrating near-wall vs. mid-stream flow

Image 3. Representation of particle movementillustrating near-wall vs. mid-stream flow

Other areasare also susceptible to wear and are generally associated with the fineclearance of the wear ring annulus. There are two types of abrasive/erosivewear that increase the wear ring clearance and deteriorate pump performance.

Image 4 (left). Particles pass across the wearring clearance driven by differential pressure and their shape and directionimpact onto the wear ring surfaces increasing the clearances. Image 5 (right).If the wear ring clearances are smaller than the maximum particle size,material is removed from the clearance.

When the wear ring clearance islarge enough to pass the particles through—the gap is bigger than the maximumparticle size—the particles pass across the wear ring clearance driven bydifferential pressure and their shape and direction impact onto the wear ringsurfaces increasing the clearances (Image 4). If the wear ring clearances aresmaller than the maximum particle size, material is removed from the clearancedue to crushing and abrasion until the clearance increases (Image 5).

Wear rings also play a significantpart in another common form of wear that occurs in the volute casing sidewalls. One of a wear rings’ primary functions is to provide a restrictiveannulus between different pressure regimes within a centrifugal pump. Becauseof this function, one side of the wear ring operates at higher pressure thanthe other. The differential pressure across the ring annulus drives the fluidacross wear ring clearance.

This effect has stiffeningproperties that improve the rotordynamics of the machine. Within an erosiveenvironment this can cause a particular wear pattern of damage. As the liquidexiting the wear ring annulus jets into the low pressure passage carryinghigher energy particles, with it the particles impact the casing side wall andcut into the pressure boundary. This is known as Taylor vortex damage.

Tactics for Limiting Wear

Limiting tip speeds on serviceswhere particles may be present within the pumped fluid limits damage. If thetip speed cannot be limited, choose a machine that has a high tip clearancebetween the impeller outside diameter and the volute lip. This givestime/distance/velocity the chance to act on any particles, thus reducing theirvelocity before they impact the casing.

Modify the volutelips. By profiling the volute lips into a horseshoe shape, it is possible tolimit the wear seen at the intersection between the lip and casing side wall.The large, smooth radius on the lip means that the consequence of the doubleboundary layer that holds the particles and causes them to spin due to thepassing flow is minimized as the particles are forced into the mean flow streamby the geometry of the radius and cannot be stuck in corner intersections(Image 6).

Volute lip modification to ease damage

Image 6. Volute lip modification to ease damage

Modify the casing to limit Taylorvortex damage. Sometimes the geometry of the wear rings and casing wall can bemodified to move the distance from the wear ring jet farther away from the wearring, limiting the impact of the jet by using time/distance/viscosity. Anothertechnique is to ensure a large, smooth radius is allied to the jetting from thewear ring annulus. This uses the Coanda effect, which states that a fluid willstay attached to a curved surface, taking the direction of the fluid and theassociated particles around the radius rather than allowing them to impact intothe side wall of the casing.

Coatings can be helpful incombatting abrasive wear, but it requires a skillful eye to ensure the righttype of coating is applied in the right areas based on an assessment of thedamage that is observed. Coating with a high bond strength and a high hardnesswhen applied in conjunction with the other techniques mentioned in this articlecan be successful in limiting damage.

Particles present in centrifugalpump liquids can cause damage and performance degradation. Some damage is stillinevitable, but using modifications to geometry, combined with hard coatings,increases pump life and reliability.

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