A warm, inviting beach begins with one grain of sand. That same grain of sand can spell the beginning of the end for downhole pumping systems. Abrasives such as sand can significantly increase capital and maintenance budgets and, thus, lifecycle costs. Abrasives cause a myriad of problems for pumping systems, including worn pumps that require replacement; blocked column pipe, which requires pulling the system and servicing sand caked components; and eroded surface instrumentation, valves and tubing.
The negative effects of ingesting abrasives in centrifugal pumps can generally be divided into vane passage erosive wear and running clearance abrasive wear.
As abrasives pass rapidly through a pump, they can erode the pump vane surfaces. The rate of erosion is a function of the pump construction and abrasive characteristics. The hardness of the pump base material will influence the erosive process—the greater the material hardness, the less vane surface erosion occurs. Strategically placed anti-swirl ribs or sand dams have proven to further protect against erosive wear. Vane tip speed also influences the erosive process. As the tip speed increases, the relative impact speed of the abrasive increases.
Abrasives cause grinding wear between close-tolerance running interfaces as well.
For example, abrasive wear can occur in the bushing clearances and other pump running clearance interfaces. A pump’s running clearances are located in the bushing interface, the pump skirts and pump wear rings. Abrasive wear is a function of the pump construction and the sand’s characteristics.
The first step in any abrasive application is to study a sand sample. A sand analysis will indicate the quantity of sand, acid solubility, particle size distribution, quantity of quartz and sand geometry. These factors in turn influence the selection of pump type, pump stage material, sleeve bushing material and other pump construction characteristics.
Downhole pump construction can be a floater or compression: A floater pump absorbs the individual stage thrust at each stage location while the more common compression pump transfers the thrust to a thrust bearing located remotely from the pump, thus, removing of the primary high-wear areas in the pump.
Stage type can be mixed or radial flow. Generally, mixed flow will handle abrasives better because there are fewer abrupt changes in direction. Hardened carbide bushings can be strategically placed in the pump, or used in every bushing location, for aggressive abrasive scenarios. The material hardness of these specially designed bushings is greater than silica, so are less susceptible to silica induced abrasive wear.
It is a common misconception that two pole centrifugal pumping systems are more susceptible to abrasives than four pole systems. When two pole and four pole systems are compared, the vane tip speed does not theoretically change for the same fluid rate. Per the Affinity Laws, the change in rate is directly proportional to the change in diameter. Some changes in vane angle will yield minor variations in tip speed, but overall there is no change in tip speed or the related erosive wear between two pole and four pole systems with the same pump materials and construction.
The running clearances in pumping systems are all hydrodynamic interfaces—there is no metal on metal wear. Also, for a given horsepower when the speed is increased the torque is decreased, allowing a smaller diameter shaft to be used in two pole applications. This results in a reduced relative speed in bushing clearances.
Likewise, the stage diameters are reduced per the Affinity Laws, which results in a reduced relative speed at the running clearances.
A rocky example
A Rocky Mountain municipality was experiencing abrasive wear in its pump bearings as a result of downhole abrasives. Well analysis indicated 100 ppm of sand at pump start-up. The high sand content resulted in short run times and elevated operating costs. The city switched four of its water wells to Centrilift pumps with hardened carbide bushings and doubled pump run time. The abrasive resistant equipment provided better reliability and lower costs for the municipality.