A Remedy for Pump Cavitation

Application of a special cavitation-resistant polymer can restore a damaged pump impeller and help forestall costly pump replacement

Cavitation is the formation and implosion of vapor bubbles in a region where the pressure of a liquid falls below its vapor pressure. Cavitation is extremely damaging and can occur in any fluid-handling equipment, especially in pumps, one of the most important components of wastewater treatment systems.

Technological advances in industrial protective coatings and repair composite materials have made it possible to repair pumps suffering from cavitation, rather than simply replacing them. Cavitation-resistant (CR) elastomers can retain adhesion under long-term immersion, dissipate energy created under high-intensity cavitation, and provide outstanding resistance to corrosion and other forms of erosion.

Cavitation is a serious problem for pumps. In simple terms, a pump moves fluid from one location to another with mechanical actions that can be extreme, and can damage the internal working parts of the pump. The focal point of damage is the pump impeller vane. During operation the impeller is subject to pressure gradients that cause bubbles to form and implode, striking the surface underneath.

Knowing the mechanics

The phase diagram of water is a practical aid to understanding cavitation (Figure 1). The diagram shows the three physical states of water at different temperatures and pressures. The curves on the graph represent equilibrium states. The curve bordering the liquid and gas phases is called the vaporization curve.

At normal pressure and temperature, a fluid is at 1 atmosphere (14.7 psi) and 25 degrees C. Water is most commonly boiled by heating at a constant pressure, such as boiling a pot of water on a stovetop (follow the black arrow). As temperature increases at constant pressure, water remains in a liquid phase until it reaches the normal boiling point (100 degrees C at 1 atmosphere), at which point, it starts to boil.

Less intuitive but equally true is that water can also be boiled by dropping the pressure at a constant temperature (follow red arrow). This is what happens just behind the leading edge of a pump impeller vane. As water enters the pump, it is deflected by the vane. Above the leading edge of the vane, the fluid is compressed, creating a high local pressure area.

Directly after the leading edge, there is a small area of decreased pressure. If this drop in pressure moves below the vaporization curve at constant temperature, the water will boil, and vapor bubbles will form in it. Behind this low-pressure area, there is another high-pressure region. As the vapor bubbles entrained in the water move into this region, they condense and collapse violently against the metal, forming a “micro jet.”

Figure 2 illustrates the implosion of the vapor bubbles. The top of the bubble becomes unstable and collapses toward the metal surface substrate. During this process, pressures as high as 145,000,000 psi have been recorded. That exceeds the elastic limit for any alloy, proving that not even the most exotic alloys can prevent cavitation.

These vapor bubbles are responsible for the mechanical damage found on pump impellers after extreme service. Figure 3 shows a typical pump suffering from cavitation and some other form of erosion after normal operation.

Solving the problem

The solution to pump impeller cavitation lies in finding a material that can withstand high pressures, bear harsh environments, and be machinable. At present, no readily available alloy can do that cost-effectively. Thus the only tangible way to salvage the pump is to protect it with a sacrificial material that is readily available, easy to use, and cost-effective.

After years of research in corrosion engineering, a CR fluid has been formulated; Elastomers that can bond to virtually any substrate, including steel. With the appropriate surface preparation, adhesion strengths greater than 3,200 kg/m² are achievable. By combining elastomeric properties and great adhesive strength, the material can withstand full immersion and a harsh working environment.

More important, the material’s flexibility enables it to dissipate the enormous energy of cavitation and other erosion processes. A CR fluid elastomer has been in service for a number of years. Before it came on the market, it underwent a series of highly demanding quality checks, including laboratory tests to determine that the correct properties had been achieved.

The testing did not stop at the inception of the product — it has continued throughout the material’s market life. To ensure longevity, CR fluid elastomers are scheduled to be subjected to the ASTMG8 testing for magnesium anode and cathodic disbondment.

In one particular case, the sides and the trailing surfaces of a large impeller had suffered from cavitation and significant metal loss (Figure 3). The CR elastomer was applied by an authorized coating applicator. Here is a summary of the methodology:

• All surfaces to be coated were grit-blasted using an angular abrasive to NACE No. 2 (near white metal), ensuring a minimum 3-mil (75-µm) angular profile.

• All those surfaces were subsequently washed down with a recommended cleaner degreaser to remove residual blasting debris and contaminants.

• Masking tape was placed at the outer edges of the areas to be coated to give a neat and clean finish.

• The substrate was rebuilt and brought back to factory specification. To rebuild such a large area, an extended-working-life paste-grade polymer from a reputable manufacturer was used.

• To protect the freshly rebuilt substrate, an efficiency-improving and abrasion-resistant polymeric coating was applied, using stiff, short-bristled brushes to a maximum wet thickness of 10 mils (250 µm). Two coats of this material are required to ensure that pinholes and other defects are eliminated. This coating is used to prevent the effect of erosion and corrosion.

• With the pump rebuilt, a CR coating was applied to the entire impeller (Figure 4). An alternative solution would be to weld numerous damaged areas, or cut out a large section and weld it in a new plate.

• All the coated surfaces were allowed to cure, the coating was inspected for continuity, and the pump was put back into service.

Providing protection

A solution to eradicate pump impeller cavitation has not been discovered. The best solution at present is to coat the fluid-handling device with a high-performance material that is elastomeric and has high-adhesive strength.

The high adhesion allows the material to bond to the fully immersed substrate, while the elastomeric characteristics better dissipate the energy of cavitation. If a solution to control mechanical damage to the pump is needed, a CR fluid elastomer is the answer.

About the author

Glenn Machado is a technical service engineer with Belzona Inc., a supplier of protective coatings and repair composites based in Miami, Fla. He can be reached at 305/594-4994 or gmachado@belzona.com.



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