Cavitation In Depth
Part 4 - Cavitation Damage
Cavitation can destroy pumps and valves, and cavitation causes a loss of efficiency in pumps immediately, and also a continuously increasing loss of efficiency as the equipment degrades due to erosion of the pump components by cavitation. Therefore It is important to understand the phenomena sufficiently to predict and therefore reduce cavitation and damage from cavitation, and also to diagnose and find practical solutions to cavitation problems.
Cavitation Enhanced Chemical Erosion
Pumps operating under cavitation conditions become more vulnerable to corrosion and chemical attack. Metals commonly develop an oxide layer or passivated layer which protects the metal from further corrosion. Cavitation can remove this oxide or passive layer on a continuous basis and expose unprotected metal to further oxidation. The two processes (cavitation & oxidation) then work together to rapidly remove metal from the pump casing and impeller. Stainless steels are not invulnerable to this process.
Materials Selection
There is no metal, plastic, or any other material known to man, that can withstand the high levels of energy released by cavitation in the forms of heat and pressure. In practice however, materials can be selected that result in longer life and customer value in their ability to withstand cavitation energies, so that attention to pump construction materials is valuable and productive.
Where cavitation is not a problem or not predicted to be a problem, common materials such as cast iron and bronze are suitable for pump construction. There are millions of cast iron and bronze pumps that work fine for 20 years or more without any problem even though many of those pumps experience some cavitation.
Factors indicating use of materials with higher resistance to cavitation are:
Corrosive Pumpage - Water with chlorine, salt water, or other oxidizers. A metal that normally has no problem with erosion by a specific chemical can become vulnerable to that chemical If cavitation occurs. Cavitation can eat away the passivated layer that normally protects the metal from corrosion. Stainless steel experiences chemical erosion if the passivated surface layer of the stainless is continuously removed by cavitation, thus continuously exposing fresh unprotected metal to the oxidizing agent.
Low Flow Rate - Long term operation at low flow rates can result in both types of Recirculation Cavitation.
Low NPSHA - Long term operation with marginal or insufficient NPSHA.
Heavy Weight (High Density) Liquids - Heavy liquids such as water cause more damage in cavitation situations. Water molecules are small and dense. Since density is highest at cooler temperatures, water and similar liquids are more of a problem at temperatures below 1500 F.
High Specific Speed Pumps (Ns>9000)
High Suction Specific Speeds (Nss>9500)
Systems with high dP values across the pump.
Systems with high margins of NPSHA over NPSHR . In these situations reducing NPSH may reduce or practically eliminate the cavitation damage.
Material Resistance - The Materials below are listed in the order of their ability to withstand Cavitation Erosion, Cast Iron having the lowest resistance and Stellite the highest resistance to cavitation damage.
Cast Iron
Leaded Bronze
Cast Carbon Steel
Manganese Bronze
Monel
Cast Iron - CA-15, CA6-NM, CF-8M
Stainless Steel (Cast Precipitation, Cast Duplex)
Cast Nickel Aluminum Bronze
Titanium
Cast Carburized 12% Chromium Stainless, Chrome-manganese austenitic Stainless.
Stellite
End of Cavitation - Part 4
