Selecting the Right Pump for Chemical Applications
Pump cavitation is one of the most common and potentially damaging phenomena in fluid handling systems. This comprehensive guide explains the science behind cavitation, its effects on pumping equipment, and practical strategies to prevent it.
What Is Pump Cavitation?
Cavitation occurs when the local pressure in a liquid drops below its vapor pressure, causing the formation of vapor bubbles. As these bubbles move to areas of higher pressure, they implode violently, creating shock waves that can damage pump components. This process happens in microseconds but can cause extensive damage over time.
The telltale signs of cavitation include unusual noise (often described as a grinding or crackling sound), vibration, reduced pump performance, and eventually, physical damage to impellers and other pump components.
The Science Behind Cavitation
To understand cavitation fully, we need to consider the relationship between pressure and the physical state of liquids. Every liquid has a vapor pressure — the pressure at which the liquid begins to vaporize (boil) at a given temperature. When the local pressure in a pumping system falls below this vapor pressure, some of the liquid transitions to a vapor state, forming bubbles.
Several factors can contribute to pressure drops severe enough to cause cavitation:
- Insufficient Net Positive Suction Head (NPSH)
- Restrictions in suction piping
- High liquid temperatures
- Operation at points far from the pump's best efficiency point
- Air leaks in suction lines
Types of Pump Cavitation
There are several distinct types of cavitation, each with different causes and characteristics:
Suction Cavitation
The most common type, suction cavitation occurs when the pump inlet pressure falls below the liquid's vapor pressure. This is typically caused by insufficient NPSH, clogged strainers, or undersized suction piping.
Discharge Cavitation
This occurs when the pump operates against a very low discharge head, causing excessive recirculation within the pump and leading to low-pressure areas where cavitation can occur.
Vane Passing Syndrome/Hydraulic Cavitation
This type of cavitation results from the design of the pump itself, where the interaction between the impeller vanes and the pump casing creates low-pressure regions.
Internal Recirculation Cavitation
When a pump operates at low flow rates, internal recirculation patterns can develop, creating localized low-pressure areas that lead to cavitation.
The Damaging Effects of Cavitation
The impact of cavitation on pumping equipment can be severe and multifaceted:
Physical Damage
The most visible effect is the pitting and erosion of metal surfaces. When vapor bubbles implode near a solid surface, they create microjets of liquid that can strike the surface at speeds exceeding 700 mph, causing material erosion. Over time, this erosion leads to characteristic "honeycomb" patterns on impellers and can eventually perforate the metal.
Performance Degradation
Cavitation disrupts the normal flow patterns within the pump, reducing efficiency, flow rate, and pressure development. This results in higher energy consumption for the same output or diminished output for the same energy input.
Vibration and Noise
The shock waves created by imploding bubbles cause vibration that can damage bearings, seals, and other components. The noise generated by cavitation — often described as sounding like gravel passing through the pump — can exceed safe workplace noise levels.
