Failed centrifugal pump

 Analysis of a Failed Centrifugal Pump Impeller 

Initial Assessment

• The damage is severe, with the impeller completely shattered.

• This type of failure is rarely due to a single cause but is often the result of one or more of the following issues, possibly acting in combination.

Identifying Potential Causes

• Here are the most likely reasons for this type of damage, ordered from most probable to less probable based on the visual evidence:

• Severe Cavitation (Primary Suspect)

• The most striking feature is the pitted, spongy, and "eaten-away" appearance of the metal.

• This is the classic signature of severe, long-term cavitation damage.

Understanding Cavitation

• What it is: Cavitation is the formation of vapor bubbles in a liquid when the local pressure drops below the liquid's vapor pressure.

• In a pump, this happens on the suction side (the eye) of the impeller.

• As these bubbles travel to a higher pressure region along the impeller vanes, they collapse violently.

The Destructive Nature of Cavitation

• How it causes damage: The collapse of each bubble creates a powerful micro-jet and shockwave, which blasts away tiny particles of the impeller material.

• Over time, this repeated action erodes the metal, creating deep pits and a sponge-like texture, as seen in your photo.

• Result: This process severely weakens the impeller's structure until the normal forces of operation are enough to cause it to fracture and disintegrate.

Root Causes of Cavitation

• Common Causes of Cavitation:

• Insufficient suction pressure (Net Positive Suction Head Available, NPSH_A is less than Net Positive Suction Head Required, NPSH_R).

• A blockage or restriction in the suction pipeline (e.g., a clogged filter or a partially closed valve).

• The pump is running too far to the right on its performance curve (pumping too much flow).

• The liquid being pumped is too warm or too close to its boiling point.

Other Forms of Impeller Degradation

• Severe Corrosion or Dealloying

Material Composition and Vulnerability

• The impeller material appears to be a copper alloy, such as bronze or brass, given its golden/brownish color.

• These materials can be susceptible to specific types of corrosion.

Corrosion: A Chemical Attack

• What it is: This is a chemical attack on the metal by the fluid being pumped.

• If the fluid is seawater, aggressive chemicals, or has a very high or low pH, it can corrode the impeller material.

Dealloying and Material Degradation

• Dealloying (e.g., Dezincification): In alloys like brass (copper-zinc), the corrosive fluid can selectively leach out the more active metal (zinc), leaving behind a weak, porous copper structure.

• This severely compromises the material's strength, making it brittle and prone to shattering under stress.

• The spongy texture in the photo is also consistent with this type of failure.

Erosion-Corrosion: A Combined Effect

• Erosion-Corrosion: This is a combined effect where the high-velocity fluid (erosion) continually removes the protective passive layer on the metal surface, exposing fresh metal to be attacked by the corrosive fluid (corrosion).

Introduction to Material Degradation

• This process accelerates material lossdramatically.

Foreign Object Damage (FOD) Explained

• Foreign Object Damage (FOD)

• What it is: A solid object (like a bolt, rock, or piece of welding slag) enters the pump through the suction line.

• How it causes damage: The object gets hit by the rapidly spinning impeller, causing an immediate fracture of one or more vanes.

• While this can cause significant breakage, it usually results in cleaner fracture surfaces and impact marks, rather than the widespread pitting seen here.

• However, an initial crack from an impact could be the final event that shatters an impeller already weakened by cavitation or corrosion.

Overheating and Dry Running

• Overheating or Running Dry

• What it is: The pump operates with little or no fluid passing through it.

• How it causes damage: Without fluid to carry away heat, the friction of the rotating parts and fluid recirculation causes a rapid temperature increase.

Signs of Impeller Damage

• This can cause the impeller to expand and seize against the pump casing, leading to a catastrophic mechanical failure.

• The discoloration (dark brown/black areas) could be evidence of extreme heat.

Probable Cause of Failure

• The most likely scenario is that the impeller was severely weakened over a long period by intense cavitation and/or corrosion (possibly dealloying).

• The spongy, pitted texture is strong evidence for these phenomena.

• This underlying damage destroyed the structural integrity of the metal, making it brittle.

• Eventually, the normal operational stresses, or a small hydraulic shock, were sufficient to cause the weakened material to completely disintegrate.

Preventive Measures

• To prevent this from happening again, you should:

• Investigate the system: Check the entire suction line for blockages, leaks, or design flaws.

• Ensure all valves are fully open.

• Verify operating conditions: Make sure the pump is operating within its designed flow and pressure range.

Fluid Compatibility Check

• Check the fluid: Confirm that the impeller material (e.g., bronze, stainless steel) is compatible with the fluid being pumped, especially concerning its temperature and chemical properties.

Corrosion Resistance

• Replace with the correct material: If corrosion is a factor, select a more resistant material for the new impeller.