Three Types of Cavitation in Slurry Pumps: NPSH Cavitation, Recirculation Cavitation, and Airfoil Cavitation

Subtitle: Pitting on the impeller inlet edge is not always caused by insufficient NPSH – recirculation cavitation and airfoil cavitation are equally common, each with different causes and solutions

Introduction

Cavitation is the most common and insidious performance killer in slurry pumps. When users see honeycomb‑like pitting on the impeller inlet edge, their first reaction is often “suction lift too high, NPSH insufficient,” and they lower the pump or enlarge the suction pipe. But sometimes these measures don‘t help. Why? Because there is more than one type of cavitation in slurry pumps.

Based on years of field service, Hebei Xingou Machinery Equipment Co., Ltd. has found that besides conventional NPSH cavitation (suction cavitation), recirculation cavitation and airfoil cavitation are equally common and often misdiagnosed. The three types have different causes, damage locations, and solutions. This article systematically explains each type to help users accurately diagnose and apply the right fix.

1. Basic Concept of Cavitation

Cavitation occurs when local pressure drops below the vapor pressure of the liquid, forming bubbles that collapse and create micro‑jets that impact the flow surface. Consequences include:

• Honeycomb pitting and notching on the impeller inlet edge

• Increased vibration and noise (“crackling” sound)

• Reduced flow, head, and efficiency

• Impeller perforation and casing damage in severe cases

Traditionally, users attribute all cavitation to available NPSH (NPSHa) being less than required NPSH (NPSHr). However, even with adequate NPSHa, two other types of cavitation can still occur.

2. Type 1: NPSH Cavitation (Suction Cavitation)

2.1 Mechanism

NPSH cavitation is the classic type. When absolute pressure at the pump suction falls below the vapor pressure, dissolved gases form bubbles. These bubbles collapse when they enter high‑pressure regions, creating micro‑jets that impact the pressure side of the blade inlet edge.

2.2 Typical Characteristics

2.3 Common Causes

• Excessive suction lift (pump installed above water level)

• Elevated water temperature (higher vapor pressure)

• Long, undersized, or overly bent suction piping

• Clogged strainer

• Low atmospheric pressure (high altitude or underground)

2.4 Solutions

3. Type 2: Recirculation Cavitation

3.1 Mechanism

Recirculation cavitation occurs when the pump operates at low flow rates (typically <60% of design flow). At low flow, secondary recirculation forms at the impeller discharge. High‑velocity recirculating flow meets the main flow near the blade inlet, creating intense shear and vortices that cause local pressure drops and bubble formation. Bubbles collapse in high‑pressure regions, damaging the blade inlet edge.

3.2 Typical Characteristics

3.3 Common Causes

• Pump oversized, operating at low flow for extended periods

• VFD minimum frequency set too low (<30 Hz)

• Discharge valve severely throttled

• System demand below pump‘s minimum continuous stable flow

3.4 Solutions

4. Type 3: Airfoil Cavitation (Blade Cavitation)

4.1 Mechanism

Airfoil cavitation is independent of NPSHa. It is caused by the blade shape itself. When liquid flows over the blade inlet edge, sudden changes in thickness or curvature cause local velocity spikes and pressure drops below vapor pressure. Bubbles collapse on the middle or rear part of the blade suction side. This cavitation can occur even at design flow and is a hydraulic design defect.

4.2 Typical Characteristics

4.3 Common Causes

• Thick blade inlet edge, poor streamlined profile

• Blade inlet angle mismatched with flow angle

• Poor blade leading edge design (too sharp or too blunt)

• Casting defects causing local protrusions or depressions

4.4 Solutions

5. Quick Identification Table

6. Case Study: Misdiagnosed Cavitation at a Coal Preparation Plant

Background: A 150ZJ-50 pump used for cyclone feed operated at about 280 m³/h (design flow 380 m³/h). The impeller inlet edge showed pitting. The user assumed NPSH insufficiency, added an inducer, and lowered the pump – but cavitation persisted.

Diagnosis: Hebei Xingou Machinery field testing showed adequate NPSHa; flow was only 74% of design – recirculation cavitation. After fully opening the discharge valve to increase flow to 360 m³/h, the cavitation noise disappeared.

Correction: Process parameters were adjusted to increase system flow, and the VFD minimum frequency was raised from 25 Hz to 35 Hz. No new damage appeared on the impeller.

Conclusion

Cavitation in slurry pumps is not only caused by insufficient NPSH. NPSH cavitation occurs when suction lift is too high or NPSHa is inadequate; recirculation cavitation occurs when the pump runs at low flow for extended periods; airfoil cavitation is caused by poor blade design. The damage locations, morphology, and solutions for the three types are completely different. Field personnel should accurately diagnose by examining damage location, analyzing operating conditions, and measuring NPSHa to avoid misdiagnosis. Hebei Xingou Machinery Equipment Co., Ltd. offers cavitation type diagnostics and impeller repair services. Please contact us.