Cavitation in Double Suction Split Case Pumps: Causes, Impeller Design and Solutions

Cavitation is one of the most common hydraulic problems affecting centrifugal pumps, particularly in applications involving high flow rates, demanding suction conditions, or continuous operation. For double suction split case pumps, cavitation can reduce hydraulic performance, increase vibration and noise, damage impeller surfaces, and shorten the service life of critical components.

Although cavitation is often associated with insufficient suction pressure, the problem is not determined by suction conditions alone. Impeller inlet geometry, flow velocity, operating point, fluid temperature, and suction pipeline resistance all influence the likelihood of cavitation.

Improving cavitation resistance therefore requires a combination of appropriate pump selection, hydraulic design optimization, and correct system operation.

This article explains how cavitation develops in double suction split case pumps, examines two important impeller design improvements, and introduces practical methods for preventing cavitation in industrial pumping systems.

 

 

1. What Is Cavitation in a Split Case Pump?

Cavitation occurs when the local pressure of a liquid falls below its vapor pressure, causing part of the liquid to vaporize and form small vapor bubbles.

As these bubbles move into regions of higher pressure inside the pump, they collapse rapidly. Repeated bubble collapse can generate localized pressure impacts and damage nearby metal surfaces.

In a double suction split case pump, cavitation commonly develops around the impeller inlet, particularly near the leading edges of the blades where local flow acceleration can produce a pressure reduction.

Typical symptoms include:

Unusual crackling or gravel-like noise during operation.

Increased vibration and unstable operation.

Pitting or erosion on impeller surfaces.

Reduced pump head and hydraulic efficiency.

Fluctuating flow rate and discharge pressure.

Premature damage to bearings and mechanical seals in severe cases.

Cavitation should be distinguished from air entering the suction pipeline. Both can produce noise and unstable operation, but their causes and corrective measures are different. A proper diagnosis should consider suction pressure, fluid conditions, operating data, and internal inspection results.

 

2. Why Are Double Suction Split Case Pumps Susceptible to Cavitation?

A double suction split case pump typically uses a horizontally split casing and a double-entry impeller. Liquid enters the impeller from both sides, dividing the total flow between two inlet passages.

This arrangement offers advantages for large-flow applications and can help balance axial hydraulic forces. However, it does not automatically eliminate cavitation.

The pressure at the impeller inlet depends on several factors:

High inlet flow velocity

When liquid passes through a restricted or poorly designed inlet passage, its velocity may increase and local static pressure may decrease. If the local pressure falls below the liquid's vapor pressure, vapor bubbles can form.

Unsuitable operating conditions

Operating too far from the recommended operating range can cause unfavorable flow patterns, increased hydraulic losses, flow separation, and local pressure fluctuations.

Insufficient suction conditions

A high suction lift, long suction pipeline, undersized pipe, partially closed valve, or blocked strainer can increase suction losses and reduce the pressure available at the pump inlet.

High liquid temperature

As liquid temperature increases, its vapor pressure generally rises. This reduces the pressure margin available before vapor formation occurs.

Uneven inlet flow distribution

Poor suction piping arrangements, nearby elbows, abrupt changes in pipe diameter, or other upstream disturbances can produce a non-uniform velocity profile. This may cause unfavorable flow conditions at the impeller entrance, even when the overall suction pressure appears acceptable.

Understanding these factors is essential before deciding whether the problem requires a system modification, a pump redesign, or a combination of both.

 

3. Two Important Impeller Design Improvements for Cavitation Resistance

Improving the hydraulic design of the impeller inlet can help reduce local flow acceleration and pressure loss. Two design parameters worth evaluating are the front shroud profile and the impeller hub diameter.

3.1 Optimize the Front Shroud Turning Radius

The front shroud, also referred to as the front cover plate, forms part of the impeller's inlet flow passage.

Near the impeller blade leading edge, the geometry of the front shroud influences how liquid turns and enters the blade passages. An abrupt transition or an unsuitable curvature may contribute to local acceleration, flow separation, and unfavorable pressure distribution.

Optimizing the turning radius of the front shroud can help create a smoother inlet passage.

Potential benefits include:

Reducing abrupt changes in flow direction.

Improving the distribution of liquid velocity near the blade inlet.

Reducing local hydraulic losses.

Helping prevent unfavorable low-pressure regions near the impeller entrance.

During the design process, the turning radius should be evaluated together with inlet width, blade leading-edge geometry, and the overall impeller passage.

A larger radius is not automatically better in every pump. Excessive geometric changes may affect the flow passage, hydraulic performance, or manufacturing feasibility. The final profile should be determined through hydraulic design calculations, computational fluid dynamics (CFD), and appropriate performance testing.

3.2 Appropriately Reduce the Impeller Hub Diameter

The hub is the central part of the impeller that connects to the pump shaft and transfers rotational power to the impeller.

At the design stage, reducing the hub diameter while keeping the impeller inlet diameter unchanged may increase the available inlet flow area.

A larger effective flow area can help reduce average inlet velocity under suitable conditions, potentially improving the pressure distribution near the impeller eye.

However, hub diameter is also closely related to mechanical strength.

The following factors must be considered before changing the hub geometry:

Shaft diameter and mechanical strength.

Hub wall thickness and structural integrity.

Impeller material and rotational speed.

Stress concentration around the hub and shaft connection.

Impeller attachment and assembly requirements.

Hydraulic performance across the intended operating range.

Reducing the hub diameter without adequate structural evaluation may weaken the impeller or affect the reliability of the rotating assembly.

This modification should therefore be treated as a design-level optimization rather than a routine field repair. The final dimensions must satisfy both hydraulic and mechanical requirements.

 

4. The Importance of NPSH in Preventing Cavitation

Impeller geometry is only one part of cavitation prevention. The relationship between the system's available net positive suction head and the pump's required net positive suction head must also be evaluated.

NPSH means Net Positive Suction Head. It is used to assess whether sufficient pressure is available at the pump suction to limit vapor formation.

There are two important parameters:

NPSHa (Net Positive Suction Head Available): The NPSH provided by the pumping system under actual operating conditions.

NPSHr (Net Positive Suction Head Required): The NPSH requirement of the pump at a specified flow rate and speed, determined through pump performance testing.

A basic selection condition is:

NPSHa > NPSHr

In practical engineering, an appropriate safety margin should also be maintained. The required margin depends on pump design, operating conditions, the applicable standard, and the manufacturer's recommendations.

NPSHa can be reduced by:

Excessive suction pipeline friction losses.

High suction lift.

Low liquid level in the supply tank.

High liquid temperature.

Restricted suction valves or blocked filters.

Unfavorable system pressure conditions.

A pump may have a well-designed impeller but still experience cavitation if the available NPSH is insufficient. Therefore, both pump-side requirements and system-side conditions must be considered.

 

5. Practical Measures to Reduce Cavitation in Existing Pumping Systems

In addition to improving the pump's hydraulic design, several system-level adjustments can help reduce cavitation.

Improve the Suction Pipeline

Use an appropriately sized suction pipe to avoid excessive flow velocity and friction losses. Keep the suction pipeline as short and direct as practical, and avoid unnecessary elbows, abrupt diameter changes, and restrictive fittings.

Ensure that suction valves are fully open during normal operation and that strainers or filters are regularly inspected and cleaned.

Maintain Adequate Liquid Supply

Check the liquid level in the supply tank and review the pump's installation elevation. Where conditions permit, reducing the suction lift or increasing the static liquid level can improve NPSHa.

For closed systems, verify that the system pressure is suitable for the fluid temperature and operating requirements.

Operate Near the Recommended Operating Range

Check the actual flow rate against the manufacturer's pump performance curve.

Avoid prolonged operation at excessively low or high flow rates where hydraulic instability may develop. Select a pump whose best efficiency point and allowable operating range are appropriate for the expected system duty.

Monitor Temperature and Fluid Conditions

For hot liquids, evaluate their vapor pressure at the actual operating temperature. Changes in liquid temperature, composition, viscosity, or gas content may influence suction performance.

The pump's NPSH requirement should be evaluated for the actual fluid and operating conditions rather than relying only on water-based reference data.

Inspect Impellers and Internal Flow Passages

During scheduled maintenance, inspect impeller blade leading edges, shrouds, and other wetted surfaces for pitting, erosion, corrosion, and deposits.

If damage is found, determine whether it is associated with cavitation, abrasive wear, corrosion, or another mechanism before selecting a repair method.

 

6. How to Select a Split Case Pump with Better Cavitation Resistance

Selecting a pump with suitable suction characteristics is particularly important for large-flow water supply, cooling water circulation, irrigation, and industrial process applications.

Before selecting a double suction split case pump, engineers should evaluate the following information:

Selection factor	Information to confirm
Flow rate	Normal, minimum, and maximum operating flow
Total dynamic head	Required head under actual system conditions
Liquid properties	Type, temperature, density, viscosity, and vapor pressure
Suction conditions	Supply tank level, pressure, suction lift, and pipeline losses
NPSH	Available NPSHa and pump-required NPSHr
Pump speed	Motor speed and expected operating conditions
Installation	Foundation, pipe arrangement, and inlet flow conditions
Materials	Corrosion resistance and suitability for the pumped liquid

The pump should be evaluated using its complete performance curves, including flow, head, efficiency, power, and NPSHr.

For applications with limited suction pressure or demanding inlet conditions, a pump with suitable low-NPSH hydraulic characteristics may be more appropriate than simply selecting a larger motor or a higher-capacity model.

 

Conclusion

Cavitation in double suction split case pumps is influenced by both the pumping system and the internal hydraulic design. Insufficient NPSH, excessive inlet velocity, unfavorable flow distribution, and unsuitable operating conditions can all contribute to its development.

At the design level, optimizing the front shroud turning radius and appropriately reducing the impeller hub diameter may help improve inlet flow conditions. However, these modifications must be evaluated alongside impeller strength, shaft design, hydraulic performance, and manufacturing requirements.

For existing installations, improving suction pipeline conditions, maintaining adequate liquid supply, operating within the recommended range, and monitoring NPSH are equally important.

A systematic approach that combines sound hydraulic design, accurate pump selection, and proper system operation can help reduce cavitation damage and improve the long-term reliability of industrial centrifugal pumps.