How Can Cavitation Be Prevented in High-Pressure Pumps Operating in Highly Corrosive Media Such as Seawater?| Insights by AQUALITEK

Introduction

In seawater reverse osmosis (SWRO) systems, high-pressure pumps (HPPs) operate under extreme conditions:

•High pressure

•High salinity

•Strong corrosiveness

•Continuous operation

Among all pump-related failures, cavitation is one of the fastest and most destructive, especially in highly corrosive seawater environments. Cavitation not only damages hydraulic components but also accelerates corrosion and shortens pump lifespan dramatically.

This article explains why cavitation is more severe in seawater applications and provides best-practice prevention strategies specifically tailored for SWRO systems.

Why Cavitation Is Especially Dangerous in Seawater Pumps

Cavitation occurs when local pressure drops below the vapor pressure of water, forming vapor bubbles that collapse violently when pressure recovers.

In seawater systems, cavitation damage is intensified because:

•Seawater has lower vapor pressure margins

•Chloride ions accelerate pitting and crevice corrosion

•Cavitation erosion removes passive layers, exposing fresh metal

•Corrosion and cavitation reinforce each other (“cavitation–corrosion synergy”)

As a result, even mild cavitation can lead to rapid impeller, casing, and seal failure.

Best Practices to Prevent Cavitation in Seawater High-Pressure Pumps

1. Ensure Adequate NPSH Margin (The Most Critical Factor)

Net Positive Suction Head (NPSH) is the primary determinant of cavitation risk.

Best practices:

•Ensure NPSH Available (NPSHa) > NPSH Required (NPSHr) with sufficient safety margin

•Minimize suction-side pressure losses

•Maintain stable feed pressure from pretreatment and booster pumps

Key actions:

•Keep suction piping short and straight

•Avoid undersized suction pipes

•Eliminate unnecessary valves and fittings on the suction side

2. Maintain Stable and Sufficient Inlet Pressure

Low or fluctuating inlet pressure is a common trigger for cavitation.

Preventive measures:

•Install reliable low-pressure interlocks

•Monitor inlet pressure continuously

•Shut down the pump immediately on low-pressure alarms

In SWRO systems, unstable inlet pressure may result from:

•Pretreatment filter fouling

•Intake screen blockage

•Seawater level fluctuations

•Air entrainment at intake

3. Avoid Excessive Suction Lift and Air Entrainment

Air entering the suction line drastically increases cavitation risk.

Best practices:

•Keep pumps flooded or below seawater level

•Ensure airtight suction piping

•Eliminate vortex formation at intake structures

•Avoid leaks at flanges, gaskets, and mechanical seals

⚠️ Even small air leaks can trigger severe cavitation in high-speed pumps.

4. Control Pump Operating Point (Avoid Off-Design Operation)

Operating far from the Best Efficiency Point (BEP) increases internal pressure fluctuations.

Recommendations:

•Select pumps sized for actual operating flow and pressure

•Avoid long-term low-flow or throttled operation

•Use variable frequency drives (VFDs) carefully to stay within design limits

Off-design operation often causes:

•Localized low-pressure zones

•Internal recirculation

•Cavitation at impeller eye or diffuser

5. Optimize Seawater Temperature and Vapor Pressure Effects

Higher water temperature increases vapor pressure and cavitation risk.

Control strategies:

•Monitor seasonal seawater temperature changes

•Adjust operating margins during warm periods

•Increase inlet pressure or reduce pump speed if necessary

This is particularly important in tropical and Middle East desalination plants.

6. Use Cavitation-Resistant and Corrosion-Resistant Materials

Material selection plays a critical role in damage resistance.

Recommended materials:

•Duplex stainless steel

•Super duplex stainless steel

•Titanium alloys (for severe conditions)

•High-grade nickel alloys in extreme cases

These materials:

•Resist chloride-induced corrosion

•Maintain passive layers under cavitation stress

•Extend service life even if minor cavitation occurs

7. Maintain Clean Pretreatment and Intake Systems

Pretreatment failures indirectly cause cavitation by restricting flow.

Key maintenance actions:

•Regularly backwash and inspect intake screens

•Maintain multimedia and cartridge filters

•Monitor differential pressure across pretreatment units

Clogged pretreatment increases suction losses, lowering NPSHa.

8. Implement Gradual Startup and Shutdown Procedures

Rapid transients can create sudden pressure drops.

Best practices:

•Use controlled startup ramps

•Avoid sudden valve closures

•Synchronize energy recovery devices smoothly

•Prevent water hammer events

Transient pressure dips can induce instant cavitation, even when steady-state conditions are safe.

Early Warning Signs of Cavitation in Seawater Pumps

Operators should watch for:

•Abnormal vibration or rattling noise

•Fluctuating discharge pressure

•Sudden efficiency loss

•Increased bearing or seal wear

•Pitting damage on impellers during inspection

Early intervention prevents catastrophic failure.

Conclusion

Preventing cavitation in high-pressure pumps operating in corrosive seawater requires both hydraulic optimization and corrosion-aware design. The most critical measures include ensuring adequate NPSH, stabilizing inlet pressure, avoiding air ingress, operating near BEP, and selecting suitable materials.

In SWRO systems, cavitation is not just a mechanical issue—it is a major reliability and lifecycle cost risk. Proactive prevention is far more effective and economical than repair.