Stainless Steel Pump for Seawater Desalination Intake and Pre-Treatment

Seawater reverse osmosis (SWRO) desalination plants pump 2–5 times the product water volume as seawater intake. A 100,000 m³/day plant requires 250,000–500,000 m³/day of seawater intake. This water is corrosive (3.5% chloride), contains sediment and marine life, and must be pretreated (screening, coagulation, filtration) before reaching the RO membranes. The intake and pretreatment pumps must handle large flow, corrosive seawater, and abrasive sediment — a standard cast iron pump will corrode through in months. Duplex or super-duplex stainless steel pumps are required for this service.

This guide covers stainless steel pump selection for seawater desalination intake and pre-treatment.

Seawater Desalination Process

1. Intake: Seawater is drawn from the ocean through an intake structure (shoreline, offshore, or beach well). Traveling screens or bar racks remove large debris (fish, seaweed, plastic).
2. Intake pumps: Large vertical turbine or horizontal split-case pumps draw seawater from the intake sump. Flow: 5,000–50,000 m³/h per pump.
3. Pre-screening: Band screens or drum screens remove particles >50–100 microns. Protects downstream equipment.
4. Coagulation/flocculation: Ferric chloride or aluminum sulfate is added to destabilize colloids. Mixing pumps or inline mixers ensure uniform dosing.
5. Sedimentation / dissolved air flotation (DAF): Flocs are removed by gravity settling or flotation. Reduces SDI (silt density index) to <5.
6. Media filtration: Dual-media (anthracite/sand) or gravity filters remove remaining particles. SDI target: <3.
7. Cartridge filtration: 5–10 micron cartridge filters as final protection before RO membranes.
8. Anti-scalant and biocide dosing: Chemical dosing pumps add anti-scalant (prevents CaCO₃/CaSO₄ scaling) and biocide (controls biofouling).
9. High-pressure RO feed pump: Multi-stage pump raises pressure to 55–80 bar for SWRO. (Separate from intake/pretreatment pumps.)
10. RO membranes: 30–50% of feed water becomes product (permeate). Remainder is brine (concentrate) discharged to the ocean.

Seawater Characteristics

Intake and Pretreatment Pumps

1. Intake Pumps (Seawater Supply)

The intake pump is the largest pump in a desalination plant:

• Flow: 5,000–50,000 m³/h per pump. 2–5 pumps per plant (N+1 redundancy).
• Head: 10–30 m (lift from intake sump to pretreatment elevation + friction).
• Configuration: Vertical turbine (can-type) or horizontal split-case (double-suction). Vertical turbine is most common — mounted in the intake sump, no suction piping needed.
• Material: Duplex stainless steel (2205) or super-duplex (2507) for all wetted parts. Bronze or nickel-aluminum bronze (NAB) for less critical parts.

2. Booster / Transfer Pumps

Between pretreatment stages:

• Flow: 1,000–20,000 m³/h.
• Head: 10–40 m (overcomes filter pressure drop + elevation).
• Configuration: Horizontal split-case or end-suction centrifugal.
• Material: Duplex stainless (2205) or 316L (for filtered seawater with low sediment).

3. Backwash Pumps

For cleaning media filters:

• Flow: 500–5,000 m³/h (high flow for filter backwash).
• Head: 10–20 m.
• Configuration: Horizontal split-case or vertical turbine.
• Material: 316L or duplex stainless (filtered seawater, low sediment).

4. Chemical Dosing Pumps

For anti-scalant, biocide, coagulant:

• Flow: 10–500 L/h.
• Pressure: 3–10 bar.
• Configuration: Diaphragm metering pump with PTFE wetted parts.
• Material: PTFE/PVDF (for corrosive chemicals).

Our stainless steel chemical pump range includes duplex stainless models for seawater service. For large intake flow, our horizontal split-case pump range is also available. For chemical dosing, our diaphragm metering pump range provides precise PTFE-lined dosing.

Material Selection for Seawater

*PREN = Pitting Resistance Equivalent Number. Higher = better pitting resistance. PREN >40 recommended for seawater with crevices.

Standard for desalination intake: Duplex stainless (2205) for casings, impellers, and shafts. Super-duplex (2507) for warm seawater (>25°C) or crevice areas (gaskets, mechanical seals). NAB is a lower-cost alternative for large impellers and casings, but requires cathodic protection (zinc anodes).

Crevice Corrosion Risk

Seawater is particularly aggressive in crevices (gasket faces, seal areas, under deposits). The critical temperature for crevice corrosion depends on material:

• 316L: ~10°C (pits in most seawater applications)
• Duplex 2205: ~25–30°C (marginal in tropical seawater)
• Super-duplex 2507: ~40–50°C (suitable for all seawater)
• Titanium: >100°C (no crevice corrosion in seawater)

For plants in tropical locations (seawater >25°C), use super-duplex or titanium for crevice areas.

Sizing Parameters

Flow Rate

Intake flow = product water ÷ recovery rate + backwash + losses.

Example: 100,000 m³/day product, 40% recovery, 10% backwash/losses.

Intake flow = 100,000 ÷ 0.40 × 1.10 = 275,000 m³/day = 11,460 m³/h

With 3 pumps (N+1): each pump = 5,730 m³/h (2 running at full load). Add 5–10% margin. Select 6,000–6,500 m³/h per pump.

Head

Intake pump: Static lift (5–15 m) + pipe friction (3–8 m) + pretreatment elevation (5–10 m) = 13–33 m. Typical: 15–25 m.

Filter feed pump: Filter pressure drop (5–10 m) + elevation (3–5 m) + friction (2–5 m) = 10–20 m.

Backwash pump: Filter bed lift (3–5 m) + friction (2–5 m) + elevation = 8–15 m.

Power Calculation

Shaft power = (Q × H × SG) / (367 × efficiency)

Example: 6,000 m³/h, 20 m, SG = 1.025, efficiency = 85%

Shaft power = (6,000 × 20 × 1.025) / (367 × 0.85) = 123,000 / 312 = 394 kW

Add 10–15% margin. Select 450–500 kW motor.

NPSH Considerations

Seawater at 25°C has low vapor pressure (~0.03 bar) — NPSH is usually not critical for intake pumps. However:

• Vertical turbine pumps in the intake sump: NPSHa depends on submergence (water level above first impeller). Maintain minimum submergence per HI standards (prevents vortexing and air ingestion).
• Horizontal pumps: maintain flooded suction (sump above pump). Keep suction piping short and large.
• Warm seawater (30°C+) has slightly higher vapor pressure — verify NPSH margin.

System Design

Intake Structure

• Offshore intake: Submerged intake head with velocity caps (low velocity <0.3 m/s to prevent fish impingement). Pipeline to onshore pump station.
• Shoreline intake: Intake structure on the shore with traveling screens. Simpler, but more debris and marine life.
• Beach wells / infiltration galleries: Seawater filtered through sand. Natural pretreatment (low SDI), but lower flow and higher cost.
• Intake sump: Concrete sump with multiple pump bays. Each pump has a separate bay with uniform flow. Designed per HI standards to prevent swirl and vortexing.

Screening

• Bar rack: 50–100 mm spacing, removes large debris. Automatic raking system.
• Traveling screen: 5–10 mm mesh, removes smaller debris and marine life. Continuous cleaning with backwash.
• Band/drum screen: 50–100 micron, removes fine particles. Protects downstream filters and membranes.

Biofouling Control

Marine growth (barnacles, mussels, algae) on wetted surfaces reduces flow and efficiency:

• Chlorination: Continuous or intermittent chlorine dosing (0.5–1 ppm residual) at intake. Controls biofouling.
• Copper-based materials: NAB or copper-nickel alloys have natural antifouling properties (copper ions inhibit marine growth).
• Regular cleaning: Pigging or chemical cleaning of pipelines. Pump impeller cleaning during maintenance.
• Smooth surfaces: Electropolished stainless steel resists biofilm adhesion.

Cathodic Protection

For NAB or cast iron components in seawater:

• Sacrificial anodes: Zinc or aluminum anodes attached to wetted surfaces. Corrode preferentially, protecting the pump.
• Impressed current: External DC power supply with inert anodes. Used for large structures (intake pipelines, screens).
• Inspection: Anodes inspected and replaced annually.

Common Problems and Solutions

Problem 1: Pitting corrosion in 316L pumps. 316L stainless pumps develop pitting in seawater, especially in crevices. Solution: do NOT use 316L for raw seawater intake. Use duplex (2205) or super-duplex (2507). For existing 316L pumps, consider epoxy coating or cathodic protection (temporary fix).

Problem 2: Biofouling on impeller and casing. Barnacles and algae grow on wetted surfaces, reducing flow and efficiency. Solution: chlorinate intake water (0.5–1 ppm). Use NAB or copper-nickel components (natural antifouling). Clean pump during scheduled maintenance. Install antifouling coating.

Problem 3: Sediment wear on impeller. Near-shore seawater contains sand and silt. The impeller and casing wear, reducing efficiency. Solution: improve intake screening (finer traveling screen). Use hardened impeller (duplex with high hardness, or NAB). Locate intake offshore (lower sediment). Monitor wear and replace impeller as needed.

Problem 4: Cavitation from vortexing. The intake sump design causes vortices, drawing air into the pump. Cavitation and vibration result. Solution: design sump per HI standards (adequate submergence, baffles, floor splitters). Use suction bell with straightening vanes. Install a vortex breaker plate. Verify with model testing for large pumps.

Problem 5: Mechanical seal failure. Seawater with sediment damages seal faces. Chlorination accelerates elastomer degradation. Solution: use SiC vs SiC seal faces (hard-hard). Use FKM or FFKM elastomers (not EPDM — EPDM swells in chlorinated water). Use Plan 32 flush with filtered seawater or clean water. Consider double seal with barrier fluid for critical pumps.

Problem 6: Galvanic corrosion. Dissimilar metals in contact (e.g., 316L bolt + duplex casing) cause galvanic corrosion. Solution: use compatible materials (all duplex or all NAB). Use insulated gaskets between dissimilar metals. Apply cathodic protection. Avoid graphite gaskets (cathodic to stainless steel).

Problem 7: Pump clogging with marine life. Jellyfish, seaweed, or fish clog the impeller or suction. Solution: improve intake screening (finer mesh, continuous cleaning). Use open impeller (less clogging). Install a debris monitor (differential pressure). Clean impeller during maintenance.

Problem 8: High energy cost. Intake pumps are large energy consumers. Solution: select high-efficiency pump (≥85%). Use multiple pumps for part-load efficiency. Consider VFD for variable flow (if intake demand varies). Optimize intake design (minimize friction losses). Regularly overhaul pumps (restore efficiency).

Selection Summary

1. Define seawater conditions: temperature, chloride, sediment, marine life
2. Select pump type: vertical turbine (intake) or horizontal split-case (transfer)
3. Use duplex stainless (2205) for most seawater service; super-duplex (2507) for warm/tropical
4. Do NOT use 316L for raw seawater (pitting risk)
5. Size flow for product water ÷ recovery + backwash, divided by number of pumps
6. Size head for static lift + friction + pretreatment elevation
7. Use NAB or copper-nickel for antifouling (with cathodic protection)
8. Install chlorination system for biofouling control
9. Design intake sump per HI standards (prevent vortexing)
10. Use SiC vs SiC mechanical seals with FKM/FFKM elastomers
11. Provide N+1 redundancy (one spare pump)
12. Install cathodic protection for NAB/cast iron components
13. Plan preventive maintenance (impeller cleaning annually, overhaul 5–8 years)
14. Require material certificates and corrosion test reports

For seawater desalination pump selection, our stainless steel chemical pump range, horizontal split-case pump range, and diaphragm metering pump range provide corrosion-resistant construction for intake and pretreatment service. For pump sizing and material recommendation, talk to our engineers with your plant capacity, seawater analysis, and intake design.