Seawater Intake Pump for Coastal Desalination Plant

Scenario

A new reverse osmosis (RO) desalination plant is under construction in Al Khobar, Saudi Arabia. The seawater intake system must deliver a steady flow to the pretreatment section while operating reliably in high-salinity, warm (32°C), and biofouling-prone conditions. Key constraints include: limited civil space for pump station footprint, strict noise limits (<75 dBA at 1 m), and requirement to avoid cavitation despite low static suction head (only 1.8 m above sea level at lowest tide). A vertical turbine pump is preferred for submersible installation.

Given Data

• Flow rate: 4,200 m³/h
• Head: 28 m (includes 5 m friction loss in 1.2 km intake pipeline + 23 m static lift to pretreatment basin)
• Fluid density: 1,025 kg/m³ (measured salinity = 42,000 ppm at 32°C)
• Efficiency: 72% (derated from catalog 78% due to viscosity effects and sand abrasion margin)

Calculation

Using the Pump Sizing Calculator:

• Power = (ρ × g × Q × H) / (η × 1000)
  = (1025 × 9.81 × (4200/3600) × 28) / (0.72 × 1000)
  = (1025 × 9.81 × 1.1667 × 28) / 720
  ≈ 458.3 kW → 458.32 kW (rounded to 2 decimals)
• Pump size: Interpolated from standard ANSI/HI 10.6 performance curves — corresponds to a 16-inch (400 mm) discharge vertical turbine pump with 4-stage impeller configuration → Size 400-VT4
• NPSHr: Derived from affinity law scaling and vendor correlation for seawater service; calculator estimates 4.18 m, validated against KSB and Grundfos VT series data sheets for equivalent duty.

Result and Decision

The calculated NPSHr (4.18 m) was compared to site-specific NPSHa: 1.8 m (static) + 0.9 m (velocity head) − 1.2 m (friction loss) − 0.3 m (vapor pressure correction for 32°C) = 1.2 m — critically insufficient. To resolve, engineers redesigned the intake sump to increase submergence by 3.2 m (via deeper wet well and tidal compensation basin), raising NPSHa to 4.4 m. A KSB MULTITEC VT 400-4 was selected, with factory-tested NPSHr = 4.0 m at BEP.

Lesson

NPSH margin cannot be compromised by derating efficiency alone — always calculate NPSHa in situ using worst-case hydraulic and thermodynamic conditions before final pump selection; physical layout adjustments often outweigh equipment re-specification.