Axial Flow Pumps

Pumps ·

Axial flow pumps — also called propeller pumps — move fluid parallel to the pump shaft axis by means of a propeller-shaped impeller. They are purpose-built for applications requiring very high flow rates at very low head: large-scale water transfer, flood control, irrigation and industrial cooling water circulation where other pump types would require prohibitively large and expensive equipment.

History

The principle of using a rotating propeller to move fluid axially was studied throughout the nineteenth century, but the decisive engineering contribution came in 1913 from Viktor Kaplan, an Austrian engineer and professor at Brno Technical University in Moravia. Kaplan patented the adjustable-pitch propeller turbine — now universally known as the Kaplan turbine — demonstrating that varying the blade pitch angle during operation allows a single machine to maintain high efficiency across a wide range of head and flow conditions, a critical capability for tidal barrages and flood-control stations where water levels fluctuate. The first large-scale axial flow pumping installations using this principle were commissioned in the Netherlands in the 1920s to drain agricultural polders, and the technology rapidly spread to irrigation and flood-control infrastructure worldwide.

Operating Principle

A propeller or axial impeller rotates in a cylindrical casing (bowl) that closely surrounds the blade tips. The angled blades generate lift — exactly as an aeroplane propeller does in air — pushing fluid forward along the axis of rotation. Guide vanes downstream of the impeller remove the rotational component of velocity and convert it to axial velocity and pressure. Because there is no outward radial flow, the entire casing diameter carries fluid and the passage area is very large.

Configurations

  • Vertical axial flow pump: Standard configuration for irrigation canals, pumping stations and water treatment intake works — impeller submerged, motor above water level
  • Horizontal axial flow pump: Inline configuration for cooling water circuits, desalination intake and industrial process
  • Variable-pitch propeller: Blade pitch adjustable while running — allows efficient operation across a wide range of flows without speed change; used in large flood-control and tidal-barrage installations
  • Diagonal (mixed) flow: Intermediate between axial and centrifugal — provides higher head than pure axial flow while retaining a large flow passage

Key Technical Parameters

  • Flow rate (Q): 500 to 100,000 m³/h and beyond for the largest installations
  • Total head (H): 1 to 20 m — the lowest head range of any industrial pump type
  • Specific speed (Ns): Very high — 5,000 to 20,000 (metric units); defines the axial-flow regime
  • Efficiency: 80 to 90% at the design point — high efficiency is achievable when the duty is correctly matched
  • Blade pitch angle: Typically 12° to 25° — increasing pitch increases flow rate and head
  • Cavitation sensitivity: High — impeller tip speed is high; adequate submergence or NPSH is critical

Applications

  • Large-scale irrigation canal pumping — transferring millions of cubic metres per day
  • Flood control and stormwater management — permanently installed or emergency mobile units
  • Power plant cooling water circulation — condenser cooling and cooling tower basin circulation
  • Desalination plant seawater intake lifting
  • River intake pumping stations for water treatment works
  • Aquaculture recirculation systems requiring high water turnover
  • Industrial process cooling water in refineries and chemical complexes

Advantages

  • Unmatched capacity for very high flow rates in a relatively compact physical envelope
  • Simple, open flow path with large clearances — passes debris and large particles without clogging
  • High efficiency at the design duty point
  • Variable-pitch blades allow a single pump to cover a very wide range of operating conditions
  • Low capital cost per cubic metre per hour of capacity for large-scale water transfer

Limitations

  • Strictly limited to low-head applications — flow falls to zero at very modest back-pressure
  • Q-H curve is steep and unstable — shut-off head is lower than design head in some configurations, risking instability at low flow
  • Very sensitive to cavitation — insufficient submergence or suction velocity leads to rapid blade erosion
  • Not suitable for viscous or abrasive slurry service
  • Efficiency drops sharply away from the design point — poor part-load performance without variable pitch
  • Large physical size at high flow rates — pump intake works and structures are major civil engineering projects

Selection Criteria

  • Confirm the required duty falls within the low-head, high-flow region — if head exceeds 20 m consider mixed-flow or centrifugal
  • Specify variable-pitch blades for installations where flow varies seasonally or with tidal conditions
  • Calculate required submergence to meet NPSHr at the maximum flow rate — account for minimum reservoir or canal water level
  • Check the system curve for stability: the operating point must be on the stable part of the Q-H curve
  • Select blade material for the water quality — cast iron for clean water, bronze for saline or brackish water, stainless steel for corrosive industrial cooling water
  • Coordinate with civil engineers early — pump station dimensions, intake screen arrangement and sump geometry directly affect pump performance and cavitation risk

Top 10 Manufacturers

  • 1. Flowserve Corporation (USA) — ~14% global market share; large axial and mixed-flow for water utilities and power
  • 2. KSB SE (Germany) — ~12%; vertical axial flow pumps for pumping stations and irrigation
  • 3. Sulzer Ltd. (Switzerland) — ~11%; axial and mixed-flow for power generation and desalination
  • 4. Ebara Corporation (Japan) — ~10%; irrigation and drainage pumps for Southeast Asian markets
  • 5. Xylem Inc. (USA) — ~9%; Flygt brand vertical and mixed-flow for water management
  • 6. Andritz Hydro GmbH (Austria) — ~8%; large Kaplan turbine-pumps for hydropower and pumped storage
  • 7. Torishima Pump Mfg. Co. (Japan) — ~7%; large-diameter axial flow for desalination and power generation
  • 8. Grundfos A/S (Denmark) — ~6%; vertical mixed-flow for water treatment intake works
  • 9. Caprari SpA (Italy) — ~5%; mixed-flow and axial for irrigation and drainage in Mediterranean markets
  • 10. Wilo SE (Germany) — ~4%; mixed-flow for stormwater and flood-control pumping stations