Water is the most pumped liquid on earth, representing approximately 40% of all industrial pump applications. It is the reference medium against which all other liquids are benchmarked in pump engineering. Its behaviour changes significantly with temperature, dissolved solids content and the presence of gases — factors that profoundly affect pump selection, material choice and long-term reliability.
Key Physical Properties
- Density: 999 kg/m³ at 15 °C; decreases to 958 kg/m³ at 100 °C
- Kinematic viscosity: 1.14 cSt at 15 °C; falls to 0.29 cSt at 100 °C — important for pump efficiency at elevated temperature
- Vapour pressure: 1.7 kPa at 15 °C; rises to 101.3 kPa at 100 °C — critical for NPSH calculations in hot-water systems
- Specific heat: 4.18 kJ/kg·K — among the highest of common liquids, making water an excellent cooling medium
- Thermal expansion: Significant above 60 °C — closed systems require expansion vessels to prevent pressure build-up
- Electrical conductivity: Near-zero for demineralised water; up to 50,000 µS/cm for concentrated brine — drives electrolytic corrosion risk
Corrosion Mechanisms
- Dissolved oxygen: Primary corrosive agent in water systems — accelerated by elevated temperature; controlled by chemical oxygen scavengers or deaeration
- Chloride pitting: Cl⁻ ions break down the passive film on standard stainless steel (304, 316) — particularly above 40 °C and above 200 mg/L Cl⁻
- Carbonic acid: Dissolved CO₂ lowers pH to 5–6 — attacks cast iron and carbon steel; controlled by pH adjustment
- Microbiologically influenced corrosion: Biofilm-forming bacteria accelerate metal attack in cooling towers, stagnant lines and warm distribution systems
- Erosion-corrosion: High-velocity water strips protective oxide films at pump inlets, elbows and throttling points
Scale and Fouling
- Calcium carbonate (limescale): Deposits when water is heated above 60 °C or loses dissolved CO₂ — reduces heat-exchanger efficiency and narrows pump hydraulic passages
- Calcium sulfate: Deposits in evaporators and boilers; difficult to remove by acid cleaning
- Silica: Hard, glassy deposits — resistant to most cleaning methods, require specialist treatments
- Biological fouling: Algae, Legionella and other organisms colonise warm water circuits and cooling towers
Pump Type Selection
- Cold clean water: Standard centrifugal pump — the universal solution for municipal, HVAC and agricultural duty
- Hot water (60–180 °C): Centrifugal with high-temperature mechanical seals and controlled-clearance impellers; verify NPSH margin increases with rising vapour pressure
- Boiler feed water: Multistage centrifugal at high pressure; deaeration essential to remove dissolved oxygen before the pump
- Water with suspended solids: Semi-open or vortex impeller centrifugal; submersible sewage type for sludge
- Demineralised or ultra-pure water: All-stainless or all-plastic lined pump to avoid metallic contamination
Materials of Construction
- Cast iron: Standard for raw water, cooling water and non-potable service where pH and oxygen are controlled
- Bronze: Good for potable water and mildly brackish service — resists de-zincification; food-safe
- Stainless steel 316L: For potable water, process water and hot-water service up to 200 mg/L Cl⁻
- Duplex or super-duplex stainless: Where Cl⁻ exceeds 200 mg/L or temperature is elevated
- PVDF / polypropylene: For demineralised or ultra-pure water where metallic contamination must be zero
- FBE-lined cast iron: Fusion-bonded epoxy lining for aggressive raw water and elevated pH service
Applications
- Municipal water abstraction, treatment and distribution networks
- HVAC chilled water, hot water and condenser water circuits
- Boiler feed water in power generation, process plants and district heating
- Agricultural and horticultural irrigation and drainage
- Industrial process water — rinse water, cooling water, washdown
- Fire suppression and sprinkler systems
- Reverse osmosis pre-treatment and permeate transfer