Cooling Water Hardness and Conductivity: What Refinery Operators Need to Know

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Hardness and conductivity are two of the most monitored parameters in a refinery cooling water system, and they are frequently confused with each other. They are not the same thing, and there is no direct formula to convert one to the other.

Hardness

Water hardness measures the amount of dissolved calcium and magnesium, expressed in milligrams per liter of calcium carbonate equivalent. Hard water causes scale, primarily calcium carbonate, to deposit on heat exchanger surfaces. Scale is an insulator, and it will form in your most critical exchanger at the worst possible time. High hardness is bad.

Conductivity

Conductivity measures the ability of water to pass an electrical current, expressed in microsiemens per centimeter (μS/cm). Dissolved salts, organic compounds, and minerals all increase conductivity. Importantly, hardness is not the only contributor. Sugar water, for example, has high conductivity but zero hardness because sugar contains no calcium or magnesium.

High conductivity in a cooling water system is not automatically a problem. It typically means the system is operating at higher cycles of concentration.

Cycles of Concentration

Cooling water systems recirculate water, which concentrates dissolved solids over time through evaporation. Conductivity is used to track this. The formula is simple:

Cycles = Circulating Water Conductivity ÷ Makeup Water Conductivity

For example, if makeup water is 1,000 μS/cm and circulating water is 3,000 μS/cm, the system is running at 3 cycles. Whether that is acceptable depends on your system design and chemical treatment program.

The Bottom Line

• High hardness = scale risk. Always bad.
• High conductivity = higher cycles. Not necessarily bad. Depends on design.
• Hardness ≠ conductivity. You cannot calculate one from the other.