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Wet lay up can cause pH to change in the water that is off line

Wet lay up can cause pH to change in the water that is off line.

Case 1 – pH Increases and acid use increases as a laid up line returns to duty. Most likely to happen in spring conditions and disappear in summer

Causes of pH Increase in an Open Loop During 1  – 4 day Layup

Cause

Mechanism

Notes

Loss of CO₂ via aeration

If CO₂ degasses from solution (e.g., warming), carbonic acid drops, and pH rises

Requires agitation, heating, or open exposure in alkaline systems

Overdosing of inhibitors

Excess sodium nitrite, borate, or phosphate can increase pH if system was mis-dosed

Check for operator error or failed dosing pump

  1. Step-by-Step: How CO₂ Degasses and Raises pH
    1. Initial Condition (Cold Water)
      1. Cold water (e.g., 5–15°C) can hold more dissolved CO₂ due to higher gas solubility.
      2. Dissolved CO₂ forms carbonic acid:
        1. CO2+H2O↔H2CO3↔H++HCO3
  • This lowers pH, often into the 7.0–7.5 range in pure water or soft waters.
  1. Temperature Increase
    1. As water heats up (e.g., to 25–35°C), CO₂ solubility drops sharply (Henry’s Law).
  1. Excess CO₂ degasses into the atmosphere, reversing the equation above.
    1. H++HCO3−→CO2(gas)+H2O
  1. This removes protons (H⁺) from solution, causing pH to increase.

Typical pH Shift Magnitude

Temp Shift

 

pH Increase (Typical)

50°F  → 77°F

0 – 10      ppm M

+0.4 to +0.8 pH units

50°F  → 95°F

80 – 200 ppm M

+0.2 to +0.6 pH units – Your Tower Water M

41 °F → 86°F

20 – 60   ppm M

Up to +1.0 (if well-aerated)

 

Estimated pH Increase with Warming (Low Alkalinity Water)

These two graphs illustrate the relationship between temperature and CO₂ behavior in water:

  • Left chart: As water temperature increases, CO₂ solubility drops significantly — from ~1.45 g/L at 0°C to below 0.6 g/L by 40°C.
  • Right chart: The resulting loss of carbonic acid raises the pH, particularly in low-alkalinity water, where buffering is weak. A shift from 0°C to 40°C can cause a pH increase of ~0.6–0.8 units.

It is anticipated this will disappear as we get into summer when tower waters will be warmer vs entering the building cold and warming as lay up progresses.

 

  1. Case 2 – pH drops – no acid is needed as a result

Why pH Drops in an Open Loop During 2–4 Day Layup

Cause

Mechanism

Severity

Notes

Atmospheric CO₂ absorption

CO₂ dissolves into exposed water forming carbonic acid (H₂CO₃), reducing pH

High

Especially rapid if circulation stops and system is vented to air

Biological activity

Bacteria/fungi grow in stagnant water, generating organic acids (e.g., acetic, lactic)

High

Common in towers, sumps, or basins without biocide residual or where biofilm exists

Oxidation of nitrite

NaNO₂ oxidizes to nitrate or N₂ gas, releasing protons and lowering pH

Moderate

Accelerated in presence of dissolved oxygen, iron, and bacteria

Dilution or evaporation effects

Water loss via evaporation concentrates some ions, potentially destabilizing buffering system

Moderate

May not drop pH directly but can shift equilibrium, especially in soft water systems

Loss of buffering agents

Buffers (e.g., borates, phosphates) may degrade, precipitate, or be consumed

Moderate

Precipitation more likely if temperature or concentration fluctuates

  1. Key Contributing Factors
    1. Stagnation = oxygen stays in water, allowing corrosion and bacteria to thrive.
    2. Open exposure = CO₂ and airborne bacteria can freely enter.
    3. Biofilm = even in “laid-up” mode, pre-existing colonies go active quickly without flow or residual biocide.
    4. Lack of residual inhibitor = nitrite depletes; azoles degrade; pH falls.

 

  1. Preventive Actions

Strategy

Action

Apply biocide before shutdown

Maintain a slug of oxidizing (e.g., NaOCl) or non-oxidizing biocide before and during layup