How Limescale Forms and Why It's Dangerous for Autoclaves

Posted by Admin | 06 Jul

Content

1 How Limescale Forms
2 The Dangers of Limescale to Sterilizers
3 How to Choose Water for High-Pressure Sterilizers
4 Water Quality Requirements for Sterilizers

How Limescale Forms

Under high-temperature conditions, calcium and magnesium ions dissolved in water form poorly soluble calcium carbonate (CaCO₃) and magnesium hydroxide (Mg(OH)₂) or other precipitates, which build up as limescale.

The main factor behind limescale formation is water hardness. Water containing mineral salts such as calcium (Ca) and magnesium (Mg) is called "hard water" — for example, river water, lake water, well water, spring water, and tap water.

Water that contains no minerals is called "soft water."

The Dangers of Limescale to Sterilizers

If improper use of a high-pressure steam sterilizer leads to scale buildup, it will not only shorten the service life of the sterilizer and affect sterilization performance, but also create safety hazards.

Limescale coating the chamber and heating tubes has a very low thermal conductivity — about 2–5% that of ordinary steel — which reduces heat transfer efficiency and increases energy consumption.
Limescale particles dispersing during heating can scratch the chamber.
Limescale particles entering the piping can cause pipeline blockages.
Limescale deposits on the water level electrode and temperature sensor surfaces can easily cause poor sensing response; they can also cause metal components to burn out from localized overheating, deform the inner wall, and even lead to safety accidents.

How to Choose Water for High-Pressure Sterilizers

According to GB8599-2008 Technical Requirements for Large Steam Sterilizers for automatically controlled models, the water source used for steam sterilization should be drinking water that has undergone further softening, demineralization, or deionization treatment.

Beyond hardness requirements, sterilization water must also meet other criteria — such as limits on heavy metal impurities like iron, cadmium, and lead, as well as pH value and electrical conductivity. See the attached table for details.

Water Quality Requirements for Sterilizers

Note: Consistency check results shall comply with established analytical method requirements.
Item	Standard
Evaporation residue	≤10 mg/L
Silicon dioxide (SiO₂)	≤1 mg/L
Iron	≤0.2 mg/L
Cadmium	≤0.005 mg/L
Lead	≤0.05 mg/L
Heavy metals other than iron, cadmium, and lead	≤0.1 mg/L
Chloride ion (Cl⁻)	≤2 mg/L
Phosphate (P₂O₅⁻⁵)	≤0.5 mg/L
Conductivity (at 25℃)	≤5 µS/cm
pH value	5–7.5
Appearance	Colorless, clean, no sediment
Hardness (total alkaline metal ion content)	≤0.02 mmol/L

Limescale inside a sterilizer may seem like a minor issue, but it can cause equipment blockage and damage, and even pose safety hazards. It is therefore essential to use sterilization water that meets the required standards, and to keep up with routine maintenance and cleaning of the sterilizer. It is recommended to change the water in the sterilization chamber once a week; if the items being sterilized carry mild corrosive residue, the water should be changed daily.

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