Corrosion inhibition in a warm water heating system

Corrosion inhibition in a warm water heating system

With any filling and refilling of a warm water heating system, oxygen enters the system, too. Even if the system is vented, air bubbles and air pockets remain in a complex pipe system. Corrosion may be the result, which leads to leakage, reduction of the system efficiency, and finally to a complete failure of the system. With the introduction of the regulation VDI 2035 - Sheet 2, hot water heating systems have to be protected from corrosion. Lutz-Jesco dosing stations have been installed in the warm water return, where good mixing is guaranteed. A special injection nozzle has to withstand temperatures up to 110 °C (230 °F).

When filling or refilling the heating system (larger than 100 kW), the dosing stations feed trisodium phosphate and sodium sulphide into the circuit. The addition of trisodium phosphate leads to an increase of the pH value and a reduction of the water hardness. A pH value of 8.2 to 9.5 is required. If a heating system is made from aluminium or aluminium parts, pH values must be in the range of 7.5 to 8.2.

Sodium sulphide, on the other hand, is fed as an oxygen scavenger. A sulphide residue of 5 to 20 mg/l must be maintained after first filling. The generated buffer is sufficient to bind the oxygen released by the ageing water. The lower the temperature of the heating water, the larger the amount of sodium sulphide that must be added (at a water temperature of 100 °C (212 °F), no oxygen is dissolved in water). It is recommended to check the pH value, the concentration of phosphate and of sodium sulphide as well as the total water hardness and the conductivity of the water regularly. The recommended monitoring schedule is immediately after the filling process, after one week and after six weeks. At a minimum, testing should be done once a year.

If the heating water doesn’t meet the standards, the feeding of the chemicals can be increased manually at the Lutz-Jesco dosing stations. At a high water hardness or if frequent make-up of the heating water is expected, softened or completely desalinated water must be used. For warranty reasons furnace manufacturers require the compliance with the regulations in order to avoid damage from corrosion and lime scale. With Lutz-Jesco dosing stations, pH value adjustments and oxygen binding can be handled without problems.

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