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The service life of pipes, boiler systems, pumps and fittings in thermal hot-water and steam-boiler systems is determined to a great extent by the quality of the boiler water.
Water contains different amounts of oxygen (O2) and carbon dioxide (CO2), depending on the pressure and temperature. Both gases cause damage to metallic boiler materials in a specific way: oxygen is oxidative and carbon dioxide (carbonic acid) is aggressive.
The degassing solution
Specific degassing methods are therefore used to remove the two gases, oxygen and carbon dioxide, from the water or to reduce them to below the respective required residual concentrations (guaranteed value to be complied with). The guaranteed value is defined as the remaining quantity of oxygen or carbon dioxide that continually reaches or undershoots degassing in the middle of a longer operational stage.
Process for degassing
A differentiation is made between the following processes for the degassing of gaseous contents in the field of water treatment:
Thermal degassing
Thermal degassing of feedwater to separate out the dissolved and corrosive gases contained within requires that the water be boiling. In this state, the solubility of gases in liquids lies near zero. The mixture of condensate and fresh water is fed into the degassing dome at the top. Heating up to the boiling temperature occurs with heating steam, which flows into the degassing unit at the bottom and flows upward against the current. The gasses driven out as a result are lead away with the exhaust vapours via the exhaust vapour outlet in the degassing dome. The degassed water is collected in the feedwater reservoir.
Thermal degassing unit for heating stations, power stations and other steam-boiler systems
Thermal degassing often occurs at temperatures minor above 100°C and operating pressures around 0.2 to 0.3 bar (g). Thus an oxygen content under 0.02 mg O2/L and a carbon dioxide content of less than 1 mg CO2/L are possible in the feedwater.
Vacuum Degassing
Water can be degassed at a temperature lower than 100 °C when it boils in a partial vacuum. Degassing occurs through a degasifier under partial vacuum. The gases will be discharged with the generated water vapour. Vacuum degassing is emplyoed in the treatment of feed water in heating systems.
Chemical Degassing
Chemical degassing or residual degassing occurs through the addition of chemicals. These chemicals are dissolved in dilution water (minimum soft water quality, depending on water treatment) and are suitable for removing or converting the oxygen contained in the water. The reaction time differs when different chemicals are used. In general, the greater the water temperature, the shorter the reaction time.
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Overview
