Pin Me

Water Quality in High Pressure Boilers

written by: Dr V T Sathyanathan • edited by: Lamar Stonecypher • updated: 9/24/2010

Process for water treatment is decided dependent on the source of water of the power plant and its quality. The levels of water quality to be maintained depend upon the drum operating pressure. Boilers operating near and above critical pressure require ultra pure water with oxygenated treatment.

  • slide 1 of 1

    The main objectives of chemical level control of a water/steam circuit are (a) to minimize corrosion damage and deposition problems on the internal surfaces of the boiler tubes and feed water systems, (b) to reduce or avoid the harmful effects in the superheater, reheater and turbine due to impurities than can be present in steam, and (c) to reduce boiler tube failures due to tube overheating and tube inside corrosion.

    The guidelines for water quality control of water/steam in boiler plants of 60 kg/cm2 and above are furnished as recommended by many boiler designers.

    The feed water limits

    For drum operating pressure kg/cm2 (g) 61-100, 101-165, and 166-205 with PO4 treatment type of boilers:

    1. Hardness, in ppm, has to be nil for all pressures.
    2. pH at 25oC for Cu alloy pre boiler system plants has to be between 8.8-9.2 for all pressure ranges.
    3. pH at 25oC for Non-Cu alloy pre boiler system plants has to be between 9.0-9.4 for all pressure ranges.
    4. Dissolved Oxygen has to be maintained below 0.007 ppm for pressure range 61-165 kg/cm2 and 0.005 ppm for a pressure range 166-205 kg/cm2.
    5. Iron is to be maintained below 0.010 ppm for pressure range 61-165kg/cm2 and 0.005 ppm for pressure range 166-205 kg/cm2.
    6. Copper needs to be below 0.010 ppm for pressure 61-100 kg/cm2, 0.005 ppm for pressure 101-165 kg/cm2 and 0.005/0.003 for pressure 166-205 kg/cm2. The copper level should match with the corresponding values has to be maintained in superheated steam.
    7. Total silica should be maintained below 0.020 ppm for pressure range 61-165 kg/cm2 and below 0.010 ppm for 166-205 kg/cm2.
    8. Specific electrical conductivity at 25oC measured after Cation exchanger in the H+ form after CO2 removal in Micromhos/cm to be less than 0.50 for pressure 61-100 kg/cm2 and below 0.30 for 101-205 kg/cm2. However, the specific electrical conductivity at 250 C in the special case of AVT, can be 0.2 (Max) and sodium 0.003 ppm (Max.).
    9. Hydrazine residual to be maintained below 0.01-0.02 ppm for all pressure range and the total organic carbon in ppm to be below 0.20.

    All the measurements are to be made at the high-pressure heater outlet or economizer inlet. The pH levels required in feed water can be obtained by dosing ammonia or any proven method. It should be noted that the level of ammonia in the condensate and feed water should not exceed 0.5 ppm (as NH3) for a copper alloy pre-boiler system and 1.0 ppm (as NH3) for a Cu free pre-boiler system. Oxygen is generally measured before de-aerator and maintained below 20 ppb to minimize pre-boiler corrosion.

    Boiler water parameters

    For drum operating pressure kg/cm2 (g) 61-100, 101-165, and 166-205 with PO4 treatment type of boilers:

    1. Total dissolved solids in ppm to be below 100 for pressure 61-100 kg/cm2, below 50 for 101-165 kg/cm2 and below 15 for 166-205 kg/cm2.
    2. Specific electrical conductivity at 25oC to be maintained below 200 Micromhos/cm for pressure 61-100 kg/cm2, below 100 for 101-160 kg/cm2 and below 30 for 166-205 kg/cm2.
    3. The residual Phosphate has to be kept below 5-20 ppm for pressure 61-100 kg/cm2, below 5-10 for 101-165 kg/cm2 and below 2-6 for 166-205 kg/cm2.
    4. The pH level at 250 C has to be 9.1-10 for pressure 61-100 kg/cm2, below 9.1–9.8 for 101-165 kg/cm2 and below 9.1-9.6 for 166-205 kg/cm2.
    5. Silica, in ppm to be controlled in such a way in boiler water so that it will be less than 0.02 ppm in steam for pressure range 61-165 kg/cm2. For pressure range 166-205 silica in boiler water to be kept below 0.10 ppm.

    The phosphate and pH are given based on Co-ordinate phosphate treatment so as to prevent the presence of free hydroxide in the boiler water. Normally each power station has their own practice; the values given are only a guideline as understood from various practices. In the case of all volatile treatment for pressure range 166-205 all the values are to be more stringent.

    Not maintaining these chemical regimes of feed water and boiler water can lead to carry over of salts to superheaters and even to turbine blades causing plant availability loss.

Popular Pages

More Info