Secondary side chemical cleaning of steam generators (SGs) removes sludge piles from the tube sheet and scales from the outside of the heat exchanging tubes. Thus the heat transfer capability of the SG can be improved and the risk of future corrosion – caused by accumulated salts in the sludge pile – will be reduced. The cause of existing corrosion damage to the tubes, for example denting, is eliminated by the cleaning process.
The technique of chemical cleaning consists of two steps, one for the removal of magnetite and the other for the removal of copper. Magnetite and copper are the main components of sludge and scales in the SGs, but the process has been adapted for the removal of other impurities such as lead. The magnetite and copper removal steps are independent of each other and can be applied in combination or separately. The most suitable application procedure has to be defined according to the sludge composition.
The main advantages of the Framatome high temperature chemical cleaning process (HTCCP) include:
•The magnetite dissolution process is very fast due to the high application temperature.
•The impact of the magnetite removal on the time schedule of the outage (critical path) is minimal. For example, performing a magnetite removal of all four SGs of a Siemens-built 1300MWe plant prolongs the shut-down procedure by about 15 hours.
•The amount of equipment to be installed for the performance of the cleaning is very limited, because no external heat source nor recirculation is required.
•No excess of magnetite solvent is necessary: the amount of chemicals to be injected can be determined during the cleaning, and solvent is injected stepwise according to the progress of the magnetite dissolution. The volume of spent magnetite cleaning solution can be reduced by evaporation within the SGs directly after the cleaning process.
•The application of the magnetite solvent is qualified for a large number of C steels and does not attack high alloyed materials. The extensive experience in material compatiblilty is a result of over 15 years of cleaning nuclear and fossil boilers.
Magnetite removal
The HTCCP for Magnetite removal from the SG(s) takes place under reducing and alkaline conditions at a primary side temperature between 160°C and 200°C. The process is preferably carried out at the beginning of an outage, so an external heat source is therefore not required, facilitating the application considerably. Mixing of the cleaning solution in the SGs is performed by steaming during the injection of the chemicals, and during the dissolution reaction. This saves external recirculation which requires adequate pumps, heat exchangers and tubing.
The SGs are not cleaned all at once but in groups, depending on the total number of SGs at the plant, or the number of SGs to be cleaned. This is because the minimum number of SGs required as a heat sink for the reactor always has to be available. The SGs serving as a possible heat sink may be filled with spent cleaning solution – draining and filling them with water is not necessary – thus the required application time for the magnetite removal is further reduced, compared with other chemical cleaning processes. Up to three SGs can be cleaned simultaneously. The time required for the injection of chemicals and the dissolution reaction can be up to five hours, depending on the amount of magnetite to be removed.
Copper removal
The copper removal process is performed at temperatures between 50°C and 90°C, and at oxidising, alkaline conditions. The procedure uses another complexing agent than for the magnetite removal as well as a catalyst. Metallic copper has to be oxidised prior to complexation, therefore compressed air must be injected into the SGs during the whole copper removal process. The air injection should be sufficient to result in the mixing of the SG contents. The time required for the copper dissolution in one or several SGs in parallel is less than 24 hours.
Material compatibility
The material compatibility of the HTCCP was demonstrated by testing different SG and plant structural materials and material combinations. The compatibility tests covered not only different material types, such as carbon steels, stainless steels, and nickel-based alloys, but also different material conditions, such as forged materials, cast materials, weldments, and cladding.
The results of these numerous material compatibility tests were also verified by using in-situ corrosion coupons in field applications. They can be summarised as follows:
•SG tubing materials (Inconel 600, Inconel 690 TT and Incoloy 800), and stainless steels are completely unaffected by the iron solvent.
•Carbon steels and low-alloy steels show uniform general corrosion, depending on EDTA concentration, pH value and temperature. The figure below shows the correlation between temperature and corrosion for some typical carbon steels by the HTCCP under comparable conditions.
•No unusual galvanic coupling effects of SG tubing material with certain C steels or low alloy steels were experienced.
•At the seams of weldments and at heat-affected zones in carbon steel and low-alloy steel, no preferential corrosion was exhibited.
The standard copper cleaning solution (copper solvent) is benign to all kinds of steel. The composition of the solvent can be adapted for SGs with Monel heating tubes.
Application
Prior to the performance of the cleaning the following tasks have to be performed:
•Estimation of sludge amount to be removed.
•Comparison of materials in contact with cleaning solution to those qualified and, if necessary, qualification of materials not already qualified.
•Design review of the plant to adapt the cleaning procedure to its specific requirements. Typical questions to be answered are: what are the maximum allowed heat up and cool down rates for the SGs; how much time is needed do drain the SGs; and which sampling lines are best suited for the analytical control of the process and the corrosion monitoring.
•Preparation, discussion and adoption of the cleaning procedure including the final time schedule. Preparation of all other papers required to license and perform the SG cleaning.
•Adaptation of the mobile dosing system to the requirements of the site.
•Planning the locations for the various mobile systems (dosing system, monitoring system, sampling, waste water tanks) during the cleaning.
Magnetite Removal
The SGs are usually cleaned in parities according to the following procedure:
As soon as the cleaning temperature, 150°C or 160°C primary side temperature, has been reached, one set of SGs (two if four SGs are to be cleaned, three if six SGs are to be cleaned) is drained to the appropriate water level, usually the lowest water-level allowed. The other set of SGs is available for residual heat removal. The concentrated cleaning solution, an aqueous solution of ammonium-EDTA and hydrazine, is injected in steps into the drained SGs and auxiliary feed water is injected afterwards to dilute the cleaning solution in the SGs to the application concentration. Thus the SGs are filled up until the predetermined level is reached. Following the last chemical injection step, de-ionised water is injected to flush the injection lines.
To ensure good mixing during the cleaning process, slight steaming during the injection of the chemicals and at least one short, thorough steaming after each injection of chemicals is performed. Samples of the SG contents are taken to check the distribution of the complexing agent and to monitor the iron dissolution.
For the final quantity of chemicals required for a complete cleaning, but avoiding the addition of an excess of chemicals, the hydrogen to nitrogen ratio in the gas phase of the SGs is monitored in addition to the liquid sampling. If the analysis shows that the hydrogen to nitrogen ratio is above 1.4 and the iron concentration is near the achievable saturation (sludge dependent, 80% or more of the theoretical saturation), the cleaning of the first set of SGs is finished. In case the saturation is higher and the hydrogen to nitrogen ratio is below 1.4, the amount of sludge in the SGs has probably been underestimated. In order to be sure of removing the whole sludge inventory, fresh concentrated cleaning solution has to be injected and homogeneously distributed by steaming.
After the first set of SGs has been cleaned, these SGs, filled up to the top of the tube bundle, are available as heat sink and the second set of SGs is drained and then cleaned as described above.
When the cleaning is finished, the primary side is cooled down to a temperature between 140°C and 120°C or directly to below 100°C. Between 140°C and 120°C the volume of the spent cleaning solution can be reduced by evaporation. The spent cleaning solution can be concentrated up to an EDTA concentration of about 30%. Afterwards, the cleaning solution can be cooled down further to below 100°C and the SGs drained.
Copper Removal
To start the copper cleaning, the SGs to be cleaned have to be drained and dried. The concentrated cleaning solution, an aqueous solution of EDTA and a catalyst, is injected and the SGs are then filled up with demineralised, hydrazine-free water. During the cleaning a sufficient flow of compressed air has to be injected to oxidise elemental copper and mix the solution. Air overpressurising and depressurising performed in succession enhance mixing and increase the copper dissolution rate considerably. The average application time is in the range of one day.
After the cleaning step, the SGs are flushed with demineralised water to remove the spent cleaning solution completely.
Between 1985 and the end of 1990 a total of 52 SGs were cleaned worldwide using the old KWU high temperature magnetite removal process using NTA instead of EDTA as complexing agent. The copper removal process was applied worldwide for the cleaning of 70 SGs during that time. The more recent applications with EDTA as complexing agent in the magnetite removal step are listed in the Table.
TablesHTCCP applications since 1991
