EXPERIENCE WITH ACID–ALKALINE CLEANING OF WATER ECONOMIZERS

I. K. LEBEDEV

Mechanical scaling, driven by an electric motor, a hydraulic, pneumatic, or steam turbine, does not always ensure reliable removal of scale from the main heating surfaces of modern water-tube steam boilers, and is entirely unsuitable for cleaning the coil heating surfaces of once-through boilers and boilers with forced circulation.

Chemical cleaning of heating surfaces by washing them with a solution of chromic anhydride or hydrochloric acid has still not found wide application, because these reagents often only soften or destroy the scale without removing it from the boiler; this either necessitates subsequent mechanical cleaning or causes serious complications in operation due to the products of scale destruction.

One effective and, as practice has shown, reliable method of chemical cleaning is the acid–alkaline washing method proposed by us; the results of its application in cleaning the coil-type water economizers of modern boilers are presented below.

At one of the Ural thermal power stations, because of a breakdown in the water-treatment regime, boilers of the KO-III-200 and TP-9 types, which had boiling-type coil water economizers, became fouled with scale deposits. This led to a number of serious accidents involving rupture of screen and evaporating tubes. The scale was removed from the evaporating and screen tubes by scaling, but on the internal surfaces of the water economizers, consisting of coils with an internal tube diameter of 31 mm, it proved impossible to remove the scale by mechanical means. For a long time the scale remained in the tubes without causing complications in the operation of the units, although its thickness in the outlet parts of the coils reached 1.5–2 mm, since it was hard and adhered firmly to the tube surface.

After 2–2.5 years the scale began to soften on the side of the internal surfaces of the tubes and to flake off. The products of scale destruction, in the form of flakes, accumulated in individual parts of the coils, caused clogging of the tubes with subsequent cessation of water circulation in them, which caused warping and burnout of the coils. Initially this phenomenon was detected on a TP-9 boiler \((D = 160/200\ \text{t/hour},\ p = 34\ \text{atm})\) with a total heating surface of the water economizer \(H_{\text{ec.}} = 1480\ \text{m}^2\). Periodic individual washing of the water-economizer coils with a jet of water during boiler shutdown did not eliminate the formation of blockages in the same coils during subsequent operation, and within 4–5 months the coils of the upper tier of the water economizer (the boiling section) became completely unfit for further operation; during this period the boiler was shut down in an emergency 3–4 times per month because of ruptures of water-economizer tubes.

It was soon found that the process of formation of blockages and corrosion of the coils had also begun on the KO-III-200 boilers \((D = 160/200\ \text{t/h},\ p = 34\ \text{atm})\). Since preventing the failure of the water economizers of all boilers consisted in removing scale, and the operating staff had no methods available for cleaning the coils mechanically, it was decided to flush the water economizers with a hydrochloric-acid solution.

Installation layout

For carrying out the acid flushing, the author proposed and implemented a layout (Fig. 1) with an intermediate tank, which was intended to serve as an intermediate vessel, a sludge settler, a heater, and a vessel for preparing the solution. To heat the solution, a coil heated by steam from the steam-atomizing line was introduced into the tank, and a perforated—

Fig. 1. Layout for acid-alkaline flushing of water economizers.

1—upper headers of the water economizers of boiler KO-II-200;
2—the same, for boiler TP-9;
3—lower headers of the water economizer of boiler TP-9;
4—the same, for boiler KO-III-200;
5—acid storage tanks;
6—intermediate tank;
7—solution feed pump \(Q = 150\ \text{m}^3/\text{h}\);
8—acid feed pump;
9—flanged connections.

—pipe for additional heating. Circulation of the solution was carried out by means of a pump with a capacity of \(Q = 150\ \text{m}^3/\text{h}\).

The entire installation, with all pipelines, was assembled in advance, and only the isolation of the boiler and its connection to the layout, which was called “assembling the layout,” was carried out after it had been shut down for flushing of the economizer. The diameters of the pipelines are shown in the layout (Fig. 1); the same layout also shows some elevation marks characterizing the arrangement of the installation elements.

The connection of the solution inlet and outlet lines, as well as the overflow lines, to the headers was carried out through special manhole fittings proposed by the author (Fig. 2), which were installed during assembly of the flushing layout in place of the ordinary manholes, in the quantity required for connecting the indicated pipelines. Isolation of any boiler for flushing was accomplished by installing blank flanges, for which the layout provided—

the necessary number of flanged connections. The water economizer was disconnected from the boiler on the feed line by means of shut-off gate valves, and from the boiler drum by wooden turned plugs driven from the headers into the feed pipes connecting the economizer with the boiler drum.

The solution was initially supplied to the upper headers and discharged from the lower headers into an intermediate tank by gravity. This arrangement was chosen on the basis of the following considerations:

1) removal of scale flakes that may separate from the tubes during washing will proceed more easily when the solution circulates from top to bottom;

Fig. 2

Manhole fitting: 1—manhole; 2—locknut; 3—bracket; 4—nut; 5—pipe

2) when circulating from bottom to top, the possibility is not excluded that solution may enter the boiler drum through the wooden plugs, since their tightness is doubtful. Subsequently it was established that the above-mentioned concerns were unfounded. Supplying the solution from bottom to top, however, eliminates the possibility of the formation of air pockets in individual coils and makes it possible to use the pump capacity most fully. Therefore, jumpers were made that allowed the direction of circulation of the solution to be changed.

Preparation of the solution and washing of the economizer

After the acid-washing arrangement had been prepared, raw water from the water supply was fed into the tank, and, as the tank was filled, the pump was started in order to fill the system (economizer, tank, pipelines) with water. When the system had been filled, the supply of raw water to the tank was stopped, the pump was adjusted to full capacity, and normal circulation of water in the system was established. Then a reagent known as “Unicol”¹, used in acid washing, was poured into the tank, and

steam for heating and the pump supplying hydrochloric acid into the system; the supply of acid was stopped when the required concentration of the solution was reached. For preparing the solution, technical hydrochloric acid was used

Fig. 3. Operating schedule for acid-alkaline cleaning of water economizers.

with a strength of 12 to 25%. The strength of the solution was maintained within the range of 4–4.5%, and the temperature at 65–75°C. “Unikol” was used in an amount of 1 kg per 1 m³ of solution. The circulation time of the solution is shown in the schedule (Fig. 3).

After completion of the acid circulation, the solution was clarified. For this purpose, raw water was fed into the tank, and a corresponding amount of solution from the tank was discharged to the drain. Clarification of the solution was continued until ordinary raw water began to circulate in the system. After clarification of the solution, a caustic-soda solution with a strength of up to 1° N was prepared, for which the appropriate amount of crystalline caustic was loaded into the intermediate tank. The temperature of the solution during alkali circulation was maintained at the maximum and reached 95–98°C. Then the solution was again clarified and the system was washed with raw water. This, in fact, completed the washing process.

By means of the alkali it was intended to neutralize the acid and prevent corrosion processes on the scale-free inner surfaces of the water economizer. However, practice showed that alkalization is one of the necessary technological elements in washing surfaces free of scale. Hydrochloric acid, reacting with the iron oxides that constitute the greater part of the scale, converts them into ferric chloride. The latter glues together the products of scale destruction, turning them into a sticky mass. An insignificant part of this mass is washed away by the hydrochloric-acid solution and passes into it in the form of turbidity, but the greater part of this mass remains on the heating surface, retaining the form of scale. It is known that in a solution with an excess of alkali, ferric chloride turns into a flocculent precipitate; therefore, after the alkali solution is introduced, the solution rapidly darkens, a large quantity of sludge is released in the form of flakes, and it is removed from the heating surfaces. When a sample of the solution is taken during washing with alkali, the sludge settles rapidly, and the solution becomes completely transparent. The amount of sludge in such a sample is up to 10–12% by weight of the solution. Thus it is clear that the principal washing and removal of scale from the heating surfaces takes place during circulation of the alkali, while during circulation of the acid only the destruction of scale and formation of ferric chloride occur.

Results of the Washing and Conclusions

1. Acid-alkaline washing made it possible to remove scale completely from the water economizers. After washing, the inner surfaces of the tubes had a metallic luster and showed a complete absence of traces of scale or any sludge. A check by individual washing of the coils with a jet of water also revealed no signs of scale or products of its destruction.

2. Accidents involving the water economizers ceased completely, and the boilers subsequently operated for about 3–4 years without any complications involving the water economizers.

3. No sharply aggressive action of the acid on the metal of the coils, headers, or fittings was detected.

4. Leaks at the rolled joints, flange connections, etc., were not found either during the washing or afterward, during subsequent operation of the boiler units.

5. All operations of the acid-alkaline washing of the economizer are shown in the graph (Fig. 3), from which it is evident that the entire washing process can be carried out during a 2–3-day boiler shutdown.

1. Acid cleaning must be combined with alkaline cleaning; the latter is an essential technological element of the cleaning process, in which mechanical cleaning of the surfaces after washing is eliminated.

2. By means of acid-alkaline cleaning, it is possible to free from scale both individual elements and entire units of any design, including once-through boilers of Prof. Ramzin’s system.

1. “Unicol” is a chemical reagent produced by our industry. It is used in acid washing as a passivator that prevents the aggressive action of acid on metal. Joiner’s glue or casein glue is also used as such a passivator. ↩