Acid cleaning

Back-flushing is much more effective when it is done once a week, not as required, ft prevents individual tubes from overheating due to low water flow and plugging off with hardness deposits. Once tubes are plugged with carbonate hardness deposits, the next step is acid cleaning, as such tubes cannot be cleared by back-flushing. 

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The most widely used cleansing agent is the chromic acid cleaning mixture. It is essentially a mixture of chromic acid (CrOj) and concentrated sulphuric acid, and possesses powerful oxidising and solvent properties. Two methods of preparation are available —... 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Another approach is the use of polyphosphate-based blends including proprietary chemicals. The exact mechanism of the observed effect is not completely understood. In the polyphosphate mode of operation, the polyphosphate is dosed ia quantities of 2—5 ppm periodically, sludge resulting from phosphate treatments is removed by acid cleaning (see Dispersants). 

Some of the observed wastage was caused by past acid cleanings. However, much of the attack was caused by long-term underdeposit corrosion. 

Reexamination of the surface after a brief acid cleaning showed that about 90% was still covered by resin, although the resin had been blistered and ruptured at small tubercular sites (Fig. 5.17). 

During acid cleaning or severe acid upsets, ferric-ion concentration may increase, albeit much more slowly than ferrous-ion concentration, to high levels. Resulting corrosion can be severe. Iron is oxidized, and... 

Hence, copper heat exchanger tubes handling acetic acid can he more seriously corroded at low temperatures than at high temperatures. Sulfuric acid at room temperature is handled routinely in carbon steel drums and tanks when water concentration is low, but it becomes extremely corrosive as water concentration increases. As ferric-ion concentration increases during acid cleaning of industrial systems, the corrosion rate of steel increases rapidly. 

Irregular grooving can occur, especially on copper alloys after acid cleaning. Tubes can be only partially filled with cleaning solution. Condensation and running of the fluid down the tube interior cuts tortuous channels (Fig. 7.9). 

Figure 7.8 Striations on the internal surface of admiralty brass condenser tube after acid cleaning. (Magnification 15x.)...

Figure 7.9 Network of shallow ditches caused by condensation of acidic fluids during acid cleaning of a copper condenser tube.

Figure 13.12 Close-up of a 90 10 cupronickel condenser s internal surface after acid cleaning. Elemental copper stains mark regions of shallow denickelification.

Figure 14.1 illustrates one of many similar pinhole perforations found in this and other tubes within the exchanger. Figure 14.2 shows the typical appearance of the internal surface immediately after removing the tube from the exchanger. Laboratory acid cleaning of the internal surface revealed a defective weld seam (Fig. 14.3). 

Figure 14.3 Close-up of the internal surface following acid cleaning. Note the perforation in the tube wall along the weld seam.

Figure 14.5 Severe seam defect apparent after acid cleaning of the internal surface.

Acid cleaning The use of acids to clean scale or other deposits from pipes or tanks. 

Ultrasonic cleaning (all grades). An ultrasonic cleaner suitable for cleaning wire rope is permitted in lieu of the acid cleaning methods described previously. 

A method which is frequently used consists in filling the apparatus with chromic acid cleaning mixture (CARE), a nearly saturated solution of powdered sodium dichromate or potassium dichromate in concentrated sulphuric acid, and allowing it to stand for several hours, preferably overnight the acid is then poured off, the apparatus thoroughly rinsed with distilled water, and allowed to drain until dry. [It may be mentioned that potassium dichromate is not very soluble in concentrated sulphuric acid (about 5 g per litre), whereas... 

Some manufacturers of this type of boiler claim that water treatment of any kind is not necessary and that no chemicals are required, especially with smaller units below, say, 240 kWh (800 lb/hr). Instead they rely on the on-off expansion-contraction cycles within the vessel to drop scale from the heating elements, or on a daily manual flush of the boiler vessel together with frequent replacement of elements and periodic acid cleaning. (Some improved design models thankfully have an automatic flush system that is actuated upon boiler shutdown.)... 

Most of these claims are spurious. In the presence of hard water and the absence of adequate condensate return, no matter how small the boiler, the elements quickly scale up, reducing efficiency and life-span. These boilers may fail in service well before a scheduled acid clean or element replacement service occurs. In addition, regular acid cleaning on small boilers can be risky and is not recommended. 

Arrangements for chemical feed and control also were limited. In these boilers, 1/2 inch to perhaps 1 inch (12.5-25 mm) of multilayered scale could easily deposit during the period between waterside inspections. It was often assumed that these operating conditions were the norm, that regular acid cleaning was a way of life, and that nothing better could be expected. 

Although relatively uncommon, low BW pH may occur for a variety of reasons under online operating conditions, resulting in either general or localized low pH corrosion. Under offline conditions, low pH corrosion usually is simply a result of improper acid cleaning procedures. 

Acid cleaning is not a precise process and many variable factors are involved, which, if not fully controlled, may result in acid corrosion of boiler steel. Under some circumstances, even with the use of acid cleaning inhibitors, acid corrosion (i.e., oxidation via hydrogen ions originating from the acid) may take place ... 

The typical results of poor control over an acid cleaning process are irregular pitting and a very rough and jagged appearance on the various boiler surfaces that may resemble Swiss cheese. ... 

Most acid cleaning processes do not totally dissolve the available scales, oxides, and other debris, and where localized crevice shielding and redeposition takes place, it may be difficult to subsequently effect the complete neutralization of pockets of acid. These areas are then subject to vigorous attack once the boiler is bought back online. 

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