Hardness deposits

Acid pickling This can be done under the following operating conditions, either with sulphuric acid (H2SO4), or hydrochloric acid (HCl). H2SO4 releases a lot of fumes and is ineffective under cold conditions. It forms iron sulphate, which forms a hard deposit at the bottom of the tank and is difficult to remove (see table on next page). 

The spiral-plate exchanger usually should not be used when a hard deposit forms during operation, because the spacer studs prevent such deposits from being easily removed by mechanical cleaning. When, as for some pressures, spacer studs can be omitted, this limitation is not present. 

Fig. 1.51 Crevice corrosion resulting from the crevice produced between the gasket and the flange of a titanium pipe used for conveying a hot hypochlorite solution. The attacked areas are coated with a hard deposit of titanium oxides, whilst the unattacked area of metal outside...

Magnesium typically is present as the phosphate [magnesium hydrox-yphosphate, 3Mg3(P04)2Mg(0H)2] or the hydroxide [brucite, Mg(OH)2] in boiler section soft sludges, and in hard deposits as the silicate (serpentine, 2Mg0-2Si022H20). 

If the water then evaporates, the dry calcium bicarbonate decomposes, recreating calcium carbonate, which precipitates and forms hard deposits and incrustations while carbon dioxide and water are released into the atmosphere ... 

Many release agents build up on the metal mould surface and can form hard deposits combined with compound degradation products. This leads to the need for frequent mould cleaning. 

Colin G. Fink and P. Deren of Columbia University in 1934 perfected a process for electroplating rhenium as a bright, hard deposit which is surprisingly resistant to hydrochloric acid (47). Dr. William F. Meggers of the United States Bureau of Standards has made a thorough study of the arc spectrum of rhenium (41). 

Fig. 10.2.2 Schematic drawing of a carbon arc apparatus. Cylindrical hard deposit (A) grown on the end of the cathode, chamber soot (B) deposited on the ceiling of the evaporator. and cathode soot (C) around the cathode surfaces are shown.

A cylindrical hard deposit, which is grown on the tip of a cathode (A in Fig. 10.2.2), consists of two regions an inner fibrous black core and an outer gray hard shell (15). The inner core had a columnar structure that was made up of bundles of nanotubes and flocks of polyhedral graphitic particles (32). On the other hand, the hard shell was made of stacked graphitic flakes. 

Fig. 10.2.3 TEM image of MWNTs grown in the cylindrical hard deposit. A carbon nanoparticle (also called nanopolyhedron) is indicated by an arrow. A high-magnification image in the inset shows another nanoparticle on a MWNT.

The dissolved air left in boiler feedwater (BFW) is stripped out with steam, in the deaerator shown in Fig. 8.9. The cold BFW has been taken first from the Mississippi River and then filtered to remove sand and sediment. Removal of the bulk of the calcium salts that would cause hardness deposits in the boilers is often accomplished by hot-lime softening. If excess C02 gas appears in downstream units consuming the steam, it is the fault of the lime softening, not the deaerator. 

Let s say that the cooling-water outlet temperature from the condenser was 140°F. This is bad. The calcium carbonates in the cooling water will begin to deposit, as water-hardness deposits, inside the tubes. It is best to keep the cooling-water outlet temperature below 125°F, to retard such deposits. Increasing the pumparound heat removal will lower the cooling-water outlet temperature. 

The bigger the radiator, the more heat is provided to a room. The bigger the radiator, the faster the steam condenses to water inside the radiator. A larger radiator has more heat-transfer surface area exposed to the condensing steam. Unfortunately, the radiator shown in Fig. 13.1 is suffering from a common malfunction. Water-hardness deposits have partly plugged the condensate drain line. Calcium Carbonate is a typical water-hardness deposit. 

Incidentally, the correct way to remove hardness deposit is by chemical cleaning. Violent banging on radiators is considered bad form in South Brooklyn. 

Also, the hot-lime-softened water has variable amounts of carbonate contamination. When boiler feedwater is converted to steam, the carbonate deposits will break down into carbon dioxide and hardness deposits. 

The hardness deposits coat the inside of the boiler s tubes, interfere with heat transfer, and overheat the tubes. The carbon dioxide, which is also generated from the dissolved solids, creates more serious corrosion problems in downstream heat exchangers. When the steam condenses, the carbon dioxide may remain trapped in the reboiler or preheater as a noncondensable gas. Actually, there is no such thing as a noncondensable gas. Even C02 is somewhat soluble in water. As the C02 dissolves in the condensed steam, it forms carbonic acid, a relatively weak acid (pH typically between 5 and 6). Strong acids will have pH values of 1 to 2. Pure water has a pH of seven. Carbonic acid is particularly corrosive to carbon steel heat-exchanger tubes. 

The amount of hardness deposits in steam is a function of entrainment of water, as well as the TDS of the boiler feedwater. A well-designed boiler, then, often is equipped with a mesh-type demister pad to remove entrainment from the produced steam. 

Finally, the steam turbine s buckets can foul with hardness deposits from the steam. This reduces the turbine efficiency, and may prevent a pump from running at its rated speed. Injecting steam condensate into the steam supply can remove such deposits. 

Some tubes were split open and subsequent examination showed the presence of very hard deposits on the surface (Figure 7.32). Isolated, but deep pits were present under the hard deposits (Figure 7.33). The measured thickness of the tube, mechanical tests, chemical analysis and etching showed the tubes to conform to the properties specified for SA 179 tubes. 

Figure 7.30 Hard deposits inside GE3B tubes1...

Figure 7.34 Hard deposit inside GEI6 tube1...

The inspection revealed significant reduction of the nozzle area produced by hard deposits. The deposits were much thicker on the concave portions of first stage blades than on the second stage blades. The intervening first stage rotor... 

And they line the arteries causing hard deposits on the arterial walls—ha ha. 

Most modern marine lubricants are prepared from good-quality paraffinic base oils although, traditionally, naphthenic basestocks were preferred. Paraffinic base oils have better oxidation resistance, a higher viscosity index and lower volatility but give harder carbon deposits. However, modern additive technology can modify the hard deposits allowing paraffinic base oils to be used and thereby making use of their other superior properties. 

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