Steam blanket

Ca.ustic Corrosion. Concentration of caustic (NaOH) can occur either as a result of steam blanketing (which allows salts to concentrate on boiler metal surfaces) or by localized boiling beneath porous deposits on tube surfaces. 

Steam blanketing is a condition that occurs when a steam layer forms between the boiler water and the tube wall. Under this condition, insufficient water reaches the tube surface for efficient heat transfer. The water that does reach the overheated boiler wall is rapidly vaporized, leaving behind a concentrated caustic solution, which is corrosive. 

When steam blanketing is occurring, corrosion can take place even without the presence of caustic, due to the steam—magnetite reaction and the dissolution of magnetite. In such cases, operational changes or design modifications may be necessary to eliminate the cause of the problem. 

When a water-based fluid makes contact with a flame or a hot surface its water component evaporates and forms a steam blanket that displaces oxygen from around the hot area, and this obviates the risk of fire. Water-based products all contain at least 35% water. Because water can be lost by evaporation, they should not be subjected to operating temperatures above about 60°C (140°F). Table 52.8 shows a comparison of oil and FR fluids. 

In the dry-out mode of concentration, the steam blanket causes the tube to become superheated, so that the temperature rises in proportion. In the second mode of concentration, i.e. at crevices, it arises initially because the confined space impedes coolant ingress so that the liquid component is impoverished, and insoluble species are precipitated. The deposit causes a rise in temperature and steam blanketing follows, as described above. 

High heat-transfer rates at boiler surfaces promote rapid nucleate boiling and other forms of convective boiling, which in turn may cause steam blanketing. 

Under these conditions, localized mineral supersaturation will lead to the precipitation of various scales and their ultimate deposition on the heat transfer surface without the possibility of scale resolubliza-tion. This is because surface washing by bulk BW (with or without the presence of water treatment chemicals) cannot take place due to the presence of the insulating steam blanket. 

The mechanics of deposition in a boiler are often a cycle of cause and effect, wherein an initial low level of scale deposited on boiler surfaces causes a rapid localized rise in wall temperature. The temperature increase leads to localized steam blanketing, which in turn prevents the deposit from resolubilizing. Consequentially, conditions then exist for the further buildup of deposit on the heat transfer surface. 

The formation of complex silicate scales takes place at high temperature (usually they are only found in boilers operating at over 300 psig) and high heat-flux density points in the boiler section. The presence of complex silicates such as analcite and acmite may indicate steam blanketing problems. 

NOTE Steam blanketing is the static steam-water condition that occurs when boiler water circulation is impeded byfoulant or when initial deposition prevents adequate heat transfer and the dispersion of lower density steam or water. 

Steam blanketing results in a departure from nucleate boiling (DNB) and typically dryout (localized total evaporation) conditions. It also may result in the formation of secondary (silicate-based) deposits that cannot be rinsed or resolublized, overheating problems, and eventual boiler tube or furnace shell rupture. 

Stratification and steam blanketing may add further to the insulating effects and also promote both thermal stress in excess of design limits and waterside corrosion. Over the longer term, considerable metal oxide is formed and cracking and creep develop. 

Where chelant corrosion metal failure occurs, an investigation may determine that contributing factors also include localized overheating, steam blanketing, or DNB. 

Analcite NajOAljOj SiO HjO Similar to acmite. Presence can indicate steam blanketing problems. 

Steam atmosphere pyrolysis, 27 466 Steam balances, 70 146, 147, 148t Steam blanketing, 26 130 Steam bleeds, 70 160 Steam-chest expanded food packaging, 73 48... 

An important consideration in the initial water addition is the use of water at the boiling point. This is a safety consideration, since the chemical reaction heat of any unremoved sodium will locally boil the water, thus quickly steam blanketing the region and probably minimizing the local sodium-water reaction vigor. 

CORROSION, CONCENTRATION ATTACK - A form of corrosion caused by the concentration of caustic or phosphate salts under porous deposits, generally iron oxide. Sometimes found at welded tubes and due to steam blanketing. 

Tanks and pipes should be electrically earthed. It is necessary to equip storage tanks with steam blanketing to be used in case of fire and preferable to incorporate a high temperature alarm in the upper section of the tank to warn of fires. 

Primary coolant pressure is maintained sufficiently to prevent hoiling at any point In the system hy means of a pressurlzer vessel. This vessel Is maintained about 6o par cent full of water with a steam blanket. Pressure Is regulated by a combination of electric heaters in the water fill l and spray cooling In the steam filled portions of the vessel. Ihe pressurlzer also serves as a surge tank to accommodate part of the water volume fluctuations resulting from density variatior due to temperature transients This is augmented by a wstar injection system and a spill cooler for the more extreme tran sients. 

For AT> 30 K, we get film boiling. The bubbles become so numerous that the surface is partially blanketed with them. If the surface temperature continues to increase, the heat transfer decreases due to the insulating effect of the steam blanket. Thus, this region is characterized by an increase in AT but nevertheless a decrease in heat flux. 

Self-restriction of pressure change rate in the primary circuit due to damping features of the steam blanket in the reactor vessel, supported by properties of a boiling coolant. 

Free convective heat transfer often involves boiling. When this is the case, heat transfer increases when bubbles of vapor form and are free to rise to the surface. This is called nucleate boiling. When bubbles become very numerous, they coalesce to form an insulating steam blanket, and heat transfer decreases as the temperature of the surface increases leading to surface damage. This leads to what is called burnout. 

Steam blanketing Gouging and tubercles Deposit amount Deposit constituents... 

Traces of caustic can become concentrated in boiler feedwater and cause SCC (caustic embrittlement). This occurs in boiler tubes that alternate between wet and dry conditions (steam blanketing) because of overfiring. Locations such as cracked welds or leaky tube rolls can form steam pockets with cyclic overheating and quenching conditions. These freqnently lead to caustic embrittlement. 

The most likely place for cracking to occur is in a stainless steel tubed steam generator, where high chloride concentrations and steam-blanketed areas develop. In addition, considerable free oxygen is likely to be present. Oxygen has an adverse effect on chloride stress corrosion, and both oxygen and chloride must be present for stress corrosion to occur. 

Fuel leakage Tank capacity free from a fuel liquid. It is sometimes called a steam blanket. 

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