Corrosion carbonic acid

Steam produced from demineralized water is largely free of carbonates. Steam produced from lime-softened water will be contaminated with carbonates that decompose in the boiler to carbon dioxide. As the steam condenses in a reboiler, the CO accumulates as a noncondensable gas. This gas wiU be trapped mainly below the channel head pass partition baffle, shown in Fig. 12.6. As the concentration of COj increases, the CO will be forced to dissolve in the water  

That is, carbonic acid will be formed. Carbonic acid is quite corrosive to carbon steel. Reboiler tube leaks, associated with steam-side corrosion, are almost certainly due to carbonic acid attack. 

Venting the channel head through the balance line shown in Fig. 12.6 will prevent an excessive accumulation of CO. This is done by continuous venting from the top of the condensate drum. For every 10,000 Ib/h of steam flow, vent off 50 Ib/h of vapor through a restriction orifice placed in the condensate drum vent. This is usually cheaper than controlling reboiler steam-side corrosion with neutralizing chemicals. 

Often the problem with CO accumulation inside the channel head results not so much in corrosion, but in the loss of heat transfer. This is due to the tubes below the bottom pass partition baffle filling with non-condensable CO gas. I have observed this problem even in plants using demineralized boiler feedwater. Venting from beneath the bottom pass partition baffle will restore heat transfer rates. Venting above the pass partition baffle or from the top of the channel head is futile. Unfortunately, most such channel head vents are installed on the top of the channel head, and thus are completely ineffective. 

Varying the steam-to-condensate interface level to control the reboiler duty will promote steam leaks in the channel head-to-shell flanged 

Varying the steam-to-condensate interface level to control the reboiler duty will promote steam leaks in the channel head-to-shell flanged closure. This is caused by the thermal cycling and stresses that result from constantly varying the level of condensate in the channel head. However, when low-pressure steam ( 60 psig) is used, this becomes a minor problem, which may be safely ignored. 

When high-pressure steam ( 100 psig) is used, rather significant leaks of hot condensate and steam can be caused by a variable condensate level in the channel head. For such higher-pressure steam sources, control of steam flow with condensate backup, as shown in Figs. 11.5 and 11.6, is best avoided. 

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Economizers are not usually designed to generate steam, and any deposits found in them therefore are not likely to be a result of carbonic acid corrosion or contamination from steam. Rather, the transport and buildup of corrosion debris within an economizer tends to originate from corrosion processes occurring either in the economizer itself or in some upstream part of the pre-boiler system. Economizer deposits typically develop in the presence of oxygen and possess a high iron content. 

Although the need to properly soften the FW control BW variables such as alkalinity, TDS, and sludge and limit carbonic acid corrosion remains vital, these other problems generally are secondary to effective control over DO. If severe boiler plant damage and system failure are to be avoided, it is necessary to continuously maintain waterside reducing conditions and properly passivated boiler surfaces. 

Corrosive gases including Oxygen Carbon dioxide Results in Oxygen corrosion. Carbonic acid corrosion. Greatly enchanced... 

In lower pressure commercial and industrial boilers, handling and safety are issues of more concern than relative chemical effectiveness, and the fact is that because of such problems, ammonia simply is not used in these smaller plants. Caustic generally provides the necessary FW alkalinity and amines are then employed to overcome the problems of carbonic acid corrosion in the steam-condensate system. 

Condensate lines and vented receivers are vulnerable to oxygen and carbonic acid corrosion. 

Figure 8.6 Venting below the pass partition baffle stops carbonic acid corrosion.

The problem with carbonated steam-gas injection is the carbonic acid corrosion. The surface lines and injection coluinns within the well must be properly protected. 

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