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Introduction Of Condensate Corrosion And Protection

Introduction Of Condensate Corrosion And Protection
Condensate recovery is one of the areas of energy conservation so widely practiced in industries today. Condensate still contains about twenty percent of saturated steam, and it is distilled water, almost free of dissolved solids. Therefore, its recovery offers advantages to boilers such as reducing fuel costs, reducing chemical treatment, conserving water, and increasing boiler efficiency. Non-contaminated condensate, which is the product of non-contact steam application, returns to steam boiler's system as feed water. There are two types of condensate receiving system in boilers - the pressurized condensate system and the gravity-vented condensate system. The flash steam from high pressure condensate is recovered by returning this condensate to the deaerator while the gravity-vented condensate returns to the boiler feed tank.
Oxygen attack
Often people said due to its pureness in nature; steam condensate requires no chemical treatment, but is that so? Condensate is corrosive and can be contaminated by rusts, something that not many people are aware. Sometimes, condensate can be slightly acidic. This is why condensate piping deteriorates faster than steam piping. Oxygen corrosion occurs in condensate piping as pitting, which is the most destructive form of corrosion.
The reaction of oxygen attack is: 4Fe + 6H2O + 3O2 ? 4Fe(OH)3
Carbonic acid thins condensate pipes and causes grooving at the bottom section of the pipes. The outcome is, if a condensate pipe has predominantly pitting, the condensate is high in oxygen, but if the pipe wall is thinning, the condensate pipe suffers from carbonic acid attack. However, where do oxygen and carbon dioxide in condensate piping originate from? Are not all dissolved oxygen and carbon dioxide in boiler water arrested mechanically by the deaerator and chemically by sodium sulphite or DEHA in the boiler? Actually, there are two main reasons for this phenomenon. First, when steam condenses, the flow volume shrinks thereby allowing oxygen to infiltrate and dissolve. Secondly, oxygen can also be drawn into condensate system through condensate pumps, leaking heat exchangers, and vents of condensate receiving tanks.
Acid corrosion
Carbon dioxide can infiltrate in the same way as oxygen, nevertheless, the main source of carbon dioxide in steam is the breakdown of bicarbonate and carbonate alkalinity in boiler water to form carbon dioxide. Here are the reactions that take place:
Carbonate reaction: CO3 + H2O ? CO2 + 2OH
Bicarbonate reaction: 2HCO3 + heat ? CO2 + 2H2O
Carbon dioxide mixes with steam, condenses in condensate to form weak carbonic acid, and lowers the pH. Condensate is so pure that even small amount of weak carbonic acid can drop the pH to 4.5. The reaction is:
CO2 + H2O ? H2CO3
In Malaysia, there was a case in a petrochemical industry where less than twenty percent of the condensate was returned into the boiler because of carbonic acid infiltration. In boiler, acidic condensate lowers pH, erodes metal, increases dissolve iron, and increases the total dissolved solids (TDS) as iron bicarbonate. Iron bicarbonate mixes with boiler water to form iron oxide and carbon dioxide, and consequently the carbon dioxide mixes with steam again. That is why some people cannot maintain a high pH in their boilers despite doubling the dosing of caustic soda. This problem in condensate recovery is disadvantageous to steam boilers and must be prevented.
Protection of after-boiler condensate lines is required. Corrosion in condensate line can be minimized by volatile amines as corrosion inhibitors. Volatile amines commonly used for condensate line protection today are neutralizing amines and filming amines. Neutralizing amines react with carbonic acid and carbon dioxide to increase condensate pH to 9. One of the commonly used neutralizing amines is cyclohexylamine. Filming amines, on the other hand, form protective films on metal surfaces and protect from oxygen attack, the same way magnetite film does. An example of filming amines is octyldecylamine. Having oil-like property, filming amines coat the metal surface with one-molecule thick. Filming amines have no effect on condensate pH. Carbon dioxide and oxygen are more corrosive in combination. Accordingly, if a system faces both oxygen and acid attacks, a blend of both amines is always recommended.
Besides chemical treatment, condensate quality monitoring should be incorporated on the daily or weekly basis. Take a test of condensate to monitor iron content, hardness, and TDS. Check for colour and order. Any change indicates improper treatment or contamination. Condensate is not treated to remove contamination, therefore, to prevent iron hydroxide infiltration; magnetic filter can be applied to remove it from condensate. Total iron in boiler feed water for medium pressure boiler should be lesser than 0.05 ppm, and any increase could mean iron hydroxide infiltration from the condensate line. For boiler operating below 20 bars, a recommended alkalinity rating is 140 to 700 ppm. If the alkalinity level exceeds 700 ppm, bicarbonate and carbonate will break down producing carbon dioxide and entrain in steam, as stated before. However, the best method is to remove the alkaline hardness (carbonate and bicarbonate) conclusively by using the reverse-osmosis system or demineralization.
Finally, condensate is corrosive and damaging to condensate piping. The danger happens when many steam engineers, based on good boiler water field-test results, feel comfortable with their boiler water treatments. Some engineers never seriously consider or even realize the potential damage that could happen to their condensate lines. This is a serious negligence. When leaks after leaks are detected, it is too late and costly repairs would be inevitable. It is the treatment of condensate return that makes it unnecessary to replace condensate piping for many years.

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