Hazards from Air Cooled Condenser
Power Plants with Air Cooled Condenser Condenser can suffer from f rom corrosion product transport issues which can be detrimental det rimental to plant performance and reliability. Due to very large surface area of an Air Cooled Condenser even small corrosion rates can result in significant levels of iron oxides being transported into the condensate system. If these corrosion products are not removed removed from f rom the condensate, they can be transported into the feed water which may result in deposition within the boiler water walls and super heaters. This can lead to boiler tube failures due to over heating of the tubes and an increased requirement to chemically clean the boiler if not proactively addressed.
The following are the two major ill effects of ACC 01. minimal particulate transport ( iron oxide ) 02. minimal through wall leakage
Consequences of Particulate Transport
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Steam generating tube require chemical cleaning Steam generating tube failure Frequent filter element replacement replacement in case condensate filter is installed Resin contamination contamination or difficulty difficulty in in regeneration regeneration if condensate polisher is installed in the system
Consequences of through wall leaks
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Air ingress : Potential vacuum deterioration : increased steam cycle contamination with oxygen and carbon dioxide. : Rapid loading of Anion resins in case of condensate condensate polisher
Contributing issues for Iron corrosion -
Early condensate environment environment with pH of <8 Vacuum conditions
Solutions
Several strategies were developed to both reduce corrosion within the ACC and also to minimize the transport of corrosion products from the condensate into the feed f eed water and boiler. The power plant can install 2 x 50% duty condensate filters (5um nominal), followed by 2 x 50% duty mixed bed condensate polishers to remove particulate and ionic impurities from the condensate. This system will provide p rovide good condensate quality with respect to ionic contaminants. Cycle chemistry changes were implemented at the plant to reduce corrosion within the ACC; however it was decided that corrosion product removal from the condensate also needed to be improved improved to meet plant requirements for feed water quality, minimizing minimizing the risk of deposits forming in the boiler. Testing at the power plant indicated that the condensate condensate filter system was not performing as desired and that replacement of the existing filter elements with a high h igh quality back-washable back-washable filter element was likely to provide p rovide the increase infiltration performance performance required by the power plant. The method is preventing significant significant quantities of corrosion products from being transported into the boiler, while also reducing the amount of iron fouling on the polisher resins. The reduction in resin fouling has resulted in time and water savings while cleaning the resins. To avoid boiler problems, water must be periodically discharged dischar ged or blown down from the boiler to t o control the concentrations of suspended and total dissolved solids in the boiler. Surface water blow down is often done d one continuously to reduce the level of dissolved solids, and bottom blow b low down is performed periodically to remove sludge from the bottom of the boiler. The importance importance of boiler blow down is often overlooked. Improper blow down can cause increased increased fuel consumption, consumption, additional chemical treatment requirements, requirements, and heat loss. In addition, the blow down d own water has the same temperature and pressure as the boiler water. This blow down d own heat can be recovered and reused in the boiler operations. Boiler water blow down at regular intervals will limit the corrosion particulate to transport to the system. It is also best practise to follow low pressure blow down through headers when ever opportunity arises.
Condensate drain from the condensate condensate reserve tank for the initial hours of start up will prevent the entry of iron oxide into the system. And it is recommended practise to drain the boiler water completely and fill up with fresh DM water prior to light up of boiler which will avoid short term over heating of the system. The predominant term for the use of pH adjusting treatments is "Condensate "Condensate Treatment" which refers to the addition of chemicals to the steam (through either the boiler water or direct d irect injection to the Steam Header) to prevent steam/condensate steam/condensate line corrosion due to tthe he acidic nature of Carbon Dioxide contaminated steam. The Carbon Dioxide is a normal by-product of the breakdown of Boiler Water Alkalinity, and occurs in most boilers to varying degrees. The Carbon Dioxide, upon dissolution in condensate forms Carbonic Acid, which can dissolve metal rapidly. rapidly. In systems where oxygen in leakage also occurs, metal dissolution can occur at a rate ten times that of a system with only one or the other contaminant. The most commonly used material for Condensate pH conditioning is the family of volatile amines known as "neutralizing amines". amines". These materials are volatilized into the steam and by their alkaline nature re-dissolve in the condensate to effectively neutralize the effects of carbonic acid. Volatile Corrosion Inhibitors (VCI), also called Vapour Phase Inhibitors (VPI), is compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, bo ilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH towards less acidic and corrosive values. In closed vapour spaces, volatile solids such as salts of dicyclohexylamine, dicyclohexylamine, cyclohexylamine and hexamethylene-amine hexamethylene-amine are used. On contact with the metal surface, the vapour of these salts condenses and is hydrolyzed by any moisture to liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly while lasting for long periods. Both qualities depend on the volatility of these compounds, compounds, fast action act ion wanting high volatility while enduring protection requires low volatility.
