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Condenser tube failures and high temperature corrosion

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Condenser tube failures and high temperature corrosion

Post by ioncube on Tue Mar 20, 2012 7:30 am

The loss of heat transfer can be quite serious and can cost a plant hundreds of thousands of dollars or more if a problem occurs during a peak generating period or becomes a long-term event. Just as serious, however, is the potential effect condenser tube fouling/scaling can have on tube integrity and how tube leaks can cause extreme damage in steam generators.
Tube deposits: A bane to tube integrity
Deposits on condenser tubes can set up a nightmare scenario. In the first place, deposits might initiate corrosion by establishing differential oxygen cells where the area underneath the material becomes anodic to exposed metal.
The formation of small anodes in a large cathodic environment generates one of the most insidious types of corrosion – pitting – in which a small metal loss by weight can result in through-wall penetrations
. When hard scale is the deposition material, oxygen differential scale is possible, but if the deposit is porous in nature, impurities in the water, like chloride (Cl) and sulfate (SO4), can concentrate under the deposit and further exacerbate the corrosion mechanism. This is quite common underneath the tubercules that form when carbon steel corrodes in oxygenated waters.
If deposits are of a microbiological nature, they can induce pitting not only from oxygen differential cell formation, but also from a more direct mechanism. Microbial colonies in condenser tubes typically contain a number of organisms, including anaerobic bacteria that grow and flourish underneath the protective slime layer produced by the colony. Anaerobic organisms utilize natural chemicals in the water, most notably including sulfate and nitrate (NO3), in their metabolic process to thrive. Unfortunately, the by-products of these processes include acids and other deleterious compounds such as hydrogen sulfide (H2S). These compounds directly attack tube metal, with pitting again as the result.
Other mechanisms that might initiate tube damage and failure include fretting at tube sheets induced by tube vibration, steam-side attack of copper alloy tubes, particularly in the condenser air-removal section, improper rolling of tubes into the inlet tube sheet, steam erosion of upper condenser tubes located near the turbine exhaust, and failure of tube plugs.
The upshot of these issues
Cooling water from a lake or river typically contains a few hundred ppm of cations and anions, most notably calcium, sodium, magnesium, potassium, bicarbonate, chloride, silica and sulfate, as well as other materials, including suspended solids. In cooling towers, of course, these impurities are cycle up in concentration. As these contaminants enter the boiler, a number of temperature-induced reactions will occur. Two common reactions are shown in Equations 1 and 2.

Ca+2 + 2HCO3- → CaCO3↓ + CO2↑ + H2O
Eq. 1

Ca+2 (or Mg+2) + SiO3-2 → CaSiO3↓ (or MgSiO3↓)
Eq. 2

Equations 1 and 2 are typical scale-forming mechanisms. Even a relatively thin deposit layer will significantly reduce heat transfer, and a boiler must be fired harder to achieve the same level of steam production. This also can lead to overheating of the boiler tubes, which will shorten tube life.
Much more frightening is the effect that cooling water in-leakage has with regard to rapid and potentially catastrophic corrosion. The reaction shown below is a prime example.

MgCl2 + 2H2O + heat → Mg(OH)2↓ + 2HCl
Eq. 3

Magnesium salts react with water to produce a magnesium hydroxide precipitate, plus hydrochloric acid. While HCl might cause general corrosion in and of itself, the compound will concentrate under deposits, where the reaction of the acid with iron generates hydrogen, which can lead to hydrogen damage of the tubes. In this mechanism, hydrogen gas molecules (H2), which are very small, penetrate into the metal wall and react with carbon atoms in the steel to generate methane (CH4).

2H2 + Fe3C → 3Fe + CH4↑
Eq. 4

Formation of the gaseous methane and hydrogen molecules causes cracking in the steel, greatly weakening its strength. Hydrogen damage is very troublesome because it cannot be easily detected.
After hydrogen damage has occurred, the plant staff might replace tubes only to find that other tubes continue to rupture.
Hydrogen damage failures might occur in a matter of days, without any appreciable metal loss, following a significant condenser tube leak. In addition, the impurities introduced by tube leaks to the condensate and steam generator might carry-over to the turbine and initiate pitting corrosion and stress corrosion cracking (SCC) of turbine blades and other turbine components. Needless to say, these mechanisms are very unwelcome in a finely-designed and balanced piece of machinery that costs millions to repair.

Hydrogen damage failure. Note the thick-lipped fracture with little noticeable metal loss
Minimizing these difficulties
Without going into excessive detail, proper treatment of condenser cooling water is absolutely vital to minimize fouling and scaling of condenser tubes. For once-through cooling systems, scale formation is normally not a major problem, and often scale treatment chemicals are not required. In some cases, calcium carbonate (CaCO3) formation in the condenser might be a concern, especially during warm weather periods, but this potential difficulty can be countered by a feed of simple polymer in small dosages.
Scale formation in open recirculating systems, i.e. cooling tower-based systems, is usually much more complex and requires the selection of a well-designed treatment program. The reputable water treatment vendors now have computer programs that utilize sophisticated algorithms to determine the interaction of all major impurities in water and select suitable programs for scale and corrosion prevention. Makeup water and/or sidestream filtration of cooling tower water is a supplemental alternative that can assist with fouling prevention.
The most problematic issue with virtually all cooling water systems is microbiological fouling. In addition to bacteria fouling of condenser tubes, fungi will attack cooling tower wood in an irreversible manner, and algae will foul cooling tower spray decks, potentially leading to reduced performance and unsafe working locations.
The core of any microbiological treatment program is feed of an oxidizing biocide to kill organisms before they can settle on condenser tube walls, cooling tower fill and other locations. Chlorine was the workhorse for many years – when gaseous chlorine is added to water the following reaction occurs:

Cl2 + H2O↔HOCl + HCl
Eq. 5

HOCl, hypochlorous acid, is the killing agent. The functionality and killing power of this compound are greatly affected by pH due to the equilibrium nature of HOCl in water.

HOCl↔H+ + OCl-
Eq. 6

OCl- is a much weaker biocide than HOCl, probably due to the fact that the charge on the OCl- ion does not allow it to penetrate cell walls. The killing efficiency of chlorine dramatically declines as the pH goes above 7.5. Thus, for the common alkaline scale/corrosion treatment programs, chlorine chemistry might not be efficient. Chlorine demand is further affected by ammonia or amines in the water, which react irreversibly to form the much less potent chloramines.
Due to safety concerns, liquid bleach (NaOCl) feed has replaced gaseous chlorine at many facilities. The major difficulty with bleach is that the product contains small amounts of sodium hydroxide, so when it is injected into the cooling water stream it raises the pH, if only by a small amount.
A popular alternative is bromine chemistry, where a chlorine oxidizer and a bromide salt, typically sodium bromide (NaBr), are blended in a makeup water stream and injected into the cooling water.
The chemistry produces hypobromous acid (HOBr), which has similar killing powers to HOCl, but functions more effectively at alkaline pH.
Another factor in favor of bromine is that ammonia does not have as negative an effect as it does with chlorine. This is due to the facts that bromine does not react irreversibly with ammonia as does chlorine, and that bromamines are more effective than chloramines. The primary disadvantages are that an extra chemical is needed and feed systems are a bit more complex than for bleach alone.
A potential method to further help control microbes is a supplemental feed of a non-oxidizing biocide. Typically, feed is needed on a temporary but regular basis, perhaps once a week.
Any chemical feed must be approved by the environmental authorities who regulate the plant, and, as is critical with all chemical usage, safety is of utmost concern. These chemicals will kill micro-organisms, but they also can harm humans as well.
Brad Buecker is a process specialist with Kiewit Power Engineers in Lenexa, Kan. He has nearly 30 years of experience in, or affiliated with, the power industry, much of it in chemistry, water treatment, air quality control and results engineering positions with City Water, Light & Power, in Springfield, Ill., and Kansas City Power & Light Company’s La Cygne, Kan., station.

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