how steam sys ocr

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HOW STE M SYSTEM CHEMISTRY ND CONTROL FFECT STE M SIDE COIL CORRO SION

Michael A. BrennerTechn ical Specialist

GE Water TechnologiesPortland, O regon

The p roper operation of the steam system is critical to maintaining the integrity of thesteam coils for the kiln operator. This paper serves to review the fundamentalmec hanisms of the steam system w ith regard to steam coil corrosion protection.

Problems

Let's first look at the corrosion of iron in water-specifically in the con densa te system.

Fe + 2H20 = Fe(OH)2 H2iron water ferrous hydroxide hydrogenThe above reaction tends to be self-limiting as the ferrous hydroxide increases theconde nsate pH thus retarding the reaction rate.Let's add a b it of oxygen:

4Fe OH) 2 02

2Fe2O3H2O

2H20

ferrous hydroxide

oxygen

hydrated ferric

wateroxide hematiteFerric oxide, hematite, red rust , it has a lot of nam es but there is no argument that it isundesirable. It is a loosely adhering oxide layer which has poor heat transfer capabilities.It also tends to deposit elsewhere in the system . It can deposit in the coil, in steam trapsand piping d ownstream, and can even tually reach the boiler. In the boiler it can formfurther deposits, which in turn retards hea t transfer, which can lead to tube failures.

All is not lost. We ha ve a little help here. At tempera tures above 12 0F :

3Fe OH) 2 Fe3 4 2H20 + H2ferrous ferrous oxide water hydrogenhydroxide magnetiteThis reaction is desirable. M agnetite is a tightly adhe ring oxide layer that protects thebase metal as well as offering excellent heat transfer qualities. However, the abovereaction is slow and is temperature depen dent as we ll as time depen dent.

At temperatures above 300F a slightly faster reaction occurs - no intermediate step here.

3 Fe 4H20 Fe304 4H 2iron water ferrous oxide hydrogenA lot here, but the one thing to rememb er is: the red ox ide (hematite) is formed und eroxidizing con ditions that exist, for example, in the con densate system or in a boiler thatWe stern Dry Kiln Association

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is out of service. The black oxide (magnetite) is formed under reducing conditions thattypically exist in an operating boiler. Hem atite has been called bad rust and magnetitehas been called good rust.

Let's switch gears a bit. We noted that oxygen p lays a large part in the corrosion process.So, how did it get into the system? Ox ygen, as well as carbon dioxide, are both presentas dissolved gasses from boiler makeup supply. Howe ver, the prima ry source of carbon

dioxide is from specific minerals in the b oiler water. Carbonates (C O 3 - 2) and b icarbonates(HCO3 ) are commonly present in the makeup water combined we call these anions alkalinity ) and in the boiler undergo the following reactions:

2NaHCO 3 heat = Na22CO3 CO2 H20sodium sodium carbon waterbicarbonate carbonate dioxideNa2CO 3 H2O = 2NaOH

CO2

sodium

water

sodium

carbon

carbonate

hydroxide

dioxide

Both the oxygen and carbon dioxide are considered non-condensable gasses. Due totheir poor solubility in hot water, they leave the boiler and travel with the steam and as thesteam cond enses they un dergo the following reaction:

H 2O CO2 H2CO3water carbon dioxide carbonic acidIn simple terms, carbon dioxide is formed wh ich then forms carbonic acid. Carbonic acid

depresses the cond ensate pH which increases the general corrosion of ferrous m etals.We aren't done yet. As steam is condensed in the condensate system, the followingreactions occur.

H 2CO 3 11 HCO3 -carbonic acid hydrogen ion bicarbonate ionHCO 3 H CO3 -2bicarbonate ion

hydrogen ion

carbonate ion

2H HC O3 Fe

=

Fe(HCO3)2

H2

hydrogen ion

bicarbonate

iron

ferrous

hydrogen

ion bicarbonateThis reaction is also self-limiting as the ferrous bicarbonate increase s the conde nsate pHthus retarding the reaction rate.Please note that for those with hard boiler makeup water (water high in calcium andmagnesium), the above m echanisms stand out even m ore as you most likely have higherconcentrations of carbon ates and bicarbonates.

Finally, oxygen and carb on dioxide can be d rawn into the coils through leaks. The mostcomm on occurrence is when the steam supply valve is shut to the coil and the residualsteam cond enses. This will cause a slight vacuum in the system d rawn in outside air andorganics (typically acidic in nature).

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a protective film (black magnetite) on system metal surfaces. The reaction mechanismis as follows:

2(C 2H 5)NOH

6Fe2O3 = 4Fe3O4+ 4CH3COOH + 3H20DEHA

ferric oxide

ferrous oxide

acetic acid

waterhematite

(magnetite)

DEHA renders metal surfaces less active, reducing overall corrosion rats. Therefore,corrosion product loading to boiler feedwater is reduced. DEHA is usually employed inconjunction with a neu tralizing am ine program, as it is most effective above a pH of 8.0DEHA also has a very high distribution ratio of and therefore will distribute m ore effectivelythroughout a complex steam and conden sate system. Due to DEH As high distributionratio, effective conden sate protection can often be ach ieved from its application to theboiler feedw ater versus the more traditional steam heade r.

Back to O perations. How the kiln operator cycles the boiler is also important. For themost pa rt, the kiln ope rator controls boiler cycles of concentration (also know n b y itsabb reviation cycles ) by boiler water condu ctivity. Cycles too high, another topic, as wecan be causing carryover as well as causing retrograde soluble salts to come out ofsolution (also known as scaling ). Cycles too low: for a given steam rate that means yourmakeup rate is higher than normal, which in turn introduces additional oxygen andcarbonates and bicarbonates. More issues downstream in the steam/condensate system.Also, your water b ill is higher and fuel b ill is higher as you are sending pe rfectly good(heated/treated) water d own the d rain.

If the kiln operator has a choice pick a boiler water makeup source that is low inalkalinity. The lower the alkalinity, the less chem ical addition may be n eed ed to lessenthe corrosion rate in the condensate system. Please note that water softener willexchange calcium and magnesium ions with sodium but will not affect boiler alkalinity.Dealkalizers and to a d egree Reverse O smosis units can reduce the alkalinity of the waterwhich in turn can reduce the carbonic acid formation in the conden sate system.

Proper operation of steam traps can prevent condensate from backing up in the coil. Ifthe steam trap is not operating properly, condensate ca n collect in the coil and b egin tocool. The condensate can actually drop below the tem perature at which it condensed .

This is known a s subcooling. As the temperature drops, the condensate can more readilyabsorb oxygen and carbon d ioxide which w ill result in a localized corrosive environme nt.The kiln operator can actually be doomed with a system in which the condensatecollection tank is above ground. At low steam flows, condensate will back up into the coildue to the lack of pressure. S ubcooling occurs, localized corrosion can occur.

Next, it goes without saying, fix your leaks. First, repairing leaks helps reducesteam/condensate losses which reduces the amount of makeup water needed. Also,repairs will help minimize the intrusion of oxygen and carbon dioxide as well as organicswh eneve r a vacuu m is formed . Looking at the bigger picture, fixing leaks w ill allow thekiln operator to control the drying process more accurately.

In summary, the corrosion of steam side coils can be prevented via mechanical,operational, and chemical means. Knowledge is important. Please confer with yourcounterparts or ask for assistance from experienced consultants if you have anyquestions.

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how steam sys ocr

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