penthouse spring 1998

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1998

DAVID No FRENCH! INCot METALLURGISTS

ONE LANCASTER ROAD

NORTHBOROUGH MASSACHUSETTS 01532

TEL: (508) 393-3635 VOL. mt O.

FAX: (508) 393-3914

A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOILERS

TALLURGICAL PROBLEMS WITH

For the past many years theation of choice for new

been

. On the back end is a heat-(HRSG) to

e the sensible heat in theimprove overall

l efficiency. These HRSG unitst exchangers that

designed to heat water in theter heater or economizer, boiler in the evaporator, and super-

tion. There is usually more than

evaporators as well.increase the output by

than 35%. The principalf this combination is the

ble heat rate, perhaps as lowbtulmegawatt of electricity

A second important feature is thef natural gas as the primary. Oil is the back-up fuel.

gas is inherently lessng than either coal or oil-boilers. Since methane (orl-_gas) s one atom of _carbon to --_

atoms of hydrogen, a significant(more than 50%)n of hydrogen to

which limits the amounte gas (carbon dioxide)

d per megawatt generated. For

f the thermodynamic advantages,r, these HRSG devices come with

problems.Natural gas is colorless andless. For safety reasons, aquantity of an odorant, acompound, is added. Gas leaks

d by the smell.ugh odorant is added that some

sulfur dioxide and sulfur trioxideare part of the combustion turbineexhaust. To prevent dew-pointcorrosion by sulfurous and sulfuricacids, the feedwater heaters havebeen fabricated of 304 or 304Laustenitic stainless steel. Thesealloys are also chosen to minimizethe oxygen-pitting corrosion probleon the water side of the tubes. Thwater inlet temperature to thefeedwater heater is between 70° and100°F, well blow the dew-point of

sulfuric acid. The exact dew-pointdepends on the concentration ofsulfur trioxide in the exhaust but around 280°F. The dew-point ofsulfurous acid (sulfur dioxide andwater) is lower, around 230°F.

Unexpected, however, were thefailures of the feedwater heaters bchloride-induced, stress-corrosioncracking that initiated on theoutside or gas side. A combustionturbine is, in effect, a very largevacuum cleaner that ingests all of

the atmospheric contaminants alongwith the combustion air. Coolingtowers use chlorine or chlorinecompounds as a biocide and contribuchlorine and chlorides to theenvironment. When the wind is in tI8wrongl8 irection, cooling-towerspray becomes a part of the turbineinlet air. Other sources ofchlorides are seawater and perhapsroad deicing salt, as well asindustrial smog. Whatever thesource, chlorine compounds then fin

their way into the combustionexhaust. The feedwater-heater inletemperature is around 70-100°F, welbelow the hydrochloric-acid dew poiof around 130°F. Inevitably thecoldest portion of the inlet to thefeedwater heaters is below thehydrochloric-acid dew-point. When

the conditions are just right (orwrong, depending on your

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hydrochloric acid (andps other chloride species)

e feedwater heater.

have occurred in the

s steel by

ing. The cracks initiated onas side of the tubes.

In order to conserve space and

e manufacturing costs, someheaters or economizers have

pattern through a split inletwith up-flow and down-flow in

same module. Water enters the

er header from one end, rises to

pper header, across the header,d flows back down the other half of

and down-legof the flow path. The result

is higher in halfheat exchanger. A baffle at the

oint of the lower header assuresproper flow pattern. Thus there

ure differenceween the two halves of the heat

hanger. This temperature

ed strain. The hotter tubes arer than the cooler tubes, and thebetween the two creates

lly at thed at the

of the headers. The cracks

develop in any under-cut leftcket or attachment weld.

r a fairly short time, leaks willy the temperature

and

A third unexpected problem hasn the oxygen pitting noted in someal locations of the feedwaterr or economizer. The design ofheat exchangers may contain a

s with a slight bend justow or above the header to permit a

radial entry of the tube. While thbends may only be 30°, or less, the

are made to a very tight radius. Textrados or outside of the bend is

left with a residual stress equal tthe yield stress. The cold workassociated with these tight-bendradii lead to preferential locationfor corrosion, both oxygen pitting

and general attack.Oxygen pitting may be viewed as

localized solution of the steel by

the oxygen-contaminated water. Themore highly stressed or the more

severely cold worked the steel is,

the easier it is to dissolve. Thuspreferential attack will occur inthose regions of most severedeformation. This form of attack isometimes called stress-enhanced orstress-assisted corrosion. In

cycling units, it has been referred

to as corrosion fatigue. Any highlstrained site is an open invitationto corrosion. It does not matterwhether the strain is from an appliload (at a weld attachment forexample) or residual strain from co

bending.During start-up, the pH may not

always be stabilized within thecontrol range. The pH is adjusted

minimize the corrosion rate; and andeviation, either up toward more

basic, or down toward more acidic,will temporarily increase corrosionAs with oxygen attack, generalcorrosion is more rapid at regions

higher localized stress.These close-radius bends,

especially at the inlet, are afavored spot. This problem may beprevented by a stress-relief annealafter cold bending. For ferriticsteels, a temperature in the range 1100°-1350°F, depending on the allois satisfactory.

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