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2001

DAVID N. FRENC H, INC., METALLURG ISTSONE LANCASTER ROADNORTHBOROUGH MASSACHUSETTS 01532TEL. NO. 508-393-3635FAX NO. 508-393-3914 VOL. XVIII, NO

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

The use of water lances seems to be increasing as theed method of removing tenacious coal-ash

Depending on the application, good successuble-free operation is reported. On the other

e also frequentlyOne of two explanations is possible for thefirst, of course, is that they trouble

The second is that the time of operation or theshort for problems to develop.

line between successtheir use. As is always the case, there ishas been submitted for analysis.

What seems clear, however, is that cold water on al tube or at a higher temperature of a

e, will lead to thermal-ks. Cracks form when the surface of therapidly cooled. When it cools it contracts

material is still atThe contraction is "resisted"

y the h o w , and hence longer steel, and the surface ist into tension. Over several to many cycles,

'The general appearance is one of a crazed pattern, see1. Here there are the remnants of the ash. In

ion: circumferential dagger-elop. All of the cracks are oxide-filled,leak

Figures 2 and 3 show damage on a waterwall tube.

Figure 1

Figure 2

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. .- . : *.I. ., ' . .. - -; :- . . . 2 .+# . -. . - .- ..%< < - I..

. .. . - -. . . -.. . .. . . - . .. -,: . ,. . . , . . . , , . , . ' 3 ,, . - --. . . * . .-... ..., : i- . --- . . :. .,. . - ,. ,< ', ,, .

Figure 5Figure 3

Apparently the secret to success with water lancesSimilar damage develops on steam-cooled circuits as not to remove all of the ash, but leave a thin insulatiFigure 4, fiom a reheater shows the similar layer in place to prevent the thermal shock on the st

All of the preceding examples are fiom water lancbut similar damage may develop fiom steam-actuatsoot blowers early in the blowing cycle. Steam hascondensed at the end of the prior cycle. The first fluon the subsequent steam-blowing cycle is water, andthe first tubes or pendants in the path are quenched the water. Thermal-fatigue cracks form, but odj7 othose elements hit by water. Once steam reaches thnozzle, the rest of the tubes do not develop cracks adamage. To prevent cracking with steam lances,condensate should be drained after each operational

Figure 4 cycle.Depending on the severity of the quench by the

lls, steam-side cracks may also develop, albeitFigure 5.

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