Note: Descriptions are shown in the official language in which they were submitted.
CASE 4007
~037~
BACKGROUND OF THE INVENTION
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Since the development of high-pressure boilers,
operating at both sub~critical and super-critical pres-
sures, the origin, formation and effects of internal
corrosion product deposits have been a vital concern
to the designer. Although, for sustained operation,
an extremely limited degree of internal scale forma-
tion is desired to protect the base metal from ~xida-
tion, the deposition of excess corrosion products in
porous form and in a unique wave-like pattern can re-
sult in increased frictional and heat transfer resis-
tances and the requirement for frequent chemical
cleaning. This requirement of chemical cleaning re-
sults in lower availability and costly outage time
but is necessary if tube overheating, particularly in `
the high heat absorbing zones of the furnace, and
forced outages are to be prevented. Efforts have been
concentrated in the areas of cycle design and water
treatment in an attempt to minimize the deposition of
water contaminants. Presently5 condensate polishing -
demineralizers are used for total solids control, and
dissolved oxygen and carbon dioxide are controlled by
deaeration and hydrazine treatment, but the gradual
build-up of internal deposits persists. It has been
found that even with optimum boiler water conditions,
the basic iron and water reaction takes place continu-
ously within a closed cycle. This reaction is a con-
tinuous process of deposition, release, transport and
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CASE 4007
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; redeposition of corrosion products generated within
the boiler, in addition to the corrosion produc-ts and
water contaminants carried into the boiler from ex--
ternal sources.
SHMMARY OF THE INVENTION
As a r~sult o~ extensi~,- field and la~oratory
testing on operating units it has been found that, in
accordance with the invention, chrome plating the in-
ternal surface of the tubes, particularly in the burner
or high heat absorbing zones of the furnace, markedly
reduces the deposition of internal corrosion products.
In this regard it is important to differentiate between
chrome plating to provide corrosion resistance to a
base metal and chrome plating to inhibit the deposition
of corrosion products from the fluid flowing within the
tubes. Chrome plating has been used to provide corro-
sion resistance to such items as autGmobile bumpers
and trim and noble metals such as gold and silver have
been plated on base metals for oxidation resistance and
decorative effects. However, in accordance with the
present invention, a novel and unexpected effect has
been determined when chrome plating is applied to the
internal surface of tubes conducting a fluid in a high
heat absorption zone of a furnace which is manifested
by a great reduction in deposition of corrosion products
from the fluid flowing within the tube as compared to an
adjacent unplated portion of the same tube.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a sectional side elevation of a forced
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flow once-through vapor generator embodying the inven-
tion.
Fig. 2 is a plan view in section of a portion
of a furnace division wall taken along the line 2-2
- of Fig. 1.
Fig. 3 is a sectional view of a test section
installed in the division wall.
DESCRIPTION OF THE PREFERRED EMBODIMENT
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In the drawings, the invention has been illus-
trated as residing in a forced flow once-through vapor
generator intended for central station use and speci-
fically in the high heat absorbing zone of the furnace.
The unit, as depicted in Fig. 1, comprises a
setting 10 having a gas passageway situated therein
and including a furnace chamber 12 whose upper end
opens for discharge into a horizontal gas passage 14
with the latter, in turn, discharging into a down flow
or convection gas passage 16. The setting 10 includes -
an upper chamber or penthouse 18 disposed above the
furnace chamber 12 and the gas passages 14 and 16~ and
isolated therefrom by a substantially gastight parti-
tion wall or furnace roof 20. The penthouse 18 ex-
tends upwardly from the partition wall 20 and laterally
beyond the furnace and gas passage boundary walls to
form an insulated chamber for housing headers, tubes
and piping associated with the vapor generator. The
roof 22 over the setting 10, which also serves as the
penthouse roof, accommodates the passage therethrough
of a plurality of upright members or rods 24 used to
supportingly connect the vapor generator to cross beams
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~ASE 4007
7~ 911V
26, the latter being part of the structural steel work
which includes upright columns 28 transmitting the
weight load of the vapor generator to ground G.
The furnace chamber 12 is formed with a sloped
bottom 13 which includes a discharge opening 30 com-
municating with an ash hopper 32 disposed thereunder
and ground supported by a plurality of stanchions 34.
The fuel firing equipment comprises independently
operable fuel burners ~not shown) extending horizon-
tally along the center line of respective burner ports36 located on the rear and front walls at the lower
portion of furnace chamber 12. The furnace chamber 12
is bounded by high heat absorbing surfaces including
the sloped bottom 13, the front wall 15, the rear wall
17, the secondary superheater 38, the partition wall
or furnace roof 20, and side walls 19. Within this
high heat absorbing zone are contained a multiplicity
of division walls 21 spaced between side walls 19.
The gas passage 14 contains a secondary super-
heater 38 and a reheater 40 arranged in series withrespect to gas flow. The gas passage 16 contains a
primary superheater 42 having two banks also arranged
in series with respect to gas flow.
The penthouse 18 is shown as housing the fur-
nace front wall upper header 44, the secondary super-
heat inlet and outlet headers 46 and 48, respectively,
the reheater outlet and inlet headers 50 and 52, re-
: spectively, the primary superheater outlet header 54
and the convection gas pass rear wall header 56. It ;::
should be understood that the penthouse normally houses
numerous other headers as well as tube sections and
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~ Q37790
piping. These were omitted from Fig. 1 so as not to
encumber the illustration.
In Fig. 2 there is shown a plan sectional
view along the line 2-2 of Fig. 1 of a portion of
division wall tubes 21. Individual division wall
tubes 23 are shown in tangent relationship and with
interior plating 25.
Fig. 3 shows a test section of a division wall
tube 23 approximately 8 feet long which was installed
in an operating unit and removed after approximately
1-1/2 years in service. The test section consisted of
four different internal tube surface materials which
are tabulated in Table 1 with the weight of deposit on
each section after this period of service.
Table 1
Piece Number ~aterial Deposit Wei~ht
(Fig. 3) grams/ft
1 Croloy 5 - ITnplated 27.4
2 Croloy 1/2 - Chromium Plated 3.3
3 Croloy 1/2 - Nickel Plated 12.7 ;
4 Croloy 1/2 - IJnplated 18.0
This clearly shows the marked decrease in depo-
sition of corrosion products on the plated sections
with the chrome plated section superior to the nickel
plated section although this also shows a decrease over
the unplated sections. The deposit is porous and con-
sists principally of magnetite, Fe304, in crystalline ~-
form with a well-defined regular rippled or wave-like
surface pattern with the wave crests at right angles to
the longitudinal axis of the tube.
An additional test program was initiated on a
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' CASE 4007
7790
second operating unit in a different plant. A 10-foot
section o~ unplated Croloy 1/2 tubing was removed from
the division wall after 6 months service and a magnet-
ite deposit weighing 20 grams per sq. ft. was found.
This section o~ tubing was replaced with a 10-foot
section o~ chromium plated Croloy 1/2 tubing and after
6 months service under similar conditions the plated
section was removed and a deposit weighing 1 gram per
sq. ft. was determined, confirming the earlier results
on the effectiveness of chrome plating in inhibiting
internal deposits.
It will be understood that the invention may '
- be generally applied to the internal surface of all
the high heat absorbing tubes of the furnace or wher-
ever the deposition of porous corrosion products on
~ the internal surface of the tubes presents a problem.
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