Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DUAL PATH PARALELL SUPERHEATER
FIELD AND BACKGROUND OF INVENTION
[0001] The present invention relates generally to methods and devices for
effectively
increasing the delivery of steam in a controlled and efficient manner.
[0002] It is commonly required that temperature and/or steam flow
(capacity) of an
existing boiler be increased. Pressure drop across the superheater increases
as the
steaming capacity increases. High pressure drop is often the limiting factor
for a
capacity increase. As a result, the complete superheater regularly needs to be
replaced
to provide a lower pressure drop.
[0003] In a typical scenario, an operator requires that steam flow be
increased (e.g.,
543.4kpph). Standard practice is to arrange the superheater such that there is
only one
path by which steam can become superheated. In order to superheat at the
increased
rate of steam, additional surface is added. Figure 1 hereof shows a typical
prior art
arrangement 10 for a single-path series superheater, in a new surface 12 is
added to an
existing surface 14 to process the increased capacity. There is a provided a
drum 16 for
delivering steam to surfaces 12 and 14 and a turbine 18 for ultimately
receiving steam
from surfaces 12 and 14
[0004] Table 1, below, predicted steam temperatures and pressures at the
locations
as defined by Figure 1.
[0005] Table 1: Typical Prior Art Arrangement ¨ Steam Temperature and Pressure
Profile
=
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WM = 543.4 itpph steam
Pressure Temp
Location psig deg F
A 1403 589
1384 744
1346 697 47F spray
1322 842
1275 840 2F spray
F 1236 900
[0006] Desired outlet pressure is 1300 psig and desired outlet temperature
is 900 F.
[0007] To control steam temperature there are spray attemperators at two
interstage
locations, the first between locations B and C and the second between
positions D and
E. The prior art arrangement is predicted to make full steam temperature with
a total of
49 F of spray attemperation. However, the arrangement does not achieve the
target
outlet pressure of 1300 psig. The best achievable outlet pressure is only 1236
psig.
The traditional remedy for this is to increase the number of parallel steam
flow paths in
the existing surface. This requires the replacement of all the existing
superheater
tubes, superheater headers, roof seals, etc. and often requires that
sootblower cavities
be relocated.
[0008] Thus, there is a need for increased steaming rate without the need
for
replacement of the existing superheater.
SUMMARY OF INVENTION
[0009] The present invention is drawn to a dual-path parallel superheater
includes a
drum for delivering steam, a steam receiving apparatus opposite the drum for
receiving
steam, a first surface and a second which receive steam from the drum to
provide first
and second paths for superheating the steam before delivering it to the steam
receiving
apparatus. There are also spray attemperators along the first and second
paths.
[0010] The present invention is a system and method in which steam is
divided into
two paths at the drum outlet. One path is defined by existing superheater
surface and
the other by new surface overhanging the furnace. Each path is independently
controlled with spray attemperation and independently achieves full steam
temperature.
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The streams are re-combined to a single path at the superheater outlet. The
present
dual-path parallel superheater ("DPPS") allows for an increased steaming rate
without
requiring the replacement of the existing superheater.
[0011] The various features of novelty which characterize the invention are
pointed
out with particularity in the claims annexed to and forming part of this
disclosure. For a
better understanding of the present invention, and the operating advantages
attained by
its use, reference is made to the accompanying drawings and descriptive
matter,
forming a part of this disclosure, in which a preferred embodiment of the
invention is
illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] In the accompanying drawings, forming a part of this specification,
and in
which like reference numbers are used to refer to the same or functionally
similar
elements:
[0013] FIG. 1 is a schematic view of a prior art single path series
superheater; and
[0014] FIG. 2 is a schematic view of the present dual path parallel
superheater.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] With reference to the FIG. 2 the dual path parallel superheater
("DPPS")
according to the present invention is shown, the superheater arranged such
that there
are two parallel paths by which steam becomes superheated. FIG. 2 shows the
DPPS
arrangement, in which a new surface 22 is added to the original surface 24 to
process
increased capacity. As in the prior art structure, there is a provided a drum
30 for
delivering steam to surfaces 22 and 24 and a steam receiving apparatus 32 such
as a
turbine for ultimately receiving steam from surfaces 22 and 24.
[0016] Table 2 below shows predicted steam temperatures and pressures at
the
locations A1-A4 and B1-E34, defined in FIG. 2.
[0017] Table 2: Steam Temperature and Pressure Profile for the present
DPPS:
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Path A = 50% = 271.7 ITO steam Path B = 50% = 271.7 itpph steam
Pressure Temp Pressure Temp
Location psig deg F Location psig deg F
Al 1378 586 81 1378 586
A2 1344 706 82 1333 839
A3 1324 665 41F spray 83 1320 778 61F Spray
A4 _ 1300 900 84 _ 1300 900
[0018]
Desired outlet pressure is 1300 psig and desired outlet temperature is
900 F.
[0019] FIG. 2
reflects two paths: Path A, marked by locations A1-A4, and Path B,
marked by locations B1-134. To control steam temperature, each path has a
spray
attemperator 26, 28 at one interstage location.
[0020] As
shown in FIG. 2, Path A, including locations A1-A4, is arranged in a
side by side orientation in order to utilize interstage spray 26 while only
requiring that
one new bank be installed. The interstage spray attemperator 26 is located
between
positions A2 and A3. The attemperator 26 controls steam temperature and
combats
high metal temperatures inherent to low steam flow.
[0021] The
tubes in the Path A bank may be made of a steel compound such as
SA213-T22, a plurality of rows of stainless steel tubes may be employed in the
outlet
legs.
Additionally, the side by side design of the present invention minimizes the
amount of new heating surface required because hot steam is reintroduced to
the front
of the furnace, where the flue gas is hottest.
[0022] Path
B, including locations B1-B4, reuses the unit's existing superheater
surface and existing interstage spray 28 location between positions B2 and B3.
The
interstage spray 28 controls steam temperature and combats high metal
temperatures
inherent to low steam flow. Similar to Path A, metals in the Path B banks may
be made
of materials well-known to those of skill in the art. The exception is the
outlet rows of
the Path B primary superheater: These rows generally require replacement with
stainless steel tubes.
[0023] Both
Path A and Path B achieve full steam temperature independently.
Path A has 41 F of spray margin and Path B has a 61 F of spray margin. After
being
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controlled to the same temperature, steam from Path A and Path B recombine to
form a
single outlet.
[0024] The
parallel paths A and B are designed for the same pressure drop. This
can be accomplished initially by under drilling headers in the new surface or
installing
orificed Dutchman in the existing surface. Under drilling headers and the
installation of
orificed Dutchmen are techniques known to those of skill in the art. However,
as the
unit becomes dirty, and spray flow changes, the pressure loss in each line may
change.
As a means of control, a trim valve may be installed in at least one of the
lines. With the
ability to dynamically adjust pressure drop, steam flow is enabled to remain
as designed
in each path. Thereby, steam temperature and pressure can also be maintained
as
designed.
[0025] The
present invention offers numerous advantages. The present invention is
for industrial boilers undergoing capacity increases. When steaming rate
increases the
amount of pressure drop between the drum and superheater outlet increases. If
the
newly-desired steaming rate is high enough, a new superheater with additional
flow
paths is required to maintain outlet pressure. A new surface is required
regardless of
the existing superheater condition. As a result, operators are often forced to
scrap
tubes before they reach end-of-life, or, abandon their projects all together
due to high
project costs. The present DPPS allows for increased steam flow without
replacing
existing surface.
[0026]
Operators continuously strive to get as much as possible from existing
equipment before investing in replacements. This is especially true when the
existing
equipment is in good operating condition. The present invention provides cost
savings
to operators through the re-use of the existing surface. The present invention
allows
satisfaction of an increased steam demand at a lower cost than traditional
solutions.
The present invention may be applied to many surface different arrangements,
offering
flexibility in its application.
[0027] The present DPPS arrangement may be applied to several boiler types,
including but not limited to, process recovery in the paper industry, stirling
power
boilers, waste-to-energy applications, and biomass combustion technologies.
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[0028] A comparison of Table 1 and Table 2, above, shows that the present DPPS
allows an increased steam flow to be controlled to a target steam temperature
while
maintaining the desired outlet pressure.
[0029] Under increased flow conditions the DPPS design provides ability to
re-use
existing superheater surface without lowering outlet pressure; ability to
reach full steam
temperature with less heating surface than prior art designs; and ability to
control
pressure drop across each steam path.
[0030] Alternative methods for processing an increased flow condition
include
allowing outlet pressure to decrease and removing the existing superheater
(tubes,
headers, roof seals, etc.) and installing new surface with additional parallel
flow paths.
[0031] In another alternative, all or a portion of capacity increases may
be derived
from increases in operating temperature. In these embodiments the method
described
herein may further be used to maintain a desired pressure drop while
maintaining a
desired superheater outlet temperature. While specific embodiments and/or
details of
the invention have been shown and described above to illustrate the
application of the
principles of the invention, it is understood that this invention may be
embodied as more
fully described in the claims, or as otherwise known by those skilled in the
art, including
any and all equivalents, without departing from such principles.
