Liquid heaters and vaporizers – Separators – Boiler circulation
Reexamination Certificate
2000-06-01
2002-01-08
Wilson, Gregory (Department: 3749)
Liquid heaters and vaporizers
Separators
Boiler circulation
C122S00600B
Reexamination Certificate
active
06336429
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates generally to large commercial utility boilers or steam generators and, in particular, to a new and useful natural circulation steam generator which uses a plurality of large diameter vertical pressure vessels at the top of downcomers of the steam generator, instead of a conventional single large steam drum.
A conventional natural circulation boiler or steam generator system, generally designated
10
, is schematically illustrated in FIG.
1
. The system
10
comprises a steam drum
12
, downcomers (DC's)
14
provided with downcomer bottles (DCB's)
15
at a lower end thereof, supply tubes
16
, furnace wall tubes
18
, riser tubes
20
and steam/water separators
22
inside the steam drum
12
. Typically, heated feedwater
24
enters the drum
12
via a feedwater distribution system whose task it is to thoroughly mix the feedwater
24
with the saturated water in the steam drum
12
which has been separated from the steam-water mixture supplied to the separators
22
in the steam drum
12
via the riser tubes
20
.
The resulting water mixture (usually subcooled, i.e., the temperature of the water is below the saturation temperature corresponding to the operating pressure within the steam drum
12
) enters and flows down through the downcomers
14
and is distributed, via a number of supply tubes
16
, to inlet headers
26
of the furnace circuits, e.g. the wall tubes
18
.
Circulation of the water in the furnace circuits or wall tubes
18
(also shown in the FIGS. as front wall (FW), side walls (SW's), and rear wall (RW), is established through the difference in fluid density between the subcooled water in the downcomers
14
and the steam/water mixture in the heated furnace circuits
18
. The fluid velocity in the wall tubes comprising the furnace circuits must be sufficient to cool the furnace wall tubes
18
, which are typically exposed to combustion gases B whose temperature may reach 3500° F. in the burner zone
28
of the furnace.
As soon as the heated fluid reaches saturation conditions, steam begins to form and the fluid within the wall tubes
18
becomes a two-phase mixture. The fluid velocity must be sufficient to maintain nucleate boiling (bubble-type boiling) within the tubes
18
, as this is the regime which generates the highest possible heat conductance, i.e. the best cooling between the fluid within the tubes
18
and the inside wall of the tubes
18
on the heated (furnace) side. Insufficient fluid velocity in combination with high heat flux and an excessive percentage of steam in the steam-water mixture leads to steam blanketing on the inside of the tubes
18
. This is equivalent to an insulating-type steam film along the heated, inside tube wall, which causes rapid tube failure. The danger of film boiling increases with increasing boiler pressure. The fluid temperature in the boiling (two-phase) regime is strictly dependent on the local internal pressure and is nearly constant from the point where boiling starts to the point where the saturated water leaves the separators.
The steam-water mixture eventually reaches outlet headers
30
of the furnace circuits
18
. From here, the steam-water mixture is conveyed to the steam drum
12
and distributed along a baffle space therein and from there to a plurality of steam/water separators
22
located inside the steam drum
12
.
The steam/water separators
22
separate the saturated water from the saturated steam, usually through centrifugal force generated through either tangential entry of the two-phase fluid into cyclones or through stationary propeller-type devices. The centrifugal action literally “squeezes” the steam out of the steam-water mixture.
The saturated steam leaves the top of the steam drum
12
through saturated connecting tubes
32
which supply the steam to the superheater(s)
34
of the boiler or steam generator system
10
, where the steam is further heated to the desired final temperature before being sent to a turbine or a process. The saturated water, as stated earlier, leaves the bottom of the steam/water separators
22
and mixes with the continuously supplied feedwater.
The crucial element in a conventional steam generator or boiler circulation system
10
is the steam drum
12
. In high-pressure boilers with natural or pump-assisted circulation, such steam drums
12
may be over 100 feet long, with a
6
foot inside diameter, and shell thicknesses over 7 inches. Thus, the steam drums
12
are very large and extremely heavy and must be lifted in place as soon as the boiler and its structural steel and columns are erected, prior to erecting all other boiler pressure parts. Accordingly the steam drum
12
is on the critical path of the overall schedule for such boiler and power plant projects. For a more detailed description of conventional steam generators and steam drums as generally described above, the reader is referred to the 40th edition of
Steam/its generation and use
, 40th Edition, Copyright© 1992, The Babcock & Wilcox Company.
SUMMARY OF THE INVENTION
Elimination of large steam drums in favor of reduced size separating vessels, according to the present invention, is the logical consequence of a steady reduction in the so-called Circulation Ratio. The Circulation Ratio (CR) is defined as (total water flow to the furnace circuits/steam flow to the superheater). For many years, the minimum CR for natural (or pump-assisted) circulation high pressure (>2500 psig drum operating pressure) boilers was 4.0. However, the invention and successful introduction of multi-lead ribbed furnace tubes made it possible to reduce the CR, as ribbed tubes can safely operate at much lower water flow rates than internally smooth tubes exposed to furnace heat. Therefore, the drumless boiler concept according to the present invention becomes economical at CR's below 3.0.
At the low end of the CR spectrum would be the types of steam generators known as subcritical once-through boilers which have a CR of 1.0 and—typically no separating equipment, except perhaps that used for removal of residual moisture. The present inventors believe, therefore, that it is only logical that with decreasing CR, a natural (or pump-assisted) circulation boiler design should more and more resemble a once-through subcritical boiler. As the following description of the drumless natural circulation boiler concept will demonstrate, this philosophy, and its attendant benefits, is realized by the present invention. As experience with the new type of drumless natural circulation boiler design becomes available, the present inventors believe that the future trend will be towards ever-decreasing CR's, since lower CR's require fewer and smaller, and therefore less expensive, connections (supply tubes, riser tubes, downcomers, etc.) and steam/water separators in the circulation system of such boilers.
An object of the present invention is to provide a drumless natural circulation boiler system. A crucial difference between such a system and a conventional natural circulation system with steam drum is that the single large steam drum is eliminated and the tops of the downcomers are modified into large vertical steam/water separators in the form of large diameter, vertically extending vessels. Phase separation is achieved through a suitable number of tangential nozzles which lead the steam-water mixture from the riser tubes into the separators where the saturated steam is separated from the steam-water mixture through centrifugal action along the separator's cylindrical inside periphery. The nozzles must be suitably inclined against the horizontal plane to avoid interference between the multiple fluid jets. The tangential velocity is a function of the total flow to each separator, the boiler pressure, the number and size of the nozzles, the allowable pressure drop across the separators, and the inside diameter of the separators, and must be sufficient to effect separation, like in other types of separators. Preferably, the upper porti
Albrecht Melvin J.
Mayer Ernst H.
Wiener Murray
Marich Eric
The Babcock & Wilcox Company
Wilson Gregory
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