U.S. patent application number 12/690197 was filed with the patent office on 2011-07-21 for controlling variables in boiler pressure vessels.
This patent application is currently assigned to ALSTOM TECHNOLOGY LTD.. Invention is credited to Donald W. Bairley, Wesley P. Bauver, II, Francois Droux, Ian J. Perrin, Christoph Ruchti, Falk Ruecker, Glenn T. Selby.
Application Number | 20110174240 12/690197 |
Document ID | / |
Family ID | 44276604 |
Filed Date | 2011-07-21 |
United States Patent
Application |
20110174240 |
Kind Code |
A1 |
Bairley; Donald W. ; et
al. |
July 21, 2011 |
CONTROLLING VARIABLES IN BOILER PRESSURE VESSELS
Abstract
A method of controlling stress in a boiler pressure vessel
comprises limiting the diameter of a drum (10) of the boiler
pressure vessel and preheating at least a portion of the wall (12)
of the drum (10). Limiting the diameter of the drum (10) allows
pressure in the drum (10) to be increased for a given mechanical
stress. Furthermore, preheating the wall (12) of the drum (10)
reduces peak thermally induced stresses in a material from which
the drum (10) is fabricated.
Inventors: |
Bairley; Donald W.;
(Farmington, CT) ; Bauver, II; Wesley P.;
(Granville, MA) ; Droux; Francois; (Oberrohrdorf,
CH) ; Perrin; Ian J.; (North Granby, CT) ;
Ruchti; Christoph; (Uster, CH) ; Ruecker; Falk;
(Nussbaumen, CH) ; Selby; Glenn T.; (Simsbury,
CT) |
Assignee: |
; ALSTOM TECHNOLOGY LTD.
Baden
CH
|
Family ID: |
44276604 |
Appl. No.: |
12/690197 |
Filed: |
January 20, 2010 |
Current U.S.
Class: |
122/406.5 ;
122/363; 122/365; 122/491 |
Current CPC
Class: |
F01K 3/22 20130101; F22B
5/04 20130101; F22B 37/22 20130101; F22B 35/04 20130101 |
Class at
Publication: |
122/406.5 ;
122/363; 122/365; 122/491 |
International
Class: |
F22B 37/02 20060101
F22B037/02; F22B 37/22 20060101 F22B037/22; F22B 37/26 20060101
F22B037/26 |
Claims
1. A method of controlling stress in a boiler pressure vessel, the
method comprising: limiting the diameter of a drum of the boiler
pressure vessel; and preheating at least a portion of the wall of
the drum; wherein limiting the diameter of the drum allows pressure
in the drum to be increased for a given mechanical stress; and
wherein preheating the wall of the drum reduces peak thermally
induced stresses in a material from which the drum is
fabricated.
2. The method of claim 1, wherein limiting the diameter of the drum
comprises using a drum having an inside diameter of less than about
1,775 mm.
3. The method of claim 1, wherein preheating at least a portion of
the wall of the drum comprises locally preheating penetrations in
the wall of the drum.
4. The method of claim 3, wherein locally preheating penetrations
in the wall of the drum comprises heating nozzles extending into
the wall of the drum.
5. The method of claim 3, further comprising locally preheating
areas of the wall of the drum adjacent to the penetrations.
6. The method of claim 1, wherein preheating at least a portion of
the wall of the drum is undertaken at at least one of a startup of
the boiler pressure vessel and during an operation of the boiler
pressure vessel.
7. The method of claim 1, wherein preheating at least a portion of
the wall of the drum is undertaken during a shutdown of the boiler
pressure vessel.
8. A method of operating a boiler pressure vessel, the method
comprising: applying local heating to a portion of the boiler
pressure vessel during at least one of prior to a startup operation
of the boiler pressure vessel, during an operation of the boiler
pressure vessel, and during a shutdown operation of the boiler
pressure vessel; wherein applying local heating to the boiler
pressure vessel reduces thermally induced stresses in the boiler
pressure vessel.
9. The method of claim 8, wherein applying local heating to the
portion of the wall of the boiler pressure vessel comprises at
least one of, heating a penetration extending into a surface of a
drum of the boiler pressure vessel; and heating an area surrounding
the penetration extending into the surface of the drum.
10. The method of claim 8, further comprising limiting a diameter
of a drum of the boiler pressure vessel, wherein limiting the
diameter of the drum reduces mechanical stresses in the boiler
pressure vessel.
11. A method of controlling variables in a boiler pressure vessel,
the method comprising: providing a steam drum of a boiler;
controlling mechanical stress in a wall of the steam drum by
limiting the diameter of the steam drum; and controlling thermal
stress in the wall of the steam drum by heating at least a portion
of the steam drum; and wherein the heating of the portion of the
steam drum is effected by preheating at least one of penetrations
in the steam drum and an area surrounding a penetration in the
steam drum during at least one of a startup period and a shutdown
period of the boiler pressure vessel.
Description
TECHNICAL FIELD
[0001] The present application is generally directed to systems and
methods for controlling variables in boiler pressure vessels. More
particularly, the present application is directed to systems and
methods for reducing stresses in the walls of boiler pressure
vessels.
BACKGROUND
[0002] A boiler pressure vessel (hereinafter "boiler") is a closed
vessel comprising a shell and containing a liquid that can be
heated under controlled conditions using a fuel or hot gases. The
shell is a drum (hereinafter "drum" or "boiler drum") that is
defined by one or more walls. Chemical energy contained in the fuel
is converted into thermal energy, which heats the liquid in the
boiler and causes it to vaporize. The mixture of liquid and vapor
enters the drum. The walls of the drum are designed to withstand
pressures exerted by the vaporized liquid. The vaporized liquid can
be taken from the drum and used to provide work or as a source of
heat.
[0003] Starting a boiler that is initially at ambient conditions
often causes rapid temperature changes to be experienced across the
walls of the drum. These temperature changes can generate thermal
stresses within the walls. Such stresses can cause crack initiation
and growth in the material of the wall. In some cases, such
stresses can also cause crack initiation and growth in a magnetite
layer that forms on the inside of the walls of the drums that
contain water.
[0004] In both natural circulation boilers and assisted circulation
boilers in which water is heated and vaporized into steam, the drum
is a steam drum utilized to separate steam from the water. In
boilers that operate at high pressures and/or have large drum
diameters, the wall thickness is greater (as compared to boilers
that operate at lower pressure and/or have small drum diameters) to
maintain acceptable pressure stress levels. Increased wall
thickness results in increased thermal stresses within the walls.
High stresses within the walls of the drums also occur at various
sites or penetrations that extend through the wall. Typical
penetrations include nozzles and the like. Since the penetrations
are points of weakness in the drum walls, the maximum operating
pressure of the boiler is effectively restricted due to limitations
imposed by the European Norm (EN) code on maximum stress ranges in
boilers (and more particularly in boiler drums). The range of
stress also limits the number of rapid startups that the boiler may
be subjected to as well as the total number of startups over the
life of the boiler.
[0005] Thick-walled boiler drums are generally heated only on their
inside surfaces, which results in temporary and uneven temperatures
in the wall, particularly during the startup period. As the wall
thickness increases, so does the temperature gradient through the
wall. The induced thermal stress increases for a given rate of
internal temperature change as the drum wall thickness increases.
Over time, the wall heats up to a uniform temperature, thereby
eliminating this type of thermal stress. The pressure stress then
dominates. Such stresses due to thermal gradients and internal
pressure, when applied and removed repeatedly, can cause crack
initiation and growth in the component material. The need to limit
stresses to prevent such cracks can effectively limit the rate of
temperature change in the drum. By limiting the rate of temperature
change, the operational flexibility (e.g., maximum pressures
attainable) of the boiler is decreased. Such flexibility is
desirable to provide for rapid start-ups to respond to changes in
power demand.
[0006] An additional constraint on boiler drums in compliance with
EN code requirements is the limitation on stress range to avoid
magnetite cracking. To avoid magnetite cracking, the difference
between the highest compressive stress and the highest tensile
stress should not exceed 600 mega pascals (MPa). This stress range
is illustrated in FIG. 1, which illustrates a typical stress
history for a steam drum during a boiler startup. Thermal stress
that occurs early in the startup process is shown as diminishing as
the temperature of the drum wall becomes more uniform as steady
state operating conditions are approached. As steady state
conditions are approached, the stress due to internal pressure
dominates the thermal stress. For a given drum diameter, the
positive hoop stress (tensile) can be reduced by increasing the
drum wall thickness, but this increases the negative stress due to
through wall temperatures at start up and limits the rate or number
of starts.
SUMMARY
[0007] According to aspects illustrated herein, there is provided a
method of controlling stress in a boiler pressure vessel. This
method comprises limiting the diameter of a drum of the boiler
pressure vessel and preheating at least a portion of the wall of
the drum. Limiting the diameter of the drum allows pressure in the
drum to be increased for a given mechanical stress. Furthermore,
preheating the wall of the drum reduces peak thermally induced
stresses in a material from which the drum is fabricated.
[0008] According to other aspects illustrated herein, there is
provided a method of operating a boiler pressure vessel. This
method comprises applying local heating to a portion of the boiler
pressure vessel prior to a startup operation of the boiler pressure
vessel, during an operation of the boiler pressure vessel, and/or
during a shutdown operation of the boiler pressure vessel. In
applying local heating to the boiler pressure vessel, thermally
induced stresses in the boiler pressure vessel are reduced.
[0009] According to other aspects illustrated herein, there is
provided a method of controlling variables in a boiler pressure
vessel. This method comprises providing a steam drum of a boiler;
controlling mechanical stress in a wall of the steam drum by
limiting the diameter of the steam drum; and controlling thermal
stress in the wall of the steam drum by heating at least a portion
of the steam drum. The heating of the portion of the steam drum is
effected by preheating penetrations in the steam drum and/or an
area surrounding a penetration in the steam drum during at least
one of a startup period and a shutdown period of the boiler
pressure vessel.
[0010] The above described and other features are exemplified by
the following Figures and Detailed Description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Referring now to the Figures, which are exemplary
embodiments, and wherein like elements are numbered alike:
[0012] FIG. 1 is a graphical representation of a typical stress
history for a steam drum;
[0013] FIG. 2 is a schematic representation of a vertical section
of a steam drum of a boiler; and
[0014] FIG. 3 is a perspective view of a vertical section of a
steam drum of a boiler.
DETAILED DESCRIPTION
[0015] Referring now to FIG. 2, one exemplary embodiment of a steam
drum of a boiler is shown generally at 10 and is hereinafter
referred to as "drum 10" or "steam drum 10." The drum 10 can be
from a natural circulation boiler, an assisted circulation boiler,
or any other type of boiler. The drum 10 is of an elongated
cylindrical shape and has a wall 12 that is penetrated by nozzles
14 that receive a high temperature steam/liquid mixture and
discharge this mixture into an annular space 16 between a drum
liner or baffle 18 and an inner surface 15 of the wall 12. The wall
12 also has an exterior surface 17. The nozzles 14 may extend
beyond the inner surface 15 of the wall (FIG. 2) or they may
terminate at the inner surface 15 (FIG. 3). A liquid 26 such as,
for example, water pools in the bottom of the drum 10. One or more
steam separating units 24 are located outside the volume enclosed
by the baffle 18. Steam from the steam/liquid mixture 34 and from
the vaporization of the water 26 passes through a drying assembly
32 and is removed through an outlet 30. The configuration of FIG. 2
is not limited to that as shown, as other configurations are
possible.
[0016] Upon operation of the boiler, particularly at startup from
ambient conditions, the nozzles 14 and the areas 15a of the inner
surface 15 of the wall 12 surrounding the nozzles 14 are affected
by the steam/liquid mixture 34. Temperature transients (e.g., the
movement of heat from one area to another) through the materials of
the nozzles 14 and the wall 12 produce thermal stresses.
Accordingly, the nozzles 14 and the areas 15a surrounding the
nozzles, namely, the drum wall 12 and particularly at the inner
surface 15, are subjected to stress from the high temperature
steam/liquid mixture 34. Mechanical stresses such as hoop stress in
the wall 12 of the drum 10 are also encountered as the result of
pressure.
[0017] Mechanical stress in the wall 12 is a function of various
process variables, namely, the radius of the drum 10, the thickness
of the wall 12, and the internal pressure of the drum 10. This can
be described by the equation:
.sigma..sub.m=f(PR/t)
where:
[0018] .sigma..sub.m is the hoop stress of the drum;
[0019] P is the internal pressure;
[0020] R is the drum radius; and
[0021] t is the drum wall thickness.
For a given internal pressure and stress, reducing the drum radius
or diameter results in the thickness of the wall 12 of the drum 10
being reduced.
[0022] One approach to accommodating mechanical stress that is
applicable to both natural circulation boilers and assisted
circulation boilers with steam production greater than 50 kilogram
per second (kg/s) to enable operation at higher pressures, which is
desirable due to the resulting higher cycle efficiency, is to limit
the thickness of the wall 12 of the drum 10. The thickness of the
wall 12 is limited by using a relatively small diameter steam drum,
for example, a steam drum having an inside diameter of between
about 1,000 millimeters (mm) and about 1,775 mm. When the diameter
of the drum 10 is reduced and the thickness of the wall 12 is
limited to a value that is consistent with drums having inside
diameters of greater than about 1,775 mm, the value for P can be
increased for a given hoop stress. Typical wall thicknesses could
range from about 70 mm to about 150 mm.
[0023] Thermal stresses within the wall 12 of the drum 10 also
occur at the nozzles 14 or other penetrations through the wall 12
to the inner surface 15 as well as at the inner surfaces 15a
proximate the nozzles 14. Referring to FIG. 3, a localized high
stress range area is shown at 20. This localized high stress range
area 20 is located on the inner surface 15 proximate the area at
which the nozzle 14 penetrates the wall. The stress in this
localized high stress range area 20 is at least twice the stress in
any other area in the rest of the drum.
[0024] It has been discovered that applying local heating to at
least portions of the drum 10 in a controlled manner can reduce the
temperature transients and thermal stresses within the drum 10.
[0025] One approach to applying local heating to accommodate
thermal stress is to preheat the nozzles 14 and the area 15a
adjacent thereto (e.g., the inner surface area 15a of the wall 12
in the area of the nozzle 14) prior to boiler startup when the drum
10 is at ambient pressure conditions. In one embodiment, the local
heating may be applied on the exterior surface 17 of the drum 10
proximate the area at which the nozzle 14 enters the drum 10 (e.g.,
area 17a). This would reduce the peak thermally induced stresses in
a material from which the wall 12 of the drum 10 is fabricated that
would otherwise limit the number of startups from ambient
conditions or even prevent use of drum-type boilers above certain
pressure ranges due to the EN code limits of stress ranges. Locally
preheating of the nozzles 14 and/or the wall 12 may be used as an
alternative to or in conjunction with limiting the diameter of the
drum 10.
[0026] It should also be appreciated that the approach is not
limited to being undertaken at startup of the boiler, as the
nozzles 14 and the wall 12 could be heated during a shutdown
operation. In doing so, the rate at which heat is dissipated from
the nozzles 14 and the wall 12 would be reduced, thereby reducing
the thermally induced stresses in the material of the nozzles 14
and the wall 12.
[0027] In addition to reducing thermally induced stresses by using
local heating, it is contemplated that local heat uses much less
energy than would be required to heat the entire drum 10 (e.g., the
entire inner surface 15) and the fluid 26 that it contains, thereby
reducing operational costs. Without any sort of preheating feature
in place, the number of cold starts could potentially be limited to
an absolute maximum in the specification (e.g., 300) as compared to
an essentially unlimited number of cold starts with preheating.
[0028] The maximum possible thermal stress for a given ramp up in
temperature (temperature transient) is also a function of various
process variables and varies approximately as the square of the
thickness of the wall. Reduced thickness would result in reduced
thermal stress for the same rate of temperature change. This is
described by the equation:
.sigma..sub.t=f(T.sub.rt.sup.2)
where
[0029] .sigma..sub.t is the thermal stress;
[0030] T.sub.r is the rate of temperature change; and
[0031] t is the drum wall thickness.
[0032] Starting a boiler that is initially at ambient conditions
results in rapid temperature changes in the drum 10 as well is in
other components of the drum 10 (e.g., nozzles 14 and the like).
These temperature changes can generate thermal stress within these
components. Such stresses can cause crack initiation and growth in
the material of which the component is fabricated and in some cases
in a magnetite layer that forms on the inner surface 15 of such
drums 10 that contain water 26. Preheating at least portions of the
drum 10 or other components of the pressure vessel in a controlled
manner can reduce the rate of temperature change, thereby reducing
the thermal stresses within the component. Preheating the drum 10
can be effected by electrical resistance heating or other means
readily available.
[0033] Although this invention has been shown and described with
respect to the detailed embodiments thereof, it will be understood
by those of skill in the art that various changes may be made and
equivalents may be substituted for elements thereof without
departing from the scope of the invention. In addition,
modifications may be made to adapt a particular situation or
material to the teachings of the invention without departing from
the essential scope thereof. Therefore, it is intended that the
invention not be limited to the particular embodiments disclosed in
the above description, but that the invention will include all
embodiments falling within the scope of the appended claims.
* * * * *