Note: Descriptions are shown in the official language in which they were submitted.
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START-UP SYSTEM FOR
ONCE-T~ROUGH BOILERS
~AC~GROUND OF THE rNVENTION
Field of the Invention:
The present inven~ion relates ~o a once-through vapor
generator, and in par~icular to a ~o~el start-up bypass
system for a once-through vapor generator.
A typical once-through vapor generator, o the type to
which the pTesent invention pertains, will include an inlet
end and an outlet end, wi~h a plurality of heat transfer
surfaces between ~he ends. As a general rule~ these will
include an economizer pass, urnace passes defining ~he high
temperature ~adiant heat transfer portion of the generator, a
reheater, and primary and finishing superheating passes~ ~he
ou~let end of the generator being connected to a suitable
point of use such as a high pressure steam turbine. The exhaust
flow from the turbine or turbines is transmitted to condens-
ing meansj a deaerator, and from ~here through heat recovery
surfaces to the inlet end of the generator.
During start-up of the vapor generator, the low enthalpy
fluid cannot be handled by the high pressure turbine, and or
this reason, the generator usually is provided with a bypass
system to recirculate the flow until the flow is at the
enthalpy level required by the turbine. It is known to trans-
mit this flow to heat recovery surfaces where it is passed
in heat exchange with the feed ~low to the Yapor generator
inlet end. It is also known to position a flash tank or
separator in the bypass system designed to separate the flow
entering the bypass system in~o vapor and liquid streams and
to transmit the vapor stream back to the main flow path for
warming and roll;ng the high pressure turbine. Other uses
are known for the bypass flow, including turbine gland sealing,
and pegging the deaera~or.
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Depending upon the design of the vapor genera~or, there
may be no flow during at least ~he initial s~ages of star~-up
through certain surfaces, for ins*ance the reheater surfaces
of the generator, and perhaps e~en through primary and finish
ing superhea~ing surfaces. These surfaces usuall~ are posi-
tioned in lower temper~ture or convectîon heating zones, so
that during the initial stages o start-up~ cooling of the
surfaces is not necessary. Accordingly, the bypass system
usually is connected to the main flow path upstream of at least
the finishing superheater surface. This has the advantage
that ~he vapor flow returned to t~e main flow path from the
~ypass ~ystem flash tank or separator can be subjected to
further heating and superheating for earlier warming and
rolling of the high pressure turbine~ reducing the star*-up
lS period.-
In a once-through boiler~ prior to firin~, sufficien~ -
flow is required through the boiler pressure par~s which are
exposed to high gas temperatures during start-up. With
smooth furnace tubes, a minimum flow at 25 percent of full
load flow generally represents a good balance be~ween urnace
tube cooling requirements over the load range and furnace
enclosure pressure drop. In the low load range, tube fluid mass
flow cannot be reduced apprec;ably without ~he possibility
of "pseudo film boiling~'l a condition much iike that known
as "departure from nucleate boiling" (DNB) at subcritical
pressures. Like DNB, ps~udo film boiling represents a
sudden deteriora~ion of heat transfer a~ the in~ernal tube
surface, which results in unacceptably high levels of tube
metal temperatures and must be avoided under all operating
conditions.
The multi-lead ribbed tube has found broad acceptance
as a tool to prevent ~NB in 2400 psî boilers and has pro~ed
very effective in preventing pseudo film boiling at super-
critical pressures. Rib~ed tubing use enables reduction of
the minimum flow to 15 percent of full load flow of 3500 psi
boilers. Lowering the minimum flow means reduced start-up
time, reduced heat loss to the condenser~ and reduced auxiliary
steam and auxiliary power requirements during start-up. The
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lower m;nimum flow also offers the abili*y to operate the
boiler oveT a wider load range, 20 ~o 100 ~, wi~hout
going on the bypass sys.tem. This- wider load range can be
~andled for most domestic co~ls wit:hout oil support and wi h
S reasona~le net plant heat rates.
On manr units, the high pressure turbine has been sub-
jected to ~emperature dips during start-ups when the steam
temperature dropped while ramping throttle pressure. The
temperature con~rol problems during.star~-up s~em from the
practice of ramping superheater pressure to full operating
pressure at relatively low loads. W~ile ramping pressure,
superheater flow is raised.from approximately 7. to 25~, and
s-multaneously7 flow ~s shifted from:-*he flash .tank-to the
normal path through khe boiler. All of.this happens a* very
l~w loads where iong ~ime lags and-large changes in fluid .
storage and heat storage in ~he koiler preclude effective
steam temperature con~rol.
Developments hav0 ~aken place ~o orercome these short-
comings; specifically, to reduce minimum start-up flow,
simplify and speed up start-up; permit controlled shutdowns~
provide positive control of steam conditions 9 and enable
quick load changes over a wide load range. Some of these
.developments are; the use of internally ribbed tubes, the
. concept of dual pressure operation-(i.e., capabili~y to
operate at variable thrott7e pressure while maintaining
fluid.pressure in the economizer, boiler enclosure, and
primary superheater), capability o ~ariable throttle pressure
operation over a wide load range, and use of saturated steam
for attemperation of main and reheat steam during star~-up
and at ~ow loads.
One problem experienced with conventional bypass systems
is that as the vapor generators become larger in size, a~d
of much larger capacity, the bypass systems of necessity must
~e designed to handle ever greater quantities of 10w; that
35 , is, the 30~ minimum flow ~ecomes increasingly greater in
terms of total mass flow. The flash tanks or separators
positivned in the bypass systems also must be capable of
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43~9
handling the increased flows as capacities of the generators
increase, and since these flash tanks or separators are
heavy walled vessels designed to withstand high pressures,
and temperatures, it is apparent that the separators or
flash tanks become major items in the capital costs of the
generator, particularly in the cost of the bypass system.
It is known, to use a plurality of smaller sized flash tanks
OT separator vessels in place of one large very hea~y walled
vessel. Howeve~, whether one or seve~al vessels are used,
the expense of this part of the system is high and can be
out of proportion when compa~ed with the ~emainder of the
system.
A further disad~antage experienced with conventional
bypass systems concerns switch-over of flow from the bypass
system to th~ main flow path at the point in the start-up
period when the bypass system is isolated ~rom the flow.
Although the bypass systems and flash tanks sr separato~s
can be designed to handle flows up to full operating pressures
and temperatures in a once-through vapor generator, which
may be in the order of 3,600 psi and about 1,100F., respec-
tively, economics (as discussed above) dictate that the bypass
system be designed for and utilized up to only about 1,000 psi,
at which time or point in the staTt-up period the flow is
switched over to the main ~low path. Since the bypass system
is positioned upstream of one OT more of the superheating
sections, for shorter staTt-up time~ there usually is insuffi-
cient surface upstream of the bypass system to produce a ~ully
vaporized flow at the normal swi~ch-over pressure of about
1,000~psi, at this point in the start-up period. The result
is that the surfaces downstream of the bypass system~ which
prior to switch-over, will have received a vapor flow from
the flash tank or separator, will now receive a vapor-liquid
mixture flow from the upstream surfaces 9 resulting in a tempera-
ture drop in the surfaces downstream of the bypass system and
an undesirable tlemperatu~e shock to these surfaces.
DESCRIPT_ON OP PRIOR ART
U. S. Patent No. 3,529,580 ~Stevens) describes a once-
through vapor generator comprising a main flow path and a
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bypass system which includes a first and second conduit means
connectecl to the main flow path, a flash tank means, vapor and
liquid return lines from the flow tank, and the second conduit
means leading to the heat recovery surfaces and including a
valve means therein so as to apportion the flow from said main
flow path between the first and second conduit means.
U.S. Patent No. 3,954,087 (Stevens et al) discloses an
apparatus and method of start-up of a subcritical and super
critical once-through vapor generator This system comprises a
plurality of separators which are capable of handling full load
flow and auxiliary flow paths, one to the condenser and the
other to the main flow path between the condenser and vapor
generator.
Other relevant prior art consists of UOS. Pakent Nos.
3,338,053 and 3,338,055 (Gorzegno, et al). Disclosed is a
start-up apparatus and methods for a subcritical and super-
critical vapor generator. This system is comprised of a
pressure reducing means in the main flow path between the vapor
generating surface and superheaters, a flash tank means
connected between ~he two superh~ater surfaces, and liquid and
vapor bypass conduit means from said flash tank means.
In accordance with a preferred embodiment of the present
invention, there is provided a once-through vapor generator
comprising a main flow path, heating surfaces~ heat recovery
surfaces and a bypass system. The bypass system includes a
flash tank means sized to handle flow up to at least 25% of
full load, a variable superheater bypass stop valve located
between the superheating surfaces and sized to handle
variable superheating pressure from approximately 15% to full
load, and two steam spray attemperators from the flash tank;
one to the outlet of the second superheater and the second to
the outlet of a reheater located between a first high pressure
turbine and a second low pressure turbine.
It is an object of the present invention to overcome the
above problems, and in particular to provide a simplified
bypass system capable of avoiding the temperature shock
experienced in conventional svstems during switch-over
from the byvass system to main path flow.
It is an object of this invention to maintain
minimum flow through the boiler furnace parts exposed to
high temperature combustion gases during start-up.
It is an object of this invention to provide means
of positive control of steam conditions during ~tart-up
and shutdown to suit turbine metal requirements.
It is an object of this invention to recover heat
during start-up and low load operation. It is an object
of this invention to provide for water cleanup during
start-up without delays in boiler and turbine warming
operations.
It is the final object of this invention to provide
means of operating at variable throttle pressure over the
load range while maintaining the necessary supercritical
pressure in the furnace circuits.
There is provided in accordance with the present
invention a once-through vapor generator start-up system
comprising a main flow path including, in series ~low
relationship, a vapor genera*ing section, a superheating
section having a primary and a secondary heating surface,
a turbine section, a reheater section, and a condensing
section, a collecting and separating section in flow
relationship wi.th the main flow path, and a first and a
second spray attemperator flow path in flow relationship
with the main flow path whereby the first steam spray
attemperator flow path is connected from the collecting
and separating section to a point downstream of the
secondary heating surface and the second steam spray
attemperator flow path is connected from the collecting
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and separating section to a point downstream of the
r~heater section.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a once~through
boiler start-up system.
FIG. 2 is a schematic diagram of a once-through
boiler start-up system with superheater and reheater
steam attemperation.
FIG. 3 is a schematic diagram of a once-through
boiler start-up system for variable pressure operation.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
Shown in Figure 1 is a schematic diagram of a once-
through boiler start-up system for constant pressure unit 10.
Included in the unit 10 is a vapor generating section 12 in
parallel flow relationship with a col:Lecting and separating
section 14, and in series flow with a superheating section
16, a turbine section 18 and a condensing section 20. It is
understood that the connections between these sections, which
are shown schematically in the drawings, are achieved by fluid
circuitry in the form of tubesr condu:its, risers, headers, etc.
to transfer heat exchange fluid either in a liquid form or a
vapor form throughout the various sections.
The vapor generating section 12 consists of an economizer
22 adapted to receive the heat exchange 1uid/ which is
preferably water, and pass it to a furnace 24, after which it
is passed to the superheating section 16.
The superheating section 16 includes a primary superheater
26 and a secondary superheater 28 which are connected in the
vapor circuit in series flow relationship in the vestibule
section and the convection section of the vapor generating
unit. A full capacity stop valve 30 and a stop by-pass pressure
reducing valve 31, in parallel flow with each other, are
located in the vapor circuit between the primary and secondary
superheater, 26 and 28 respectively. Note that although only
one stop or steam flow valve 30 and one stop bypass valve 31
are shown there could be a plurality of each valve. The
function of valves 30 and 31 could be combined into one inline
valve.
The vapor output from the secondary superheater 28 is
adapted to be connected to the turbine section 18 which
includes a high pressure turbine 32 and a low~pressure turbine
34. A reheater 36 which is also located in the convection
section, is flow connected between the high pressure turbine
32 and the low pressure turbine 34. The turbines 32 and 34
are driven by the vapor from the secondary superheater 28 and
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4399
the reheater 36 respectively and are adapted to drive an
electric generator or the like (not shown) in a conventional
manner. A drain line 38 is connec:~ed to the vapor circuit
between the secondary superheater 28 and the high pressure
turbine 32 to enable the ~apor circui~ to be warmed prior
to rolling of the high pressure turbine 32 as will be e~plained
later.
The outlet fTom the low pressure turbine 34 is connected
to the condenser section 20 which includes a condenser 40, a
pump 41 and a condensate polishing system 42. The exhaust
vapor from the *urbine section 18 is passed to the condenser
40 where it is condensed, then pumped by pumps 41 through the
condensate polishing system 42 and then on to a plurality of
external low pressure heaters designated by reference numeral
44. A deaerator 46 is connected to the output of the low
pressure heaters 44 for ~eceiving the condensate before it
is circula~ed, via feed water pump 4g, to hig~ pressure heater
S0 to further heat the csndensate before it enters ~he econo-
mizer of the vapor generating section 12.
The boiler feed pump 48 supplies the minimum required
flow of feedwater during start-up and low load operation
as required to the furnace circuitry. The addition of multi-
lead internal ribs of the kind well known in the art to the
enclosure tubes in the high heat input zones within the con-
vection pass permits lower velocity limits, so the same size
enclosure tubes can be used, and the minimum flow reduced
from 25% to 15%. Since the velocities of the liquid in the
tubes at full load are the same as before the addition of
the ribs, there is only a slight increase in the pressure
drop due to the slightly higher friction factor created by
the ribbed tubing. During early start-up, all of the flow
goes through thle primary superheater 26 and bypasses the
secondary superheater 28 through the bypass system's pressure
reducing valve 52 to the 1ash tank 54 where the water-steam
mixture is separated. The water flows to a deaerator 46 and
a condenser 40. The steam flows from flash tank 54 through
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two attemperator lines 56 and 58. The overall water level in
the flash tank 54 is controlled by drain valves 60 and 62. The
drain valve 60 controls the flow to the deaerator 46 for
maximum heat recovery. Excess water above the capacity of the
deaerator 46 is discharged to the condenser 40 through drain
valve 62. If the flow through drains 60 and 62 is not within
water quality limits, all of the flow is through valve 62 to
the condenser 40 and a polishing system 42.
The return block valve 64 from the flash tank 54 remains
closed until a level is established in the flash tank 54 to
assure that water will not enter the steam attemperator lines
56 and 58. Once a level is es-tablished the block valve 64 is
opened and then the deaerator steam pegging line 66 from the
flash tank 54 can be used to hold pressure in the deaerator 46
(controlled by valve 66). This permits returning all of the
excess flow to the condenser 40 through the drain valve 62
during a hot cleanup without using an auxiliary steam source
for maintaining deaerator pressure and also serves to recover
the heat in the steam during cleanup.
After a predetermined water~steam level is established in
the flash tank 54 the bypass return steam valve 68 is opened
and the dry steam flows to the secondary superheàter 28. Steam
separated in the flash tank 54 in excess of that required is
relieved through the steam relief valve 70 to the condenser 40.
The steam relief valve 70 also acts as an over pressure relief
valve to avoid popping spring loaded safety valves (not shown)
on the flash tank 54. Steam relief valve 70 has an adjustable
set point which can be set to hold flash tank 54 pressure at
desired levels at particular load points during start-up.
The entire bypass system is sized to handle 25% flow
during start-up and to permit up to 25~ load on the flash tank
54. The transition from operation on the flash tank 54 to
straigh through flow is made at approximately 25~ load. As the
steam entering and leaving the flash tank 54 at this time is
dry steam, the .....
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4399
transition from flow through the pressure reducing valve 52
to flow through the steam flo~ valve 30 is accomplished with-
out fluctuations in steam temperature. The transition is
accomplished by opening the superheater stop valve by-pass
valve 31 and the closing of the by-pass valves 52 and 68. As
no other transients are occurring at this time, there are no
changes in steam temperature and pressure.
Above 25% load the steam flows through the steam flow
valve 31 bypassing stop valve 30. The steam flow valve 31
permits variable pressure operation of the superhe~tex section
16 up to full load.
The variable throttle pressure feature is well known in
the art and permits operating the unit with the throttle
valves almost wide open. This feature eliminates the turbine
metal temperature changes resulting from valve throttling and
permits rapid load changes without being limited by turbine
heating-cooling rates. Shutdown with variable throttle
pressure maintains high temperature in the turbine metals
and permits rapid ~estarting.
The start-up system shown in Figure 2 includes provision
for steam attemperation to the secondary superheater and
reheat steam outlet headers, points 57 and 59 respectively,
for positive control of the st~am conditions during start-up
to meet the turbine metal requirements. The superheater out-
let steam attemperator line 56, is used at low loads to
introduce saturated steam from the flash tank 54 to the
superheater outlet header 57. Initial rolling of the turbine
32 may be started with saturated steam through steam attempera-
tor line 56, mixed with a limited quantity of steam passing
through the return bypass steam valve 6~ and the secondary
superheater 28 to control the high pressure turbine 32 inlet
temperature down to about 550F.
The steam return control valve 68 is used to acquire the
necessary pressure drop between the flash tank 54 and the
secondary superheater outlet 57 for attemperation. The
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4399
reheat outlet steam attemperator :Line 58 is used at low
loads to introduce flash tank ste~m to the reheat outlet
header 59. The ratio of flow through the attemperator
line 58 and the high pressure turbine 32 is limited to 1:1,
for turbine consideration mainly to limit windage heating
in the high pressure turbine 32.
Figure 3 is a schematic diagram of a power plant with
a variable pressure once-through vapor generator unit ll.
Included in the unit 11 is a vapor generating section 12
connected in a series flow relationship with a collecting
and separating section 14, a superheating section 16, a
turbine section 18, and a condensing section 20~
The collecting and separating section 14 is composed of
vertical separators 55 and is connected in series flow with
the vapor generation section 12 and the superheating section
16.
The superheating section 16 includes a primary superheater
26 and a secondary superheater 28. A stop valve 30 and a
stop bypass valve 31, in parallel flow with each other, are
flow connected between the two superheaters.
In the vapor generating unit 11 a pressure reducing
valve 52 is connected to the main steam line before the
primary superheater 26. Flow through valve 52 is used as
the source of steam attemperation, as will be discussed later.
The vapor output from the secondary superheater 28 is
adapted to be connected to the turbine section 18 which is
shown to include high pressure turbine 32 and low pressure
turbine 34. An intermediate pressure turbine 33 can be used
if desired and located between the high and lo~ pressure
turbines, 32 and 34 respectively. The reheater 36 is flow
connected between the high pressure turbine 32 and the inter-
mediate pressure turbine 33. The outlet from the low pressure
turbine 34 is f:Low connected to the condenser section 20.
The boiler fesdwater pump 48 supplies the minimum required
flow of feedwater during start-up and low load operation as
required to protect the furnace circuitry. Included in the
i
~39g
.
circuitry for start~up are an economi~er 22, a urnaee
enclosure 24, and a separating vessel 55. The water-steam
mixture leaving the fu~nace is separated in the separating
vessel 55 during start-up and low load operation.
Drain valves 60 and 62 control the liquid drainage rom
the separator 55. Valve 60 contTols the flow to the deaerator
46 for maximum heat recovery. Excess water no~ sent ~o the
deaerator 46 is discharged to the condenser 40 thTough valve
62, which controls the water level in the separator 55. If
the flows through the drains are not within water quality
limits, all of the flow is through Yalve 62 to the condenser
40 and then to polishing system 42.
The dry steam 7eaving the separator 55 flows through the
convection pass to the primary and secondary supeTheaters J
26 and 28 respectively. During the initial phase of start-
up, the flow through the main steam line drain 38 is used
for warm up of the steam lines.
Generally during hot starts, including starts ollowing
overnight shutdowns, the gas temperature leaving the furnace
24 has to be kept high to maintain high main steam and reheat
temperature. This results in too rapid a rise in thro~tle
pressure which is undesirable because of the resulting large
throttling temperature drop when admitting steam to the
turbine 32. By means of the superheater stop valve 30 and
bypass means, the saturation pressure surface can be isolated
from the secondary superheater. The overfiring required to
raise and maintain steam temperatures can be allowed to raise
satura;tion temperature or boiler pressure while maintaining
the desirable low pressure entering ~he high pressure turbine
32 and in the secondary superheater 2~.
When reaching the maximum desired boiler pressure or when
starting up with the superheater stop valve 30 open, the
superheater by-pass system to the condenser 40 (or low pressure
auxiliary steam system) provides the means to con~rol or limit
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boiler pressure durin~ hot start conditions. During the
transient loading period for the unit following a hot restart~
the superheater by-pass valve 70 may be opened to permit higher
firing rates and thus sustain rated steam temperature until -the
boiler control load is reached. Thus the by-pass and control
system has the capability to continuously maintain desired
steam temperatures during the transient time of loading the
unit 11 from start-up to full load following a hot restartO
Before steam is taken to the turbine 32, the reheater 36
is without flow. The reheater metal absorbs heat from the flue
gas and eventually reaches the flue gas temperature whlch can
be as high as 1000F (538C). When steam is first admitted to
the turbine 32 and thereafter passing through the reheater 36,
reheat outlet steam temperature rises very rapidly to the gas
temperature level, resulting in a poor match with turbine metal
temperatures for cold starts.
Reheat steam attemperation with saturated steam from the
separator 55 permits reducing reheat outlet steam temperature
when steam is first admitted to the turbine 32, and offers
positive reheat steam temperature control at low loads. Control
of the saturated steam flow to the reheater outlet attemperator
is provided by line 5~. The ratio of flow through the
attemperator line 58 and the high pressure turbine 32 is
limited to 1:1 for turbine considerations.
For cold starts andstarts following weekend shutdowns,
the requirements of low steam temperature for a temperature
match with the metal component and high heat input for a quick
start-up, are not compatible. With low steam flows, the main
steam temperature approaches the gas temperature in the vicinity
of the secondary superheater outlet 57. Therefore -to keep the
steam temperature low, the gas temperature must be kept low,
however the heat input must be high enough to generate
sufficient steam for rolling and initial loading of the turbine
section 18.
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The superheater outlet steam attemperator 56 utilizing
steam from the separator 55, overcomes this problem by
permitting steam temperature control independent of heat input
to the vapor generating section 12. In order to by-pass
saturated steam from the vapor generating section 12 to the
secondary superheater outlet 57, it is necessary to use stop
valve 30 and stop by-pass valve 31 between the primary and
secondary superheater, 26 and 28 respectively, -to provide flow
resistance and to control flow through the secondary superheater
28.
The superheater outlet steam attemperator line 56 is
used at low loads to introduce saturated steam from the
separator 55 to the superheatex outlet 57 for rolling the
turbine 32. Initial rolling of the turbine 32 may be started
with saturated steam through the superheater outlet steam
attemperator line 56, mixed with a limited quantity of steam
passing through the stop by-pass valve 31 and through the
secondary superheater 28 to control the high pressure turbine
32 inlet temperature to the desired value.
While in accordance with the provisions of the statutes
there is illustrated and described herein a specific embodiment
of the invention and those skilled in the art will understand
that changes may be made in the form of the invention covered
by the claims, and that certain features of the invention may
sometimes be used to advantage without a corresponding use of
the other features.
