Steam Turbine Aux. Steam System
Steam Turbine Aux. Steam System
Steam Turbine Aux. Steam System
Module 2344
AUXILIARY STEAM
SYSTEMS
OBJECTIVES:
After completing this module you will be able to:
4.1
4.2
a)
For each of the two types of the reheat system. explain how the
flow of reheater heating stearn is regulated through the whole
range of turbine load.
~Pages
b)
<=>Page 4
c)
~Page5
d)
~Page5
a)
For each of the two types of the reheat system. describe how
the Donna! drains level is controlled.
~Page6
b)
~Pages
6-7
~Pages
6-8
c)
d)
4.3
a)
i)
il)
il)
il)
3-4
Pages 8-9
Pages 9-10
Page 1
APPROVAL ISSUE
Pages 10-11
b)
i)
Pages 12-13
a)
Page 13
b)
Pages 13-14
c)
Page 14
4.5
water hammer.
Pages 15-16
4.6
Pages 17-18
4.7
a)
Pages 18-19
b)
i)
c)
Pages 19-20
d)
INSTRUCTIONAL TEXT
INTRODUCTION
In this module. the following auxiliary stearn systems are discussed:
In some stations, the name
of this system is slightly
different. Examples: the
turbine Aland steam system, the gland steam system or the gland sealing
system.
Page 2
APPROVAL ISSUE
The previous turbine courses describe the major functions and the layout of
these systems. as well as the functions of their major components. Based
on this general knowledge, this module discusses operation of these systems. In the discussion. emphasis is placed on operational upsets.
Live steam reheat systems l"here boiler steam is the only heat input;
Two-stage reheat systems where two different heat inputs are used: HP
turbine extraction steam in the ftrst stage, and boiler steam in the second
stage.
The first pullout diagram (on page 29) shows both these systems. For simplicity. part a) of this diagram shows steam supply to only one reheater.
Likewise. part b) illustrates only one two-stage rebeater with its steam supply and drainage equipment. The remaining reheaters are equipped identically.
In this module. you wilileam about the following aspects of reheater operation:
Obj. 4.10)
In both types of reheat system. the reheating steam flow changes with
turbine load.
1. In live steam rebeat systems and the second stage or the twostage reheat systems. this happens as follows.
Page 3
APPROVAL ISSUE
valve takes part in this process, ie. all valves in the steam supply piping
stay fully open. Here is how this self-regulation happens.
At any steady load. only as much steam enters the reheater as condenses
inside the tubes. The rate of condensation depends on. among other .
factors, the turhine steam flow rate. In the extreme case, where no
steam flows through the turbine, the rate of condensation is, in principle, zero and hence no reheating steam is taken. When the turbine load
increases, so does the rate of heat transfer through the reheater tubes because more turbine steam flows through this heat exchanger. If the rate
of condensation exceeds the flow of incoming reheating steam. the pressure inside the reheater tubes drops. As a result, more steam is drawn
through the reheater steam supply piping until a new equilibrium is established.
.. Pressure losses in the
piping are only about
3-5% of boiler pressure
at full power, and less
at partial loads.
The pressure drop that is necessary to increase the reheating steam flow
is very small" because the reheater steam piping has a very small resistance to flow. This is achieved by proper sizing of the piping such that
steam velocity is kept reasonably low.
The opposite changes in the reheating steam flow occur when the turbine load decreases.
Obj. 4.1 b)
Typically, below
20-30% FP.
Page 4
APPROVAL ISSUE
Because the control valve position is linked to turbine load - whose rate
of changing is limited during turbine startup and power manoeuvres the reheaters are valved In/out gradually.
Obj. 4.1 c)
Obj. 4.1 d)
Typically, the reheating steam flow is isolated during turbine startup and
operation at light turbine loads, throttled at medium turbine loads, and
self-regulating at high loads.
During turbine startup and at light loads, reheating must be limited in order to prevent overheating of the LP turbine exhaust
Exceeding the limit on slde-to-side I1T at the LP turbine inlet can result
in large thermal deformations of the LP turbine casing. The deformations can cause rubbing in the turbine, as well as increased vibration.
Page 5
APPROVAL ISSUE
Normal control
In both types of reheat system. condensate of the reheating steam is collected in one or more drain tanks. In live steam reheat systems (see Fig.
4.5 a) on page 29), there is only one tank which is shared by both reheaters.
Their drains are pumped to the boilers, and the drains level In the tank is
norrnaily maintained by a control valve whiclt adjusts the drains now to
the boliers. When the level rises, the valve opens more, increasing the
outflow from the tank. A recirculatinn line back to the tank is provided to
prevent overheating of the drains pump due to too small a flow. The recircuiation line operates when the dntios flow to the boiler is below a certain limit. This happens when the control valve opening is small in response to a
low level in the drains tank.
Note in Fig 4.5 a) that some water is supplied from the discharge of the
boiler feed pumps to the suction of the reheater drains pumps. The purpose
of this water - whose temperature is well below the drains temperature - is
to subeool the dntios, thereby preventing pump cavitationlvapourlocking.
If not isolated when necessary. this water may. however, flood the reheat-
ers and their steam piping. Such an incident has happened in a CANDU
unit.
In two-stage reheat systems (see Fig. 4.5 b) on page 28), separate
drain tanks are used for each stage because of their different operating pressures and temperatures. Typically, each individual reheater has its own set
of two drain tanks. The ftrst stage drains cascade to the HP feedheaters.
whereas the second stage drains are pumped to the boilers. The drains levels are normally maintained in the same way as described above. ie. by
adjusting the drains outflow.
Obi. 4.2 b)
Obi. 4.2 c)
Page 6
APPROVAL ISSUE
-..I:i.
...l:1
LeV
Opono
Applie~
only to some
stations .
------------~~~~~~--LCV
CIooea
-v Low LevelAlarm
-v
Even when the reheater tubes are still not flooded, the typical protective
action on too high a drains level is to dump the drains to the condenser.
Usually, this action can quickly restore the nonnalleve!. However,
dumping hot drains to the condenser reduces the overall thennal efficiency which can be of concern when such operation is continued.
2. Flooding of the reheater tubes would result in a partial or total loss
a) Large thermal stresses in the piping if the drains are much cooler than the piping.
Note that attemperating water temperature' is far below the reheat
stearn piping temperature. When this water is allowed to reach the
12S-1S0'C, depending.
on the station.
Page 7
APPROVAL ISSUE
Obj. 4.2 d)
a) Control or mechartical problems with the level control valve resulting in too small opening of the valve;
b) Tripping of the rehealer drains pump combined with failure of the
standby pump (if any) to start up.
Page 8
APPROVAL ISSUE
When reheater drains level is abnormally high, an alarm is given and the
dump valve (if any) to the condenser opens. The standby drains pump
(if applicable) starts up as well. In some stations, the steam supply is
automatically isolated when the drains level reaches a very high limit
Too Iowa reheater drains level gives an alarm. When applicable, the
drains pump trips as well.
Too high a reheater drains level can result in the following adverse consequences, listed in the order of rising level. First, the overall thermal
efficiency is reduced when hot drains are dumped to the condenser.
Second, flooding of the reheater tubes results in a partial or total loss of
reheat - with all attendant consequences. Finally, flooding of the reheater steam piping can damage the piping due to water hammer or
quenching. Water induction to the HP turbine can also occur.
Too Iowa reheater drains level can cause reheater drains pump cavitationlvapourlocking. The overall thermal efficiency would be reduced if
hot drains were dumped to the condenser due to the dump valve stuck
open. In the extreme case, water hammer in the drains piping can result
if the level has dropped enough to cause steam to drive slugs of water
through the piping.
'>
Pages 21-23
'>
Obj. 4.3 a)
LOSS OF REHEAT
Page 9
APPROVAL ISSUE
the turbine, ego when only ooe reheater is experiencing tube flooding. In such a case, an excessive side-to-side dT is produced at the
LP turbine inleL The resultant thenna! deformation of the turbine
casing can cause high turbine vibration and possible blade and!
or seal rubbing.
b) The affected reheater(s).
A rapid loss of the reheating steam subjects the reheater tubes to fast
cooling by the turbine steam. The resultant thermal stresses, if repeated a sufficient number of times. can eventually cause a reheater
tube or gasket failure.
... In the reactor leading mode,
BPe raises the setpoint to
the turbine governing system. The GYs open more.
If they can accommodate the
extra flow, the MW output
increases. But if they cannot, the small SRVs open,
forcing a manual reduction
in reactor power to conserve
makeup water. This action,
combined with decreased LP
turbine effICiency, may reduce the generator MW output somewhat.
APPROVAL ISS1JE
Mitigating actions
Some of the above consequences and concerns can be minimized if the operator takes proper actions. First, thermal stresses in the LP turbine can be
kept at a safe level if the side-to-side aT at the turbine inlet is within its limit. To ensore this, loss of reheat on one side of the turbine must be
accompanied by vaIvlng out a proper number of reheater tube bundIes on the other side of the turbine.
Second, a snbstuntIaI loss of reheat should be followed by a proper reducllon In turbine load as specified in the appropriate opetating manual. Operation at full load can be continued only when absolutely necessary (to supply the grid load at the time when other sources of generation are
unavailable), and then only over a limited period of time (usually, up to 12
hours).
When the torbine is unloaded, following a substantialloas of reheat, the excessive steam wetness In the LP turbine Is reduced back to the acceptable level. This effect is shown in Fig. 4.2, where sample values of
steam preasore, temperatore and wetneas are plotted, and torbine unloading
is aasorned to reduce load to about 60% W*.
2SO'C
SpIc.1c EnII'OPY
Fig. 4.2. Effects of 1088 at reheat on simplified turbine ....m expansion line:
Operation at full poNef wtth fuY reheat available;
- - - - Operation at full power with reheat capacity SUbstantially reduced;
- - Operatlon at partial load with the same reduction In the reheat capacity.
Page
i1
APPRUVAL 1:>:>U.I
Notice in Fig. 4.2 that turbine unloading is effective in reducing the LP turbine exhaust steam wetness for two reasons:
1. Reduced LP turbine inlet pressure (and hence, the pressure ratio) causes
the turbine to extract less heat from the steam;
2. In the case of partial loss of reheat. the remaining reheat capacity can superheat the reduced flow of turbine steam to a higher temperature. Naturally, this effeCt does not apply to the total loss of reheat
The leak. if large enough, can raise the HP turbine exbaust pressure
and iower the LP turbine Inlet steam temperature. The latter effect perhaps a bit surprising - stems from the throttling process that the leaking
steam undergoes. During this process. steam temperature drops substantially: from about 2S0-25S'C (assuming that the leaking reheater is supplied
with boiler steam) to about 170-180'C, depending on the station. Note that
the latter temperature is well helow the normal LP turbine inlet steam temperature (22S-240'C at full power, dependi,ng on the station). This is why
the leak tends to lower this temperat)lre. Of course, the leak rate must be
large enough for this temperature reduction to be measurable.
This brings us to adverse consequences/operating concerns caused
by a large Internal leak:
1. Reduced steam temperature on one side of the LP turbine (it is
assumed here that a large tube leak appears only in one tube bundle at a
time) may result In an excessive side-tn-slde ~T at the turbine inlet.
You will recall that the ~Tlimit would force valving out a similar tube
bundle on the other side of the turbine in order to prevent damage due to
excessive deformations of the turbine casing.
2. Increased moisture content of the LP turbine steam due to reduced Inlet temperature.
Accelerated erosion and corrosion, reduced overall thennal efficiency.
and increased overspeed potential result from it
Page 12
APPROVAL ISSUE
components.
b) Reduced overall thermal efficiency and loss of generator output due
to the following automatic protective actions which intend to
prevent overpressure of the moisture separators. reheaters and interconnecting pIping:
Opening of the release valves (if there are any);
Operator actions
While a small tube leak creates no acute problem, a large leak requires the
following operator actions to prevent further equipment damage:
I. Identification and Isolation of the leaking lube bundle.
Note that in some stations equipped with a two-stage reheat system, isolation of the frrst stage bundle may force valving out the second stage.
This action may be necessary to prevent excessive thennal stresses in
the second stage of the reheater.
2. Isolation of another tube bundle(s) on the other side of the turbine.
This action may be necessary to prevent an excessive side-to-side AT at
APPROVAL ISSUE
This precaution is taken because some drains might enter the steam piping during the drains level excursion. To remove this water, drain
valves in the piping must be open for a sufficiently long period of time.
You will recall that prevention of a very low drains level (such that steam
could enter the drains dump pipmg to the condenser and drive slugs of water) is also important to prevent water hammer in the reheat system.
Page 14
APPROVAL ISSUE
A loss of reheat increases thermal stresses in the LP turbine and the affected reheater(s), raises the steam wetness in the LP turbine and disturbs boiler pressure and level control.
To prevent equipment damage in the event of a large interoaI.leak in a reheater, the leaking bundle, as well as another tube bundle on the other
side of the turbine, must be isolated. The turbine may have to be unloaded, depending on the extent of the loss of reheat
~Obj.
Pages 2325
4.6
Most of the information about this system is provided in the previous turbine courses. The only topic that is left over is the use of attemperating
sprays. Fig. 4.3 on the next page shows the part of the system where the
sprays are installed, whereas the whole system is shown in a pullout diagram at the module end (Fig. 4.6 on page 29).
Page 15
APPROVAL ISSUE
t"i:l atmosphere
... Other names of this equipment, that are used in
some stations, are listed
on page 29 (Fig. 4.6).
GLANO*
EXHAUST
FANS
\..J
10 LP feedhealers .....
~ __ ~
_~-J.-
.,,/''''-
GLAND EXHAUST
CONDENSER
-------1"
ATTEMPEAATING SPRAYS **
!
*
, ..../...........
L.
.~_
From CEP
discharge
... In the units where no attemperating sprays are fitted, the gland exhaust
condenser has an overflow
sized to handle the con
densate from a tube leak.
Therefore, the gland exhaust condenser with a
tube leak does not have to
be valved out, and operation can be continued.
Note that the greatly increased flow rate would exceed the capacity of
the fans, causing their suction pressure to rise. As a result. the pressure
in the gland exhaust manifold would rise enough to cause steam outleakage from the glaild seals connected to this manifold-. Recall that the
manifold pressure must be maintained a few kPa below abnospheric in
order for the gland seals to function properly.
3. Possible steam hammer in the condensate system.
Page 16
The flow of hot steam through the gland exhaust condenser, combined
with a loss of condensate flow. would cause the condensate inside the
tubes to boil. The relief valve installed on the condensate line should
open, preventing overpressure of the tubes. But the steam pockets
formed inside the tubes would implode when the condensate flow is restored. The resultant collisions of water columns previously separated
by the pockets would produce steam hammer-.
APPROVAL ISSUE
Failure to do this would result in steam flowing through the gland exhaust fans which normally handle air only. The fans could suffer damage due to overheating. In addition, their suction pressure would rise
because the actual flow would greatly exceed their flow capacity. As a
result, steam egress from the turbine and steam valve gland seals would
occur. Finally, water hammer in the condensate system could occur due
to implosion of the steam pockets created (due to beat input from the
gland leakoff steam) inside the gland exhaust condenser tubes.
Possible LP turbine damage due to overheating of its exhaust if the system failed to provide adequate cooling;
Operating parameters that should be monitored to prevent such damage;
Operator actions that should be taken wben the turbine exhaust beating
is excessive;
Operating concern caused by excessive use of the LP turbine exhaust
hood cooling sprays.
For easy reference. the system is shown in a pullout diagram (Fig. 4.7) on
page 29.
You will recall that prolonged motoring" or operation at very light load promotes overheating of the LP turbine exhaust During these operating conditions, large windage losses occur in the turbioe last stage(s). and the small
stearn flow cannot provide adequate cooling. As a result, the moving blades
and (to a smaller extent) the shaft, diaphragms, casing and exhaust cover
become hotter. The ttansient heating produces increased thermal sttesses.
It also results in reduced radial and axial ciearances in the turbine due to the
rotating and statiooary components expanding at different rates.
Motoring is discussed in
detail in module 23413.
Db}. 4.7 a)
If proper condenser vacuum is maintained and the LP turbine exhaust cooling system operates satisfactorily, LP turbine exhaust temperature - while
elevated as.compared with normal operation - stays at a safe level. Otherwise, overheating of the LP turbine exhaust may develop, causiog the following adverse consequences/operating concerns:
Page 17
APPROVAL ISSUE
Monitored parameters
To prevent turbine damage, the following paramelers must be carefully
monitored:
I. LP turbine exhaust temperatures.
Several temperature sensors are installed in the six LP turbine exhausts.
The indicated temperatures should be checked against the operating limits (as specified in the appropriate operating manual) to make sure the
turbine trip limit bas not been exceeded and that the cooling waler sprays
in the LP turbine exhaust hood operate properly.
2. LP turbine bearing vibrations.
They are monitored to ensure that hearing of the LP turbine exhaust has
not resulted in excessive bearing misalignment and/or internal rubbing in
the turbine.
3. LP turbine axial differential expansions.
These parameters (typically, one for each LP turbine) are monitored to
make sure that the axial clearances in the turbine have not been reduced
excessively.
4. Condenser vacuum.
APPROVAL ISSUE
status of the isolating valves (should be open) and the pressure drop across
the strainer (should not be excessively high). During motoring, similar
checks should be made to ensure proper supply of motoring cooling
steam and Its attemperatlng sprays (if installed).
Operator actions
Obj. 4.7 c)
Obj. 4.7 d)
If the LP turbine exhaust overheating has become excessive (as indicated by some of the monitored parameters), prevention of turbine damage
requires the opemtor to take either of the fonowing actions:
2. Trip the turbine (if loading is impossibie. ego due to reactor problems).
BLADES
MOVING
BLADES
ROTOR
APPROVAL ISSUE
Note in Fig. 4.4 that the recirculating steam enters the moving blades at their
trailing edge close to the blade root The stearn carries the sprayed water
droplets that have not been fuHy evaporated. Collisions between these
droplets ood the trailing edge of the blades eventually cause blade erosion.
To minimize this erosion. it is important not to use the sprays when they are
not necessary for turbine protection from overheating.
If ooy safe limit has been reached. turbine load sbould be slowly increased. When loading is impossible. the turbine should be tripped in
order to prevent damage.
Pag 2527 ~
Page 20
Excessive use of the LP turbine exhaust cooling sprays can result in erosion of the traliing edge of the moving blades in the last stage.
APPROVAL ISSUE
ASSIGNMENT
I.
a)
b)
i)
Ii)
iii)
bine load.
2.
a)
b)
3.
Valving out the reheat during turbine startup and operation at light
loads (does I does not) result in excessive steam webless in the
LP turbine.
b)
c)
4.
5.
Page 21
APPROVAL ISSUE
6.
The following actions (other than the nonnal control) are canied out in
response to:
a)
iv)
b)
7.
a)
Even when the reheater tubes are still not flooded, too high a
drains level reduces the overall thennal efficiency due to
b)
c)
i)
ti)
iii)
8.
Too Iowa .reheater drains level can cause the following adverse conse~
quencesloperating concerns:
a)
b)
9.
a)
Page 22
APPROVAL ISSUE
il)
b)
il)
10. a)
Valving out some or all of the rebeatertube bundles while operating at a high load can cause the following adverse consequences!
operating concerns:
i)
il)
iii)
b)
il)
11. a)
b)
A large leak can (decrease I increase) the temperature of the superheated steam supplied to the LP turbines.
c)
Page 23
APPROVAL ISSUE
d)
12. a)
i)
il)
b)
il)
iii)
iv)
c)
When a large reheater internal leak results in increased HP turbine exhaust pressure, the following operating concerns arise:
i)
il)
iii)
Page 24
APPROVAL ISSUE
d)
A large rehealer intemalleak requires the following operator actions to prevent further equipment damage:
i)
til
iii)
13. Water hammer in the reheat system is prevented by the following general operating practices:
a)
b)
14. a)
b)
til
iii)
15. a)
Overhealing of the LP turbine exhaust is promoted during the following turbine operating states:
i)
til
Page 25
APPROVAL ISSUE
b)
il)
16. a)
il)
iii)
Parameter.
Parameter:
Parameter:
Reason Why it is monitored:
Page 26
iv)
_
_
APPROVAL ISSUE
b)
c)
ti)
17. The major operating concern caused by excessive use of the LP turbine
exhaust cooling sprays is
Before you move on to the next module, review the objectlves and make
sure that you can meet their requirements.
Prepared by:
J. Jung, ENTD
Revised by:
1. Jung, ENID
Revision date:
May, 1994
Page 27
APPROVAL ISSUE
...., ,k1....-_
Live st611,~--1t>lq.-
REHEAT
ISOLATING
VALVE
REHEAT *
CONTROL
VALVE
REHEATER
From
moisture - -
separato~
ToLP
turbines
Drains from
other rehealer
;;p
~=--~-
.0'
.'."
From boiler
foodWaler pump
discharge
~
:.
.,'
=
=
OR~T~~tT~~(S)
,.f;
_._._._..~~:?~F~..JL..,!f:"";.;,:;;;;;;::
f
l
....
10 boilers
REHEATER DRAINS
LEVEL CONTROL VALVE(S)
To condenser
REHEATER DRAINS *
DUMP VALVE
..j.
p
Live sleam
AEHE.od
ISOLATli'll3
HP turbine
extraclio
steam
"
~"
ISZC()fld -'''II''
V
REHEAT
",m""
.P-"
REHEAT
STOPCHECI(
~'"
REHEATER
IA
Firsts!..;
t
I
I
s8plvator in Ina
same \/essel
,;:..~...
o~"
DUMP
REHE.oJER
FIRST
O~"
~~,
DUMP
VALVE
LEVEL
,~
~'"
To condenser
-.
'....~"i".
l.............. o-
~"'
EHEIUER SECON : \ . .
STAGE DRAINS TANK
@ ..
- -
10 HP feedheaters
,..-QTo condense,
~""""
""
DAAlNS
PUMP
REHE."JER
SECONO sto.GE
9- LEVELVALVE
CONTROl.
Tooolers
APPROVAL ISSUE
.. ofl'OfP/'lO'"
'-
FromollomOltOOllf'l>Ot 1)
ol ''90111am
i PF\V
STJV,INEA
TURBINE
S~"IA
VAIYES
LP TURBINES
At
t" Iln'IOCpIItrll
.lrTEMPEPATING SPRAYS 1)
GLANDEXIi,o.USTFANS2)h
\.J - - - ,
llLPf.._.....
"'",
./'.........
P_
...... '
~
~,,,
GLAND EXHAUST 3)
CONDENSER
:...- _ . - ~"" 10
F"""CEP
di~_'9'
mo,n condon..,
3)
!uItJ.'M QJRtId 8J8wn VlIf'OiH ~r,idots. (ff gI.Md sleam ax/Wtst foWs.
A few otller n!ll!ll!l& III" <1180 in U!JJ9, ""mp/lls: the gland sream
1)
STRAINER
~
CEP
dlachatQll
OJ
MOTORING STEAM
ISOLATINQ VALVE
;-~-_i}-----,
:
I
I
PRESSURE"
f'.EOUClOO.
ESV
GV
Notina!!stmW/18.
OSPPtAY
.""""
LP tUrb
RING
To olher
L?turolnlljl
EXHAUST
L_
.. --- --_ .. 1
I
I
To .m
OR\F~E
'rom
.eheaters
'0'"
51'1'....'1'$
LPTURBINE
"
I"
"
N0t9:
In soms sfBtlons. motoring cooling steam;s not used at all or is
SIJPP/isd to (fie HP turbine, bypassing m@cfos/JdGItS. In the latter
case. the steam follows the normal fIr:Nv path tllrough tfl9 tur/)ifle set.
Page 29