Table of ContentsTurbine Gland Seal Steam System
(TSS)21Functions22Design Bases22.1Safety Design Bases22.2Power
Generation Design Bases22.2.1Power Generation Design Basis
One22.2.2Power Generation Design Basis Two22.2.3Power Generation
Design Basis Three23Description23.1General Description23.2Pressure
System53.3Suction System54System Operation64.1Labyrinths94.2Rubbing
in Turbine Seals104.3Abnormal Pressure of gland sealing
steam115Instrumentation Application135.1Gland Steam Condenser
Exhausters135.1.1Pressure135.1.2Level135.1.3Effluent
Monitoring135.2Sealing Steam Header136Troubleshooting And Failure
Modes146.1Causes of System Failure147Safety Issues157.1Personnel
Safety Issues157.2Equipment Safety Issues15
Turbine Gland Seal Steam System (TSS)1 FunctionsThe Turbine
Gland Sealing System (TSS) prevents the escape of steam/radioactive
steam from the turbine shaft/casing penetrations and valve stems
and prevents air in-leakage through sub-atmospheric turbine
glands.2 Design Bases2.1 Safety Design BasesThe TSS does not serve
or support any safety function and has no safety design bases.
2.2 Power Generation Design Bases2.2.1 Power Generation Design
Basis OneThe TGSS is designed to prevent atmospheric air leakage
into the turbine casings and to prevent radioactive steam leakage
out of the casings of the turbine-generator.2.2.2 Power Generation
Design Basis TwoThe TGSS returns the condensed steam to the
condenser and exhausts the non-condensable gases, via the Turbine
Building compartment exhaust system, to the plant vent.2.2.3 Power
Generation Design Basis ThreeThe TGSS has enough capacity to handle
steam and air flows resulting from twice the normal packing
clearances.
3 Description3.1 General DescriptionThe turbine gland seal
system consists of a sealing steam pressure regulator, sealing
steam header, a gland steam condenser, with two full-capacity
exhauster blowers, and the associated piping, valves and
instrumentation.The steam seal system is designed to do the
following: Seal the shaft where it penetrates the turbine casing
Seal and isolate each turbine element from atmospheric conditions
in order to optimize its thermodynamic performance Seal feedwater
turbine shafts where they penetrate the casing Seal associated
turbine valve stems on both the main and feedwater turbines
The seal forces the escaping steam or the incoming air to travel
through a long and torturous path, which increases drag and thus
reduces leakage. In analytical terms, steam passing through a
labyrinth seal is throttled with a resultant pressure loss as it
passes through a consecutive series of restrictions.As an
additional improvement, the labyrinth seal is split into two
sections and a suction chamber is introduced between the two seal
sections, as shown in Figure.
Fig: Labyrinth Seal with Suction Chamber
The suction chamber, where the pressure is maintained at
approximately 12 psia, or -2.3 psi vacuum, draws the high-pressure
steam leaking through the labyrinth path. Some system designs
maintain the suction chamber at about 10 inches wg below
atmospheric pressure. This pressure is set during initial unit
startup to a level that just prevents steam leakage from the shaft
ends. The pressure is set by adjusting the butterfly valve controls
on the gland seal condenser exhauster fan.If the pressure is set
too low, steam leaks into the atmosphere and possibly into the
bearing pedestals; if it is set too high, oil vapor and
non-condensables can be drawn into the glands and into the
condenser.As a further improvement, a three-piece seal is used
throughout the LP turbine unit. In this design, in addition to the
suction chamber, a pressure chamber is added to the steam seal
system (gland seal system), as shown in Figure
Labyrinth Seal with Suction and Pressure Chamber in an LP
Turbine
Steam at controlled pressure is supplied to the pressure chamber
located between the suction chamber and the turbine exhaust.
Consequently, steam from the inboard pressure chamber, where the
pressure is set between 2 psi and 5 psi above atmospheric, leaks
toward the suction chamber and to some extent, toward the
condenser. This steam is subsequently drawn into the suction
chamber, as shown in Figure above. Any air that enters from outside
is also drawn into the suction chamber. The pressure in the
pressure chamber should be as low as practicable, consistent with
preventing air leakage into the condenser and minimizing steam
leakage into the condenser.
Fig: Labyrinth Seal with Suction and Pressure Chamber in an HP
Turbine at Full Load
Above figure illustrates the operation of the three-segment seal
in an HP or IP turbine under full load condition. As soon as the
pressure inside the turbine exceeds the pressure in the pressure
chamber, steam is drawn out of the turbine and into the pressure
chamber. Most steam leakage will be drawn from the high-pressure
zone into the pressure chamber.However, some steam will continue to
leak past the middle section and into the suction chamber, where it
will be removed along with the air leaking from outside.
Fig: Sectional View of the Rotor Shaft Area with Gland Seals
ExposedThe gland steam supply often features two stages: a primary
steam supply station takes the main steam from ahead of the turbine
stop or throttle valves, and a number of additional supply stations
augment the primary pressure control station. In addition, only the
HP turbine has the spillover station. Further, the high-pressure
leakoff from the main steam valve leads into the HP turbine steam
seal system and the pressure header.
3.2 Pressure SystemThe supply stations draw steam from several
sources. High steam pressure is usually drawn directly from the
boiler, before the turbine stop valves, during a startup. To
improve plant performance during normal operation, the cold reheat
station typically supplies steam to the header. Under normal
operating conditions, steam is supplied to the steam header through
open regulating valves at the supply stations. However, if the
pressure at the header increases above the normal level, the
normally closed pressure-sensing regulating valve in the spillover
station will open. This will allow the excess steam to escape to
the condenser or to a heater. As a safety precaution, the supply
header can also feature a safety relief valve. A rupture disk on
the supply header serves as a final safety mechanism.
3.3 Suction SystemThe suction header supplies a small vacuum for
the steam seal system. It does so by delivering the mixture of
steam and air leakage from the gland seals to the gland seal
condenser. The condenser for the steam seal system is designed to
maintain pressure below atmospheric pressure in the suction
header.The vacuum is maintained by the action of exhauster fans,
and varying the opening of the butterfly valve controls the actual
vacuum level. As the steam passes over the condenser pipes, the
steam condenses to water that in turn is returned to the main
condenser hotwell. The condensate from the main condenser system
serves as cooling water. The air that passes through the steam seal
system condenser is released to the atmosphere by the exhauster
fans.4 System OperationIn general, the steam seal system must be
operational in order to start the unit. Vacuum must be established
inside the turbine casings before the unit can be started. This is
normally accomplished on turning gear operation while water
circulation is initiated through the main condenser. The condensate
pump is started concurrently to push cooling water into the steam
seal system condenser. Subsequently, steam is diverted through the
steam supply system into the gland areas while the
high-pressure-regulating valve maintains adequate header pressure
of ----MPa. TSS is supplied steam from the main steam header in the
start and with the increase in power, steam leak off starts from
the HP turbine to the LP turbine. This will cause the TSS pressure
controller to close on sensing the increasing pressure downstream.
During pre-start, both the cold reheat and the
high-pressure-regulating valves are wide open, whereas the swing
check valve in the cold reheat station prevents reverse steam flow
from the header.As the pressure increases in the header, the
exhauster fans in the steam seal system condenser are turned on
manually to prevent steam leakage to the atmosphere. Thus, vacuum
is established in all turbine elements. When vacuum has been
established in the main condenser, the rotor can be accelerated to
the rated speed as per turbine manufacturer instructions by
following the curve. With increase in load and cold reheat
pressure, the swing check valve in the cold reheat station is
opened, allowing cold reheat steam into the header. The high
pressure-regulating valve closes at a predetermined pressure
----MPa. Some turbine designs use a single integrated controller
that controls the main steam supply, the cold reheat supply, and
the spillover valves. This integrated controller provides for a
progressive change-over from one to another rather than at a
predetermined pressure. Drawing steam from the HP turbine exhaust
is more economical. With further load increase, the steam pressure
inside the HP turbine exceeds the pressure chamber and the steam
supply header. Consequently, steam will start leaking from the
turbine through the inner gland seal segment into the pressure
chamber and, eventually into the steam supply header. This is
referred to as a self-sealing condition and is usually reached
below 25% of rated load. At this point, steam from the HP turbine,
combined with the steam from the cold reheat station, supplies the
LP gland seal pressure chamber.As the load and the pressure inside
the HP turbine increase even further, the steam flow from the HP
turbine into the steam supply header also increases. Typically, the
temperature difference between the sealing steam and the turbine
rotor in the HP/RHT-IP gland area should be less than 200F. At some
predetermined pressure, or if an integrated controller is
controlling, the cold reheat regulating valve will close. If the
pressure increases even further, the spillover regulating valve
will open at apressure of ------MPa, venting some steam to the main
condenser. This usually happens at approximately one-half load.
Some manufacturers provide a single diverting valve that shuts the
cold reheat steam supply and redirects the excess steam to the
extraction heater or the condenser. (From a cycle efficiency
standpoint, it is best to discharge this steam to the extraction
line.)The steam seal system should be in operation at all times
when: There is vacuum in the main condenser. Turbine temperature is
above the ambient temperature. Either the turning gear or steam is
rotating the turbine. This should apply only if the unit is being
prepared for startup. If the turbine has been shut down and is
scheduled for maintenance, it can remain on gear without the steam
seal system in service.
The steam seal system should not be in service when the turbine
shaft is stationary.
The temperature difference between the sealing steam and rotor
surface can vary under different operating conditions. A large
temperature difference will cause thermal cracking. Thus, to
protect against rotor damage in gland seal areas, the difference
between sealing steam temperature and the rotor surface should be
kept to a minimum during startup and shutdown. To provide an
assessment of potential damage, some manufacturers supply a curve
showing a number of cycles that will initiate thermal cracking for
a given temperature difference.Sealing steam should not be applied
to a turbine shaft for a long period of time when the unit is
stationary. This is because in addition to thermal cracking, the
hot steam can also result in unacceptable thermal expansion and/or
bowing of the shaft. A bowed rotor is eccentric and, in general,
cannot be safely operated. Temporary or elastic rotor deformation
can result from a short exposure of the rotor to steam at a
temperature that exceeds normal operating temperature.Elastic
deformation/bow can be corrected by turning the rotor on a turning
gear until the bowing is rolled out and the rotor is straight.The
annular space through which the turbine shaft penetrates the casing
is sealed by steam supplied to the shaft seals. Where the gland
seals operate against positive pressure, the sealing steam acts as
a buffer and flows either inwards for collection at an intermediate
leakoff point or, outwards and into the vent annulus. Where the
gland seals operate against vacuum, the sealing steam either is
drawn into the casing or leaks outward to a vent annulus. At all
gland seals, the vent annulus is maintained at a slight vacuum and
also receives air in-leakage from the outside. From each vent
annulus, the air-steam mixture is drawn to the gland steam
condenser.The seal steam header pressure is regulated automatically
at ----MPa by a pressure controller. During startup and low load
operation, the seal steam is supplied from the main steam line or
auxiliary steam header. At all loads, gland sealing can be achieved
using auxiliary steam so that plant power operation can be
maintained without appreciable radioactivity releases even if
highly abnormal levels of radioactive contaminants are present in
the process steam, due to unanticipated fuel failure in the
reactor.The outer portion of all glands of the turbine and main
steam valves is connected to the gland steam condenser, which is
maintained at a slight vacuum by the exhauster blower. During plant
operation, the gland steam condenser and one of the two installed
100% capacity motor-driven blowers are in operation. The exhauster
blower to the Turbine Building compartment exhaust system effluent
stream is continuously monitored prior to being discharged. The
gland steam condenser is cooled by main condensate flow.
The TSS is designed to prevent leakage of radioactive steam from
the main turbine shaft glands and the valve stems. The
high-pressure turbine shaft seals must accommodate a range of
turbine shell pressure from full vacuum to approximately 1.52 MPa.
The low-pressure turbine shaft seals operate against a vacuum at
all times.The gland seal outer portion steam/air mixture is
exhausted to the gland steam condenser via the seal vent annulus
(i.e., end glands), which is maintained at a slight vacuum. The
radioactive content of the sealing steam, which eventually exhausts
to the plant vent and the atmosphere, makes a negligible
contribution to overall plant radiation release. In addition, the
auxiliary steam system is designed to provide a 100% backup to the
normal gland seal process steam supply. A full capacity gland steam
condenser is provided and equipped with two 100% capacity
blowers.Relief valves on the seal steam header prevent excessive
seal steam pressure. The valves discharge to the condenser shell.At
high and medium loads, the HP turbine exhaust pressure is well
above atmospheric so that air ingress into the turbine is of no
concern. Instead, high pressure steam egress (out) into the turbine
hall must be prevented. But at very light loads, during turbine
startups, as well as those rundowns and shutdowns when condenser
vacuum is still maintained, the HP turbine exhaust pressure drops
below atmospheric and the seal must then prevent air ingress.When
the turbine operates at high and medium loads, the seal is self
sealing (i.e. it does not need any sealing steam) and its leak-off
is typically utilized as sealing steam in LP turbine gland seals.
Whenever turbine load drops below a certain level, this seal
requires sealing steam to prevent air ingress. The pressure
distribution and steam/air flow paths in the seal then become
similar to those in the LP turbine glands.
The steam seal system is designed to prevent steam from leaking
into the atmosphere from the HP or IP turbine and to prevent air
from leaking into the steam path in the LP turbine. Without a
system for sealing the rotor shaft, where the shaft penetrates the
cylinder, high-pressure steam would escape from the HP and IP
turbines, and air would leak into the LP turbine. Both conditions
would be unacceptable because of the potential for: Inability to
start the turbine-generator unit (condenser vacuum cannot be
attained) Decreased thermal efficiency Loss of condenser vacuum
during operation (which can cause blade vibration) Overheating due
to increased windage caused by the loss of vacuum Bow of the
turbine shaft caused by large temperature differential
Vibration/rub due to excessive rotor axial elongation caused by hot
steam leak Damage of sealing and/or bearing surfaces caused by dust
or debris Contamination in the condensate system Steam or water in
the turbine bearing oil or pedestal Contamination in the
lubrication system High levels of non-condensable gases in the
condensate system Uncontrolled radiation risk at the BWR
turbine
It would not be possible to establish an adequate condenser
vacuum without a steam seal system and, therefore, it would not be
possible to start a large turbine. Interlocks and computer logic
incorporated in the power plant design would prevent starting the
turbine. Eliminating leakage of air into the LP turbine from
outside the LP turbine is necessary because the LP turbine exhausts
at the end of the LP shell to a condenser that operates under a
vacuum. Eliminating air leakage into the HP and IP turbines during
startup and low-load operations is essential to prevent
non-condensables from entering the steam path.4.1 Labyrinths Sets
of labyrinth packing are employed along the turbine rotor where the
rotor exits the turbine casing to maintain this pressure
differential.a. The labyrinths create many little chambers causing
pressure drops along the shaft. The number of labyrinth sets
depends greatly on the steam pressure possible in that area.
Labyrinth packing alone will neither stop the flow of steam from
the turbine nor prevent air flow into the turbine.
4.2 Rubbing in Turbine SealsAll turbine seals, both external
& internal are designed with the clearances between the
rotating and the stationary parts as small as possible. While this
minimizes steam leakage and hence keeps the turbine efficiency
high, it also promotes rubbing.Rubbing is the major operational
problems with all turbine seals because their small clearances can
be closed up relatively easily. This usually happens due to high
vibration or excessive thermal or mechanical deformations of the
turbine casing and/or rotor.Seal rubbing is particularly likely
during the initial startups of the turbine. This is because some
manufacturers make the radial clearances within turbine seals
slightly too tight in order to avoid undue leakage losses caused by
excessive clearances. Thus, during a few initial startups, some
mild rubbing is expected to increase clearances to their proper
values. Needless to say, these startups must be carried out
extremely carefully.All modern turbine seals are designed to
accommodate light rubbing without damage to the turbine. For
example, the stationary parts of gland seals and diaphragm seals
are flexibly supported and they are made of soft materials (e.g.
lead bronze) to prevent/minimize damage to the shaft. Similarly,
the tips of free standing (i.e. shroudless) moving blades are
thinned so that rubbing can wear them out without bending or
breaking the blades themselves. While these measures are effective
against light rubbing, they cannot protect the turbine in case of
more intensive rubbing. Following are the adverse
consequences/operating concerns caused by such rubbing in order of
severity:1. Damage to the rubbing seals:Damage to turbine seals
results in increased clearances between the fixed and moving
components. Therefore, turbine efficiency is permanently reduced
due to increased steam leakage through the damaged seals. This
adverse consequence applies to both internal and external seals.
But severe damage to an external seal has additional consequences,
which are the same as those caused by insufficient sealing steam
pressure.2. High turbine rotor vibration due to:Direct of the
forces generated by the rubbing.3. Thermal bending of the rotorWhen
rubbing occurs in a turbine seal, only a small arc of the shaft
surface is at any given time, in contact with the stationary part
of the seal. Therefore, frictional heat at the site of rubbing does
not heat the shaft evenly. Instead, it produces a hot spot on the
shaft surface. The resultant thermal expansion of the shaft causes
it to bow. This increases its unbalance and hence, vibration.High
rotor vibration can force a turbine trip and result in damage to
the machine.4. Severe damage to other turbine internalsDeep grooves
can be cut on the shaft surface by the stationary parts of the
rubbing seals or some blades can get bent or even broken off if
their seals are involved in the rubbing. Also, localized heating of
the shaft may produce thermal stresses so large that the shaft can
bow permanently. In case, damage would be accompanied by high
vibration.In the extreme case, a long outage may be necessary for
costly repairs to the turbine.4.3 Abnormal Pressure of gland
sealing steamTrouble free operation of any turbine gland seal
requires proper adjustment of its sealing steam pressure. When this
pressure increases above its normal value, the sealing steam flow
through the seal increases and pressure at port B rises as more
steam flows out of the seal and into the gland exhaust condenser.
Eventually, pressure at port B can rise above atmospheric, causing
hot steam to blow out of the seal.
In the above figure, an HP turbine gland seal whose leakoff is
utilized as sealing steam in LP turbine gland seals.In either case,
the steam blowing out of the seal can overheat the adjacent bearing
(if leakage is large enough). Contamination of oil in the bearing
with water can also result because the leaking steam can penetrate
the bearing seal and condensate inside. The axial distance between
turbine gland seals and bearing is very small (to reduce the total
length of the turbine generator). Therefore, it is not difficult
for leaking gland steam to reach the nearest bearing. Of course,
any steam leak represents a safety hazard and causes increased
consumption of makeup water. In this case, however, the safety
hazard is nearly nonexistent because during turbine operation it is
very unlikely that someone will be close to the leaking seal.
Likewise, the cost of increased makeup is small in comparison with
the cost of a bearing repair which would require a turbine
shutdown.When the sealing steam pressure at the inlet to a gland
seal is too low, air can leak into the turbine through the
malfunctioning seal. This problem can occur in any LP turbine gland
seal at any power level or in an HP turbine gland seal whenever the
HP turbine exhaust pressure is below atmospheric.Note that the
sucking action of vacuum inside the turbine can lower pressures at
ports A and B much that the malfunctioning seal can actually draw
an air/steam mixture from other seals via common headers in the
gland sealing steam system that connect all the glands
together.
Air in-leakage that results from a loss of gland sealing steam
pressure to one or more glands causes the following adverse
consequences/operating concerns:1. Increased air concentration in
the condenser, resulting in:a. Reduced condenser vacuum with all
its adverse consequences. If many seals are leaking and no
corrective action is taken, condenser pressure may quickly (within
several minutes) rise enough to cause automatic turbine unloading
or even trip, both representing a costly loss of production.b.
Increased concentration of dissolved oxygen in the condensate. If
The gland malfunction does not force a turbine trip and operation
is continued, accelerated corrosion in the condenser and the
condensate system is our major concern. The feed water and steam
systems, as well as the boiler and the turbine, are also affected
if the hydrazine injection rate is not increased properly.To
minimize corrosion damage, the allowable duration of operation is
limited and proper actions must be taken when certain limits on the
oxygen content are reached. In the extreme case, the unit must be
shutdown.2. Quenching of hot parts of the malfunctioning seals by
cool in-leaking air. The quenching can produce thermal stresses
large enough to cause cracking of the seal segments and/or
increased turbine vibration. Also abnormal axial differential
expansion can occur.3. A total loss of sealing steam to a turbine
gland can cause condenser vacuum to suck in lube oil from the
adjacent bearing. The resultant contamination of steam generator
feedwater can cause foaming and asphalt like deposit in the steam
generators.
5 Instrumentation Application5.1 Gland Steam Condenser
Exhausters5.1.1 PressureGland steam condenser exhauster suction
pressure is continuously monitored and reported to the main control
room and plant computer. A low vacuum signal actuates a main
control room annunciator.5.1.2 LevelWater levels in the gland steam
condenser drain leg are monitored and makeup is added as required
to maintain loop seal integrity. Abnormal levels are annunciated in
the main control room.5.1.3 Effluent MonitoringThe TGSS effluents
are first monitored by a system-dedicated continuous radiation
monitor installed on the gland steam condenser exhauster blower
discharge. High monitor readings are alarmed in the main control
room. The system effluents are then discharged to the Turbine
Building compartment exhaust system and the plant vent stack, where
further effluent radiation monitoring is performed.
5.2 Sealing Steam HeaderSealing steam header pressure is
monitored and reported to the main control room and plant computer.
Header steam temperature is also measured and recorded.
Turbine gland seal system pressure is maintained by the
pneumatic controller according to the set point set by the
operator, with the help of pressure transmitter downstream of
controller.
In case the main gland seal controller failed to control the
pressure of the gland seal header with in the specified limits then
emergency spill over controller will release the steam to the
condenser to decrease the header pressure. This way this spill over
controller will protect the system as well as its piping.
6 Troubleshooting And Failure ModesIn general, all
troubleshooting should be based on the following three steps:1.
Identify the problem and what has changed. Without an accurate
identification of the problem and what has changed, the probability
of resolving the problem is not good. Some problems are intuitively
obvious, and some require extensive troubleshooting and testing to
completely understand the problem.2. Define the desired approach
and outcome. Is a temporary repair the best way to proceed? Should
the component be replaced or repaired? Is a modification or change
the best way to proceed? Are there Code or regulatory implications
involved in the resolution?3. Define the best corrective action
approach. Identify the immediate, short-term, and long-term actions
to resolve the problem and (one hopes) minimize the chance of a
reoccurrence.
6.1 Causes of System Failure
1. Since most gland seal regulators are air operated reducing
valves, improper pressure settings on the air pilots for the
regulating and unloading valves can cause system pressure to be too
high or low, or both valves may be open at the same time. Ruptured
diaphragms may occur in these air pilot controllers and air
operated valves. Oil and water in the air lines to the pilots or
air operated valves can cause erratic operation and deterioration
of the rubber diaphragms. Upon loss of air pressure, both valves
fail open and the unloader valve must be operated with the manual
handwheel to control gland seal pressure.2. Painted valve stems or
improper packing installation can cause binding of the stem,
restricting valve operation.3. Improperly calibrated gages can
cause the system to be improperly operated.4. In the event of a
jammed gland seal regulator, the operator should take control of
gland seal pressure by using the regulator bypass valve.
7 Safety Issues7.1 Personnel Safety IssuesOperation of steam
seal valves should be performed by knowledgeable, trained
individuals to prevent burns from hot components or leaking steam.
All the pertinent Occupational Safety and Health Administration
(OSHA) regulations must be adhered to when working on the steam
seal system.7.2 Equipment Safety IssuesDo not admit steam to the
glands of an idle turbine because varying degrees of corrosion or
erosion or a bowed rotor can result. Ensure that the steam seal
system is in operation prior to establishing a condenser vacuum.
Dirt and debris could be drawn into the turbine glands if the seal
system is not in operation.
Fahad KhalilPage 15