Table of Contents Turbine Gland Seal Steam System (TSS) ......................................................................... 2 1 Functions ................................................................................................................ 2 2 Design Bases .......................................................................................................... 2 2.1 Safety Design Bases ................................................................................... 2 2.2 Power Generation Design Bases .......................................................... 2 2.2.1 Power Generation Design Basis One ..................................................... 2 2.2.2 Power Generation Design Basis Two ..................................................... 2 2.2.3 Power Generation Design Basis Three ................................................... 2 3 Description ............................................................................................................ 2 3.1 General Description ................................................................................... 2 3.2 Pressure System ......................................................................................... 5 3.3 Suction System ........................................................................................... 5 4 System Operation ................................................................................................. 6 4.1 Labyrinths................................................................................................. 10 4.2 Rubbing in Turbine Seals.......................................................................... 10 4.3 Abnormal Pressure of gland sealing steam .............................................. 11 5 Instrumentation Application ............................................................................. 13 5.1 Gland Steam Condenser Exhausters ................................................................ 13 5.1.1 Pressure ........................................................................................................ 13 5.1.2 Level ............................................................................................................ 14 5.1.3 Effluent Monitoring ..................................................................................... 14 5.2 Sealing Steam Header................................................................................... 14 6 Troubleshooting And Failure Modes ................................................................... 14 6.1 Causes of System Failure ................................................................................. 15 7 Safety Issues......................................................................................................... 15 7.1 Personnel Safety Issues.................................................................................. 15 7.2 Equipment Safety Issues ................................................................................ 15
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Turbine Gland Seal Steam System (TSS)
1 Functions The 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 Bases 2.1 Safety Design Bases The TSS does not serve or support any safety function and has no safety design bases. 2.2
Power Generation Design Bases
2.2.1
Power Generation Design Basis One The 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 Two The 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 Three The TGSS has enough capacity to handle steam and air flows resulting from twice the normal packing clearances.
3 Description 3.1 General Description The 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 Fahad Khalil
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• 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
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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.
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Fig: Sectional View of the Rotor Shaft Area with Gland Seals Exposed
The 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 System The 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 System The suction header supplies a small vacuum for the steam seal system. It does so by Fahad Khalil
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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 Operation In 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 Fahad Khalil
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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 200°F. 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 Fahad Khalil
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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. Fahad Khalil
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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 Fahad Khalil
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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 Seals All 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:
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Direct of the forces generated by the rubbing. 3.
Thermal bending of the rotor When 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 internals Deep 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 steam Trouble 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.
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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 Fahad Khalil
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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 Application 5.1
Gland Steam Condenser Exhausters 5.1.1 Pressure Gland 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.
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5.1.2 Level Water 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 Monitoring The 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 Header Sealing 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 Modes In 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
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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 7.1
Safety Issues Personnel Safety Issues Operation 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 Issues Do 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.
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