1 Overview
The turbine steam seal system is primarily used to prevent steam from leaking outwards along the shaft ends of HP and IP cylinders and even flowing into the bearing box and resulting in water polluting lubricating oil; it is also used to prevent air from seeping into the steam cylinder and as a result destroying the vacuum of the unit.
The self sealing turbine steam seal system refers to the system in which the steam escaped from the shaft end steam seals of HP and IP cylinders is, after spray desuperheating, used for the gland sealing steam supply for LP shaft ends during the normal operation of a unit. The surplus leaked steam flows to the LP heater or condenser through an overflow station. During the unit's start or low load phase, the gland sealing steam supply comes from the outside. From the start to the operation at full load, the turbine steam seal system can conduct automatic switching in accordance with the unit's gland sealing steam supply requirements.
The system is featured by simplicity, safety, reliability, and good applicability.
2 System composition and major equipment
The system consists of the steam supply and leakage pipes of gland seal, the valve-stem steam leakage pipes of stop valve and control valve, the valve-stem steam leakage pipes of IP combined steam valve, and other related devices. (for details, see Figure 0-1-2)
A three-valve system is adopted for gland steam supply, that is, under all operation conditions, the steam supply pressure for a steam turbine is controlled via three control valves, namely, HP steam supply control valve, subsidiary source supply control valve, and overflow control valve, all of which ensure that the set steam pressure in the steam supply main pipe can be maintained automatically under any operation condition. The subsidiary source steam supply station has two steam supply sources. Except the reheating section of the unit itself, subsidiary steam supply from other than the unit is also available. During start or operation at low load, the subsidiary steam enters into the self-sealing system through the control valve of the subsidiary source station. The above three
control valves, their cut-off valves, and the necessary by-pass valves make up three pressure control stations. Furthermore, in order to meet the LP cylinder gland steam supply's temperature requirement, a direct-contact desuperheater is set up in the main pipe of low-pressure shaft gland steam supply. Temperature control is used to control the water spray, which as a result realizes that the desuperheating steam meets the low-pressure shaft gland's steam supply requirement.
The actuators for all control valves in the system runs in an electrically way and are controlled by DCS. Import components are adopted for all the control valves and actuators, which can perform stably and reliably.
In order to ensure that the main steam supply station is at a hot bank status all along the normal operation, a bypass with throttling orifice plates is set up in front of the control valve. During a normal operation, the steam goes from the gland sealing steam supply main pipe to the pressure control station through the bypass, which keep the station in a hot bank state.
In the system there are also set up a JQ-110-3 gland seal heater and two shaft gland blower fan (one of them as a standby), which are used to extract the leaked steam (or air) in the final section of shaft gland chamber and to keep the chamber run at a minor negative pressure.
In order to prevent foreign substances from entering into the shaft gland, a Y-type steam filter is set up at each branch pipe of steam supply. In the steam supply main pipe there is set up a safety valve with a set-pressure of 0.3MPa(a), which can prevent the occurrence the over high steam supply pressure.
3 System operation 3.1 Preparation for start
3.1.1 Shut down all pressure regulating stations, put through the steam supply source, and adjust the temperature of the steam supply line to a superheating temperature.
3.1.2 Confirm that the instruments and meters of the system are normal.
3.1.3 Confirm that the turning of steam turbine has been put into service.
3.1.4 Confirm that the recirculation of condensate has been established.
3.1.5 Open the manual and electric check valves of all the pressure control valves
and temperature control valves.
3.1.6 Put through the electric control valves and the corresponding EPSs.
3.1.7 Open the manual gate valve in the cooling water (condensate) line of the gland seal heater, which puts the gland seal heater into service.
3.1.8 Open the shaft gland blower fan, and open the electric butterfly valve in the fan's admission line. The fan is put into service normally (one for service, the other for standby). The gland seal's steam return line maintains a negative pressure, and the pressure is adjusted to about 95～ 99kPa (absolute).
3.2 Start
3.2.1 Cold start
3.2.1.1 In the cold start, the steam is supplied by the subsidiary source station.
3.2.1.2 After closing the control valves in both HP source station and overflow station, open the electric check valves in the control valve line of the subsidiary steam supply source station; confirm that the system is normal input and runs automatically in accordance with the following steps:
A) Turning, rolling, and low load phase
The gland sealing steam supply comes from the subsidiary source, and the pressure of the steam supply main pipe is maintained at 0.124MPa (absolute).
B) 25%-load to 60%-load phase
When the unit load ascends to 25% of the rated load, the cool head of reheat pipe can fulfill all the requirements of gland sealing steam supply, the steam is supplied by the cool head of reheat pipe, and the main pipe pressure is automatically maintained at 0.127MPa (absolute).
C） 60%+ load phase
When the load is increased to more than 60%, the quantity of steam leaked from HIP cylinder shaft ends into the supply main pipe exceeds that required by the LP cylinder gland seal. When the main pipe pressure ascends to 0.130MPa (absolute), the control valves of all steam supply stations are closed automatically; the control valves in the overflow station are opened automatically and discharge the redundant steam to the condenser through the overflow control station. Up to this point, the turbine steam seal system enters into a self sealing state and the
pressure of main pipe maintains at 0.13MPa. During a normal operation, the electric check valve in the cool head of reheat pipe shall be shut down.
3.2.2 Hot start
3.2.2.1 If there is a qualified subsidiary steam supply source, the gland sealing steam is supplied by the subsidiary source station.
The parameters of the subsidiary source fail to meet the requirements, the gland sealing steam is supplied by the HP source station.
3.2.2.2 If adopting the HP steam source, confirm that after the control valves of the subsidiary source station and the overflow station are closed and the electric check valve in the control valve line of the main steam supply station is opened, the steam supply system shall be put into service normally and run automatically in accordance with the following steps:
A) Turning, rolling, and low load phase
The gland sealing steam supply comes from the main steam supply station, and the pressure of the steam supply main pipe is maintained at 0.118MPa (absolute).
B) 25%-load to 60%-load phase
When the unit load ascends to 25% of the rated load, the cool head of reheat pipe can fulfill all the requirements of gland sealing steam supply, the steam is supplied by the cool head of reheat pipe, and the main pipe pressure is automatically maintained at 0.127MPa (absolute).
C) 60%+ load phase
When the load is increased to more than 60%, the quantity of steam leaked from HIP cylinder shaft ends into the supply main pipe exceeds that required by the LP cylinder gland seal. By this time the main pipe pressure ascends to 0.130MPa (absolute), the control valves of all steam supply stations are closed automatically, and the control valves in the overflow station are opened automatically and discharge the redundant steam to the condenser through the overflow control station. Up to this point, the turbine steam seal system enters into a self sealing state and the pressure of main pipe maintains at 0.13MPa.
During a normal operation, the electric check valve in the cool head of reheat pipe shall be shut down.
3.2.3 Unit load rejection phase
When load rejection occurs, it shall be treated by the following two means:
a) If there is a standby subsidiary steam supply source conforming to the temperature requirement, the pressure of the main pipe for gland sealing steam supply can be down to 0.124MPa. Close the overflow governing valve and the gland sealing steam is supplied by the subsidiary source station.
b) If the unit has no standby subsidiary steam sources or the parameters of the subsidiary sources fail to meet the requirements, the subsidiary sources and cool head of reheat pipe cannot be used. The electric check valve in front of the control valve of the subsidiary source station must be closed. The pressure of the main pipe for gland sealing steam supply is down to 0.118MPa. The overflow governing valve has already been closed automatically, and the HP steam supply control valve is opened automatically. The steam is supplied by the main steam supply station, namely, the HP steam source adjustment station.
3.2.4 Putting into service of temperature adjustment station
Under all operation conditions, a temperature adjustment station can maintain the temperature of the LP gland seal chamber at 121～ 177℃.
3.2.5 Putting into service of by-pass valve
When the throttle pressure of a subsidiary steam supply control valve is 25%
lower than the rating, or when the steam turbine starts with a wearing gland seal, in order to get adequate steam flow to seal the steam turbine, the by-pass valve of the subsidiary steam station will be open for steam supplement. When the throttle pressure is high enough to maintain the steam for gland seal, the by-pass valve will be closed. If the by-pass valve is still open, the surplus steam will be automatically discharged into LP heater or condenser by way of the overflow valve. Such a case does not made any difference to the turbine operation, but it will lower the efficiency of the power plant slightly.
3.2.6 Steam temperature requirements
3.2.6.1 Subsidiary steam parameters' requirements No. Application Steam pressure
（MPa）
Temperature (℃)
Used for Steam source
1 Warming About 150~260
Cold start
About 208~375
Hot start 2 Steam
turbine gland sealing
0.588~0.784MPa
About 150~260
Cold start
Subsidiary steam header
3.2.6.2 Steam turbine gland sealing's temperature requirements
In order to avoid the inconsistency between the temperature of seal steam and that of turbine rotor, the steam supply temperature must meet the required needs.
In general, in the main pipe of gland seal with a absolute pressure of 0.127MPa, it is allowed that the temperature of seal steam is 167℃ higher or 167℃ lower than that of the metal. Excessive temperature differences between seal steam and rotor will result in a high heat stress on the rotor surface in the gland seal zone.
Each heat stress cycle will result in the alternate life loss of part of the metal, while the recurrence of excessive temperature difference of rotor will give rise to heat fatigue cracking on rotor surface; meanwhile, the excessive temperature difference will also bring about excessive expansion difference between the rotor and the stator.
3.2.6.3 Specific steam temperature requirements
In a cold state (when the first shell temperature of steam turbine is lower than 150℃);
When the maximum steam temperature under the main pipe pressure is 260℃ and when the minimum steam temperature under the main pipe pressure is 150℃. In a hot state (when the first shell temperature of steam turbine is higher than 150℃);
When the maximum steam temperature under the pressure in the main pipe for gland sealing is 375℃ and when the minimum steam temperature under the pressure in the main pipe for gland sealing is 208℃ (the maximum temperature of gland-packing leakage minus 167℃).
The above parameter requirements are shown in Figure 0-1-1.
3.3 Emergency adjustment
3.3.1 In order to prevent the occurrence of steam supply overpressure accident,
there are set up a full-open-type safety valve with radiator spring with a set pressure of 0.3MPa in the main pipe for steam supply. In order to guarantee a safe operation, there shall be set up sound and light alarm devices for overpressure alarming with an alarm pressure of 0.25MPa in the main pipe for gland sealing steam supply, so that the system operation can be monitored at any time.
3.3.2 Gland sealing back-steam pressure adjustment
If end steam leakage is found during operation, the throttle of gland sealing blower fan can be adjusted to guarantee a certain negative pressure is maintained in the gland sealing back-steam chamber, at about 95~99kPa (absolute).
3.3.3 When there are failures in the steam supply stations and overflow stations, control valve hand wheels and by-pass valves can be used to adjust the system.
3.3.4 Under an abnormal working condition, if the bypass channel of the steam supply control valve is opened or the control valve of the steam supply station is open, excessive steam will enter into the main pipe for steam supply by way of the steam supply station. No matter which case occurs, the overflow control valve will be automatically opened. If the overflow control valve also goes wrong, the motor operated gate valve in the bypass of overflow station can also be opened.
3.4 System shutdown
3.4.1 Confirm that the steam turbine is in the shutdown and turning phase.
3.4.2 Cut off the electric check valves of all steam supply pipes.
3.4.3 Cut off the motorized valve in the water inlet of the desuperheating station.
3.4.4 Cut off the power supply of the gland sealing control valve.
3.4.5 Confirm that all the drainage points are free from obstruction.
4 Precautions for installation and operation
4.1 In order to ensure that the system can run safely, the components such as control valve bodies and actuators have been calibrated and aligned in the manufacturer's plant, disassembly of them is not required during installation.
4.2 A drain pipe shall be set up the low level point after the valve of a steam supply control station, and water in the drain pipe flows into the condenser by way of a throttling device.
4.3 A drainage point shall also be set up after the direct-contact desuperheater, with its position at least 2.5m after water spraying, and the drained water flows
into the condenser by way of a throttling device.
4.4 The minimum distance from the spraying part of the direct-contact desuperheater from the LP gland seal shall be 15~17m. After water spraying, a straight section at least 5m longer shall be available, and its diameter shall be the same as that of the direct-contact desuperheater shell.
4.5 In a turbine steam seal system the pipes before the steam supply control station shall have continuous incline towards the steam source (main steam, subsidiary steam, and the steam in the cool head of reheat pipe) direction with a gradient of slope of 1/50. If these pipes are connected to the source in a inclined way, a drain pipe must be set up at the inlet side of each valve to prevent the water from being accumulated. The drain pipe must be a continuous one with throttling orifices.
4.6 All the pipes for gland sealing steam supply shall be inclined continuously downwards to the gland sealing pressure control station direction with a slope of 1/50. If there is a low level point in the piping system, a continuous drain pipe with throttling orifices shall be used to drain the water to the condenser.
4.7 All the pipes of steam-air mixture from gland seal and valve stem leakage shall be inclined towards the direction of the gland seal heater, with a slope of 1/50. If there is a low level point in the piping system, a drainage point shall be set up, and the drained water shall be discharged into the condenser.
4.8 The condensate from the gland seal heater is discharged into the condenser through a water-sealed pipe.
4.9 Before the vertical climbing of the exhaust duct of a shaft gland blower fan, a downward slope starting from the fan and ending in a low level drainage point shall be available for the duct. During operation, it shall be guaranteed that the drainage is clear. Two fans (one for standby) shall be connected in parallel.
4.10 The steam filter in a steam supply line shall be fitted on a horizontal position.
4.11 The pressure control signals for all electric control valves come from the main pipes for steam supply, and the temperature control signals from the gland seal chambers. Due to such reasons as arrangement, the temperature measuring
point after a direct-contact desuperheater shall be set up on the main pipe, and the distance from the measuring point to the desuperheating spraying part shall be no less than 15m. It shall approach the gland seal chamber as near as possible.
4.12 The steam pipelines to the same place can be combined, but the pipe diameter shall be big enough.
4.13 Purging is required for all pipes. The cleaning for all pipes and equipment in the system shall be performed in accordance with clauses 3 and 2 in JB/T4058-1999 Steam Turbine Cleanliness.
4.14 The inlet of the shaft gland blower fan for standby must be closed.
4.15 The steam filters shall be washed regularly.
4.16 When the pipe medium temperature is lower than 400℃, No.20 steel pipes shall be adopted; when it is higher than 400℃, alloy-steel pipes shall be adopted.
5 The calibration and operating conditions of the control valves in all control stations are as follows (see Table 0-1-1).
Table 0-1-1
adjust Close Vacuum phase
(cold start)
adjust Close Close Load rejection
0.118 Open and
adjust Close Close Vacuum phase
(hot start)
Note: The valve set pressures in Table 0-1-1 are only for onsite adjustment reference. Power plants can adjust the pressure setting value based on actual operating conditions. If no smoking occurs in all HP, IP and LP gland seals and the vacuum is not unaffected by the adjustment, it is appropriate.
6 Simple manipulation measures for common troubles (see Table 0-1-2) Table 0-1-2
Failure Failure source Simple manipulation measures Too high
1 The steam supply control valve is not tightly closed.
2 External steam goes into the system.
3 There are unclear leakage points near the gland seal.
1 Check the control signals of the control valve and its tightness. If having confirmed that it is not tight, inform the manufacturer or the support plants.
2 Identify the external source and cut off it.
3 Find out the leakage points near the gland seal.
Smoking in the gland seal
1 The outlet valve of the gland seal blower fan is
1 The outlet valve of the gland seal blower fan is