7/26/2019 TURBINE GLAND STEAM CONDENSER
1/4
Function of the Gland Steam Pressure Control System:
This system ensures the sealing of glands in HP, IP and LP
turbine under various
Load conditions.
The Turbine glands are self seal type. Upto approximately 40%
load steam from auxiliary
source (CRH/Aux.steam header) through gland steam valve is taken
to seal all the HP, IP
and LP glands. During this period the steam leakage valve
connecting this header to
condenser is kept closed. After approx. 40% of load, gland steam
valve is closed and
steam leakage valve is opened. Pipings are so sized that the
leak-off steam from Front
and Rear end of HP turbine goes to the condenser through steam
leakage valve while
leak-off steam from the IP front and rear glands is utilised for
sealing the LP glands,thus
ensuring proper temperature of sealing steam. The leak-off steam
and air from the last
chambers of each rotor is sucked into a gland cooler.
Building up of vacuum in the condenser is the first step during
start-up. For building up
vacuum it is necessary to seal the turbine gland by supplying
steam to the shaft through
gland steam valve. The control system opens the gland steam
supply valve until the
pressure in the header acquires a preset value. Subsequently
when the set picks up Load,
the pressure of steam inside HP and IP turbine builds up
resulting in the leakage of steam
from the Turbine into gland steam supply header which in turn
would result in increase of
pressure. The controller gradually closes the gland steam supply
valve and opens gland
leak-off valve, if required.
Electro-Hydraulic valve actuators:
The actuators of the two control valves are continuously under
the control commands
from the electric controller through 2 Electro hydraulic
converters.
The +ve voltage output opens the gland steam supply valve while
the -ve voltage output
opens the gland leak-off valve.
Each hydraulic actuator has its own motor driven oil pump
supplied from station AC syste
In the event of pump failure, the valve remains in stayput
position. Subsequent operation
of the valve can be carried out locally though hand pump.
Pressure measurement:
The pressure in the header is measured by means of a pressure
transducer. This
measurement is used for control as well as display purposes. The
measurement range is -1
to +300mmWC
Control system equipment:
The control system is comprised of solid-state control system.
The modules, including
power supply, operating and monitoring modules are arranged in a
3 tier group frame and
housed in a cubicle.
The pressure controller has PI characteristics. Its output is
fed to electro-hydraulic
actuators via a PID position controller. The function of this
controller is to set the
position of the valve as determined by the pressure controller
as quickly and accurately
as possible.
7/26/2019 TURBINE GLAND STEAM CONDENSER
2/4
The actuators incorporate magneto-resistor potentiometer to give
contactless
non-wearing position indication. The output voltage of the
potentiometer varies
proportionally with the valve position. The voltage is used to
give an actual value
signal to the position controller as well as for valve position
indication in the control
room. In order to ensure stable control characteristics during
the transition period of
Gland steam supply valve closure to Leakage steam supply valve
opening or vice-versa,the valve travel of both the valves have got
slight overlap.
The reference value for the steam pressure in the header can be
set between 0-220mmW
by means of potentiometer on the controller.
The pressure reference value is compared with the actual value
supplied by the
transducer at the input to control of the deviation amplifier.
The press controller is
given the progressive static characteristics so that its action
is increased in the event
of large disturbance to the press control system eg., Load Trip.
For small deviation
uptp +10mmWC, the characteristics is chosen to ensure stable
control under all
operating conditions. The controller action is increased for
larger deviations.
Progressive characteristics of the press controller indicates
that it is possible to
adjust the amplifications between the press controller and
position controller
independently for each valve. This feature enables to take care
of any inaccuracies in
the sizing of the supply and leakage valves and the pipe
lines.
