Design Criteria for
LP BYPASS SYSTEM
LP Bypass system provide a very
effective means of controlling reheat
–steam flow during
Startup
Low-load / part load operation
Sudden load rejection
It enhances fast starting
capability of turbines from
all initial conditions e.g.
Cold condition
Warm condition
Hot Condition
LP steam-bypass system operate in
parallel with the turbine
Protect Condenser from excessive
temperature and pressure that could
result from sudden injection of
superheated steam
Ensure coordinated control between HP
and IP turbine control valves and the
steam bypass systems to operate at any
remaining load
Subsequently reloaded in a controlled
manner
Schematic Diagram of Bypass System
• The controls for LP Bypass system
are combination of electrical and
well proven hydraulic system
• Electro-hydraulic converter
provides the necessary link between
hydraulic actuator and electrical
system
• The LPBypass Controller acts as a
maximum pressure Controller
• It regulates the reheater pressure
either to the fixed set value or
suitably derived variable set value
• If the reheater pressure exceeds this
set value, the Controller causes the
electro-hydraulic converter to
operate and initiate bypass
operation
Set Value formation
The Two set values – fixed and variable
are formed for the LPBypass control
system
set value to be used is determined by a
maximum value selector
For the formation of variable set point,
a pressure transducer is used to measure
the steam pressure in HP turbine
SEQUENCE OF EVENTS RESULTING
FROM LOAD REJECTION
Upon receiving load rejection signal from
electrical system:
 Turbine Speed Controller assumes control
of the TG set
 It initiates fast closure of HP & IP
turbine control valves through position
controller
 Fast closure of turbine control valves
cause an immediate decrease in HP 1st.
Stage pressure
SEQUENCE OF EVENTS RESULTING
FROM LOAD REJECTION
 Pressure increases in main and
reheat steam systems
 These pressure changes are sensed
by HP and LP Bypass controllers
 Respective bypass valves are opened
If LP steam- bypass system fails
or the condenser cannot
accept LP bypass steam , Safety
Valves open and dump steam
to atmosphere
Hydraulic Mode of Operation
• The controller controls plunger coil of Electro
Hydraulic Converter
• With increase in voltage, the jet pipe is deflected
towards right
• Converter piston moves downwards leading to
generation of LPB signal oil through Follow-up
pistons provided in the hydraulic control system
Hydraulic Mode of Operation
• Two Follow-up Pistons actuate and open first stage
of water injection valve
• Remaining two Follow-up Pistons generate signal
oil for opening LPBypass stop and control valve
ADJUSTMENT
OF
LPBYPASS
CONVERTER
Design of LPBypass Valves for 210 MW
Steam Turbine
• Controlled
by 8 bar
mineral oil
Design of LPBypass Valve for 500 MW
Steam Turbine
• Controlled by
36 bar FRF
Protection Devices
 Low Vacuum Condensor Protection
 Spray Water Pressure Switch
 Condensor Temperature Protection
Protection Devices cause closure of
bypass station in case of following
eventualities
a) Condensor vacuum is low ( > 0.6 bar
absolute )
b) Spray water pressure is low (< 8.5 / 9.5
bar )
c) Temperature of condenser body is high
(> 90° C )
Water Injection Valves operate in
two stages
• Second stage Water Injection Valves
open when steam flow through LPBypass
Valves increases more than 45 %
CURVE FOR LPBYPASS CONTROLLER
• Damping device