1. Design Criteria for LP BYPASS SYSTEM

2. Low-load / part load operation Sudden load rejection
LP Bypass system provide a very effective means of controlling reheat –steam flow during
Startup
Low-load / part load operation
Sudden load rejection

3. Cold condition Warm condition Hot Condition
It enhances fast starting capability of turbines from all initial conditions e.g.
Cold condition
Warm condition
Hot Condition

4. 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

5. 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

6. Schematic Diagram of Bypass System

7. 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

8. 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

9. 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

10. 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

11. 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

12. If LP steam- bypass system fails or the condenser cannot accept LP bypass steam , Safety Valves open and dump steam to atmosphere

13. 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

14. 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

15. ADJUSTMENT OF
LPBYPASS
CONVERTER

16. Design of LPBypass Valves for 210 MW Steam Turbine
Controlled by 8 bar mineral oil

17. Design of LPBypass Valve for 500 MW Steam Turbine
Controlled by 36 bar FRF

18. Protection Devices
Low Vacuum Condensor Protection
Spray Water Pressure Switch
Condensor Temperature Protection

19. Protection Devices cause closure of bypass station in case of following eventualities
Condensor vacuum is low ( > 0.6 bar absolute )
Spray water pressure is low (< / 9.5 bar )
Temperature of condenser body is high (> 90° C )

20. Water Injection Valves operate in two stages
Second stage Water Injection Valves open when steam flow through LPBypass Valves increases more than 45 %

21. CURVE FOR LPBYPASS CONTROLLER

22. Damping device