HP/LP BY PASS SYSTEM
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HP/LP BY PASS
SYSTEM
During short trip outs the supply of steam to
turbine has to be totally cut off from the
boiler. Once short tripping is over, turbine
requires steam at high temperature as the
temperature of the turbine parts have not
reduced much during the short period due to
effective insulation and high mass of
materials.
BY PASS SYSTEM
But boiler will not give in a sudden during
restart. If low temperature steam admitted
turbine will be affected. Other words, TPS
will idle until the turbine cool and match
with steam parameters. This period will be
very long since cooling rate is very low for
the turbine.
BY PASS SYSTEM
*By pass system comprises a pipe work system
with valves and pressure dropping devices to
enable live steam from the boiler to by pass the
turbine completely and be dumped into the
condenser.
* For matching live steam and turbine metal
temperature for a quick start-up, By pass
stations have been provided.
BY PASS SYSTEM
WHEN BY PASS IS REQUIRED
1.During Startup
2.During a Turbine Trip
3.Higher Steam Pressure (Exceeding
design limit)
4 Flexible load and ramp up change
WHEN STEM FLOW IS NECESSARY
THROUGH REHEATER
1.The higher steam temperature required
by the turbine during Startup
2. During sudden trip
CLASSIFICATION OF BY PASS
SYSTEM
1. HP By Pass System
2. LP By Pass System
HP BY PASS SYSTEM
FUNCTIONS OF HP BY PASS
SYSTEM
1. To establish flow at the outlet of super heater for
raising boiler parameters during startup.
2. To maintain or control steam pressure at preset
valve in main steam line during startup.
3. To warm up steam lines.
4. To control steam temperature down steam of HP
By Pass at the present value.
5. To dump steam from boiler into condenser in case
the generator circuit breaker opens.
FUNCTIONS OF LP BY PASS
SYSTEM
1. Control of steam after reheater
2. Establish flow of steam from reheat line to
condenser by its opening, proportional to the
opening of by pass valve.
3. Release of stem entrapped in HP and
reheater circuit in case generator circuit
breaker opens.
HP BY PASS SYSTEM
1. HP By Pass Valve
2. Spray water pressure control valve
3. Spray water flow control valve
4. Break down orifice
Design Pressure 200 Kg/Sq.cm-
Design Temperature545°C
(Parameters based on the system
requirement of unit)
Combined Pressure
reducing and
Desuperheating Valve
Spherical Shaped body
to optimize Stress
levels
Zero leak valve
HP BYPASS VALVE
HP BYPASS VALVE- one type
COMPONENTS OF HP
BY PASS VALVE
1. Valve body
2. Valve stem
3. Valve seat
4. Jet case
5. Pressure seal plug
6. Bearing carrier
7. Packing net
HP BYPASS SPRAY VALVES
• Design Pressure
350 Kg/Sq.cm
Design Temperature
300°C
Provide spray Water
control to HP
Bypass valves
Zero Leak Valve
HP BYPASS SPRAY
VALVE
COMPONENTS OF SPRAY WATER PRESSURE
CONTROL VALVE AND FLOW CONTROL VALVE
1. Valve body
2. Valve stem
3. Valve seat
4. Guide bush
5. Gland nut
6. Pressure seal plug
LP BY PASS SYSTEM
1. LP By Pass valve
2. Spray water flow control valve
3. Cooler
LP BYPASS VALVE
Design Pressure
64 Kg/Sq.cm
Design Temperature 540°C
Pressure Reducing valve
Spherical Shaped Body To
Optimize the Stresses
Zero Leak valve
COMPONENTS OF LP
BY PASS VALVE
1. Valve body
2. Valve stem
3. Valve plug
4. Valve seat
5. Bonnet
6. Bearing carrier
7. Packing net
Design Pressure
50 Kg/Sq.cm
Design temperature
300°C
Provides Spray Water Control
To LP Bypass valve
Zero Leak valve
LP BYPASS SPRAY VALVE
ADVANTAGES OF HP AND LP
BY PASS SYSTEM
1. To cut down unit startup time.
2. To allow boiler operation during short turbine trips.
3. To warm up steam lines during startup.
4. It gives smooth warm up of boiler components,
including reheater. A most rapid boiler run-up may be
possible. It is saves fuel.
5. Enables the boiler to take up heavy rejection trip to
house load operation advantageously.
ADVANTAGES OF HP AND LP
BY PASS SYSTEM
6. To allow better matching of boiler and turbine.
7. Water loss during startup is minimised.
8. To help conserve demineralised water during start
up and sudden load sheddings.
9. Independent operation of boiler is achievable.
10. To prevents frequent flowing of safety valves.
INTERLOCKS IN BY PASS
SYSTEM
1. Generator Circuit Breaker
2. Condenser vacuum too low
3. HP by pass valve position less than 2%
4. Temperature too high
5. HP by pass valve position more than 2%
6. Close - signal for spray water control
PROBLEMS WITH BY PASS
SYSTEM
1. Noise:
a. Reduced by employing adiabatic flow with friction.
b. Increasing thickness and covering acoustic
insulation.
2. Water ingress:
Pressure control valves are shut, a large build up
condensation will occur in the steam lines. To avoid
this drains are located at suitable positions.
PROBLEMS WITH BY PASS
SYSTEM
3. Thermal shock
4. Leakage flows
5. Heating of turbine blade
6. Turbine Exhaust Pressure Increasing
BY PASS VALVE COMPONENTS
1. Valve Body:
Low alloy steel, 2.25% Cr, 1% Mo (with stand high
pressure & temperature)
2. Valve Stem:
High alloy steel, 2.25% Cr, 1% Mb (Valve throttle from
high pressure to low pressure)
3. Valve Seat:
Low alloy steel (it is pre-stressed before welding, to
avoid high heat transfer stress to the welding position).
BY PASS VALVE COMPONENTS
4. Jet Case: Low alloy steel (it should be flexible for
avoiding direct impingement of spray valve)
5. Pressure seal plug: Low alloy steel
6. Bearing Carrier: Stellite
7. Packing Net: Low alloy steel, sufficiently tightened.
SPRAY WATER FLOW CONTROL
VALVE AND PRESSURE VALVE
Cylindrical nature, flow medium is erosive in nature,
total pressure drop in the valve has been divided
between 3 stages.
1. Valve Body:
Carbon molybdenium steel, 2.25% Cr, 1% Mo
2. Valve Stem:
13% Cr. (High chromium alloy steel)
SPRAY WATER FLOW CONTROL VALVE
AND PRESSURE VALVE
3. Valve Seat: High alloy steel
4. Guide Bush: Monel material (Hardened
quality)
5. Gland Net: Low alloy steel
6. Pressure Seal Plug: Graphite, Asbestos & oil
mixed loosely
BREAKDOWN ORIFICE
Limit outlet velocity by increasing pressure low
vibration.
Low alloy steel, 1%Cr, 1/2 % Mo
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