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Why does vacuum in steam condenser reduce or drop??

  1-High exhaust temperature: Vacuum drops or maintains at lower side due to high exhaust steam temperature flow into steam condenser. This high exhaust temperature is mainly due to 1-Operation of Turbine at lower loads 2-More clearance in labyrinth seals 3-Not operating exhaust hood sprays 4-More load on condenser 5-Breaking of ejector U loop 2-Low circulating cooling water flow Vacuum in condenser reduces due to inadequate cooling water flow through steam condenser. This is mainly due to; 1-Problems associated with pumps 2-Air pockets in pipe line 3-Leakages in cooling water line 4-Stuck of discharge valve of pump 3-High cooling water temperature at condenser inlet Higher cooling water temperature at condenser inlet results into reduction of vacuum due to poor heat transfer from steam to water 4-Poor heat transfer in condenser Very less or poor heat transfer in steam condenser reduces vacuum to very low level resulting into high exhaust temperature &am

Protections & Interlocks in power plants

 Interlocks: Are the programmed or hardwired control systems to protect systems and improve the operation reliability.

Protections: Are the programmed or hardwired control systems to protect the equipments, man power and systems from failure/harm.

The interlock and protection system is used to ensure safety of equipment and personnel as well as smooth & trouble free operation of the plant

This system initiates automatic corrective actions to stabilize the unit quickly. The protection scheme is developed to trip the equipment automatically with or Class A trip involves a serious electrical fault like differential, stator earth fault etc. and is considered to be the most dangerous in terms of the shock on the unit. Since it involves serious electrical faults, connections from both generator and the HV bus is immediately switched off to limit the damage at the fault point and also to isolate the healthy system. Hence the unit (turbine, generator and boiler) has to be tripped without time delay. Alarm & buzzers are generally used to alert the operator.

POWER PLANT PROTECTIONS & INTERLOCKS AND THEIR SIGNIFICANCE

Sl No.

Interlock description

Significance

A

Boiler

 

1

FD & SA fan trip/stop on tripping of ID fans

To avoid furnace pressurizing

2

Fuel feeding system trip/stop on trip/stop of SA fans

To avoid jamming of fuel feeding system due to no spreading air

3

FD fans trip/stop on high furnace pressure (>25 MMWC)

1-To avoid furnace leakage
2-To avoid furnace explosion
3-To avoid buck stay damage

4

ID fans trip/stop on low furnace pressure (-25 MMWC)

1-To avoid carryover of fuel at secondary combustion zone
2-To avoid back end flue gas ducts explosion due to accumulation of unburnt (Unburnt results into formation of CO gas)

5

FD fan trips on low drum level (On tripping ID fans, boiler all systems like FD,SA & fuel feeding system trip)

To avoid boiler pressure parts over heating & failure

B

Steam Turbine

1

Turbine trips on high main steam pressure

To protect turbine internals & casing from high pressure damage

2

Turbine trips on low main steam pressure

To protect turbine internals from  saturated  steam (water particles in steam)

3

Turbine trips on high main steam temperature

To protect Turbine internals from creep failure (Turbine internals fail on prolonged exposure to temperature more than recommended)

4

Turbine trips on low temperature

1-To protect Turbine from uneven expansion
2-To protect Turbine internals from water particles in steam (Low pressure & temperature steam will have water particles in it)

5

Turbine trips on high bearing temperature (>110 deg C)

To protect turbine bearing failure & other secondary system/operation interruption for long time

6

Turbine trips on high vibration (>5 mm/sec or >110 microns)

To protect turbine bearing failure & other secondary system/operation interruption for long time

7

Turbine trips on high axial displacement

To protect turbine internals from rubbing & damages

8

Turbine trips on high differential expansion

To protect turbine internals uniform thermal expansion & from rubbing & damages

9

Turbine trips on low control oil pressure

To ensure reliable operation of HP & LP actuators

10

Turbine trips on low lube oil pressure

To avoid damages to the bearings

11

Turbine trips on low trip oil pressure

 

12

Turbine trips on low vacuum or high exhaust pressure

To avoid damages to the rotor blades
Note: High back pressure on rotor creates reaction force to rotation of turbine rotor

13

Turbine trips on high back pressure

 

14

Vacuum breaker valve opens on activation of trip interlocks like
1.High bearing temperature
2-High bearing vibration
3-High axial displacement
4-High differential expansion
5-Low lube oil pressure

To reduce the speed of rotor within minimum time to avoid damages to the bearings & internal parts.
Note: High back pressure on rotor creates reaction force to rotation of turbine rotor

15

High hot well level

To avoid entry of water into Turbine

C

Fuel handling

 

1

Belt conveyor trips on operation of Zero speed switch (ZSS)

1-To avoid the further damage to the belt conveyor
2-To avoid system disturbance & major damages to the conveyor structure
Note: ZSS operates when belt gets cut or slips on pulley

2

Belt conveyor trips on operation of belt sway switch (BSS)

1-To avoid swaying of belt
2-To avoid belt side edges damage
3-To avoid fuel spillage

3

Belt Pull cord Switch (PCS)

To stop the belt conveyor during emergency situations to avoid damages to the man & system

D

Boiler feed pumps

 

1

Pump trips on high bearing temperature

To avoid bearing damage & secondary system damage/disturbance

2

Pump trips on high bearing vibrations

To avoid bearing damage & secondary system damage/disturbance

3

Pump trips on low suction pressure

To avoid pump cavitation

4

Pump trips on high differential pressure

To avoid pump cavitation

5

Pump trips on high balance leak off pressure

To avoid further damages to the balance & counter balance discs

6

Pump trips on lower cooling water temperature

To avoid failure of pump's bearings & seal

7

Pumps trips on over load

To avoid damages to the pump internals

8

BFP trips on Deaerator level low

 

E

Boiler fans

 

1

Fan trips on high bearing temperature

To avoid bearing damage & secondary system damage/disturbance

2

Fan trips on high bearing vibrations

To avoid bearing damage & secondary system damage/disturbance

F

Motor

 

1

Motor trips on higher bearing temperature

To avoid bearing damage & secondary system damage/disturbance

2

Motor trips on higher winding temperature

To protect winding

3

Motor trips on over load

To protect winding

G

Generator

 

1

Over current protection

Protects the generator from over load, short circuit & earth faults

2

Earth Fault Protection

To protect the generator from earth faults & short circuits

3

Generator Differential Protection

To protect the generator from winding faults or unbalance currents in winding

4

Reverse Power Protection

To avoid motoring of generator during reverse flow of power to generator from other source

5

Low Forward Power Protection

To protect the generator running under load

6

High bearing temperature

To avoid bearing damage & secondary system damage/disturbance

7

High bearing vibrations

To avoid bearing damage & secondary system damage/disturbance

8

Higher winding temperature

To protect winding

9

Higher core temperature

To protect core

10

High air temperature

To limit winding temperature

 

Other protections

 

11

High & Low voltage protections

 

12

High & low frequency protection

 

13

Rotor earth fault protection

 

14

Loss of excitation

 

 

Classes of STG Trips:

Class A trip

This involves serious electrical faults and is considered to be the most dangerous in terms of the shock on the unit. Since it involves serious electrical faults, connections from both generator and the EHV bus is immediately switched off to limit the damage at the fault point and also to isolate the healthy system. Hence the whole unit need to be tripped.

Class B trip

Class B primarily relates to mechanical problems. This results in tripping of turbine followed by generator.

Class C

Class C involves basically external system related problems like frequency, overvoltage etc. This does not involve instant tripping of the unit. CPP unit operates on house load

CLASSES OF GENERATOR PROTECTIONS

SL NO.

CLASS A

CLASS B

CLASS C

1

Generator Differential Protection

Loss of Excitation

Under Frequency

2

100% Stator Earth Fault Protection

Rotor Earth Fault

 Over Frequency

3

Generator Over Voltage Protection

Over excitation

Pole Slipping Protection

4

95% Stator Earth Fault Protection

 

Tripping of unit transformer

5

Starting Over Current Protection

 

 

6

Over fluxing Protection of Generator

 

 

7

  Differential Protection of GT

 

 

8

Buchholz Relay of GT

 

 

9

Trip from oil & winding temperature of generator transformer

 

 

 

These protection when operated initiate tripping of Generator Circuit Breaker, Field Circuit Breaker, Generator Transformer Circuit Breakers & Unit Transformer LV Circuit Breakers and turbine.

This results in tripping of turbine followed by generator.

Class C involves basically external system related problems like frequency, over voltage etc. This does not involve instant tripping of the unit. 

 Why do the Boilers explode


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