Why do the Boilers explode???

 



Now days, we are hearing more on explosion in Boilers & related auxiliaries. This is harming man, machines & system and creating unrecoverable situations in power plant. Upon thinking on this following major causes or reasons come into picture.

The main reasons for Boiler explosions are;

  • Operating the Boiler more than the design pressure for long time
  • Operating the Boilers at lower water level in drum
  • High furnace pressure
  • Poor water quality
  • Poor maintenance of Boilers in shutdowns
  • Ignoring the aging factors of Boiler parts
  • Operating the Boilers at higher main steam or metal temperature

1-What are the reasons for operating the Boilers at more than operating pressure?

  • Malfunction of pressure transmitter or leakages in its impulse line
  • Faulty local pressure gauges
  • Faulty safety valves or safety valves set at higher pressure
  • Improper control of combustion system
  • Operating the Boiler at higher pressure set point
  • Varying fuel quantity & quality
  • Frequent load fluctuation on Boiler
  • Leakages
  • Improper spreading of fuel or no control on fuel feeding system

2-What are the potential reasons for low or no water level in the boilers?

  • Feed water pump is not running, but showing running indication in DCS
  • Mis-operation of feed water control valve or control valve is not opened as per requirement
  • False reading from feed water or steam flow meters
  • Malfunction of drum level transmitters/controllers
  • Low water level in Deaerator or mal function of Deaerator level sensors
  • Feed water discharge pump is closed
  • More load on Boiler than design
  • Under capacity feed water pumps
  • Cavitation or steaming in feed water pumps
  • Steaming in Economizers
  • Long time lifting of Boilers safety valves-(Higher blow down of Safety valves)
  • Wrongly set Boiler safety valves (Super heater line safety valve should be set at lower pressure than drum safety valves)
  • Heavy passing in blow down valves
  • Bypassing the instrument process interlocks like alarm value, low level trips etc

3-How does no water in pressure parts lead to explosion?

No water in pressure parts for long time leads into overheating of pressure parts & eventually failure.

Sudden entry of cold water into no water operating parts leads to flashing of water resulting into high volume steam. This high volume steam suddenly lead to explosion, as there is no sufficient space for steam to expand.

4-How do wrongly set Boiler safety valves lead to explosion?


POWER PLANT PROTECTIONS & INTERLOCKS

Requirement is SH outlet Steam line safety valve is to be set at lower pressure than steam drum safety valves.

If drum safety valves set at lower pressure, on lifting of drum safety valves before lifting of SH line safety valves lead to starvation of super heater coils leading to overheat & eventually explosion.

Questions & Answers on AFBC Boilers

5-What are the reasons for high furnace pressure?

  • High furnace pressure is due to;
  • No control on FD fans
  • Bypassing or not working of Fans interlocks
  • Sudden closure or miss-operation of ID fans dampers
  • Variation of fuel moisture
  • Improper spreading of fuel into furnace
  • Wrongly set over fired air
  • Draught transmitter malfunction & its impulse pipe leakages
  • Choke up of open portion transmitter

6-How the Boiler does lead into explosion due to high furnace pressure

Boiler calculations for Boiler operation engineer (BOE) exam

High furnace pressure leads into damages to the sealing, buck stay & membranes if exceeds leads to furnace & duct explosion.

7-How does the poor quality of feed water lead into Boiler explosion?

Poor quality of feed water leads to scaling & corrosion of pressure parts.

Scaling will lead into poor heat transfer & overheating of tubes, this over heating of tubes cause failure of pressure parts followed by explosions.

8-What are the reasons for overheating of pressure parts?

  • Low water level
  • Continuous flame impingement on particular area of pressure parts
  • Localized heating
  • Fuel (Coal) deposition on pressure parts area
  • Pressure parts choke up
  • Poor material quality
  • More Excess air than requirement
  • Internal scaling
  • Lower feed water temperature at economizer inlet

9-What is the significance of shutdown maintenance of Boilers to avoid explosions?

  • In order to take proper care on Boilers to avoid damages & explosion, need to concentrate on following listed areas of maintenance
  • Thorough cleaning of pressure parts
  •  Inspection of pressure parts thickness & replacement of less thickness
  • Proper setting of air & gas nozzles
  • Proper setting of mechanical spreaders to avoid direct hitting of coal particles to rear wall tubes
  • Inspection of steam line spring hangers
  • Cleaning & inspection of buck stays
  • Safety valves maintenance & proper floating
  • Removing all obstacles & temporary supports in boilers for boilers free expansion
  • Proper setting of burners & air nozzles to avoid localized heating & flame impingement
  •  Cleaning & inspection of ducts
  • Overhauling of steam drum gauge glasses
  • Maintenance & calibration of pressure & level transmitters, pressure gauges & temperature gauges
  • Proper maintenance of soot blowers 
Note:Bend lance tube inside the furnace/SH zone/Economiser can harm the pressure parts
Lance tubes if not rotating may damage the pressure parts by continuous impingement of steam jet.

Why & How these in Boilers???

10-What are the reasons for Boiler back end flue gas ducts explosion?



It is due to secondary combustion & formation of carbon monoxide at ducts such as APH & ESP.

During start up or abnormal operation of Boilers, fuel will carry through flue gas  & settle into APH & ESP ducts, where its incomplete combustion starts & form CO gas.CO is very explosive gas, when external Oxygen combines with this gas results into explosion.


Also read 16-Perfect reasons for increasing the fuel consumption of Boilers

11-What precautions can be taken to avoid high metal temperature operation?

  • Installation of metal skin temperature sensors for Super heater coils
  • Avoiding wet fuel intake into the furnace
  • Avoiding more excess air than design for long time
  • Maintain required or designed feed water temperature at economizer inlet
  • Operate boilers more than 50% of its MCR

  

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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

To reduce the speed of rotor within minimum time to avoid damages to the bearings & internal parts.

1.High bearing temperature

Note: High back pressure on rotor creates reaction force to rotation of turbine rotor

2-High bearing vibration

 

3-High axial displacement

 

4-High differential expansion

 

5-Low lube oil pressure

 

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. 


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