Challenging situations & troubleshooting during Boiler light up & start up

 

1-Unavailability of start-up equipments/Competent Man power etc:

As we aware after long shutdown or after maintenance activities, we face following common problems

• Not closing of work permits,
• Not normalizing equipment like Boiler fans, Fuel feeding system equipments from electrical department
• No trial of equipments prior to Boiler light up.

Solution:

• Preparation of check lists for pre light up of Boiler
• Equipments trial & interlock testing
• Ensuring competent persons for light up activities

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2-Boiler light up failure:

The main reasons for Boiler light up failure are;

• Improper selection of fuel (coal, bagasse) : High moisture content fuel may lead to late light up or complete failure
• Foreign materials in fuel may lead to clinker
• Improper fuel: air mixture can lead to clinker formation
• Following wrong SOP of light up
• Not achieving 3T’s of combustion (Temperature. Time & Turbulence)
• Failure of fuel feeding system
• Choke up of fuel feeding system

Solution:

• Following SOP
• Selecting good quality fuel
• Deputing competent person for Boiler light up

3-Drum level fluctuation:

The reasons for drum level fluctuations are;

• Not following boiler pressure raise curve (Fast raising of Boiler pressure)
• Sudden opening or closing of start-up vent control valve
• Improper controlling of blow down valve
• Improper Boiler loading
• Uncontrolled & uneven firing rate

Solutions:

• Following Boiler start up SOP
• Maintaining good control over Start up vent control valve & IBD valves
• Deputing competent Boiler Engineer for operation

4-Air Pre Heater (Tubes jamming):

Reasons for APH tubes jamming;

• Not bypassing the APH during initial start-up (Some Boilers have 100% APH bypass arrangement & some Boilers have 25-30%).If APH not bypassed during Boiler light up, cold FD air will cause condensation of flue gas passing through APH tubes. Initially APH inlet flue gas quantity & temperature is less and FD air quantity will be more. This causes condensation of flue gas in tubes resulting into tubes jamming.
• Maintaining lower flue gas temperature at APH inlet
• Leakages in APH tubes: APH tubes leakage may also lead o jamming
• Operating Boiler at more excess air

QnA on AFBC Boilers

Solutions:

Bypassing the APH during light up & start ups.Generally APH is taken online when APH outllet flue gas temperature crosses 140 Deg C

5-Draft fluctuation & Boiler back fire:

Initially there will not be any control systems working in Auto mode, so controlling the draft & combustion is totally based on communication with field people

Probable reasons are;

• Unbalanced draft
• Poor communication of field people & control room engineers
• Uncontrolled fuel feeding

Solutions:

Maintaining proper communication of field & control room engineer is good remedy to control back fire & draft fluctuation

6-Incomplete & Secondary combustion:

Incomplete combustion is mainly due to poor air fuel mixture leading to more unburnt & secondary combustion at Super heaters, APH & ESP.

Incomplete combustion is also due to not achieving 3T’s of combustion (Lower bed temperature, over feeding, low excess air)

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Solutions:

• Maintaining 3Ts during boiler light up & start up
• Maintaining balanced draft to avoid secondary combustion due to carryover of fuel
• Ensure OFA nozzles are open & dampers opening is optimized as per requirement

7-Flue gas duct explosions:

This is mainly due to more negative draft, carry-over of fuel particles & secondary combustion. Secondary combustion at the lack of O2 lead to formation of CO, this CO when exposed to rich oxygen leads to explosion.

How CO formation leads to explosions??

During Light up & start-ups unburnt fuel particles get carryover & accumulate at corners of ducts (APH, ESP etc).As we know incomplete combustion leads to formation of Co instead of Co2.This CO formed by unburnt particles when comes in contact with oxygen start to burn (combustion), due to low volume at ducts corners, ESP , this combustion leads to sudden explosion.

Solution:

• During light ups & start up maintain balanced draft, or draft slightly positive (up to + 2 to 4 mmwc) in furnace.
• Maintain sufficient turbulence for combustion
• Ensure all flue gas & air ducts are clean & free from obstacles
• Get feedback from field people on furnace draft, combustion & chimney condition etc.

Note: More black smoke at stack outlet shows unburnt carryover & incomplete combustion or insufficient air

8-No water flow to Boiler:

This condition exists;

• On tripping of pumps,
• Instruments malfunctions
• Faulty flow meter
• Stuck up of control valve etc
• BFP suction strainer choke up

Solution:

• Ensure pre start up checks have been followed properly
• Keep standby equipments healthy & should be available readily

9-Stuck up of Main Steam Stop valve (MSSV):

• Main steam valves stuck up due to long shutdown & no frequent operation
• Unequal pressure at inlet & outlet of MSSV
• Problems related to actuator (low torque set)
• Not operating MSSV bypass valve before opening MSSV

Solution:

• Carryout preventive maintenance (PM) of MSSV periodically
• If PM not conducted, follow valve operation schedule during long shutdowns
• Ensure actuator open & close torques have been set properly
• Before opening the Main valve, do open the bypass line valve (1” or 1.5”), which helps for pressure equalization at inlet & out let side of main valve.

Note:

Functions of Main steam line bypass line:

• Pressure equalizer
• Initial line charging

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10-Main steam line hammering

Steam line hammering is the phenomenon when steam & water mix at high pressure. Water hammer is a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change).

As soon as steam leaves the boiler, it starts losing heat. As a result, steam stats condensing inside the pipe work. The rate of condensate formation is high particularly during the start ups when the system is cold. As a result of the condensation, the droplets of water are formed. These droplets of condensate get built up along the length of steam pipework forming a solid slug. When this slug encounters any obstacle such as a bend, it will be brought to a halt abruptly. All the kinetic energy of the condensate slug will get converted into pressure energy which has to be absorbed by the pipe work. This gives rise to the phenomenon of water hammer.

Understanding the term water hammer:

After condensate is formed, the flow inside the pipe has two components, steam and the condensate. The flow velocity of steam is much higher than that of the condensate. During such dual phase flow, the heavy condensate which flows at the bottom of the pipe is pulled by high speed steam. This results in formation of water slug which is much denser than steam travelling with the velocity of steam. When this slug is stopped by any abruption like a bend or equipment, the kinetic energy of the slug will be suddenly converted into pressure energy which will create a shock wave in the entire pipework. The pipework will keep on vibrating until this energy is dissipated in the structure.

Solution:

• Charge main steam line by opening bypass valves only, if line is cold give sufficient time to warm the line
• Keep open all drains of main steam line & ensure condensate is draining from those line
• Ensure operator opens the valve very slowly (if MOV is not provided)
• Ensure all steam traps are working properly
• Ensure all steam lines are covered with insulation to avoid steam condensation
• Ensure NON return valves of attemperator (desuper-heating line) water line is working properly

11- Boiler & steam line uneven thermal expansion:

This condition exists when;

• Boiler light up & start-ups are not done as per OEM recommendation
• Quick light up & start ups
• Improper refractory during carrying out maintenance activities
• Obstacles or foreign material left in Boiler during maintenance
• Damage of steam line supports or stuck up
• Overloading the Boiler or steam line

Solution:

Following post shutdown Boiler maintenance checks

• Ensuring proper refractory during shutdown
• Ensuring no any pressure parts are welded with external parts of boilers like platform, columns, beams etc. which do not undergo thermal expansion
• Conduct preventive maintenance steam line supports & spring hangers
• Strictly following SOPs for Boiler light up & start-ups as per time curve given by manufacturer

12-Not achieving the Boiler rated parameters (Pressure, temperature & load)

This is condition is due to;

• Improper combustion
• Selecting improper fuel (high moisture. High ash content fuel)
• Problems related to combustion system
• Improper air & fuel distribution
• Choked fuel feeding system
• Choked air nozzles
• Leakages in & out of the Boilers
• Not operating the Boilers as per SOP

Solution:

• Check issues related to combustion system like proper opening of air & flue gas duct dampers, direction of rotation of FD, PA & ID fans
• Check & ensure field & DCS instruments are showing correct readings
• Check & ensure proper air & fuel distribution in furnace/bed
• Arrest leakages.

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13-Frequent jamming of fuel feeding system

• This is due to wet fuel
• Operating the fuel feeding at lower speed
• Not providing sufficient air pressure
• Foreign materials in fuel

Solution:

• Ensure correct moisture fuel is being used for Boiler light up
• Ensure proper air pressure for fuel distribution

14-Pressure parts failure

During initial start-up of Boilers there is more chances of failure of super heater coils & water wall tubes.

This is due to;

• Less steam flow through super heater coils: This happens if pressure rising is done without opening the start-up vent valve, or super header drains.
• Low water level
• High firing rate
• High drum level (>100%) can cause water hammering in super heater coil & eventually failure

Solution:

• Follow Boiler light up & start up SOP
• Ensure enough steam is flowing through super heater coils coils during boiler start up
• Control firing rates
• Ensure proper water level & cross check DCS level with local gauges

Why does Boilers explode???

15-Clinker formation:

• Reasons for clinker formation;
• Improper air fuel mixture
• Not achieving 3TS during light up
• Foreign materials in coal
• Foreign materials in bed material or poor quality bed material

Solution:

• Using low moisture coal
• Maintaining proper air fuel mixture
• Achieving proper combustion
• Using coal with no foreign materials
• Avoiding frequent startups and stops
• Use coal with high ash fusion temperature

Other General problems associated during Boiler light up & start-ups are

• Fuel handling chutes jam due to wet coal
• Coal crusher jamming
• Low water level in DM storage tanks
• Power supply constraints for running WTP & other BOPs
• Ash handling chutes & hoppers jam

 

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