Boiler Emergencies

Procedures during the following emergency conditions1. Boiler Explosions2. Boiler Implosions3. Low Water Level4. High Water Level5. Boiler Tube Leaks6. Master Fuel Trip

Boiler EmergenciesEmergency boiler procedures : A) Boiler explosions a. Causes 1. Furnace explosions can be caused by an accumulation of unburned

fuel in the furnace due to incomplete combustion, loss of ignition, or fuel valve leakage.

2. With a mixture of unburned fuel with air in explosive proportions, and the application of heat sufficient enough to raise the temperature of the mixture to the ignition point, explosions can occur.

3. The accumulated fuel is ignited resulting in a greater than normal pressure increase.

4. The fuel may enter the furnace in the unburned state in a number of ways, for instance:

a) Through leaky main fuel or ignition fuel inlet valves on idle wind box/burner compartments.

Boiler Emergenciesb) If the fires are extinguished and if the fuel is not shutoff promptly.c) If the fuel is not burning as rapidly as it is entering the furnace.d) In starting up, if difficulty is experienced in establishing ignition. b. Prevention 1) During start-ups maintain a high air flow (up to 30% of full load air flow) to ensure an air rich furnace atmosphere and prevent accumulation

of explosive mixtures.2) Be sure that the main fuel and ignition fuel inlet valves on idle fuel

compartments or burners are closed tightly and do not leak. It is advisable to remove idle oil guns from the guide pipes, to avoid dripping.

3) Watch the fires so that the fuel may be shut off without delay if extinguished unexpectedly. If the fires are lost, trip all main and ignition fuel immediately. Close all trip and fuel shut off valves. Purge the furnace at least for 5 minutes after shut down with 30% (minimum) air flow.

Boiler Emergencies4) When a boiler has been idle, purge the furnace before putting igniters in service. Be sure that the warm up fuel firing rate is high enough to produce a flame not easily extinguished.5)Always use an associated igniter to light off a main oil or gas nozzle.6) Regularly check proper function of FSSS or BMS (interlocks, trips)B) Boiler Implosions : Destructive negative pressure.a. Causes 1) Tripping or sudden loss of fuel results in mass temperature reduction

resulting in rapid decrease in furnace pressure.2) Rapid evacuation of heated gases by ID fans.3) Failure of fan controls.b. Prevention1) The air flow to the furnace must be maintained at its pre trip value

and must not be prevented from increasing by following natural fan curves; but positive control action to increase air flow is not allowed.

Boiler Emergencies – water levels2) The flow of combustion products from a furnace must be reduced as quickly as possible following a unit trip.3) If the removal of fuel from the furnace can be over a 5 to 10 seconds period (rather than instantaneously) there will be a reduction in the magnitude of a furnace pressure excursion that follows a unit trip.4) Further prevention of implosion can be accomplished by keeping protective control systems in proper working order and well tuned.C. Low Water Level a. Causes 1. Control failure 2. BFP failure 3. Tube leaksb. Action 1) Compare indication.

Boiler Emergencies- water levels2) If the water level falls out of sight in the water gauge, due top failure of the feed water supply or neglect of the operator, except in cases of momentary fluctuations that might occur with extraordinary changes in load, appropriate action should be taken at once to trip the fuel. Any decision to continue to operate, even if only for a short time at a reduced rating, would have to be made by someone in authority who is thoroughly familiar with the circumstances that led to the emergency and positively certain that the water level can be restored immediately without damaging the boiler. In the absence of such a decision : a) Trip all fuel immediately.b) Shut off all steam discharged from the unit( trip turbine, steam driven

auxiliaries etc.)c) Simultaneously, if feed water has become available and the operator is

assured that no pressure parts have been damaged, gradually reduce the flow of feed water to the boiler by manual regulation. This will avoid quenching of hot pressure parts with relatively cold water. The feed water regulating valve should be closed completely when all the steam flow from the unit has ceased.

Boiler Emergencies-water levelsd) Maintain a high air flow at first to hasten the cooling process.e) If pressure parts damage is suspected, reduce the main steam pressure gradually by opening the super heater startup drain. Open the drum vents when the pressure drops below 1.75 bar. As the boiler cools, reduce the air flow. Shut down the fans as soon as the unit is cool enough for a man to enter. Drain the boiler when the drum metal temperature is 93C. Determine the cause of low water and examine the boiler for the effects of possible overheating such as leaks and distortion of pressure parts.D. High Water Level :Water impingement may cause turbine damage.a. Causes1) Feedwater control malfunction2) Operator errorb. Action1) Abnormally high water level should be avoided as it may lead to

carryover and even priming. If the water level rises above rises above the recommended normal operating range proceed as follows

Boiler Emergencies-water levelsa) Reduce the water level immediately by opening the intermittent

blowdown valves. b) Reduce the steam rate, if necessary, and place feedwater control on manual.2) If priming should occur, as indicated by rapid fluctuations in outlet steam temperatures in outlet steam temperature, proceed as follows:a) Reduce the steaming rate.b) If the water level is abnormally high, reduce the level by opening the

intermittent blowdown valves and place feedwater control on manual.

c) Investigate the water condition (alkalinity and solids)d) Investigate the condition of the drum internals as soon as an

opportunity is afforded.

Boiler Emergency-Tube failure

Boiler tube failures : Operating a boiler with a known tube leak is not recommended. Steam or water escaping from a small leak can cut other tubes by impingement and set up a chain reaction of tube failures. By the loss of water or steam, a tube failure can alter boiler circulation or flow and result in other circuits being overheated. A tube leak can also cause loss of ignition and, if reignition occurs, a furnace explosion. An investigation of tube failure is very important so that the conditions causing the tube failure can be eliminated and future failures prevented.The investigation should include a visual inspection of the failed tube. In some cases a laboratory analysis or consideration of background information leading up to the tube failure is required. This information should include the location of the failure, the length of time the unit has been in operation, load conditions, start up and shutdown conditions, and feed water treatment.

Boiler Emergencies-tube leaks5. Tube Leaksa. Indications of the tube leaks are 1) Feedwater flow greater than steam flow2) Excessive desuperheater flow3) Wetted insulation of water running down casing with no apparent

cause.4) Excessive make up.5) Noise from the boiler (Acoustic noise level indicators)b. Action6) Furnace tube leaks a) The action taken is dependant on the size and location of the tube

leak 1) A large leak may extinguish fire or cause loss of drum level. 2) A medium size leak may force an immediate shut down.

Boiler Emergencies-tube leaks3) A small leak may allow continued operation after considering its consequences on continued operation.b) If at all possible locate the leakc) In the vent of failure of one or more water carrying tubes, the best method of shutting down the unit will be dictated by the size of the failure, the ability to maintain normal water level and the demands for the service of the unit.d) The following instructions regarding tube failures are of general nature. It must be understood that conditions may arise which will require exercise of judgment by the operators.e) In case of a leak or tube failure which does not involve a serious drain on the feedwater supply, the water level should be maintained and the unit taken out of service in the normal manner.f) If the tube failure results in a loss of water so great that the water level cannot be maintained with the feedwater supply available, use the method outlined below. 1) Trip all fuel

Boiler Emergencies-tube leaks 2) Shut off feedwater to the boiler. 3) Maintain only enough air flow to carry the escaping steam up the stack. 4) Leave the fans in service until pressure is off the unit. g) After the unit has cooled enough to permit a man enter it, make a thorough inspection of pressure parts for any indication of damage resulting from loss of water level. After the necessary repairs have been made, apply a hydrostatic test and obtain the approval of the proper authorities before putting the unit back in service.2) Economizer Tubesa) An economizer tube leak can be detected by sound and/ or increased

make up water requirements. The leak should be detected at the earliest possible time and the unit shutdown in the normal manner.

b) Water leaks in the economizer can cause considerable erosion damage to the adjacent tubes. Water carried over from economizer tube leak may cause plugging of hoppers and airheaters.

Boiler Emergencies-tube leaks3) Super heater Tubes a) A small leak in a superheater element should be investigated at the earliest possible time, and the unit should be shut down in a normal manner. Steam leaks in the superheater can cause considerable erosion damage to the adjacent tubes. Any decision to continue operation with leaks should be made this in mind.b) A major superheater tube failure may require an emergency unit shutdown.6. Master Fuel Tripa) Under conditions requiring a master fuel trip through normal interlocks

or operators action (manual trip), all fuel should be tripped instantaneously.

b) The following steps should be taken immediately following an emergency fuel trip.

1) Maintain the unit air flow at the pre-trip value for at least 5 minutes to purge the system.

2) Check to ensure that all the fuel nozzles shut off valves are closed.

Boiler Emergencies-tube leaks

3) If all auxiliary power sources are lost during a trip, upon restoration of power, start the ID and FD fans and purge the furnace for five minutes.

Other boiler emergenciesAir preheater fires/tripping of APH ( one fan operation)Coal mill explosions ( may lead to boiler trip)Loss of furnace sealLoss of scan supply of any elevationLoss of instrument airLoss of cooling water to scaffold doorBending of long retractable soot blower / furnace probe

BOILER TRIP Causes

With the exception of the operator push button trip, following conditions will automatically generate Master Fuel Trip (MFT)• Loss Of All ID Fans•Loss Of All FD Fans•Less Than Two Circulating Pumps Running•Circulating Δp Less Than 0.6 Bar•Drum Level Low-low (-330 mm).•Furnace Pressure More Than 330 Mmwc Or Less Than -250 Mmwc•Loss Of All Fuel•Air Flow Less Than 30%•Turbine TripReheater Protection Trip Unit Flame FailureLoss Of Unit Logic PowerEmergency Push Button.Cause of trip system : The first boiler trip command that causes a MFT will illuminate the appropriate indicator in the cause of trip section on the console insert. Any successive boiler trip commands to the other indicators are blocked. There will be only one indicator that will be glowing