How small ignorance could cause huge damage in APH of boiler

Hello!! all the power plant professionals,today we will discuss about causes of AHP fire and precautions need to be taken to avoid such mishaps and after that I will share one of the real incident of APH fire which I have witnessed then you will come to know that how it could be disastrous.Function of APH is to increase temperature of air supplied to boiler by recovering waste heat from flue gas to increase overall thermal efficiency of boiler.Installation of APH helps in following ways to boiler;

• Increase in thermal efficiency of boiler as most of the waste heat is absorbed by air from flue gas.
• Increase in temperature of air stabilises ignition and improves combustion efficiency of boiler.
• Decrease in fuel consumption and also make it possible to use low grade fuel.
• Helps in increase in steam generation capacity and gives stability during load fluctuations.

These are the advantages of APH but when things are going in wrong direction then it could be harmful to both man and machine.

Now causes of APH fire are as follows,

• The important reason for APH fire is continues accumulation of unburnt fuel fines in gaps of tubes or surrounding duct area;when that accumulated unburnt fuel gets favourable conditions for secondary combustion it starts burning in that area and causes APH fire.
• Maintaining low excess air supply for combustion in furnace; which is ultimately responsible for insufficient amount of Oxygen for combustion and resulting in improper combustion in furnace and results in carry over of unburnt fuel along with flue gas to 2^nd  pass duct and will accumulate and may catch fire.
• Due to air ingress from leakage in flue gas passing duct there will be sudden drop in temperature of flue gas and moisture in flue gas gets condensed and water droplets will forms and resulting in stagnant condition for  particles in the flue gas and will get accumulated and when suitable conditions occurs it may catch fire.
• Maintaining improper furnace pressure which will restrict  flue gas flow and causes accumulation of unburnt in APH and surrounding duct and may catch fire when it gets proper resources for burning.
• If ash handling system for APH is not working properly then hoppers will get full and it may catch fire if it gets.

To avoid APH fire precautions need to be taken are as follows;

• Maintain sufficient amount of excess air for complete combustion of fuel in furnace i.e.try to maintain proper O₂ at the time of operation of boiler.
• Find any leakage in duct and arrest them as soon as possible.
• Try to maintain proper negative furnace pressure for free flow of flue gas.
• Check ash handling systems on regular basis.

If suppose there will be fire in APH then ,How to identify it. There are several conditions which are pointing out towards APH fire while normal operation as follows;

• Rise in temperature of air at APH outlet than normal operating conditions.
• Sudden rise in temperature of flue gas after APH.
• Rise in temperature of flue gas at Super heater.
• Increase in opening of steam temperature controlling attemperator control valves.
• Rise in casing plate temperature from outside.May be checked with temp measuring gun.

Now we will discuss about one of such incident of APH fire which I have witnessed.

This incident I have faced when I was commissioning boiler.Before explaining  actual incident I will tell History in brief.So commissioning of boiler was in its last phase i.e steam blowing ,we were giving steam blow downs as per procedure by the gap of 2 hours with targate plate at its position.And finally at 6:00 pm after 77 steam blows we got clear targate plate upon which turbine personal and customer side manger agreed and signed protocols. So now we all have decided to stop boiler at 7:30 pm so that we can wrap up balanced erection work  as soon as possible to start it again for commercial steam generation.So boiler has stopped by shutting fuel feeding of and and told operator to keep all the fans in running condition till all the temperatures inside boiler comes down to around 50 ⁰ C and we left site at 8:00 pm. But in the name of hurry to leave plant operator stop all the fans at 9:00 pm and by mistake left SA fan in running condition and left plant . When next shift person came and he also hasn't checked that APH temp is rising.Now at around 11:45 pm when we reached plant and found that temperature of APH risen so high that APH tubes started melting and outer casing warped due to heat and protective aluminium sheet also melted.And in control room we found only SA fan running ;immediately I have started all the fans and keep them running for cooling of boiler.  

Melted outer Aluminium cladding sheet

Melted APH tubes clicked from menhole

In night we were not able to identify intensity  of damage done and APH outlet temperature found reached to 550 ⁰ C. And after cutting outer casing plate we found the real damage as shown below,

Damaged APH section of boiler

Almost 6000 no of tubes found damaged.After analysis and investigation we come to conclusion that this thing was happened due to accumulation of unburnt fuel in between gap of APH tubes. Amount of moisture in fuel was high because which fuel was not burning properly and carried over to APH section and accumulated over there. After stopping boiler slowly moisture in the fuel got veporized and dry fuel starts burning as all the fans were stopped before sufficient reduction in temperature of boiler. And natural draft created due to ID fan provided air for combustion and unburnt fuel catches fire.But fortunately half of the section saved due SA fan was running because APH was divided in two sections i.e. SA APH and PA APH .As SA fan was running air was carrying heat along with it and saved metal of tubes from getting overheated.As PA fan was not running PA APH tubes got overheated and melted.

Saved section of SA APH

So I am suggesting all the power plant professionals that be alert on small signals because it could cause sever damage afterwards.Follow all the instructions given above to prevent such incident to be happened in your plant.

Hope this article will help you all ,feel free to ask and if you have any suggestion always welcome.Please subscribe and follow blog as a motivation for more such important articles.