Tricks for lowering tube failures in FBC, AFBC boilers

Tricks for lowering tube failures in FBC, AFBC boilers

My Experience & view

Boiler not yet delivered : 

Go in for bigger steam drum size, which should be 50 to 60% of boiler capacity

Boiler already running :

Now the case becomes complicated.

In many installations, the boiler drum size is small which has major implication on the tube life as follows

Low Boiler Drum size creates the following condition

1) Low natural water re-circulation rates

2) Increased exposure of tubes to thermal stress

3) Tube surfaces receive more heat & become red hot & soft

4) Increased erosion due to point no. 3

5) Increased erosion due to increased recirculation rate of the bed material

What is bed material recirculation rate :

Imagine a coin which is flipped in air for HEADS or TAILS. Similar way for FBC & AFBC the fuel particle & bed material particle is flipped.

The maximum average height the bed material has to reach is called Bed Expansion Height

When due to low PA or low FD or high WBP or due to high bed material density, the bed expansion height reduces, it is known as a condition which promotes increased bed material recirculation.

Consider this

When bed material reaches a particular height it takes time & then returns back to same starting position & then again returns to the expanded height earlier

One cycle where it starts & ends at the same point is called recirculation & time take is the recirculation time

Now recirculation is inversely proportional to the time i.e. more the time, lesser the recirculation & vice versa

When bed expansion is lesser the recirculation rate increases & bed expansion is more the recirculation rate decreases

Heat retention in the bed is also directly proportional to recirculation rate, i.e. heat retention increases which higher recirculation rates & reduces with lower recirculation rates due to heat trapping

Tube failures occurrence the causes :

1) KEY or 90% reason is tube surface gets more heated due to smaller drum sizes, low drum level or low water recirculation rate

2) balance is increased recirculation rate of the bed which increases erosion

How to know, whether the current settings are increasing the bed recirculation ?

If the bed material density is over & above the stipulated value of 1000 to 1100 grams per liter then bed recirculation rate has increased

Higher bed recirculation increases mutual bed particle friction leading to increase in density

How to avoid tube failures :

1) Change smaller drum size to bigger drum size if possible

2) Run drum level at 70 or 75%, there will be no water hammering (explained in Steam dryness blog)

3) Set the right conditions for bed expansion by proper setting of PA, FD, SA, WBP etc.

4) Check drained bed density values frequently, once in a day

5) Avoid bed over draining

6) Drain the bed when the level reaches 20 mm above the set point & stop the drain at the set point

For example : WBP is say 500 mmwc, then Start drain at 520 mmwc & Stop draining at 500 mmwc

7) Operate at design furnace draft to keep the exact heat retention time by design (lower furnace draft increases retention time & therefore the thermal stress on tubes)

Results :

I have changed several boiler operations & reset the PA, FD, WBP, draft & drum levels, viz., all the above 

The result is there are no tube failures in AFBC or FBC boilers for over 5 years now & tubes display excellent characteristics to last another 3 years

The tube life has increased even in tubes which are not studded

The trick is to ensure to take care that tubes do not get thermally stressed due to excess heat retention in the bed, low bed expansion & low water recirculation rates, low drum levels.

Wish you happy increase in tube life

for any questions please contact me at

sap@chargewave.in

PS Anand Prakash

Director Technical

Chargewave Energykem Pvt. Ltd.