7/30/2019 Air Heaters choking

1/2

Home Jun July Months Article

TUBULAR AIR HEATER TUBES CHOKING.... AN EXPERIENCE

PROBLEM:

In one of the commissioning site having 3 units of 210 MW rating, the boilers have been supplied with

Tubular Air Heater. During the process of loading of the First unit for Full load operation, the unit had to be

shut down while cutting in the first mill due to non-achieveable of the Air temperature after air heater. Thiswas causing problems in getting the desired Mill outlet temperature required for proper combustion. Site

pesonnels reported that water was dripping from the bottom of the air heater hopper openings during the

boiler operation. This led to a doubt that there could be a tube leakage in the second pass and so while

checking the boiler the Air-Heater tubes were found completely blocked. No tube leakages was seen while

carrying put the Hydro-Test on the boiler. The chocked tubes were the reason for the very low air

temperature at air-heater outlet.

OBSERVATIONS :

The tube blockages were concentrated only in the MIDDLE and the FINAL block. The FIRST block

was absolutely clear.

1.

The concentration of the blocked tubes were in the LOW TEMP zone i.e. in the area where flue gas

temps are LOW and the air temperature are also low.

2.

The boiler had been operating for a number of days on oil firing only for pre-commissioning and

commissioning and the air temperatures are also low.

3.

Steam Coil Air Pre Heater (SCAPH) was not in service during the entire oil firing period due to non

completion of jobs.

4.

The ambient temperature was also very low (being Winter season).5.

ANALYSIS :

Since SCAPH was not in service, hence the secondary air entering the air-heater block (FINAL BLOCK) was

very low, being winter season. With the boiler being operated only on oil for a very long time, the flue gas

temperature at the FINAL BLOCK air heater was very low at around 60 deg.cent. This combination of low

air and low flue gas temperatures resulted in LOW AIR HEATER TUBE METAL TEMPERATURE, lower than

the DEW POINT resulting in CONDENSATION of the vapors in flue gas. This explains the observations of

the site personnels of water dripping at the air-heater hopper bottom. The condensate with the ash

resulted in chocking of the tubes. The FIRST block did not experience the tube chocking as no

condensation took place here due to higher flue gas temperature and also higher air temperature.

SOLUTIONS RECOMMENDED :

The basic solution is to raise the air-heater tube metal temperature above the Dew Point to avoid the

Condensation mechanism. Hence it was decided that boiler would be taken into operation only when

SCAPH is made ready and available for operation.

FEEDBACK :

The unit was taken into service only after the SCAPH was made available and the unit is operating without

any further report of the said problem.

EXPERIENCES OF OTHER SITES :

A few of our 110MW boilers has been provided with similar tubular air heaters and it has been informed that these sites

never experienced such degree of tube blockage any time.

FEEDBACK FROM BOILERS OF OTHER MAKE :

Boilers of a few manufacturers always goes with Tubular air heaters. They do not provide Steam Coil Air Pre-heater and

the following are incorporated either alone or together for start-up purposes.

Air Heater AIR-BYPASS system.a.FD Fan Hot Air RECIRCULATION system.b.

FEEDBACK OF SITES WITH OTHER MAKE :

ember Article http://www.jantermanter.com/octproblem.ht

2 13-01-2013 22:26

7/30/2019 Air Heaters choking

2/2

In one of the station with a number of 210 MW units, the boiler has been provided with FD fan Hot-Air

Recirculation system. Feedback gathered is that these are not being used and thay do not face tube blockages

experienced are cleared off in annual shut-downs.

A.

In one of the stations having a number of 60 MW units, the boiler has been provided with both FD fan Hot Air

Recirculation and Air-Heater AIR BYPASS system. These systems are not being used and they are not facing

large scale chocking till date.

B.

INFERENCES :

Based on the DATA as stated above, it is inferred that this unique large scale tube blockage experience resulting in unitshut down is because of operation of the boiler on oil at very low loadings for long period.

ember Article http://www.jantermanter.com/octproblem.ht

2 13-01-2013 22:26