Cftest by Navneetkrishnan

7/27/2019 Cftest by Navneetkrishnan

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CASE STUDY ON CLEAN AIR FLOW TEST OF MILLS

BY: Navneetha Krishnan

INTRODUCTION:

Coal fired thermal power plants with corner fired burner arrangements

are having different lengths of coal piping emanating from the

pulveriser. The primary air supplied, to pulverisers for drying the coal

powder and transporting the powders through the coal piping to the

burners, shall have equal distribution for effective combustion in boiler.

The equal distribution of the transportation air is ensured by

conducting the Clean air flow test prior to the initial admission of coal

in to the pulveraisers. The test is carried out in boiler with status

confirming to the coal firing conditions, i.e. running of one set of ID fan,

FD fan, PA fan, Pulveriser and seal air fan.

TESTING CONDITIONS:

Boiler furnace Air Tightness Test is completed and furnace

bottom seal trough is in service

Primary air and secondary air ducting from fans to boiler and

pulveriser is completed and ATT is completed

Flue gas ducting from furnace to chimney is completed with ATT

ESP is ready after air in leak test completion with all internals

assembled

Pulveriser and seal air fan trial run completed

Coal piping from pulveriser to burner is completed with all

supports and the tapping points provided (2 nos. at 900 apart) as

in Figure 3, in piping at all four corners for taking measurements

of air velocity

The pneumatic gates and control dampers provided in the

primary air circuit should be operable from control room



Erection of Raw coal feeder and the piping to pulveriser should

be completed

Instrumentation of air, flue gas and pulveriser system should be

completed and parameters shall be available in control room

Proper approach and platforms shall be provided in all corners

nearer to the measurement locations of coal piping

Pitot tube, manometer and thermometer are available for

measurements

Markings are done on Pitot tube with values, 0.0436R, 0.135R,

0.238R, 0.358R, 0.507R, 1.492R, 1.642R, 1.762R, 1.816R and

1.956R (R- inner radius of coal pipe) to ensure measurements

from equal area inside the coal pipe (Figure 2)

TEST PROCEDURE:

One set of ID fan, FD fan, PA fan, Seal air fan and the Mill are

started and PA header pressure (750 – 800 mmwc), furnace draft

(-5 mmwc) and secondary air flow (30 %) are adjusted

Pitot tube measurement is taken from inside the coal pipe at the

centre of the coal pipe and air flow is adjusted to around 110 %

of the design air flow by regulating hot and cold primary air

control dampers.

The condition established as above should be continued for the

complete period of test.

Velocity profile in each coal pipe shall be obtained by traversing

the pitot tube at two directions (Figure 3) wrt the markings made

on the pitot tube

Mill air temperature, airflow, mill DP and PA header pressure

indications are noted from control room.

The pitot tube velocity head readings are converted into air

velocity inside the coal pipe using the formula,

V = 0.2365227 √h(t)



where V = air velocity in m/sec

h = DP measured using pitot tube and manometer in mmwc &

t = temperature in deg Kelvin

Average velocity is calculated for each coal pipe. The maximum

and minimum velocity of air in the pipe and their deviations from

the average of the four pipes are also calculated

The deviations shall be within ± 5% of the average velocity for

successful completion of the test.

The velocity measured can be converted into the air flow and can

be compared with control room readings

Airflow = velocity x area x air density

PROBLEMS FACED IN BHILAI UNIT 1:

⇒ The tests were conducted on 01/08/08 on mills A & B maintaining

110 % of design air flow and the deviations from average were

calculated as 10 % & 8 %

⇒ The tests were repeated on 10/10/08 on mills A & B maintaining

140 % of design air flow and the deviations calculated were 7 %

& 8 %

⇒ Mills A & B were thoroughly inspected for choking in MDVs,

classifier vanes and rechecking of orifices at mill & burner ends

also done. Coal pipes of Mill A was cut at two locations and

cleanliness was also ensured

⇒ The tests were repeated on 20/10/08 on mills A & B maintaining

140 % of design air flow and the deviations calculated were 7 %

& 8 %

⇒ The test was conducted on Mill C on 01/09/08 and Mill D on

10/09/08 and deviations calculated from the readings, were not

within acceptable limits



⇒ The measurements taken and calculations done were sent and

clarifications regarding the orifice sizes’ selection were obtained

from BHEL Trichy Designers. BHEL Trichy confirmed that the

selection of orifices was in order

⇒ PA fan was kept running more than a day to ensure the

cleanliness of the coal pipings and the test was repeated in Mills

A & B on 01/11/08. The test results were not satisfactory and the

deviations calculated from the pitot tube measurements were

not within acceptable limits.

SOLUTION OBTAINED:

√ Pitot tube was arranged from BHEL site/ JPL, Raigarh and the

test was done on Mills E, F & G. The calculated results were

within acceptable limits.

√ The clean air flow tests were carried out in other mills ( A, B, C &

D) also and the test results were all within acceptable limits.

√ Customer insisted to repeat the tests with the new PITOT tube

and after getting repeatability of results, cleared all the tests and

cleared for coal firing operation of the unit.

CONCLUSION;

Lot of efforts were put to identify the problem faced in the clean airflow

tests conducted and finally the instrument used (PITOT TUBE) for

measurement was found defective. Hence it is concluded that the

calibration, accuracy and correctness of Instruments used for all tests

are to be doubly verified and ensured for successful completion of the

tset.

Markings done End to beput insidecoal pipe



Connections to U tube Manometer

FIGURE 1



FIGURE 2

STUBS AT 900 APART FOR PITOT TRAVERSE

FIGURE 3

Cftest by Navneetkrishnan

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