STEAM BLOWING STAGES PART-II (BOILER COMMISSIONING) ABC OF THERMAL POWER PLANT

STEAM BLOWING -II

Stage 1b --  Super heater, Main Steam Lines up to ESVs, temporary lines from ESVs to EOTVs, temporary loop from EOTV exit temporary line to overload lines and from there to exhaust. Tap off lines from CRH for Deaerator, TDBFPs, PRDS, HP Heaters 6A & 6B, Gland sealing etc., shall remain closed / isolated. Blowing of this stage shall be concluded by observing the indents on the target plates.

Stage 1c -- Superheaters, Main Steam Lines, HP bypass lines and CRH lines upto Reheater inlet and temporary lines upto exhaust. EOTV provided in temporary loop in HPBP lines will be used for steam blowing. 8 (eight) nos. of blows shall be given through HP Bypass lines to ensure cleanliness of the limbs.

Stage 2a -- Superheaters, Main Steam Lines upto ESVs, temporary lines from ESVs to EOTVs, temporary lines from EOTVs to CRH lines, CRH lines, Reheater, HRH Lines to IVs, temporary lines from IVs to exhaust. RH Desuperheater shall be replaced by temporary spool piece before start of stage 2a  blowing. LP bypass lines shall remain blanked. Blowing of this stage shall be concluded by observing the indents on the target plates.

Stage 2b -- Superheaters, Main Steam Lines upto ESVs, temporary lines from ESVs to EOTVs, temporary lines from EOTVs to CRH lines, CRH lines, Reheater, HRH Lines, LP bypass lines with temporary exhaust pipe. RH Desuperheater shall be welded prior to this stage. 8 (eight) nos. of blows shall be given through LP Bypass lines to ensure cleanliness of the limbs.

The Boiler is started in a normal manner following start up procedure.

Stage  1a, 1b, 1c, 2a & 2b

First steam blow for each of stage 1a, 1b, 1c, 2a & 2b is given at a lower pressure (Drum pressure 25 kg / cm2) and the pipe lines including temporary piping are surveyed for proper expansion and supports. Subsequent blows are given at 50 kg / cm2 Drum pressure.

Boiler pressure is raised to 50 kg / cm2.  The steam lines to be steam blown are warmed up prior to blowing. Firing is shut off and at the same time EOTVs are opened. The opening of temporary valve allows steam to escape to atmosphere at high velocity carrying with it loose and dislodged debris. To prevent thermal stresses in the thick walled shell like Separator and headers the saturation temperature change in Separator is limited to maximum 400C. Hence, for the purpose of steam blowing of all critical piping, the valve is opened at 50 kg / cm2 and closed at 25 kg / cm2 Separator pressure.

The water level in the Separator swings beyond visible limits at the start, during and at the end of steam blowing. There are chances of water droplets getting carried over to superheaters. Hence optimum water level is maintained in the Separator.

The furnace exit gas temperature should be limited to 560 deg. C to safeguard superheaters and reheaters.

Boiler water treatment is carried out with liquid ammonia and hydrazine hydrate to maintain pH above 9 in Boiler water and 5 to 25 ppm of N2H4 in feed water.

Hot tightening of flanges, studs / bolts is done wherever such connections have been made.

During steam blowing MS, CRH & HRH line drains are kept open to atmosphere without any valve for achieving free and uninterrupted flow.

While carrying out stage 2a blowing, suitable dummies are put at LPBP outlet temporary line. Similarly, during stage 2b blowing, suitable dummies are put at IVs outlet temporary lines. Drains of suitable size are provided at the upstream of flange local blanks for the purpose of warming up.

CONTINUED PART-III