Steam Temperature Control Loop

Attemperator spray control is designed to maintain the steam temperature at 485 deg C. As the heat pickup in the superheater increases with load, the spray water requirement increases with load. The spray water line for the Attemperator, spray water is obtained from the Boiler Feed water main, before the ßow transmitter 11-FE 137. The spray water line consists of the following.

An manually operated isolation valve DSW-VG-101. The isolation valve needs to be opened when attemperator is to be taken into service.

Drain valves DSW-VG-112/113, these drain valves are opened to drain the line for maintenance.

Pneumatically operated ßow control valve 11-TCV-153. The ßow control valve is provided with inlet/outlet Isolating valves DSW-VG-102/108. The inlet/out Isolating valves remain normally open. The drain valves DSW-VG-103/104/106/107 which remain normally closed. These drain valves are opened after closing inlet/outlet Isolating valves, when control valve is to be taken for maintenance. The spray water line connects to the spray nozzle of the attemperator through a non return valve DSW-VC-111. Pressure gauge 11-PI-159, indicates pressure of the spray water ßowing to the nozzle.

Temperature Indicators 11-TE-151 & 11-TE 152 provides steam temperature indication before and after the attemperator to judge the effectiveness of attemperation.

4 Main Steam Piping

The SH steam line connecting the top header of Superheater II to the plant steam main.

This line incorporates the following.

• ELECTRICALLY OPERATED SH STEAM STOP VALVE 11-MV 302This valve Isolates the WHRB from the plant / Common steam header. This valve is provided with an electrically operated, integral by pass valve 11-MV 303.

• SAFETY VALVE 11-PSV-003 This is a spring loaded, valve set at 71 Kg/cm², pressure to protect the boiler against over pressures. The safety valve is similar to Drum safety valves described earlier. The exhaust of the safety valve is piped to a silencer to reduce the noise levels when the safety valve is operating. The silencer is mounted on a separate structure on top of the WHRB.

• START UP VENT VALVE 11-PCV 154 is an pneumatically operated regulating start up vent valve. 11-MV 301 is an electrically operated Isolating valve preceding 11-PCV 154. The outlet of the start up vent valve is exhausted to atmosphere through a silencer. The start up vent valve is to be kept open while start up. If provides initial steam ßow for the superheaters. • STEAM LINE DRAIN The steam line drain

consists of the following valves manually operated MSS-VG-101/102/108/109. These

valves are kept opened during start up upto 5 kg/cm2 pressure.

• FLOW NOZZLE Flow nozzle 11-FE-157 is installed on the steam line to provide impulse to upstream & down stream pressure readings to steam ßow transmitter 11-FT-157. The ßow transmitter reading, after steam pressure & temperature compensation is used for the following,

– Steam ßow reading. (11-FI 157)

– Steam ßow compensation for feed ßow, steam temperature controllers

• SH STEAM TEMPERATURE INPUT

Temperature transmitter 11-TE 153 provide the steam temperature input for the following – Temperature Indicating controller 11-TIC-153

which provides steam temperature High & low alarms and also controls positioning of the attemperator spray control valve as described earlier.

– Temperature compensation signal to the feed ßow, steam ßow instruments.

5 Operational Control

This section explains the major operational control points described in this chapter.

Steam Drum

• Maintain Feed water, Boiler water quality, and phosphate concentration.

• Maintain water level in the drum within permissible low and high levels. The protection system envisages boiler trip at very high and very low levels, which should not be by passed. • Maintain drum level gauge glasses LI 3401 &

PI 34102 in good working condition. Operators may verify the readings of level transmitters with the readings of the drum level gauge glasses once a day.

• Drain superheaters thoroughly during startup. • THERMAL STRESSES IN DRUM DURING

START UP AND SHUT DOWNSteam Drum is a large cylindrical shell. Before light up of a boiler, the inner and outer surfaces of the drum are at the same temperature. When boiler is lighted up, the inner surface gets heated up Þrst by the water (and then by steam) and transmits heat to the outer surface of drum. The heat transfer is by conduction and is a bit slow. For short time after light up, there can be

differences of temperature between steam and water surfaces of the drum. Such a difference can set up thermal stresses, which are not desirable, and an alarm sounds at DCS. To minimize the thermal stresses, the operator must restrict the Þring rate when starting the WHRB by modulating the diverter damper. Boiler water temperature rise rate must not be above 56 0C per hour till operating pressure is reached.

• SWELLING During WHRB startup, as the Boiler water temperature reaches 90°C, there is an increase of water level caused by increase in the volume of hot water. Such swelling, if not controlled, can cause a High Level trip. To avoid this, initial Þlling is normally restricted to low level (say – 100 to 150 mm) and the smart Operator anticipates a swell and uses the IBD to drain and control the level. • Do not operate the WHRB with safety valves

gagged. Passing safety valves must be attended during the next planned shut down.

Super Heaters & Attemporator

• Super heaters must be drained after shut down and cooling of the boiler. They must also be kept open before a cold start up till 2 - 3 kg/cm2 pressure is built up. During hot light ups they are opened for a few minutes.

• Soot blowing of Super Heaters may be done once a shift to keep their surfaces clean, it liquid fuels are burnt.

• Super heat steam temperatures at exit of Super Heater- 1, Super Heater 2 main steam temperatures must be monitored to see there is no excessive heat pick up. Compare these Þgures with predicted performance values. High steam temperatures may mean high metal temperatures.

General

• Boiler water can be drained after a shut down only after depressurizing to 2 kg/sq. cm and after cooling to 80 °C

• Draining of Boiler water must preferably done through the blow down tank.

• If a tube failure is detected, it is advisable to plan for an early shut down. It may be possible to quickly repair the failed tube and return to service. If the shut down is in-ordinately delayed, there are possibilities of larger secondary damages, which may prolong the shut down, required for repairs.

• Manually operated valves must be closed hand tight only. Use of levers on hand wheels is not desired.

5.1 Steam and Water System