Automatic Control

AIM

To describe the automatic controls provided for operation of the WHRB.

Steam Temperature Control

The steam temperature control has following function:

To position the spray water control valve such a manner that S.H. steam temperature at outlet of SH II is controlled at 540 Deg C.

The control acts on spray control valve, which is a pneumatically operated control valve, positioned by positioner.

Drum Level Control

It includes:

• Single Element Control • Three Element Control

SINGLE ELEMENT CONTROL After density compensation the drum level signal is used for indication, control and generation of, Low alarm, and High alarm.Input goes as PV to the single element Controller 11 LIC-142A. Out put of this controller is feed to the level control valve 11-FCV-137. Single element controller should be used up to 30% load of the boiler. Above that select three element control to maintain and control the Drum level with respective to boiler load.

In case of bad PV both control should go to Manual mode automatically. And alarm should be generated.

PV tracking and SP tracking should be provided to this controller for Auto/ Manual bump-less transfer.

In Three element control O/P of 11 LIC-142B goes to the summation block where the steam ßow signal (0-100%) is added to 11 LIC-142B output. Action of 11 LIC-142B controller is reverse. The output of summation block is used as a remote set point for the feed water ßow controller 11 FIC-137. Remote set point calculation is given below.

Remote SP for Water Flow Controller (11-FCV 137) = Drum Level Controller (11-LIC 142B) O/P in % + (Steam ßow signal (11-FT-157) in % * Gain) + (Bias in %)

On Bad value of PV, the controller 11-FCV -137 shall be automatically switched to manual mode. • If (11-FCV 137) is put in manual mode, then

(11-LIC 142B) is to be forced to manual mode. • SP tracking shall be provided for (11-LIC 142A

& B) in manual mode.

• (11-LIC 600A & B) SP shall track its PV in manual mode and shall have its local setpoint. Water ßow is measured using transmitter 11- FT-137 which receives DP from ßow nozzle 11- FE-137, the square rooting to be done at DCS side and is used as process variable for Feed water ßow controller 11- FIC-137. The output of 11- FIC-137 is used to control the feed water control valve 11- FCV-137.

Steam ßow is measured using a steam ßow transmitter 11 FT-157 connected to ßow nozzle; the steam ßow transmitter shall be square rooted at DCS side.

Pressure and temperature compensation of steam ßow is carried out to take care of errors arising due to density variation of steam, when boiler is operating at conditions different from design conditions.

Comp. Steam ßow = Indicated steam ßow *[ Square Root ({ (P1+1.033) * (T2+273) } / { (P2+1.033) * (T1+273) } ]

where

P1- Measured pressure signal (Kg/Cm2) T1- Measured temperature signal in deg C. P2- Design pressure (Kg/Cm2)

T2- Design Temp of steam, in deg C

[P2 = 66Kg/cm2 absolute & T2 = 485+5 Deg C] 11 LIC-142A Controller action should be direct i.e. Controller output will increase if PV increases.

The action of the 11 LIC-142B (Three Element controller) is Reverse i.e. out put of the controller will decrease if the PV increases.

The action of 11 FIC-137 controller shall be direct action i.e. as the Feed water ßow goes above set point; the 11-FCV-137 should close (i.e. the 4-20 mA output should increase).

(11 FIC 137) shall have only two modes of operation. 1. Manual mode 2.Cascade mode (11 LIC 142A & 11-LIC-142B shall have only two modes of operation namely. 1. Manual mode 2. Auto mode

SP shall track its PV in manual mode and shall have its local setpoint.

Indications and alarms to be conÞgured as shown in the control schematic. Trends, Totalizers, indicators and alarms shall be conÞgured in the DCS as indicated the Control Schematic drawing. Totaliser 11 FIQ-137, 11 FIQ-157 to be conÞgured in DCS.

Following are trip interlocks on Drum level Low Low

• a) ID fan trip

• b) Open ßue gas process stack inlet damper (11-DI-004)

• c) Close ßue gas damper to boiler inlet (11-DI-003)

• d) Close damper in recirculation line (11-DI-005)

FURNACE PRESSURE CONTROL

The furnace pressure is measured using pressure transmitter 11 PT 160. This is used as a process variable (PV) and is then compared with a Þxed set point in the Proportional-Integral controller 11 PIC-160. The resultant control signal actuates the damper actuator of the ID fan or VFD of ID fan motor.

ID fan actuator close limit switch 11 ZSC-160 is taken to DCS as start-up interlock to start ID fan motor when started in Bypass damper mode. This is to ensure that the motor starts under no load condition.

• Action of controller 11 PIC-160 is Direct. The action of the PID can be changed at site depending on linkage

• HIC-160A and HIC-160B shall have Auto/ Manual selection.

• On Selection of VFD mode damper of same fan should not go to Auto mode

• Furnace presses Low & high alarms are generated from 11 PIC-160. • Trends, indications and alarms shall be conÞgured in the DCS.

Note

• The ID fan not running signal is taken from drive logic to close the ID fan damper when ID fan not running. • When furnace pressure is controlling

through VFD mode then ID fan damper will be 100 % open & controlling is through VFD

• When furnace is controlling through damper then ID fan motor is in full speed & controlling is through Damper.

STEAM TEMPERATURE CONTROL

The temperature of Þnal steam is controlled by attemperation i.e. by spraying feed water into the steam after the primary super heater.

11 TIC-153 receives Þnal steam temperature as the process value from 11 TT-153. The temperature controller shall have 485 °C set point for the Þnal steam temperature. The out put of the TIC will be given to the Attemperator temperature control valve 11 TCV-153. The controller shall have Reverse action. The controller shall have PV tracking for manual to auto bump less transfer. • Steam Temperature Low,High and High High

alarms are to be conÞgured from 11 TIC-153 • The action of the Steam Temp. Controller 11

TIC-153 is Reverse

• The actuator of Steam Temp. Control valve: 11 TCV-153 is air to close type.

• Fail safe action is Stay put /Tends to Open. • Trends, indications and alarms shall be

conÞgured in the DCS.

SOOT BLOWER PRESSURE CONTROL

The soot blower pressure is measured using pressure transmitter 11 PT 147. This is used as a process variable (PV) and is then compared

with a Þxed set point in the Proportional-Integral controller 11 PIC-147. The resultant control signal actuates soot blower control valve 11-PCV-147. • Soot blower pressure Low & High alarms are

to be conÞgured from 11 PIC-147 • The action of the soot blower pressure.

Controller 11 PIC-147 is Reverse. • Fail safe action is close.

• Trends, indications and alarms shall be conÞgured in the DCS.

DEAEARATOR PRESSURE & LEVEL CONTROL

Pressure transmitters 11-PT-105 measures the deaerator pressure & fed to the 11-PIC-105 for Deaerator Pressure Controller as a PV . The output of 11-PIC 105 acts on the deaerator Pressure Control Valve 11-PCV-105. The controller shall have Reverse action. The controller shall have PV tracking for manual to auto bumpless transfer.

• The action of the Deaerator Pressure Controller shall be reverse.

• Action of DA Pressure control valve 11-PCV -105 is air to open type.

• Fail safe action is air fail to close.

Deaerator pressure High & Low alarms are generated from 11-PIC 105 signal.

Level transmitter 11-LT-102A & 11-LT-102B measures the deaerator level.The output of these given to 11-LY-102 1002 block .The output of 11-LY-102 fed to the 11-LIC-102 as the PV for Deaerator Level controller. The output of 11-LIC 102 acts on the Deaerator Level control Valve 11-LCV-102. The controller shall have direct action. The controller shall have PV tracking for manual to auto bumpless transfer.

• The action of the DA Level Controller shall be direct.

• Action of DA Level control valve LCV-002 is air to close type.

• Fail safe action is open on air fail..

Deaerator Level High & Low alarms are generated from LIC 102

Section C

Topics Covered in this Chapter

♦ Section Overview ♦ Operation Procedure

♦ Pre-requisites to Be Attended Before Start up ♦ Boiler Start Up

♦ Boiler Shutdown

♦ Paralleling WHRB To The Plant steam Mains ♦ Cooling of Shutdown WHRB & Its Preservation ♦ Do’s and Dont’s

♦ WHRB Log Sheet ♦ Emergency Procedures ♦ Alarms and Interlocks ♦ Troubleshooting Chart

♦ Water Quality Recommendations ♦ Safety In WHRB House

1 Section Overview

This section describes the start up, shut down procedures of the WHRB. WHRB operation & safety are also described here.

2 Operation Procedure

(Please refer the P & I Diagram for Waste Heat Recovery Boiler Drg No.- D12-1WH-6203P for the valve references described in this section)

3

Pre-requisites to Be Attended

Before Start up

• All pre-commissioning activities like Refractory Dry Out, Alkali Boil Out, pressure test of the complete system, Safety Valve ßoating check and steam blowing of pipe lines should have been completed prior to start up of boiler for continuous operation

• All instruments should be taken on line and checked for proper functioning. Switch on power supply to MCC and control panel. Ensure all the safety interlocks are in operation and functioning. Necessary personal protection equipment and Þeld safety gear should be kept in place.

• Provide all manholes and hand-holes with proper gaskets. Use sealant for proper holding of gaskets.

• Flush all piping like feed water piping, drain lines, CBD / EBD piping etc with water to ensure no clogging in pipelines.

• Open isolation valves for level transmitters & level gauge for steam drum. Run the feed water pump with minimum opening of ßow

control valve. Fill the system with fresh boiler feed water up to common below normal water level. Close the manual valve at pump discharge and stop the pump.

• Check system for any leaks or mal-functioning equipment.

• Reduce the water level by operating blow-down valve and check the operation of level low alarms from transmitter are functioning properly. Also check readings on level transmitter and level gauges are matching. • Keep water level 2 inches below the normal

operating level.

• Adjust valves for boiler operation according to VALVE SETTINGS.

• Ensure all retractable soot blowers are in rest positions.

• Check electrical and pneumatic motorised operation of valves and dampers

• Ensure ßow and pressure of instrument air and cooling water is as per requirement.