Governing System Writeup Tdbfp

GOVERNING SCHEME WRITE-UP HYDERABAD

PROJECT CONSULTANT Reference scheme

: : :

PROJECT

PRAYAGRAJ BARA

M/c. TYPE

NK 63/71-3

W.O.No.

1-0-192-164 to 169

PRAYAGRAJ STPP AT BARA (U.P.) TATA CONSULTING ENGINEERS LTD. HY-TC-MPA1006-0104 (1-306-00-96401)

INTRODUCTION: The boiler feed pump drive turbine (Machine Type: NK 63/71 -3) for the 660 MW turbine is featured with two stop valves, yoke designed five control valve set. The BFP drive turbine design is optimized for minimum inlet losses for the given set of varying steam parameters (viz. pressure, temperature and flow) sourced from auxiliary steam source / cold reheat line or bleed steam line of 660 MW turbine. The boiler feed pump drive turbine is equipped with Electro-hydraulic governing system, which maintains the speed of the turbine corresponding to the set point signal from the feed water controller. During start-up / shut down and low load operation of 660 MW turbine, steam to BFP drive turbine is supplied from auxiliary steam header or cold reheat line. After sufficient load is achieved on the 660 MW turbine, steam is sourced from the main turbine bleed. The governing systems for the BFP drive turbine is designed for the change over from auxiliary steam /cold reheat lines or the bleed line and vice-versa as per the logic diagram. The other features of the BFP drive turbine governing system are a quick closing stop valves, over speed protection, rotor axial motion protection, manual trip, remote trip though solenoids and remote engagement of tripping device. The system also facilitates for on load testing of emergency stop valve and over speed governor pin.

SPEED CONTROL: The speed transmitters (SE 3400A,B & C) sense the turbine speed digitally and feed the actual speed signal to speed controller. The speed controller gives output signal to Electro hydraulic converters through ratio modules. The ratio module splits the out put signal of speed governor for main turbine control valve and auxiliary control valve for smooth change over of steam source from bled steam header to CRH header & vice versa as explained below. The split signal and over lapping (if required) can be fine tuned during commissioning. The Electro hydraulic converters convert the electrical signal into hydraulic signal (secondary oil pressure). CHANGE OVER OF STEAM SOURCE: 1) The admission of steam to BFPDT is controlled by EHTC (Electronic governor) which gives an output to regulate the main steam inlet governing control valve (0801) and Auxiliary steam inlet governing control valve (0802). 2) The controller output is a function of the deviation between demand (speed set-point) and actual speed. 3) The governing control valve (0801) and governing control valve (0802) operate in split range i.e. 0 to 50% of the controller output corresponds to 0 to 100% opening of governing control valve (0801) and 50 to 100% of the controller output corresponds to 0 to 100% opening governing control valve (0802). 4) If the demand (speed set-point) is achieved by throttling the governing control valve (0801) then the governing valve (0802) remains closed. i.e. the demand is catered by the IP turbine Exhaust steam.

STEAM TURBINE ENGINEERING T & C ENGINEERING FORMAT No REV PREP. BY TER-9235-00

00

(SG)

DEPT CODE CKD. BY (KHS)

TCED 415 APPD. BY

DOCUMENT No T0964GSW.DOC (HY-TC-MPA1006-0105) REV 00

(KHS)

SHT 1 of 4

GOVERNING SCHEME WRITE-UP HYDERABAD

PROJECT

PRAYAGRAJ BARA

M/c. TYPE

NK 63/71-3

W.O.No.

1-0-192-164 to 169

5) However if the demand (speed set-point) is not achieved even after fully opening the governing valve (0801) then the controller output increases beyond 50% and governing valve (0802) starts to open. i.e. the demand is catered by the CRH steam or Auxiliary steam (Source with Higher pressure). 6) When the IP exhaust steam pressure is sufficient to cater to the BFPDT then the CRH source is isolated by closing the isolation MOV (LBQ72 AA201).

ELECTROHYDRAULIC CONVERTER (1800 A&B): The Electro-hydraulic converters are connecting links between electronic and hydraulic parts of Electro hydraulic governing system. The output signal of electronic governor is fed to the torque motor of Electro hydraulic converter (I/H). Torque motor positions the control slide, which in turn builds up the secondary oil pressure proportional to the signal. This secondary oil pressure generated is transmitted to the control valve actuator. I/H convertor 1800A caters to 0801 (main control valve on turbine) and 1800B caters to the actuation requirements of 0802 (auxiliary control valve).

CONTROL VALVE ACTUATORS (Servomotors) (1910 & 1911): Servomotors (1910 & 1911) position the main & Aux. control valves (0801 & 0802) corresponding to the main and auxiliary secondary oil pressures generated by hydraulic amplifier. Servomotor mainly consists of a pilot valve, a power cylinder and a feed back system. Upon change in secondary oil pressure, pilot valve provides oil passages to and from power cylinder. The movement of power piston is transmitted to the control valves through levers. The feed back lever resets the pilot valve to its neutral position, once the power piston occupies new position corresponding to changed secondary oil pressure. The system is such that the increase in secondary oil pressure causes the control valves to open and vice versa for control valves to close.

EMERGENCY STOP VALVES (2301A & B) AND SOLENOID VALVES (2220, 2221A & B): The identical emergency stop valves (2301A & 2301B) are of quick closing type and consist of a spring-loaded piston, a piston disc, which is in turn connected to the valve cone through a spindle. The emergency stop valve is actuated by means of solenoid valves (2220, 2221A & B) and a control circuit built in the electronic control cubicle. The logic and interlocking part permits the opening of emergency stop valve only when the control valve (0801) of the turbine is closed. When the solenoid valves (2220 & 2221B) are energized, the start-up oil pressure builds up to the right stop valve and PT (3328, 3329) will activate on pressure high. On activation of PT (3328, 3329), solenoid valve 2221A will energize to build up the start up oil pressure to the left stop valve and PT (3313, 3316) will activate on pressure high. On activation of PT(3328,3329) &PT(3313,3316) the solenoid valve 2220 will de-energize and there by trip oil pressure builds up below the piston disc and PT(3312,3317)will activate on pressure high. Solenoid valve 2221B is now de-energized, and the start up oil of the right stop valve is drained through orifice (5611B).Stop valve right now opens and limit switch (ZSH3303) signal is activated. On activation of limit switch (ZSH 3303) solenoid valve 2221A is de-energized to drain the start up oil of the left stop valve and activating the limit switch (ZSH 3301).On opening of right and left stop valves, limit switches(ZSH 3303 &ZSH 3301) are taken for start permissive of control valve(0801).The STEAM TURBINE ENGINEERING T & C ENGINEERING FORMAT No REV PREP. BY TER-9235-00

00

(SG)

DEPT CODE CKD. BY (KHS)

TCED 415 APPD. BY

DOCUMENT No T0964GSW.DOC (HY-TC-MPA1006-0105) REV 00

(KHS)

SHT 2 of 4

GOVERNING SCHEME WRITE-UP HYDERABAD

PROJECT

PRAYAGRAJ BARA

M/c. TYPE

NK 63/71-3

W.O.No.

1-0-192-164 to 169

opening and closing speeds of emergency stop valves can be set accurately with the aid of adjustable orifices(5610,5611A &B) in the oil drain lines of the solenoid valves. Stop valves are tripped mechanically on the loss of trip oil which holds the piston disc against the spring force. The stop valve is tripped mechanically on the loss of the trip oil which holds the piston disc against the spring force.

TESTER (2309A & B) FOR EMERGENCY STOP VALVE: Testing devices (2309A & B) are provided to check the proper functioning of emergency stop valves (2301A & B) during normal operation of turbine. On operation of the testing device (a three way valve), pressure oil is admitted into the space behind the test piston of the emergency stop valve, which will then push itself against the piston and move the piston and valve spindle towards the closed position partially. After ensuring the piston movement, the three way valve is turned off and normal position of the stop valve is retained.

DAMPING DEVICE (5620A & B ) : Damping device (5620A &B) damp out the quick oscillations in secondary oil pressure generated by hydraulic amplifier. This helps in improving the stability of the governing system.

SOLENOID VALVES (2222A & 2222B) FOR REMOTE TRIPPING : The turbine can be tripped from remotely from control panel by de-energizing the solenoid valves (2222A&B) positioned in series. When solenoid valves are de-energized, it interrupts oil supply to governing system and thus de-pressurizes it. Turbine trips immediately on loss of governing oil pressure.

HAND TRIP VALVE (2250): Whenever the turbine is to be tripped, the governing oil pressure (trip oil line after trip valve) is drained by trip valve. Thus, pressure in front of stop valve piston disc and secondary oil pressure falls, resulting in closure of stop valve and control valves. The hand trip valve (2250) is operated either manually or by loss of source oil pressure.

REMOTE ENGAGEMENT OF TRIP VALVE: Hand trip valve (2250) can be put into operation remotely with the help of a control switch, i.e. a pressure transmitter (PT 3324, PT3325& PT3326) and a solenoid valve (1050). As soon as control switch is pressed, solenoid valve (1050) gets energized and oil flows into hand trip valve (2250) and reset the trip lever. Soon after trip oil pressure is built up, solenoid valve gets deenergized through the pressure transmitter (PT3324, PT3325& PT3326).

DUPLEX FILTER (4520): The Duplex filter with 225 lpm 20 micron filtration is consist of filter head with change over valve and two filter bowls which are removable for maintenance. The change over mechanism

STEAM TURBINE ENGINEERING T & C ENGINEERING

DEPT

CODE

TCED

415

DOCUMENT No T0964GSW.DOC HY-TC-MPA1006-0105 REV 00 SHT. 3 OF 4

GOVERNING SCHEME WRITE-UP HYDERABAD

PROJECT

PRAYAGRAJ BARA

M/c. TYPE

NK 63/71-3

W.O.No.

1-0-192-164 to 169

facilitates redirection of fluid stream from the dirty to clean filter housing without reduction in flow. The arrangement also provided with a maintenance indicator, which monitors the degree of clogging of the filter elements. Arrows indicate which filter bowl is in operation. Closing of both filter bowls simultaneously is not possible.

STEAM TURBINE ENGINEERING T & C ENGINEERING

DEPT

CODE

TCED

415

DOCUMENT No T0964GSW.DOC HY-TC-MPA1006-0105 REV 00 SHT. 4 OF 4