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Heat Recovery Steam GeneratorI&C Training Program

LPG TRAIN-4 PROJECT _ AT MINA AL-AHMADI REFINERY - HRSG

NOOTER/ERIKSEN s.r.l.

Giuseppe Battaglia

TOPICS

• Main reference documents• Process Description and P&ID• Involved systems• Operating modes• Instruments• Control & Automation• HRSG Protections• HRSG Status definition, Ramp rate, Sequences

Main Reference Documents

The presentation refers to the following documents:1) Piping and Instrumentation Diagrams• HP SH System• Exhaust Gas System• Fresh Air Fan System• Duct Burner System• Sampling System• Service System2) HRSG Control and Automation Narrative and Diagrams3) HRSG Protection and Operating Mode Narrative and Diagrams

Process Description

The Steam Generator characteristics are:1. Drum type, Natural circulation & vertical tube2. One pressure level3. Horizontal gas flow4. Heat Recovery Boiler for use with Gas Turbine5. Duct Burner for supplementary firing6. Fresh Air firing facility for Steam Generation without Gas

Turbine7. T-box / by-pass Stack to connect GTHeat Recovery Steam Generator supplies HP superheated steam into the steam headers for process use.

HRSG, Drum type, Natural circulation & vertical tube

Process Description

Process Description

Process Description – HRSG mail parts1. Inlet duct (GT

connection)2. By-pass System3. Transition duct &

Duct Burner4. Horizontal Duct with

H.E.5. Exhaust gas stack

6. Economizer7. Evaporator8. Boiler drum9. Superheater 1&2

10. Fresh Air Fan11. Sampling skid12. DB skid13. Sealing sys.

12

34

5

67

8

9

1011 12 13

Exhaust Gas Circuit

Water/Steam Circuit

Main Equipments

P&ID – Exhaust gas system

HP DrumBy-Pass Stack

Burner MainStack

Multi LouverDampers

TEGSH-EVA-ECO coils

P&ID – HP EVA system

BFW control valve

HP Drum

BFW

ECOEVA

Sat. Steam

P&ID – HP SH system

DSH Control Valve

DSH nozzle

Start Up VentControl Valve

Sat. Steam

SH1SH2

HP SH Steam

DSH Water

P&ID – Fresh Air (FA) System

P&ID – FA Fan Steam Turbine

Steam Turbine

ST Bearings oilcircuit

ST Trip Valve

ST Governor Vlv

MP steam

LP steam

P&ID – Duct Burner system

Local control panel To Pilots

To BurnerElements

NG Burner control & protection skid

P&ID – Duct Burner Elements

Burnerelements/runners (7)

Pilot

Main burner

Flame scanners

P&ID – Sampling SystemFeedwater

Boiler water

Sat. steam

SH steam

9,5 pH;0-1 µS/cm

9,5 pH;50-100 µS/cm 0-1 µS/cm0-1 µS/cm

P&ID – Service System

Instrument air

Nitrogen

Cooling water for ST and sampling

Process Flow Diagram

Boiler Safety Valves

Involved Systems

The following control & protection systems are involved during HRSG operation:

• Distributed Control System (DCS)• Gas Turbine Safeguarding System (GTS)• Emergency ShutDown System (ESD)• Burner Management system (BMS)

Systems Not supplied by NE

DCS FunctionsThe DCS performs the following functions:

• Supervises HRSG start up/shut down• Measures or detects process conditions• Displays the status information from ESD, BMS and GTS• Automatically computes required adjustments, based on the

deviations between the process conditions and the relevant set points and records/trends of the process conditions

• Outputs references to control elements in order to control the process.

• Supervises, controls and managements Burner elements• Transfers status information to the BMS and ESD

GTS Functions

The GTS performs the following:

• Supervises GT operation and control• Protects the GT• Transfers status information to the DCS, BMS and ESD• Purge sequence (GT+By-pass Stack)

ESD FunctionsThe ESD performs the following:

• Supervises HRSG protection, automatically by-pass the TEG or trips the GT (via GTS) or trip the Duct burners (via BMS) in emergency situation

• Detects process conditions (trip initiators)• Transfers status information to the DCS, BMS and GTS• Purge sequence (HRSG Duct)

The BMS performs the following:

• Supervises Duct Burner operation and performs relevant burner elements light off / shut down sequences

• Detects process conditions (duct burner trip initiators) and protects the Duct Burner

• Transfers status information to the DCS and ESD• HRSG/Duct Burner Purge

BMS Functions

MODES OF OPERATION HRSG in FA mode (GT in OPEN CYCLE)

• In this operating mode the Turbine Exhaust Gases (TEG) produced by gas turbine (GT) are directly discharged to the atmosphere by means of the by-pass stack.

• HRSG operation is allowed in fresh air (FA) operation with duct burner only (the HRSG stack damper is open ).

• In FA mode the duct burner purge and light-off logics will be inhibited by BMS until the HRSG trip causes (monitored by ESD system) and the duct burner trip causes (monitored by BMS and ESD systems) are removed.

• When the HRSG and duct burner safe condition has been restored it is possible to purge and start the HRSG and duct burner in fresh air operation and then change over in combined cycle mode operation (GT+HRSG) when required by Operator.


FA fan is provided by two drives:1. main drive is an electric motor2. stand-by drive is a steam turbine.The drives changeover from electric motor to steam turbine is manually performed by local operator, whereas from steam turbine to electric motor can be in auto mode from DCS.Electric motor has local/remote start-up and shut down facilities. Steam turbine has local start-up facilities only as well as local shut down and remote ESD command.

During the HRSG FA mode operation it is recommended to change over from electric motor to steam turbine, in order to avoid an HRSG trip due to electric motor failure.FA fan clutches automatically couple the actual driving machine (Steam turbine or electric motor)


STEAM TURBINE LOCAL (MANUAL) START UP PROCEDUREPrior to start-up (locally) the Steam Turbine, all the following conditions has to be checked:

1. MP Steam turbine header pressure > min

2. MP Steam turbine header temperature > min

3. LP Steam turbine header pressure > min

4. LP Steam turbine header temperature > min

5. FA fan damper close (231-ZSL-6644A) (If FA fan is not running by means of the electric motor)

6. Turbine exhaust steam side open (HV-403A manual isolation valve open)

7. Check oil level in the reservoir (H-231-001A-V2) (fill if necessary)

MP SteamHeader

LP SteamHeader

HV-403A

Oil tank

Woodward GovernorPeak 150


1. Turbine Oil pump start up (either by local or remote command from DCS)

2. Reset trip from ESD. Depress local emergency trip reset button to energized solenoid valve. Then reset emergency trip lever (steam turbine trip valve 231-HV-6650A opened)

3. Check MP steam line steam trap in operation and MP steam pipe to turbine free of condensate

4. Since all process and other permissive to start conditions are met, depress local start button; the governor valve (SCV-6651A) goes wide open

5. Open MP steam to turbine system manual bypass isolation valve (HV-401A) and slowly open MP steam to turbine system main manual isolation valve (HV-400A), so that the inrush of steam starts the rotor spinning

6. Check steam turbine drains steam traps in operation

MP steam by-pass valve

Trip Reset

Oil Pump

With the above permissive, the Steam Turbine Manual start up procedure consists of:

Reset valve trip lever

Local Start

Governor ValveMP steamMain vlv

ST drainsMP drain


7. Since the governor senses the speed, it closes the governor valve (SCV-6651A) and adjust the steam flow so to ramp up the speed up to the programmed rated speed

8. Fully open MP steam to turbine system main manual isolation valve (HV-400A) then back one-quarter turn to prevent the valve from sticking in open position and close relevant bypass valve (HV-401A)

9. Adjust the speed set point to the desired operation point by depressing speed raise or lower buttons onto the ST governor

Governor Valve

Woodward GovernorPeak 150

ST by-pass and mainSteam valves

MODES OF OPERATIONCOMBINED CYCLE MODE

(GT+HRSG)After the GT+HRSG purge, in this operating mode the TEG are directly sent to the HRSG. The by-pass stack damper is closed; the HRSG damper is open as well as HRSG stack damper. There isn’t provision for diverter dampers system in intermediate position.The combined cycle operation includes:• simple recovery• recovery with supplementary-firing In the first mode the HRSG receives enough TEG to produce the required steam.When the GT has reached the 60% of the load it is possible to light-off the HRSG supplementary firing (duct burner) in order to increase the steam production and satisfy the steam demand.

MODES OF OPERATION – CHANGE OVER

HRSG Sequences into DCS will allow manual change over (initiated by operator) between the two HRSG operating modes: HRSG on in FA mode and HRSG in CC mode with supplementary firing on.

PURGE REQUIREMENT(GT+HRSG)

• A cold purge of both the HRSG enclosure and GT+By-Passsystem shall be completed prior to the light-off of the GT and prior to the admission of combustion turbine exhaust gas into the HRSG.

• The purge sequence shall be in accordance to the GT manufacturer’s requirements and NFPA 85 code, considering HRSG volumes to be purged.

• Purge shall be accomplished by at least five volume changes and for a duration of not less than 5 minutes. During the purge, the flow rate shall be not less than 8% of GT full-load mass air-flow.


Two different phases compose this purging sequence:1. phase 1, the GT purge sequence purges the GT and the By-

pass stack2. phase 2, after the above phase the ESD monitors the purge

of the HRSG duct using the Unfired GT airflow

As alternative and during purge phase 1, the phase 2 (HRSG purge) could be performed by BMS (HRSG/Duct Burner purge) using fresh air from FA fan and then from DCS the operator can change over the HRSG from FA mode to CC mode (with GT stillnot firing and in purging condition).After the HRSG purge and during combined cycle mode, the GT load ramp has to respect the permissive for the HRSG warming up, following the temperature ramp rate (Start up curves) indicated in the HRSG manufacturer’s manual.


During the combined cycle operation, and following the GT+HRSG purge operations as above indicated, the GT is permitted to switch back in open cycle mode (by closing the diverter dampers system by DCS) and to purge and operate the GT in open cycle through the by-pass stack.

When the GT is operating in open cycle through the by-pass stack, then the HRSG Purge completed condition (Purging phase 2) is reset by ESD. In this case the TEG is not permitted to re-enter the HRSG at a later time without HRSG re-purging (the opening of the diverter system dampers is not allowed by DCS and ESD).


When the GT is running in open cycle mode through the by-pass stack, in order to include the HRSG and operate in CC mode the following steps shall be necessarily performed:1. HRSG duct shall be purged in FA mode (by means of the FA

duct burner purge sequence, managed by BMS logic).2. In FA mode it shall be possible to light-off the duct burner and

start up the HRSG (following the FA start up curve/ramp rate).

3. Then the TEG shall be permitted to re-enter the HRSG at a later time (by activating the manual change over sequence from FA mode to CC mode) without lowering the GT load.

4. TEG shall not be permitted to re-enter the HRSG (logic by ESD) until the HRSG is in service, pressurized and in hot status (i.e. the Boiler drum pressure is higher than 27 barg).

As alternative GT shall be shut down and an integrate (GT+HRSG) cold purge shall be performed.

DUCT BURNER PURGE REQUIREMENT

The duct burner purge (BMS) shall accomplish eight volumes change of the HRSG volume with a fresh air flow not less than 25% of full-load mass flow rate or the minimum flow necessary for operation of the duct burner, whichever is greater.The duct burner purge takes 5 minutes.

HRSGBy-Pass System

By-pass system is realized by means of two dampers (multi louver type), one to isolate the by-pass stack and another to isolate the HRSG inlet. The system includes sealing air also, in order to guarantee multi louvers tightness of 100 % when in close position. The two dampers have a single pneumatic actuator and are mechanical linked in order to have the following two configurations:

• System open: By-pass stack close and HRSG inlet open• System close: By-pass stack open and HRSG inlet close

Intermediate positions are not allowed.

INSTRUMENT– General Notes

• Instruments are conventional type, 4-20 mA signal, I.S., Smart Hart protocol.

• Instruments related to the safety functions are in voting 2 o o 3 or 2 o o 2 and are connected to the safety systems (ESD or BMS)

• Instruments related to control and monitoring functions are in voting 1 o o 1 and are connected to the DCS

Control & Automation

Main Control Loop

HRSG main control loops and automations are:

• HP Drum Level Control• HP Drum Continuous Blow Down Control• HP Steam vent control• HP Steam temperature control• Duct Burner Load Control and elements managements• Condensate drain valves automation

General Notes

• The controller function blocks within the DCS provide auto/manual facilities on the outputs to the final control devices.

• Auto/Manual selection is indicated to the operator.• The bump-less switching is provided during the Automatic to Manual control

and tracking functions.• Flow signals from the field transmitters are linearized in the DCS (square

root inside DCS).• Controllers actions: the error between a set point (SP) and the measure of

the same variable (PV) is calculated as: “SP – PV”, thus positive if set point is bigger than measure; this error is the input of the controllers in the control loops logic.

• The PID controllers is considered “direct” if a positive error in input, generates an increment of the PID output, “reverse” if generates a reduction of the PID output.

• Final Tuning of controllers and threshold settings will be done during the commissioning.

The purpose of the control is to maintain a constant water level inside the HP steam drum during any load condition (during start-up and in the transient states due to load changing) by modulation of the boiler feed water control valve.

HP DRUM LEVEL CONTROL

• Operation below Low-Low level in any of the drums could cause overheating of the tubes and headers, and must initiate a HRSG trip.

• Low level alarms the operator to allow corrective action to be taken before Low-Low level is reached.

• Operation above High-High level in any of the drums could cause water droplets carryover and subsequent fouling of superheatertubes or other downstream equipment. High- High level initiates automatic action to immediately reduce water level.

• High level alarms the operator to allow corrective action to be taken before High-High level is reached.



Involved process variable

P compensated


P, T compensated

Involved processvariable


Involvedprocessvariable

T compensated

• During startup, when water starts to boil, a drum level swelling takesplace due to the sudden formation of steam bubbles in the evaporator

• This swelling can cause a high-high drum level trip

• To prevent this problem, level setpoint during startup is lowered and goesback to normal value as drum pressure reaches operating levels


HP Drum Set Point Generation

050

100150200250300350400450500550600650700750800850900950

1000

0 10 20 30 40 50 60 70 80 90 100HP Drum Pressure [%]

HP

Dru

m L

evel

(mm

)

NWL

Set Point

Start-up Set PointTo be adjusted during

Commissioning

NWL (640 mm)

100% Boiler pressure is 64 barg [Case 10 Fired FAF]

Control philosophy could be single or three elements base on operating condition.The control philosophy will be:• SINGLE ELEMENT TYPE during the start-up or during normal operation

when the steam flow is less than 30%• THREE ELEMENTS TYPE for higher steam flow (>30%).An automatic switch between single element and three element [in both direction] is foreseen by the status of a threshold [about 30% - 27 t/h] on the compensated steam flow.Single element control is automatically selected in case of a discrepancy between HP Feedwater total ingoing compensated flow (see after) and compensated superheated steam higher than 10%. Return to three element control is manually done by the operator.The operator can, however, choose single element control even at steam flows more than 30% by selection.Suitable tracking logic is required for the single and three element controllers to ensure a bumpless transfer between the control modes.


• Single-element control isbased on the action of aPID controller which actsaccording to the level PVand SP.

• Three-elements control isbased on the action of aPID controller which actsaccording to the level PVand SP; the output isscaled and added to theHP main steam flow,generating the SP withwhich the PI controlleracts according to the HPFW flow.


Single elementcontrol

Three elements control

ECO

Boiler Drum

Bo iler feed water

LCVM

LT

Steam Flo w

DCS

General typical control loop, not related to the plant


ECO

Boiler Drum

Bo iler Feed water

LCVM

LT

Steam Flo w

DCS

PT

LEVEL COMPESATION



ECO

Boiler Drum

Bo iler feed water

LCVM

FT

LT

PT

Steam Flo w

DCS

FT FLOW BALANCE:

FT steam- FT water



SP function generator – start up


FF/Bias

One element

Three element


HP DRUM CONTINUOUS BLOWDOWN CONTROL

The purpose is to maintain a water chemistry inside the drum to correct value by modulation of the blowdown valve .

Operator fixes continuous blowdown flow SP, about 1% of HRSG steam flow.The FIC compares the SP with the PV. The error is processed by a PI algorithm.The FIC output is the valve opening/closing request.

Involved process variable

HP DRUM CONTINUOUS BLOWDOWN CONTROL

HP STEAM VENT CONTROLThe purpose of the HP Steam vent control valve is to control the rate of water temperature (pressure in drum, water saturated condition) into the HP drum during start up phase, by modulation of the HP steam vent control valve. More over at the end of the start up phase the controller SP is set in guard of pressure, in order to prevent the opening of PSV.

HP STEAM VENT CONTROL


Control element

Drum Pressure


During start-up, the HRSG has to follow the HRSG temperature ramp rate (HP drum temperature gradient), in accordance to the HRSG temperature rise gradient indicated by the HRSG Manufacturer Instruction book.

Because drum temperature is related to the drum pressure (saturation condition) and pressure measure is more easy and reliable to be read, temperature gradient is calculated starting from HP Drum pressure.

Temperature gradient set-point is generated in accordance to boiler start-up curves (8,5°C/min).

During startup phase, the controller TIC compares the set point (SP) with actual boiler gradient temperature [d/dt of saturated temperature] and processes the error by means of a PID algorithm.The TIC controller is activated only when the GT is ON and the HP drum pressure reaches 1 barg.

Once a pre-defined operating pressure is achieved (boiler drum pressure: 64 barg [Case 10 Fired FAF GT shutdown]), the vent TIC control will switch to a PIC control, in order to maintain the reached steam pressure. Set Point for PIC control is the operating HP SH pressure achieved (i.e. 62,1 barg) and his measured variable is the SH steam pressure.The resulting signal (coming from TIC/PIC) is sent as output signal to the start-up vent valve through a common Auto/Manual station.

At “GT Flame ON” the vent will be positioned according to the actual HP Drum pressure as per the following table (to be tuned during commissioning) [step function]:


HP Drum pressure

[barg]

Vent opening

[%]

< 1 50>=1 40

>=40 10>=64 0

In order to ensure a minimum steam flow through the heat exchangers, during the start-up phase a vent valve minimum opening of 10% is foreseen.The same minimum opening is applied, during the normal operation, if the main steam MOV is detected closed. This minimum opening has to be applied also in manual mode.

When boiler drum pressure is equal or higher than 64 barg [end of start-up phase] AND the main steam valve is detected not closed, then the PIC set point will switch from 62,1 to 68 barg (HP SH pressure).By this way the vent will be driven to the closed position and will stay in guard of pressure, in order to prevent the opening of PSV (PSV Set Point = 69 barg). When GT is OFF the vent is driven to closed position.


Vent valve positioning during start up


start up ramp rate control

Vent valve positioning and guard of pressure after start up


HP STEAM TEMPERATURE CONTROLThe purpose of the control is to maintain constant the temperature of the high pressure steam by means of the desuperheatersituated between the HP SH1 and HP SH2, on the basis of the final HP steam temperature.

Control element


HP STEAM TEMPERATURE CONTROLDuring normal operation the HP main steam temperature set point value is fixed by process design at 441°C, operator can apply a bias (+/- 5°C). This set point is compared with the HP main steam temperature and elaborated by a TIC controller [master-direct]. The output of this controller is passed as a Remote Set Point to the temperature controller TIC [slave-reverse] to position the desuperheater valve to maintain a temperature downstream the desuperheater.The Remote Set Point to slave controller is scaled and limited to the operating range of the desuperheater. Even if between the operating range, the Remote Set Point to slave controller is also limited to a value that is at least 30°C above the steam saturated temperature [calculated from HP drum pressure].Suitable tracking logic is to be provided for the master controller whenever the slave controllers are limited or forced to the closed position.

The output of the slave TIC is by-passed completely and the relevant desuperheater valve forced to close in the following cases:

1. When compensated HP steam flow is less than 25%.2. To prevent excessive opening of the desuperheater valve, when the

difference between the HP steam temperature downstream of the desuperheater and the saturated steam temperature is less than 20°C. Saturated steam temperature is calculated starting from the HP drum pressure.

3. When the final steam temperature is 10°C below set point.

HP STEAM TEMPERATURE CONTROL

SH-1HP drum

SH-2

Vapore A.P.

Attemperating water

TT

Control system

TV


SH-1HP drum

SH-2

Vapore A.P.

Attemperating water

TT

Control system

TT

TV


Cascade Control

Master Controller

Slave Controller


DUCT BURNER LOAD CONTROL

Control element


The purpose of this control loop is to control the duct burner firing by modulating the fuel valve on the basis of the plant load control.

Set Point definition:• Set point [Nmc/h] is calculated as a function of the remote set point

[MW] (from Balance Of Plant load demand).• A set point limitation is foreseen depending on the CC/FA operating

mode: in case of FA operating mode the maximum allowed set point is 44 MW, in case of CC operating mode is 32.3 MW with (GT >=60% load, 0 MW with GT at reduced loads).

• Set Point is also limited if any of the 7 rows of the burner is out of service to avoid either loss of the flame or too long flame

• During Start-up phase the duct burner gas control valve is positioned in the “Start-up” position


HRSG limits are provided by DCS in order to limit the Duct burner gas control valve maximum opening: • HP Main Steam volumetric flow, to avoid too high steam velocity in

the steam pipe• HP Main Steam pressure, to avoid too high steam pressure in the

steam pipe.• HP SH2 Outlet temperature, to avoid too high steam temperature in

the steam pipe.


HRSG limits

Burner limits


DUCT BURNER ELEMENTS MANAGEMENT

Drain Pot Condensate RemovalThe unit have been supplied with condensate pots downstream of the desuperheating stations and at the bottom of the SH banks.

The purpose of these pots is protection against quenching of tubes from over spraying (incomplete evaporation of water) or steam condenses, improving the life time of the components as well as avoid the water hammering in the steam lines during GT start-up.The pots are fit with temperature detectors to automatically operate the pneumatico on-off valves in the case that excessive water builds up in the pots. During the commissioning of the boiler, the frequency of operation (for DSH drain pot) should be noted by plant personnel.If any significant change to this frequency occurs, plant operators are responsible for diagnosing and correcting the root cause of the over spraying of water. It is advised that an alarm signal be generated upon the opening of the drip leg condensate pot.

Drain pot condensate removal

Pneumatic on-off vlv

DrainPots

Temperature elements

The valve can be opened/closed either:

1. manually by the operator from DCS interface or

2. in automatic mode when required by process conditions.

The valve working mode (automatic or manual) will be selected by the operator.The valve is open/close type, and will travel full stroke when the open/close signal is initiated.


Drain pot condensate removalOPEN PERMISSIVE(temperature in drain pot is lower than saturation temperature at HP Drum pressure + 20°C) OR (HP Drum Pressure is lower or equal to 0,4 barg)

OPEN REQUESTBy the operator in manual mode or in auto mode if:(temperature in drain pot is lower than saturation temperature at HP Drum pressure + 20°C AND [GT in Crank Mode OR GT Running OR FA fan running]) OR (HP Drum Pressure is lower or equal to 0,4 barg).

CLOSE PERMISSIVEAlways present

CLOSE REQUESTBy the operator in manual mode or in auto mode if:(Temperature in drain pot is 30°C higher than saturation temperature at HP Drum pressure OR [GT NOT Running AND FA fan not running AND GT start-up sequence NOT initiated]) AND (HP drum pressure higher than 0,4 barg).

HRSG & DUCT BURNER PROTECTION

HRSG shutdown will be initiated by one the following initiators:

1. plant shut-down2. HP Boiler drum high-high level3. HP Boiler drum low-low level4. Cooling air to flame detector low low pressure, delayed to enable

cooling air fan change-over5. Sealing air to FA duct damper low low pressure, delayed to enable

fan change-over of FA operation change over6. SH steam HH temperature, 90 sec delayed (duct burner trip 60 sec.

Delayed)7. HRSG stack damper not open8. HRSG instrument air low-low pressure9. HRSG emergency trip (push button in Control Room)10. Diverter damper system open & GT running & FA duct damper not

closed, (with time delay 15 sec to enable the change-over from CC to FA mode)

HRSG Protection (ESD)

11. HRSG change over from CC mode to FA mode initiated and time expired

12. HRSG change over from FA mode to CC mode initiated and time expired

13. GT trip; GT trip initiator shall be masked in case of HRSG in FA mode operation

14. Diverter damper system discrepancy (diverter damper system NOT closed & NOT open – signal delayed to enable the CC/FA mode change over)

15. BFW pumps trip


EFFECT1. Emergency close diverter system (*)2. Duct Burner Shutdown request

(*) Close HRSG damper and open By-pass stack damper (de-energizing the ESD solenoid valve of the diverter dampers system). In the mean time the ESD will start a timer of 12 seconds (shall be defined during design and commissioning) to check correct by-pass stack damper opening and HRSG damper closing. If the time expires and the HRSG inlet damper is not closed OR the by-pass stack damper is not open, a signal of “GT shutdown for HRSG shutdown and diverter system failure to close” will be transmitted hardwired to GTS.

When the above trip initiators have been restored and HRSG tripcondition are reset from ESD, the diverter system will be released to beoperate from DCS (in manual mode by the operator or in auto mode bylogics) for the purge sequence (logic from GTS, BMS and ESD) andoperation in CC or FA mode.


Duct Burner Shutdown from ESD will be initiated by one of the following initiators:

1. HRSG shutdown2. SH steam HH temperature (60 sec. delayed)3. HRSG flue gas (right or left side) high-high temperature (60 sec

delayed)4. HRSG-HP EVA differential pressure < min

This initiator is masked during either CC to FA or FA to CC changeover

5. Duct Burner emergency trip (push button in Control Room)

Duct Burner trip request from ESD

EFFECTDuct Burner Shutdown request (Hardwired signals to BMS)

Moreover, the DCS shall track the gas control valve (231-FCV-6756A) at the light-off position and shall force the relevant controller in manual mode.The DCS track shall be removed when the duct burner is confirmed IN OPERATION

The BMS shall provide the closing of the pilot/burner valves and the main pilot/burner double block & vent valve (vent will be open), and the opening of pilot/burner header vent valves (for further details please refer to the Duct Burner - BMS Function Specification). Moreover, the “DUCT Burner purged” condition shall be reset.

Duct Burner trip request from ESD

The duct burner shutdown will be initiated by the BMS by one of the following initiators:• Duct burner Shutdown from ESD• Emergency trip from BMS local push button• Others duct burner trip initiators from duct burner skid or runners

(Pressure, Flame, valves position, etc) see doc - Duct Burner - BMS Function Specification

EFFECT:BMS:• The BMS shall provide the closing of the pilot/burner valves and the

main pilot/burner double block & vent valve (vent will be open), and the opening of pilot/burner header vent valves

• Moreover, the “DUCT Burner purged” condition shall be reset.• The HRSG can remain in service as simple recovery (if, before the

duct burner trip, HRSG were in CC mode).

Duct Burner Protection (BMS)

DCS:• The DCS shall track the gas control valve (231-FCV-6756A) at the

light-off position and shall force the relevant controller in manual mode. The DCS track shall be removed when the duct burner is confirmed IN OPERATION.

NOTE• The “Duct Burner Purge completed” condition is reset through

intervention of one of the above initiators.• When all the trip initiators have been removed, then it will be

possible to perform the duct purge and the duct burner light-off.

Duct Burner Protection (BMS)

FA Fan shutdown will be initiated by ESD by one of the following initiators:• Fixed bearing MDE high high temperature • Floating bearing STDE high high temperature• Fixed bearing MDE high high vibration• Natural Draft

EFFECT• FA Fan motor drive emergency stop• FA Fan Steam turbine emergency stop

Fresh Air Fan Protection (ESD)

Motor drive shutdown will be initiated by ESD by one of the following initiators:• Motor drive bearing NDE high high temperature• Motor drive bearing DE high high temperature• FA Fan shutdown

EFFECT• FA Fan motor emergency stop signal to MCC


Steam Turbine shutdown will be initiated by ESD by one the following initiators:• Bearing NDE high high temperature• Bearing DE high high temperature• FA Fan steam turbine emergency trip• FA Fan Fan shutdownEFFECTEmergency close Steam turbine trip valve, by means of ESD SOV (de-energize 231-HV-6650A) Emergency shutdown Steam turbine signal to pick 150 local panel (ST governor, for logic alignment purpose only).NOTE:In case of steam turbine over speed, the local speed governor control unit will close the relevant steam inlet control valve 231-SCV-6651A, as well as the steam turbine built-in over speed protection system will force to close (emergency closure) the turbine trip valve 231-HV-6650A.


During FA operation with Duct burner in operation, in case of Duct burner trip caused by the loss of fresh Air (FA fan not running OR FA downstream damper not open) the condition “natural draft” shall be set by ESD and the following trip actions shall be kept for 15 minutes:• Burner trip shall be request from ESD • FA duct damper, HRSG stack damper and the FA fan damper shall

be locked-out open• FA fan motor shall be locked-out stopped• Diverter dampers system shall be locked-out in close position• Steam turbine shall be tripped via valve 231-HV-6550A

After 15 minutes, the “natural draft” condition will be reset by the ESD and the trip actions will be removed.The Duct burner trip has to be reset also by operator before of the restart in FA mode operation with the FA purge and duct burner operation.

Natural Draft (ESD)

HRSG STATUS DEFINITIONSTART- UP & SHUT-DOWN

SEQUENCES

Actual HP drum operating temperature compared with normal base load operating temperature define the HRSG status [Hot/Warm/Cold]. Following situations are possible :HRSG Cold StatusHP drum temperature up to 110°C (pressure up to 0.4 barg)HRSG Warm StatusHP drum temperature between 110°C and 230°C (0.4<pressure<27 barg)HRSG Hot StatusHP drum temperature higher than 230°C (pressure>27 barg)

During start-up HP drum water temperature gradient shall be maintained below 8.5°C/min during boiler start-up/shutdown.The thermal gradient in the drum is controlled by the HRSG start-up vent.

HRSG status definition, ramp rate

This sequence prepares the HRSG to be purged and to receive the exhaust gas from GT or, in alternative, to be purged and started in FA mode.It is activated on operator request. Manual start-up of the boiler is also possible.

Sequence steps shall be integrated, customized and harmonized to plant procedures and standard.

HRSG START-UP SEQUENCE

Step 1• Verify no trip initiators present and sequence start req. by operator.Step 2• This step checks that all equipments [not only the ones involved in

the sequence] are available and in auto mode.• If some equipment is not in auto mode, operator shall be requested

to put in auto mode or, alternatively, the sequence may put the equipments in auto mode (according to the general plant philosophy).

• Only the equipments that require a manual startup by operator could be left in manual mode.

Step 3Following actions are required:• HP Desup. Interc. Valve Close

HP Interm. Blow-down Interc. Valve CloseHP Cont. Blow-down Interc. Valve CloseHP Main steam Stop. Valve CloseHP Drum Water Isolation Valve Close


Step 4Once previous step is finished, sequence will start the sealing air blowers.• Main sealing air blower StartStep 5Once previous step is finished the operator is required to put in service Feed Water system (not in NE supply scope)• FW system: HP FW pumps and valves in service Step 6Once previous step is finished, sequence will open the stack damper.• Stack Damper OpenStep 7Once previous step is finished, sequence will start the cooling air blowers.• Main cooling air blower Start


Step 8Once previous step is finished the operator is required to reset the HRSG trip condition and drum start-up level shall be checked.• Start-up level is a function of the actual drum pressure and it is

considered acceptable if it is +/- 50 mm from actual set-point [refer to drum level start up SP curve].

HP Drum manual refilling• If water level is too low, the HP Drum control valve [231-LCV-6601A]

and HP Drum water isolation Valve [231-MOV-6610A] will be opened by operator to fill in required amount of water. If water level is too high the intermittent blow down intercept valve [231-MOV-6605A] shall be opened by operator to reduce the level.

Step 9When level is acceptable and the level control valve 231-LCV-6601A is closed. • 231-LCV-6601A HP drum level control will be put in AUTO mode.

Operator can proceed with the unit purge and start up.


This sequence is activated on operator request. Manual shut-down of the boiler is also possible. Sequence steps shall be integrated, customized and harmonized to plant procedures and standard.

Sequence can be started only if the GT and the Duct Burner are both off.Step 10• This step checks that all equipments [not only the ones involved in

the sequence] are in available and in auto mode.• If some equipment is not in auto mode, operator shall be requested

to put in auto mode or, alternatively, the sequence may put the equipments in auto mode (according to the general plant philosophy). Only the equipments that require a manual startup by operator could be left in manual mode.

HRSG SHUT-DOWN SEQUENCE

Step 11Once previous step is finished following actions are required:• HP Drum water Interc. valve CloseStep 12Once previous step is finished, following actions are required :• HP System:

HP Desup. Interc. Valve CloseHP Interm. Blow-down Interc. Valve CloseHP Cont. Blow-down Interc. Valve CloseHP Main steam Interc. Valve Close

Step 13• Once previous step is finished, and GT is OFF AND Duct Burner is

OFF AND [HRSG HRSG flue gas temp. left side right side <100°C] the duct burner air Blowers can be switched OFF

• Stack Damper Close

HRSG SHUT-DOWN SEQUENCE

_maa refinery - daelim - training - i&c

Documents

control elements

control protection systems

hrsg operation

horizontal duct

gas turbine7

gas turbine5

transition duct duct

exhaust gas stack6