C&I part of SC

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May 24, 2012 PMI Revision 00 1


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ADDITIONAL MFT CONDITIONS

Furnace vertical wall tube temperature highIf water wall tube metal temperatures exceedthe set point, automatically trip the fuelfollowing a maximum 3 second time delay.

This trip protects the tubes from overheatingand potent failure. High water wall tubetemperatures are an indication of over-firing,low water wall flow, or a combination of both.

Over-firing could be a result of extreme loadchange rates. Low water wall flow could be aresult of excessive SH spray flow.


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Superheater pressure (both leads) high.

Superheater temperature (either side) high for

more than twenty seconds.

Reheater outlet (either side) temperature high for

more than twenty seconds.


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Turbine trip conditions causing vacuum break like

axial shift very high, bearing vibration high


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REHEATER PROTECTION LOGIC

Arming - The reheat protection logic is armed or activated once operatingconditions are such that there is a potential for steam generator materialdamage due to overheating. Specifically, total fuel flow > X % (where Xcorresponds to 538 C furnace flue gas temperature) or steam flow > X % ormeasured furnace temperature > 538 C.

Initiation of a master fuel trip A loss of main steam flow path (no path through HP turbine or HP bypass,

or no flow path through LP turbine or LP bypass).

The boiler remains in an over fired condition (total fuel 5% above steam

flow) 50 seconds after a turbine trip or fast cutback.

HPBP & LPBP not open after generator tripped.

HPBP & LPBP not open after turbine tripped.


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Heavy Oil Leak Test/ Light oil leak test

All of the following conditions must be present before oil

leak test can be started:All heavy oil nozzle valves closed.

Heavy oil trip valve closed

Open the oil control valve , After fifteen (15) second,open command is sent to the oil trip valve, When the oiltrip valve is fully open, the oil return valve is commandedto close.

The oil piping downstream of the oil trip valve is nowbeing pressurized, the oil header pressure is indicative atpresent value.


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Start-Up System with Recirculation

HPH

BFP

Deaerator

C

C

WW

ECO

To Condenser

C

HWL

SH

Start-Up SystemRecirculation Pump in Main Bypass Line

S

eparator

Flash

Tank


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May 24, 2012 8

If the water system of the boiler is empty (economizer,

furnace walls, separators), then the system is filled withapproximately 10% TMCR feed water flow.

When the level in the separator reaches set-point, theWR valve will begin to open.

When the WR valve reaches >30% open forapproximately one minute, then increase feed water flowset-point to 30% TMCR. As the flow increases, WR valve

will reach full open and ZR valve will begin to open.


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The water system is considered full when:

The separator water level remains stable for two(2)

minutes

and

The WR valve is fully opened and ZR valve is

>15% open for two(2) minutes.


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May 24, 2012 10

Water flows through the economizer and evaporator, anddischarges the boiler through the WR valve to the flash tankand via connecting pipe to the condenser.

Start BCP and open the UG valve to establish minimum waterwall flowat 30% TMCR.

As the pressure is raised, first the WR and then the ZR valves will openwhen

separator water level increases due to boiler water swell. As pressurefurther

increases, the WR and ZR valves will start to close as the water level

decreases.


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The steam temperature at the separator inlet will reach astable superheated

condition at app. 30% TMCR, causing the level in the

separator to decrease and

eventually disappear. The boiler is now in once-through mode(dry mode). The

BCP (Boiler Circulating Pump) will be stopped automatically.

It is extremely important that minimum water wallflow be maintained at all times when firing the boilerto prevent tube damage due to overheating.


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SEPARATOR STORAGE TANK LEVEL

CONTROL

Separator level is maintained by the combined action of aseparator storage tank level feed water demand and thepositioning of WR and ZR drain valves.

Feed water demand is developed in response to separatorstorage tank level error and total fuel flow so as to preventtank level from dropping too low.

The WR and ZR valves are controlled in a split range

manner to maintain the liquid level once the level reachesa high limit.


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SEPARATOR STORAGE TANK LEVEL

CONTROL

The WR valve will respond first and then theZR when the WR exceeds its linear operatingrange.

Tank geometry is such that fluctuations intank level are very dynamic, for this reason,only proportional control action established

through the WR/ZR function curves is usedto position these valves in response to levelerror.


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UG VALVE CONTROL

Control objective:

Maintain minimum economizer inlet flow.

Control action:

The boiler circulating pump is started following the startof a turbine-driven feed water pump and the final clean-upcycle. This pump circulates feed water from the evaporatoroutlet back to the economizer inlet.

Located at the outlet of this pump is the UG valve which

controls economizer inlet flow during the start-up phase ofoperation. Demand for this recirculation control valve isestablished based on measured economizer inlet flowcompared to a minimum boiler flow set point.


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FEEDWATER CONTROL LOOPControl objective:

Develop total unit feed water demand as required to supportunit load.

Adjust feed water demand to maintain desired separatoroutlet temperature.

Adjust separator outlet temperature set point as required tomaintain acceptable platen superheat spray control range.

Incorporate separator storage tank level (wet mode) feedwater demand.

Maintain minimum required economizer inlet flow.

.


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FEEDWATER CONTROL LOOP contd..

Optimization of the feed forward in this manner minimizestemperature fluctuations that may otherwise result fromvarying dynamic response between the firing and feed watercontrol systems (as they relate to evaporator heat transfer)thereby lessening the dependence on feedback correction.

Demand for feed water is established predominately by theBoiler Master demand.

This signal, processed though a boiler transfer function

provides the feed forward component of the total feed waterdemand.

The boiler transfer function is a tunable dynamic elementproviding a means to dynamically match the feed water feed

forward demand to actual evaporator heat transfer.


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FEEDWATER CONTROL LOOP

contd..

The first controller acts on a load dependent average platenspray differential temperature. Its output represents the required adjustment to evaporator

heat transfer/steam generation to maintain both the steamconditions and flue gas temperatures entering the platen

superheat section so as to ensure adequate platen spraycontrol range.A second controller acts on a load dependent separator outlet

temperature set point corrected by the platen spraydifferential temperature controllers output.

This controller acts to adjust feed water in response to firingsystem disturbances and the relatively fast effect they haveon separator outlet steam temperatures.

The overall combined feed water feedback control action issuch that feed water demand is responsive to changes in theoverall unit heat transfer profile.


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FEEDWATER CONTROL LOOP

contd..

The combined feed forward/feedback demand signal issubject to a minimum economizer inlet flow set point (wetmode) activated if the boiler circulation pump is not inservice and the unit is being fired.

This ensures the minimum economizer inlet cooling flow ismaintained by the feed water supply system in the eventthe start-up system is not available.

The feed forward/feedback demand signal is subject to asecond wet mode feed water demand developed tosupport separator storage tank level control.

The resulting demand provides the set point to a feedwater master controller.The fuel/feed water ratio protection logic providesoverriding control of individual feeder speed demands inthe event of an excessively high fuel to feed water ratio.


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SUPERHEAT STEAM TEMPERATURE CONTROL

Overall superheat steam temperature control is

accomplished with adjustment of spray water control andmanipulation of feed water flow.

The basic concept is such that two final spray watercontrol valves act to maintain final super heater outlet

steam temperatures and two platen spray water controlvalves respond to final spray differential temperatures andfeed water flow is adjusted in response to average platenspray differential temperatures.

In the short term, final superheat temperature fluctuationsare minimized by the fast acting final spray water controlvalves.


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SUPERHEAT STEAM TEMPERATURE CONTROL

In the long term, control action automatically re-adjusts steam generation at each control point(evaporator, platen, final) in response to changes in

corresponding heat transfer rates. This approach provides a high level of disturbancerejection and ensures the platen and final spraywater control valves remain in control range by

ultimately adjusting evaporator outlet steamconditions/heat transfer and consequently fire sideheat passed to the superheat sections.


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Platen Superheat Temperature Control

The primary objective is to keep the final superheat spray water control

valves in their desired operating range.

The control structure is a cascade arrangement where the mastercontroller acts on the differential temperature measured across thecorresponding final spray station as compared to a load dependentdifferential temperature setpoint. The output of this controller

represents the required temperature entering the platen superheatsection to achieve the desired temperature at platen outlet (i.e.corresponding final spray station inlet).

An over/under firing feedforward is added to the masters output forimproved response. This signal is developed by comparing the rate

of change in steam flow to the rate of change in fuel flow. Over firing(rate of change in fuel flow in excess of rate of change in steamflow) decreases the masters output. This is an anticipatory action tooffset the tendency for increased steam temperatures resulting froma temporary imbalance between cooling (steam flow) and availableheat (firing rate) when over firing.


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The signal has the opposite effect when under firing(increases masters output) since in this case the temporarycooling/heat imbalance tends to decrease steamtemperatures

The slave controller output, subject to an overriding saturationprotection circuit provides the spray valve position demand. Saturationprotection prevents over spraying by limiting the final valve demand.Saturated steam temperature is established from measured separatoroutlet pressure, adding X degrees C establishes the minimumpermissible degree of superheat.

Increased spray is prevented by limiting the final spray valve demand inthe event measured desuperheater outlet temperature drops below theestablished minimum level of superheat.


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contd

The resulting modified master output provides the setpoint to a slave controller. This controller acts on this setpoint as compared to steam temperature measured at thespray stations outlet (i.e. inlet to platen superheat section).

S


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Final Superheat Temperature

Control

Control objective: Control final superheat steam temperature.

Control action: The master controller for each valve acts on the

corresponding final steam outlet temperature ascompared to a load dependent set point.

The slave controller positions the final spray valve

(subject to saturation limit) in response to the masteroutput (with over/under firing feedforward) as comparedto the associated spray station outlet temperature.


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