Modification improvement in Booster Fan Control in Tanjung Jati B Power Plant Units 3-4 Revision 5 Muhammad Arif Susetyo 1. Background of proposed of proposed Study – unit 3&4 incident 16 June 2012 In 16 June 2012, a forced de-rating occur in TJB power plant unit 4, 4 , reducing the load from 695 MW gross to 285 MW gross (40% Load runback). It is understood that the de-rating de-rating wa was caused by the Trip of Booster fan B. The logic control is set to reduce the load of the unit in case of the of the loss of either of the booster fans fans. The further events of Unit 4 will then force force the unit into Forced outage.
This study will focus on the elimination of unit de-rating in the event of Booster Fan Trip Another incident that occurred on the same same day is trip of Booster Fan 3A & 3B Unit 3. However, this event did not cause Runback to the Power Plant Unit due to automatic opening of FGD Bypass Damper. The occurrence of the aforementioned aforementioned booster fan trip without any runback in unit 3 is the empirical basis for proposed improvement. improvement. Detail of unit 3 Booster Fan trip will be described in Chapter 4. Aside from the Root Cause Problem Solving currently being conducted to eliminate the cause of the of the Booster fan Trip, it is proposed that a study is also conducted to mitigate the effect of the trip of a Booster fan to the Unit production production.. The main purpose of this th is propose proposed study is to eliminate, eliminate, or minimize, minimize, the Runback of a of a Power plant unit, in the case of a of a Booster Fan Trip.
2. What is a booster Fan? The booster Fans Fans is are a pair of axial-flow of axial-flow fans fans located between Induced Draft Fan (ID Fan) and the Flue Gas De-sulfurizer (FGD). The main function of the of the Booster Fans Fans is to overcome the additional head loss exerted by the addition of (FGD) Absorber in the gas flow-path prior to reaching stack, and additional drag due to the indecrease indecrease of buoyancy of cooled gas.
Bypass Damper A Bypass Damper B ID Boiler
ESP
Booster Fan A
Fan Fa n
FGD
Stack
Booster Fan B
Diagram 1 : Simplified diagram of TJB Unit 3-4 Booster Fan Location
1
Atmosphere
3. Scope Of Study : The scope of study is to determine the possibility of effecting modification in DCS control, control, which allows the trip of one of one or two booster fan, fan , without runback and/or trip of the of the Unit. The philosophy behind the proposal is that the main function of booster fan is to cope with the additional head loss exerted by the FGD absorber. In FGD full bypass mode, there is little need for the booster fan, because the bypass line exerts little head loss, loss , and the hot gas remains very buoyant in the flue duct .
3.1
Analysis of Unit 4 Booster Fan Runback Operation
In the event of one Booster Fan Trip, the existing DCS logic wi ll trip one train of ID Fan, FD Fan, PA Fan, and some selected Pulverizers will be tripped, tripped , to maintain with boiler furnace in a mild vacuum condition.. The Boiler furnace pressure needs to be kept at a vacuum to prevent Main Fuel Trip due condition to Boiler Furnace going into a Positive Pressure. After runback operation operation has has been completed, the FGD bypass damper can be opened and allows an increase of load, load, with single or no booster fan operation is possible via partial FGD bypass. The Sequence Of Unit 4 Runback (focused only on the booster Fan Runback) is as fo llow; Booster Fan 4B Trip
Runback Sequence commenced : ID Fan 4B, PA Fan 4B, FD Fan 4B Trip
Unit Load Reduced to 40 % Load
Diagram 2 : Unit 4 Runback Sequence due to booster fan trip on 16/6/2012 3.2 Booster Fan 3A&3B trip that did not cause unit runback On the same day of trip incident, electrical fault occurred that caused Booster Fan 3A and 3B to trip. In the normal logic, the trip of 2 Booster Fans will cause Unit Trip, due to trip of ID Fan, PA Fan, and FD Fan train A&B. However due to the same electrical fault, the Lube Oil Pump going to the ID Fan damper is not powered. powered. This causes the runback to be not permitted by the logic. The operation of single booster fan increases the Booster Fan inlet Pressure, and triggers Bypass Damper Forced Open to be activated. The bypass damper open open signal is set for 35 seconds lag before it is triggered, triggered, and the Bypass opens. The incident causes Furnace Pressure Pressure to increase but not to the point of Main Fuel Trip (MFT).
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Booster Fan 3A Trip
Runback Sequence not Permitted due to ID Fan 3A & 3B Damper In manual Mode (Loss Power)
Furnace Pressure & Booster Fan Inlet Pressure Increase
Booster Fan Inlet pressure high alarm activated
FGD Bypass Damper 3A Open
Furnace Pressure & Booster Fan Inlet Pressure Normalized
FGD Bypass Damper 3B Open
Booster Fan 3B Trip
Unit Full Load Maintained (684 MW Gross)
Diagram 3 : Unit 3 Booster Fan Trip Sequence on 16/6/2012 Based on diagram 3, it is deemed possible for TJB Unit 3&4 to have booster fan tr ip without compromising the unit load. However, a risk of the experience is that that if the bypass damper does not not open soon enough, then the furnace pressure will increase up to a point where the furnace pressure is positive, and MFT is activated causing unit trip. A basic modification to the logic would be by instantaneously opening the bypass damper in the event of booster fan trip, to minimize the time delay between the booster fan trip, and Bypass Damper Open. This will reduce the risk of MFT.
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3.3 Conclusion of Booster Fan Logic based on Unit 3&4 incident on 16 June 2012-07-09 Based on the explanation in subchapters 3.1 and 3.2, it can be concluded that the existing c ontrol philosophy of booster fan control in relation to unit load is as follow ;
Unit Full Load
Single Booster
Both Booster
Fan Trip
Fans Trip
Runback Permit?
Runback Permit?
Yes
Yes
st
1 layer protection
st
1 layer protection Trip
No
Trip single train of ID
No
Both train of ID Fan, PA
Fan, PA Fan, FD Fan.
Fan, FD Fan, & Main Fuel Trip
Booster fan inlet
Booster fan inlet
pressure increase
pressure increase
2nd layer protection
2 layer protection
FGD Bypass Open
FGD Bypass Open
(after 35s
(after 35s
integration time)
integration time)
nd
Unit Runback 40%
Unit Load Maintained
Load
-PROVED
Load Maintained
(Risk of Main Fuel Trip
(Risk of Main Fuel Trip
due to 35s integration
due to 35s integration
Unit Trip
POSSIBLITY of Unit
Diagram 4 : Unit 3&4 Booster Fan control in relation to unit load It is understood based on diagram 4 that the first reaction of control in case of booster fan trip is st
reducing the load of Unit by Unit trip (1 layer protection). In the case runback permit unachievable, unachievable, then the control opens the FGD Bypass line (2 nd layer protection) to maintain the furnace pressure in a mild vacuum. It needs to be noted that that there is a lag between the the trip of booster fan and opening signal of FGD Bypass line due to D CS control that includes 35 second integration time. This control
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configuration increases risk of boiler positive pressure. The failure of opening of FGD bypass line will result in increase of furnace pressure, resulting in Unit trip via Main Fuel Trip (MFT).
3.4 5 Why Analysis Based on the aforementioned condition, root cause analysis of BUF failure can be conducted, using 5 why analysis method;
Why?
Logic is set to runback
EPC configure
Unit in case of BUF
DCS logic in
Trip, instead of Why? opening FGD bypass
favour of SO2 emission instead
damper Unit 4 Runback
of Unit Load
Why?
16 September 2012
Booster Fan Trip Why?
Why?
3 Phase voltage
Low pressure
imbalance in Low
Silicon control
Pressure Seal Air
Rectifier
Low Pressure seal control box inadequate cooling
Based on the 5 why analysis above, it can be concluded that there are 2 root that cause the derating of booster fan : 1. Cause of Booster fan trip : Low Pressure Seal Air Silicon Control rectifier overheat due to inadequate cooling. 2. Cause of De-rating in case of booster fan trip : D CS logic that is set to runback the unit due to emphasis of SO2 instead of unit load. From the 5 why analysis, it can be concluded that both of these root cause need to be eliminated.
4.
Modification of Logic & method of improvement (elimination of root cause #2)
The proposed modification is to modify the DCS control, control , to send open signal to the FGD bypass damper, damper, in the case of booster Fan trip to to maintain full load of Unit. This will include excluding the customary trip of booster fan from the Runback logic, but still include trip of booster fan in the case of other items included in the runback operation trips.
4.1. Feasibility Analysis To enable such feasibility analysis, the following parameters need to be determined: -
The Time length required for the FGD bypass to fully Open (mechanical lag)
-
SO2 emission considerations due to FGD bypass in case of BUF trip 5
4.1.1.
Determining Time Length of FGD Bypass full open (Mechanical Lag)
Determining the Time Length of FGD Bypass damper Full open operation can be done by opening the FGD Bypass dampers to full open, and plotting the opening time and Corresponding opening angle ionto io nto a graph (X Axis : Time, Y Axis : Opening Percentage). The result of FGD bypass damper opening test is as follow; Opening of FGD Bypass damper A : Approximately 15 seconds Opening of FGD Bypass damper B : Appproximately 42 Seconds
4.1.2.
Booster fan control modification logic diagram
Based on the existing booster fan logic depicted in diagram 4, The Booster fan control logic is modified in a way that in the event of booster fan trip does not cause runback, as described in the diagram below;
Unit Full Load
Single Booster
Both Booster
Fan Trip
Fans Trip
1st layer protection
1 layer protection
FGD Bypass damper
FGD Bypass damper
forced open
Open
After 5 seconds, FGD
After 5 seconds, FGD
st
Bypasss damper A&B still fully closed? No
Bypasss damper A&B
Yes
No
still still full full close closed? d?
2nd layer protection
2nd layer protection
Booster Fan
Booster Fan Yes
Runback Operate
Runback Operate
1 Train of ID, PA, FD
1 Train of ID, PA, FD
Fan Trip
Fan Trip
Unit Load
Unit Runback
Maintained
40% Load
Unit Load
Possibility of Unit
Maintained
Trip (described below)
Diagram 5 : Logic diagram Modification of booster fan c ontrol in relation to unit load
6
st
As described in diagram 5, it is understood that in case of booster fan trip, the 1 layer of protection is the opening the bypass damper. The failure of opening of bypass damper will result in the 2
nd
layer
of protection, which is unit runback or unit tip, based on the amount of booster fan that tripped. This will increase availability of unit in case of booster fan trip. Based on the proposed modification, the success of the control maneuver is determined by the success of opening of FGD Bypass damper. Because the FGD bypass damper is normally closed, it is deemed necessary for the damper to be opened regularly to check the opening capability.
4.1.3 Modification of DCS logic ladder diagram and testing (14 september 2012) The logic modification of booster fan trip in relation to runback control in Tanjung jati B unit 3 D CS is as follow :
Picture : DCS Modification Part 1
Picture : DCS Modification part 2
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There are two parts of modification. In Logic 1, from point A to B can be described by the following IF – THEN logic ;
DCS modification Part 1 – Point A to Point B if BUF A trip or BUF B trip then FGD Bypass damper forced open This modification of logic will open both bypass dampers in the case of any, or both Booster Fan trip.
DCS Modification Part 2 – Point A to Point B if BUF A trip or BUF B trip and after 5 seconds FGD Bypass damper A fully closed and FGD Bypass damper A fully closed then Runback 40% Operate Note : This logic will send the unit into 40% runback in the event of 1 Booster fan trip, and both FGD bypass dampers are still fully closed after 5 seconds (complete opening failure of both dampers). In the event of both booster fan trip and both dampers are still fully closed after 5 second, and both FGD bypass dampers are still closed after 5 seconds, the 40% runback will be started.
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Result of Test
Picture :FGD Bypass Operation @ 45 % Load
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“Blindspot” in testing
No test have been done to test the operation of plant without any booster fan, and both FGD bypass damper still closed. However, based on the modification, the following events can be predicted; predicted; Unit Full load
Both Booster Fans Trip
st
1 layer protection FGD Bypass damper Open
After 5 seconds, FGD Bypasss damper A&B
Yes
still fully closed?
Booster Fan Runback Operate
No
Can ID fan overcome FGD transient head Yes
Unit Load
Unit 40% Load
No
Boiler positive Pressure
Main Fuel Trip
Maintained
Such predicted of events is based on the modification. The certainty of event is up to the triggering of 40% unit load, because such condition has been tested tested in 14 september 2012. However, the further state of furnace pressure is yet to be known. If the ID fan is capable of overcoming the head loss of the FGD absorber, then Unit Unit can be run in 40% load. If ID fan is not capable to overcome the FGD absorber, then furnace pressure will increase increase up to the point of Main fuel trip. trip. In the original logic, trip of 2 booster fans will trip the unit. Based on the explanation, it can be be concluded that the logic modification does not pose additional risk in comparison to to the existing logic. Because, in the original logic, the simultaneous trip of BUF A & B will immediately trip the unit, whereas with the modification of unit, there is a chance that ID Fan will able to overcome the head loss of FGD absorber, and unit can be run in 40% load continuously. continuously. Furthermore, it must be noted that simultaneous trip of BUF is also a rare occurance. 10
5. Conclusion Based on the modification of Booster fan logic control, and 14 September 2012 testing, it can be concluded that the modification is an improvement to the unit, and will reduce risk of unit de-rating. The benefit of the modification is as follow: -Elimination of runback in case of single or double BUF trip, provided that the FGD bypass dampers can open. -Possibility of unit 40% load operation in case of simultaneous BUF A&B trip, and FGD bypass damper not open.
6. Suggestions Based on the modification, it can be concluded that the maintaining of load in case of booster fan trip is determined by the success opening of FGD Bypass damper. It is suggested that Operator conduct periodic FGD bypass opening test to ensure damper is capable to be opened in any given time. Furthermore, the LP seal air control panel needs needs to be improved. The air circulation within the control panel needs to be improved, for the stable temperature of Silicon Control Rectifier.
-Input From KPJB #1 : To enable trip of Absorber Recirculation Pump (ARP) Trip without Trip of booster fan, causing runback of unit. Currently, the FGD is run with two or one ARP Operation. In single ARP operation, the trip of ARP will result in Booster Booster fan trip, thus in resulting unit runback. It is proposed to include the ARP operation in the logic modification i n a way that in the st
event of ARP Trip, it will automatically change over (1 layer protection). The failure of change over will result in Opening in FGD Bypass (2 rd
nd
layer protection), and if that is not
possible, then refer to trip of booster fan (3 Layer Protection).
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