Early Identification of Boiler Tube Leakage Using BI2 Analysis

1. “BI 2 Analysis” for Early Identification of Boiler Tube Leakage in Second Pass of Coal Fired Boilers of Power Plants” By- V K Deshmukh, DGM(CC-OS),NTPC Ltd WRHQ2,Raipur 13.07.2019 NSPCL O&M CONFERENCE

2. OUTLINE 1. Traditional ways of Identifying BTL ,and need of early identification. 2. What is second pass? What is BI2 ? How the index is calculated? Why to use it? 3. How does the BI2 , help in identifying presence of BTL in secondpass. 4. Case studies for showing the correlation. 5. Using BI2 for early identification of BTL in second pass.

3. Identifying BTL, and need of early identification. Traditional way of identifying BTL : Increased hot well make up Hearing the peculiar sound on local visit (require experience) And in recent time we have invested in ASLD Primary Damage Secondary damage Difficulty of early identification of BTL in second pass. A new way to identify BTL in second pass at very early stage is by analysing BI2 : BTL Index 2 Early identification, no additional cost, reduced secondary damage, cost &down time

4. Second pass. First pass: water wall, pendant and platen SH, Radiant RH; radiant heat transfer is predominant, high steam and FG temp, FG more inter mixing till it goes to APH Second pass: LTRH/LTSH, Economiser, steam cooled wall, Convective heat transfer is predominant, lower steam and FG temp, little FG intermixing till it goes to aph

5. BI2: BTL index for second pass Heat Transfer take place between flue gas and Air in Primary and Secondary air pre-heaters in second pass of boiler: Heat given by flue gas= Heat taken by air Mg x Cpg x (Tgi –Tgo)= Ma x Cpa x (Tao- Tai) Mg x Cpg (Tao- Tai) --------------- = --------------- = BI two or (BTL index for tube leakage in 2nd pass) Ma x Cpa (Tgi –Tgo) Heat capacity of FG gas entering APH Air temp rise in APH ---------------------------------------------------- = ---------------------------------------- = BI TWO Heat capacity of Air leaving APH Flue gas temp Drop in APH In presence of Boiler tube leakage, Heat capacity of FG increases, leading to increase in value of BI2 ie BTL Index for second pass.

6. BI2: BTL index for second pass Normally in power plant of capacity 500 mw and above, we have total 4 APHs. PAPH-A, SAPH-A, SPAH-B & PAPH- B; arranged usually in this sequence across Flue gas path.

7. How does the BI2 , help in identifying presence of BTL in second pass In presence of Boiler tube leakage, Heat capacity of FG increases, leading to increase in value of BI2 ie BTL Index for second pass. • Words of caution: • BI two” gives correct correlation with BTL in second pass, when unit is running at near full load, say 95% loading factor, and is in steady state of condition. • It is to be noted that interpretation from “BI TWO” during part load operation or transient condition may lead to wrong conclusion. So value of this index, its trend and tendencies should be observed when unit is in or comes back to full load and steady state • Moreover, rather than absolute value of “BI two index”, tendencies and trends of its variation at full load, steady state condition is the real indicator / more relevant for identifying BTL in second pass.

8. Case study-1: UNIT 2 (660 mw) 3 d before: -BI (<1.75) , max BI 1.66, trend is smooth curve, without pick and valleys , more or less straight line . 2d before: max value BI 1.73, trend is deviating from straight line, appearance of sharp especially for BI3 & BI2 1d before: max value of BI 1.77 (>1.75) FOR BI1 &BI3, disturbance in smoothness and straight nature of all the BI curves , there is an upward movement On day of sd: BI value is increasing =>1.75 and more than 1.75 in many occasion ,obvious disturbance in smoothness of BI curves, upward movement. Unit was stopped at 84th block to attend BTL in LTRH in second pass

9. Case study-2 : 660 mw u1 NORMAL RUNNING (in absence of BTL), BI <1.75 BI curves are smooth over longer duration , closer to each other. 2 days before shut down for attending BTL BI > 1.75 nearing to 2, showing repetitive upward movement (specially central BISs, BI2 &BI3), central BI (BI2 &BI3, both going upward) and outer BI (BI1 & BI4 both going downward) 1 days before shut down for attending BTL central BI; BI 2 &BI3 >1.75 frequently, moving upward, outer BI though have lesser value smoothness of curve have reduced Unit was taken under shut down next day at 03:15 A.M., to attend BTL in LTRH in second pass

10. Case study-3 : 500 mw U5 Unit # 5 BI trend : NORMAL OPERATION (in absence of BTL) BI curves are smooth, more or less straight-line without peak and valleys, close to each other& BI <1.35 U5 BI TREND 2012 BI value equal to 1 shows, unit is either in shut down condition Or at low load, when heat transfer in APH is not significant. Earlier unit has been synchronized at synchronized at 34 block, it has again tripped at 36 th block, again synchronized in 10:16 hrs in 42nd block. Unit has full load from 53rd to 65th block, however BI index has upward tendency, value going>1.35. Unit was shut down at 70 th block to attend LTSH leakage in 2nd pass.

11. Using BI2 for early identification of BTL in second pass • 1. At full load and steady state condition,: • - before BTL; BI Curves remains smooth, more or less straight line. • - due to BTL; its smoothness disturbs (and peak and valleys appears ) curve • deviates from, its earlier straight line nature • for 660 MW Units Central BI ( BI2 or BI3 or both ) moves upward towards 2 (>1.75 ), For 500 MW Unit >1.35 • departure from normal value and tendency / smoothness, if any, observed at full load and steady state condition; it can be taken as an indication of being caused by BTL in second pass.

12. Using BI2 for early identification of BTL in second pass • 2 - Different unit may have different normal pattern, • -but normal pattern are distinguishable by smoothness / straightness over • longer duration, absence of sharp peaks or valleys & • -BI value remaining less than certain level (<1.75 and <1.34 in above • discussed cases of 600 mw & 500 mw units respectively). • - it is required to recognize normal pattern(in absence of BTL) at full load • and steady condition as well as the departure from them; which may be • used as an alert for existence of BTL in second pass at early stage.

13. Using BI2 for early identification of BTL in second pass • 3. Value of BI index and it curves normal behavior may be specific for a unit type (say 500 mw,660 mw,250 mw) and for different condition of APH leakage level • -Boiler desk engineer may get acquainted over a period of time with this BI index value and their trend pattern at full load, steady state condition. • -Than they should look for type of observable departure during actual BTLs in future.. • It will increase their knack for identifying BTL at early stage by using this BTL Index and their capability for making informed decision regarding stopping the unit at the earliest to attend BTL (in second pass at Primary stage) Let us use and get benefited from this new insight to make better decision.

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