Presented at National Boiler Workshop 2015 - Presented by HPCL Vizag Refinery

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Introduction & Boiler Description
• HPCL had installed and commissioned two
high pressure boilers for GFEC in 2009,
namely SG-10 & SG-11.
• 140 TPH at 40 Kg/cm2g design steam
generation capacity.
• Bi-drum, natural circulation, dual fuel fired ‘D’
type boiler.
• Inverted loop, single pass, single super-heater.
• Super-heater metallurgy T11 alloy with 136
tubes of 2” Nominal dia.
• Three burners positioned one below another
vertically on furnace wall.

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• Superheater is of inverted loop, single pass design with 136
Nos. of 2” OD T11 material tubes arranged as shown in
diagram below.
Superheater Design

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• Both SG10 and SG11 boiler had series of superheater
tube ruptures since commissioning in 2009.
• SG10 had 7 and SG11 had 5 instances of premature tube
leak in superheater. Detailed study concluded failure due
to overheating.
• The leak occurred within 3 to 100 days after
commissioning.
• Nature of failures: Short term overheating leading to fish-
mouth rupture.
Failure history of the boiler

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• Super-heater tube skin temperature was running very
high upto 550-600°C since commissioning against OEM
provided limit of 469°C.
• Steam outlet temperature was also running high upto
380°C against limit of 365°C.
• Boiler load was limited to 60% of MCR (about 80 TPH)
due to high Super-Heater metal temperatures. Still
failures occurred at 50 TPH, 65 TPH and 80 TPH steam
load.
Failure history of the boiler

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Fish Mouth Rapture in Super Heater tube

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Metallographic Analysis
• Substantial amount of black thick oxide layer in laminated
formation.
• No steam side deposits were observed.
• 300-500 micron of magnetite layer was observed.
• Microstructure analysis suggested long term temperature
exposure to more than 727 °C.
• Burner operating philosophy and other operational
measures had no impact on failure reduction
Root Cause Failure Analysis

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Thermo-hydrolic analysis carried out under consultancy of
boiler expert Shri. V. Ganpathi.
 The superheater design pressure drop was too low 0.2-3.0 psi
at full boiler load against recommended values of 30-35 psi
which was causing overheating of superheater tubes.
 Very low pressure drop at start up conditions caused
stagnation of steam flow in hotter and longer tubes facing flue
gases from furnace.
 Lower design steam velocities are found inadequate to provide
heat sinking at higher steam loads in view of increased heat
flux and radiation.
Thermo-hydrolic analysis

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 Superheater was not preceded by enough screen tube
rows which normally prevents superheater from hot flue
gases by soaking up radiation and equalizing the flue
gas flow across the superheater cavity.
 Due to location of the superheater and single pass
design, the superheater tubes were exposed to high
radiant heat particularly at higher loads.
 Compromised design of superheater, lead to failures at
lower as well as higher loads.
Thermo-hydrolic analysis

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• Reduction in number of superheater tubes were carried
out as shown in following diagram.
• Total 7 rows (56 tubes) were removed out of originally
provided 17 rows (136 tubes).
Modifications
Original Arrangement
Modified Arrangement

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• Super heater design was changed from single stage to
double stage and attemperator has been attached
intermediate header to control the steam temperature in
desired limits.
• Super heater metallurgy was T11 & this was upgraded to
T22.
Design Modifications

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Modifications in Design

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Improved performance & reliability post-modifications
Parameters Before modification After modification Remarks
Load Avg 75.00 75.00
Stack Temp (Avg) 188.00 164.19 -23.81 degC
O2 (%) (Avg) 6.00 3.50 -2.5%
Air flow TPH (Avg) 113.00 96.58
Efficiency (Avg) 90.43 91.51 +1.1%
TST (Avg) 494.00 445.00 -50 degC
SH outlet temp (Avg) 384.73 355.70
SH pressure drop (Avg) 1.63 2.37
Failure frequency due to
overheating 3 to 100 days NIL

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Major changes in Original Design
Parameters Original design Modified design
Number of superheater tubes per pass 136 80
MCR, TPH 140 140
Steam drum pressure, Kg/cm2g at MCR 46.4 46.4
Superheater outlet pressure, Kg/cm2g at MCR 42.9 39.6
Superheater pressure drop, Kg/cm2g at MCR 3.5 6.7
Superheater outlet temperature at MCR 380-400 335-355
Superheater steam velocity, m/s at MCR 18 32.6
Superheater steam mass flux, Kg/m2/s at MCR 289.5 492
Expected Superheater TST Max, DegC at MCR >700 490
Failure frequency due to overheating 3 to 100 days NIL

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