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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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