A study on damages in alloyed super heater tubes of thermal power station

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 14, February 2007, pp A study on damages in alloyed super heater tubes of thermal power station M Azad Sohail a* & A Ismail Mustafa b a Central Chemical Research Laboratory, Ghorasal Thermal Power Station, BPDB, Narsingdi 1610, Bangladesh b Department of Applied Chemistry and Chemical Technology, University of Dhaka, Dhaka 1000, Bangladesh Received 7 November 2005; accepted 31 October 2006 In this paper, an investigation on the damages in alloyed super heater and reheater tubes materials for natural circulation water wall tubes (NCWWTs) high-pressure drum boiler units TGME-COB-206 (Russia) of Ghorasal Thermal Power Station (GTPS), Bangladesh, have been carried out at different schedule maintenances. Extensive analyses of different chemical parameters of boiler water chemical regime (BWCR), and generated steam (live) quality at rated operating steam temperature and pressure, revealed carryover of SiO 2 20 (±5) mgl -1 in steam (live) during cold startup and abrupt increase in thermal load of boiler unit. The micro and macro structural examinations of super heater tube materials, scale composition; scale thickness and composition of super heater tubes materials have been monitored very carefully. Silica is found to be the major component for the super heater scaling which has strong affinity to the chromium containing steel reheater tubes materials. IPC Code: C21D 1/00 In the long run, world-wide fossil fuel (coal, gas and furnace oil) based thermal power station (TPS) will continue to be installed due to the rapid industrialization as well as rapid increasing demand of electricity. Most of the fossil fuel high-pressure boiler of TPS in Bangladesh is operated with steam temperature 540 C (live steam) at super heater outlet and 545 C at reheat outlet. Damages in super heater tube due to scaling, corrosion, high rated heat fluxes, thermal stresses and erosion micro structural changes, spalling and exfoliation of magnetite on internal surfaces are usual problems in many TPS. Mayer 1 shown that the gain in the thermal efficiency of a power station with steam temperature of 630 C is more than 8% as compared with traditional power station with a steam temperature of 540 C. But in such a case super heater tube quality must be improved. Therefore, prevention of scaling, improvement of super heater and reheater tube materials applying different alloyed steel, with respect to increase in the steam temperature, pressure are now a significant research for power plant researchers. The scaling on the steam side also increases at increased steam temperatures and is influenced by the alloying additions of the tube materials employed, with the resistance being essentially determined by the *For coprrespondence ( masohail2004@yahoo.com) chromium content 2. In the case of the austenites, a small grain size has an additional favourable effect on the scaling resistance. Masuyama has expressed the relation of high-temperature corrosion behaviours of austenitic super heaters tubes using a bell curve 3. At GTPS, the operating temperature maintained surrounding the super heater tubes of high pressure boiler (158 kg f cm -2 ) unit No. 4 and unit No. 2 (100 kg f cm -2 ) are 800 C and 780 C respectively. And while reheater tubes temperature recorded for unit No. 4 was 760 C. In this study scaling behaviour and damages in alloyed super heater tubes materials with different length of time has been investigated to evaluate the actual facts of cause defects in super heater tubes. Experimental Procedure The micro and macro-structural examinations of boiler super heater tube materials were carried out by using an Optical Microscope (Japan). During analysis of microstructure hydrochloric acid, nitric acid with water were employed for etching of sample materials. Analyses of tube materials, steam quality as well as operational chemical parameters were carried out by using Atomic Absorption Spectrophotometer (AAS), model: AA6650, Shimadzu (Japan) and photo electric colorimeter, model: KFK-2 (Russia). Oven Modal N , (Russia) and Furnace, Model Mn-2YM, (Russia) were used during this work.

2 20 INDIAN J ENG. MATER. SCI., FEBRUARY 2007 Results and Discussion The causes of super heater tubes failure in NCWWTs high pressure boiler unit No. 2 (90 kg f cm -2, 55 MW unit) and super heater and tubes in the NCWWTs high pressure boiler unit No. 4 (140 kg f cm - 2, 210 MW unit) of Ghorasal Thermal Power Station (GTPS), Bangladesh, has been investigated at different schedule maintenance period and sudden unscheduled shut down of unit due to technical problems or damages in super heater tubes. The super heater tubes materials of GTPS boiler unit No.2 is consists of carbon steel, i.e., steel-20 (Russia) and while the super heater and reheater tubes of boiler unit No. 4 are consists low alloy steel (st. 12XIMφ Russia) and of medium alloy steel (st. 12X18 H12T, Russia) respectively. The natural gas fired the boiler units are operated at maintaining the chemical parameters as shown in Table1. The laboratory test results of super heater and reheater tubes materials for both the GTPS units (No. 2 and No. 4) are given in Table 2. The unit No. 2 of GTPS is operated at rated super heater steam temperature (live steam) 540 C and while the unit No. 4 is operated with super heater steam (live) temperature 540 C and maintained reheater outlet temperature 545 C. Unit No.2 have no reheater. The scaling behavior is demonstrated by means of investigations of tubes, which have been taken form the super heater and reheater regions after different operating periods, i.e., during schedule maintenance, or sudden shutdown of units for long instant. In such a type of fossil fuel TPS super heater and reheater tubes (steam temperature 545 C) in the martenstic steel X20CrMoV12-1 usually thick scale layers were observed after only some 1000 h operation 7. Literature shows, in operation with good experienced personnel 8 (600 C), steam side scale layer with maximum thickness of 250 µm have been measured in the case of X8CrNiNb after operating hours. The equivalent good result being a maximum thickness of 100 µm after h of operating hours in the case of X8CrNiMoNb Table 1 Supplied (Russian) parameters for the NCWWTs high-pressure drum boiler unit (140 kgcm -2 and 90 kgcm -2 ) TGME-206-COB (Russia) of GTPS, Bangladesh Parameters ph E.C.* Alk. p/m Hd. SiO 2 N 2 H 4 NH 3 Cl - Cu Fe DO Na P DemiWater Feedwater 9.1± Condensate 9.1± Boilerdrum clean (evaporation) Salt section (blow down) Steam 9.1±0.1 - EC*-µ Scm -1, Other units mgl -1 Sl. Sample No Description Trace upto upto Table 2 Chemical composition of different super heater and reheaters tubes used in GTPS, Bangladesh 1 28X4 Carbon Steel super heater (unit No. 2) 2 36X6 Alloy Steel HPSHii (High- pressure supper heater 2nd stage) (unit No. 4) 3 32X4 Alloy steel high pressure reheater (unit No. 4) Chemical composition (%) Carbon Silicon Manganese Sulphur Phosphorous Nickel Chromium Molybdenum Titanium

3 SOHAIL & MUSTAFA: DAMAGES IN ALLOYED SUPER HEATER TUBES 21 Practical experience and experimental results depicts that both the super heater tubes materials st. 20 (Russia) for unit No. 2 and (st., 12 XIMφ Russia) for unit No. 4 produces good performance with less scale layers up to h of operation. Damages in super heater tubes took place after h of operation. In case of reheater tube materials very different scale layer thickness (200µm) with local exfoliation were observed at h of operation. The practically observed behaviour of steam side low alloyed super heaters (unit No. 2 and unit No. 4) or medium alloyed reheater (unit No. 4) scaling with respect to operating hours at GTPS boiler are shown in Fig. 1. Physical investigation revealed that up to an operating period of h the steam side super heater and reheaters were covered with light gray scale layer. In the case of super heater tube sample for both unit No. 2 and unit No. 4 after h, in contrast microscopic irregularities were observed on the scale surface and huge pits were also observed as Figs 2, 3, 4 and 5 respectively. Microstructure of Fig. 3 Microstructure of ( 28X4 Carbon steel super heater, St. 20) unit No. 2 showing ferrite and pearlite, 600, (after h) Fig. 1 Scaling behaviour of super heater and reheater tube materials Fig. 4 Macrostructure of ( 28X4 Carbon steel super heater) unit No. 2 showing etch pits X60 in the transverse section (after h) Fig. 2 Microstructure of ( 36X6 Low alloy steel high pressure super heater 2nd stage) unit No. 4 showing ferrite and pearlite 600, (after h) Fig. 5 Macrostructure of ( 36X6 Low alloy steel high pressure super heater 2nd stage) unit No. 4 showing etch pits X60 in the transverse section (after h)

4 22 INDIAN J ENG. MATER. SCI., FEBRUARY 2007 super heater tube materials for unit No. 2 and unit No. 4 after h of operation (Figs 2 and 3) shows that there are large discontinuity in ferrite (white) and pearlite (dark) uniformity and distribution. These changes took place due to long-term high rate of heat flow through the steam generatative super heater tubes. The macrostructures of inside super heater tubes of unit No. 2 and unit No. 4 after h showing the flow lines in the longitudinal direction, in Figs 6 and 7 respectively. For both the units the sharp lines indicates the loss of super heater tube materials. That s might be due to erosion consequence of wet steam contaminants and high rate of steam flow through the steam generating tubes surfaces. In the case of reheater tubes similar pits and flow lines were observed after h operation. Physical observation also revealed finally spaced, wave like, parallel undulations on eroded five side external surface of super heater tubes of unit No. 2. after h of operation. In the case of unit No. 4, after h of operation damages in super heater tubes were occur with little or no wall thinning, but in most cases some metal wastage were occurred that is due to by large pit or gauge on the inside surface of the tubes (Fig. 7). The internal surfaces are found smooth and covered with very thin tenacious, dark oxide layers. No significant deposits were present anywhere on the received section. There are few numbers of pits were observe on the internal surfaces and while external lose of tube materials were also identified. Defective welding was also found to be is one of the major causes of super heated tubes failure. In many cases fracture near the welding side due to scaling were also identified. The final failures were observed with fish mouth opening with brittle appearance (Fig. 8). The failure of two adjacent platen super heater tubes of unit No. 4, GTPS has been observed after h of operation. The tubes st. 12XIMφ (Russia) Fig. 6 Macrostructure of ( 28X4 Carbon steel super heater) unit No. 2 showing flow lines in the longitudinal directions 60 (after h) Fig. 8 Failure in super heater tube with fish mouth opening with external loss of materials. Fig. 7 Macrostructure of ( 36X6 Low alloy steel high pressure super heater 2nd stage) unit No. 4, showing flow lines in the longitudinal directions 60 (after h). Fig. 9 Rupture in super heater tube st. 20, (Russia) U-band after h of operation.

5 SOHAIL & MUSTAFA: DAMAGES IN ALLOYED SUPER HEATER TUBES 23 was found to be made of Cr (1.2%)- Mo (0.61%) steel. One tube fractured with a fish mouth opening and another thinned down considerably. A significant amount of reddish-brown oxide layer (with fracture) was observed on the outer surface (out side) of the failed tube of unit No. 2. That may be due to change in internal microstructure of tube by long term high rate of heat flow. In the other tube, inner oxide layer was thin (200 µm) and the tube was thinned down due to fireside corrosion. The bulge and rupture in two super heater U bands, were observed (Fig. 9) in super heater tubes (st. 20) of boiler unit No.2 after h of operation of the unit at GTPS. In both cases internal surfaces were observed to be free of significant deposits, while external surfaces were covered with a tenacious fragmented oxide layer. The wall strength might have decreased at elevated temperature (~800 C) and tubes were thinned and weakened by thermal oxidation and turned bulged and then ruptured. In such a case chain graphitization could also occur after long term over heating. The damage began when iron carbide particle (normally present in plain carbon or low alloy steel) decompose into graphite nodules after prolonged 9 overheating above (427 C). The graphitization nodules, if distributed in the steel, rarely caused failure. However nodules sometimes chain together, forming plains of cavities field with graphite. The nodules usually form at microstructure defects in plain where there are change in chemical properties, and prolong stress lies. Carryover of silica in steam with other impurities are most objectionable for such a type of NCWWTs highpressure boiler super heaters and turbines. Therefore control of silica and other trouble some elements in boiler drum clean section should be administrate strictly. The danger of stress corrosion creaking is also reduced by substantial improvements to the water treatment installations and therefore, the feed water quality should also be maintained strictly. Extensive analyses of steam (live) revealed that a considerable quantity of 20 (±5) µgl -1 of silica content is carry over into the steam during start up of a boiler unit as well as abrupt increase of thermal load into the boiler steam water system. Therefore, excess number cold start of boiler unit also responsible for the super heater tube scaling. However, super heater contained very hard and thin layer of scaling in the interior surfaces. The composition of deposit of super heater tubes for both the units No. 2 and No. 4 were analyzed after different period of plant operation. The analyses showed approximately the same results for both units. The average composition of super heater contains maximum quantity of SiO 2 (78.3%) followed by Fe 2 O 3 (12.2%), CuO (5.4%) and other (4.1%) respectively. But, in case of reheater of unit No. 4 deposit contain also maximum quantity of SiO 2 (84.1%) followed by Fe 2 O 3 (6.7%), Cr 2 O 3 (3.9%), CuO (2.1%) and other (3.2%) respectively. Conclusions Super heater tubes of carbon steel st. 20 (Russia) and the steel (st.12ximφ, Russia) have fulfilled the expectations placed on them with respect to scaling in steam. Silica carryover takes places during cold startup and abrupt load changes. Silica is found to be the major component for the super heater scaling which has strong affinity to the chromium containing steel reheater st. 12X18H12T (Russia) tubes materials. Carryover of impurities in steam must be controlled by using polishing demineralization plant filtration method 10 Acknowledgement The authors are indebted to Bangladesh Power Development Board (BPDB) for providing scope to carryout this work at the Ghorasal Thermal Power Station, Bangladesh. References 1 Mayer K H & Kern T U, New materials for steam generation with efficiencies above 50%, VGB Power Tech, Essen, Germany, VGB Research Project 198, Teranishi H, Sowaragi Y, Kibota M & Haysa Y, Proc., 2nd Int. Con. Improved Coal-Fired Power Plants, Electric Power Research Institute, Palo Alto, CA, USA, Masuyama F, Hanedz H & Roberts B W, Pro, 1st Int Con. Improved Coal-Fired Power Plants, Electric Power Research Institute, Palo Alto, CA, USA, (1986) CS-5581-SR. 4 Jeffery G H, Bassett J, Mendham J & Denney R C, Vogels text book of qualitative chemical analysis, 5th Ed (London), (1989), Operational Manual, GTPS, BPDB, Bangladesh, Kostrikin Y, Manual on Aanalysis of Water, Steam and Sediments at Thermal Power Facilities (Energia, Russia), (1967) Horst K, Gunter K, Gerhard M & Jorg S, Damage in Power Plants, paper presented in 30th Ann Symp on Damage Analysis, Germany, September 13-14, Baumann K, Schulte J & Waltenberger G, VGB Kraftwerkstechnik, 58 (10) Port R D & Herro H M, The Nalco guide to boiler failure analysis, (McGraw Hill Inc., Kingsport USA) Burgmann F, Grunschlarger E & Fichte W, VGB Kraftwerkstechnik, 60 (3) (1980) 208.