HOW TO INCREASE HEAT TRANSFER AND REDUCE WATERWALL TUBE FAILURE IN HIGH PRESSURE BOILERS

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1 HOW TO INCREASE HEAT TRANSFER AND REDUCE WATERWALL TUBE FAILURE IN HIGH PRESSURE BOILERS Mr. Ashutosh Mehndiratta Chief Manager (Production) Offsites, KRIBHCO Shyam Fertilizers Ltd. Shahjahanpur (U.P) India Dr. Meghna Mehndiratta Consultant, KRIBHCO Shyam Fertilizers Ltd. Shahjahanpur (U.P) India Abstract: In spite of maintaining the very good water chemistry in water / steam cycle of thermal power plant, corrosion phenomena takes place in the boilers. The product of corrosion in the feed water system transported into the boiler and gets deposited on the internal surface of water wall tubes. It leads to overheating and On-load corrosion and ultimately tube failure. To achieve the almost zero tube failure in water wall of high pressure boilers, post-operational chemical cleaning is essential at the frequent intervals in the life span of power plant which will improve heat transfer and reduce under deposit on load corrosion. This paper will discuss the phenomena of localised corrosion, its remedial measures to achieve near zero tube failure. INTRODUCTION The Kribhco Shyam Fertilizers Limited has 4 numbers of fossils fired, sub critical, high pressure boilers112 kg/cm 2. The saturation temperature of boiler water 333 ºC. The capacity of these boilers is 100T/Hr each, These boilers are drum type types. Waterwall tubes of these boilers are made of carbon steel or low alloy steel. Deminerialized water is used in these boilers with proper boiler water treatment generally recommended by Original Equipment Manufacturer (OEM). Generally Non volatile treatment (NVT) i.e. TSP is used. Due to On-load corrosion in low pressure parts, condenser and heat exchangers of boilers, deposition of corrosion products and salt concentration takes place on the internal surface of waterwall tubes. Ingress of raw water due to condenser leaks into the feed water also increases the salt deposition and oxygen concentration on internal surface due to boiling. The condenser of 500 MW generators can contain 300 miles of tubing and approx tube-to-tube-plate joints, so some in-leakage of the external cooling water is inevitable from time to time. Thermal conductivity of this deposit is very low (about 2W/m/ºC) in comparison of carbon steel (about 50 W/m/ºC), thus significantly reduces the heat transfer and increases the outer metal temperature. Frequent tube failure has been observed in old boilers which ran more than 100,000 hours due to internal localised corrosion. Tube failure investigation indicated that the main reason of waterwall tube failure is either due to hydrogen damage or caustic corrosion or overheating or the effect of all. How mild steel corrodes in boiler water Water wall tubes in most of the fossil-fi red boiler are made of carbon steel. In pure DM water, or in very dilute acid or alkaline solutions at boiler temperature, it normally corrodes very slowly to form the black iron oxide known as magnetite (Fe3O4). The overall reaction is: 3 Fe + 4 H2O The corrosion rate is dependent by the rate at which the reactant (water) can reach the metal surface and the reaction product can leave the surface. In nearly neutral solutions, magnetite is very slightly soluble and it deposits as coherent and tenacious surface film, which greatly impedes this two-way chemical traffic. The 89

2 transport processes are dominated by slow state diffusion through the oxide layer and the corrosion rate is virtually independent of solution composition. The corrosion rate is also diminishes with time as the oxide thickness grows. Even after years of exposure, the layer is no more than a few microns thick. In more alkaline or more acid solutions, magnetite becomes increasingly soluble and precipitates in a different physical form. Instead of yielding a strongly coherent film, it has a more porous structure. Soluble species can now diffuse relatively rapidly through the film and the corrosion rate is much faster, although it still falls off the time as the oxide accumulated. The deposition of salts and corrosion products observed in different water wall tubes has been shown in the Photograph Figure-1 showing: The various regions of heat transfer in a once through boiler tube, with uniform heat flux. Figure-1 Showing water-steam formation in water wall tube. Photograph showing: waterwall tubes of 100T/Hr. boilers having a very thick deposit ranging thickness mm.. Regions of Heat Transfer in Boilers Recirculating drum type boilers are in use in KSFL. Drum type boiler consists of three separate components: an evaporator (waterwall area), where water is converted to a steam-water mixture: a drum, in which two phases are separated and a super heater. Of the six different heat transfer regions, five are usually present in a recirculation boiler (the exception is the dry out / post-dry out regions. Dry out should not occur in recirculation boilers, but is possible if the flow in a tube is reduced for any reason, or if one or more adjacent tubes are plugged). At the inlet, both wall and liquid are below saturation temperature and heating is by convection. At some point along the tube, the wall temperature exceeds the saturation temperature and bubbles are formed on the wall, although the bulk liquid temperature is below saturation. This is the region of sub-cooled nucleate boiling. (Continuing along the tube, the region of saturated nucleate boiling, where the bulk liquid temperature reaches saturation, is then enters). As more steam is formed, small bubbles coalesce to form large bubbles (slug flow). Eventually, these combine to give a central core of steam and leave an annular flow of water along the wall. Further along the 90

3 tube, the liquid film on the wall becomes sufficiently thin for convective heat transfer to the liquid surface coupled with direct evaporation to become the only means of heat transfer, because this method of heat transfer is highly efficient, nucleation of bubbles at the wall ceases through lack of sufficient superheat. At the same time, increasing steam velocity results in entrainment of liquid in the form of droplets. Depletion of the liquid film by evaporation and droplet formation eventually results in complete dryout of the tube wall. A dramatic increase in wall temperature occurs at the dryout point. In the post-dryout region heat transfer is by convection in the steam phase, the droplets gradually disappearing by evaporation. At low heat fluxes direct impingement of droplets on to the wall may also occur. Evaporation of all droplets finally produces single phase superheated conditions. WATER CHEMISTRY Impurities and additives present in water fed to high pressure boilers are controlled within closely defined limits for two very good reasons. # To reduce corrosion in the water-steam circuits to a minimum under normal and fault conditions # To reduce transport of iron oxide, hardness salts, silica etc. into the boiler and turbine. These substances deposit out on the heat transfer surfaces and turbine blades. Accelerated corrosion in many regions of high pressure boilers is known to be brought about by concentration of corrosive substances (chlorides, sulphates, hydroxides and phosphates) by boiling. The main water chemistry regimes used to reduce corrosion and transport of iron oxide etc. to a minimum are given in Table 1. Table1 showing: together with the advantages and disadvantages of each regime. Regime All volatile treatmen t (0.5 ppm NH3) Dissolve d Oxygen level Less< 5 ppb Protective oxide on tube surface Magnetite, Fe3O4 Advantages Presence of ammonia reduces transport of iron from feed train Disadvant ages No neutralizat ion action at high temperatu Na3PO4 or NaOH dosed High Oxygen Less< 5 ppb High >200 ppb Magnetite, Fe3O4 Hematite (Alpha Fe2O3) into boiler Ammonia is not corrosive at high temperatur e Neutralizes acid chloride and acid sulphates Reduced rate of transport of iron into boiler compared with low oxygen re. Ammonia can attack copper containing alloys in condenser and feed heaters in presence of oxygen Na3PO4 or NaOH can be concentrat ed by boiling, forming corrosive solutions No neutralizin g action Ingress of chloride results in rapid corrosion. Besides controlling the levels of dissolved oxygen and ph by means of chemical additives, as indicated in Table, very strict limits are specified on the concentrations of many other substances. CORROSION AND DEPOSITION IN VARIOUS HEAT TRANSFER REGIONS: Convective Heating region: In this region, at the inlet to a boiler, there have been no reported instances of tube failure owing to heat flux conditions. In the absence of boiling, the increase in corrosion rate should be small and related to the higher metal temperature and activation energy of the corrosion process in pure water. Studies on surfaces which have been oxidized under good water chemistry conditions show that even at high heat fluxes as high as 800 kw/m 2, negligible amounts of sodium and chlorine are taken up from NaCl, NaOH, NaHSO4, Na2SO4 and Na3PO4 solutions. Some iron is deposited from Fe (OH)2, which is likely to be the initial form of iron produced by protective or aggressive corrosion under alkaline conditions. 91

4 Nucleate boiling regions: Three types of situation are possible. Two of these are concerned with the formation of oxide deposits with different porosities on a tube surface and the effect of this on corrosion. In the first instance, all oxide is assumed to deposit from iron dissolved in solution and to form an oxide layer of low porosity (<10%). In the second situation, all oxide is considered to deposit from particles suspended in the liquid and to have a high porosity (>50%). The situation in a real boiler may lie anywhere between these two extremes. The third situation is concerned with boiling at defects on a tube surface. Oxide deposit of high porosity: Oxide of high porosity (>50%) is found to deposit in drum as well as once-through boilers under both low and high oxygen water chemistry conditions. The deposition rate is approx. proportional to the concentration of particulate iron oxide and the square of the heat flux. The best approximation to the real situation is given by D = k q2 c t Where, D = amount of magnetite deposited (kg/m 2 ) q = the heat flux (W/m 2 ) c = concentration of iron in water (kg/m 3 ) t = time (hour) k = constant ( approx. 5 X / W2 m 2 /s In a wick boiling mechanism, salts dissolved in the boiler water can be concentrated by factors > 10 4 Generally, the protective magnetite scale thickness is microns in the waterwall tube. When the corrosion rate increases due to upset of water chemistry parameters in boiler, (due to salt ingress and concentration), the deposit formation also increases due to corrosion of metal and precipitation of contaminants whose water solubility decreases at higher temperature on the evaporator tube surface. To maintain the ph in boiler water, in case of reduction of ph due to salt ingress, addition of more Tri Sodium Phosphate (TSP) is required. In this process, at some places on the internal surface of waterwall tubes, deposit thickness increases and the protective iron oxide scale becomes non protective and porous in nature. Porous, insulating types of deposits allow boiler water to diffuse into the deposit where the water becomes trapped and boils. The boiling of deposit in entrapped water produces relatively pure steam which tends to diffuse out of the deposit, leaving behind super heated non-boiling equilibrium solution of caustic, which is responsible for caustic corrosion or acidic solution, which is responsible of hydrogen damage in waterwall tubes as discussed below. CAUSTIC CORROSION Salt concentration under the deposit by Wick boiling phenomena states that If the salt concentrated under the deposit is having high ph due to concentration of caustic from TSP dosing, it start dissolution of protective magnetite (Fe3O4) layer on the evaporator tube wall inner surface and form sodium ferrite (NaFeO2) and sodium ferroate (Na2FeO2) as shown in the equation. Fe3O4 + 4 NaOH H2O Photograph showing: Caustic corrosion of boilers. ACIDIC CORROSION Solution of low ph is generated in high pressure boilers in two different ways: 92

5 * ph of the entire boiler water is reduced when contaminants which are acidic or becomes acidic when heated in to the boiler. * The bulk boiler water remains alkaline but acidic solutions are generated within corrosion pits. by the action of dissolved oxygen and chloride. The most common acid forming contaminant is sea water or a river water which is low in carbonate and sulphate. In the boiler, the acidity is increased locally to corrosive concentrations by boiling. In the acidic or highly alkaline conditions, iron reacts and hydrogen is liberated. As these micro cracks accumulate, tube strength diminishes until stresses imposed by the internal pressure exceed the tensile strength of the remaining, intact metal. At this point a thick-walled, longitudinal burst may occur depending on the extent of hydrogen damage as shown. Fe + 2 NaOH = Na2FeO2 + H2 Fe + 2 HCl = FeCl2 + H2 If the hydrogen is liberated in an atomic form, it is capable of diffusing into the steel. Some of this diffused, atomic hydrogen will combine at metal grain boundaries or inclusions to produce molecular hydrogen, or it will react with iron carbides in the metal to produce methane. Fe3C + 4 H = CH4 + 3 Fe Because neither molecular hydrogen nor methane is capable of diffusing through the steel, these gases accumulate, primarily at grain boundaries. Eventually, the gas pressure created will cause separation of the metal at its grain boundaries, forming discontinuous, intergranular micro cracks as shown in the micrograph Micrograph showing the fissures and cracks inside the metal due to hydrogen damage. Photograph showing: Waterwall tubes failure due to hydrogen damage because of localised acidic condition. EXPERIMENTAL PROCEDURE FOR DEPOSIT ASSESSMENT IN WATERWALL TUBE Boiler tube sampling: Four water wall tubes are cut from the four corner / sides of the high heat flux zone of the boilers, i.e. mainly from the uppermost portion of (2-3 meters above) burner zone. The tube sample are prepared as per ASTM D-3483, machined and cut longitudinally in two parts, one being the hot side (internal surfaces facing fire side) and the other, the cold side (internal surfaces facing remote side). Internal deposition from the samples is removed for chemical analysis mechanically by pressing the machined sample in a vice. The average quantity of internal deposits is calculated separately for both the sides from the difference between sample tube weights, measured before and after the deposits are removed chemically. 93

6 Deposit quantity assessment: Water wall tube samples are collected from site and machined on the outer surface. The outer machined surface of the tube is painted with corrosion resistant lacquer. The internal surface area (A, in cm 2 ) is measured and the initial weight (W1, in mg) of the tube samples is measured. The water wall tube sample is cleaned in 5% inhibited hydrochloric acid solvent at 65 ºC and put on a magnetic stirrer till the deposit is removed completely from the internal surfaces. Then it is washed with demineralised water and an alkaline solution and dipped in a surface passivation solution for 5-10 minutes. The final weight of the sample (W2, in mg) is measured. The quantity of internal deposit (DQ, in mg.cm 2 ) is calculated by the following formula. W1 W2 = DQ CRITARIA FOR CHEMICAL CLEANING Chemical cleaning of a boiler is suggested on the basis of the quantity of internal deposit present in the water wall tubes of the boiler. When the quantity of deposit exceeds 40 mg/cm2, the tube surfaces are considered to be very dirty surfaces as per the Indian Standard and the chemical cleaning is suggested to improve the heat transfer and reduce the overheating. The guidelines are given in Table-2. CONCLUSIONS Failure investigation studies of water wall tubes of different capacity boilers indicated the reasons of tube failure are: Formation of localised alkaline / acidic conditions is as per the mechanism of wick boiling phenomena under the deposit due to ingress of raw water into the condensate water. Acidic localised condition is responsible for hydrogen damage as observed in many old boilers where the deposit is very high and adherent type. Alkaline condition is responsible for caustic corrosion where the deposit is dense and porous in nature. Overheating of tubes due to deposition of salts on the internal surface of tube. REMEDIAL MEASURES It is suggested, to control the ingress of cooling water from the condenser tube leakage which will reduce the phenomena of salt concentration on the internal surface of boiler tubes. If the internal oxide growth increases from the limit as per the Indian Standards of IS Post operational chemical cleaning of boilers should be carried out to remove the existing porous deposit and to form a new adherent magnetite layer. Magnetite layer will work as a protective layer and reduce the possibilities of on-load corrosion. ACKNOWLEDGEMENT Author express gratitude towards Mr. O.P. Gupta(M.D)KSFL, Shahjahanpur,U.P. India Mr. N.K.Agarwal.(Ad.G.M) KSFL U.P. India & Mr.S.P.Singh (D.G.M.) KSFL U.P. India Table-2 Showing limits of deposit quantity allowed in water wall tubes at different pressure. The deposit quantity measured in high pressure boilers of different site are given here along with the trend chart and recommendations for chemical cleaning. REFERENCES 1) Test methods for accumulated deposition in a steam generator tube, 2005, ASTM International,West Conshohocken, PA, USA, ASTM Standard D ) Code of practice for chemical cleaning of boilers, 1998, Bureau of Indian Standards, New Delhi, IS ) Sugimoto A, Ueki, H. Sakuma, S., Proc. American Power Conference, Illinois 94

7 institute of Technology, Chicago, IL, USA, 34, 764 4) David E. Hendrik, Hydrogen attack on water wall tubes in a high pressure boiler, Material Performance, Aug. 1995, pp ) C. Syrett, Corrosion in fossils fuel power plant, EPRI, USA 6) L. Tomlinson and A.M. Pritchard, Effects of heat flux on corrosion of high pressure boilers, Br.corrosion J., 1985, vol.20, No.4, pp ) R.D. Port, Identification of corrosion damage in boilers, Material Performance, Dec. 1994, pp ) G.M.W. Mann, History and causes of On-load water side corrosion in power plants, Br. Corrosion J., 1977, vol. 12 No.1, pp 7-14 Websites From Linkedin.com Mr.Nikhilesh Mukherjee (Independent Consultant & Author) Mr.Arvind Desai Mr. Vinod Sharma Mr. Steven Dunn 95