A semi-analytical approach on critical thermal states in water wall tubes of a subcritical drum boiler of a thermal power plant

https://doi.org/10.1016/j.ijpvp.2021.104507Get rights and content

Highlights

  • The critical heat flux in a furnace of a subcritical drum boiler was investigated.

  • The water wall tubes in higher altitudes were more sensitive to heat flux distribution.

  • The possibility of dryness and failure of the tubes increased with increasing altitude.

  • Serious risk of overheating occurs in the whole tube length by fluid flow disturbance.

Abstract

One of the most important issues in design of water tube boilers is the prediction of the critical heat flux and the corresponding critical quality of the steam and vice versa. This phenomenon directly affects growth rate of oxide/deposit layers, rising water wall tubes temperature, and consequently tubes burnout. In the present work, the critical situations in a furnace of a subcritical drum boiler were investigated. It was shown that by moving to higher altitudes of the investigated furnace, its water wall tubes were more sensitive to heat flux distribution, which consequently increased the possibility of dryness phenomenon and failure of the tubes. Besides, this study demonstrated that in the event of a disturbance in the fluid flow inside the tubes, all the length of water wall tube are at serious risk of burning and overheating. Microstructural evolutions, existence of thick deposits, and frequent failures at higher the altitudes confirmed the semi-analytical results.

Introduction

Steam power plants are categorized into strategic industries of any country so that their development is one of the indicators of development and prosperity. In addition, the initial significant investment for construction, operational complexity, maintenance problems, complex and nonlinear dynamic behavior, interaction with power distribution grid, operation in very harsh temperature, stress, chemical and electrochemical conditions along with high expectations for high reliability and availability have led to continuous improvement of power plant elements including materials, processes, methods of operation, maintenance and evaluation, simulation of thermo-hydraulic behavior and root cause failure analysis. Most of failures that occur in boilers are due to the microstructural deterioration of tube metal or to the occurrence of invisible local stresses. In addition, the most common cause of forced shutdowns in all types of boilers has been attributed to tubes failure [1]. Approximately 90 % of long-term overheating occurs in superheaters, reheaters, and water wall tubes [2]. About 24 % of the outages of the power plants is originated from creep failure of a boiler [3,4], which can be attributed mainly to the temperature rising and formation of oxide scales growth. Increasing temperature of the tube metal and growth of the oxide layer have direct two-way effects on each other. In addition, abnormal increasing in metal temperature of the tubes can be related to combustion problems such as fuel-air ratio and direct flame impingement on the tubes. In the long term operation of boiler, this will lead to significant increase in the oxide/deposit layer growth rate on the inner surface of the tube, followed by accelerated increase in temperature of the tube metal. Obviously, the heat flux and temperature fields have the most important effect on this issue. This is probably why, in terms of ranking based on the location of failures, water walls are in second place after superheaters, and in terms of ranking by the material, carbon steels statistically have the most frequent failures [4]. It is noteworthy that carbon steels are an economic selection for manufacturing of conventional boiler water wall tubes. Generally, the riser tubes of water walls forms the combustion chamber or furnace walls which are directly exposed to combustion radiation. Therefore, one of the most important concerns in this regard is to prevent the occurrence of critical heat flux.

The critical heat flux is the starting point of a region from which the fluid is only in the vapor phase. In other words, it is a situation in which a small increase in heat flux leads to a sudden overheating of the tube due to the transition from nuclear boiling to film boiling [5]. Equivalent to the point where the critical heat flux occurs is the critical vapor quality point; that is, the minimum quality after which the phenomenon of fluid dryness or the depletion of the fluid from the saturated liquid occurs. This problem can be localized or general in low and high vapor quality vapor respectively. Critical heat flux has a significant effect on temperature distribution of boiler tubes and as a result oxide/deposit layer growth and the burning phenomenon of the tubes.

Basically, the estimation of the critical heat flux and the corresponding temperatures consists of two parts, which include the heat flux produced inside the furnace as an independent part and the thermo-hydraulic calculations of the fluid inside the tubes as the part whose results depend on the heat flux inside the furnace.

Various methods typically including analytical method [[6], [7], [8]], numerical analysis [[9], [10], [11], [12], [13], [14]] and zone method [[15], [16], [17], [18], [19], [20], [21], [22], [23]] are used to evaluate heat flux in the boiler furnace. It can be said that the imaginary planes method [24] was another form of zone method.

The zone method was first used by the Hottel and Cohen to investigate radiant heat transfer in a gas-filled enclosure [25]. In this method, the radiant medium is divided into several zones with uniform properties and the balance of mass and energy is applied in each zone. The mass flow and energy released in each zone are considered as the inputs of another zone. If a zone has a burner, is analyzed with the well-stirred model and otherwise with the plug-flow model. Later Hottle and Sarofim applied this method for complex geometries [26]. The zone method is one of the most popular methods for estimating the heat flux inside furnaces.

On the other hand thermo-hydraulic calculations inside the boiler tubes involve many complexities, mainly due to the complex nature of the boiling phenomenon, the dynamic behavior of the fluid, direction of flow, inclination of tubes, geometry of tubes, unevenness of the heat flux on the tubes, fouling and oxide/deposit layer on inside and outside of tube surfaces, circulation ration and the interaction of these factors. Therefore, researchs in this field is quite complex and significantly dependent on experiments and semi-empirical relationships. While experimental researches involves simplifications and uncertainties that cannot fully cover the real and dynamic conditions of the boiler, it is expected that they can reveal the behavior of the under study system to a good extent. Numerous data have been obtained through researches on the boiling area, which have led to many experimental relationships.

The effects of inclination on critical heat flux (CHF) and post-CHF performed by Kefer et al. [27] over a parameter range typical of fossil-fired once-through boilers and waste heat recovery boilers. Kandlikar [28] developed a correlation by expanding the data base to 5246 data points from 24 experimental investigations with ten fluids. Hall and Mudawar [29,30] conducted researches aiming to compile and assess world CHF data, identifying all known subcooled CHF correlation for water flow in a uniformly heated tube, developing a superior design correlation for accurate prediction of subcooled CHF, and evaluation this correlation using the subcooled CHF data. A comparison between nucleate boiling and film boiling in subcritical, once through power boilers performed by Swenson et al. [31]. A review on published CHF models including CHF trigger mechanisms and parametric influences was done by Liang and Mudawar [32]. Ahmed and El-Nakla [33] discussed the existing CHF experimental and modeling studies in order to understand the phenomena leading to CHF. Payan-Rodriguez et al. [34] extended sublayer dryout theory to account the effects of the axially non-uniform heat flux distribution by combining it with the shape factor method (F-factor). They also calculated the critical wall temperature (CWT) of the tubes using CHF data.

In the present study, a straightforward semi-analytical methodology, as an introductory guide for designers and steam power plant experts, is proposed to predict and evaluate the critical heat flux in two modes: normal heat flux and normal quality. In the event of any of these modes in the boiler, short-term and long-term overheating of the tubes, oxide layer growth, under deposit corrosion due to concentration of chemicals may occur significantly. It is hoped that this work can be a good help for researches working in the field of the root cause failure analysis of boiler tubes.

Section snippets

Process description and failure history

Fig. 1 demonstrates general view of the investigated boiler [35]. The boiler consists of 12 burners in one side of the wall, which are arranged in four triple rows. The path of combustion products inside the furnace is inverse U-shaped. Horizontal center axes of the first to fourth rows of the burners are 10.9, 8.7, 6.5 and 4.3 m from the furnace floor, respectively. The water enters the economizer at a temperature of 220 °C and 150 bar pressure and is delivered to the drum at the top of the

Research methodology

Table 1 shows the characteristics of the working fluid and investigated boiler. In order to investigate the behavior of the boiler in critical states, two states has been considered: state 1, and state 2. Each state is described below:

State 1: In this state, it is assumed that the steam quality distribution in the boiler is normal, but for any reason, such as deviations in the burners, disturbance of the fuel-air ratio in the boiler, and likewise, the amount of radiant heat in the boiler

Calculation steps

Step 4.1. Extraction of formulas related to modes 1 and 2.

Critical state 1. In order to calculate the critical heat flux in the boiling height section, which is in the saturated state, Katto (L-regime) model with saturation input conditions [37] was used as follows:qcGrhfg=C(σρfGr2(BH))0.043(BH)dirwhere:C=0.25,(BH)dir<50C=0.25,150(BH)dirC=0.25+0.0009((BH)dir50),50(BH)dir<150

Heat flux in two-phase region was also calculated using the relationship of Biasi et al. [37]:qc=maximumof{1883dirαGr1/6

Modeling results

Fig. 6 shows the graph of critical steam quality changes in terms of circulation ratio. For this purpose, sum of relations (4) and (5) (model 1) and the other hand relation (7) (model 2) were used for comparison. As the graph shows, model 1 seems to demonstrate a more logical behavior, because the higher the circulation ratio, the less heat flux is produced inside the furnace, and therefore the critical quality of the steam is expected to increase. The diagram in Fig. 7, which show the changes

Conclusions

  • 1.

    In the event that there is a heat disturbance in the furnace, the tubes located at higher elevations of the furnace are more prone to burn and overheat. On the other hand, if the factors such as tube clogging disrupt the flow of fluid inside the water wall tubes, the whole tube is prone to burning and overheating.

  • 2.

    According to the previous investigations, most of the boiler tube failures occurred at higher altitudes between 12 and 14 m. Inspection of the nozzle angle of the burners showed that

Author statement

Ali Hossein Assefinejad: Investigation, Methodology, Writing - Original draft, Writing – Review & Editing.; Ahmad Kermanpur: Conceptualization, Supervision, Resources, Validation, Writing - Review & Editing; Abdolmajid Eslami: Conceptualization, Supervision, Writing - Review & Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to sincerely thank the partial financial support of R&D department of Zargan Power Plant (Contract ID 95/B.Z/068).

Nomenclature

a1
Non-uniformity coefficient inside the surface
a2
Height correction factor for gas fuel boiler
a3
Heat loss in the mass of the tube
a4
Heat absorption coefficient on the internal surface
afl
Total emissivity
αfu
Overall furnace emissivity
alu
Luminous flame emissivity
atr
Flame emissivity of tri-atomic gases
Bi
Fuel consumption of each burner in row i kg/s
Bo
Boiling number
BH
Boiling head M
C
Carbon fraction in fuel (as-received basis)
Cpf
Specific thermal capacity J/(kg. K)
CR
Circulation ratio
dfu
Depth of the furnace

References (50)

  • D.D. Hall et al.

    Critical heat flux (CHF) for water flow in tubes-I. Compilation and assessment of world CHF data

    Int. J. Heat Mass Tran.

    (2000)
  • D.D. Hall et al.

    Critical heat flux (CHF) for water flow in tubes-II. Subcooled CHF correlation

    Int. J. Heat Mass Tran.

    (2000)
  • G. Liang et al.

    Pool boiling critical heat flux (CHF) – Part 1: review of mechanisms, models, and correlations

    Int. J. Heat Mass Tran.

    (2018)
  • W.H. Ahmed et al.

    Towards understanding the critical heat flux for industrial applications

    Int. J. Multiphas. Flow

    (2010)
  • L.A. Payan-Rodriguez et al.

    Critical heat flux prediction for water boiling in vertical tubes of a steam generator

    Int. J. Therm. Sci.

    (2005)
  • A.H. Assefinejad et al.

    Failure investigation of water wall tubes in a drum boiler of a thermal power plant

    Eng. Fail. Anal.

    (2020)
  • J.R.S. Thom

    Prediction of pressure drop using forced circulation boiling of water

    Int. J. Heat Trans.

    (1964)
  • S. Raghavan et al.

    State diagram-based life cycle management plans for power plant components

    IEEE Trans. Smart Grid

    (2015)
  • M. Bethmont

    Damage and lifetime of fossil power plant components

    Mater. A. T. High. Temp.

    (1998)
  • A. Kumari et al.

    Impact of boiler water chemistry on waterside tube failures

    Int. J. Inn. Res. Sci. Technol.

    (2015)
  • G.K. Abad-Sada et al.

    Enhancement the performance of steam generation power plant

    J. Eng. Develop.

    (2005)
  • H.E. Emara-Shabiak et al.

    Prediction of risers' tubes temperature in water tube boilers

    Applied Mathematical Modeling

    (2009)
  • S.A. Dudek, Z. Chen, A.N. Sayre, COMO: a computational fluid dynamics model for predicting boiler flow and combustion,...
  • C.V. Silva et al.

    CFD analysis of the combustion process in a boiler of a 160 MWe power plant: leakage influence

    J. Braz. Soc. Mech. Sci. Eng.

    (2019)
  • T.R. Johnson et al.

    Radiative heat transfer in furnaces: further development of the zone method of analysis

    Int. Symp. on Combustion

    (1973)
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