Failure Analysis of 600 MW Supercritical Boiler Water Wall

Boiler tube often causes abnormal boiler outage, bringing greater economic losses. This thesis mainly comes from the dynamics of boiler water, boiler furnace accident location of wall temperature distribution to explore the cause of the accident boiler. Calculation results show that the deformation will seriously reduce the boiler allowable maximum temperature difference between the screens. And the boiler is not over-temperature, low temperature difference between the screens, which have burst pipe. Analysis of the deformation of the boiler and propose solutions.


INTRODUCTION
The coal-fired power plants have dominated the main position of power generation in China (Cui, 2010).By the end of 2009, it's installed generating capacity has reached 10.5 billion kilowatts, in which the coal-fired power generation accounted for 72.3% (BaWei Company, 2009).Due to the needs of high efficiency and low pollution emission, the supercritical boiler has become an important clean coal-fired power generation technology, but the boiler's safety will directly impact on the security and stability of the other components (Zhou, 2006).The data show, among the accidents of Chinese coal-fired power generation, because of the shutdown, which is caused by the boiler accident, accounting for 40% and the all types of heating surface tubes accident is about 70% (Luo, 1981;Song, 1995).It shows that the destruction of the heating surface tubes is an important reason for thermal power plant accident shutdown.
The 3 rd boiler of Jinzhushan power plant is the first 600 MW supercritical anthracite "W" flame boiler in the world and began to operate in December 2009.But the destructions of water wall tubes often happen since operating, so it has practical significance to research the failure mechanism of boiler water wall.

BOILER OVERVIEW
This 3 rd boiler of Jinzhushan coal-fired power plant is produced by the Beijing Babcock and Wilcox.It use the Π type layout, single furnace, new concentrated EI-XCL burner, arch combustion and the Furnace use internally ribbed tube vertical rise Membrane water wall, circulation pump to start the system, once reheat and the thermostat addition to using coal/water and then also to use the flue gases allocation baffle, swing burner, spraying and so on.The boiler main parameters are in Table 1.
The boiler is a general rather poor equipment and it need to design a suitable boiler according to coal quality characteristics for different coals.This kind of coal designed for the boiler comes from the coal mine near the power plant.Considering changes in boiler coal and check its coal again.Coal characteristics of the boiler and checking coal characteristics are shown in Table 2.
It has 1562 water wall tube for The lower part and composed of 30 loops, Pipe specification φ 35×6.5 mm, the pitch of 55 mm, The water wall pipe material SA213T12, flat steel material for 15CrMo, the furnace water wall of lower all use the Optimization bulls threaded pipe (OMLR) and avoid the occurrence of deterioration of the heat transfer.In the middle it is composed of 110 Φ 133×20 mm material for 12Cr1MoVG connection tube to incorporated into the intermediate mixed set box.One thousand four hundred and ninety eight water wall tubes of Upper water wall compose 18 loops.Tube specifications is Φ 28×6 mm, material is 15CrMoG and the pitch is 55 mm, The flat

BOILER TYPICAL SQUIB SITUATION
Boiler furnace is in September 2009, as shown below, it is at the water wall middle of the 41 m elevation in the front wall and the water wall tube cracks 45 cm.Accident pictures in Fig. 1.Water wall tube has no significant rising crude.In November 2010, the water wall tube crack in the 38 m position in the back wall, Population explosion 10 mm, wide mouth 20 mm and no significant up rough.Accident pictures in Fig. 2.
Boiler hydrodynamic calculations: According to boiler hydrodynamic calculation standards in our country and refer to the former Soviet Union hydrodynamic calculation standards, Select several typical conditions (respectively the BMCR, TRL, 80% THA, 70% THA, 60% THA conditions) to calculate the boiler water wall voltage drop and to draw hydrodynamic characteristics curves and compared with the boiler water wall actual hydrodynamic characteristics curve (the actual data is the processed data in the DCS).The results are shown in Fig. 3.
Calculated pressure drop agrees well with the actual pressure drop on the whole.The slope of the curve of Boiler hydrodynamic characteristics is relatively large and generally does not appear hydrodynamic instability.But in the flow 1400-1500 t/h, when the separator outlet pressure is about 21.81 Mpa, In the pseudo-critical areas, the working fluid specific heat change greatly, under special circumstances may appear film boiling and may appear multivalent situation (the flow area slope is small and anti-disturbance ability is weak ).
Boiler in traffic of 100% BMCR Firstly use average flow calculation and then do the flow curve Back calculation by the pressure drop.Obtained by calculating the front, rear, side walls single tube flow ratio is 1:1.01:1.This shows that the boiler flow is more evenly distributed.
The boiler tubes pulsating checking usually choose the boiler startup load and boiler minimum load condition to calculate.If the calculated mass flow rate is greater than the boiler limit velocity that the boiler does not occur pipes pulsating.Choose flow of 974,570 t/h to carry out inter tube calculation.Through lookup   Nomographic to get limit flow rate and calculation coefficient, the calculation results are shown in Table 3.
Boiler wall temperature calculation: The highest point temperature of wall is calculated by using the following formula: The calculation results show that the front and back walls of the boiler is the bimodal temperature field, Before and after the wall about six meters away from the side walls on both sides is the highest temperature in the boiler and the Sidewall center position has the highest temperature.Wall temperature of the boiler is lower than the inner wall temperature about 10 to 50°C.
As it is calculated that the working fluid temperature is lower than tube wall temperature, pipe working fluid temperature is higher than the outer wall temperature.
max max w w gz q q t t r r Both inside and outside wall temperature difference is the biggest in the highest position of the center temperature.Front, rear and side walls all have the smallest temperature difference between the inside and outside wall of the corner position.The maximum wall temperature of boiler is about 420°C, even coupled with the maximum temperature of 50°C is only 470°C, The boiler calculation rupture strength temperature is the next furnace to 511°C, and the upper furnace to 492°C.Therefore boiler wall temperature does not exceed the temperature.The boiler Checking Calculation temperature is also lower than the boiler's calculation rupture strength temperature.Table 4 is the boiler furnace water wall operating parameters, when to DC process.As can be seen from the Table 4 boiler wall running temperature much lower than the design, there is no danger of over temperature, when boiler is running.

THE BOILER BURST PIPES REASON ANALYSIS
The squib positions concentrated in the 35-45 m where obvious deformation of the section, Fig. 10 and 11 is the basis of measurement of the upper furnace to the top position of 40 m in the deformation of the front and rear wall.Consider the deformation of the boiler water wall.
The water wall as one surrounded by a fixed sheet, the bending generated by thermal stress calculated by the following formula, the stress is the tangential stress: Among, r : Radius of curvature, m t : Inner and outer wall temperature difference, °C σ max : Bending produce the greatest internal stress, pa α : Coefficient of linear expansion µ : Elastic modulus Upper furnace of the boiler approximately 14 m, every 2.9 m horizontal fixtures.If 10 mm as the actual amount of deformation aboving calculation, the temperature difference between the inner and outer walls of 117°C, tangential stress due to bending σ max is 203 Mpa, it is significantly more than the boiler material furnace allowable stress (Upper furnace 511°C  Consider the boiler bent by the impact of the boiler water wall tubes.Boiler strength adopt the third strength theory, Boiler axial to the most dangerous the direction of the third strength theoretical, tangential stress increases to over axial stress.According to the third strength theoretical maximum shear stress is the material cause of failure, the size of the maximum shear stress is τ max = (σ 1 -σ 3 ) /2, σ 3 is tangential stress, in this process unchanged, σ 1 is the axial stress original, tangential stress currently, σ 3 constant to make τ max less than the boiler maximum allowable stress can only keep σ 1 constant large, σ 1 is composed of three parts: the boiler thermal stress, the curved shear stress and film stress, The size of the other two forces, boiler bent to form shear stress increases the boiler tube damage, we can only reduce the boiler temperature stress, reduce the allowable temperature difference.
This creates a temperature difference between the boiler screen did not exceed the boiler burst pipe allowable temperature difference.If calculated in accordance with the above formula, boiler bend increased by 3 mm, boiler screen temperature difference between allowable will be reduced to about 20°C.Actual wall temperature of the boiler is not in excess of the calculated temperature, but boiler actual shear force exceeds the calculated temperature or close to the calculated temperature of the stress causes creep acceleration during operation, resulting in the overtemperature creep phenomenon.

The reason of the pipe burst:
The boiler water wall serious deformation, deformation water wall radial stress is greater than the axial stress, boiler as the stress increases between screen allows maximum temperature difference reduced, resulting in the boiler in the deformation of the large temperature difference between serious regional screen squib.

TO TAKE PRECAUTIONS
• A reasonable temperature field: From the boiler temperature distribution, the deformation of the front and back walls of the boiler is very serious and the side wall is almost no deformation.The front and back walls of the boiler have two highest temperature values, the side wall is only a maximum temperature.This shows that to some extent different deformation bimodal temperature field expansion lead boiler deformation.Consider the flame center closer to the center to form a side wall of a single peak.• To control the temperature difference between the boiler screen: The temperature difference between the screen of the boiler recommended is 84-120°C.Consider the deformation of weakened to allow the temperature difference between the screen.In actual operation, to maintain the temperature difference between the outer wall of the screen less than 20°C or lower.This guarantees that the side near the fire of between the screen temperature below 70°C.• To monitor boiler outer wall temperature: Boiler water wall tubes calculated rupture strength at temperatures under the furnace is 511°C, upper furnace is 492°C.Check Calculation of boiler water wall tube wall than the outer wall of the maximum temperature difference is back of the water wall, at 80°C.The front wall and side walls of the maximum temperature difference is 60°C.
The front side wall of the outer wall of the boiler's upper furnace should be controlled below 430°C, under furnace is lower than 450°C.The back side wall of the outer wall of the boiler's upper furnace should be controlled below 410°C, under furnace is lower than 430°C.• Replace the deformed tube: Severely deformed under stress analysis pipe tangential stress increase, when it is greater than the axial stress of the boiler will reduce the boiler allows between screen temperature difference cause boiler force shear, creep acceleration, tube rupture.Replacement of deformed pipe to prevent pipe explosion from happening again.

CONCLUSION
Simplify the water wall to fixed flat, calculate the occurrence of severe deformation of boiler water wall.The results showed that the deformation will seriously reduce the boiler to allow the largest temperature difference between the screen.This leads to the boiler temperature difference between not over-temperature, low screen squib.Water wall deformation may be the boiler temperature difference too large between the inside and outside wall or horizontal water wall deformation varies.Replace the severe deformation water wall tube, to reduce operation allows the screen not higher than 70°C temperature difference, prevent to burst pipes happen again.

Fig. 1 :
Fig. 1: September 2009 accident pictures The calculated boiler water power curve The actual boiler hydrodynamic characteristics curve ratio q max : Wall positive heat flux density α : Coefficient of heat transfer to the working fluid from the inner wall δ : Wall thickness λ : Thermal conductivity of the tube tgz : The temperature of the working fluid u (r 1 ) : Shunt coefficientThe fin end temperature is calculated as follows: k β , k s/d : Correction factor(Zhou, 2006) According to the formula to Calculate Boiler in 100% condition, the largest and most dangerous area of the heat flux.The position of elevation is 40 m and the calculation results are shown in Fig.4to 9.

Table 1 :
Boiler main parameter

Table 2 :
Boiler fuel characteristics

Table 3 :
Tube pulse calibration Load

Table 4 :
Furnace wall temperature design values and operating values