Causes and Countermeasures of short-term bulge of

2022-08-15
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Causes and Countermeasures of short-term bulging of hot water boilers

preface

around the Spring Festival in 2003, dozens of bulging accidents of hot water boilers occurred in Zaozhuang City, including eight bulging boilers from a certain structure. On March 8 of the same year, a school in Zaozhuang City had another bulging accident of hot water boilers with this structure after only six months of use, local cracks and water-cooled wall pipe blockage and leakage. It casts a shadow on the promotion and use of this structural type of boiler. As a technician who has been engaged in boiler and water treatment inspection for many years, the author starts with the use management and boiler structure of this boiler, and through analysis and demonstration, finds that there are some common problems in drum boilers

1 basic condition of boiler

after investigation: this boiler is a dzl2.8-0.7/95/70-aii hot water boiler produced by a municipal boiler factory in our province in September 2001. Long and short water-cooled wall tubes are arranged on both sides of the furnace, and there are two rear water-cooled wall tubes at the rear of the left and right smoke chambers respectively (hereinafter referred to as wall tubes). The long, short and rear wall tubes are cross led out from the header and welded on the drum, and the short wall tubes are inserted into 240mm on both sides of the longitudinal axis of the lower part of the drum, The space formed between the long and short wall pipes is sealed and insulated to form the left and right second return smoke chambers; The regular sewage discharge device in the boiler is a tubular longitudinal sewage suction pipe, which is fixed by the front and rear supports welded at the bottom of the boiler drum. Two rows of slag suction holes with a diameter of 12mm are opened at the lower side of the sewage suction pipe (see the schematic diagram for the boiler structure)

boiler structure diagram

the boiler was installed in late November 2001 and put into use in December. By March 2003, there was a bulge accident, and the continuous operation time was only more than 6 months; Inspection of furnace condition: an insulation layer about 80mm thick and 1.6m long is set in the belly of the drum before the rear arch end. The bottom of the second return smoke chamber is covered by the insulation layer, and the upper part of the furnace rear arch is about 150mm thicker than the normal rear arch; During the inspection, it was found that the fashionable appearance brought by the second return smoke chamber insulation layer, drum insulation layer, front and rear arch end varnish and polyamide material scheme were accompanied by serious coking. Check the drum at the drum. The drum starts at the end of the insulation layer on the drum abdomen, and is about 300mm long, 100mm wide and 25mm ~ 30mm high. During the inspection, it was also found that a short wall pipe on the left side of the drum directly opposite the drum had bulged and leaked. After entering the waist hole of the drum for inspection, it was found that there was water slag with a thickness of about 60mm at the bottom of the drum, and the scaling thickness in the pot, header, long and short wall pipe and visible parts of the smoke pipe ranged from 0.5mm to 1mm, and no serious scaling of heated parts was found; The weld root of the rear support of the sewage suction pipe 450mm away from the center of the drum was inspected, and two cross thermal cracks were found; The long and short wall tubes were checked for blockage, and it was found that two wall tubes on the left and right sides of the drum facing the drum were blocked at the lower bend (including the leakage of a short wall tube on the left mentioned above)

2 accident cause inspection and laboratory analysis

2.1 the foreign matter blockage of the short wall pipe causes the water flow in the pipe to stop and aggravates the deterioration of the working conditions of the drum

the blocked short wall pipe is in the radiant heat zone of the furnace flame, and the blocked part is at the beginning of the bending of the pipe at the lower part of the smoke chamber. The water flow resistance in this part is large, which is very easy to produce scale or cause pipe blockage. In order to find out the cause of the blockage, first start with the inspection of water treatment equipment. After investigation, the water treatment equipment of the health school belongs to the downstream regeneration exchanger, which is the attached machine of the boiler factory; Check the connecting pipe and installation of the exchanger, and the appearance quality meets the requirements; Check the quality and quantity of the filled resin, and the resin factory witness data are complete. The resin height accounts for 2/3 of the length of the exchanger, which is in the middle of the mirror; Refer to the water treatment analysis records of the school: the total hardness of raw water is 10.5mmo1/1, the total alkalinity is 4.6mmol/1, and the hardness of boiler feed water is 2mmol/1 ~ 3.5mmol/1 (the basic units of alkalinity are HCO3 -, oh -, 1/2co32 -, and the basic units of hardness are 1/2ca2+, 1/2mg2+, the same below); The pH value of boiler water is 8.5 ~ 9. During the cutting inspection of the burst and blocked short wall pipe, it was found that the bending part of the pipe was blocked by white objects, and the blocking length was nearly 150mm. After the inspection of the blocking objects, it was found that the blocking objects were relatively loose, and the steel wires were quickly removed by beating. Take the blockage and put it in 10% hydrochloric acid. 2. Make the filter element shell of the water purifier for dissolution test. The blockage is not dissolved violently, and there is no large number of bubbles, indicating that the scale is not mainly carbonate. However, judging from the color of the scale sample, it is very unlikely that it is sulfate or silicate scale. To be cautious, 10% barium chloride solution was added to the scale sample solution, but no large amount of white sediment was found; In addition, 100ml of scale sample solution was taken and 20ml of hydrogen fluoride solution was added to it, which also ruled out the possibility that the blockage was sulfate or silicate scale. In order to find out the source of the suspected blockage, a detailed inquiry was organized to the stoker operators. They said that when the boiler was dried with lime in the spring of 2002, it was found that lime powder overflowed into the boiler and was removed, but the situation in the pipe was not carefully checked. Therefore, it can be said that the entry of lime has led to the local blockage of individual pipes and the weakening and stagnation of water flow rate in the pipes. The cooling capacity of the pipe wall decreased, which accelerated the formation of scale, resulting in the complete blockage of some short wall pipes

in the boiler structure, the setting of the drum short wall tube is an important guarantee for the safe operation of the boiler. The rapid flow of water in the tube plays an extremely important role in stirring the slag accumulation in the boiler, scouring and cooling the bottom of the boiler. When the short wall tube works normally, even if a certain thickness of water slag is accumulated in the pot, the drum will not bulge due to this; However, when some short wall tubes are blocked, the water flow in the tubes weakens or stops, and the drum completely placed in the radiant heat zone without heat insulation accumulates water slag of a certain thickness, the working environment of the steel plate at the lower part of the drum accelerates to deteriorate. Therefore, partial blockage of the short wall pipe directly leads to and accelerates the occurrence of drum bulging accident

2.2 impact of water treatment

although the water treatment quality of this boiler does not cause a large range of serious scaling of the boiler, the hardness of the feed water is seriously over standard, and the pH value of the boiler water is unqualified, which still accounts for a large part in the short wall tube explosion and drum bulging accident. According to statistics, this type of drum boiler generally has the problem that the hardness of the feed water is seriously over standard, which will cause the following serious consequences

(1) too high water hardness intensifies the speed of short wall pipe blockage

gb1576-2001 "industrial boiler water quality" stipulates that the feed water hardness of pressure hot water boiler is ≤ 0.6mmol/1, in order to prevent serious scaling of hot water boiler, resulting in overheating and bulge of boiler heating and pressure parts. However, at present, many hot water boiler users still have the idea of paralysis that the outlet water temperature of water heater is below 95 ℃, and the high feed water hardness is not a problem. Zaozhuang is located in the mining area, and the hardness of raw water is high. The hardness of raw water using tap water as water source has reached 10.5mmol/l. It is difficult for the primary treatment of flow regeneration exchanger to meet the feedwater hardness standard. If water treatment is not paid attention to, the feedwater hardness will seriously exceed the standard. When the pipe is exposed to radiant heat or there are foreign matters in it, the hot water in the pipe will vaporize, which will seriously scale the pressure bearing parts of the boiler, and lead to pipe blockage, overheating and bulge. The short wall pipe of this boiler is blocked, overheated and burst, and the high feedwater hardness plays a role in rapid filling of the blocked pipe holes. If the feedwater hardness of this boiler is controlled within the scope specified in gb1576-2001 at the initial stage of installation and operation, the metal surface in the pipe will be relatively smooth, making it possible for no or little lime to be left, or it will fall off at the initial stage of boiler operation, so as to avoid the occurrence of short wall pipe blockage and explosion accidents, Avoid bulging accidents at the bottom of the drum

(2) too high water hardness caused the deposition of a large number of flake slag in the furnace barrel

it was found from the slag removed from the boiler that most of the slag was formed by the falling and deposition of the thinner scale of the flue pipe. The smoke pipe is a convective heating surface, which is not heated seriously, the water temperature outside the pipe is low, and the water flow scouring force is large, so the scale is easy to fall off. The inspection found that the surface area of each scale is relatively large, about 30. This innovative technology is the first in the world × About 20 (mm2), because the pipe sewage suction is small with the improvement of people's living standards, it is difficult to effectively discharge it out of the pot. When the hardness of the feed water meets the standard, there are few scaling substances. Because there is basically no evaporation and concentration of the boiler water in the hot water boiler, the impurities in the boiler water will not precipitate due to dissolution and saturation. When the pH value of the boiler water meets the standard, a small amount of precipitated dirt has small viscosity and good fluidity. Most of it is granular. As long as the stoker discharges the sewage correctly, it will be discharged out of the boiler through the sewage pipe or through the dirt remover on the system pipe, However, due to the hardness of the feed water of this boiler exceeding the standard by 3 ~ 6 times, the smoke pipe in the boiler is scaled. However, due to the hardness of the feed water being less than the alkalinity, the scale is composed of carbonate, the scale sample is thin, and the adsorption force is weak. It is easy to fall off and deposit on the bottom of the boiler under the scouring of the high-speed water flow in the boiler. However, the pH value of the boiler water is not up to the standard, which makes the fluidity of the water slag poor, and the solid cannot be effectively discharged out of the boiler. In addition, there is no heat insulation here. When the water slag accumulates to a certain thickness, and the short wall pipe at the bottom of the boiler is blocked again, the water flow cannot scour the bottom of the drum to effectively cool it, resulting in overheating of the steel plate here and reduction of the allowable stress of the steel plate. When the bearing capacity is not reached, plastic deformation occurs, and bulges occur. Statistics show that there are large thickness flake water slag in Zaozhuang bulged boilers

2.3 shortcomings of boiler structure

boiler plant in order to prevent the occurrence of hot water boiler drum and short wall tube scaling and tube explosion accidents. During the manufacture of the boiler, the 1600mm radiant heating surface at the lower abdomen of the drum was added with an insulating layer, and the lower part of the smoke chamber composed of short wall tubes was also set with an insulating layer, and a series of measures such as thickening the back arch were taken. After calculation, this measure reduces the radiant heating surface of the boiler by nearly 1/2, and the setting of the second return flue gas chamber greatly reduces the volume of the furnace, thus improving the volumetric heat intensity of the boiler furnace. The influence of the above structure on the furnace volumetric heat intensity can be seen from the following calculation formula of furnace volumetric heat intensity

qv=bqd/v, where: QV - furnace volumetric heat intensity; B - coal consumption per unit time; QD -- low calorific value of fuel; V - furnace volume. It can be seen from the above formula that due to the increase of the insulation layer of the drum, the setting of the second return flue gas chamber and the thickening of the back arch, the furnace volume V is greatly reduced and the furnace volume thermal intensity QV is increased. The increase of furnace volume heat intensity QV and the reduction of radiation surface cause the increase and exceeding standard of furnace exhaust temperature, and the smoke and dust melt, resulting in the coking of insulation layer and furnace arch. On the one hand, coking further increases the volumetric heat intensity QV of the furnace, and the more serious consequence is to worsen the working conditions of the uninsulated heating surface of the boiler. When the hardness of feed water exceeds the standard, the heating surface will scale rapidly. When the water slag in the drum accumulates to a certain thickness, the pressure bearing parts will be deformed and bulged due to rapid overheating. It is worth noting that the cracking of the welded root of the rear support of the sewage pipe connected to the bottom of the drum by welding is also caused by the deterioration of working conditions, overheating and poor heat transfer

3 countermeasures

for the problems found in this boiler, in order to avoid similar accidents, while ensuring the basic structure of the boiler remains unchanged, the following countermeasures are proposed for this type of boiler in our city: 1. Strictly control the pH value of boiler feed water and boiler water to make it meet the requirements of national standards; 2. Remove the arch end 1600 at the belly of the drum

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