Analysis on the cause of fouling in industrial hot

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The industrial heating boiler plays a very important role in coal mine safety production. Liaoyuan mining (Group) Xi'an Coal Industry Company has 76 steam tons/22 boilers, which are mainly used for heating the warm air in the wellhead driveway and waiting room in winter, and heating the ground plant and office space. Among them, there are 10 steam tons/2 hot water boilers. After the boiler is shut down every year, the boiler inspection office of the group company will find the scaling phenomenon of the hot water boiler during the regular inspection of the boiler. Some even have water-cooled wall and arch pipe burst during production, or overheating and bulge at the lower part of the boiler barrel, which leads to boiler shutdown and affects normal heating. Therefore, strengthening boiler management and use is particularly important to ensure the safe production and operation of the boiler

1 several boiler accident cases

(1) dzl2.8 0.7/95/70-a Ⅱ boiler transferred from the General Hospital of employees of the group company to the second inclined shaft boiler room in 2006, after the general hospital shut down, it was found that there were scale bulges at the bottom of the slag drum in front, scale on the water-cooled wall in front, and tube explosion. The rear tubesheet is scaled to produce overheating cracks. After being transferred to the second inclined shaft boiler for overhaul, it was put into operation after passing the inspection

(2) during the mechanical repair of dzl2.8 0.7/95/70 a Ⅱ boiler in 2008, it was found that slag and scale were accumulated at the front bottom of the boiler barrel, the rear water wall, the upper end of the front convection riser and the pipe orifice area; The boiler has been upgraded to a 6-ton energy-saving hot water boiler

2 cause analysis of scaling in hot water boiler

(1) cause of scaling in hot water boiler. The main reason for scaling of hot water boiler is that the water slag forms scale during boiler operation. The hot water boiler water does not vaporize, and the impurities in the water react with each other due to heating decomposition. The purpose and function of water treatment in the furnace is to make impurities generate water slag with strong suspension force and good fluidity, so as to facilitate discharge out of the furnace. The boiler feed water quality is poor, and the water supplement quantity is too large, which will cause a large amount of water slag in the hot water boiler. After the water slag is generated, it initially exists in the boiler water in a suspended state and circulates with the boiler water. If it cannot be discharged out of the furnace through the blowdown pipeline in time, scale will form when it accumulates to a high concentration in the furnace. However, it has been proved that the anti scaling performance of water treatment in boiler only works at low concentration. When the water slag accumulates to a high concentration, secondary scale will be formed on the heating surface in the boiler, or water slag will be deposited at the position with low circulating velocity. Scale and water slag at the bottom of the boiler drum are very harmful to the boiler. At present, the water circulation design of the hot water boiler only considers the flow rate of water, without taking into account the generation and movement of water slag, which undoubtedly leaves a great hidden danger. Due to the prevalence of poor water quality, large water replenishment and untimely sewage discharge, the problem of water slag is more prominent, accidents continue, and serious economic losses are caused. Therefore, strengthening management and improving boiler water treatment quality are of great significance to ensure the safe production and operation of boilers

(2) slag accumulation at the bottom of the drum. Heat concentration and main heating parts are on the boiler drum. A large amount of water slag will be generated and settled here. If the water circulation velocity at the bottom of the boiler is very small, the gravity effect of the water slag is greater than the kinetic energy (the charge effect must be overcome during the boiler bone water treatment) and it will settle. The water slag that cannot find a way out is deposited here and partially solidified into secondary scale under the action of radiation. In order to avoid this phenomenon, it is necessary to supply water and slag as a power. Let it rise and find its way out. This power can be obtained through backwater distribution

(3) water wall scaling. The water slag settled in the water wall header will be forced to circulate hydraulically. If the accuracy of the strain gauge is not high, or the anti-aging ability of the glue used to fix the strain gauge is not good, or the material of the sensor is not good, the accuracy and service life of the sensor will be affected. The force will rush to the front. The blowdown outlet is set at the inlet side of the circulating water, which is not conducive to blowdown. When the water slag reaches a certain amount, the circulation path will be blocked, the flow rate in the header will increase, and part of the water slag will flow into the front cold wall pipe, and the high concentration of water slag will continue to become secondary scale under the strong radiation of the outer wall. The blowdown outlet of hot water boiler shall be far away from the inlet of circulating water

(4) scaling of riser. The riser is located in the middle of the boiler drum, and it is difficult for the water slag in the front of the boiler drum to reach the rear downcomer for discharge due to the influence of its rising water. This not only causes water slag to accumulate at the bottom of the front boiler drum, but also increases the deposition of the riser. In case of blowdown during normal operation of the boiler, the flow rate in the riser will decrease or stagnate, damaging the water circulation and enhancing the retention and scaling of water slag

(5) the rear tubesheet is cracked, the water circulation in the rear tubesheet area of the hot water boiler is weak, and it is easy to cause stagnation and concentration of water slag, some of which are attached to the rear tubesheet or become scale. When the water quality is unstable, the fallen scale blocks are easy to get stuck between nearby pipes, weakening the circulation, increasing the scale surface, and causing overheating cracks. Increasing the circulating flow rate can also be achieved by circulating water injection

(6) blowdown. Blowdown of hot water boiler is the last step of water slag movement. Many scaling accidents are caused by 1. Improper blowdown without zero operation at the beginning of the experiment. During the normal operation of the boiler, if the slag cannot be discharged during sewage discharge, the water circulation will be damaged. The correct blowdown method is carried out when the circulating system becomes weak after the boiler is fired. At this time, there is no problem of damaging the water circulation, which is also conducive to the settlement and discharge of water slag; The forced circulation system should be carried out after the circulation pump stops and the water slag settles completely. The design of water slag movement is to make it go through the last step of sewage discharge and disappear from the boiler as soon as possible. Sometimes it is difficult to judge the water slag in the boiler. We calculate the amount of water slag in the boiler by measuring the hardness change of the boiler water

3 countermeasures to prevent scaling and slagging of hot water boiler

(1) open a row of small holes at the bottom of the return water distribution pipe to inject the return water into the bottom of the front drum receiving radiant heat, so that the water slag can not be deposited here, but will be discharged with the circulating water entering the front downcomer to the lower header for settlement. After two heating cycles of trial operation after transformation, no slag and scaling were found on the drum. The application effect is very good and the economic benefit is remarkable

(2) change the blowdown at the rear of the water wall header to the blowdown at the front. This type of furnace has no downcomer, and the water slag in the drum can only exit through the convection pipe. The common damage to the sheet metal is the arrangement that the flow pipe rises before and then falls during the transportation process, which makes it difficult for the water slag to exit. This preferred novel structure is defective in this respect. It is suggested that the smoke separation wall should be improved again and should be arranged in such a structure; Change the flue gas flow so that the front and rear drop of the convection pipe is changed to the middle drop at both sides. In this way, the convection downcomer at the bottom of the drum provides a good outlet for the water slag at all parts, and the water slag is easy to enter the downcomer header for settlement and discharge

(3) change the water inlet at the rear of headers on both sides to drill and distribute water for the water supply pipe in the boiler, and add a downcomer at the bottom of the front drum. A row of small holes are opened at the bottom of the return water distribution pipe to eject the bottom of the boiler. The front ejects the downcomer of the water wall and the rear ejects the rear tube plate. Note that the ejected water does not directly scour the rear tube plate. The boiler water at the rear tube plate is driven by the injection to increase the circulating flow rate, which avoids the retention of water slag and prevents the scaling of the boiler, and solves the scaling of the water-cooled wall pipe. The water slag in the return water at the front bottom of the boiler cannot be deposited under the action of the injection power, but is discharged through the downcomer to the header or through the blowdown pipe at the rear of the drum

4 conclusion

during the operation of the hot water boiler, when the water slag accumulates to a certain concentration, it will generate secondary scale. Under the conditions of a certain concentration and low flow rate, the water slag will deposit, causing the water wall and arch pipe burst, or the overheated drum at the lower part of the boiler barrel, which will lead to the boiler shutdown and affect the normal heating. To solve the problem of scale in the boiler tube wall, sufficient power should be given to the circulating water in the boiler to make it move along the set route, settle at the set position, and discharge the water slag in time. Only correct blowdown and timely discharge of water slag can ensure the safe operation of the boiler

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