In order to ensure the safe operation of the unit, it is very important to maintain a stable steam temperature, because an excessively high steam temperature will reduce the allowable stress of the metal and endanger the safe operation of the unit. The temperature of superheated steam and reheated steam during boiler operation is affected by many factors, and temperature fluctuations are inevitable. In order to ensure the safe and economic operation of the unit, a reliable steam temperature adjustment device must be installed to correct the influence of certain operating factors on the fluctuation of steam temperature.
Water jet water heater: steam temperature regulation has steam side regulation and flue gas side regulation. The superheated steam temperature of modern large boilers is mainly adjusted by changing the enthalpy of steam by spraying water desuperheater. There are many kinds of nozzle desuperheaters, such as single nozzle desuperheater, water chamber desuperheater, swirl nozzle desuperheater and porous nozzle desuperheater.
The original design of the water chamber desuperheater spray water is self-made condensate, the water temperature is the saturated steam temperature, and the temperature difference between the superheated steam and the water chamber desuperheater is small. Most power plants nowadays are designed to use water as the temperature drop water at the outlet of the feed water pump. Cut the head of the cooler head, loosen the fixing screw of the mixing temperature tube from the outside of the head, and pull out the spray cooler. The results showed that the surface of the water chamber was cracked, and there was no crack on the inner wall of the head. The annular water chamber is welded at the throat of the venturi nozzle, the weld is grounded, and the annular water chamber is removed. A large number of cracks appeared near the cooling water inlet pipe. No cracks were found in the throat of the Venturi nozzle.
From the analysis of the structure and material of the desuperheater: the structure of the water chamber desuperheater is complex, and the cross section and welding seam vary greatly. Under variable water spray conditions, it is easy to produce a large temperature difference stress. The inlet pipe of the desuperheater is connected to the water chamber by thread. The thread size is 36 mm×2 mm. The joints and thread retraction grooves have a large stress concentration. in the middle. The design material of the water chamber and sprinkler pipe is 20G steel, the spectral analysis is carbon steel, and the material of the venturi nozzle is 12Cr1MoV. Relevant data shows that under the same alternating stress, the fatigue life of 12Cr 1Mo material is twice that of 20g steel. In the design, if the temperature is considered alone, 20G steel can meet the requirements without considering the fatigue strength.
For peak-shaving units, due to large load changes and unstable operation, the combustion changes lag behind the load changes, and the temperature of the superheated steam will rise to a certain extent. In order to protect the superheater tube wall from over-temperature, it is bound to require frequent adjustment of desuperheating water The change in the amount of desuperheating water will inevitably cause changes in the temperature of the desuperheating water inlet pipe and the wall of the annular water chamber. In terms of design, the temperature of the steam medium of the second-level spray desuperheater is 486 ℃, and the temperature of the desuperheating water is 156 ℃. The change of wall temperature depends on the change of the desuperheating water flow. When the desuperheating water flow reaches the maximum, the wall temperature is close to the temperature of the desuperheating water; when the desuperheating water flow is zero, the wall temperature is close to the temperature of the steam medium. From this point of view, the wall temperature of the desuperheating water introduction pipe and the annular water chamber varies between 156 and 486 ℃, and there is a large temperature difference stress.
To sum up, the reason for the cracks in the introduction pipe of desuperheating water is due to the large stress concentration at the connection and the thread undercut groove, and the stress cracks are generated under the action of the large temperature difference stress. The reason for the crack of the annular water chamber is that the structure is unreasonable. The desuperheating water is first introduced into the annular water chamber at the throat, which bears large alternating temperature stress. In addition, the fatigue strength is not considered in the design of the material, and the fatigue strength is lower. In the process of long-term operation of the 20G steel, under the action of high temperature and alternating stress, the inner wall of the annular water chamber will have thermal fatigue cracks.
In view of the above situation, the following corresponding improvement measures can be taken:
a) In order to improve the fatigue life of the material, when the No. 2 boiler was overhauled, the water chamber and spray pipe material of the second-grade water spray desuperheater was replaced with 12Cr1MoV, which not only improved the fatigue strength of the water chamber, but also the material of the venturi nozzle was 12Cr1MoV is better in the welding process of the water chamber and the venturi nozzle. The expansion coefficient of the same material is the same, which can relatively reduce the expansion stress.
b) In order to reduce the stress concentration of the joint and the undercut groove, the thread specification of the desuperheating water introduction pipe is changed to M36 mm×1mm.
c) Through the feasibility analysis, it is understood that some large units use the feed water after the high temperature heater as the desuperheating water, and the desuperheating water from the outlet of the feed water pump is changed to the feed water pipe after the high temperature heater. Using the feed water after the high temperature heater as the desuperheating water can increase the temperature of the desuperheating water by 90 ℃. The data shows that the temperature difference is reduced by 90 ℃, and the service life of the material is approximately doubled.
d) Improve the inspection method. In the past overhaul, the inspection of the desuperheater by a certain factory was to cut the header hand hole and check with a flashlight. The inspection part was the mixing pipe, and the connection thread of the water chamber surface and the spray pipe was Can't be checked. In each overhaul, an endoscope should be used to inspect the surface of the water chamber and the connection thread of the sprinkler pipe.
As the damage of the desuperheater eventually causes thermal fatigue cracks in the header, many power plants will often occur. The key is whether the treatment method is correct, and the endoscopic inspection method should be adopted. The surface of the water chamber and the connection thread of the spray pipe are the key points of inspection, and the lower part of the mixing pipe and the vicinity of the throat should also be listed as the key points of inspection. Once the damage of the desuperheater causes a crack in the header, the consequences are serious. Therefore, the supervision of the desuperheater should be strengthened during operation.