In the process of converter steelmaking, the vaporization flue undertakes the key task of collecting 1100℃-1600℃ high temperature flue gas and recovering waste heat. Because the converter flue system is subjected to drastic temperature change, high-temperature flue gas erosion and steel slag spattering during operation, expansion joints must be set between each section of flue to absorb thermal displacement and reduce thermal stress. However, if the selection and use of the expansion joint does not meet the standard specification, it may lead to smoke leakage at least and explosion accident at worst. Therefore, it is of great significance to clarify the standard of using expansion joint in converter flue for ensuring the safe and stable operation of steelmaking system. This paper will systematically expound the standard of using expansion joint in converter flue from four dimensions: design and manufacturing standard, material selection specification, safety prohibition regulation and installation acceptance requirement.
I. Structural characteristics of converter vaporization flue and application scenario of expansion joint
Converter vaporization flue, also known as waste heat boiler, is the key supporting equipment of converter steelmaking. The system is mainly composed of movable hood, stove mouth fixed section flue, middle section flue (divided into middle section I, middle section II) and final section flue connected in turn, and the flue deployment length can reach about 42 meters. During the blowing process of the converter, the temperature of high-temperature flue gas produced in the furnace can reach 1400-1600℃, and the inner wall of the flue is frequently subjected to drastic high temperature change and steel slag splashing and washing.
In order to absorb the thermal expansion displacement between the flue sections and reduce the damage of thermal stress to the pipeline and equipment, the flue in the fixed section of the furnace mouth and the flue in the middle section are usually connected by expansion joints. A sealing device needs to be set between the movable hood and the flue of the furnace mouth section. Common forms include water seal, sand seal and dry (nitrogen/steam) seal. These expansion joints and sealing devices work in extremely harsh environments and must be selected and manufactured in strict compliance with relevant standards.
Core technical standards for the design and manufacture of expansion joints
The standard of using expansion joint in converter flue is first reflected in the design and manufacturing link. According to the industry norms, the converter vaporization flue system itself should be manufactured in accordance with national standards and specifications such as GB1576 and GB5310. For flue expansion joints, the following technical standards are mainly implemented:
1. Metal bellows expansion joint standard
For metal expansion joints used in converter flue, GB/T 12777-2019 General Technical Specifications for Metal Bellows Expansion Joints shall be followed. This standard specifies the classification, design calculation, material requirements, manufacturing inspection and test methods for metal bellows expansion joints. The metal expansion joints in the high-temperature section of converter flue are usually made of high-temperature resistant stainless steel (such as 321, 316L, etc.). The wall thickness, wave height, wave pitch and other parameters of bellows should be determined according to the design compensation amount and pressure conditions.
2. Standards related to boilers and pressure vessels
Since the vaporization flue of converter belongs to the category of waste heat boiler in nature, the manufacture of expansion joint should also meet the relevant technical conditions of boiler, including JB/T1611-93 Technical Conditions for Boiler Pipe, JB/T1612-93 Technical Conditions for Boiler Hydraulic Test, etc. These standards ensure the safety and reliability of expansion joints under high temperature and high pressure operating conditions.
In addition, the materials used for expansion joints should meet the corresponding standards: stainless steel materials should meet ASTM A240 or GB/T 3280 standards.
3. Standard requirements for material selection
The temperature, pressure and medium corrosivity of different sections of the converter flue are significantly different. The material selection of the expansion joint must comply with the following standard specifications:
1. Material standards for metal expansion joints
In the high temperature section of the converter flue (such as the connection between the furnace mouth section and the middle section), the flue gas temperature can reach 800-1000℃, and the expansion joint bellows must be made of high temperature resistant alloy material. The material selection criteria are as follows:
- Working temperature ≤600℃: austenitic stainless steel (304, 321) can be selected
- Working temperature 600-800℃: High temperature resistant stainless steel (309S, 310S) should be selected
- Operating temperature> 800℃ or presence of corrosive smoke: High nickel alloy (Inconel 625) or Hastelloy should be selected
The inner wall of converter flue is often coated with nickel-chromium wear-resistant and high temperature resistant alloy coating to prevent slag sticking and erosion. Similar protective measures should be taken for the guide tube and lining plate of the expansion joint.
Mandatory standards and prohibited provisions for safe use
Safety is the most important consideration in the standard for the use of expansion joints in converter flues. According to the clear provisions of the Catalogue of Equipment and Processes Prohibited by Metal Smelting Enterprises (First Batch) issued by relevant state departments:
Non-metallic material is strictly prohibited for the expansion joint in front of the converter gas recovery system
The reason for this ban is that the expansion joint is installed in the negative pressure section in front of the primary fan of the converter, and its position is too high from the ground to be inspected. The fan impeller runs at high speed, and the flue gas temperature is about 60℃ or above. If non-metallic materials such as rubber hoses are used, a large amount of air will be inhaled once it is damaged, which will easily cause mixed explosive gases and cause gas explosion accidents.
Alternative: Metal materials such as stainless steel must be used instead of non-metallic materials such as rubber hose to ensure safety.
This mandatory standard means that metal expansion joints must be selected in the negative pressure section in front of the fan of the converter gas recovery system, and any form of non-metallic expansion joints is strictly forbidden. This is an insurmountable safety red line in the selection of flue expansion joint of converter.
V. Installation and acceptance standard requirements
1. Pre-offset and installation requirements
The converter flue expansion joint shall be pre-stretched or pre-compressed (cold tightened) according to the design requirements during installation. Because the operating temperature of the converter flue is extremely high, the flue will produce significant thermal elongation between the segments, and an appropriate pre-offset must be set during installation. The auxiliary components used for pre-deformation shall be removed after the pipeline is installed.
VI. Routine maintenance and regular inspection standards
In order to ensure that the expansion joint meets the standard for use throughout the life cycle, a periodic inspection system should be established:
- Appearance inspection: Check the expansion joint skin or bellows for damage, bulging and corrosion every week
- Leak detection: Check the interface tightness with a leak detector or soapy water
- Bolt re-tightening: Tighten flange bolts once in 1 month and once in 3 months after operation
- Temperature monitoring: Infrared temperature measurement to check whether there are abnormal high temperature points
- Regular replacement: Non-metallic expansion joints should be replaced in time after their life expiration, and replacement products that meet the original design standards should be selected during replacement
conclusion
To sum up, the standards for the use of expansion joints in converter flue cover five levels: design and manufacturing, material selection, safety disabling, installation acceptance and routine maintenance. At the design and manufacturing end, metal expansion joints shall comply with GB/T 12777 standard, and non-metal expansion joints shall comply with JB/T 12235 standard; At the material end, high-temperature resistant stainless steel or high-nickel alloy can be selected for the high-temperature section, and compliant non-metallic materials can be selected for the low-temperature section according to the working conditions; On the safety end, the expansion joint in front of the fan of the converter gas recovery system is strictly forbidden to use non-metallic materials, which is a mandatory standard to ensure the explosion-proof system; At the installation end, pre-bias setting and airtightness inspection should be strictly implemented; On the maintenance side, a regular inspection and replacement system should be established.
The flue of the converter is in the extreme working conditions of high temperature, corrosion and vibration, so the expansion joint is by no means an indispensable accessory, but a core component to ensure the safety, environmental protection and stable operation of the whole converter steelmaking system. The selection, installation and maintenance of the expansion joint in strict accordance with the standard specifications can not only avoid flue gas leakage and explosion accidents, but also significantly prolong the overall life of the flue system and reduce the operation and maintenance cost of the whole life cycle. It is hoped that this paper can provide clear and authoritative technical reference for equipment management and engineering technicians in the field of converter steelmaking.