In the production process of paper industry, a large number of high-temperature steam, black liquor, pulp and chemical chemicals are involved in the transportation and treatment. Piping systems are not only subjected to fluctuations in temperature and pressure, but also challenged by corrosive media and mechanical vibrations. The application of expansion joints in papermaking industry is to solve these problems-expansion joints absorb thermal displacement of pipelines, isolate equipment vibration and compensate for installation errors through their flexible structure, while corrosion-resistant materials can resist chemical attack of black liquor, green liquor and bleach. Based on the characteristics of papermaking process, this paper systematically expounds the typical application scenarios, selection points and maintenance suggestions of expansion joint in pulping, alkali recovery, papermaking and thermal power system, so as to provide professional reference for equipment management personnel in papermaking enterprises.

Special Requirements for Expansion Joints in Papermaking Process

The application of expansion joints in papermaking industry is special because the papermaking process media is complex and the working conditions are severe:

• Corrosive media: Black liquor (pH> 13), green liquor (containing Na₂S, Na₂CO₃), bleaching liquor (containing ClO₂, NaOH) strongly corrode ordinary metal materials.
• High temperature and high pressure steam: The drying part of the paper machine needs to heat the drying cylinder with steam, the temperature can reach 150-180℃, and the pressure is 0.5-1.0MPa.
• Fiber slurry: Paper pulp contains fiber and filler particles, which cause erosion and wear on the inner wall of pipeline and expansion joint.
• Continuous Operation Requirements: Papermaking production lines usually run continuously all year round, with short maintenance windows, which requires expansion joints to have high reliability and long life.

Therefore, the application of expansion joints in papermaking industry must take into account the four requirements of corrosion resistance, temperature resistance, wear resistance and long life.

Application of Expansion Joint in Pulping Workshop

1. Steaming pot inlet and outlet pipeline

The cooking pot in the pulping workshop reacts wood chips with chemical liquid under high temperature and high pressure (about 170℃, 0.8MPa) to produce coarse pulp. The cooking pot opens and closes periodically, and the pipe is subjected to severe thermal circulation displacement. Here, the application of expansion joints in papermaking industry mostly uses large-diameter axial expansion joints made of 316L stainless steel or duplex steel, and the compensation amount needs to cover the full temperature cycle. For some extremely corrosive black liquor systems, the inner wall of the bellows can be lined with polytetrafluoroethylene (PTFE) or the whole is made of Hastelloy C-276.

2. Black liquor delivery pipeline

Black liquor contains residual alkali and lignin, and the temperature is 80-100℃, which causes serious corrosion to carbon steel. Rubber soft joints or non-metal expansion joints are commonly used in the inlet and outlet pipelines of black liquor pumps, which take advantage of their corrosion resistance and vibration isolation characteristics. It should be noted that the fibers in black liquor are easy to accumulate in the trough of the expansion joint. A structure with smooth inner wall (such as single wave or large wave distance design) should be selected, or a filter should be installed upstream of the expansion joint.

Application of Expansion Joint in Alkali Recovery Workshop

Alkali recovery system is the core link to realize chemical recycling and energy recovery in paper mills. Among them, the flue temperature of black liquor burner and lime kiln can reach 800-1000℃, and the flue gas contains SO₂ and alkaline dust. The application of expansion joints in the paper industry faces the dual challenges of high temperature and corrosion here:

• High-temperature flue expansion joint: select heat-resistant alloy (such as Inconel 625) bellows, lined with heat insulation layer inside, and set air-cooled or water-cooled structure outside. The large-amplitude thermal displacement should be fully considered in the design of compensation quantity.
• Green liquor delivery pipeline: The main components of green liquor are Na₂CO₃ and Na₂S, the temperature is about 150-180℃, and it is strongly alkaline. It is recommended to use rubber-lined expansion joints lined with natural hard rubber. The rubber layer thickness is 4-6mm, which can withstand alkaline media with pH> 13.

Application of Expansion Joints in Paper Machines and Drying Parts

The drying part of the paper machine is composed of a plurality of groups of drying cylinders, which need to be heated by steam through a rotary joint, and the condensed water in the drying cylinders is discharged through a siphon pipe. The application of expansion joints in the paper industry in this region is mainly focused on:

• Steam main pipe introduction pipe: the main steam pipe from the thermal power station to the drying section, with high temperature, large pipe diameter and long distance. It is recommended to use rotary expansion joints or universal hinge metal expansion joints, which can greatly reduce the number of fixed brackets because of their large compensation and no blind plate force.
• Metal hose at inlet and outlet of drying cylinder: The steam inlet and outlet hose (usually stainless steel bellows) equipped with each drying cylinder needs to frequently follow the slight displacement of drying cylinder. Multi-layer thin-walled bellows are selected, and the fatigue life requirement is ≥10,000 times.
• Condensate recovery pipeline: The condensate temperature is about 100-120℃ and contains trace corrosive substances. 304 stainless steel axial expansion joint can be selected. Pay attention to setting slope and hydrophobic point to prevent water hammer impact.

Application of Expansion Joint in Thermal Power Station and Auxiliary System

The paper mill's own thermal power station provides steam and electricity for production, and the flue gas system and steam and water pipeline all need expansion joints. Among them, the flue and returner of circulating fluidized bed boiler (CFB) fluctuate greatly, and the dust concentration is high. Here, the application of expansion joints in papermaking industry is more inclined to non-metallic expansion joints-their corrosion resistance, vibration isolation and anti-dust accumulation characteristics are excellent in low temperature section, while alloy bellows expansion joints need to be used in high temperature section.

In addition, rubber joints and plastic expansion joints are widely used in white mud conveying pipelines in caustic sections and aeration pipelines in wastewater treatment systems. These parts have low pressure and low temperature, and the key lies in corrosion resistance and blockage prevention.

Selection Points and Common Materials

Comprehensive application scenarios of expansion joints in papermaking industry, it is recommended to select according to the following principles:

Maintenance and failure prevention

According to the application characteristics of expansion joints in papermaking industry, the maintenance work should focus on:

• Regular inspection: measure the wall thickness of expansion joints on black liquor and green liquor pipelines every quarter to monitor the corrosion thinning; Visually check the rubber joint for bulging and cracking.
• Anti-deposition measures: The expansion joint containing fibrous or granular media shall be provided with a flushing port at the lowest point, and the internal deposits shall be flushed every time the machine is stopped.
• Replacement cycle: the life of ordinary rubber joint is about 3-5 years, rubber-lined expansion joint is 5-8 years, and metal expansion joint is 8-10 years (appropriately shortened in corrosive media).
• Spare parts management: Key stations (such as black liquor burner and dryer steam main pipe) should reserve spare parts of the same type, which can be quickly replaced once it fails to avoid long-term production stoppage.

sum up

The application of expansion joints in the paper industry covers the whole process from pulping cooking, black liquor transport, alkali recovery, drying in paper machines to flue gas from thermal power stations. Different stations have huge differences in the requirements for expansion joints-cooking pot needs high temperature and high pressure resistant metal expansion joints, black liquor and green liquor pipeline needs rubber lining or rubber joints resistant to alkali corrosion, bleaching section needs PTFE lining products resistant to chloride ions, and flue gas system needs flexible non-metal expansion joints. Correct identification of media characteristics, temperature, pressure and displacement requirements, selection of matching structure and material, and establishment of regular inspection and maintenance system can ensure long-term stable, safe and efficient operation of paper production line. For papermaking enterprises, although expansion joints are small parts, they are directly related to product quality, environmental protection standards and shutdown losses-which deserve enough attention.