Denitrification units (DeNOx) are commonly used to reduce nitrogen oxide (NOx) emissions generated during combustion. In these systems, expansion joints (also known as compensators) are used to absorb pipe displacements due to temperature changes, mechanical vibrations, and more, thus protecting the integrity of equipment and piping systems. Here are some key points regarding expansion joints at the inlet of denitrification units and recommendations for their selection and maintenance:

The role of the expansion joint

• Thermal compensation: Absorbs the thermal expansion and contraction of pipes caused by temperature changes.

• Vibration isolation: Reduce the transmission of vibrations generated by mechanical equipment during operation to the pipeline system.

• Displacement compensation: Allows axial, lateral, and angular displacement of the pipe in three dimensions.

Selection Considerations

1. Work environment

• Temperature: The flue gas temperature at the inlet of the denitrification device is high, usually between 300°C and 400°C, and sometimes even higher. Therefore, it is necessary to select high-temperature resistant materials.

• pressure: Know the maximum working pressure of the inlet pipe and ensure that the expansion joint can withstand that pressure.

• Properties of media: The flue gas at the inlet of the denitrification device may contain a certain amount of corrosive components, such as SO₂, NOx, etc., and anti-corrosion materials should be selected.

• Silicone cloth or fluorine rubber cloth: Suitable for medium and low temperature environment, with good flexibility and certain corrosion resistance.

• Ceramic fiber cloth: For extremely high temperature application scenarios (such as over 800℃), ceramic fiber cloth can be used as the inner layer, and the outer layer is covered with a layer of silicone or fluororubber cloth for enhanced protection.

• S32100 (321 stainless steel): Suitable for high temperature environment, with good oxidation resistance and thermal fatigue resistance.

• S31603 (316L stainless steel): It has good corrosion resistance and is suitable for working conditions containing corrosive gases.

• N08825 (Incoloy 825): For more demanding corrosive environments, this super austenitic stainless steel can be chosen.

• Metal bellows expansion joint：

• Non-metallic expansion joint：

• Single type bellows expansion joint: Suitable for simple axial displacement compensation.

• Compound bellows expansion joint: Can carry out displacement compensation in multiple directions, suitable for complex working conditions.

• hinged or universal expansion joint: Suitable for applications requiring large angle deflection.

Installation and maintenance

1. Installation Considerations

• Correct alignment: Ensure that the expansion joint installation position is accurate to avoid offset or twist.

• Support design: Design the pipeline support system reasonably to prevent excessive stress concentration on the expansion joint.

• Seal Inspection: After installation, perform tightness testing to ensure no leakage at the flange connection.

• Appearance inspection: Check the exterior of the expansion joint regularly for deformation, cracks, or other damage.

• Connector inspection: Check that all flange bolts are tight and the sealing gaskets are intact.

• Displacement monitoring: Record and compare the actual displacement with the design value to ensure that it works within the allowable range.

• Cleaning and maintenance: Keep the surface of the expansion joint clean to prevent dust and dirt from affecting its normal operation.

• Lubrication maintenance: For expansion joints with sliding parts (e.g. hinged or universal type), lubricating oil should be added regularly.

• Replacement plan: According to the actual use situation and manufacturer's recommendation, establish a reasonable replacement cycle, and timely replace the parts that are aged or close to the service life limit.

Practical application cases

It is assumed that the temperature of the flue gas at the inlet of a denitrification device is 350℃, the pressure is slightly negative, and it contains a small amount of SO₂ and NOx. In this case, the following configuration is recommended:

• Material: The bellows expansion joint made of S32100 (321 stainless steel) is selected, because it has good high temperature resistance and oxidation resistance.

• Structural form: Considering the possibility of slight lateral displacement requirements, a compound bellows expansion joint can be selected.

• Installation and maintenance: Install in strict accordance with the instructions provided by the manufacturer, and establish a regular inspection system to discover and deal with potential problems in time.

Through scientific and reasonable selection and strict installation and maintenance measures, the reliability and service life of the expansion joint at the inlet of denitrification device can be effectively improved, and the safe and stable operation of the whole denitrification system can be ensured. If there are more specific questions or special needs, please feel free to consult a professional engineer or supplier for detailed guidance.