In sulfur acid production plant, sulfur incineration conversion section is one of the core processes, and the stress treatment of high temperature gas pipeline is directly related to the long-term stable operation of the whole plant. The expansion joint of sulfur acid pipeline is the key component to solve the huge thermal stress caused by the thermal expansion and contraction of the pipeline due to the large temperature difference and to prevent the damage of equipment。 Combined with engineering practice and failure case analysis, the selection principle, failure reason and optimization design measures of flue gas pipe expansion joint in sulfur acid production plant will be deeply discussed in this paper.

1. Why is it necessary to set expansion joints in sulfur acid flue gas pipelines

In the sulfur acid production plant, the operating temperature of the flue gas pipeline from the outlet of the sulfur incinerator to the converter and between the converter and the heat exchanger is usually as high as 400℃-600℃, but drops to the ambient temperature when the car is stopped. This drastic change in temperature difference can cause significant thermal elongation of carbon steel pipes.

Taking a 300 kt/a sulfur acid plant as an example, if a 30-meter-long high-temperature pipeline is not equipped with a reasonable flexible compensation structure, the thrust generated by its thermal expansion can reach tens of tons, which is enough to cause bracket damage, flange leakage and even pipeline weld tear。 Therefore, the core function of the expansion joint of sulfur acid production flue gas pipeline is to absorb the axial, transverse and angular displacement, reduce the pipeline stress, and thus protect the key equipment such as fan and converter。

2. Analysis of common failure modes and causes

In actual production, the failure events of the expansion joint of the flue gas pipeline of sulfur acid production occur from time to time. According to the case analysis, its failure forms mainly focus on the following aspects:

1. Low temperature dew point corrosion

It is one of the most common causes of failure. SO₂, SO₃ and water vapor are contained in the flue gas from sulfur acid production. Pipe temperatures may drop below the flue dew point when operating temperatures fluctuate or during unit shutdowns. At this time, SO₃ in the flue gas combines with water to form sulfuric acid, which condenses on the surface of the bellows of the expansion joint, resulting in serious sulfuric acid dew point corrosion。

Typical case: The expansion joint of the outlet pipeline of the primary converter of a sulfur recovery unit leaked and caused spontaneous combustion after only 21 days of operation. The analysis found that the operating temperature was below the dew point for a long time, resulting in SO₂-O₂-H₂O type corrosion and rapid thinning and perforation of carbon steel guide tubes and bellows。

2. Stress corrosion cracking of polysulfuric acid

When the sulfur acid plant is stopped for overhaul, the equipment is exposed to the air. The sulfide (FeS) formed on the bellows surface at high temperatures will react with moisture and oxygen in the air to form polysulfuric acid (H₂S₂O₆). This acidic substance can initiate stress corrosion cracking in austenitic stainless steels (e.g. 304, 316L), with cracks usually dendritic or intergranular propagation。

3. Improper design selection and installation

Part of the engineering design neglects the tension and support arrangement of pipeline dead weight. For example, if the expansion joint is too close to the inlet and outlet of the fan, or if a reasonable guide bracket is not set, the self-weight of the pipe will directly crack the corrugated structure。 In addition, manufacturing defects such as the material of corrugated pipe is inconsistent with the design, and the residual stress of welding is not eliminated, which is also the cause of early failure。

3. Optimized selection: a special scheme for sulfur acid production

In view of the strong corrosiveness and high temperature characteristics of sulfur acid-making flue gas, the selection of expansion joint cannot simply apply the common industrial standard. The optimization design should start from the following aspects:

1. Scientific Selection of Structural Types

Different types of expansion joints are used for different pipe sections according to pipe stress analysis (commonly used CAESAR II software)：

• Unconstrained axial type expansion joint: Suitable for longer straight pipe sections, absorbing axial displacement.
• Large tie rod transverse expansion joint: Suitable for "Z" or "L" pipe sections that need to absorb transverse displacement and do not allow internal pressure thrust to act on the equipment interface.
• Hinged expansion joint: suitable for in-plane angular displacement, commonly used in three-dimensional bending pipe section.

2. Material upgrade and anti-corrosion design

• Bellows material: For high temperature sections (> 400℃), it is recommended to use high temperature resistant alloys such as Incoloy 825 or 316L (containing Mo element to resist chloride ion corrosion); Avoid sensitive stainless steel or use high nickel-based alloys for areas with risk of sulfur corrosion。
• Guide tube: A corrosion-resistant guide tube must be set inside the expansion joint to avoid high-speed dusty flue gas directly washing the bellows and prevent liquid accumulation.
• Condensed acid discharge: Patented technology shows that opening a discharge hole at the lowest point of the bottom of the expansion joint and connecting corrosion-resistant pipes and valves can discharge condensed acid in time and effectively prevent corrosion and perforation at the bottom。

3. Increase wall thickness allowance

For sulfur acid-making flue gas pipeline, it has been shown that the corrosion-resistant perforation life can be increased exponentially by increasing the wall thickness of corrugated pipe from conventional 2mm to 6-8mm (such as single-wave expansion joint)。

4. Key points of installation, operation and maintenance

1. Strict pre-stretching: The expansion joint should be pre-stretched or pre-compressed according to the design calculations during installation to make full use of its displacement compensation ability.
2. Reasonable arrangement of brackets: guide brackets and fixed brackets must be arranged at both ends of the expansion joint to ensure that the displacement direction is consistent with the design, and to avoid twisting and lateral instability。
3. Driving and parking protection: the heating tube should be raised before driving to avoid sharp heating; After parking, hot air circulation or nitrogen-filled protection should be used to keep the pipeline temperature above the dew point and prevent the generation of condensed acid。
4. Regular inspection: Focus on whether there are "sweaty" leakage points on the surface of the bellows, whether the tie rod is bent, and whether the insulation layer is damp (containing acid).

5. Call to Action: Customize Safety Plan for Your Sulfur Acid Plant

Although the expansion joint of sulfur acid flue gas pipeline is a small part, it is related to the safety and continuous production of the whole device. The catalyst deactivation, equipment maintenance and output loss caused by an unplanned stop are often dozens or even hundreds of times the procurement cost of expansion joints.

If you are facing the following issues:

• The existing expansion joint leaks frequently and is repaired constantly;
• The calculation of pipeline stress in new projects is complicated, and the selection is uncertain;
• Concern about condensate acid corrosion risk during unit shutdown.

Contact my team of Sulphur Acid Piping Technical experts today.