1. The importance of flue gas desulfurization flue expansion joint condensation discharge

In the wet flue gas desulfurization system, the original flue gas becomes clean flue gas after being washed by the absorption tower, and the temperature drops to a saturated state of 50-60℃, carrying a large amount of water vapor. As the net flue gas passes through the flue and expansion joints, the water vapor condenses into an acidic condensate as the temperature decreases or the pressure changes. If not effectively discharged, this condensate can accumulate within the troughs of the expansion joint bellows, causing severe stainless steel corrosion (pitting, stress corrosion cracking) while increasing flue loads. Therefore, whether the condensation design of flue gas desulfurization flue expansion joint is reasonable or not is directly related to the service life of expansion joint and the stable operation of desulfurization system. However, this detail is neglected in many projects, resulting in leakage failure of the expansion joint within 1-2 years after it is put into operation. This paper will systematically expound the main points of condensation design from the mechanism of condensate generation, the structure of condensation drainage device to the construction and installation.

2. Generation and harm of condensate in expansion joint of desulfurization flue

2.1 Formation mechanism of condensate

To answer the necessity of flue gas desulfurization flue expansion joint condensation, first of all, we need to know where the condensate comes from. The net flue gas after wet desulfurization has the following characteristics:

• Relative humidity: close to 100% (saturated state)
• Temperature: 45-60℃ (depending on desulfurization process and whether GGH is added)
• Composition: contains small amounts of SO₂, SO₃, Cl⁻¹, F⁻¹, condensate pH usually 1.5-4.0

When the saturated flue gas comes into contact with the lower temperature expansion joint metal shell, a film of condensed water is formed on the inner wall. These water films flow downward under gravity and eventually collect at the lowest point of the expansion joint — the trough of the bellows or outside the bottom guide tube.

2.2 Hazards of condensate accumulation

Prolonged accumulation of condensate inside the expansion joint can have a series of serious consequences:

Therefore, the reasonable design of flue gas desulfurization flue expansion joint is the fundamental measure to prevent the above problems.

3. Typical structure of expansion joint condensation discharge device

3.1 Single corrugation lowest point condensation discharge

The simplest way to drain condensation is to open a drain hole at the lowest point of the expansion joint (usually the bottom trough), and connect a short tube and valve. Suitable for small diameter, low pressure flue expansion joints. Design Points:

• The condensation discharge hole is opened at the lowest position of the bellows trough, generally within ±15° of the bottom center
• The hole diameter should be small rather than large, usually φ 10-φ 20mm, so as not to weaken the strength of the bellows
• The short condensation discharge tube shall be inclined downward ≥5°, and the length shall not exceed 200mm
• Equipped with ball valve or plug valve at the end, slightly open continuous drainage during normal operation

3.2 Liquid collecting tank type condensation drainage device

For large diameter rectangular expansion joints or working conditions with extremely high moisture content in flue gas, it is recommended to adopt liquid collecting trough structure. In the structure, a liquid collecting groove lower than the lowest point of the bellows is specially arranged at the bottom of the expansion joint, and the condensate is first fed into the groove and then discharged through the condensate discharge pipe. The advantage is that no holes will be opened in the bellows to avoid stress concentration.

Liquid sump design parameters:

• Volume: Designed at 2-3 times the amount of condensate produced per unit time
• Depth: ≥50mm, ensure liquid seal height
• Material: Stainless steel of the same grade as the expansion joint or more corrosion resistant alloy

3.3 Condensation drainage device with water seal

When the flue is operated under negative pressure (typical value-1~ -3kPa), the ordinary condensation discharge port cannot rely on gravity to freely drain water, but may suck air from the condensation discharge port. At this time, it is necessary to set up a water-sealed flue gas desulfurization flue expansion joint condensation discharge device. The water seal height should be greater than the water column height corresponding to the maximum negative pressure of the flue, usually 200-300mm water column. The relationship between the height H (mm) of the vertical section of the water seal pipe and the flue negative pressure P (kPa) is as follows: H = P ×102+50 (safety margin).

3.4 Automatic drain valve

In the desulfurization system with a high degree of automation, a float-type automatic drain valve or a solenoid drain valve can be installed at the end of the condensate drain pipe. The automatic drain valve can open and close automatically according to the condensate level without manual operation, especially suitable for unattended occasions.

4. Design and calculation of condensation discharge device

4.1 Estimation of condensate production

When designing the flue expansion joint for flue gas desulfurization, it is necessary to estimate the amount of condensate generated by a single expansion joint. It can be estimated by the following empirical formula:

Q = k × A × (T_gas-T_wall) × RH

Among them:

• Q: Condensate generation amount (kg/h)
• K: Comprehensive heat transfer coefficient, taken as 5-10 W/ (m²·K)
• A: Inner wall surface area of expansion joint (m²)
• T_gas: flue gas temperature (℃)
• T_wall: wall temperature (take ambient temperature +5℃)
• RH: relative humidity correction factor (1.0 in saturated state)

For the desulfurization net flue of a typical coal-fired power plant, the condensate production of a single DN2000 circular expansion joint is about 0.5 – 2 kg/h.

4.2 Determination of discharge tube diameter

The diameter of the condensate discharge pipe should meet the discharge requirements of the maximum condensate generation, and at the same time, avoid clogging by debris. Recommended caliber:

• Condensate volume
• Condensate volume 1-5 kg/h: φ 20mm (3/4 inch)
• Condensate volume 5-20 kg/h: φ 25mm (1 inch)

Thick-walled tubes (Sch40 and above) shall be used for condensation drainage pipes, threaded or welded, and thin-walled tubes shall be strictly prohibited.

V. Key Points of Construction and Installation of Condensation Discharge Device

5.1 Position Determination and Opening

The installation position of the condensation discharge device of the flue gas desulfurization flue expansion joint should be at the lowest point of the expansion joint. During construction, attention should be paid to:

• The condensation discharge port is reserved in the manufacturing stage of the expansion joint, and it is strictly prohibited to open the hole by itself after arriving at the site
• Stress analysis shall be carried out for the opening in the bellows, and the edge of the opening shall be rounded with a corner R ≥3mm
• Argon arc welding is used for the welding of the condensation exhaust port to ensure good root formation
• 100% penetration test is carried out after welding is completed, grade I qualified

5.2 Arrangement of condensation discharge tube

The layout of condensate discharge pipe should follow the principle of "short, straight and slope":

• Length as short as possible to reduce the risk of clogging and fouling
• Go straight, avoid right-angle elbows (45° elbows or simmer bends should be used)
• Maintain a slope of ≥5% towards drainage
• When passing through the flue insulation layer, the condensation discharge pipe shall be provided with heat insulation to prevent freezing in winter

5.3 Installation of water sealing device

For the occasion where water seal is used for condensation drainage in negative pressure flue:

• Vertical deviation of water sealing cylinder ≤2mm/m
• The sewage valve shall be set at the bottom of the water seal cylinder to facilitate the cleaning of sediment
• Add water to the overflow port before first operation and check the water seal height
• When running in winter, the water sealing cylinder should be tracked by heat preservation or added with antifreeze

5.4 Cooperation with flue insulation

Both the expansion joint and the condensation discharge pipe shall be insulated according to the design. Special attention should be paid to the insulation box at the part where the condensation drain pipe passes through the insulation layer to prevent the local condensation from being aggravated by cold bridge. The condensation discharge valve should be arranged on the outside of the insulation layer for easy operation.

Operation and maintenance and common fault handling

6.1 Main points of daily inspection

After being put into use, the flue gas desulfurization flue expansion joint condensation discharge device shall be included in regular inspection:

• Check whether the condensation drainage port is blocked and whether the drainage is continuous and unobstructed
• Observe the color and pH value of the condensate (it should be transparent or pale yellow normally, and check the desulfurization system if the pH value drops abnormally)
• In winter, check whether the heat insulation and heat tracing are normal, and whether the condensation drain pipe is frozen
• Water seal type condensation drainage needs regular water replenishment, check the water seal liquid level

6.2 Common Troubles and Handling

6.3 Maintenance Cycle and Replacement

The condensation discharge device should be inspected simultaneously with the overhaul of the expansion joint, and it is recommended that it be disassembled and inspected every 2-3 years. When the wall thickness of stainless steel condensation drainage pipe is reduced by more than 30% or penetrating pitting corrosion occurs, it should be replaced as a whole. Local repair welding is not recommended (re-corrosion is more likely to occur in the repair welded area).

VII. Consideration of condensation drainage design under special working conditions

7.1 Condensation drain protection during furnace shutdown

If the desulfurization system is shut down for more than 24 hours, the condensation discharge valve at the bottom of the expansion joint should be opened to empty the internal accumulated liquid and purged clean with compressed air. For systems that may be out of service for a long time, the moist air in the flue can be replaced with nitrogen to prevent static corrosion from increasing.

7.2 Expansion joint discharge of flue climbing section

When the expansion joint is installed at the climbing section of the flue instead of the horizontal section, the lowest point is not at the expansion joint itself, but at the bottom of the downstream flue. At this time, a condensation discharge pipe should be added at the lowest point of the downstream flue adjacent to the expansion joint, and a guide plate should be set to guide the condensate in the expansion joint to the condensation discharge point.

VIII. Summary

Flue gas desulfurization flue expansion joint condensation is a detail that cannot be ignored in the design of desulfurization system, but it is precisely the key factor that affects the life of expansion joint. Corrosion failure caused by condensate accumulation tends to occur faster and more concealed than thermal stress fatigue failure. Successful condensation design can be summarized into four principles: "Find the lowest point, set sufficient slope, prevent negative pressure suction, and select corrosion-resistant materials". Specifically, gravity condensation discharge or water seal condensation discharge should be selected according to the flue pressure state. The lowest point of the bellows should be provided with a liquid collecting tank instead of a conventional opening. The condensation discharge pipe should be kept short and straight with a slope of ≥5%. The material of the components contacting the condensate should be raised to no lower than the expansion joint body. Through standardized condensation design and operation and maintenance, the corrosion rate of expansion joint of desulfurization flue can be reduced by more than 80%, the actual service life of expansion joint can be prolonged from less than 2 years to 8-10 years, and the environmental protection risk caused by leakage and the frequency of maintenance can be reduced. For desulfurization projects under construction, it is recommended that the expansion joint condensation discharge device be included in the detailed design scope during the design stage, so as to avoid omission or random disposal during the construction stage.