In the application of non-metallic expansion joints for wet desulfurization systems, the flue gas expansion joint drain hole is a seemingly unremarkable but crucial detail design. Many operation and maintenance personnel have found that expansion joints with no drainage holes or blocked drainage holes often have problems such as skin seepage and acid dripping within a few months after being put into operation. This paper will systematically explain the functional principle, design specification and daily maintenance points of the drainage hole of the flue gas expansion joint, and help technicians solve the stubborn problem of "water leakage" of the expansion joint from the source.

1. Why do flue gas expansion joints need drainage holes?

1. Formation mechanism of condensed acid solution

In the wet desulfurization system, the temperature of flue gas drops to 45-55℃ after spraying and washing, and it is in a completely saturated state. When the saturated wet flue gas passes through the non-metallic expansion joint, it will condense into liquid water on the inner wall of the skin and the surface of the metal frame when exposed to cold. Because the flue gas contains acidic components such as SO₂, SO₃, Cl⁻¹, the pH of these condensates can be as low as 2-3, which is highly corrosive.

2. Consequences of no drainage holes

If the expansion joint is not provided with the drainage hole of the flue gas expansion joint, the condensed acid liquid will accumulate in the annular groove formed by the skin and the pressure plate for a long time, causing the following hazards:

• Skin penetration: slow penetration of acid through layers of fabric, resulting in moist patches or even dripping on the outer surface
• Bolt corrosion: Acid penetrates along the gaps of the bolts, causing carbon steel or stainless steel bolts to corrode, loosen or even break
• Frame corrosion: Damage of metal frame coating where acid accumulates, corrosion and perforation of matrix
• Insulation layer failure: After acid penetrates into the insulation layer, the insulation performance is reduced, and the external surface temperature is abnormally increased

The expansion joint at the inlet of the desulfurization tower of a thermal power plant had serious water leakage only 4 months after it was put into operation because there was no drainage hole. Disassembly and inspection found that more than 2 liters of acid was accumulated in the skin groove, and half of the pressure plate bolts were loosened. After adding the drainage hole and replacing the skin, the expansion joint has been running normally for 2 years without leakage.

2. Design specification for drainage hole of flue gas expansion joint

1. Installation position of drainage hole

The flue gas expansion joint drain hole must be set at the lowest point of the expansion joint frame. For horizontally mounted expansion joints, the lowest point is located near the bottom centerline; For an inclined mounted expansion joint, the end is located on the lower side.

Specific positioning method:

• After the expansion joint is installed in place, use a level to measure the elevation at each point of the frame
• Find the lowest point position and mark the center of the drill hole
• For expansion joints exceeding 2 meters in length, it is recommended to set 2-3 drainage holes at equal intervals at the bottom to ensure that all condensate can be discharged in the full length

2. Aperture design of drainage hole

Aperture design should take into account both drainage efficiency and structural strength:

If the pore size is too small, it will easily be blocked by dust; Excessive hole size will weaken the frame strength, so it is necessary to add welding reinforcing ring at the opening.

3. Drain pipe joint design

The drainage hole of the flue gas expansion joint shall be equipped with drainage short pipe and valve. The specific requirements are as follows:

• Short tube material: the same material as the frame or higher grade (it is recommended to install stainless steel lined short tube for carbon steel frame)
• Length of short tube: 50-100mm, extending out of the outer edge of the frame
• Valve configuration: It is recommended to install ball valve or plug valve to facilitate regular dredging and cleaning. Normally closed state, regularly open drainage
• Drainage measures: The outlet of the drainage pipe shall be led to the gutter or acid collection system through a hose or pipe, and shall not be directly discharged on the platform ground

4. Anti-clogging design

In order to prevent dust accumulation from blocking the drainage hole of the flue gas expansion joint, the following measures can be taken:

• Dust cover: Install stainless steel dust net (hole diameter 3-5mm) at the drainage hole on the inner side of the frame
• Diversion groove: Machining a shallow diversion groove along the lowest point on the inner side of the frame to guide the condensate to collect into the drainage hole
• Flushing connection: Reserve the flushing water connection on the drain pipe, and reverse flushing and dredging can be done if necessary

Key control points during installation stage

1. Timing of drilling

Drilling of the flue gas expansion joint drainage hole should be performed after the expansion joint is installed in place, not prefabricated in the factory. The reason is:

• After on-site installation, the actual lowest point of the expansion joint may differ from the theoretical design due to pipe support settlement, flue connection deviation and other factors
• Field-positioned drilling ensures drainage holes are truly at their lowest points

2. Drilling process requirements

• Mechanical drilling (bench drill or magnetic drill) is adopted, and gas cutting is strictly prohibited
• Remove burrs and smooth after drilling
• Anti-corrosion coating on the edges of the openings to prevent rusting of the exposed metal

3. Level check

When installing the expansion joint, the horizontal deviation of the expansion joint should be strictly controlled ≤3‰. If the installation is skewed, the condensate will accumulate to one side, and the drainage hole on one side cannot be effectively emptied.

IV. Inspection points in operation and maintenance

1. Regular drainage operations

Flue gas expansion joint drain hole and very open design. Because the system operates under negative pressure, if the drain valve is normally open, the outside air will be sucked into the flue, which will affect the oxygen control of the system and the power consumption of the induced draft fan. The correct way to do this is:

• The valve remains normally closed
• Turn on drainage once per shift (8 hours) for 1-2 minutes each time
• Observe the properties of the discharged liquid: clear water is normal, turbid or containing solid particles indicates that the internal heat insulation layer is damaged
• Close valve after drainage

2. Blocking inspection and dredging

If no water flows out or the flow rate is very small after opening the valve, the drain hole may be blocked. Treatment method:

• Dredge from the outside to the inside with a thin steel drill
• If the blockage is severe, it can be blown back with compressed air from the inside
• After dredging, rinse with water to confirm that it is clear
• Check for detachment or blockage of the inner dust net

3. Corrosion inspection

Observe the color of the discharged liquid and sediment at each drainage:

• Colorless and transparent: normal condensate
• Yellow or reddish brown: Frame has begun to corrode, anti-corrosion coating needs to be checked
• White crystals: sulfate deposition, suggesting insufficient drainage frequency

V. Frequently asked problems and solutions

Typical Application Cases

In the desulfurization system of sintering machine of an iron and steel group, the non-metallic expansion joint has a serious skin water seepage problem one year after it is put into operation, and it can't be cured by repairing the leak once a month. On-site investigation found that the drainage hole of flue gas expansion joint was not set in the original design, and acid liquid was accumulated in the groove for a long time.

Modification scheme:

1. Drill a DN50 drain hole at the lowest point at the bottom of each expansion joint
2. Installation of 304 stainless steel drainage short pipe and ball valve
3. Drain pipe leads to gutter
4. Synchronous replacement of aging skin

After the transformation, the operator performs the drainage operation once per shift, and discharges about 0.5-1.5 liters of acid each time. Up to now, it has been running for 18 months, and there is no skin seepage phenomenon.

sum up

The drainage hole of flue gas expansion joint is the key design to ensure the long-term reliable operation of non-metallic expansion joint, and its value is often underestimated by designers and operation and maintenance teams. The correct drainage hole design should follow the four principles of "lowest position, appropriate hole diameter, anti-blocking and anti-corrosion, and regular operation":

• LOWEST LOCATION: Locate the drilling hole on site to ensure that the drainage hole is at the actual lowest point of the expansion joint
• Suitable hole diameter: Select DN25-DN100 drainage hole according to the diameter of the expansion joint, and set multi-hole for large size
• Anti-blocking and anti-corrosion: Install dust-proof net, select stainless steel short tube, and set up flushing interface
• Regular operation: Drain water for 1-2 minutes per shift, observe the properties of discharged liquid, and find abnormalities in time

A drainage hole of less than 100 yuan may avoid the early replacement cost of expansion joints of tens of thousands of yuan. Attention to this detail from the design source will significantly improve the operation reliability of flue gas desulfurization system.