Baffle door, also known as shut-off door or air duct baffle door, is the key equipment used for intercepting flue pipe and air duct medium in power plants and industrial fields. The pipeline can be fully opened or closed by 90° opening and closing。 Its main functions can be summarized as three core tasks: system isolation-quickly cutting off high-temperature corrosive flue gas during overhaul or failure of the desulfurization system; Flow regulation-cooperate with DCS system to achieve precise control of flue gas quantity; Sealed and leakproof-ensures zero leakage of SO₂, dust and other media in the closed state。

According to the location distribution in the desulfurization system, the baffle door can be divided into original flue gas baffle door, clean flue gas baffle door and bypass baffle door. It should be noted that according to the requirements of the documents of the Ministry of Environmental Protection, the bypass baffle door of the newly built unit is no longer provided, and the bypass baffle door of the existing unit has also been sealed。

2. Classification and structural characteristics of baffle doors

1. Classification by shape

• Square baffle door: Suitable for rectangular smoke ducts, it is the most commonly used form in power plants
• Round baffle door: suitable for round pipes, commonly found in small and medium-diameter systems

2. Classification by Drive Mode

3. Classification by structural form

• Single panel flap door: suitable for small diameter pipes, low cost but limited sealing
• Double shutter type: Zero leakage by staggering two baffles, recommended for high pressure differential conditions
• Airtight flapper door: Equipped with an air-filled seal, the leakage rate can be controlled below 0.5%

4. Core structure composition of baffle door

The baffle door is mainly composed of the following components：

• Frame: Steel plate compression molding or steel section welding to provide overall structural strength
• Blades: Streamlined cross-section design to ensure minimal dielectric pressure loss
• Sealing sheet: Special elastic sealing structure is arranged between the blades and between the blades and the frame, ensuring that the sealing rate can reach more than 99% when closed
• Main shaft and bearing: Special sealing structure and self-aligning bearing are used at the journal to ensure flexible rotation and no sticking
• Crank-linkage mechanism: The large baffle door adopts double-arm heavy-duty crank-linkage mechanism to ensure foolproof opening and closing action
• Actuator: Manual, electric or pneumatic, can be directly connected to the drive spindle or driven by a crank-link mechanism

3. Design principle of zero leakage of baffle door

The core technical path to achieve "zero leakage" of baffle door is the combination scheme of "mechanical seal + sealed air system"。

1. Mechanical seal

The mechanical seal consists of a sealing sheet, a pressing plate and its matching fasteners, and forms an elastic sealing structure with the main body blade. The principle is that the elastic deformation of the sealing piece itself is used to compensate for the flatness error between the blades, and the clearance is small and the sealing is reliable。

2. Sealed air system

This is the key technology to achieve zero leakage of baffle doors. The system is usually equipped with two fans (one for use and one for standby) and one heater, which are sealed by the following paths：

• The double louver blades and the baffle door frame constitute a sealed air cavity
• Introduce sealing air higher than the flue gas pressure (≥500Pa) to form an isolation layer
• The sealing air forms an air curtain between the blades to block the flue gas leakage channel

According to industry data, the baffle door with "double-layer sealing structure + sealed air system" has an air tightness of more than 99%, which effectively isolates corrosive media。

4. Selection points and material standards of baffle doors

1. Key selection parameters

Correct selection of baffle doors requires clarification of the following working condition parameters：

• Interface dimensions: Width (W) and Height (H) refer to the dimensions of the inner edge of the frame
• Maximum working pressure: generally
• Maximum working temperature: generally
• Working medium: flue gas composition (SO₂, Cl⁻¹ content, dust concentration)
• Line Direction: Horizontal/Vertical
• Drive mode: manual/electric/pneumatic/electro-hydraulic
• Opening and closing speed: full travel time requirements
• Sealing effect: conventional> 99%, special can reach over 99.5%

2. Material selection strategy

The material selection of the baffle door should be determined according to the smoke gas working condition：

3. Selection decision tree

1. Confirm flue gas parameters (temperature, pressure, composition)
2. Select Structural Form (Priority Double Blinds + Inflatable Seals)
3. Matching actuator (peak shaving unit electro-hydraulic)
4. Determine the substrate (select C276/2205 for high temperature and strong corrosion)

V. Common leakage causes and treatment measures of baffle doors

1. Causes of leakage in actual use

According to industry analysis, the main causes of baffle door leakage can be summarized into three categories：

Production quality problems: The baffle door produced by the manufacturer itself has quality problems, such as uneven blade surface, inadequate installation of sealing sheet, etc., resulting in poor mechanical sealing.

Commissioning problem: The quality of on-site installation and commissioning directly affects the sealing performance. Frequently asked questions include: the professional skills of engineering generation are not up to standard; The installation period is short, and the construction party will install it by itself before the supplier's worker arrives for commissioning; The actuator switch limit is not consistent with the action of the baffle door body.

Sealed air system problems: selection deviation (wind pressure and air volume do not meet the requirements); Unreasonable pipeline design (too many elbows lead to excessive pressure loss); "Pressure relief" conditions such as soft connection damage and pipeline leakage in actual use.

2. Treatment measures

Mechanical Seal Adjustment:

• Take the main blade as the reference, adjust the crank connecting rod mechanism to ensure that the surface of each blade is at the same horizontal plane
• Adjust the mechanical seal structure to ensure that there is no gap between the blade and the seal sheet
• Reset the actuator switch position command to ensure that the actuator action is consistent with the baffle door body

Sealing system problem handling:

• Selection problem: Re-select, replace sealing system
• Pipeline Leakage: Find the Leakage Point and Repair it on Site
• Fan/Accessories Damaged: Replace as appropriate
• Pipeline design problem: Before selection, the supply and demand parties should fully communicate and set up a reasonable pipeline direction

VI. Installation, maintenance and acceptance standards of baffle doors

1. Installation Points

• In the pipeline arrangement, the spindle of the blade should generally be kept in a horizontal position
• Large flapper doors feature a split structure that can be transported in sections and assembled on site
• The extension length of baffle shaft should fully consider the thickness of insulation layer

2. Maintenance recommendations

• Regularly check the wear of the sealing sheet and replace it in time
• Check the bearing lubrication status regularly and use high temperature resistant grease
Conduct full-stroke opening and closing test 1-2 times a month to prevent long-term immobility and sticking
• Sealed fan system periodically inspect filters and lines

3. Acceptance criteria

• Leakage rate test: refer to GB/T 13927-2018, test by bubble method at 1.1 times design pressure
• Actuator life: Electric device needs to pass 5000 trouble-free opening and closing tests
• Machining accuracy: baffle clearance ≤1mm, blade parallelism error

sum up

Baffle door is the key equipment to ensure safe operation and reliable isolation in flue gas system. Proper type selection, standard installation and regular maintenance are the three core elements to ensure its long-term reliable operation:

• Selection according to working conditions: according to flue gas temperature, corrosivity, pressure, diameter to determine the structure form and material, conventional selection of double shutter type + air seal, high temperature strong corrosion selection of 2205/C276 alloy
• Zero leakage depends on combination design: mechanical elastic seal + sealed air system is a two-pronged approach, and the wind pressure of the sealed fan should be higher than the flue gas pressure by more than 500Pa
• For leak troubleshooting, first look at the sealing sheet and sealing air system: for mechanical problems, adjust the flatness of blade and actuator limit; for system problems, check the wind pressure and pipeline leakage
• The standard basis should be clear: NB/T 42046-2015 specifies the technical specifications of flue gas baffle doors, DL/T 5121-2020 is the design specification basis

A baffle door with reasonable design and standard installation can run stably for a long time under harsh working conditions such as high temperature and corrosion, providing reliable "door handling" guarantee for flue gas system.