1. The importance of the direction of non-metallic expansion joints

In flue system, non-metallic expansion joints are widely used in flue gas pipelines in power plants, metallurgy, chemical industry and other industries because of their excellent compensation ability, heat insulation performance and corrosion resistance. However, many installers do not know how to correctly judge the direction of the non-metallic expansion joint on the flue. Unlike metal expansion joints, non-metal expansion joints are usually composed of multi-layer composite materials, including a sealing film of fluororubber or silicone rubber, a thermal insulation layer of glass fiber and a reinforcing layer of stainless steel wire mesh. Its structure has obvious "internal and external distinction" and "upstream and downstream distinction". Once the installation direction is wrong, the sealing film will be overheated and aged, leakage will be aggravated, and the whole expansion joint will be burned out in a short time, resulting in the shutdown of the flue system. This paper will systematically expound the complete method of orientation judgment from the aspects of structural characteristics, orientation marking, installation orientation and common errors.

2. Understanding the directional structure of non-metallic expansion joints

2.1 Why do non-metallic expansion joints have directional requirements

To answer the question of the direction of the non-metallic expansion joint on the flue, we must first understand why its structure is directional. The design of non-metallic expansion joints usually considers the following directional factors:

• Airflow direction: The scour of the material by dusty flue gas is uneven, and the deflector and insulation layer are usually biased upstream of the flue gas
• Deflector plate setting: The opening direction of the deflector plate must face the incoming flue gas, otherwise the flexible ring belt cannot be protected
• Condensate drainage: The bottom condensate drainage port shall be located at the downstream side or lowest point of flue gas flow
• Anti-loosening of bolts: The installation direction of flange bolts should consider anti-loosening under vibration environment

2.2 Main directional components of non-metallic expansion joints

2.3 Distinction of "Inside and Outside" of Circle Belt

The non-metallic band is usually a multilayer composite structure, which is in order from the outside to the inside:

• Outer layer: weather-resistant rubber or silicone rubber, resistant to UV and ozone aging
• Intermediate layer: Glass fiber cloth or ceramic fiber cloth for insulation and reinforcement
• Inner layer: fluororubber or PTFE membrane, resistant to high temperature, corrosion and gas sealing

If the non-metallic expansion joint on the flue is installed in the reverse direction, the inner high-temperature resistant layer faces outward, and the outer weather-resistant layer directly contacts the high-temperature flue gas (up to 200-400℃), it will age and crack rapidly, and it will fail within a few hours to a few days.

3. Method for judging the direction of non-metallic expansion joint on flue

3.1 Judgment by Deflector

Deflectors are the most reliable direction indicators for non-metallic expansion joints. Proper installation shall satisfy:

• The short end is upstream: the fixed end of the deflector connected with the flange of the expansion joint should be facing the direction of flue gas flow
• The long end is downstream: the freely extending end of the deflector is pointed in the direction of where the flue gas goes, which is consistent with the flow direction
• Overlapping length: The deflector shall cover the entire width of the ring belt, and the overlapping length shall not be less than 100mm

When it is impossible to view the ring belt mark, you can quickly judge whether the direction of the non-metallic expansion joint on the flue is correct by observing the deflector.

3.2 Identification according to the flow direction on the belt

For non-metallic expansion joints produced by regular manufacturers, the following information is usually marked on the outside of the ring belt:

• Arrow mark: directly marks the allowable flue gas flow direction
• The tape surface should be carefully inspected before installation, these logos are usually printed with high temperature resistant ink or riveted with metal labels. If the identification is blurry or missing, contact the manufacturer for confirmation before installation.

3.3 According to the opening of the guide tube or the position of the drain port

For rectangular or round non-metallic expansion joints:

• The bottom condensate discharge port shall be located on the downstream side to facilitate the smooth discharge of the condensate
• If the expansion joint is provided with a temperature measuring hole or a detection hole, it is usually located in the middle and downstream position
• Pressure relief holes (if any) in deflectors should be downstream

3.4 Unified judgment using standard flue gas flow direction

When the direction of the expansion joint itself is not completely identified, it can be judged according to the unified flow direction of the whole flue system:

• Flue gas flow direction: from boiler/furnace → dust collector → desulfurization tower → chimney
• The convex surfaces of non-metallic expansion joints (belt drum outward) are usually facing away from the direction of flue gas flow
• Location of bead and fastening bolts of ring belt: The overlapping direction of bead shall be consistent with the flow direction (along the stubble direction)

4. Correct installation orientation of non-metallic expansion joint on flue

4.1 Installation direction of horizontal flue

When installing non-metallic expansion joints on horizontal flues:

• Circular expansion joint: The ring band joint should avoid the top and bottom of the flue, and should be arranged in the direction of 45°
• Rectangular expansion joint: When the long side is arranged horizontally, the deflector should point downstream horizontally; When the long side is arranged vertically, the deflector shall point vertically downward
• The bottom drain port is located at the lowest and aligned with the deflector notch

4.2 Installation direction of vertical flue

In the vertical flue, the determination of the direction of the non-metallic expansion joint on the flue should consider whether the flue gas flows upward or downward:

• The flue gas flows upward: the fixed end of the deflector is in the lower flange, and the free end is pointed upward
• Flue gas flow downward: The fixed end of the deflector is on the upper flange, and the free end is pointed downward
• In either case, the inclined surface of the deflector should face the flow of smoke

4.3 Special operating conditions in which bidirectional air flow exists

There may be bi-directional airflow (alternating flow directions) in some bypass flues or switching flues. At this point:

• Non-metallic expansion joints with symmetrical structure should be selected, and the deflectors should be double-sided or center-mounted deflectors
• The inside and outside of the circle must still be correct, but the flow direction arrow has no absolute meaning
• It is recommended that the "two-way air flow conditions" be specified to the manufacturer at the selection stage

Common Installation Errors and Corrective Measures

5.1 Install deflector in reverse direction

Error manifestation: The fixed end of the deflector is downstream, and the free end is pointing upstream
Consequence: The flue gas directly washes the non-metallic ring belt, the temperature exceeds the standard, and the ablation and perforation within a few days
Correction: After discovery, the machine must be stopped immediately, and the expansion joint must be removed and reinstalled. Do not use it reluctantly

5.2 Inner and outer layers of the loop are installed backwards

Error manifestation: rubber seal facing inward, insulating fiber layer facing outward
Consequence: The sealing layer becomes brittle after high temperature aging, and it cracks and leaks after 2-3 months
Correction: Re-install, flip the ring belt or replace the expansion joint as a whole

5.3 Vertical flue reverse up and down

Error: When smoke is flowing upward, the fixed end of the deflector is on the upper flange
Consequence: The deflector plate loses the deflecting effect, and the ring belt is subjected to high temperature and high concentration dust scour
Correction: Adjust the up and down direction of the expansion joint and install it according to the correct flow direction

5.4 Ignore bottom drainage direction

Error manifestation: The drain port is located on the upstream side or side of the flue gas
Consequence: Condensate does not drain completely, accumulates at the root of the deflector, corrodes flanges and bolts
Correction: If it cannot be reinstalled, a separate hole should be opened at the lowest point for drainage

6. Direction check list after installation

After completing the installation of the non-metallic expansion joint direction on the flue, it is recommended to confirm item by item:

• The fixed end of that deflector is point to the direction of flue gas flow
• The flow direction mark on the outside of the ring belt is consistent with the actual flow direction of the flue
• The bottom drain is located on the downstream side and at the lowest position
• The opening direction of the loop strip is the same as the flow direction (straight, no ride)
• The pre-compression direction of the limit tie rod is consistent with the main displacement direction
• The flange bolts are oriented for easy tightening and loosening (nuts are generally located on the lower side of the airflow)
• If there are more than one expansion joint, make sure that the direction of each expansion joint is consistent or coordinated according to the design

VII. SUMMARY

The correct judgment and installation of the direction of the non-metallic expansion joint on the flue is the prerequisite to ensure the normal compensation and sealing function of the expansion joint. The core points are summarized as follows: "The deflector has a fixed flow direction, the inside and outside of the circle, the flow vane is marked with arrows, and the drainage outlet faces downward". The most reliable basis for judgment is the deflector-the fixed end is always facing the direction of flue gas flow, and the free end is pointing downstream. The inner and outer surfaces of the ring belt can not be installed backwards, and the heat-resistant layer must face the smoke side. For new installation projects, it is recommended to permanently mark the flow direction arrow on the expansion joint housing and write the installation direction requirements into the construction plan and acceptance record. Once it is found that the direction is reversed, it must be corrected in time. Don't have the lucky mentality of "as long as it can be used", otherwise serious failures such as leakage and burn-through will occur in a short time, resulting in greater downtime losses. Through standardized direction confirmation and installation control, the non-metallic expansion joint can operate safely and stably for more than 5 years, significantly reducing the maintenance cost of flue system.