How to ensure the fire safety of rigid beams in boiler steel structures during design?

Fire protection design for rigid beams in boiler steel structures is crucial for ensuring structural integrity under fire or high-temperature conditions. It requires comprehensive consideration from multiple dimensions, including material selection, protective measures, and structural design. The following are specific design points and technical solutions:

I. Selection and Application of Fire-Resistant Materials

1. High-Temperature Main Materials

High-Temperature Strength Retention:

Ordinary carbon steel (such as Q235B) shows a significant decrease in strength above 300℃ (yield strength drops to 50% of room temperature at 400℃). Heat-resistant steel or refractory alloys should be used:

Low-alloy heat-resistant steel: 15CrMo (oxidation resistance temperature ≤580℃), 12Cr1MoV (≤560℃), suitable for rigid beams in high-temperature furnace areas;

Austenitic stainless steel: 304, 316L (oxidation resistance temperature ≤850℃), used for key parts in contact with flames or high-temperature flue gas;

Refractory cast iron: such as high-silicon cast iron (HTSi5), suitable for extreme high temperatures (≥1000℃) and scenarios requiring resistance to thermal shock (such as rigid beams in gasification furnaces).

Material Fire Resistance Limit:

According to the "Building Fire Design Code" (GB 50016), the fire resistance limit of industrial boiler rigid beams must meet ≥1.5 hours (Class 1 fire resistance rating). The fire resistance limit of heat-resistant steel at 600℃ can reach more than 2 hours.

2. Fire-Resistant Coating Materials

Inorganic Insulation Layer:

Rock wool/mineral wool: thermal conductivity ≤0.04W/(m・K), wrapping thickness 50~100mm, fixed with galvanized iron sheet or stainless steel plate, suitable for flue and air duct rigid beams that do not directly contact flames;

Lightweight refractory concrete: such as perlite concrete (density ≤800kg/m³), coating thickness 30~50mm, high-temperature resistance ≥1000℃, used for rigid beams around the furnace;

Fire-resistant board: calcium silicate board (fire resistance limit ≥4 hours), vermiculite board, fixed to the beam surface with bolts to form a heat insulation barrier.

Intumescent fire-resistant coating:

When exposed to fire, the coating expands to form a porous carbon layer (expansion ratio ≥10 times), with significant heat insulation effect (such as ultra-thin steel structure fire-resistant coating, fire resistance limit ≥1.5 hours with a coating thickness of 2~3mm), suitable for outdoor equipment with high appearance requirements.

II. Structural Fire Protection Design Optimization

1. Fire Separation and Thermal Bridge Interruption

Thermal Bridge Isolation:

The connection between the rigid beam and the shell is prone to forming thermal bridges. Ceramic fiber gaskets (thermal conductivity ≤0.03W/(m・K)) or refractory concrete can be used to fill the gaps and prevent heat conduction to non-fire-resistant areas;

Example: When a rigid beam passes through a fire wall, fire-resistant sleeves (such as refractory brick masonry) should be installed on both sides of the wall, and the gap between the sleeve and the beam should be filled with fire-resistant sealant.

Zoned Fire Protection Design:

Divide the boiler area according to the fire hazard level (such as the furnace as a high-risk area, and the flue as a medium-risk area). High-risk area rigid beams use double-layer fire protection (inner layer refractory concrete + outer layer rock wool), and medium-risk areas can use single-layer coating.

2. Heat Deformation Resistant Structure

Flexible Support Design:

High-temperature area rigid beams use a "hinge + fire-resistant support" structure, for example:

Support nodes use fire-resistant ceramic ball hinges, allowing for thermal expansion of the beam while maintaining support stiffness;

Expansion joints (such as corrugated expansion joints) are installed in the middle of the beam to absorb axial thermal displacement (compensation ≥50mm), preventing cracking of the fire protection layer due to thermal expansion and contraction.

Crack-Resistant Construction of Fire Protection Layer:

Embed wire mesh (grid ≤50mm×50mm) in refractory concrete or fire-resistant coating to enhance the crack resistance of the coating; a galvanized wire mesh (diameter ≥1.2mm) is installed outside the rock wool coating to prevent vibration or impact from causing the insulation layer to fall off.

III. Fire Performance Verification and Monitoring

1. Fire Resistance Test and Calculation

Standard Fire Resistance Test:

Conduct fire resistance tests according to ISO 834 or GB/T 9978, simulating the fire temperature rise curve (such as the standard temperature rise curve: 10 minutes to 500℃, 30 minutes to 800℃), and test the load-bearing capacity retention rate of the rigid beam within the specified time (target ≥80%).

Finite Element Thermal-Mechanical Coupled Analysis:

Simulate the temperature field distribution of the beam under fire conditions (such as a sudden temperature rise to 1200℃ during furnace explosion) using software such as ANSYS, calculate thermal stress and deformation, and optimize the thickness and structural form of the fire protection layer.

2. Temperature Monitoring and Early Warning

Built-in Temperature Sensors:

Pre-embed K-type thermocouples or fiber optic sensors at key parts of the rigid beam (such as the inner side of the fire protection layer, welding nodes) to monitor the temperature in real time (accuracy ±5℃). When the temperature exceeds the allowable value of the material (such as 600℃ for heat-resistant steel), an alarm is triggered.

Fire Protection Layer Status Monitoring:

Regularly inspect the integrity of the fire protection layer using an infrared thermal imager to identify defects such as shedding and cracking (areas with abnormal temperature increase in the coating may have damage).

IV. Fire Protection Design for Special Scenarios

1. Explosion and Fire Protection for Gas Boilers

Explosion-proof and fire-proof integrated design:

The rigid beam of the gas boiler needs to simultaneously resist explosion impact (pressure peak ≥0.5MPa) and flame burning. A “high-strength steel + double-layer fireproof layer” design is adopted:

Q460 high-strength steel (yield strength ≥460MPa) is selected as the main material to improve explosion resistance;

Impact-resistant firebricks (such as high-alumina bricks, compressive strength ≥50MPa) are used on the inner side of the fireproof layer, and the outer side is wrapped with fireproof cotton to prevent explosion fragments from penetrating the fireproof layer.

2. High-temperature protection of waste heat boiler

Thermal radiation shielding:

When the rigid beam of the waste heat boiler is close to the high-temperature flue gas side (temperature ≥800℃), a removable heat-resistant shielding plate (such as 1Cr25Ni20Si2 stainless steel plate, thickness ≥5mm) is installed. The distance between the plate and the beam is ≥100mm, forming an air insulation layer to reduce the impact of thermal radiation.