How can the fire‑resistance performance of rigid beams in boiler steel structures be ensured during design?

Category: Company News

Time: 2025-06-12

Fire‑proofing design for rigid beams in boiler steel structures is critical to ensuring structural integrity under fire or high‑temperature conditions, requiring a comprehensive assessment across multiple dimensions, including material selection, protective measures, and structural design.

Fire‑proofing design for rigid beams in boiler steel structures is critical to ensuring structural integrity under fire or high‑temperature conditions, requiring a comprehensive assessment across multiple dimensions, including material selection, protective measures, and structural detailing. The following outlines specific design considerations and technical solutions:

I. Selection and Application of Fire-Resistant Materials

1. High-temperature-resistant base material

High-temperature strength retention:

Ordinary carbon steel (such as Q235B) experiences a significant reduction in strength above 300°C (with its yield strength at 400°C dropping to 50% of its room-temperature value), necessitating the use of heat-resistant steels or refractory alloys.

Low-alloy heat-resistant steels: 15CrMo (oxidation resistance up to 580°C) and 12Cr1MoV (up to 560°C), suitable for rigid beams in the high-temperature zone of the furnace.

Austenitic stainless steels: 304, 316L (oxidation resistance up to 850°C), used in critical components exposed to flames or high‑temperature flue gases.

Refractory cast iron, such as high-silicon cast iron (HTSi5), is suitable for applications involving extremely high temperatures (≥1000°C) and requiring resistance to thermal shock, such as the rigid beams of gasifiers.

Fire-resistance rating of the material:

According to the Code for Fire Protection Design of Buildings (GB 50016), rigid beams in industrial boilers shall have a fire-resistance rating of no less than 1.5 hours (for Class I fire resistance). Heat-resistant steel can achieve a fire-resistance rating of more than 2 hours at 600°C.

2. Fireproof Coating Material

Inorganic insulation layer:

Rock wool/mineral wool: thermal conductivity ≤ 0.04 W/(m·K), insulation thickness 50–100 mm, fixed with galvanized steel sheet or stainless steel plate; suitable for flue ducts, air ducts, and rigid beams that are not directly exposed to flame.

Lightweight refractory concrete: such as perlite concrete (density ≤ 800 kg/m³), applied at a thickness of 30–50 mm, with a high-temperature resistance of ≥ 1000°C, used for the rigid beams surrounding the furnace chamber.

Fireproof panels: calcium silicate boards (with a fire-resistance rating of ≥4 hours) and vermiculite boards, which are bolted to the beam surface to form an insulating barrier.

Intumescent fire-retardant coating:

When exposed to fire, the coating expands to form a porous carbonaceous layer (with an expansion ratio of ≥10 times), providing significant thermal insulation. For example, ultra-thin intumescent fire‑protective coatings for steel structures can achieve a fire resistance rating of ≥1.5 hours at a coating thickness of 2–3 mm, making them suitable for outdoor equipment with stringent aesthetic requirements.

II. Optimization of Structural Fire-Resistant Design

1. Fire Separation and Thermal Bridge Breakage

Thermal bridge isolation:

The junction between the rigid beam and the shell is prone to forming thermal bridges; ceramic fiber gaskets (with a thermal conductivity ≤ 0.03 W/(m·K)) or refractory concrete can be used to fill the gap, thereby preventing heat transfer into non‑refractory areas.

Example: When a rigid beam passes through a fire wall, fire‑resistant sleeves (e.g., constructed from refractory bricks) shall be installed on both sides of the wall, and the gap between the sleeve and the beam shall be filled with fire‑resistant sealant.

Zoned Fire Protection Design:

The boiler area shall be classified according to fire‑hazard levels (e.g., the furnace chamber is designated as a high‑hazard zone, while the flue is a medium‑hazard zone). In the high‑hazard zone, rigid beams shall be provided with double‑layer fire‑protective insulation—comprising an inner layer of refractory concrete and an outer layer of rock wool—whereas in the medium‑hazard zone, a single‑layer insulation may be used.

2. Heat-Deformation-Resistant Structure

Flexible Support Design:

In the high-temperature zone, the rigid beams are designed with a “hinged connection + fire-resistant support” system, for example:

The support nodes employ refractory ceramic ball joints, which accommodate thermal expansion of the beam while maintaining structural stiffness.

An expansion joint (such as a corrugated expansion joint) is installed at the midspan of the girder to accommodate axial thermal displacement (compensation ≥ 50 mm), thereby preventing cracking of the fire‑protective coating due to thermal expansion and contraction.

Crack-resistant structure of the fireproof layer:

In refractory concrete or fire‑resistant coatings, steel wire mesh (with mesh openings no larger than 50 mm × 50 mm) is embedded to enhance crack resistance; on the exterior of the rock wool insulation layer, a galvanized steel wire mesh (with wire diameter ≥ 1.2 mm) is installed to prevent the insulation layer from detaching due to vibration or impact.

III. Fire Performance Verification and Monitoring

1. Fire Resistance Testing and Calculation

Standard fire-resistance test:

Fire-resistance tests are conducted in accordance with ISO 834 or GB/T 9978, using a simulated fire‑rise temperature curve (e.g., the standard heating curve: 500°C in 10 minutes and 800°C in 30 minutes). The load‑carrying capacity retention rate of rigid beams is evaluated over the specified test duration, with a target value of ≥80%.

Finite Element Thermo-Mechanical Coupling Analysis:

Using software such as ANSYS, the temperature field distribution in beams under fire conditions is simulated—e.g., during a furnace deflagration, when local temperatures can rapidly rise to 1200°C—allowing for the calculation of thermal stresses and deformations, and facilitating the optimization of fire‑protective coating thickness and structural configuration.

2. Temperature Monitoring and Early Warning

Built-in temperature sensor:

K-type thermocouples or fiber-optic sensors are embedded at critical locations of the rigid beam—such as the inner side of the fireproofing layer and weld joints—to monitor temperature in real time with an accuracy of ±5°C. An alarm is triggered when the temperature exceeds the material’s allowable limit (e.g., 600°C for heat-resistant steel).

Fireproofing Layer Condition Monitoring:

Regularly inspect the integrity of the fire‑protective coating using an infrared thermal imager to identify potential hazards such as delamination or cracking (e.g., areas where the coating temperature has abnormally risen may indicate damage).

IV. Fire Protection Design for Special Scenarios

1. Explosion- and fire-proof design for gas-fired boilers

Integrated blast-resistant and fireproof design:

The rigid beam of a gas-fired boiler must simultaneously withstand both explosive impact (with a peak pressure ≥ 0.5 MPa) and flame exposure, employing a “high-strength steel + double-layer fireproof coating” design:

The main structural material is Q460 high-strength steel (yield strength ≥ 460 MPa), enhancing blast resistance.

On the inner side of the fireproof layer, impact‑resistant refractory bricks (such as high‑alumina bricks with a compressive strength ≥ 50 MPa) are used; the outer side is wrapped with fire‑resistant insulation to prevent explosive fragments from breaching the fireproof barrier.

2. High-Temperature Protection for Waste Heat Boilers

Thermal radiation shielding:

When the rigid beam of the waste‑heat boiler is located on the high‑temperature flue‑gas side (temperature ≥ 800°C), a removable heat‑resistant shielding plate—such as a 1Cr25Ni20Si2 stainless steel plate with a thickness of ≥ 5 mm—shall be installed. The gap between the plate and the beam shall be ≥ 100 mm, creating an air insulation layer to mitigate the effects of thermal radiation.

Keywords: How can the fire‑resistance performance of rigid beams in boiler steel structures be ensured during design?

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