What factors need to be considered in the design of rigid beams for boiler steel structures?

Category: Company News

Time: 2025-06-05

The design of rigid beams in boiler steel structures must comprehensively account for mechanical performance, the equipment’s operating environment, material properties, and other relevant factors, ensuring that they both enhance the shell’s stiffness and accommodate complex service conditions.

The design of rigid beams in boiler steel structures must comprehensively account for mechanical performance, the operating environment of the equipment, material properties, and other relevant factors, ensuring that they both enhance the shell’s stiffness and accommodate complex service conditions. The following analysis focuses on the key design elements:

I. Load Condition Analysis

1. Static Load

Shell and Medium Weights: It is necessary to calculate the self-weight of the shell, the weight of the internal medium (such as water, steam, or flue gas), and the loads imposed by the insulation layer and ancillary components (e.g., piping, platforms), ensuring that the beam’s strength is sufficient to support the vertical gravitational forces.

Internal pressure / External pressure load:

The combustion pressure within the boiler furnace (positive or negative) and the negative pressure in the flue (induced draft fan suction) can impose circumferential or axial stresses on the shell. The stiffening beams must be designed to distribute these stresses, thereby preventing shell buckling.

For example, during a furnace explosion, the instantaneous pressure can reach 0.2 to 0.5 MPa; the rigid beam must withstand this impact and restrain deformation of the shell.

2. Dynamic Load

Wind and Seismic Loads: For tall boilers or equipment installed in open-air settings, wind loads—such as wind pressure coefficients and windward projected areas—and seismic intensity—including horizontal and vertical seismic forces—must be taken into account. Rigid beams shall transmit dynamic loads to the foundation to prevent structural resonance.

Vibration Loads: Vibrations generated during the operation of equipment such as fans and pumps may be transmitted through piping to the enclosure. Rigid beams should be used to increase structural stiffness and minimize vibrational deformation.

II. Structural and Mechanical Design

1. Cross-Section and Material Selection

Section type: Select structural steel (angle steel, channel steel, I-beam) or composite sections (plate‑welded H‑sections) based on the magnitude of the applied load; for high‑load scenarios, a box‑section may be employed to enhance torsional stiffness.

For example, flue gas ducts typically use 10# to 20# channel steel for their rigid beams, while furnace chamber rigid beams, due to the heavy loads, may employ double‑spliced I‑beams.

Material Strength:

In ambient-temperature environments, carbon steels such as Q235B and Q355B are recommended; in high-temperature environments (e.g., the furnace’s high-temperature zone), heat-resistant steels—such as 15CrMo and 12Cr1MoV—should be used to prevent strength degradation under prolonged exposure to elevated temperatures.

Under the trend of green transformation, high-strength steels (such as Q460) can be used to reduce material consumption and lower the structure’s self-weight.

2. Arrangement Method and Spacing

Circumferential and Axial Arrangement:

Circumferential beams are uniformly distributed around the shell’s circumference, with typical spacings of 1.5 to 3 meters; the exact spacing shall be determined based on the shell diameter and external pressure calculations (larger diameters require smaller spacings).

Axial beams are perpendicular to circumferential beams, forming a grid structure that is used in large-diameter shells—such as furnace chambers with diameters exceeding 4 meters—to enhance overall structural stability.

Spacing Calculation: The critical buckling pressure of the shell is determined using Euler’s formula, ensuring that the spacing between stiffening beams remains below the critical value. For example, for thin-walled shells (with wall thickness ≤ 10 mm), the beam spacing typically does not exceed twice the square root of the shell diameter.

III. Thermal Expansion and Displacement Compensation

1. Thermal Expansion Gap Design

During boiler operation, the shell undergoes thermal expansion due to rising temperatures (e.g., when the furnace temperature exceeds 1000°C, linear expansion can reach several tens of millimeters). Therefore, the connection between the rigid beams and the shell must either provide an expansion gap or employ sliding supports.

Long oval holes are provided at the bottom of the sliding bearings to permit axial movement of the girder, thereby preventing additional stresses caused by thermal expansion and contraction that could lead to weld cracking.

Flexible connection structure: In the high-temperature zone, the rigid beam may adopt a combined “hinge–spring” configuration, which can both transmit loads and accommodate thermal expansion.

2. Expansion Direction Simulation

Finite element analysis (FEA) is used to simulate the expansion of the shell in all directions, enabling the determination of optimal constraint locations for the stiffening beams and preventing structural deformation caused by improper restraint. For example, the stiffening beams at the four corners of the furnace chamber should permit radial expansion of the shell while constraining only displacements in critical directions.

IV. Welding and Connection Design

1. Welding Process Requirements

Weld Strength: The welds between the rigid beam and the shell shall be continuous fillet welds or butt welds, with a weld throat not less than 1.2 times the thickness of the thinner member, to ensure effective load transfer.

Deformation‑prevention measures: During the welding of large rigid beams, thermal deformation is prone to occur; therefore, processes such as symmetrical welding and segmented welding should be employed, followed by post‑weld stress‑relief treatments (e.g., annealing).

2. Connection Method Selection

Welding vs. Bolted Connections:

Welded joints offer high strength and low cost, making them suitable for non‑removable applications (e.g., furnace‑chamber stiffening beams).

Bolted connections (such as high-strength bolts) facilitate installation and maintenance, making them suitable for flues or rigid beams of storage tanks that require disassembly; however, anti-loosening measures—such as locknuts and lock washers—must be implemented.

V. Corrosion and Protection Design

1. Material Corrosion-Resistant Matching

When exposed to outdoor conditions or corrosive media (such as flue gas after desulfurization and denitrification), rigid beams shall be fabricated from corrosion‑resistant materials (e.g., stainless steel, galvanized steel) or coated with protective finishes (e.g., epoxy zinc‑rich paint, polyurea coating) to prevent electrochemical corrosion arising from material mismatches between the shell and the beam.

2. Protective Layer Design

Rigid beams covered with insulation must be sealed to prevent moisture ingress and subsequent internal corrosion; in coastal areas, equipment should be upgraded to a higher corrosion‑protection grade (e.g., by increasing coating thickness to 200 μm or more).

VI. Ease of Installation and Maintenance

1. Modular Prefabrication

Rigid beams with complex structures can be fabricated using modular prefabrication—such as factory-welded grid units—to reduce on-site installation effort and enhance accuracy, with prefabricated units typically controlled to within ±5 mm.

2. Provision for Maintenance Space

The rigid beam layout shall avoid equipment access openings, instrument interfaces, and other such locations, while also providing sufficient clearance for subsequent nondestructive testing (e.g., ultrasonic inspection of welds) and corrosion‑protection maintenance.

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