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

The design of rigid beams in boiler steel structures requires comprehensive consideration of mechanical properties, equipment operating environment, material characteristics, and other factors to ensure that they can enhance shell stiffness and adapt to complex operating conditions. The following is an analysis of the core design elements:

I. Load Condition Analysis

1. Static Load

Shell and medium weight: It is necessary to calculate the self-weight of the shell, the weight of the internal medium (such as water, steam, flue gas), and the load of the insulation layer and accessories (pipes, platforms) borne by the rigid beam to ensure that the beam strength is sufficient to support the vertical gravity.

Internal pressure / External pressure load:

The combustion pressure (positive or negative pressure) in the boiler furnace and the negative pressure in the flue (induced draft fan suction) will generate hoop or axial pressure on the shell. The rigid beam needs to disperse the pressure through structural design to prevent shell buckling.

For example: The instantaneous pressure during furnace explosion can reach 0.2~0.5MPa, and the rigid beam needs to resist this impact force and limit the shell deformation.

2. Dynamic Load

Wind load and seismic load: Tall boilers or outdoor equipment need to consider wind load (such as wind pressure coefficient, windward area) and seismic intensity (horizontal/vertical seismic force). The rigid beam needs to transfer the dynamic load to the foundation to avoid structural resonance.

Vibration load: The vibration during the operation of equipment such as fans and pumps may be transmitted to the shell through pipes. The rigid beam needs to enhance the structural stiffness to reduce vibration deformation.

II. Structural and Mechanical Design

1. Section and Material Selection

Section form: Select steel sections (angle steel, channel steel, I-beam) or composite sections (steel plate welded H-beam) according to the load size. Box sections can be used in high-load scenarios to enhance torsional stiffness.

Example: 10#~20# channel steel is commonly used for flue rigid beams, while double I-beams may be used for furnace rigid beams due to the large load.

Material strength:

Q235B and Q355B carbon steel are selected for normal temperature environments; heat-resistant steel (such as 15CrMo, 12Cr1MoV) is required for high-temperature environments (such as high-temperature areas in the furnace) to avoid strength degradation under long-term high temperatures.

Under the trend of green transformation, high-strength steel (such as Q460) can be used to reduce material usage and self-weight.

2. Arrangement and Spacing

Circumferential and axial arrangement:

Circumferential beams are evenly distributed along the circumference of the shell, with a spacing of usually 1.5~3m, which needs to be determined according to the shell diameter and external pressure calculation (the larger the diameter, the smaller the spacing);

Axial beams are perpendicular to circumferential beams, forming a grid structure, used for large-diameter shells (such as furnace with diameter > 4m), enhancing overall stability.

Spacing calculation: Calculate the critical buckling pressure of the shell using the Euler formula to ensure that the spacing of the rigid beams is less than the critical value. For example, the spacing of rigid beams for thin-walled shells (thickness ≤10mm) generally does not exceed 2 times the square root of the shell diameter.

III. Thermal Expansion and Displacement Compensation

1. Thermal Expansion Gap Design

When the boiler is operating, the shell expands due to temperature increase (e.g., the furnace temperature reaches over 1000℃, and the linear expansion amount can reach tens of millimeters). The connection between the rigid beam and the shell needs to reserve an expansion gap or use a sliding support:

A long oval hole is set at the bottom of the sliding support to allow the beam to slide axially, avoiding additional stress caused by thermal expansion and contraction leading to weld cracking.

Flexible connection structure: In high-temperature areas, the rigid beam can adopt a "hinge + spring" combination structure, which can both transmit load and absorb thermal displacement.

2. Expansion Direction Simulation

Simulate the expansion amount in various directions of the shell through finite element analysis (FEA) to determine the constraint point position of the rigid beam, avoiding structural deformation due to improper constraints. For example, the rigid beams at the four corners of the furnace need to allow the shell to expand in all directions, only constraining the displacement in key directions.

IV. Welding and Connection Design

1. Welding Process Requirements

Weld strength: The welding of the rigid beam and the shell needs to use continuous fillet welds or butt welds, and the weld height should not be less than 1.2 times the thickness of the thinner component to ensure effective load transfer.

Deformation prevention measures: Large rigid beams are prone to thermal deformation during welding. Symmetrical welding, segmented welding, etc., should be used, and stress relief treatment (such as annealing) should be carried out after welding.

2. Connection Method Selection

Welding vs. bolted connection:

Welding connection has high strength and low cost, suitable for non-disassembly scenarios (such as furnace rigid beams);

Bolted connection (such as high-strength bolts) is convenient for installation and maintenance, suitable for flue or tank rigid beams that need to be disassembled, but attention should be paid to anti-loosening design (such as anti-loosening nuts, stop washers).

V. Corrosion and Protection Design

1. Material Corrosion Matching

For outdoor equipment or equipment in contact with corrosive media (such as flue gas after desulfurization and denitrification), the rigid beam should use corrosion-resistant materials (such as stainless steel, galvanized steel) or surface coatings (epoxy zinc-rich paint, polyurea coating) to avoid electrochemical corrosion between the shell and the beam due to material differences.

2. Protective Layer Design

The insulation layer covering the rigid beam must be sealed to prevent water vapor from penetrating and causing internal rust; coastal area equipment needs to enhance the anti-corrosion level (such as increasing the coating thickness to more than 200μm).

VI. Installation and Maintenance Convenience

1. Modular prefabrication

Complex structured rigid beams can adopt modular prefabrication (such as factory welding into grid units), reducing on-site installation workload and improving precision (such as prefabricated unit error control within ±5mm).

2. Reserved space for inspection

The rigid beam layout should avoid equipment maintenance openings, instrument interfaces, etc., and reserve operating space for subsequent flaw detection (such as ultrasonic testing of welds) and anti-corrosion maintenance.