How to test the quality of low-pressure boiler tubes?

Low-pressure boiler tubes are seamless steel pipes used to manufacture low-pressure boilers, with a pressure rating of 2.5 MPa or less. These pipes are typically produced from steel ingots or solid tube billets through perforation, hot rolling, cold rolling, or cold drawing processes. Low-pressure boiler tubes are primarily used for components such as superheated steam tubes, boiling water tubes, water-cooled wall tubes, smoke tubes, and arch brick tubes in low- and medium-pressure boilers.

Made from high-quality carbon structural steel, common materials for these tubes include No. 10 and No. 20 steel. These materials provide high strength, good plasticity, and can withstand various working conditions, making them suitable for transporting fluids in industrial and domestic boilers. So, what are the quality standards for these tubes, and how is their quality assessed?

Quality Standards for Low-Pressure Boiler Tubes

- Size and Shape
The outer diameter and wall thickness of low-pressure boiler tubes are subject to allowable deviation ranges. For instance:

Outer diameter D1: ±1.5% (minimum deviation of ±0.75 mm)

Outer diameter D2: ±1.0% (minimum deviation of ±0.50 mm)

Outer diameter D3: ±0.75% (minimum deviation of ±0.30 mm)

Outer diameter D4: ±0.50% (minimum deviation of ±0.10 mm)

- Chemical Composition
The chemical composition is strictly defined to ensure the tube's mechanical properties and resistance to corrosion. For example, No. 20 steel, a high-quality carbon structural steel, must have a carbon content of no more than 0.17%.

- Mechanical Properties
The mechanical properties, including tensile strength, yield strength, and elongation, are evaluated to confirm the tube’s ability to perform under various operating conditions.

- Metallographic Structure
The structure of the material at the microscopic level plays a crucial role in the strength and toughness of the boiler tube. The standard specifies the required metallographic structure for optimal performance.

- Nondestructive Testing
Nondestructive testing is employed to detect any internal or surface defects in the steel pipes, ensuring their quality and reliability without causing damage.

Methods for Quality Inspection of Low-Pressure Boiler Tubes

- Appearance Inspection
A visual inspection is conducted to assess the appearance of the tube. This includes checking the gloss of the outer surface, ensuring flatness, and identifying any visible defects like scratches, cracks, or deformations. Special attention is given to inspecting the ends for issues like desoldering, corner bending, depressions, or bulges.

- Dimension Detection
Measuring tools like calipers and micrometers are used to check the tube's diameter, wall thickness, length, and other critical dimensions. These measurements are compared to manufacturing standards to ensure they fall within the specified range.

- Internal Inspection
The internal quality of the tube is evaluated using fluid or optical detection methods. Fluid detection can assess the flow performance of liquids or gases within the pipe to detect potential leaks or blockages. Optical inspection, using tools like endoscopes, allows for a close examination of welds, edges, and internal surfaces for defects.

- Chemical Composition Analysis
The chemical composition of the boiler tube is analyzed to verify it meets the required standards. Spectral analysis, electron probe analysis, and other methods are commonly employed to determine the exact composition of the material.

- Stress Test
Stress testing, such as pressure and tensile tests, is used to determine whether the tube can withstand pressure without deforming. These tests ensure the tube’s stability and strength under operational conditions.