Steel Plate Air Duct Testing

Steel plate air duct inspection mainly conducts scientific testing and analysis on the appearance and dimension inspection, material and thickness inspection, connection strength and sealing testing, mechanical strength and stiffness testing, anti-corrosion coating performance testing and fire resistance testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Appearance and dimension inspection: Mainly inspects the surface quality, flatness and geometric dimensions of the air duct plates. The inspection content includes checking whether there are the defects such as the cracks, dents, corrosion and galvanized layer peeling, and measuring the side lengths, diagonal differences and end face flatness of the air ducts, ensuring that they meet the tolerance requirements of the design drawings and the relevant standards (such as GB 50243). It is the basis for guaranteeing the installation quality of the air ducts.

Material and thickness inspection: Through the sampling or with the professional thickness gauges, the designations, models and actual thicknesses of the materials used for the steel plate air ducts are verified. It needs to be confirmed whether the steel plates are the specified galvanized steel plates, stainless steel plates or ordinary carbon steels, and the micrometers or ultrasonic thickness gauges are used for the multi-point measurement, ensuring that the plate thicknesses are not lower than the lower limits of the design and specifications (such as GB/T 12754), to meet the structural strength and durability requirements of the air ducts.

Connection strength and sealing testing: Evaluates the mechanical strength and sealing performance of the connection parts of the air ducts (such as the flanges, seamed joints and welds). The tensile testing is carried out on the seamed and riveted parts to check their firmness; for the flange connections, the light leakage method or air leakage testing can be adopted to check the tightness of the sealing gaskets, preventing the air leakage, noise or energy losses during the system operation.

Mechanical strength and stiffness testing: Verifies the ability of the air duct to resist the deformation and damage under the working pressures (positive or negative pressures). The static pressure testing is usually carried out in the laboratory or on site, observing whether there are the permanent deformations, cracks or connection failures on the air duct walls under the standard test pressures. Its strength and stiffness indicators need to meet the provisions of the construction quality acceptance specifications for the ventilation and air conditioning works such as GB 50243.

Anti-corrosion coating performance testing: For the air ducts with the special anti-corrosion requirements (such as the painted and epoxy resin coated), the adhesion, thickness and corrosion resistance of their coatings need to be detected. The cross-cut method is adopted to test the adhesion, the coating thickness gauges are used to measure the dry film thickness, and the salt spray testing can be carried out to evaluate the corrosion resistance, ensuring that the coatings can effectively protect the substrates under the expected environments.

Fire resistance testing: For the air ducts of the smoke control and exhaust systems with the fire protection requirements, the fire resistance limit testing must be carried out in accordance with the relevant standards such as GB 17428 "Fire dampers for building ventilation and smoke exhaust systems". The air duct specimens are placed under the standard fire temperature-rise curve, testing whether they can maintain the structural integrity (no collapse) and insulation (the temperature rise of the unexposed surface not exceeding the limit) within a certain period.

Testing Scope

Metal rectangular air ducts: This is the most common form of the steel plate air ducts, and the testing scope covers the whole process of their fabrication, connection and installation. Including all the components such as the straight duct sections, elbows, tees and reducers, the testing emphatically checks their seaming forms (such as the combined angle seams and button punch seams), flange flatness and bolt spacings, ensuring the air tightness and structural stability of the whole system.

Metal circular air ducts: Mostly used for the high-speed air supply systems or industrial exhaust. The testing focuses on the forming quality, seam tightness and the firmness of the spigot connections of the spiral air ducts. The continuity of the spiral welds and the concentricity between the circular flanges and the air ducts need to be checked, preventing the vibrations and leakages caused by the eccentricity.

Composite material air ducts (metal inner/outer walls): The testing includes the composite air ducts with the steel plate linings or outer sheaths, such as the metal flange connection parts of the phenolic aluminium foil composite air ducts. In addition to the routine dimension testing, the bonding strength between the metal layers and the core materials, and the air tightness at the connections need to be particularly focused on, preventing the peeling of the composite layers.

Special air ducts for the smoke control and exhaust systems: The testing scope of this type of air ducts strictly follows the fire protection regulations. In addition to the routine items, the core lies in the verification of the fire resistance grades of the materials, whether the flange gaskets are the non-combustible materials (such as the ceramic fibre gaskets), and the tightness testing of the fire stopping at the places where the air ducts pass through the fire partition walls and floors.

Air ducts for the clean air conditioning systems: Used for the environments such as the clean rooms, with the extremely strict testing standards. The scope includes the cleanliness of the inner walls of the air ducts, the smoothness of the welds or seamed joints (to avoid the dust accumulation), the dust-proof and mould-proof properties of the flange sealing gaskets, as well as the final purging and air leakage testing, ensuring that they meet the cleanliness grade requirements.

Industrial process exhaust air ducts: Used to exhaust the corrosive, high-temperature or dust-laden gases. The testing scope is expanded according to the medium characteristics. For example, the intergranular corrosion tendency testing is carried out on the acid-resistant steel air ducts, the heat insulation layer thicknesses and fixing methods of the high-temperature air ducts are checked, and the reinforcement measures at the wear-prone parts of the wear-resistant air ducts are checked.

Testing Methods

Visual and rule inspection method: The most basic and important method. The surface defects of the air ducts are observed through the naked eyes or with the aid of the magnifying glasses, and the tools such as the steel tape measures, vernier callipers, right-angle rulers and spirit levels are used to directly measure the external dimensions, flatness, perpendicularity and flange diagonal differences. This method is intuitive and fast, and is the first choice of the on-site acceptance.

Light leakage method testing: Applicable to the preliminary inspection of the air tightness of the air ducts of the medium and low pressure systems. In the dark environment, a safety light source is set on one side inside the air duct, and the other side is observed. If the light can be observed at the seams and seamed joints, it indicates that there are the leak points. This method is simple and easy to implement, but is not accurate enough for the high-pressure or clean systems, and is usually used as the qualitative screening.

Air leakage testing method: A method for the quantitative detection of the air tightness of the air duct systems. A dedicated air leakage testing device is used. The air duct system is sealed, and under the action of the fan, a certain test pressure (such as 1000 Pa) is maintained. The air leakage is directly obtained by reading the flow meter at the outlet of the measuring device. The result needs to be compared and judged with the allowable values in the specifications (such as the values corresponding to the different pressure grades in GB 50243).

Ultrasonic thickness measurement method: A non-destructive testing method, used to measure the thicknesses of the steel plates, coatings or anti-corrosion layers. The ultrasonic probe transmits the sound waves on the material surface, and the thickness is calculated by measuring the time difference of the echo reflections between the upper and lower surfaces of the material. This method has the high precision, especially suitable for the installed air ducts or the situations where only one side is accessible.

Pressure testing method (strength and stiffness): The two ends of the air duct section are sealed, and the pressure source (such as the fan) and pressure gauges are connected. The pressure is slowly increased to 1.5 times the design working pressure or the test pressure specified in the specifications. The specified time is maintained, and it is observed whether there are the permanent deformations (monitored with the dial indicators for the deflections) or abnormal sounds on the air ducts. After the pressure relief, the residual deformations are checked, thereby evaluating their mechanical properties.

Laboratory fire resistance testing method: Carried out in a standardized fire resistance test furnace. The full-scale air duct specimens are fixed on the furnace body according to the actual installation methods, and the air supply state is simulated inside. According to the standard time-temperature curve (such as ISO 834), the heating is carried out, and the times when the air ducts lose the integrity and insulation are recorded, thereby determining their fire resistance limit grades.

Testing Standards

Domestic Standards:

1. GB 17428, Fire dampers for building ventilation and smoke exhaust systems

2. GB 50243, Code of acceptance for construction quality of ventilation and air conditioning works

3. GB/T 12754, Prepainted steel sheet and strip

4. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

5. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

6. GB/T 232-2010, Metallic materials — Bend test method

International Standards:

1. ISO 834, Fire-resistance tests — Elements of building construction

2. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Air leakage tester: This equipment is the core instrument of the air tightness testing of the air ducts, usually composed of a variable frequency fan, flow measuring device, micro differential pressure sensor and control system. It can precisely control and stabilize the test pressures, automatically measure and display the real-time air leakage, with the objective and reliable data, conforming to the requirements of the quantitative testing in the standards such as GB 50243.

Ultrasonic thickness gauge: Used for the fast and non-destructive measurement of the thicknesses of the steel plate substrates and coatings. The instrument contacts the tested surface through the probe, and directly displays the thickness values on the LCD screen by utilizing the ultrasonic pulse reflection principle. By selecting the suitable probes and couplants, the thicknesses of the galvanized layers, paint layers or the steel plates accessible from one side can be accurately measured.

Digital anemometer and pressure gauge (micro manometer): Used to measure the static pressure, dynamic pressure and wind speed in the air ducts, and are the auxiliary equipment for calculating the air leakage and evaluating the system performance. Combined with the Pitot tubes, the wind speed distributions of the air duct cross-sections can be measured at the multiple points, thereby evaluating whether the fabrication and installation quality of the air ducts affect the airflow organization.

Laser range finder and electronic spirit level: Used for the efficient and precise measurement of the installation dimensions and spatial positions of the air ducts. The laser range finder can quickly measure the lengths and installation elevations of the air ducts; the electronic spirit level is used to detect the levelness, perpendicularity and the flatness of the flange planes of the air ducts, with the precision far higher than the traditional spirit levels.

Coating thickness gauge and adhesion tester: The coating thickness gauges are divided into the magnetic method (measuring the non-magnetic coatings on the steel substrates) and eddy current method (measuring the insulating coatings on the non-ferrous metal substrates), used to measure the dry film thicknesses of the anti-corrosion coatings. The adhesion testers (such as the cross-cut testers and pull-off testers) are used to quantitatively or semi-quantitatively evaluate the bonding strength between the coatings and the steel plate substrates.

Material testing machine and hardness tester: Used to detect the mechanical properties of the air duct plates and connectors. The material testing machine can carry out the tensile and bending testing, obtaining the data such as the yield strength and tensile strength of the materials; the Leeb or Rockwell hardness testers can be used for the fast detection of the steel plate hardness, indirectly evaluating its material and work hardening conditions.