GB/T 8625-2005Test method of difficult-flammability for building materials (English PDF)

3.1 Afterflame

Flaming combustion that persists after the ignition source has been removed.

[SOURCE. ISO 13943.2023, 3.12]

3.2 Afterglow

Phenomenon in which the combustion zone continues to glow after the ignition source has been removed and any flaming combustion has ceased.

[SOURCE. ISO 13943.2023, 3.14, modified]

3.3 Smoldering

Slow combustion of a material without visible light.

NOTE. Typically accompanied by the generation of flue gas and a rise in temperature.

[SOURCE. GB/T 5907.1-2014, 2.28, modified]

4.1 Vertical combustion furnace

4.1.1 General

The vertical combustion furnace consists of a combustion chamber, burner, specimen holder, stable airflow layer, and flue. Its external dimensions are (1000 ± 40) mm × (1000 ± 40) mm × (3930 ± 40) mm, as shown in Figures 1 and 2.

4.1.2 Combustion chamber

The combustion chamber consists of furnace walls and furnace doors; the internal dimensions of the chamber are (800 ± 20) mm × (800 ± 20) mm × (2 000 ± 20) mm. The furnace walls feature an insulated sandwich structure comprising, from inside to outside. a 2 mm thick stainless-steel plate, a (6 ± 2) mm thick refractory fiber blanket, two layers of rock wool insulation [each (40 ± 3) mm thick], a (10 ± 2) mm thick calcium silicate board, and a 2 mm thick stainless-steel plate.

The furnace doors are divided into upper and lower sections, each connected to the furnace body by hinges; the door structure matches that of the furnace walls (see Figure 2). A tight seal against the furnace body is achieved for both doors through the combined action of handwheels and securing screws.

Observation windows are provided on both the upper furnace door and the rear wall of the combustion chamber.

4.1.3 Burner

The burner (see Figure 3) has a square configuration measuring (200 ± 2) mm × (200 ± 2) mm.

Each of the four side tubes of the burner is fitted with eight stainless-steel nozzles with a diameter of (3.5 ± 0.1) mm; the upper edges of all nozzles must lie in the same horizontal plane, with a levelness deviation not exceeding ±0.5 mm. The burner shall be installed horizontally at the center of the combustion chamber, at a height of (1 000 ± 10) mm above the furnace floor.

The gas used for the burner is a mixture of air and methane with a purity of no less than 99%.

When using volumetric flow meters, the volumetric flow rates under standard conditions (0°C,

101.325 kPa) are (35.0 ± 0.5) L/min for methane and (17.5 ± 0.2) L/min for air; when using mass flow controllers, the mass flow rates are (418 ± 6) mg/s for methane (with density of 0.717 g/L under standard conditions) and (378 ± 4) mg/s for air (with density of 1.293 g/L under standard conditions).

The stable airflow layer consists of a square frame made of angle steel, positioned horizontally directly beneath the burner, see Figure 2.A wire mesh is laid at the bottom of the frame; and multiple layers of glass fiber mat are placed over the mesh. To establish uniform airflow within the combustion chamber, air is introduced through a duct with a diameter of (200 ± 10) mm at the lower part of the furnace body; the supplied air shall be maintained at a constant temperature and flow rate.

4.1.6 Flue

The flue of the vertical combustion furnace is a square duct with a cross-sectional area of (500

± 10) mm × (500 ± 10) mm. The lower part of the flue connects to the combustion chamber, while the upper part connects to the exhaust system.

4.2 Exhaust system

The exhaust system connects to the flue. To maintain negative pressure within the vertical combustion furnace throughout the test, the exhaust capacity of the system shall be adjustable.

4.3 Measuring instruments

4.3.1 Flow meter

4.3.1.1 Mass flow controllers or volumetric flow meters shall be used to control the flow rates of methane and compressed air. If mass flow controllers are used, the measurement range shall be at least 0 mg/s ~ 520 mg/s for methane and 0 mg/s ~ 470 mg/s for air, with a reading accuracy of 2%. If volumetric flow meters are used, the maximum measurement value shall be no less than 40 L/min for methane and no less than 20 L/min for air, with an accuracy class of 2.5 or better.

4.3.1.2 An orifice flow meter, a bidirectional pressure probe, or a vane anemometer shall be used to measure the airflow rate entering the vertical combustion furnace. The flow measurement device shall be installed in a straight section of the air supply pipe with a smooth inner surface, with straight pipe lengths of at least five times the pipe's inner diameter on both sides of the device. The measurement accuracy for the flow rate shall be no less than 0.4 m³/min.

4.3.2 Thermocouple

4.3.2.1 The insulated K-type sheathed thermocouple meeting the Class 2 tolerance requirements

of GB/T 16839.1 shall be used to measure the temperature of flue gas and furnace wall. The thermocouple wire diameter shall be 0.5 mm, and the outer diameter shall be no greater than 3 mm.

4.3.2.2 Five thermocouples of the same diameter shall be used to measure flue gas temperature;

they shall be installed at a height of (1 560 ± 10) mm above the top of the burner, see Figure

2a).

4.3.2.3 Two thermocouples of the same diameter shall be used to measure furnace wall temperature; they shall be installed at a height of (460 ± 10) mm above the top of the burner, see Figure 2b).

4.3.2.4 One thermocouple shall be used to measure the temperature of the air entering the vertical combustion furnace; the thermocouple is installed at the same height as the centerline of the air inlet pipe, with the temperature measurement point positioned at the center of the vertical combustion furnace.

4.3.3 Furnace pressure test tube

The furnace pressure test tube shall have a T-shaped structure, with an internal tube diameter of (10 ± 2) mm and a head width of (100 ± 5) mm. The T-shaped furnace pressure test tube shall be installed at a position (2 700 ± 10) mm above the furnace bottom and (100 ± 5) mm away from the inner wall of the flue.

The furnace pressure test tube shall be connected to a pressure sensor with an accuracy of at least 0.5%.

4.3.4 Anemometer

A hot-sphere anemometer or another anemometer with equivalent accuracy, with an error not exceeding 10%, shall be used to measure the air velocity above the wire mesh of the stable airflow layer.

4.3.5 Data acquisition system

The data acquisition system shall be capable of automatically recording the output values from the thermocouples, furnace pressure test tube, etc. The measurement accuracy shall be ±0.5°C for temperature data, ±0.1 s for time data, and ±0.1% of the full-scale output value for other data.

5.1 Thermal load uniformity test

Place four stainless-steel plates (1 000 mm × 190 mm × 3 mm) on the specimen holder. Securely mount one insulated K-type sheathed thermocouple on the back of each plate, positioned on the centerline at a height of 200 mm from the bottom. Conduct the test in accordance with the provisions of Clause 8.During the test, the deviation of the temperature measured by each of the four thermocouples on the backs of the plates from their average value shall not exceed

30°C. After 10 min of testing, the average temperature measured by the four thermocouples on the stainless-steel plates shall be (540 ± 15) °C; and the average temperature measured by the five thermocouples in the flue shall be (105 ± 5) °C. Otherwise, the apparatus shall be adjusted.

......