The portable tank instructions T1 to T22 apply to liquid and solid substances of class 1 and classes 3 to 9. The general provisions of section 4.2.1 and the requirements of section 6.7.2 shall be met. The instructions for portable tanks with FRP shells apply to substances of classes or divisions 1, 3, 5.1, 6.1, 6.2, 8 and 9. Additionally, the provisions of chapter 6.10 apply to the portable tanks with FRP shells. T21 Minimum test pressure (bar): 10 Minimum shell thickness (in mm - reference steel) (see 6.7.2.4): 10 mm Pressure-relief provisions *) (see 6.7.2.8): Normal Bottom opening provisions **) (see 6.7.2.6): Not allowed *) When the word "Normal" is indicated, all the requirements of 6.7.2.8 apply except for 6.7.2.8.3. **) When the word "Not allowed" is indicated, bottom openings are not permitted when the substance to be transported is a liquid (see 6.7.2.6.1). When the substance to be transported is a solid at all temperatures encountered under normal conditions of transport, bottom openings conforming to the provisions of 6.7.2.6.2 are authorized.