This part of ISO 28721 specifies requirements for the thermal shock resistance of, and heating and cooling procedures for, glass-lined apparatus, components, accessories and pipes primarily used for process equipment in chemical plants. It specifies the limits of thermal shock resistance using diagrams (see Figure 1 and Figure 2). For glass-lined steel, a distinction is made between a thermal shock on the glass-lined side (produced by charging an apparatus) and a thermal shock on the steel side (produced by heating or cooling an apparatus). This part of ISO 28721 applies to operating temperatures from - 25 °C to + 230 °C. It is only applicable to enamelled unalloyed and low-alloy carbon steels.