ISO 28721-5:2016

Overview - ISO 28721-5:2016 (Glass-lined apparatus: defects)

ISO 28721-5:2016 is part of the ISO 28721 series covering vitreous and porcelain enamels for glass-lined apparatus used in process plants. This Part 5 establishes a systematic cataloguing and characterization of detectable enamelling defects (visual and otherwise observable defects) and provides a consistent language for describing and recording those defects. It is limited to detectable defects, does not attempt to evaluate acceptability, and aligns its classifications with ISO 28721-1 where possible.

Key topics and technical content

• Terms and definitions: clear definitions for enamelling defect, reparable vs non-reparable defects, refiring, and re-enamelling.
• Defect taxonomy: structured descriptions of defect families, including:
  • Colour lines (strain lines, tearings, pearl lines, shore lines, drying cracks)
  • Boiling defects (origin, causes, classification and treatment suggestions)
  • Surface defects (rough enamel, underfired areas, eggshell)
  • Local defects (dimples, pinholes)
  • Contamination (chamotte/refractory clay, scale, ceramic fibres)
  • Colour deviations (water marks, colour stains)
  • Repaired areas (polished areas, enamel-covered areas after grinding)
• Origins and causes: for many defect types the standard describes likely origins (thermal/mechanical stress, incorrect material combinations, firing-control issues, weld issues, contamination).
• Classification approach: defects are categorized (e.g., reparable vs non-reparable) based on experience; non-reparable defects such as strain lines and tearings typically require re-enamelling.
• Suggested remedial actions: where applicable, guidance on defect treatment (e.g., polishing, refiring, re-enamelling) is provided, with emphasis on addressing root causes.

Practical applications - who uses ISO 28721-5:2016

• Manufacturers and enamellers: standardizes defect reporting during production of glass-lined vessels and pipes.
• Quality inspectors and third‑party auditors: provides a common defect vocabulary for inspections and acceptance reporting.
• Plant engineers and asset owners: aids assessment of in-service glass-lined equipment and selection of remedial measures.
• Repair and maintenance technicians: helps determine whether defects are reparable (polishing/refiring) or require full re-enamelling.
• Coating and materials engineers: supports root‑cause analysis and material/process optimization to reduce defect rates.

Related standards

• ISO 28721-1 - Quality requirements and acceptance criteria for glass-lined equipment (referenced for acceptance of glass-lined equipment).
• ISO 28721 Parts 2–4 - complementary parts covering chemical resistance, thermal shock resistance and quality requirements for flanged components.

Keywords: ISO 28721-5:2016, glass-lined apparatus, vitreous enamels, porcelain enamels, enamelling defects, defect characterization, re-enamelling, strain lines, pinholes, process plants.