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ISO 28721-4:2015 — Quality Requirements for Glass-Lined Pipes and Fittings
Vitreous and porcelain enamels — Part 4: Quality requirements for glass-lined flanged steel pipes and fittings
1. Scope and Applicability of ISO 28721-4
ISO 28721-4:2015 specifies comprehensive quality requirements for glass-lined flanged steel pipes and fittings used in chemical process plants and similar industrial applications where corrosion-resistant piping systems are essential. It complements the general quality requirements of ISO 28721-1 by addressing the specific challenges of applying and maintaining glass-lined coatings inside narrow bore pipes, at flange faces, and on complex fitting geometries. The standard covers straight pipe sections, elbows, tees, reducers, crosses, and special fittings within the dimensional framework of DIN 2873, which specifies the standard series for glass-lined piping components.
Glass-lined piping systems offer the unique combination of glass-like chemical resistance across virtually the entire pH range (except hydrofluoric acid and hot concentrated phosphoric acid) with the mechanical strength of carbon steel, at a cost that is competitive with high-alloy metallic piping materials for large-diameter corrosive service. They are particularly cost-effective for pipe diameters exceeding DN 80 where the cost of alternatives such as stainless steel, Hastelloy, or titanium becomes prohibitive.
The standard applies to nominal pipe sizes from DN 25 (1 inch) to DN 600 (24 inches), with pressure ratings typically PN 10 (1.0 MPa) for larger diameters and PN 25 (2.5 MPa) for smaller diameters per DIN 2873. The glass-lined piping system is designed to handle the full range of process chemicals at temperatures up to 230 °C and pressures up to the rated value, with full vacuum capability at elevated temperatures subject to the thermal shock limitations of ISO 28721-3.
2. Material and Manufacturing Requirements
The steel substrates for glass-lined pipes and fittings must be selected from materials suitable for the enamelling process, as specified in EN 13480-2. Suitable steel grades typically include P235GH, P265GH, and similar low-carbon steel grades with controlled carbon content (maximum 0.20 %) and silicon content (maximum 0.35 %) to ensure proper enamel adhesion and minimize defect formation during firing. All welds must be full-penetration welds conforming to ISO 5817 Class B quality, the highest quality classification for fusion-welded joints. The joint coefficient must be at least 85 % per EN 13480-3, ensuring that the welded joints retain sufficient strength relative to the base metal. Surface preparation before enamelling requires blast-cleaning to Sa 2½ grade per ISO 8501-1, which is the near-white metal standard providing a clean surface with a minimum surface roughness profile suitable for enamel adhesion.
| Parameter | Requirement | Reference Standard |
|---|---|---|
| Steel grade | Low-carbon steel (C ≤ 0.20 %) | EN 13480-2 |
| Welding quality | Class B (full penetration) | ISO 5817 |
| Joint coefficient | ≥ 85 % | EN 13480-3 |
| Surface preparation | Sa 2½ (near-white metal) | ISO 8501-1 |
| Dimensions and tolerances | Per DIN 2873 series | DIN 2873 |
Weld quality is particularly critical for glass-lined pipes. Any weld defects such as porosity, slag inclusions, or incomplete penetration will be amplified during the high-temperature enamelling firing process (typically 800-900 °C), potentially causing severe enamel defects at the weld zone. It is recommended that all welds be inspected by radiographic or ultrasonic testing before enamelling to ensure Class B quality.
3. Enamel Coating Quality Requirements
The enamel coating thickness on glass-lined pipes and fittings must be maintained within 0.8 mm to 2.0 mm, with a minimum of 0.6 mm permitted at locations with small radii such as the internal corners of fittings and the bore transitions at flange connections. This thickness range is slightly thinner than the 1.0-2.2 mm specified for vessels in ISO 28721-1, reflecting the greater difficulty of applying uniform enamel coatings inside narrow pipes where drainage effects, restricted access for spray application, and complex flow patterns during firing create additional challenges. The coating must be free of cracks, bubble lines (continuous chains of bubbles that could interconnect to form leakage paths), improperly fused areas (where the enamel has not fully melted and flowed to form a continuous glass layer), and fireclay particles or other contamination that could compromise chemical resistance.
Flange faces require special attention because they serve as the sealing surfaces for the piping system. The enamel coating on flange faces must be smooth and continuous, with no defects that could create leakage paths when the flange joint is assembled with a gasket. The standard allows for the flange face to be either fully enamel-coated or ground flat to expose the steel face, depending on the manufacturer’s design and the service requirements. When the flange face is ground flat for gasket sealing, the exposed steel face must be protected from corrosion by the gasket material and the flange bolting torque must be carefully controlled to prevent damage to the enamel on the adjacent surfaces.
When designing glass-lined piping systems, pay special attention to the gasket selection. The gasket must be soft enough to seal effectively against the glass-lined flange face without damaging the enamel, yet chemically resistant to the process fluid at the operating temperature. PTFE envelope gaskets or flexible graphite gaskets with a suitable filler are the most commonly used options. The bolting torque for glass-lined flanges is typically 30-50 % lower than for equivalent metallic flanges to prevent over-compression of the gasket and damage to the enamel facing.
4. Testing and Certification Requirements
Each glass-lined pipe or fitting must undergo a comprehensive testing regimen. Visual inspection under adequate lighting identifies surface defects. The high-voltage spark test is performed at 20 kV as a preliminary factory test to detect gross defects, and at 12 kV as the acceptance test after final handling and inspection. Coating thickness is measured per ISO 2178 with a measurement uncertainty less than 5 %. Chemical corrosion resistance is verified through testing per ISO 28706-2 (boiling HCl for acid resistance) and ISO 28706-4 (cylindrical vessel method for alkaline resistance), or ISO 28706-5 (autoclave method) where elevated temperature service is specified. A test report type 2.2 per EN 10204 is provided, documenting the manufacturing inspection results and confirming compliance with the standard’s requirements. Marking includes the manufacturer’s identification mark, nominal inner diameter, pressure rating, and the designation “ISO 28721-4”.
In glass-lined piping systems, the most common failure points are the flange joints and the transition areas between straight pipe and fittings. During system design, ensure that all flanged connections are accessible for maintenance and re-torquing, and provide adequate pipe supports to prevent excessive bending moments at flange connections. Following ISO 28721-3 thermal shock limits is essential for piping systems that undergo thermal cycling during normal operation.
5. Frequently Asked Questions
Q1: Can glass-lined pipes be cut or modified in the field?
A: No. Glass-lined pipes are factory-manufactured to exact specified lengths, and the enamel coating is applied and fired at high temperatures (800-900 °C) in controlled conditions. Field cutting, welding, or bending would destroy the glass lining and cannot be re-enamelled on site. Any field modifications require removal of the glass lining, re-welding, and factory re-enamelling of the modified section. It is essential to have accurate piping isometric drawings prepared before ordering to ensure all pipe lengths and fitting configurations are correct.
A: No. Glass-lined pipes are factory-manufactured to exact specified lengths, and the enamel coating is applied and fired at high temperatures (800-900 °C) in controlled conditions. Field cutting, welding, or bending would destroy the glass lining and cannot be re-enamelled on site. Any field modifications require removal of the glass lining, re-welding, and factory re-enamelling of the modified section. It is essential to have accurate piping isometric drawings prepared before ordering to ensure all pipe lengths and fitting configurations are correct.
Q2: What gasket materials are recommended for glass-lined flanged joints?
A: PTFE envelope gaskets (with a compressible filler such as flexible graphite or ceramic fibre) are the most common choice, offering broad chemical resistance and sufficient compressibility to seal without damaging the enamel flange face. Flexible graphite gaskets with a stainless steel inner ring are also suitable for higher temperature services. Standard compressed asbestos-free (CAF) gaskets are generally not recommended as they may be too hard for the glass-lined flange surface. The gasket must completely cover the flange face and should not extend into the pipe bore.
A: PTFE envelope gaskets (with a compressible filler such as flexible graphite or ceramic fibre) are the most common choice, offering broad chemical resistance and sufficient compressibility to seal without damaging the enamel flange face. Flexible graphite gaskets with a stainless steel inner ring are also suitable for higher temperature services. Standard compressed asbestos-free (CAF) gaskets are generally not recommended as they may be too hard for the glass-lined flange surface. The gasket must completely cover the flange face and should not extend into the pipe bore.
Q3: What is the difference between the 12 kV and 20 kV high-voltage tests?
A: The 20 kV test is a preliminary factory test conducted before final assembly and handling. Its higher voltage is designed to detect gross defects such as complete exposure of the steel substrate, large pinholes, or thin spots in the enamel coating. The 12 kV test is the acceptance test performed after all handling, inspection, and preparation for shipment are complete. The lower voltage reflects the practical consideration that a second 20 kV test could damage sound enamel that has been stressed during handling. Both tests are essential quality control steps in the manufacturing process.
A: The 20 kV test is a preliminary factory test conducted before final assembly and handling. Its higher voltage is designed to detect gross defects such as complete exposure of the steel substrate, large pinholes, or thin spots in the enamel coating. The 12 kV test is the acceptance test performed after all handling, inspection, and preparation for shipment are complete. The lower voltage reflects the practical consideration that a second 20 kV test could damage sound enamel that has been stressed during handling. Both tests are essential quality control steps in the manufacturing process.
Q4: What is the maximum operating pressure for glass-lined piping systems?
A: The maximum operating pressure depends on the nominal diameter and the pressure rating per DIN 2873. Typical ratings are PN 10 (1.0 MPa or 10 bar) for nominal diameters DN 100 and above, and PN 25 (2.5 MPa or 25 bar) for smaller diameters up to DN 80. The actual maximum allowable working pressure (MAWP) must consider the operating temperature, as the pressure rating decreases at elevated temperatures per the applicable pressure vessel code. Always consult the manufacturer’s pressure-temperature rating chart for the specific pipe size and enamel grade being used.
A: The maximum operating pressure depends on the nominal diameter and the pressure rating per DIN 2873. Typical ratings are PN 10 (1.0 MPa or 10 bar) for nominal diameters DN 100 and above, and PN 25 (2.5 MPa or 25 bar) for smaller diameters up to DN 80. The actual maximum allowable working pressure (MAWP) must consider the operating temperature, as the pressure rating decreases at elevated temperatures per the applicable pressure vessel code. Always consult the manufacturer’s pressure-temperature rating chart for the specific pipe size and enamel grade being used.
