Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ISO 28721-3:2008 — Thermal Shock Resistance of Glass-Lined Chemical Apparatus
Vitreous and porcelain enamels — Part 3: Thermal shock resistance
1. Understanding Thermal Shock in Glass-Lined Equipment
ISO 28721-3:2008 specifies thermal shock resistance requirements for glass-lined chemical process apparatus operating within the temperature range of -25 °C to +230 °C. Thermal shock in glass-lined equipment occurs when a sudden temperature change creates differential expansion between the enamel coating and the steel substrate, generating stresses that can cause cracking, spalling, or complete delamination of the enamel layer. The standard distinguishes between two fundamentally different thermal shock scenarios: glass-lined side shock, where the sudden temperature change occurs on the product-contact surface (e.g., charging hot product into a cold vessel or cold product into a hot vessel), and steel side shock, where the temperature change occurs on the jacket side (e.g., switching heating medium to cooling medium or vice versa). Each scenario has different tolerance limits because the stress distribution through the enamel-steel composite structure differs significantly.
The two thermal shock scenarios have very different tolerance limits. Glass-lined side shock is generally more severe because the enamel surface experiences the full temperature differential instantaneously, placing the brittle enamel layer under maximum tensile or compressive stress. Steel side shock is moderated by the thermal mass and conductivity of the steel substrate, which buffers the temperature change reaching the enamel layer.
The key parameters used in thermal shock analysis are the wall temperature TW (the average temperature of the steel wall at the moment of shock), the product temperature TP (the temperature of the fluid being charged into or out of the vessel), and the heating/cooling medium temperature THC (e.g., steam at 0.6 MPa has a condensation temperature THC = 165 °C). These three parameters define the thermal shock severity and determine whether a given operating condition falls within the safe limits defined by the standard’s thermal shock diagrams.
2. Thermal Shock Diagrams and Operating Limits
ISO 28721-3 provides thermal shock diagrams that define the safe operating envelope for glass-lined equipment. The diagrams plot the wall temperature TW against the product temperature TP, with the region between the curves representing permissible operating conditions. For the scenario of hot product entering a cold apparatus, the allowable temperature differential decreases as the wall temperature drops, reflecting the increased brittleness of enamel at lower temperatures. For the reverse scenario of cold product entering a hot apparatus, the allowable temperature differential is more generous but still constrained by the tensile strength limits of the enamel coating.
| Scenario | TW (°C) | TP (°C) | ΔT (°C) |
|---|---|---|---|
| Hot product into cold apparatus | -20 | 130 | 150 |
| Hot product into cold apparatus | 0 | 150 | 150 |
| Hot product into cold apparatus | 20 | 165 | 145 |
| Hot product into cold apparatus | 40 | 180 | 140 |
| Cold product into hot apparatus | 120 | -25 | 145 |
| Cold product into hot apparatus | 140 | -5 | 145 |
| Cold product into hot apparatus | 160 | 20 | 140 |
| Cold product into hot apparatus | 180 | 50 | 130 |
Products with low heat-transfer coefficients such as gases, solid powders, viscous fluids, and non-Newtonian liquids may allow higher temperature differentials than shown in the standard diagrams. This is because the slower heat transfer rate reduces the instantaneous thermal stress on the enamel. However, any deviation from the standard limits must be agreed upon in writing between the manufacturer and the purchaser, with supporting thermal analysis documentation.
3. Heating and Cooling Procedures for Safe Operation
When the required temperature change exceeds the limits defined by the thermal shock diagrams, stepwise operation is essential to prevent enamel damage. The standard provides specific guidance for multi-step heating and cooling procedures. For example, when heating a cold apparatus (initial wall temperature 0 °C) with steam at 0.6 MPa (165 °C), the temperature differential of 165 °C exceeds the permissible single-step limit. The procedure requires: Step 1, introduce steam at a reduced pressure of 0.3 MPa (approximately 134 °C) until the product temperature exceeds 25 °C and the wall temperature has stabilized; Step 2, switch to full-pressure steam at 0.6 MPa and continue heating to the target temperature of 165 °C. This two-step procedure limits the maximum instantaneous thermal stress on the enamel by allowing intermediate temperature equalization.
For batch processes with frequent temperature changes, the operating sequence must be designed with the thermal shock diagrams as a primary reference. A well-designed sequence may include automatic interlocks that prevent charging product at temperatures outside the safe envelope. Many glass-lined equipment failures occur not during steady-state operation but during the transition phases of heating and cooling. Proper attention to thermal shock limits during process design can dramatically extend equipment service life.
The same stepwise principle applies to cooling operations. When cooling a hot apparatus with a cold jacket medium, the temperature must be reduced in controlled stages, allowing the wall temperature to equalize between stages. The standard recommends maximum cooling rates of 0.5 °C to 1.0 °C per minute for typical glass-lined equipment, with slower rates recommended for larger vessels and more sensitive enamel grades. The specific cooling procedure depends on the apparatus design, enamel type, and the thermal shock resistance classification.
4. Design Considerations Affecting Thermal Shock Resistance
The thermal shock resistance of a glass-lined apparatus depends on multiple interrelated factors including the enamel composition and its coefficient of thermal expansion, the steel grade and thickness, the apparatus design geometry (particularly at nozzles, dished ends, and jacket connections), the enamelling process quality, and the intended service conditions including temperature ranges and cycling frequency. ISO 28721-3 applies to apparatus designed per ISO 28721-1 using unalloyed or low-alloy carbon steel with minimum yield strength of 210 N/mm², operating at pressures from -0.1 MPa (full vacuum) to +0.6 MPa. Equipment designed outside these parameters requires individual thermal shock analysis.
When designing glass-lined equipment for cyclic thermal service, consider the cumulative effect of repeated thermal shocks. While a single shock within the diagram limits may not cause immediate failure, repeated cycling can lead to fatigue crack initiation and propagation in the enamel layer. For applications with more than 100 thermal cycles per year, consider reducing the maximum allowable temperature differential by 10-20 % to provide an additional safety margin against thermal fatigue.
5. Frequently Asked Questions
Q1: What is the fundamental difference between glass-lined side and steel side thermal shock?
A: Glass-lined side shock occurs when the product contacting the enamel surface undergoes a sudden temperature change, placing the enamel under direct compressive or tensile stress. Steel side shock occurs when the heating/cooling medium in the jacket changes temperature, with the stress transmitted through the steel wall to the enamel. Glass-lined side shock is generally more severe because the enamel experiences the temperature differential directly, without the buffering effect of the steel substrate. The standard provides separate thermal shock diagrams for each scenario.
A: Glass-lined side shock occurs when the product contacting the enamel surface undergoes a sudden temperature change, placing the enamel under direct compressive or tensile stress. Steel side shock occurs when the heating/cooling medium in the jacket changes temperature, with the stress transmitted through the steel wall to the enamel. Glass-lined side shock is generally more severe because the enamel experiences the temperature differential directly, without the buffering effect of the steel substrate. The standard provides separate thermal shock diagrams for each scenario.
Q2: Why is multi-step heating or cooling required when the temperature differential exceeds the diagram limits?
A: Multi-step operation allows the thermal stress to equalize across the enamel-steel composite structure at intermediate temperature plateaus. If a large temperature differential is applied in a single step, the instantaneous thermal stress can exceed the tensile strength of the enamel (typically 30-90 MPa), causing immediate crack formation. By introducing intermediate steps, the stress is distributed over time, allowing viscoelastic relaxation in the enamel and more uniform temperature distribution through the wall thickness.
A: Multi-step operation allows the thermal stress to equalize across the enamel-steel composite structure at intermediate temperature plateaus. If a large temperature differential is applied in a single step, the instantaneous thermal stress can exceed the tensile strength of the enamel (typically 30-90 MPa), causing immediate crack formation. By introducing intermediate steps, the stress is distributed over time, allowing viscoelastic relaxation in the enamel and more uniform temperature distribution through the wall thickness.
Q3: Does the -25 °C to +230 °C temperature range apply universally to all glass-lined equipment?
A: No, this range applies specifically to apparatus manufactured in accordance with ISO 28721-1 using unalloyed or low-alloy carbon steel with minimum yield strength of 210 N/mm². Equipment made from different materials or with different design parameters may have different temperature limits. Additionally, the standard’s thermal shock diagrams define safe operating transitions within this overall range, and not all transitions are permitted even within the -25 °C to +230 °C envelope.
A: No, this range applies specifically to apparatus manufactured in accordance with ISO 28721-1 using unalloyed or low-alloy carbon steel with minimum yield strength of 210 N/mm². Equipment made from different materials or with different design parameters may have different temperature limits. Additionally, the standard’s thermal shock diagrams define safe operating transitions within this overall range, and not all transitions are permitted even within the -25 °C to +230 °C envelope.
Q4: How is the wall temperature TW determined in practice?
A: The wall temperature TW is typically approximated as the temperature of the heating or cooling medium in contact with the steel side of the apparatus. For steam heating, TW is taken as the condensation temperature at the operating pressure. For liquid heating media, TW is the bulk fluid temperature. For electrical heating (heating mantles), TW is measured by thermocouples attached to the external steel surface. In critical applications, direct measurement via embedded thermocouples in the steel wall provides the most accurate TW determination.
A: The wall temperature TW is typically approximated as the temperature of the heating or cooling medium in contact with the steel side of the apparatus. For steam heating, TW is taken as the condensation temperature at the operating pressure. For liquid heating media, TW is the bulk fluid temperature. For electrical heating (heating mantles), TW is measured by thermocouples attached to the external steel surface. In critical applications, direct measurement via embedded thermocouples in the steel wall provides the most accurate TW determination.
