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ISO 28300: Venting of Atmospheric and Low-Pressure Storage Tanks
Normal and emergency vapour venting requirements for petroleum storage tanks
Introduction to ISO 28300: Tank Venting Standards
ISO 28300:2008, with its 2009 Technical Corrigendum, specifies the normal and emergency vapour venting requirements for aboveground liquid petroleum and petroleum-products storage tanks, as well as aboveground and underground refrigerated storage tanks. Developed by ISO/TC 67 (Materials, equipment and offshore structures for petroleum, petrochemical and natural gas industries), this standard covers tanks designed for operation at pressures from full vacuum through 103.4 kPa (ga) [15 psig].
Proper tank venting is essential for safety: during normal operations, vents prevent pressure buildup from thermal expansion and product transfer; during emergencies, they provide critical relief capacity for fire exposure scenarios, preventing catastrophic tank failure.
The standard addresses two fundamental venting scenarios: normal (operational) venting and emergency venting. Normal venting covers pressure and vacuum variations caused by liquid movement (filling/emptying) and thermal breathing (temperature changes). Emergency venting provides additional capacity for fire exposure or other abnormal conditions where heat input generates vapour at rates exceeding normal vent capacity.
| Venting Category | Cause | Design Basis | Typical Capacity |
|---|---|---|---|
| Normal – Inbreathing | Liquid outflow, thermal contraction | Maximum empty rate + thermal | Based on pump-out rate |
| Normal – Outbreathing | Liquid inflow, thermal expansion | Maximum fill rate + thermal | Based on pump-in rate |
| Emergency – Fire exposure | External heat from fire | Wetted surface area calculation | Per API 2000 / ISO equations |
| Emergency – Other | Equipment failure, process upset | Scenario-dependent | As determined by risk analysis |
Engineering Design Insights for Vent Sizing
The Technical Corrigendum 1:2009 made several important corrections to the original standard. Notably, it revised the pressure range in Clause 1 to clarify that the standard applies from “full vacuum through 103.4 kPa (ga)” — correcting the upper limit from 103.4 kPa (absolute) to gauge pressure. This distinction is critical for engineers sizing relief systems.
One of the most significant corrigendum changes was in Table 6 (Clause 4.3.3.3.3), where a condition was corrected to affect the equation selection for vent sizing calculations. Engineers must ensure they use the corrected version for all design work.
Heat Absorption and Wetted Surface Area
Figure B.1, which was replaced in the corrigendum, provides the critical relationship between wetted surface area and heat absorption during fire exposure. The X-axis represents wetted surface area (ft2 or m2), while the Y-axis shows heat absorption (BTU/h or W). The wetted surface area is the portion of the tank shell that is wetted by liquid and therefore subject to heat transfer during a fire. The total heat input to the tank determines the vapour generation rate and consequently the required emergency venting capacity.
The corrigendum also corrected the units for a variable in Equation (22) and revised variable naming (T to Ti, Z to Zi) for clarity. These corrections eliminate ambiguity in vent capacity calculations.
Reference to API 2000
ISO 28300 is harmonized with API Standard 2000, which is referenced in the revised bibliography. The 2009 corrigendum added “First edition” qualification to API 2000. Engineers working in jurisdictions that reference API standards should consult both documents for comprehensive guidance.
Practical Applications in the Petrochemical Industry
ISO 28300 is used worldwide for the design and sizing of tank venting systems on storage tanks containing flammable and combustible liquids. The standard applies to both new installations and modifications to existing tanks. Key design inputs include tank dimensions (diameter, height), product characteristics (vapour pressure, latent heat), ambient temperature range, fill/empty rates, and fire exposure scenarios. The venting system must protect the tank against both overpressure and vacuum conditions that could cause structural failure.
Frequently Asked Questions
Q1: What is the pressure range covered by ISO 28300?
A: The standard covers tanks designed for operation at pressures from full vacuum through 103.4 kPa gauge pressure (15 psig), as clarified by the 2009 corrigendum.
A: The standard covers tanks designed for operation at pressures from full vacuum through 103.4 kPa gauge pressure (15 psig), as clarified by the 2009 corrigendum.
Q2: What is the difference between normal and emergency venting?
A: Normal venting handles routine pressure variations from liquid transfer and thermal effects. Emergency venting provides additional capacity for fire exposure or other abnormal conditions that generate vapour at rates exceeding normal vent capacity.
A: Normal venting handles routine pressure variations from liquid transfer and thermal effects. Emergency venting provides additional capacity for fire exposure or other abnormal conditions that generate vapour at rates exceeding normal vent capacity.
Q3: How is emergency vent capacity determined for fire exposure?
A: Based on the wetted surface area of the tank shell, as shown in Figure B.1. The wetted area determines heat absorption, which determines vapour generation rate, which determines required vent capacity.
A: Based on the wetted surface area of the tank shell, as shown in Figure B.1. The wetted area determines heat absorption, which determines vapour generation rate, which determines required vent capacity.
Q4: Is ISO 28300 compatible with API standards?
A: Yes, ISO 28300 is harmonized with API Standard 2000. The corrigendum added the specific reference to API 2000 First edition in its bibliography.
A: Yes, ISO 28300 is harmonized with API Standard 2000. The corrigendum added the specific reference to API 2000 First edition in its bibliography.
