A vessel venting assessment was carried out for two identical 60,000-litre stainless steel ethanol storage tanks under same operating conditions. The objective of the assessment is to verify that the existing venting arrangement provides adequate protection against excessive pressure and vacuum conditions during both normal operation and emergency scenarios.
The assessment considered normal inbreathing and outbreathing, thermal effects, and emergency venting due to external fire exposure. Vent sizing calculations were primarily undertaken with reference to NFPA 30 and API 2000 (ISO 28300). BS EN 14015 was also considered because the tank General Arrangement drawing identified this as the original tank design standard.
Vessel Description
The assessment covered two identical vertical ethanol storage tanks, with the detailed calculation undertaken for one tank and the results considered applicable to the second, subject to confirmation of identical conditions. The existing tank venting arrangement comprises two main roof nozzle connections: 3in open atmospheric vent / vacuum vent connection for normal breathing and 8in bursting disc connection for emergency pressure relief.
Key design information included:
- Tank capacity: 60,000 litres
- Stored product: 99% ethanol
- Tank material: Stainless steel
- Operating pressure: Atmospheric
- Maximum filling rate: 30,000 litres per hour
- Design pressure: 400 mbar(g)
- Design vacuum: -20 mbar(g)
- Normal vent: 3-inch atmospheric vent
- Emergency vent: 8-inch bursting disc connection
Design Basis & Assumptions
The assessment was based on the available vessel information, process conditions, existing vent arrangement, and defined engineering assumptions.
Key assumptions included:
- The bursting disc is assumed to have a burst pressure equal to, or below, the vessel design pressure of 400 mbar(g).
- The tanks operate at atmospheric pressure and ambient temperature during normal service.
- No inert gas blanketing system is installed.
- During emergency venting, both the atmospheric vent and bursting disc connection are assumed to be available following bursting disc activation.
- Bursting disc accumulation pressure has not been considered in this preliminary assessment due to the absence of certified manufacturer data.
- No accumulation vacuum has been specified for the normal venting device. Therefore, no vacuum accumulation allowance has been considered in this assessment.
- Pressure-drop calculations are intended as a preliminary hydraulic assessment and use estimated loss coefficients in the absence of manufacturer data.
- Emergency venting adequacy will ultimately be verified against the bursting disc manufacturer's certified relieving capacity and performance data.
These assumptions established the basis for evaluating both normal and emergency venting performance.
Applicable Standards
The venting assessment considered the requirements and calculation methodologies of:
- NFPA 30 – Flammable and Combustible Liquids Code
NFPA 30 provides comprehensive guidelines for the storage, handling, and use of flammable and combustible liquids, by considering venting requirements, vent pipe design and installation, material selection, safety measures and inspection and maintenance.
- API 2000 (ISO 28300) – Venting Atmospheric and Low-Pressure Storage Tanks
While NFPA 30 remains the governing standard for system compliance, API 2000 serves as the technical basis for vent sizing calculations and equipment selection, ensuring that all relief paths are sized to safely accommodate anticipated vapor flows under both routine and emergency scenarios.
- BS EN 14015 – Site-built vertical cylindrical storage tanks
BS EN 14015 permits the use of various venting arrangements, including free vents, pressure/vacuum relief valves, vent pipe systems, emergency venting devices and flame-arresting vent systems, provided the selected arrangement is capable of accommodating the required venting capacity.
- ATEX principles
As ethanol can generate flammable vapours, the storage and venting arrangements have been designed with consideration of the requirements of the ATEX Directive 2014/34/EU and the principles of hazardous area classification. The vent pipework itself is not considered ATEX equipment, as it does not contain an ignition source and serves only as a passive pressure-relief pathway. However, any in-line components installed within the venting system shall be assessed for ATEX compliance where applicable.
Vent Sizing Analysis
The venting assessment considered the operating and emergency conditions that could result in excessive tank pressure or vacuum. The analysis included:
- Normal venting during filling and withdrawal
- Thermal inbreathing and outbreathing
- Emergency venting due to external fire exposure
- Vent nozzle capacity and flow velocity
- Pressure-drop assessment
- Review of the existing atmospheric vent and bursting disc arrangement
Normal Venting Analysis
Normal venting requirements were assessed for both inbreathing and outbreathing.
The assessment was done for:
- Inbreathing due to liquid withdrawal and thermal contraction
- Outbreathing due to liquid filling and thermal expansion
Using the NFPA 30/API 2000 methodology, the calculated normal venting requirements were:
- Normal inbreathing: 38.34 m³/h
- Normal outbreathing: 70.74 m³/h
The BS EN 14015 methodology was also assessed, resulting in different requirements because of its treatment of pump capacity and thermal effects.
Emergency Venting Analysis
Emergency venting was assessed for an external fire exposure scenario, where heat input could cause rapid vapour generation and an increase in tank pressure.
The tank's calculated wetted surface area was approximately 80.04 m², which was used as the basis for determining the required emergency venting capacity.
The assessment produced different emergency venting requirements under NFPA 30 and BS EN 14015 due to the different methodologies used by the standards.
The governing combined emergency outbreathing requirement was approximately:
- 13,691 m³/h (3.803 m³/s) using the NFPA 30 basis
- 4,412 m³/h (1.226 m³/s) using the BS EN 14015 basis
For this assessment, the NFPA 30 case governed because it resulted in the greater required emergency venting capacity.
Vent Capacity, Velocity & Pressure Drop
The existing vent connections were assessed to determine their ability to accommodate the calculated flow requirements.
The assessment included:
- Vent nozzle cross-sectional area
- Vapour velocity
- Normal vent capacity
- Emergency vent capacity
- Preliminary pressure-drop calculations
- Combined venting through the atmospheric vent and bursting disc connection
The 3-inch atmospheric vent was found suitable for the calculated normal venting duties.
For emergency conditions, the assessment considered the combined area of the 3-inch atmospheric vent and 8-inch bursting disc connection. The governing NFPA 30 case produced a preliminary calculated pressure drop of approximately 185 mbar, remaining below the tank design pressure of 400 mbar(g).
Results & Findings
The assessment indicated that:
- The existing 3-inch atmospheric vent was suitable for normal inbreathing and outbreathing requirements
- The 8-inch bursting disc, together with the atmospheric vent, provided the emergency venting arrangement
- NFPA 30 produced the governing emergency venting requirement
- Calculated normal vent velocities and pressure losses were within the assessment criteria
- The preliminary emergency pressure-drop assessment remained below the tank design pressure
- The existing venting arrangement was considered suitable in principle for the assessed normal and emergency conditions
- Final emergency venting verification remained dependent on the manufacturer-certified bursting disc capacity and performance data
Outcome & Value
The vessel venting analysis provided:
- Verification of normal inbreathing and outbreathing requirements
- Assessment of emergency venting during external fire exposure
- Evaluation of the existing atmospheric vent and bursting disc arrangement
- Vent velocity and preliminary pressure-drop assessments
- Comparison of venting requirements determined using different applicable standards
- Identification of the governing venting condition
- Engineering evidence to support decisions regarding the existing venting arrangement
- Identification of the manufacturer data required for final emergency venting verification
This case study demonstrates how Aetherium Engineering can assess existing vessel and storage tank venting arrangements, determine normal and emergency venting requirements, and provide clear engineering evidence to support safe and informed design decisions.