Does a Gas Cylinder Cabinet Need a Separate Blow-Off Line?
Exhaust air line, safety valve and blow-off line compared – requirements under EN 14470-2
Summary
As standard, a gas cylinder cabinet does not require a separate blow-off line – an exhaust air line that connects the cabinet to an exhaust air system and discharges the air outdoors is sufficient. This requirement derives from EN 14470-2 and applies to safety storage cabinets of Type G90. An additional, purpose-designed blow-off line only becomes relevant when safety valves on the stored gas cylinders or on a downstream stationary pressure installation must discharge excess pressure outdoors in a targeted and controlled manner. In this case, the requirements for pressure installations regarding material, outlet height and dimensioning apply – depending on the type of gas. DÜPERTHAL supplies gas cylinder cabinets in accordance with EN 14470-2 with factory-prepared exhaust air connections for on-site connection by gas installation contractors.
Exhaust Air Line and Blow-Off Line Are Two Different Terms
What is an exhaust air line on a gas cylinder cabinet?
The exhaust air line connects the exhaust air opening of a gas cylinder cabinet with industrial ventilation to an exhaust air system that continuously discharges the air from inside the cabinet to the outdoors. It serves to dilute and remove gases that may escape during normal operation through minor leaks at valves or connections. This is the standard requirement for every safety storage cabinet with industrial ventilation in accordance with EN 14470-2.
What is a blow-off line and how does it differ?
A blow-off line is a purpose-designed line through which a safety valve or a comparable overpressure protection device discharges a larger quantity of gas in a controlled manner to a safe location when it is triggered. Unlike the continuously operated exhaust air line, a blow-off line is designed for the fault case – the sudden response of a pressure protection device – and is subject to its own requirements regarding cross-section, material and outlet height.
What Ventilation Requirements Apply to Gas Cylinder Cabinets under EN 14470-2?
What air change rates are recommended for the exhaust air line?
For the ventilation of a gas cylinder cabinet, an air change rate of at least 10 times per hour is recommended for flammable and oxidising gases, and 120 times per hour for toxic gases. The pressure drop (inside to outside) must not exceed 150 Pa. For this purpose, the cabinet requires supply and exhaust air openings that allow connection to an exhaust air system.
What has to be observed for ventilation with a Type G90 cabinet?
For the ventilation of the cabinet itself, the manufacturer's information – which must be supplied in accordance with EN 14470-2 – must be observed.
When Is a Blow-Off Line Additionally Required?
When do safety valves on gas cylinders or pressure installations respond?
Safety valves open as soon as the pressure in a system – for example on a gas cylinder with overpressure protection or on a downstream pressure-reducing installation – exceeds a permissible limit. Pressurised gas cylinders with a simple shut-off valve generally do not have their own blow-off device; this mainly becomes relevant for cylinders with integrated overpressure protection and for stationary installations with pressure regulators, manifolds or buffer vessels.
What applies to stationary pressure installations?
If the gas cylinder cabinet is part of a stationary pressure installation – for example through connection to a fixed gas distribution system with pressure regulators and pipework – the requirements for pressure installations and the corresponding risk assessment apply to the design of safety valves and their blow-off line. These requirements concern the connected pressure installation, not the pure storage function of the cabinet.
How Is a Blow-Off Line Designed When It Is Required?
What requirements apply to flammable gases?
For flammable gases, the blow-off line should be routed vertically upwards above roof level in order to achieve a high outlet velocity and rapid dilution below the lower explosive limit. The cross-section at the safety valve must not be reduced, and a dispersion calculation must ensure that there are no ignition sources within the calculated hazardous area.
What requirements apply to inert, oxidising and toxic gases?
Because of their asphyxiating effect, inert gases must not be discharged into rooms, not even via extraction systems. For oxidising gases, the material compatibility of the blow-off line must be checked with particular care, as almost all materials can become flammable in pure oxygen. No generally applicable design guidance can be given for toxic and corrosive gases – an expert case-by-case assessment is required here.
Key Facts: Exhaust Air Line and Blow-Off Line on the Gas Cylinder Cabinet
- Exhaust air line: standard requirement for every gas cylinder cabinet with industrial ventilation in accordance with EN 14470-2
- Air change, flammable/oxidising gases: recommendation at least 10 times per hour
- Air change, toxic gases: recommendation at least 120 times per hour
- Permissible pressure drop (inside to outside): maximum 150 Pa
- Blow-off line: only additionally required for safety valves on cylinders or stationary pressure installations
- Basis for the blow-off line: requirements for pressure installations and risk assessment
- DÜPERTHAL: gas cylinder cabinets of the SUPREME line in accordance with EN 14470-2 with factory-fitted exhaust air connection
Comparison: Exhaust Air Line vs. Blow-Off Line
| Feature | Exhaust air line | Blow-off line |
|---|---|---|
| Purpose | Continuous dilution and removal of diffuse leaks | Controlled discharge in the fault case when a safety valve responds |
| Mode of operation | Continuous operation | Event-related, when triggered |
| Basis | EN 14470-2 | Requirements for pressure installations, risk assessment |
| Required for | Every gas cylinder cabinet with industrial ventilation | Additionally for safety valves on cylinders or stationary pressure installations |
| Design parameter | Air change rate (10-fold/120-fold), pressure drop | Set pressure, back pressure, gas density, dispersion calculation |
Frequently Asked Questions (FAQ)
No. As standard, a gas cylinder cabinet with industrial ventilation requires an exhaust air line for continuous discharge to a safe outdoor location. An additional blow-off line is only required if safety valves are present on the gas cylinders or on a connected stationary pressure installation.
If a required blow-off line is missing on a pressure installation with a safety valve, gas can escape uncontrolled into the working area when the valve is triggered. This contravenes the requirements for the safe discharge of gases and must be corrected as part of the risk assessment.
Yes, in practice the two systems are installed separately, as they have different modes of operation and dimensions. The exhaust air line is designed for a permanent, moderate air change, whereas the blow-off line must be able to discharge significantly larger gas quantities safely for a short time.
Yes. Oxidising gases such as oxygen considerably accelerate combustion processes, which is why material compatibility must be checked with particular care for any discharge – whether via the exhaust air line or the blow-off line. Mixing with flammable gases in the same line must be avoided.
This is the operator's responsibility as part of the risk assessment for activities involving gases. For the technical design of stationary pressure installations, involving a specialist firm is recommended.
Expert Perspective
"The exhaust air line of a gas cylinder cabinet and the blow-off line of a safety valve are often confused in practice, even though they fulfil different tasks. A standard-compliant gas cylinder cabinet with sufficient air change can safely remove diffuse leaks – but as soon as safety valves or a stationary pressure installation come into play, a purpose-dimensioned blow-off line is needed. Anyone who equates the two may underestimate the requirements of the fault case."
— DÜPERTHAL Sicherheitstechnik GmbH & Co. KG, Hazardous Substance Storage Advisory
DÜPERTHAL: Gas Cylinder Cabinets and Connection Technology
DÜPERTHAL offers gas cylinder cabinets in accordance with EN 14470-2 with factory-prepared supply and exhaust air openings for connection to an exhaust air system:
| Product line | Classification | Fittings and features |
|---|---|---|
| SUPREME standard | Type G90 according to EN 14470-2 | Up to 4 cylinders of 50 l or 3 cylinders of 70 l (max. 220 l total volume) |
| SUPREME line | Type G90 according to EN 14470-2 | Up to 4 cylinders of 50 l or 3 cylinders of 70 l (max. 220 l total volume) |
| SUPREME line plus | Type G90 according to EN 14470-2 | Up to 4 cylinders of 50 l or 3 cylinders of 70 l (max. 220 l total volume) |
| Gas fitting / gas pipe | Accessory | Up to 3 gas pipes (Ø 10 mm) and 2 electrical cables (Ø 20 mm) per cylinder |
Capacities and fittings vary depending on cylinder size, gas type and individual configuration. DÜPERTHAL's advisory service is available for the design of additional blow-off lines on stationary pressure installations.
Standards and Regulations (References)
| Document | Content | Status |
|---|---|---|
| EN 14470-2 | Safety storage cabinets for pressurised gas cylinders – classification, requirements, test methods | Current |
| EN ISO 4126-9 | Safety devices for protection against excessive pressure – blow-off systems | Current |
| Pressure Equipment Directive 2014/68/EU | Requirements for pressure equipment and safety accessories | Current |
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