Compressed and liquified gases

When Fire and Rescue Services are called to restore order at an industrial site, such as a warehousing complex, production/manufacturing plant or a public highway, as part of the dynamic risk assessment the incident commander must consider the hazards that the incident team may face upon entry.

One hazard is the presence of cryogenic or compressed gases on the site. The dangers of packaged gas (especially high-pressure gas cylinders) exposed to heat/fire must never be underestimated. This article gives a basic insight into the chemistry of industrial, medical and speciality gases, as well as considerations for their presence on a site requiring emergency response, and the restoration of order.

Where do gases come from?

The obvious answer is from the air that surrounds us, but that is only partially correct. We do indeed extract what we term atmospheric gases (or ‘air gases’) from the air around us, but some gases are either not present in sufficient quantity for extraction from the air, or the technology to do so is not economic, so other methods need to be deployed.

Gases are split into the following categories, and can be supplied in various forms, from pipeline, bulk tanker, tube trailer, dewar vessel, cylinder, lecture bottle and aerosol.

LNG/LPG

Liquefied natural gas and Liquefied Petroleum Gas, propane, butane, refrigerants.

Industrial gases

Nitrogen, oxygen, argon, hydrogen, carbon dioxide (and mixtures of), acetylene, industrial hydrogen.

Special gases

Hydrogen, helium, methane, ethane, noble gases, electronic/semiconductor gases, mixtures (span and zero gases), calibration gases, chemical gases e.g. ammonia, nitric oxide, sulphur dioxide, hydrogen sulphide et al.

Medical gases

Oxygen, nitrous oxide, nitrous oxide in oxygen, 5% helium in oxygen, medical air, medical carbon dioxide, breathing/diving gases, nitric oxide mixtures for lung function et al.

Firstly, let’s consider the composition of the air around us:

Gas Chemical symbol %

Nitrogen N₂ 78.084

Oxygen O₂ 20.947

Argon Ar 0.934

Carbon dioxide CO₂ 0.0350

Neon Ne 0.001818

Helium He 0.000524

Methane CH₄ 0.00017

Krypton Kr 0.000114

Hydrogen H₂ 0.000053

Nitrous oxide N₂0 0.000031

Xenon Xe 0.0000087

Ozone O₃ Trace

Carbon monoxide CO Trace

Sulphur dioxide SO₂ Trace

Nitrogen dioxide NO₂ Trace

Ammonia NH₃ Trace

In terms of tonnage of gases produced, air separation (coupled to fractional distillation) is a technique that is deployed in extracting what we term the ‘Air or Atmospheric Gases’, namely nitrogen, oxygen and argon, while some air-separation plants have secondary columns/processes for collecting and concentrating trace quantities of noble gases such as neon, xenon and krypton.

The other gases required for industry (and therefore logistics management) are produced by physico-chemical processes, and by ‘by-product extraction .

To understand the air-separation process, we need to examine the boiling points of the major constituents of air: nitrogen, oxygen and argon. The basic process of air separation consists of purification of the air that is sucked out of the atmosphere, purified and cooled cryogenically until in the liquid phase [‘liquid air’] from which it is passed through an insulated fractional-distillation column, from which liquid nitrogen, oxygen and argon are separated and collected. Due to the high energy usage required in the air-separation process, these plants are normally situated close to both lower energy agreements and a large-volume user, e.g. near steel works, chemical or oil refinery complexes (where large volumes of nitrogen and/or oxygen are required). It should be noted that the air surrounding a steel works, chemical plant or refinery has many impurities so purification of the air that is sucked from the atmosphere needs to be considered.

This type of air-separation plant situated close to a large volume user is often termed a merchant plant: one that can supply the local customer(s), as well as draw-off excess product, especially the higher-value argon – and in some cases secondary distillation, concentration of the noble gases such as neon, xenon and krypton. Some of the cryogenic gases are piped to the user with insulated pipelines, others are pumped into insulated tanks, and from there loaded into cryogenic tankers, and if the (local) user requires gaseous product like nitrogen for ‘blanketing’, then the cryogenic product can be pumped into a vaporiser (basically a ‘finned-evaporator’ that has a high surface area to volume ratio) that flashes off the cryogenic liquid, into a gaseous phase.

Gas cylinder filling plants for the base industrial gases such as nitrogen, oxygen and argon use cryogenic product from the tank, then ‘pushed/pumped’ through a vaporiser and the gas pumped into cylinders. Naturally other factors such as cylinder preparation, post-fill analysis and leak checking and other factors need to be considered. Atmospheric gases such as nitrogen, oxygen and argon can be supplied by pipelines, cryogenic tankers, dewar vessels (basically a super-insulated vacuum flask, which can vary from 50-litre to 1,000-litre capacity) and cylinders.

How are the non-atmospheric gases produced, transported and stored?

Helium: the majority of the world’s helium is produced from natural gas from plants in the US, Algeria and Russia where natural gas streams have between 0.4% and 4% helium, which can be extracted, purified and then cryogenically cooled – no mean feat as liquid helium has a boiling point a few degrees above absolute zero. When liquid helium is supplied in bulk cryogenic tankers, these are specially designed iso-tanks that have super-insulation (and ultra-vacuum insulation), as well as an ‘encapsulated/over-tank’ of liquid nitrogen to maintain the deep cold. The tanks are designed with relief valves as well as bursting-discs in the event of a serious heat leak, and over pressurisation as liquid helium will boil violently and rupture the tank (if the tank and/or insulation fails or malfunctions). It is crucial to keep an eye on the pressure readings to ensure that the tank is stable, because you will see steady pressure rise when there is a malfunction in the tank or insulation. Another useful tip is to watch out for ‘frost spots’, i.e. ice formation on the outer tank, as this is indicative of a heat leak, caused by an imperfection in the insulation. When liquid helium tanks are shipped from the US to Europe, the tanks are held as deck cargo, and the pressure readings monitored of both the inner helium and outer liquid nitrogen, to keep check on the stability of the tank. Helium is also supplied as bulk liquid in dewars but also as a gas in skid-mounted tube trailers, and of course cylinders.

Hydrogen is normally produced through ‘natural gas reforming’ (aka ‘steam reforming’), the decomposition of hydrocarbons using steam in the presence of a catalyst. It can also be produced through electrolysis as an electrical current is used to separate the atoms and produce both gaseous hydrogen and oxygen. Hydrogen plants are often built for high-volume users, such as vegetable-oil refining (for hydrogenation), or as a by-product from an on-site electrolytic plant. It can be then either liquefied in an analogous method to liquid helium (see above) and also as a gas, in skid-mounted tube trailers, and cylinders.

Carbon dioxide comes via two major production streams. It is supplied for food use, as a by-product from the brewing process after purification and liquefaction, as well as for non-food use, as a chemical by-product from chemical processes and the wood industry and again, after purification, it is liquefied. Supply of CO₂ comes in bulk tanker, cylinder (including liquid withdrawal via dip tube), and also as a solid form ‘dry ice’.

Methyl bromide is extracted and purified from the Dead Sea in Israel and the Great Lakes in the US and Canada and supplied in bulk tanks as well as cylinders.

Nitrous oxide is produced by the catalytic heating of ammonium nitrate over 250°C.

Acetylene: 10% of the world’s acetylene is produced from petroleum refining, with 90% still being produced by the calcium carbide process.

As acetylene is a highly unstable molecule (as the triple bond between the two carbon atoms has huge potential energy, which is released when burned) so it is not a gas one can compress, hence gaseous acetylene is bubbled and dissolved in acetone (a solvent) that is saturated within the porous material contained within the cylinder. Acetylene cylinders must always be stored and transported upright due to the flammable liquid acetone which is saturated with acetylene gas within the cylinder body.

Ammonia and syngas: from the petroleum refining process comes many products via synthesis gas (carbon monoxide and hydrogen), which via the Haber process forms ammonia. Synthesis gas is also crucial in the production of petroleum products and petrochemicals and gases such as ethers, alcohols, acids, aldehydes and ketones from which more complex products can be produced.

Considerations in the transportation and storage of gas cylinders

It is crucial for the logistics organisation to ensure they are trained by the supplier/consignor on safety issues related to the gases they will manage within the supply chain, as well as risk assessments and contingency planning for emergency situations. I’ve listed a few key points for consideration, though this is not a complete list, as specific issues are relevant to the specific gases handled. The logistics supplier should also use the services of their Dangerous Goods Safety Advisor [DGSA] in ensuring regulatory compliance.

Ventilation issues must be considered when handling compressed gases, with storage to be in secure ventilated areas (i.e. outdoors) to reduce risk of asphyxia as well as leakage into a confined space, which when it comes to toxic and flammable gases is a serious risk, not only to staff but the emergency services should they be called to an incident in a warehouse. Gas cylinders must also be kept way from sources of heat and/or ignition, as well as combustible material.

In-transit considerations are crucial, as gas cylinders must be transported on a ventilated vehicle. On a curtain-sider, staff should be trained to open the curtains gingerly and then allow the vehicle to ventilate for at least 15 minutes before unloading. It is not recommended that gas cylinders be carried in closed vans, but if sent in a van, the rear doors must have a ‘ventilate before entry’ sign on the outside. Gas cylinders must also be robustly palletised and if using a double-deck trailer, never placed on the top deck.

Aerosols/disposable cylinders – when stored in a warehouse, they must be internally caged and must also be kept way from sources of heat and/or ignition, as well as combustible material.

Cryogenic issues: when considering tanks/dewar vessels (insulated) for the storage of refrigerated gases it is important to factor tank rupture by heat, on cold liquid, and the properties of the liquified gas. Examples being pressure release of asphyxiants such as liquid nitrogen (LN2) and liquid argon (Lar) or liquid oxygen (LOX), anhydrous ammonia (NH3) et al. The dangers of the presence of oxygen in a fire situation are obvious, and serious. With the presence (and risk) of fire, cooling becomes important to prevent ‘overheating’ and rupture.

COMAH – the Control of Major Accident Hazards regulations apply to many gases, both as named substances, restrictions on products such as ethylene oxide, phosgene, dichloro-silane, tungsten hexafluoride, fluorine, hydrogen sulphide et al, as well as total tonnages and storage restrictions with flammable gases, aerosols etc.

Security issues must be considered when handling Toxic Gas Cylinders (Class 2.3) which fall under ADR High Consequence Dangerous Goods (HCDG). Many of these are special gases such as silane SiH4 (silicon tetrahydride), which is pyrophoric (i.e. catches fire when the gas is in contact with atmospheric oxygen without the need of a spark or heat), phosgene (carbonyl chloride), which is used in pharmaceutical manufacture, but most notorious as mustard gas from the First World War. Related to phosgene is sarin, which was released on the Tokyo underground by the Aum Shinrikyo doomsday cult in 1995 resulting in 13 deaths and many taken to hospital.

‘Empty’ cylinders will contain residual product, and sometimes this is not insignificant, so they will fall under ADR* regulations (as they are termed ‘empty dirty’). The only time an empty gas cylinder is outside of ADR regulations is when the valve has been removed and no ADR diamond/hazard markings are present, or presented with a ‘gas free’ certificate and no ADR diamond/hazard markings are present, e.g. new cylinders from manufacture.

Vertical standing on pallet (or secured) is required with gases that are in the liquid phase, such as acetylene, propane, butane, carbon dioxide, nitrous oxide, ammonia, etc. This is to prevent the valve inadvertently coming open in storage or transit, and liquid gas escaping and potentially pooling in a void space. Some liquid-phase gases have internal dip-tubes from the valve to the cylinder bottom to allow liquid product to release on opening the valve, by pressure siphon action. These often have a white stripe on the outside of the cylinder like liquid carbon dioxide.

Horizontal position on a pallet is allowed when the gas is in the gaseous phase such as nitrogen, helium, oxygen, hydrogen, argon, mixtures and many others. If cylinders are presented in the horizontal position, they must be secured to the pallet, and not overhang the pallet edge – to avoid impact damage in movement.

Valve guard: gas cylinder design is regulated, with the weak point being the position where the valve is screwed into the cylinder shoulder. To prevent impact damage, and high pressure and gas release the valve needs to be guarded while in the supply chain. There are two general types: the ‘tulip’, which encapsulates the valve with a gap to allow a regulator to be attached to the valve for usage, or the ‘bell-end’ which is screwed into the top of the cylinder to fully protect (and encapsulate) the valve.

Manual handling and Personal Protective Equipment (PPE) are key considerations when gas cylinders are de-palletised or palletised, and your consignor should advise you of training requirements and ensure you never have vertical cylinders ‘free-standing’ – as they can fall and a domino effect occur. Cylinders must be on pallets, or chained/restrained for stability rationale. Considerations on PPE must be risk assessed.

And finally, planning: when managing the logistics of gas cylinders, planning for emergency situations is vital, which comes from product training, risk assessments and contingency for emergencies.

More information regarding transportation and storage of compressed and liquefied gases is available from the following trade associations:

British Compressed Gas Association

www.bcga.co.uk

European Industrial Gas Association

www.eiga.eu

The LPG Association

www.uklpg.org

The British Aerosol Manufacturers Association

www.bama.co.uk

This information is provided in good faith, as an insight only and is not comprehensive. The Fire and Rescue Operational Manual and Guidance must always be followed in the restoration of order.

For more information, contact akarim1462@aol.com

Reference

ADR* Accord Dangereux Routier (European regulations concerning the international transport of dangerous goods by road). It is a UN treaty concluded in 1957 and updated ever since, according to new rules and regulations in the logistics industry.