8 Glossary and definition of terms

Certain terms used within this document have specific meanings and these meanings have certain implications. Furthermore the terms ‘dangerous fluid’ and ‘dangerous substance’ have specific legislative impact. In addition the terminology of carbon dioxide phases is explained.

8.1 Dangerous substance

The term ‘dangerous substance’ is a term applied to substances as classified under the Dangerous Substances Directive (67/548/EEC)123. A substance that is considered a ‘dangerous substance’ falls into any of the following classes:

  • explosives;
  • oxidising agents;
  • flammable substances or preparations;
  • toxic substances or preparations;
  • harmful substances or preparations;
  • corrosive substances or preparations;
  • irritants;
  • sensitisers ;
  • carcinogens;
  • mutagens;
  • substances or preparations that are toxic for reproduction, and
  • substances or preparations that are dangerous for the environment.

General classification of carbon dioxide is that it is excluded from the list of toxic substances with exposure limits of 5 000 ppm time weighted average (TWA) and 30 000 ppm short term exposure limit (STEL) and is not listed as a dangerous substance under the directive. However carbon dioxide does have a known toxicity and is considered an asphyxiant but at high concentrations. There are two key definitions: the United Nations publishes model regulations124 that classify substances that are transported, and this is the accepted classification that appears on material safety data sheets; the other is the UK Chemicals (Hazard Information and Packaging for Supply) (CHIP) Regulations125 and supporting directive126. The definition of Toxic from the Chemicals (Hazard Information and Packaging for Supply) Regulations 2009127 is:

“Substances and preparations which in low quantities cause death or acute or chronic damage to health when inhaled, swallowed or absorbed via the skin.”

and the definition of toxic and non-toxic under the UN Recommendations on the Transport of Dangerous Goods Model Regulations128 is:

Division 2.3 Toxic gases

Gases which:

  1. are known to be so toxic or corrosive to humans as to pose a hazard to health; or
  2. are presumed to be toxic or corrosive to humans because they have an LC50 value (as defined in section equal to or less than 5 000 ml/m3 (ppmv).

The UN classification of carbon dioxide is Division 2.2 - non-toxic gas and given the Chemicals (Hazard Information and Packaging for Supply) Regulations definition, carbon dioxide would not qualify as toxic. The United Nations has set out to harmonise such definitions internationally. The Globally harmonized system of classification and labelling of chemicals (2007) discusses the definition of ‘toxic’ material in Part 3: Health Hazards; this standard methodology of classification, like the transportation and CHIP regulations would indicate that carbon dioxide is not toxic.

The LC50 value for carbon dioxide is considered by the US National Institute of Occupational Safety and Health (NIOSH) to be 40 000 ppm129, although some studies130 indicate the value may be 100 000 ppm, far above the threshold required for the UN classification of toxic.

In the UK the HSE has stated131 that carbon dioxide is to be considered a dangerous substance when considering carbon capture and storage schemes. This instruction automatically applies the requirements of the Seveso II directive132 or in the UK the COMAH (Amendment) regulations133 2005. It should be noted that the current carbon dioxide production, storage and handling facilities of the industrial gases sector do not fall under COMAH.

However this document recommends, as with all hazards, debated or not, that competent engineering and the ALARP approach to risk abatement and management and the proper consideration of all legislation, particularly COMAH, is the key to safe delivery of these projects.

8.2 Dangerous fluid

Like ‘dangerous substance’, ‘dangerous fluid’ also has a specific legislative meaning in terms of the Pipeline Safety Regulations 1996134. The term ‘dangerous fluid’ is used in the regulations to indicate that pipelines carrying a dangerous fluid are to be considered major accident hazard pipelines.

Dangerous fluids are described as:

  • A fluid which:
    • is flammable in air;
    • has a boiling point below 5 °C, at 1 bar absolute, and
    • is, or is to be conveyed in, the pipeline as a liquid.
  • A fluid which is flammable in air and is, or is to be conveyed in, the pipeline as a gas at above 8 bar absolute.
  • A liquid which has a vapour pressure greater than 1,5 bar absolute when in equilibrium with its vapour at either the actual temperature of the liquid or at 20 °C.
  • A toxic or very toxic fluid which:
    • is a gas at 20 °C and 1 bar absolute, and
    • is, or is to be, conveyed as a liquid or a gas.
  • A toxic fluid which:
    • at 20 °C has a saturated vapour pressure greater than 0,4 bar, and
    • is, or is to be, conveyed in the pipeline as a liquid.
  • Acrylonitrile.
  • A very toxic fluid which:
    • at 20 °C has a saturated vapour pressure greater than 0,001 bar; or
    • is, or is to be, conveyed in the pipeline as a liquid at a pressure greater than 4,5 bar absolute.
  • An oxidising fluid which is, or is to be, conveyed as a liquid.
  • A fluid which reacts violently with water.
  • For the purpose of this definition:
    • a liquid is oxidising, and
    • a fluid is toxic or very toxic, or reacts violently with water.
  • Classified, pursuant to regulation 5 of the Chemicals (Hazard Information and Packaging for Supply) Regulations 2009, as, as the case may be, oxidising, toxic, very toxic or as reacting violently with water.

The above descriptions include carbon dioxide in a number of ways by being toxic and by the classification as a ‘dangerous substance’ under the CHIP regulations.

In considering this classification the implications are significant on the pipeline design process with specific criteria that must be met including emergency shut off valves, consideration of incident prevention, accident planning and specific planning and permitting requirements.

8.3 Phases of carbon dioxide

The phases of carbon dioxide are discussed at length through out this text. But the definition of supercritical and dense phase needs to be considered. Figure 1 shows the phase diagram for carbon dioxide. The fluid state above critical pressure is referred to as the dense phase, above both critical pressure and temperature, supercritical. Whilst the term supercritical is to specific conditions, dense phase is an umbrella terminology, and encompasses supercritical state and liquid state.

8.4 Glossary of terms

AGA: American Gas Association
API: American Petroleum Institute
ASME: American Society of Mechanical Engineers
ASTM: American Society for Testing and Materials
BCGA: British Compressed Gases Association
BLEVE: Boiling Liquid Expanding Vapour Explosion
CCGT: Combined cycle gas turbine
CCS: Carbon capture and storage
CFR: Code of Federal Regulations
CGA: Compressed Gas Association
CONCAWE: Conservation of clean air and water in Europe
COSHH: Control of substances hazardous to health
CR: Chloroprene
Critical point: the temperature and pressure point above which carbon dioxide gas and liquid phases cannot exist as separate phases
Cu: Copper
DMEPEG: Dimethyl Ether of Polyethylene Glycol
Dry ice: solid carbon dioxide
DWTT: Drop Weight Tear Testers
EAF: Electric Arc Furnace
ECBMP: Enhanced Coal Bed Methane Production
EGR: Enhanced Gas Recovery
EIGA: European Industrial Gases Association
EOR: Enhanced Oil Recovery
EOS: Equations of State
EPDM: Ethylene-Propylene
ESOV: Emergency Shut-off Valve
EU SACS: European Union Saline Aquifer Carbon Dioxide Storage Programme
FGD: Flue gas desulphurisation
FKM: Chloroflurocarbons
HSE: Health and Safety Executive
HSL: Health and Safety Laboratory
IGCC: Integrated gasification combined cycle
IGC Code: International Gas Carrier Code
IIR: Butyl isobutene isoprene rubber
IPCC: Intergovernmental Panel on Climate Change
LNG: Liquified Natural Gas
LOC: Loss of Containment
LPG: Liquified Petroleum Gas
MAOP: Maximum allowable operating pressure
MMSCFD: Million standard cubic feet per day
Mt: Million tonnes
MW: Megawatt
MWth: Megawatt thermal
MSS: Manufacturers Standardisation Society
NACE: National Association of Corrosion Engineers
NBR: Nitrile rubber
NFPA: National Fire Protection Association
Ni: Nickel
NIOSH: National Institute for Occupational Safety and Health
NIST: National Institute of Standards and Technology
NORSOK: Norsk Sokkels Konkuranseposisjon - Standards developed by the Norwegian Technology Centre
OSHA: Occupational Safety and Health Administration
PA: Polyamide
Pb: Lead
PCTFE: Polychlorotrifluoroethylene
PEEK: Polyetheretherketone
P-h diagram: Pressure-enthalpy
PHMSA: Pipelines and Hazardous Materials Safety Administration
PP: Polypropylene
ppmvd: Parts per million volumetric dry
PSR: Pipeline Safety Regulations
PTFE: Polytetrafluoroethylene
PVTF: Vinylidene polyfluoride
QA: Quality Assurance
REDOX: Reduction-oxidisation reaction
RGD: Rapid Gas Depressurisation
ROSOV: Remotely Operated Shut Off Valve
SACROC: Scurry Area Canyon Reef Operators Committee
SCADA: Supervisory Control and Data Acquisition
SLOD: Significant likelihood of death
SLOT: Specified level of toxicity
Sublimation point: a temperature/pressure combination at which solid can vaporise directly into gas e.g. −78,5°C at 1atmosphere
Supercritical: above the critical point where carbon dioxide has some characteristics of a gas and some of a liquid
Triple point: the temperature & pressure where carbon dioxide exists as a gas, liquid and solid simultaneously
T-s diagram Temperature-entropy diagram
UHMWPE Ultra High Molecular Weight Polyethylene
UN-IMO United Nations International Maritime Organisation
Zn Zinc





127‘The Chemicals (Hazard Information and Packaging for Supply) Regulations, 2009’, Statutory Instrument No. 716.

128‘UN Recommendations on the Transport of Dangerous Goods, Model Regulations’, 15th Edition, 2007.

129http://www.cdc.gov/niosh/idlh/124389.html NB. Described as the IDLH Level (Immediately Dangerous to Life or Health).

130 ACGIH [1971]. Carbon dioxide. In: Documentation of the threshold limit values for substances in workroom air. 3rd ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, p. 39.


132‘Directive on the control of major-accident hazards involving dangerous substances’ (96/82/EC).

133 ‘The Control of Major Accident Hazards (Amendment) Regulations 2005’ Statutory Instrument No. 1088.

134‘The Pipelines Safety Regulations 1996’, Statutory Instrument No. 825.