safety in high pressure oxygen systems.pdf

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Prepared by the Safety Advisory Group Safety Info 15/00/ERevision of Safety Info 15/97/E

Safety Principles of High pressure Oxygen Systems

1. INTRODUCTION:

1.1 Scope and field of application

2. OXIDISING GASES

3. IGNITION SOURCES

4. CONTROLLING FIRE HAZARDS IN OXYGEN SYSTEMS

5. CONSIDERATIONS IN SHUTDOWN AND REPAIR SITUATIONS

6. SURVEY OF IMPORTANT PUBLICATIONS ON OXYGEN

Safety Information 15/00/E- page 2Revision of Safety Information 15/97/E

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1. INTRODUCTION

Oxygen system design should only be carried outby engineers who have professional experienceand knowledge of the design principles andmaterials involved. There are also well establishedstandards and regulations in many countries whichmust be followed where appropriate.

The following internationally recognisedorganisations have issued oxygen safetyinformation which should be referred to whendesigning oxygen systems.

ASTM- American Society of Testing & Materials (USA)

NASA - National Aeronautics and Space Administration (USA)

CEN - Standardisation bodiesISO - Standardisation bodiesCGA - Compressed Gas Association

(USA)EIGA - European Industrial Gases

Association (Europe)(See also paragraph 6 regarding nationalorganisations, e.g. VBG (1a)1, BSI (1b), NFE(1c))

Normally each time an oxygen system is planned,suitable company standards should be availablewhich specify components and materials to beused, rather than referring to the above sources indetail. This document therefore outlines only thebasic recommendations for oxygen systems.

If established company standards are notfollowed, or do not exist, then additional researchmust be carried out to avoid accidents. Note that itis the responsibility of the company and thesystem design engineer to ensure that safetyaspects have been properly covered.

1 Refer to references at the end of the document

1.1 Scope and field of application

This Safety Information, Safety Principles ofHigh Pressure Oxygen Systems comprisesgeneral recommendations related to the selectionof components and materials for use in systemsfor compressed gaseous oxygen, or mixtures ofoxygen and inert gases if the mixture containsmore than 25% oxygen by volume and a pressureabove 30 bar. It explains the main causes of firesin oxygen systems and gives references toimportant publications on oxygen whichpreferably should be studied by technicalpersonnel who are writing company standards.

The risk of toxic contamination in conjunctionwith combustion or decomposition in supply linesor of components used in medical systems has notbeen considered in this document.


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Ignition Temperatures of Materials as Function of Oxygen Pressure


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2. OXIDISING GASES

The most common oxidising gas is oxygen (seealso the oxidising properties of ozone, fluorineand nitrous oxide (2)). Oxygen itself is non-flammable, but it supports combustion. It is highlyoxidising, reacts vigorously with combustiblematerials, and enhances a fire or explosion whichwill generate a large amount of energy in a shorttime.

Oxygen

Both the risk of ignition and the rate ofcombustion increase with higher concentrations ofthe oxidising gas. Higher pressure usually resultsin a lower ignition temperature and increasedcombustion rate. Furthermore a higher pressurewill in the case of adiabatic compression create ahigher temperature. Increased temperature willalso make the risk of ignition more likely, as theamount of energy that must be added to start afire, decreases. The ignition temperatures ofpolymeric materials are lower than those of metalsand they become significantly lower withincreasing pressure. Refer to Fig. which showsapproximate ignition temperatures for selectedmetals, plastics and elastomers. For ranking ofmaterials to be used in oxygen systems refer totesting organisations and regulating authoritiesaccording to paragraph 6.

With sufficient pressure and ignition energy,nearly all substances can be made to burn in pureoxygen, including substances which are notusually regarded as flammable, e.g. metals(compare with Oxy-fuel cutting). Accidentalignition of metals in oxygen normally requires akindling chain of reaction, i.e. ignition of impuri-ties or non-metallic materials such as soft seatinserts in valves, O-rings etc. Organic substancese.g. oils and grease are ignited extremely easily

and can function as an ignition source for othermore difficult to-ignite materials.

Gas mixtures containing oxygen

For gas mixtures containing 21-25 % (Vol.)oxygen, the oxidising properties must be takeninto consideration when regarding lubricants,sealing materials and cleaning requirements. Foroxygen concentrations exceeding 25 % the samerules may apply as for oxygen (3).

3. IGNITION SOURCES

Single or repeated violent pressure shocks(heat from adiabatic compression).

Excessive gas velocity in pipes or components. Particles impacts. Jammed valve, rough valve handling (galling

and friction energy). Resonance. Contamination with e.g. grease or oil in

combination with an ignition (kindling chainof reaction).

Ignition energy in oxygen systems often comesfrom adiabatic compression. At customer sitese.g., high pressure cylinders should be openedagainst regulators or closed manifolds and hoses(4) with special end fittings with good heat sinkused. Adiabatic compression can also be aproblem at filling stations since there is apossibility of connecting returned cylinders at fullpressure to the filling manifold or operating thevalves in the wrong order. The hose hasconnections at both ends which should bedesigned to absorb the compression heat in thesame way as the distance pieces, used with thehoses for emptying manifolds, will do. Pressureshocks can be prevented by not using quickopening valves e.g. ball valves and alwaysopening manual or automatic valves slowly.


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4. CONTROLLING FIRE HAZARDS INOXYGEN SYSTEMS

Safety philosophy of oxygen system design:

No system will be safe unless suitable materialsand safe components are used: i.e. materials andcomponents that have a trouble-free history inoxygen service or have been properly tested andwhere applicable approved by a recognised testingorganisation.

No component will be safe unless it has beendesigned, manufactured and tested according toprinciples which ensure safe operation in oxygenservice.

Design principles and material cannot beselected unless knowledge of oxygencompatibility, system design and test methods areavailable.

Reducing the fire hazard in oxygen systemsinvolves the control of the ignition mechanismand the propagation of the fire. For example, it isimportant that heat is removed from the reactionzone. The mass of non-metallic material should bekept small, and must be well embedded insurrounding metal for heat conductivity. Thesystem or component must be cleaned for oxygenservice, i.e. not be contaminated with e.g.hydrocarbons. Only tested (5) and approvedlubricants should be used and applied in thesmallest possible amounts. The system shall bedesigned, when appropriate, to protect operatorsin case of a fire e.g. by using shields (panels) infront of valves and other components or byoperating remote controlled valves, for example inlarge distribution pipelines, from a safe distance.

Engineering guidelines regarding choice ofmaterials:

Metallic materials:

Aluminium and some aluminium alloys shall notnormally be used in pipelines or other componentswhere there is a history of ignition e.g. in pumps,regulators, valves etc. Al-seals must be wellembedded. Aluminium is however considered tobe an appropriate material for gas cylinders,vaporisers etc.

Pipes and components :

Copper, copper alloys (e.g. brass, tin bronze),Nickel, Monel etc. are the preferred materials forvalves, where ignition is more likely to occur, inhigh pressure oxygen systems i.e. above 30 bar.These metallic materials and high alloy steels (6)are also used in pipe systems where the flow rateexceeds 25 m/s.

Carbon steel (7) may be allowed in pipesdepending on pressure and velocity according toguidelines from national authorities and fromCGA/EIGA (e.g. DOC. 13/82)

Cleanliness is a prerequisite. Sections of copperalloy or similar material should be inserted whereparticle impact, pressure shock or extremevelocities are expected.

Internal parts of thin sections made from stainlesssteel should be avoided especially where adiabaticcompression may occur and where non-metallic oreasily ignited materials are present.

Non-metallic materials:

Materials with high ignition temperature and lowcombustion heat are preferred. Good design mayeffectively reduce the probability of a fire.

When selecting a sealing medium, as a firstchoice, use metallic or inorganic material which isnot combustible in an oxygen atmosphere.Organic materials must be tested and approvedbefore use in the actual operating conditions.Refer to section 6 for a survey of tests and testingorganisations which are used. See also Attachment2: Summary of Material Properties according toIGC Document 13/82 and note no 5.

Good practice when designing and buildingoxygen systems:

When planning or working with high pressureoxygen systems always be aware of the great riskof reaction between oxygen and incorrect or con-taminated materials. A fire will often causeextensive damage. Therefore the following fivedesign rules should be met, as well as definingsafe operational procedures.


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1. Use only oxygen compatible materials andapproved products. When selecting materialsand dimensions, consideration must be givento pressure and gas flow velocity (8).

2. Only use components (valves, non - returnvalves, filters, hoses, regulators etc.) whichhave been burn out tested and approved (9)at the pressure and direction, for which theywill be used or exposed to, if the pressure willbe 30 bar or higher. For a survey of burn outtesting see section 6.2.

3. Pipes and other components used in oxygenservice shall be cleaned and degreased using asuitable solvent and procedure. All dangeroustraces of the solvent shall be removed beforeoxygen is admitted. It is a good practice to dothe cleaning before assembling the oxygensystem and to prevent the ingress of impuritiesduring the work.

4. Foreign particles such as mill scale, rust, dirt,pieces of PTFE tape, turnings and welddroplets must be carefully removed beforesystem start-up. Particles shall as far aspossible be arrested in filters and not beallowed to transfer from one part of thesystem to another. Such filters shall becleaned after purging and prior to admission ofoxygen into the system. Materials for filtersmust be chosen with great care as they have avery large exposed surface. Sintered bronzemay be a good choice for systems which areoperated at somewhat higher pressures e.g incylinder filling or emptying manifolds.

5. Although non liquefied (permanent) gases donot cause static electricity charging, the fillingmanifold, the hoses and the connection to gascylinder valve shall be connected to the plantbond (earth) (10) in order to prevent internalelectrostatic charging and sparks. The reasonis that it is very difficult to avoid foreignparticles especially if the same equipment isused for cylinder emptying. Foreign particlesin a gas stream will increase the risk ofelectrostatic charging of e.g. the plastic in theinner tube of a steel braided flexible hose (11)

5. CONSIDERATIONS IN SHUTDOWN ANDREPAIR SITUATIONS

During repair of oxygen systems the risk ofoxygen enrichment and injuries due to fire mustbe considered. All work in confined spaces mustbe thoroughly planned and supervised to ensurethat there is not too high oxygen concentration oroxygen deficiency after purging operations.Before welding or cutting is performed on anoxygen pipe system the oxygen supply must beshut off, the pipe section if possible blind flangedand its pressure must be relieved. A single shut offvalve cannot be relied on. Double block and bleedvalves shall be considered. The pipe section mustbe purged with an inert gas. Purging is usuallycarried out with nitrogen until the oxygenconcentration has fallen to 21 %. Note that duringcutting work, severely rusted pipe systems canstart burning even after they have been purgedwith an inert gas.

The risk of oxygen enrichment or oxygendepletion during maintenance and purgingoperations refer is described in the followingIGC Documents:

04/00 Fire Hazards of Oxygen and Oxygen enriched Atmospheres

44/00 Hazards of inert gases40/90 Work Permit Systems


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6. SURVEY OF IMPORTANT PUBLICATIONS ON OXYGEN

Method, procedure orpractice used

Testing Organisation /Regulating Authority

BAM 1) /VBG 2) ASTM 3) /NFPA 4)

Other Testing Organisation orStandards, e.g.

NASA 5) , BSI 10) , DIN 7) Others14)

1. Pneumatic Impact Test(PIT) of materials - GOX

Main test method, seeListe 6)

G74 Standard testmethod

2. Pneumatic Impact Test ofcomponents e.g. valvesregulators etc. - GOX.(Burn-out test)

Main test method.Conditions forapproval

G74 Std. testmethod can be usedalso for components

TRG (DIN) 7) , DIN 8546, DIN8545,TRG770 Anlage 1ISO 2503 8)EN 849-CEN/TC 23SC 2N 138 9)

3. Auto Ignition Temp (AIT)-GOX (high pressure)

Optional G72 Standard testmethod

BS 3N 100: 1985 (Bomb test) 10)NF E 29-763/94 11)ISO TC 58/WG7, Oxygencompatibility

4. Oxygen Index (OI) Optional D2863 Standard testmethod

ISO 4589-1984 (DIN 22117)Standard test method

5. Heat of Combustion(HoC)

Optional D4809 Standard testmethod

DIN 51900

6. Mechanical Impact Test -LOX 5) (MIT-LOX)

Optional D2512 Standard testmethod

BS 3N 100 (Lox Impact Machine)

7. Design of systems for oxy-gen service. (Oxygen)

Regulation/VBG 62 G88 Standard guideG63, G94(evaluation of ma-terials)

EIGA-IGC 12) Doc. 13/82CGA 13) Pamphlet G4.4-1993BS 3N 100

NASA 5) (e.g. SP 3090, 8060)

8. Cleaning Methods forMaterials and Equipmentin Oxygen Service

Regulation/VBG 62 G93 Standard IGC Doc. 33/97CGA Pamphlet G-4.1 - 1996

9. Fire Hazards in OxygenEnriched Atmospheres.

NFPA Manual (53M 19909)

EIGA-IGC Doc. 44/00 and 04/00

Footnotes:1) BAM- Bundesanstalt fr Materialforschung und P rfung,

Germany.8) ISO-International Organisation for Standardization.

2) VBG-Unfallverhtungsvorschriften der Berufsgenossenschaftder chemischen Industrie.

9) CEN-European Committee for Standardization.

3) ASTM-American Society for Testing and Materials, USA. 10) BSI-British Standard Institute. BS-British Standard.4) NFPA-National Fire Protection Association, USA. 11) NFE-French Standard.5) NASA-National Aeronautics and Space Administration, USA. 12) EIGA/IGC-European Industrial Gases Association/Industrial

Gases Council (Doc 13/82 temporary out of print).

6) Liste der nichtmetallischen Materialen -BG Chemie (see note 2).Test results by BAM.

13) CGA-Compressed Gas Association, USA.

7) DIN- Deutsches Institut fr NormungTRG- Technische Regeln Druckgase (DIN)

14) a) Air Liquide, Testing Centre, Franceb) DNV, Norway


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Re: The following references are found in the text of the document.(1a) VBG-Berufsgenossenschaft der chemischen Industrie (Germany).(1b) BSI-British Standardisation Institute (United Kingdom).(1c) NFE-French Standard.(2) The same rules apply in principle to nitrous oxide systems as to oxygen. Oxidation reactions with

nitrous oxide give off more heat than oxygen. On the other hand, nitrous oxide reactions require ahigher activation energy, which makes them more difficult to initiate. Violent reactions can take placeonce the ignition is started since nitrous oxide can decompose. Hot work must not be carried out onany equipment under pressure or which have not been purged (also compare with the decompositionproperties of Ozone).

(3) Reference: ASTM G88-84, Oxygen enriched atmosphere.(4) Reference IGC Doc. 42/89 Prevention of Hose Failures in High Pressure Gas Systems.(5) A list of Oxygen compatible lubricants is prepared by BAM in Germany, Liste der Nichtmetallischen

Materialen, can be ordered from Jedermann-Verlag KG-Fax No. + 49 6221-278 70.(6) E.g. Cr Ni steel with 18-22% Cr by weight (some national standards require at least 22% Cr).(7) Carbon steel may be allowed in pipes for a pressure below 40 bar and a flow rate not exceeding 25

m/s. Grey iron (cast iron) may be used for valves and components if the pressure is 10 bar or less.Please refer to national authorities for guidelines.

(8) Ref. IGC DOC. 13/82-The Transportation and Distribution of Oxygen by Pipeline.(9) Valve manufacturers marking of oxygen compatibility may not always be relied upon. If in doubt

check the test certificate from the testing organisation.(10) Ref. ASTM G 88-84.(11) A measured resistance of 10 M Ohm or less will prevent electrostatic charging of e.g. a pipe or hose.

CEN- Standardisation bodiesISO- Standardisation bodies2. OXIDISING GASES