Why is oil and grease-free so important in oxygen systems?

Transcription

1 Why is oil an grease-free so important in oxygen systems? 1. Forewor Oxygen this vital gas is more complex in its use than one woul imagine when consiering how matter-of-factly we inhale an exhale every ay. Inustry, in particular, is constantly confronte with new challenges in the hanling of oxygen. SERTO AG, a manufacturer of tube connecting elements an valves, meets these upmarket emans an has evelope a special cleaning process for this purpose. This technical bulletin is a summary of intensive research in the subject matter. It aresses the properties of oxygen as a gas an focuses on the essential points of working with this gas. 2. Oxygen (O 2 ) Oxygen is foun in large quantities in nature. Approximately half of the Earth s mass is mae of chemically combine oxygen. Atmospheric air contains about 21% oxygen (oxygenium). The chemical element oxygen is a colourless an oourless gas. Pure oxygen oes not burn; it is, however, absolutely necessary for combustion an breathing. Oxygen bons with all elements. When oxygen bons with another substance, this is referre to as oxi. Combustion is an oxi. If this process is rapi, a flame occurs; if it is suen, we call it an explosion. Most substances burn fast an intensely with compresse or pure oxygen. This is also true of substances which cannot be combuste in atmospheric air. Some Airsepar unit substances react so violently with oxygen that they either combust after igniting or even self-ignite, such as for example oils an greases. The Professional Associ of the Chemical Inustry has taken issue with this subject matter among other. In their accient prevention guielines 1 they efine oxygen as follows: Pure oxygen All compouns with a volume content of > 21% oxygen source: Line 2.1. Manufacture an applic The cryogenic process use for oxygen recovery was evelope over 100 years ago by Carl von Line an name after him. In this process, air is free of water vapour, ust an carbon ioxie, compresse, an coole own to a very low temperature. Subsequently, it is separate into components through istill (fraction). At -196 C nitrogen is first to vaporise an nearly pure oxygen remains, which only transforms into the gaseous state at 183 C. Toay, other physical processes are also use to separate an clean air gases, for example separ through a membrane or asorption. In these methos, some elements are asorbe via a special material while others pass through unhinere. 1 Unfallverhütungsvorschrift 62 Sauerstoff, Fassung vom 1.Januar 1997 (Accient prevention guieline 62 Oxygen, version ate 1 January 1997) 2003 SERTO AG, Switzerlan 1/7

2 Oxygen is use in iverse applics. Besies being utilise as a respiratory gas in meicine, avi an space travel, oxygen is also use in inustry mainly as an oxiising agent in combustion processes where high temperatures are neee: in metallurgy, for manufacturing pig iron an steel an for refining copper in chemical processes, for olefin oxi, for partial oxi of coal an heavy oil to create hyrogen an synthetic gases, for the prouction of sulphuric an nitric acis, of acetylene,... in autogenous technology for fusion, cutting, flame treatment, for thermal cutting, e.g. of concrete in rocket technology in treatment of rinking water an waste water for ozone prouction Other significant fiels of applic are measurement technology, fuel cells, semiconuctor technology an biological processes. 3. Hazars in oxygen systems The EIGA 2 examines this subject matter in its IGC 04/00 3 ocument because oxygen is more angerous than one woul think! Even though oxygen itself oes not burn, an is actually essential for life, it harbours a high risk potential. Hence, a great eal of caution is require In hanling oxygen. Oxygen oes not only support combustion, but in its purest form it will cause most known substances to combust rapily or even suenly (explosion) when uner pressure. It is not possible to etect the presence of high oses of oxygen without technical instruments What is necessary to ignite a fire? Generally speaking, three elements are necessary: fuel, oxygen an a source of ignition. If one element is missing, there can be no fire. combustible material Ignition Principally, the higher the oxygen content, the greater the risk of an oxygen fire the lower the ignition temperature the higher the flame temperature an thus the more estructive the fire Oxygen The fire triangle is a common metho use to illustrate 2 European Inustrial Gases Associ 3 Fire Hazars of Ox ygen an Ox yge n-enriche Atmosphere 2003 SERTO AG, Switzerlan 2/7

3 3.2. Combustible materials Basically all materials can burn with oxygen; this also hols true for most metals an metal alloys. Oils an greases consist to a large extent of carbon an are particularly angerous in oxygen systems since these are highly combustible an explosive. Igniting oils an greases in oxygen plants often leas to a chain reaction, which finally ens in metals melting or burning. Some of the melte material may splatter aroun, possibly causing oxygen to be emitte which woul in turn cause the fire to sprea more rapily Oxygen Oxygen is inseparable in these systems, i.e. the fuel (tubes, unions, valves or seals) comes unequivocally into contact with oxygen Source of ignition In systems that are uner pressure, the source of ignition is not as obvious as an open flame or hot surface. The fire may have the following origins: friction mechanical influences electric sparks high flow velocity simultaneous with the presence of particles heating-up from turbulence heating-up from aiabatic compression Aiabatic compression takes place when oxygen uner high pressure is injecte abruptly into a system with low pressure. In such instances, the gas can flow at the spee of soun. This situ often occurs with valves an fittings. When a gas collies at great spee with a resistance, the temperature rises very quickly ue to aiabatic compression. This is always the case when gases are compresse so fast that no thermal energy can get lost. The general rule is: the higher the initial pressure, the higher the temperature. This circumstance is put to use in the iesel engine. Most often, sai resistance is represente by close valves. Aiabatic compression close open open 2003 SERTO AG, Switzerlan 3/7

4 4. Design an materials in oxygen systems 4.1. General Safety in oxygen systems begins with the construction, respectively the esign, of the system. Owing to the risk potential of such systems, it is absolutely necessary to work exclusively with specialists an professionals qualifie in this matter. The inform containe herein is only a recommen; the builer is in no way release from consulting a competent an traine professional. The ASTM G88 4 stanar efines a professional as follows: Qualifie professional: person, such as a chemist or engineer, who on the grouns of his or her training an experience knows how to apply physical an chemical principles that occur in the reaction of oxygen with other materials. Shoul there be in such professional in your vicinity, your oxygen supplier can surely be of assistance Risks an hazars in oxygen systems To increase the safety of oxygen systems an to avoi potential risks, it is important that the possible sources of anger are recognise Flow velocity The flow velocity etermines whether the gas can catch fire or not. At high velocities, particles can self-ignite when colliing with pipes an fittings an thus start a chain reaction. It is therefore essential that there are no high velocities. Suen transitions from large to small iameters are to be avoie in particular. Abrupt changes in irection or turbulence can also lea to high temperatures, which in turn can cause the gas to ignite. For these reasons, it is important to ensure that fittings, tees an other similar fixtures are not place too closely to the pressure unit, e.g. the pressure pump. V1 V Materials Only flame resistant materials shoul be use. The alloying composition, the component strength (wall thickness), the temperature, the pressure an the purity of the oxygen are all key elements which influence flammability. If the pressure is uner a certain limit 5, the velocity is no longer a risk as such Aiabatic compression This can occur, for example, when a valve is opene quickly an oxygen uner high pressure hits on a close valve. To prevent this from happening, it is important that manually operate valves are only opene slowly Cleanliness The cleanliness of the system an the components is ecisive. All reputable institutions, organiss an manufacturers have guielines, specifics an norms which treat this subject explicitly. Especially noteworthy in this regar is ASTM G Stanar Guie for Designing Systems for Oxygen Service 5 IGC 13/02 Anhang D 2003 SERTO AG, Switzerlan 4/7

5 The safety of the system can be improve consierably by the respective esign. In one particular instance, a ball valve that was uner high pressure on one sie was opene accientally uring oper. The gas flowe at high spee into the low pressure area behin it an hit upon a tee. This impact cause the system to ignite, burning the stainless steel tube an the valve. There are ifferent ways of esigning an oxygen system so that oxygen fires can be prevente; it is nevertheless avisable to consult a specialist in each case. 5. Materials The correct materials selection is extremely important because how an where oxygen fires sprea epens greatly on the flame resistance of the materials use. The right materials in the right places can, for example, prevent a potential fire from spreaing Metallic materials Copper-base alloys, such as brass, bronze or copper-nickel alloys, have a long history of use in oxygen systems an are very suitable for such purposes. Stainless steels can also be implemente. The combustion resistance of stainless steel alloys (per DIN 1.4xxx) lies between the copper alloys an C steel alloys. C steel shoul only be use, if at all, in systems with less than 2 bar operating pressure Non-metallic materials These are not as suitable for oxygen systems an are usually use only for gaskets, lubricants or valve packing to create a better seal or to reuce friction. Accoring to IGC Doc 13/02 6, PTFE an FEP are best suite for oxygen applics. Elastomers, such as Kalrez, Viton, etc., are also suitable. However, toxic gases may be emitte when burne. Threa seals are often mae of plastics; it is therefore essential that a sealant is selecte which is compatible with oxygen. Threa wrappe with PTFE tape has proven very effective in practice. 6. Cleaning an cleanliness As inicate above, the cleanliness of the components is a very important factor in renering oxygen systems safe. A system is consiere clean when the removal of organic an inorganic contamin has been warrante. The removal of such contamin, for example greases, oils, threa seals, lubricants, shavings, etc., is ecisive. Perioic inspection of the system is recommene. To ensure the necessary egree of cleanliness, all reputable institutions, such as ASTM, CGA, EIGA, NPF, specify that the components (valves, unions, tubes) must be pre-cleane by the supplier Organic contamin Mineral oils an greases are largely compose of carbon compouns. The carbon bons with many substances, but especially with hyrogen an oxygen. Since carbon oxiises very easily together with oxygen, extremely high temperatures can occur rapily (to some extent explosively) which again can lea to self-ignition of the materials in its environment an thus to a chain reaction. It is therefore inispensable that all component surfaces which come into contact with oxygen are oil an grease-free. For this reason, oils an greases may never be use as lubricants in oxygen systems SERTO AG, Switzerlan 5/7 6 Oxygen Pipeline System

6 7. Cleaning an assembly of system parts an components from SERTO AG in piping systems with enriche oxygen As a manufacturer of metal-to-metal sealing, raial (is)mountable, compression ferrule tube unions an valves, we strive to meet the requirements of the market. There are numerous norms an guielines pertaining to the cleaning of system parts, which emonstrates how important this is. The ASTM 7 norm G is eicate to this subject matter an serves as the basis for the entire cleaning process use at SERTO AG Contamin In Paragraph General conservative target it is state: For the majority of systems, a cleanliness target parameter of ca. 1-5 mg/ft 2 (11-55mg/m2) or less of unesirable oils an greases is recommene.... SERTO has set a target of 33 mg/m2 of non-volatile resiual contamin. This correspons to the cleanliness level B 9 accoring to ASTM Cleaning the components In orer to achieve this high level of cleanliness of the surfaces which come into contact with the meia, we have evelope, in collabor with specialists, a multi-level process. The components to be cleane are place iniviually in special baskets; this ensures optimal off-flow of the cleansing an rinsing agents. Depening on the material brass or stainless steel the components go through a series of alkaline an aci cleaning baths. The SERTO company takes special care that only environmentally frienly substances are use. The parts are rinse several times, but particularly at the en of the process, in warm DI-ultrasoun water baths an then rie with filtere air to warrant a resiue-free cleanliness. The selecte cleaning process is escribe in ASTM G During implement of the process, the EMPA 11 was consulte an the surface cleanliness of the cleane components was teste by them using iverse methos. These tests are one perioically. Cleaning st SERTO AG 7.3. Monitoring an control The process stability is very important, which is why various measures an proceures were efine for this purpose. For example, the quality of the cleansing an rinsing agents is monitore electronically in the entire cleaning system. All the prouction steps are carrie out by specially traine personnel an are ocumente accoringly. The quality of the surface cleanliness is checke an warrante perioically by means of the test methos containe in ASTM G Each step in the cleaning process is ocumente an traceable. 7 American Society for Testing an Materials 8 Stanar Practice for Cleaning Methos an Cleanliness Levels for Material an Equipment Use in Oxygen-Enriche Environments 9 ASTM Kapitel Stanar Practice for Cleaning of Materials an Components by Ultrasonic Techniques 11 Eigenössische Materialprüfungsanstalt (Swiss Feeral Materials Testing Institute) 12 Stanar Test Metho for Determin of Resiual Contamin of Materials an Components by Total Carbon Analysis Using a High Temperature Combustion Analyzer 2003 SERTO AG, Switzerlan 6/7

7 7.4. Assembly To make sure that the components are not re-contaminate after cleaning, a separate prouction an assembly cell, which is irectly ajacent to the cleaning system, was set up especially for the assembly of these proucts. Personnel working in this area must follow strict reguls Lubricants Accoring to IGC 13 13/02, lubricants shoul be avoie whenever possible. If this is not feasible, a lubricant which has been teste an approve for use in oxygen systems must be use. SERTO uses a lubricant teste an approve for use in oxygen system components by BAM 14. The lubricant was selecte so that the normal applic upper limits of the proucts, +200 C an 220 bar, i not nee to be reuce Seals O-rings an other seals are necessary in regulating an check valves. Since these are generally mae of non-metallic materials, special attention must be given to these parts. SERTO only uses materials from manufacturers whose proucts are BAM-approve Testing The valves are teste in such a way as to prevent re-contamin after function testing Packaging In orer to maintain the cleanliness uring transport an storage until the components are use on site, the parts are package iniviually in wel-seale plastic bags. The bags are specially marke so that the contents can be ientifie without having to open the bags. The oil an grease-free cleane an package components from SERTO AG, are safe for use with oxygen provie that all the necessary precautions were taken, especially in the final assembly, so that the components are not re-contaminate. The entire cleaning, assembly an monitoring process is containe in SERTO s own CSO-OX cleaning reguls. Attention The content of this technical ocument is intene to make our customers more aware of the angers involve in hanling oxygen. We consier this a service to our customers. For safe, trouble-free oper, the complete system esign must be taken into consier when selecting a prouct. The system builer an the user are responsible for the functioning of the proucts, the material compatibility, the respective performance characteristics an applic limits as well as aherence to the reguls concerning hanling, oper an maintenance. We strongly recommen consulting a specialist in this case. SERTO AG is not a specialist in this fiel an enies all liability an responsibility. 13 Inustrial Gas Council 14 Bunesanstalt für Materialforschung un Prüfung (German Feeral Institute for Materials Research an Testing) 2003 SERTO AG, Switzerlan 7/7