Ctef Sts Group 4

8/3/2019 Ctef Sts Group 4

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Comit Technique Europen du FluorCTEF

Working GroupStorage, Transport and Safety (STS)

Recommendation onMaterials of Construction for

Anhydrous Hydrogen Fluoride (AHF)and Hydrofluoric Acid Solutions (HF)

Group 4First editionApril 2009

This document can be obtained from:

CTEFAvenue E. Van Nieuwenhuyse 4

Box 2 - B-1160 BRUXELLES

[email protected]

WWW.EUROFLUOR.ORG


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MATERIALS FOR USE WITH HYDROFLUORIC ACID page 2 of 23

PREFACE

Hydrogen Fluoride (HF) is essential in the chemical industry and there is a need forHF to be produced, transported, stored and used.

The HF industry has a very good safety record; nevertheless, the European HFproducers, acting within CTEF have drawn up this document to promote continuousimprovement in the standards of safety associated with HF handling.

This Recommendation is based on the various measures taken by membercompanies of the CTEF.

It in no way is intended as a substitute for the various national or international

regulations, which should be respected in an integral manner.

It results from the understanding and many years experience of the HF producers intheir respective countries at the date of issue of this particular document.

Established in good faith, this recommendation should not be used as a standard ora comprehensive specification, but rather as a guide which should, in each particularcase, be adapted and utilised in consultation with an HF manufacturer, supplier oruser, or other experts in the field.

It has been assumed in the preparation of this publication that the user will ensure

that the contents are relevant to the application selected and are correctly applied byappropriately qualified and experienced people for whose guidance it has beenprepared.

The CTEF does not, and indeed cannot, make any representation or give anywarranty of guarantee in connection with material published in CTEF publicationsand expressly disclaims any legal liability or responsibility for damage or lossresulting from the use, or misuse, of information contained in this document.

The contents of this recommendation are based on the most authoritativeinformation available at the time of writing and on good engineering practice, but it isessential to take account of appropriate subsequent technical developments orlegislative changes. It is the intent of the CTEF that this guideline be periodicallyreviewed and updated to reflect developments in industry practices and evolution oftechnology. Users of this guideline are urged to use the most recent edition of it, andto consult with an HF manufacturer before implementing it in detail.

This edition of the document has been drawn up by a Working Group "Storage,Transport and Safety" to whom all suggestions concerning possible revision shouldbe addressed through the offices of CTEF. It may not be reproduced in whole or inpart without the authorisation of CTEF or of members companies.


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SUMMARY

1. Introduction - General remarks

2. Materials2.1. Metallic materials2.1.1. Non alloyed Carbon steel2.1.1.1. For Use with Anhydrous Hydrofluoric Acid (AHF)2.1.1.2. For Use with Hydrofluoric acid solutions (HF2.1.2. Cast Iron2.1.3. Stainless Steels2.1.4. Monel, Nickel, Inconel

2.1.5. Copper and Bronze2.1.6. Lead2.1.7. Magnesium2.1.8. Titanium, Tantalum2.1.9. Aluminium, Aluminium Alloys (without Copper), Tin, Zinc, Yellow Brass,

Zirconium2.1.10. Silver, Platinum2.2. Ceramics Glass Bricks Mineral Fibres2.3. Organic materials2.3.1. Rubber and Ebonite2.3.2. Polyethylene, Polypropylene, PVC

2.3.3. Cotton, Asbestos free fibres2.3.4. Graphite Carbon2.3.5. Fluorinated Polymers2.3.5.1. PTFE2.3.5.2. FEP, PFA, ETFE2.3.5.3. PTFCE (Polytrifluorochloroethylene)2.3.5.4. PVDF (Polyvinylidene fluoride)

3. Applications3.1 Tanks3.1.1 Tanks, vessels and ISO Container for AHF3.1.2 Tanks, vessels and ISO Container for more than 70% and less than

85% HF3.1.3 Tanks, vessels and ISO Container for aqueous HF3.1.3.1 Lined steel tanks and vessels3.1.3.2 ISO Container or railway tank for more than 70% and less than 85% HF as

lined steel tanks3.1.3.3 Plastic tanks3.2 Piping and pumps3.2.1 Piping and pumps for HF concentrations of 85% or higher3.2.2 Piping and pumps for HF concentrations of 40% HF to 85%

3.2.3 Articulated arms for anhydrous HF loading and unloading3.3 Connections (i.e. branches, flanges, nuts and bolts)


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3.3.1 Branches and Flanges3.3.2 Nuts and Bolts3.4 Valves3.4.1 Valves for AHF3.4.2 Valves for HF solutions3.5 Gaskets3.6 Hoses and flexible steel pipes3.6.1 Hoses3.6.2 Flexible Steel Pipes3.7 Thin section applications (i.e. flexible bellows, bursting discs)


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1. Introduction - General remarks

This recommendation is written to provide general advice on the suitability of variousmaterials for industrial application with hydrofluoric acid (HF) in terms of corrosionresistance concerning new and replacement facilities. It does not attempt to define thevarious corrosion processes but indicates the conditions under which certain materialscan be used or should be avoided. Care must however be taken to consider thepossibility of the presence of other constituents in either the HF or the materials ofconstruction, as the presence of certain trace components can considerably influencethe corrosion behaviour. Practical testing under service conditions is therefore the bestguide to the suitability of any particular material.

The recommendation does not cover the question of mechanical properties, which are

more appropriately dealt with in specific engineering recommendations. Theserecommendations, in any case of uncertainty, should not be taken as a firm guide butreference should be made to a HF manufacturer to confirm the suitability of anymaterial for a given duty.

Hydrogen fluoride is an extremely hazardous material, in the liquid or vapour form.Prolonged exposure to even a few ppm can cause painful irritation. Clearly, greatcare must be taken in handling any material that has been exposed to AHF

This document does not include all acceptable materials of construction. Processvessels and piping constructed for AHF service should follow recognized design

codes


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2. Materials

2.1 Metallic materials

When a small AHF leak mixes with moisture in the air it forms a highly corrosiveaqueous HF acid. This will corrode both Carbon steel and austenitic Stainless Steelfasteners. The corrosion rate on Carbon steel is much higher than that on theStainless Steel.

2.1.1 Non-alloyed Carbon Steel

2.1.1.1 For Use with Anhydrous Hydrofluoric Acid (AHF)

Carbon steel is the most commonly used material for handling liquid anhydrous HF ordry gas. All steel components must be thoroughly cleaned and dried, before cominginto contact with HF. Carbon steel when used in HF is then protected by a layer offerric fluoride, and is resistant up to about 100C in liquid (above 100C somecorrosion must be expected which increases drastically with temperature). If theprotective layer of ferric fluoride is removed by erosion or due to the presence ofreactive material or where the metal has a large specific surface area, this uppertemperature limit is reduced. For practical purposes, the maximum recommendedtemperature for Carbon steel in the presence of anhydrous HF liquid is 100C, and thelimit for dry gas is higher.

In order to avoid destruction of the protective layer of ferric fluoride due to erosion, thelinear velocity of liquid HF at the vessel wall should be limited. The normal practice forpipe work is to limit HF velocities to 1,0 m/sec on average at ambient temperature andlower velocity, i.e. less than 0,6 m/sec, for higher temperature. This limitation applies toall metals in connection with HF.

Forged or cast steel can be used on condition that the mechanical property of thefabricated components have been studied for the range of temperatures and stresswhich might be encountered. For cast steel inclusions shall be reduced as far aspossible.

2.1.1.2 For Use with Hydrofluoric Acid (HF)

The corrosion resistance of steels in hydrofluoric acid is reduced as the HFconcentration in solution in water is decreased. However, at ambient temperatures,Carbon steel is used on HF solutions of more than 70 % weight. Under theconcentration of 85 %, some corrosion must be expected. The corrosion rate willincrease dramatically below 70 % HF. (For example: HF 70 % < 0.5 mm/year;HF 65 % < 1.5 mm/year).These corrosion rates are significantly increased by lower concentration of HF anderosion.

The use of Carbon steel on HF solutions of less than 70 % is not recommended. Dilutesolutions under 60 % will attack steel rapidly even at low temperatures.


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Corrosion of Carbon steel in HF will result in hydrogen formation. Therefore, care mustbe taken to prevent any explosive mixture between hydrogen and air.

Even in concentrations of more than 85 % and with anhydrous HF, some atomichydrogen migration will occur into the steel sheet. This may cause blistering if the steelsheet is not free of defect of lamination or inclusions.

A special grade which is vacuum degassed during the steel making process, which isdesulphurized to below 0.010 % and which has alumina particles modified incomposition and shape may be recommended. The steel sheets used in theconstruction of HF tanks should be checked ultrasonically to ensure the steel islamination free. All reinforcements welded on the steel must be provided with vents for

hydrogen venting.

Steels having a hardness exceeding 225 HB (Brinell) after welding may be subject tohydrogen assisted stress corrosion cracking in anhydrous HF, particularly if HFcontains high levels of surface poisons such as arsenic.

Corrosion of steel in HF solutions will increase with temperature.

2.1.2 Cast Iron

Its use in HF is not advisable due to risks of porosity and embrittlement and in some

cases corrosion rates much higher than Carbon steel or cast steel. Under specific welldefined circumstances specific grades malleable or nodular grades can however beused where there will be no problem due to mechanical shock or tensile forces. WithCast Iron components, the material needs to be checked for absence of defects whichcould cause porosity with HF.

2.1.3 Stainless Steels

Austenitic chromium Nickel steels do not have a better resistance to anhydrous HFthan Carbon steels, and cannot be used at higher temperatures.

Dilute solutions of less than 10 % show moderate corrosion rates at roomtemperatures for austenitic steels stabilized. Monel and Hastelloy C will have normallythe best resistance and therefore should be recommended. For Alloys 20, seamlesspipes should be preferred to welded pipes.

Austenitic steels may be subject to hydrogen assisted stress cracking in hot (> 100C)diluted solutions.

Fluoride ions could promote stress corrosion cracking even at low temperature,especially with AISI 304 SS if insufficient stress relieved. Stainless Steel should be

used only with a low Carbon content and Titanium or niobium stabilization.


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Special attention for cast pieces must be taken on the silicon content which issometimes used to increase the cast ability.

Ferritic Stainless Steel should not be used in a hard condition (Brinnel over 225 HB)due to hydrogen stress cracking.

2.1.4 Monel, Nickel, Inconel

Monel has a good resistance to HF at room temperature in HF solutions of more than50 % HF where velocity or agitation is limited in order to prevent the removal of thepassivation.

Increase of temperature up to 80C will cause some corrosion which increases as the

concentration decreases from anhydrous to 50 %.

Monel may be subject to stress corrosion cracking, especially if cold formed or if weldsare not stress relieved. Do not apply Monel 60 wires for welding, these welds corrodesin HF.

Oxidising agents and reducing sulphur compounds increase the corrosion rate of allthese materials.

Copper-Nickel Alloys (with 30 % Nickel) are less resistant than Monel itself.

Nickel and Inconel are less resistant to HF solutions than Cooper and Monel.

Monel, Nickel and Inconel are suitable for HF gas up to 600C even in the presence ofmoisture.

2.1.5 Copper and Bronze

Copper has sometimes been used in the range of 60-90 % concentration of HF up tothe boiling point with a moderate corrosion rate.

Bronze has also an acceptable resistance at room temperatures and is sometimesused for pump castings in HF solutions. But it is not suitable for higher temperatures.

These materials are subject to embrittlement.

These Alloys should be used with great caution in AHF service.

2.1.6 Lead

Chemical Lead is fairly resistant to HF solutions up to 40 % at ambient temperatureand Lead coated steel has been used in the past for metal fluorides fabrications.

Corrosion increases with temperature and oxidizing agents, such as sulphuric acid.


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2.1.7 Magnesium

Magnesium is resistant to HF at all concentrations at room temperatures. Magnesiumalloy (6 % Al - 1 % Zn) is fairly resistant (< 0.2 mm per year) but the corrosionincreases with temperature.

2.1.8 Titanium, Tantalum

Titanium and Tantalum are strongly attacked by hydrofluoric acid at all concentrations.

2.1.9 Aluminium, Aluminium Alloys (without Copper), Tin, Zinc, Yellow Brass,Zirconium

None of these materials should be used with hydrofluoric acid, not should Alloys basedon these metals except Copper Alloys (see Copper) despite the fact that pureAluminium has some resistance to anhydrous HF.

2.1.10 Silver, Platinum

Silver is resistant to HF solutions up to 50 % at temperatures up to the solutions boilingpoint.

Platinum is resistant in all concentrations at temperature up to the boiling point of the

solution.

2.2 Ceramics, Glass Bricks Mineral Fibres

All these materials containing silica are strongly attacked by HF even diluted and at lowtemperatures and must not be used in HF service.

Only Carbon bricks are suitable for HF.


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2.3 Organic materials

The following comments deal only with the use of the materials on construction dutiesor as a lining. Their behaviour when used on specialised duties are dealt with inparagraph 4. Most of these organic materials are slowly attacked and require aschedule of inspection and replacement before they become defective. Many of theplastic materials listed are liable to stress corrosion cracking and the fabrication ofcomponents or systems from plastics should be done so as to avoid regions of highstress during manufacture or service.

2.3.1 Rubber and Ebonite

All forms or synthetic or natural Rubber lack mechanical strength and are attacked by

HF. The slow rate of attack of Rubber in very dilute solutions of HF means that it ishowever frequently used as a lining material (natural, Rubber, butyl Rubber) for weaksolutions lining (less than 50 %) and room temperatures. Care must be taken of thequality of the filling materials and their resistance to HF.

2.3.2 Polyethylene, Polypropylene, PVC

Polyethylene, Polypropylene and PVC are fairly resistant to weak hydrofluoric acidsolutions up to 50 % at ambient temperatures up to 40C.

High density Polyethylene can be used up to 75 % at room temperature.

Polypropylene and PVC can be reinforced with polyester fibber-polyester resins(Fibreglass must be avoided for this application).

Polyethylene and Polypropylene are suitable for weak acid bottles construction butcare must be taken to the resistance to shock (wall thickness, reinforcements, ...) andto the UV light and heat sensitivity of the materials.

Small drums made of Polyethylene are used for HF solutions up to 70% with amaximum service of two years.

Under certain conditions of stress and in the presence of some environments includingHF, Polyethylene may exhibit mechanical failure by cracking. Therefore it is notrecommended to build large vessels with this material for HF solutions.

Unless Polypropylene is less susceptible of stress corrosion cracking, the aboverecommendation should be extended to this material.

Polyethylene in the presence of concentrated HF solutions will allow penetration of HFin material and therefore reduction of the mechanical properties. Penetration increaseswith the concentration. Polyethylene and Polypropylene should be avoided on

anhydrous HF. If bottles are used for sampling the total contact time must be limited to24 hours maximum.


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2.3.3 Cotton, Asbestos free fibres

Mineral fibre filled plastics, silicone rubbers, polyurethane's, methacrylates, furanesresins, polyamides:

Do not apply in HF service.

2.3.4 Graphite - Carbon

Graphite equipment (especially with PTFE impregnation) can be used on HF up to50 % concentration and up to the boiling point.

Graphite treated with PTFE for impermeability is resistant to higher liquidconcentrations (60 to 70 %). Normal impregnation should only be used for weaksolutions and low temperatures.

Pure Graphite can be used for gaskets, packing, etc...

2.3.5 Fluorinated Polymers

Below are listed some of the fluorocarbon plastics that have been used successfullyin loose lined pipe, valves, and other equipment, and for gaskets and seals in AHFservice.

PTFE polytetrafluoroethyleneFEP copolymer of PTFE and HFP (hexafluoropropylene)PFA copolymer of PTFE and a fluorinated etherMFA copolymer of PTFE and a fluorinated etherETFE copolymer of PTFE and PolyethylenePVDF Polyvinylidene fluoride, also a copolymer of PVDF with HFPCTFEpolychlorotrifluoroethyleneECTFE copolymer of CTFE and PolyethyleneTFM PTFE with PFA

2.3.5.1 PTFE

PTFE resists hydrofluoric acid anhydrous and solutions pretty well. It should be notedhowever that PTFE is not completely impervious. This problem of permeability can bereduced by the use of a suitable thickness. Special care needs to be taken with hightemperature. Where PTFE is used as a protective layer, care must be taken to guardagainst attack on the supporting material. For supporting material and lined vessels, itshould be considered that the material is provided with vent-holes to purge out thehydrogen coming from the unavoidable corrosive attack.

2.3.5.2 FEP, PFA, ETFE


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These fluorinated copolymers are also used because of their corrosion resistance toHF and their better possibility than PTFE for protective layers.

2.3.5.3 PTFCE (Polytrifluorochloroethylene)

PTFCE has a good resistance to HF and can be used for fittings or pump pieces(particularly for sight Glass as the material is transparent).

2.3.5.4 PVDF (Polyvinylidene fluoride)

PVDF has a good resistance to HF solutions up to, at least, 60 % and up to theirboiling points.

PVDF can be reinforced with polyester fibbers and polyester resins to give improvedmechanical strength for applications at higher temperatures and pressures. (Care mustbe taken of the differential expansion of the materials).


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3. Applications

The following comments concern the pure materials, without plasticizers, fillers,coatings, greases or other potentially reactive ingredients. The choice of materials ofconstruction for any systems therefore, must be controlled to avoid the introduction ofmaterials which could react with hydrofluoric acid.

It is also important when using any component (which is itself made from a satisfactorymaterial for use with HF) to separate HF from other fluids, that account is taken of thepotential for leakage occurring between the two fluids. This could occur incomponents associated with heat transfer or hydraulic systems, where it is important toensure that the fluid is not reactive to HF.

All ancillary equipment (instruments, sealing arrangements etc...) should always beverified to be made from components of materials which are compatible with HF.Materials used for thermal insulation should also be selected from those whichgenerate corrosive products under service conditions.

3.1 Tanks

3.1.1 Tanks, Vessels and ISO containers for AHF

The steel chosen for the construction of HF storage tanks should be of fine grain

steel, readily welded and which has the appropriate level of impact strength at theminimum design temperature after welding. In order to guarantee the optimalconditions for welding, the tensile strength of the steel should be limited. The steelplate should be subjected to acceptance tests according to national or internationalcodes. These tests are particularly important concerning the impact strength beforeand after welding.

The use of conventional steel has sometimes resulted in hydrogen blistering. Thisblistering occurs when galaxies of inclusions and laminations are present in the steel.A special grade which is vacuum degassed during the steel making process, which isdesulphurized to below 0.010 % and which has the Al2O3 particles removed shouldbe recommended.

Carbon steel approved for use with HF shall be used to construct the tank andinternal fittings. Low hardness steel is required. Steel sheets should be checkedusing ultrasonic to ensure it is lamination free.

Storage tanks must be designed to withstand unloading pressure as well as vacuumin the event internal pressure drops below atmospheric pressure when the acidcools.

Storage tanks for hydrofluoric acid should be manufactured and controlled to achievea high standard of quality.


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A corrosion allowance of 2 mm (for carbon-steel tanks) is considered as a minimumrequirement. Higher corrosion allowance should be considered depending on the HFconcentration, temperature and specific risks depending on the application.For transportation tanks, a safety margin of 2 mm is recommended by CTEF.

This rate can highly increase in case of high velocity of the flow and because oftemperature. Local turbulences must be avoided for the same reason. Anotherproblem can arise from moisture ingress. If a tank vents to the atmosphere, themoisture in even low humidity air can dilute the surface layer of acid so this layer canbecome more corrosive. This is suggested to avoid maintaining the level at constantvalue for long periods of time.

The same could happen if the moisture ingress causes an acid dew point in thevapour space with subsequent condensation. For these reasons the purging of thevapour space by a dry inert gas or the use of closed vessels under slight overpressure which does not need to breathe is recommended.

In addition the thickness of the wall shall be monitored periodically by ultra sonictests.

3.1.2 HF Tanks, Vessels and ISO Container for more than 70% and less than85 % HF

The material of construction depends on the concentration of the acid and the storagetemperature.

Between 60 % and 85 % advice from the supplier is necessary.

The minimal design pressure of the shell is 10 barg, as requested by RID/ADR.

The 10 barg design pressure of the shell allows a minimum thickness of the shell as agood protection against mechanical impact in case of accident.

The concentration of the acid and the expected ambient temperature must be takeninto account. High temperatures increase the rate of corrosion and low temperaturesmay lead to embrittlement of the Carbon steel.

The steel must have good weld properties, good resilience to -20 C after welding andgood ductility characteristics. A minimum elongation of 22% at fracture is required. Themaximum ultimate tensile strength at rupture should be less than 63 Kg/mm2.

Carbon steel may suffer hydrogen stress corrosion cracking and hydrogen blistering incontact with aqueous HF.

The steel should be tested for resistance to hydrogen assisted stress corrosion


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Construction of ISO tanks or Railway tanks for HF should only be carried out by anapproved contractor.

Carbon steel approved for use with HF shall be used to construct the tank and internalfittings. Steel sheets should be checked using ultrasonic to ensure they are laminationfree.

Higher corrosion allowance should be considered depending on the HFconcentration, temperature and specific risks depending on the application.

Example for calculation:Wall thickness according to mechanical strength and designpressure x mm.

Lower limit of the wall thickness (x value) for atmosphericpressure operation 6 mm.

Corrosion allowance for 20 years 25 C (20 x 0,4 mm.*) 8 mm.

Total minimum thickness 8 + x(minimum

14)

mm.

(*) 0.4 mm. corrosion per year for HF 70% at 25 C. (Additional thickness to be addedfor higher temperatures).For transportation tanks, a safety margin of 2 mm is recommended by CTEF.

In addition, the thickness of the wall shall be monitored periodically by ultra sonic tests.

3.1.3 Tanks, Vessels and ISO containers for aqueous HF

For concentrations up to 60 % lined tanks are required.

3.1.3.1 Lined steel tanks and vessels

The lining must consist of a suitable material:1. A double layer of i.e. butyl Rubber or chlorobutyl that should have at least athickness of 3 mm each, in order to avoid HF-diffusion through the lining, thusattacking the steel associated with formation of hydrogen.Or:2. A single layer of polyethylene that should have at least a thickness of 6 mm,.

In case of organic impurities, special care must be taken to ensure the durability of thelining.

Note: it is very important to choose carefully the manufacturer who will prepare and


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carry out the lining on the metallic wall. The quality of the lining sticking is fundamentalbecause it affects significantly the mechanical resistance and the porosity of the plasticliner, and, consequently, the final resistance of the lined tank against corrosion.A successful construction starts from metallic surface preparation: high care must alsobe taken to correctly prepare the surface to support the lining (e.g. welds must belevelled by grinding and sharp connections rounded). This requires a deep interactionbetween the vessel constructor and the manufacturer of the liner.

3.1.3.2 Plastic tanks

CTEF does not recommend the use of Polypropylene as material of construction

because of the relatively quick sunlight ageing.

Polyethylene can be used as material of construction for the storage of solutions up toaround 70 - 75 % HF at ambient temperature.

For outdoor installation, the choice of Polyethylene stabilized against sunlight ageing isrecommended.

The wall thickness depends on the hydrostatic pressure: this plastic constructionshould be limited to atmospheric pressure storage.

The use of Glass fibre reinforced plastic is not recommended because of the potentialrisk of degradation of the Glass fibre reinforcement by HF corrosion:Note that this risk increases with temperature, pressure and concentration.

Polyethylene can be used for small drums and containers for transportation of limitedquantities of solutions up to 70 - 75 % HF, with a life-time of maximum 2 years, and inaccordance with RID/ADR regulations limitations.

Such drums and containers exist for concentrations:

- up to 60 % HF, containers of maximum 1000 litres and a maximum lifetime of 2years ;

- up to 75 % HF, drums of maximum 200 litres and a maximum life-time of 2 years.

Note: we recall that the lifetime of the drum / container is calculated from the date ofmanufacture, whatever the tank has been or not in contact with HF. This life-timelimitation is due to the loss of flexibility of the plastic with time.

3.2 Piping and Pumps

3.2.1 Piping and pumps for HF-concentrations of 85 % or higher:


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All materials and components used in liquid HF piping systems should at leastcomply with national codes and standards.This guide, however, is not intended to limit the use of other codes and standards,but only to provide information as to possible choice.All components used in piping systems should be fabricated in materials which arecompatible with anhydrous liquid HF.Carbon steel is however the most commonly used material for the temperatureoptions stated below, but care should be taken to avoid corrosion.

Mechanical properties

Pipe wall thickness for liquid HF lines

Various standards are available which provide guidance to the designer forcalculating the mechanical strength and wall thickness of pipes. However, wherecalculations have not been carried out, the following minimum wall thickness isprovided for guidance. These thicknesses take into account mechanical robustnessand commercial availability.

Line Wall thickness 40S 80S1" NS 3.38 4.551 " NS 3.68 5.082" NS 3.91 5.54

3" NS 5.49 7.62

The mechanical properties of materials used for construction should be satisfactoryfor use at a low temperature according to option warm or cold (see below) and forthe stresses encountered in designing the pipes or pumps.

Moreover certain national codes can require impact tests to be carried out todetermine the impact strength of materials to be used in construction as well as allother parts and fittings undergoing static or dynamic loads.

Piping systems which handle liquid HF should be capable of resisting all static ordynamic forces for the temperature ranges stated below. There are two optionsaccording to whether the HF handled is at ambient conditions "warm" or "cold"

Option warm Min Temperature = - 10C Max Temperature = + 120COption cold Min Temperature = - 40C Max Temperature = + 120C

If one is seeking to handle liquid or gas at a temperature below 40C, one shouldseek a quality of steel which gives the same quality of impact resistance at the

temperatures required as that quoted above.


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Generally Carbon steel seamless pipe is recommended for anhydrous HF and isnormally a readily welded, fine grain steel that is suitable for the temperature optionsstated above. Where welded pipe is used it should be obtained from approvedsuppliers and the quality controlled using the documented quality assuranceprocedure.

Velocity in the pipes for liquid HF should be limited to 1 meter per second.

3.2.2 Piping and Pumps for HF solutions from 40 % HF to 85 %:

Only PTFE lined steel pipes with a thickness of at least 3 mm PTFE should be put inservice for HF solutions from 40 % HF to 85 %. For concentrations below 40 %Polypropylene lined steel pipes can be used as well as PTFE lined pipes.

The design of the steel pipe must be the same as for non-lined steel pipes and inaccordance with national codes and regulations. Below 40 % HF Polypropylene-linedGRP (Glass fibre reinforced plastic) pipes are not recommended.,

Pump is made of plastic material of construction, or, internally lined (e.g. PTFE) steelconstruction for proper corrosion protection.

3.2.3 Articulated arms for anhydrous HF loading and unloading:

Metallic components in the bearing surfaces of the articulations should preferably be

manufactured from material, which is resistant to gaseous and liquid HF, either dry orslightly moist (for example Hastelloy C or Inconel, Incoloy 825).In addition, any metallic component which can be in contact with HF due to sealfailure should be preferably made in materials resistant to slightly moist HF, e.g.Monel 40, Hastelloy C, Incoloy 825 or PTFE or PFA (Poly fluorinated alkoxy) liningon steel when possible.

3.3 Connections (i.e. branches, flanges nuts and bolts)

3.3.1 Branches and Flanges

The metal used for flanges, branches, nuts and bolts should have the same quality asthe material used in the construction of the stock tank. The overall arrangement offlange and jointing material should be studied in such a way to provide a system fromwhich the joint cannot be expelled by excess pressure.

High tensile strength steel for bolts and nuts must be avoided in order to be protectedfrom hydrogen stress corrosion (for example B 7 M ASTM Standard A 193 can beused). The hardness of the steel should be limited.

The connections to the liquid phase of the mobile tank should be made of steel pipes,

PTFE flexible hoses (see 3.7) or articulated arms (see 3.2), all confirmed byexperience. A maximum pipe diameter of 50 mm is recommended for the liquid phase


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connection. Corrugated metallic hoses without internal lining are not recommendedbecause they do not drain fully. Hoses made of Stainless Steel, Monel, Inconel, orHastelloy lined with PTFE can be used but care must be taken for the porosity of thePTFE lining. Therefore permanent use is not recommended and for intermittent usesperiodic checks (twice a year) and change if necessary.

If splash guards fixed over the flanges are used, these guards must be designed toavoid corrosion of the bolts, the flanges and the pipe work.

The flanges in the connection between valve and tank should preferably be in tongueand groove type with cold flow treated PTFE gaskets or PFA gaskets. PTFE loadedwith CaF2 (or another mineral compound) can be used, but the Glass filled type mustbe avoided. The flanges for the connection with the flexible pipe work for filling and

unloading should be of the same type.

The resilience of metals used must be at least equal to that defined by internationalrules.

The materials of the flanges should be suitable for the selected temperature conditionsand be compatible with the associated pipe and fittings. Metal shall be checked forlamination and inclusions defects and suitable hardness.

Flanges should preferably be of the welding neck type, made from forged steel with aminimum rating of 16 bars. Screw type unions should be avoided. Gaskets are not

required when using lined steel pipes. On Polypropylene lined GRP, PTFE gaskets canbe used, with special care on the risk of cold flow. The same can be used in the PTFEor Polypropylene lined steel pipes if necessary.

3.3.2 Nuts and BoltsBolting for securing flanges must have adequate resistance to hydrogen assistedstress corrosion cracking (HSCC), sufficient strength for use with the type of selectedgasket and must be quality controlled.

Bolts and nuts should conform to a recognised national standard and be suitable forthe selected temperature conditions.

The manufacture and testing of the various Alloys recommended above are covered inthe appropriate national standards. It is recommended that, as an added precaution,the supplementary requirement of 100% hardness testing of the bars of ASTM B 7 Mas an example is requested.

For example the mechanical properties of ASTM B 7 M are listed below:

The stud bolts should be delivered in the heat treated state after the rolling of the barto obtain the following mechanical characteristics:

Grade B 7 M


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ultimate tensile strength0.2% proof stresshardness

69 kg / mm2 minimum55 kg / mm2 minimum

235 H B or 99 H R B maximum

The strength of these various metals varies considerably. The strength must beconsidered as different gasket materials require different loading. The grade B 7 M ofthe ASTM is suitable for use with plain PTFE (polytetrafluoroethylene) gaskets.

3.4 Valves

3.4.1 Valves for AHF

The valves used in the pipe work in the liquid phase must be confirmed by experience,for example:

- steel body with Monel or austenitic steel stem and plug, PTFE seating andstuffing box or

- steel plug cock valve with PTFE lined plug, PTFE membrane and packing.

If cast steel bodies are used, precautions have to be taken to avoid imperfections inthe casting.

All pieces concerned with static or dynamic stresses should be made of a materialsuitable to HF service and should present sufficient mechanical and resiliencecharacteristics to withstand the conditions of pressures and temperatures.

In order to take into account the potential risks associated with AHF, it isrecommended that a maximum of 80% of the nominal pressure is allowed for whichthe valve has been constructed.

As a consequence the maximum operating pressure is given by the following table asa function of the nominal design pressure of the valve.

Nominal pressure Maximum service pressureauthorised at ambient

temperature

PN 16 12,8 bars effectivePN 25 20 bars effective

PN 40 32 bars effective


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Minimum pressure

0,20 bar absolute

The resilience of metals used must meet the standards.

The body and the cap of the valve should not be made of Cast Iron but preferablymade of forged or cast steel.

The plug cock should be made of Stainless Steel or Monel.

The inserted casing ensuring the tightness between plug cock and body should be inPTFE (plain or Carbon filled).

The body and the valve head should preferably be constructed from forged steel.

All parts of the valve should be constructed in materials which are compatible withAHF.

The seat of the valve, the disk, the bellow and the other parts of the closure exposed tocorrosion by intermittent contact with the atmosphere must be made of materialresistant to gaseous and liquid HF of more than 85% concentration

All the parts subjected to a static or dynamic stress must be made from a material with

a resilience and tensile strength sufficient to withstand the conditions of temperatureand pressure. (which excludes the use of Cast Iron).

In the case of valves without a bellow, the spindle should be made of Monel 400, orInconel, or austenitic steel.

When the bellow is employed it should be constructed in austenitic Stainless Steel,(austenoferritic) Stainless Steel, or Monel or in Hastelloy C. The bellows should bedesigned to ensure at least 10.000 operations of opening and closing at the nominalpressure of the valve and to be able to resist without permanent distortion the variousstatic and dynamic forces to which it will be subjected, as well as the hydraulic testpressure of the valve.

Do not use plug and ball valves as a primary liquid isolation of a HF storage tank, butonly globe valves.

3.4.2 Valves for HF solutions

Plug or ball valves, preferably lined with FEP, are normally used.PTFE can also be used. For plug valves the valve body must be of one piece. Globevalves can cause problems, because of abrasion of the valve seating. Precautions

must be taken regarding the risk of quick corrosion of the steel structure in case offailure of the lining. Periodic testing is recommended.


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It is recommended the use of pneumatically actuated valves fitted with an internalstop/check valve and an outlet valve, for example size PN25-DN40 DIN 2501, Form C.

Alternatively, FEP or PFA (PTFE can also be used) lined plug or ball valves, may beused.The liquid valve shall have a dip-pipe connected to the check valve body (if welded, itmust be stress relieved and radio graphed). The pipe may be PTFE coated, with athickness of at least 3 mm. The dip-pipe will be sized to terminate in a sump, such thatthe end of the dip-pipe is 10 mm below the bottom of the tank and at least 10 mmabove the bottom of the sump. The bottom end of the dip-pipe shall be secured bysteady welding to the shell. It shall be possible to remove the dip-pipe without having toenter the tank.

3.5 Gaskets

Gaskets must be made of material resistant to HF (e.g. pure PTFE), capable of goodservice proven by experience, especially for flanges in the liquid phase.

Note, all gaskets must be replaced after each use by a new gasket.

Note: Even during construction and testing the gaskets used must meet this standard.This is to prevent the possibility of wrongly specified gaskets finding their way into HFservice.

Note: Packing materials, gaskets and pump seals will absorb AHF, and are difficult todecontaminate. After washing and drying, droplets of HF may form on such materialsby drawing moisture from the air.

For preference connections should be performed with tongue and groove flanges inconjunction with PTFE gaskets.

Under no circumstances gaskets should contact surfaces be machined in a mannerwhich leaves tool marks extending radially across the seating surface. Gaskets shouldnot be reused.

Sealing material of the swivel joints should be made of pure virgin PTFE (with a captivejoint).

Gaskets containing Asbestos free fibres, Asbestos substitutes or enveloped gasketscontaining silicate should not be used as silicate is attacked by HF.

3.6 Hoses and Flexible Steel Pipes

3.6.1 Hoses

Transfer hoses are high-risk, critical components and must be handled carefully.


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The use of Stainless Steel wire braid hoses lined with PTFE (minimum 3 mm) isrecommended. The design of the Stainless Steel hoses should be the same as fornon-lined types and comply with national codes.

PTFE hoses designed for general industrial use for conveying liquids and gases aresuitable for the service of filling and unloading anhydrous HF, provided the conditionsset forth hereafter are met.Protective armour may further be used where hose lines are subject to excessiveabrasion or to help prevent kinking damage.The diameter of the hose will be limited to 50 mm.

3.6.2 Flexible Steel Pipes

All the parts subject to a static or dynamic stress must be made of a material with aresilience and sufficient resistance to withstand the conditions of temperature andpressure indicated below (the pipes must be connected hermetically to the tank for thefollowing conditions of temperature and pressure :

Minimum temperature : - 20C Maximum temperature : + 50C Minimum working pressure : 0,20 bar absolute Maximum working pressure : 11 bars absolute)

Pipes must be of seamless drawn steel tubes and with a quality according to 3.2. Pipesmust have a minimum thickness of 4 mm.

3.7 Thin section applications (i.e. flexible bellows, bursting discs)

Certain duties such as flexible bellows, bursting discs etc... necessitate use of thincross section components. In these circumstances, the material must be effectivelynon-reactive to HF and not relying on a protective fluoride surface layer. The mostcommonly used materials are PTFE and fluoropolymers elastomers (bellows) andMonel or Inconel 600 or 625 in certain applications. Nickel, Silver and Graphite arecommonly used for bursting discs.

Documents

Ctef Sts Group 4