Piping Material & Metallurgy

PIPING MATERIAL & METALURGY   

Selection of basic Piping Metallurgy and Material (viz. CS, LTCS, AS, SS, etc.) for piping specification lies with Process/Metallurgy Engineer The above selection is based on process, licensor and/or intrinsic metallurgy requirement to suit process medium, like corrosion, high temp., pressure, etc. This basic selection shall also address special considerations for PWHT, special valve trim for NACE, corrosive services like acids, amines, etc. and hazardous services like Hydrogen, Chlorine, Phosgene, Oxygen, etc.

This article will try to provide basic guidelines for piping material selection. The article is published in three parts. This is part 1 of the article. Material Basics:

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Metals rarely used in their purest form as they have low mechanical strength Alloying helps increase its properties like strength and ductility. (Easiest eg. is adding Carbon to Iron to produce ferritic Carbon Steel) Addition of alloying elements in proper proportions along with appropriate metal processing and heat treatment, results in optimization and improvement of it’s mechanical properties Alloying also helps in improving corrosion & oxidation characteristics, machinability, weldability, etc. Complex alloyed material is also being engineered for use in aero space programs and applications Metallic glasses and crystalline alloys have also been developed and metal alloys are sometimes even bonded with graphite, ceramic and organic materials as composites for wider and complex applications

Mechanical Properties:

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Modulus of Elasticity (Young’s Modulus) – ratio of stress to strain and measured using tension tests Elastic range: Material returns to original shape after load is released Plastic range: Material is permanently deformed even after load is released Based on Stress-Strain Curve Yield Strength – It defines the transition from elastic to plastic phase and it establishes the limiting value at which this transition occurs Ultimate Tensile Strength – It defines the limit to which any further addition of load under constant strain would arrest the specimen elongation or thinning and would result in its failure. Ductility – expressed in elongation of a specimen and its reduction in cross sectional area before it’s failure. Established by measuring specimen length before elongation and minimum diameter before failure. Hardness – Ability of a material to resist deformation. Hardness is tested by Brinell or Rockwell Hardness tests, both of which are indentation type tests Toughness – Ability of a material to resist sudden and brittle fracture due to rapid application of loads. Measured using Charpy V-Notch test. Fatigue Resistance – Ability of a material to resist failure or crack initiation and its further propagation under repeated cyclic loading conditions

Terms and definition:

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Creep Strength – Ability of a metal to withstand constant weight or force at elevated temperatures without yielding Brittle fracture – sudden & rapid failure of a metal due to application of energy with hardly any deformation Stabilization – Addition of alloying elements to prevent carbon-chromium precipitation and formation of carbides, which reduces corrosion at higher temps Intergranular Corrosion (IGC)– Corrosion occurring at grain boundaries of metals due to depletion of chromium by formation of Cr Carbide layer, after reacting with carbon, which protects from further corrosive environments. (Min 12% Cr in SS). IGC is caused by reducing acids, oxidizing acids and organic acids Reducing acids – In Chemistry reduction means loss of oxygen and gain of Hydrogen – examples are Hydrochloric acid, Hydrofluoric acid and hydrobromic acid Oxidizing acids – Oxidation (chemical reaction between metal & Oxygen) means gain of Oxygen and loss of Hydrogen – examples are Sulfuric acid, Nitric acid and Chromic acid Organic acids – are of carboxyl (COOH) group containing hydroxide (OH) – examples are Acetic acid, Formic acid, Citric acid Stress Corrosion Cracking (SCC) – Failure of a metal through a combined action of tensile stress and chemical corrosion. SCC also depends on service temp, solution environment, exposure duration and metal properties High Temp Hydrogen Attack – Results in degradation of Carbon and Low Alloy Steel due to depletion (decarburization) of carbon (strengthening agent) in steel due to reaction with Hydrogen at high temps, thus causing loss of strength in metal. Hydrogen Blistering – A low temp phenomenon where atomic hydrogen diffuses into steel and is trapped as nonmetallic inclusion, which builds up pressure and eventually bulges and blisters steel.

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Hydrogen Induced Cracking (HIC) – Phenomenon similar to hydrogen blistering but HIC occurs in pipelines operating in sour services. Hydrogen blistering and HIC can be controlled by restricting sulfur content in steel to 0.005% or 0.010% max. Oxidation – Chemical reaction of metal and alloys with oxygen in the metal in air, to form oxides is called oxidation. This process results in scaling.

Effects of Alloying:

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Carbon (C) – More carbon means more strength and hardness but less ductility and toughness Phosphorus (P) – High content decreases shock resistance & ductility making material brittle Silicon (Si) – Increases high temp properties making metal more stable by increasing tensile strength without increasing brittleness when under 2%. It also resists oxidization & is used as a deoxidizing agent Manganese (Mn) – It improves hot working characteristics by increasing hardening when combined with sulfur Nickel (Ni) – It improves hardenability by increasing the strength and toughness in steel. Combined with Chromium it improves impact and fatigue resistance. Improves low temp properties. Higher nickel content improves resistance to chloride cracking Chromium(Cr) – It is a hardening element & improves material strength at higher temp. Improves high temp oxidation & corrosion resistance of steel Molybdenum (Mo) – It makes the steel harder and stable by increasing its creep resistance at higher temp. 2% Mo in steel also reduces high temp oxidation rate Columbium/Titanium (Cb/Ti) – Commonly used stabilizing elements to improve sustained high operating temp properties of steel by reducing carbide precipitation. SS Type 321 and 347

Typical Material Selection: Cast Iron (CI) ASTM A126, A436:

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Cast Iron/Ductile Iron/Malleable Iron – Are brittle, low strength material used for low temperature applications and basic utilities like air, water, drains, etc. Low cost material. CI shall not be used on severe cyclic condition services, excessive heat, thermal shock DI & MI cannot be used at temp below -29° C & above 343°C (ASTM A47, A536) Austenitic DI (ASTM A 571) may be used at temp up to -196°C max but not lower

Carbon Steel (CS) ASTM A53-B/A106-B/API 5L-B:

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Better than CI and has higher strength Used for higher temperatures (up to 800° F or 427° C) Most process services including steam piping

Low Temp Carbon Steel (LTCS) ASTM A333-Gr 1, 3, 4, 6, 8, 11, etc.:

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Used for low temp services like chilled brine, chlorine liquid/gas, propylene, etc. (Bet -45°C to 485°C) Has more of Carbon and no alloying elements like Cr and Mo and contains Nickel which improves low temp properties Impact properties/values at low temp is better than in CS (Charpy N Notch test) Refer to ASTM 01.01 for impact test requirement for low temp/cryogenic services

Galvanized Carbon Steel:

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Use limited to about 200° F or 93° C for basic utilities like water, air, nitrogen Normally piping connections are screwed to avoid damage to galvanizing due to welding

Lined Piping:

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Normally metallic, glass, non-metallic, cement lined Used for highly corrosive services like acids, caustic, process limited services, etc. CS Cement lined pipe normally used in sea water applications

Alloy Steel (AS) – Also known as Cr-Mo steel:

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Used for high temp applications in CS base like process services, superheated steam, reformer gases, etc. above 400° C design temp (ASTM A335 Gr P1, 5, 11, 22, etc.) C-1/2Mo steel can be used between -29°C up to 454°C design temp Cr-1/2Mo steels can be used bet -29°C and up to 550 to 600°C PWHT or stress relieving is a must after welding

Stainless Steel (SS) – Austenitic Grade Cr-Ni-Mo:

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Used for high temp and process critical services and for cryogenic applications Selection governed by process for specific service needs ASTM A312 Gr TP 304 and 316 are normally used SS grades for pipes Presence of 2% Mo in SS316 gives better overall corrosion resistance properties than SS304 SS316 has higher resistance to pitting and crevice corrosion in chloride environments Grade L series has lower C (0.035%) which improves its use for higher temp up to 1100°F (600°C), has higher resistance to IGC and better weldability, Better mechanical strength at elevated temps & good high temp oxidation resistance up to 925°C. Grade H – Controlled C between .04 to 0.1% & lower Ni provides improved high temp strength above 815° C. Common applications of SS304 are food, steel utensils, beverages, dairy industry, etc. Common applications of SS316 are food, pharma, marine, medical implant steel, etc. Grade 317 – use dictated by licensor/process Grades 321 and 347 are metallurgically very stable in high temp applications because of the addition of Columbium and/or Tungsten Impact testing is not required if C < 0.1% Refer to ASTM 01.01 for impact test requirement for low temp/cryogenic services

PIPING MATERIAL & METALURGY Exotic Grades – Not commonly used – High cost!!! Duplex SS: Grade 2205/2207 (UNS No. S31803/32760)

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Cr-Ni-Mo steel Has excellent strength & corrosion resistance, improved resistance to acids and chlorides and good weldability (ASTM A928)

Nickel 200/201: (UNS No. N02200/N02201)

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Pure Nickel-very good mechanical properties Excellent resistance to corrosive media Good mechanical strength at high temp Good ductility at low temp, good weldability Nickel 201 has little more Carbon than 200 which makes its use ideal for highly corrosive caustic soda (Sodium Hydroxide) applications up to 300°C (ASTM B163).

Monel 400/500: (UNS No. 04400/05500)

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Ni-Cu Alloy – ASTM B165 Good resistance to saline and acidic conditions Ideal for high velocity sea water/brackish water applications High resistance to cavitation and corrosion Monel 400 has oxidation resistance upto 550°C Monel 500 has higher tensile strength and hardness and resists oxidation upto 650°C Ideal for use with H2SO4 and other acids Ideal material for valve trims

Inconel 600/601: (UNS No. N06600/06601)

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Ni-Cr-Fe Alloy – ASTM B167 Excellent oxidation and scale resistance properties up to 1200°C Excellent resistance to corrosive media Good weldability and is resistant to chloride stress corrosion cracking Inconel 600 is used extensively in power plants Higher Cr content in Inconel 601 offers better oxidation and carburization resistance to sulfur applications

Inconel 625: (UNS No. N06625)

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Ni-Cr-Mo-Fe Alloy – ASTM B705 Excellent strength and ductility between 700°C to 1100°C Presence of Mo further increases corrosion resistance at higher temps Ideal for phosphorus acids, organic acids, sea water, boiler tubing, etc. Good weldability

Inconel 800: (UNS No. N08800)

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Ni-Cr-Fe Alloy – ASTM B407

Inconel 800H: (UNS No. N08810)

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Ni-Cr-Fe Alloy – This is a solution heat treated high carbon version of Inconel 800 with improved elevated temp properties and strength

Inconel 825: (UNS No. N08825)

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Ni-Cr-Mo-Fe Alloy – ASTM B423 High resistance to sulfuric acid, phosphoric acid, solvents, reducing acids and sea water Cupro Nickel: C70600, ASME B466/B467, B111Is an alloy of Cu & Ni, Cu 70-95% & Ni 30-95%

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Has very good corrosion resistance, especially to sea water applications Good mechanical strength & weldability B466 smls and B467 welded pipes used in marine applications like sea water piping and fittings B111 used in exchanger/condenser tubes Also used extensively in the automobile industry especially for brake tubing

Hastalloy C276: (UNS No. N10276)

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Ni-Cr-Mo-W Alloy – ASTM B619/622 Best alloy for extremely corrosive conditions Good for reducing and oxidizing applications Presence of Tungsten (W-Wolfram) imparts excellent resistance to strong oxidizing services, hot contaminated acids, solvents, chlorides, etc. Ideal for strong acids, formate acids, acetic hydride solutions, sea water, saline solutions. Corrosion resistant to wet HCL, hydrochloride solutions, etc.

Alloy 20: (UNS No. N08020)

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Ni-Cr-Mo-Cu Alloy – ASTM B464 Excellent mechanical properties, strength and machinability Excellent stress corrosion resistance to boiling 20-40% Sulfuric acid Widely used in 98% Sulfuric acid service for valve trims Ideal in food & pharma applications where product purity has to be guaranteed

Aluminium and Al Alloy Pipe (ASTM B345)

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Excellent mechanical properties like strength, weldability and formability Good surface finish High corrosion resistance Used in Gas & Oil transmission and distribution piping systems Also used in aircraft applications as it is light but strong

Titanium: ASTM B861 – Seamless Pipe/ ASTM B862 – Welded Pipe

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These are practically pure grades of Titanium with about 6% Nitrogen Has high strength, impact toughness, fabricability, formability and weldability Exceptional corrosion and erosion resistance which allows zero corrosion allowance Used a lot in aerospace applications and engine components besides chemical, marine, refinery, chlorine, food processing and pharma applications

Non Metallic Piping: PVC, UPVC, CPVC, PP, LDPE, HDPE, PVDF, HALAR, PTFE, PFA, FRP, RTRP, etc. Polyvinyls

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(PVC, UPVC, CPVC) ASTM D1784/1785 Polyvinyl Chloride (PVC) is the most common material for plastic pipe Unplasticized (U-PVC) is the same as PVC but has resins added as additives to make it harder Service temp up to 60°C (140°F) Joining methods are solvent welding, threading or flanging Chemically inert, good corrosion and weather resistance, high strength, good electric and thermal insulator Used in chemical processing, chilled water distribution, hemical drains, etc. Not suitable for oxidizing agents like conc. sulfuric acid, nitric acid, esters and amines Polyvinyls will burn but does not support combustion because of its high chlorine content, will extinguish immediately upon flame removal Chlorinated PVC (CPVC) has added chlorine which makes it suitable for use at higher temps up to 100°C (210°F) and offers better corrosion resistance to liquids Used for hot process piping, corrosive liquids, hot and cold water lines, etc. Joining methods are solvent welding, threading or flanging

PIPING MATERIAL & METALURGY Polyolefins (PP, CPP, PE)

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Polypropylene (PP) is the lightest thermo plastic piping material. Good strength and chemical resistance, resistant to sulfur bearing compounds. May be used up to 80°C (180°F) applications. Excellent material for industrial drainage, petroleum industry, salt water disposal, chilled water and demineralized water lines. Copolymer Polypropylene (CPP) is a copolymer of propylene and polybutelene. Has excellent dielectric and insulating properties, high chemical resistance, toughness and strength between freezing to 93°C (200°F) operating temps. Excellent abrasion resistance and good elasticity. Joining by socket fusion or butt welding. Polyethylene (PE) has four classifications: Low Density Polyethylene (LDPE) has more branching and less compact molecular structure, Lower mechanical strength than polyethylenes, Ideal for food handling services, brine tanks, etc. Good for temps up to 60°C (140°F). Joining by hot gas welding. All polyethylenes have excellent chemical resistance to wide range of common chemicals Medium Density Polyethylene (MDPE) is a thermo plastic which has lesser density than HDPE. It has good shock and drop resistance and better stress cracking resistance than HDPE. It has lower hardness and rigidity when compared to HDPE. Used a lot in gas piping and fitting and in packaging films. Joining by hot gas welding High Density Polyethylene (HDPE) has minimal branching and more compact molecular structure. More rigid and less permeable than LDPE. Good for temps up to 71°C (160°F). Used for abrasion resistant piping, caustic storage tanks, control tubing, etc. Joining by hot gas welding Cross Linked High Density Polyethylene (XLPE) is a 3 dimensional polymer of extremely high molecular weight and close molecular structure. Superior resistance to environmental stress cracking and very high impact strength. Good for temps up to 71°C (160°F). Ideal material for large storage tanks for outdoor service

PIPING MATERIAL & METALURGY Polyvinylidene Flouride (PVDF) is a strong, tough, abrasion resistant fluoroplastic material. Resists distortion and retains strength up to 135°C (275°F). Ideal to handle wet and dry chlorine, bromine and other halogen services. Also withstands most acids, bases and organic solvents. PVDF not recommended for strong caustics. Best material for high purity piping such as deionized water. Joining methods are thermal butt, socket or electrofusion. Halar (ECTFE) Ethylene Chlorotrifluoro Ethylene is a very durable copolymer of ethylene and chlorofluoroethylene. Excellent resistance to wide variety of strong acids, chlorine, solvents, aqueous caustics. Excellent abrasion resistance, electric properties, low permeability, temp capabilities from cryogenic to 170°C (340°F). Resistant to radiation. Halar has excellent application for high purity hydrogen peroxide application. Joining by thermal butt fusion. Polytetrafluoroethylene (PTFE) PTFE offers most unique and useful characteristics of all plastic materials. PTFE can handle liquids or gases up to 232°C (450°F). PTFE flows and is used as excellent sealant material. Normally an opaque white material. Fluorinated Ethylene Propylene (FEP) This fluoroplastic was invented by DuPont. Can be melt extruded and fabricated by conventional methods which allows more flexibility in manufacturing. Excellent dielectric and chemical resistance properties similar to PTFE. Use limited to temp between -54°C (-65°F) to a maximum of 150°C (300°F). Has a glossy surface and transparent when in thin section. FEP is widely used for its high ultraviolet light transmitting ability. Perfluoroalkoxy (PFA) Similar to PTFE and FEP. Better properties than PTFE, permits conventional thermoplastic molding and extrusion. Good flexibility for tubing purposes. Higher mech strength up to 260°C temp. Acrylonitrile Butadiene Styrene (ABS) A family of engineered thermoplastic with a range of performance characteristics. Acrylonitrile imparts chemical resistance and rigidity to this thermoplastic. Butadiene endows it with impact strength and toughness. Styrene contributes to ease of processing. Good for hostile environments like esters, ketones, alcohols and hydrocarbons up to 93°C (200°F). Sulfone Polymers These are clear thermoplastics used in corrosive environments. Temp range up to 150°C (300°F). High resistance to acids, alkali, salt solution. Not suited for ketones, chlorinated hydrocarbons and aromatic hydrocarbons . Used a lot in flow meters and sight gauges Forms of Sulfone Polymers VITON – A fluoroelastomer compatible with many chemicals at varied temp ranges. Used for sealing in valves, pumps and instruments. Excellent for mineral acids, salt solutions, chlorinated hydrocarbons and petroleum oils. Maximum temp limit is 120°C (250°F). EPDM – Is a terpolymer elastomer made from ethylene-propylene-diene monomer. Good abrasion and tear resistance. Excellent chemical resistance to a variety of weak acids and alkalies. Not recommended for applications involving petroleum oils, strong acids or alkalies. Maximum temp limit is 100°C (212°F). Nitrile BUNA-N– Nitrile rubber is a copolymer of butadiene and acrylonitrile. It has excellent elastometric properties. It has excellent resistance to aliphatic hydrocarbons and aromatic solvents. Excellent material for valve seating. Maximum temp limit is 100°C (212°F). HYPALON – This is a DuPont registered name for its elastomer of chlorosulfonated polyethylene. Used widely for valve seats and seals. Maximum temp limit is 100°C (212°F). NEOPRENE – Is a chlorinated synthetic rubber used primarily as a seating and sealing surface for valves. Maximum temp limit is 100°C (212°F). NATURAL RUBBER – Is a polymer isoprene with the highest molecular weight. Derived from Hevea (Rubber) tree. Used as diaphragm and sealing material because of its elastometric properties and resistance to abrasion. Maximum temp limit is 100°C (212°F) Thermoset Plastics Fiberglass Reinforced Plastic (FRP) is a highly valuable engineering material for piping and vessels (epoxy glass fiber). Very vast industrial use because of low initial cost & low maintenance. Broad range of chemical resistance. High strength to weight ratio. Ease of fabrication and flexibility in design. Good electrical insulation properties. Can be used up to temp of 150°C (300°F). Such epoxy piping is commonly used in oil, mining and chemical industries. Sometimes used for steam condensate systems. Used in industrial cooling towers. Also used in Chlorine gas, chlorine water and brine services. Reinforced Thermosetting Resin Pipe (RTRP) Vinylester resin epoxy based thermosetting resin material cured by free polymerization. Better tensile strength, elongation and fatigue resistance. Material has excellent alkali resistance of the epoxy and acid and oxidation chemical resistance of the polymer. Used a lot in large bore sea water piping. Non-metallic piping material use generally restricted to about 120°C (250°F). Check with vendor for specific application for services and corrosion, sizes, fitting dimensions, jointing procedures, etc. Typical Material/Service checks: Caustic :

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Always check NaOH or KOH concentration before material selection Use Baume’s scale (Caustic Cracking Curve RP0403) for reference Typical material is CS and PWHT is required Typical valve trim is monel If PWHT is not mandatory, do not allow steam out

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Services include MEA (Mono Ethanolamine), DEA (Di Ethanolamine), DIPA (Di iso propanolamine), DGA (Di Glycolamine), etc. Typical material is CS and PWHT is required Fresh amines do not cause SCC; PWHT not required, but if exposed to lean and rich amines, they require PWHT Restrict fluid velocity to < 2 m/sec for CS Preferred valve trim is SS 316 Use SS for higher velocity and temps

Wet H2S:

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If H2S concentration in water is > wt% 50 to 75 ppm, it is susceptible to SCC PWHT required and material hardness to be restricted to 200 BHN NACE MR-0103 valve are not required but acceptable. Many use NACE valves as a norm CS with 300 series SS trim is typical for valves Always consult client/process for specific metallurgy requirements

Hydrogen (API 910/941):

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Hydrogen service defined as a combination of H2 partial pressure and temp above the curve for CS per API 941 Use Nelson’s Curve for material selection limits CS can be used up to 232°C 232° to 330°C use 1.25Cr or 2.25Cr/Mo steel or Duplex steel 330° to 400°C use SS 321 400°C use SS 347 or Alloy 800 For higher pressure rating (Class 600 & higher) valve casting inspection by radiography is a must and may need to go for non standard valves – long lead item!!! Check for PWHT requirement for piping & NACE requirement for valve trim based on pp of Hydrogen and wet H2S concentration

Hydrofluoric Acid (HF) – Concentration > 1ppm:

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Very critical and hazardous service Typical material is CS PWHT is normally required, restrict hardness to 200 BHN Use special precaution during purchase of piping components, shielding and color coding Strictly follow piping specifications

Chlorine (Dry and Liquid):

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Follow Chlorine Institute guidelines for material selection and design. Leakage is hazardous Strictly follow pipe specification requirements Ball valves with soft seats are typically used CS is normally used in liquid chlorine services PWHT is normally not required Exercise high precaution in purchase spec for piping components w.r.t. cleanliness, packing and shipping requirements

Ammonia (Aqueous and Gaseous application):

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In aqueous (Ammonia with water) CS is commonly used material Strictly follow pipe specification requirements In gaseous service, low temp and cryogenic material and codes shall be used Do not use CS if temp is < -29°C

Oxygen (For concentration > 31%):

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CGA (Compressed Gas Association) standard shall be followed Strictly follow pipe specification requirements, special notes for design and material selection Commonly used material is CS, SS and Monel

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Velocity in pipe is a critical factor for material selection PTFE is preferred material as gasket filler and for valve stem packing Avoid threaded piping Cleanliness requirement very critical during material handling/shipping after fabrication

Stainless steel METALLURGY WRITTEN BY:

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The Editors of Encyclopaedia Britannica See Article History

Stainless steel, any one of a family of alloy steels usually containing 10 to 30 percent chromium. In conjunction with low carbon content, chromium imparts remarkable resistance to corrosion and heat. Other elements, such as nickel, molybdenum, titanium, aluminum, niobium, copper, nitrogen, sulfur, phosphorus, or selenium, may be added to increase corrosion resistance to specific environments, enhance oxidation resistance, and impart special characteristics.

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steel: Stainless steels

This outstanding group receives its stainless characteristics from an invisible, self-healing chromium oxide film that forms when chromium…

Most stainless steels are first melted in electric-arc or basic oxygen furnaces and subsequently refined in another steelmaking vessel, mainly to lower the carbon content. In the argon-oxygen decarburization process, a mixture of oxygen and argon gas is injected into the liquid steel. By varying the ratio of oxygen and argon, it is possible to remove carbon to controlled levels by oxidizing it to carbon monoxide without also oxidizing and losing expensive chromium. Thus, cheaper raw materials, such as high-carbon ferrochromium, may be used in the initial melting operation. There are more than 100 grades of stainless steel. The majority are classified into five major groups in the family of stainless steels: austenitic, ferritic, martensitic, duplex, and precipitation-hardening. Austenitic steels, which contain 16 to 26 percent chromium and up to 35 percent nickel, usually have the highest corrosion resistance. They are not hardenable by heat treatment and are nonmagnetic. The most common type is the 18/8, or 304, grade, which contains 18 percent chromium and 8 percent nickel. Typical applications include aircraft and the dairy and foodprocessing industries. Standard ferritic steels contain 10.5 to 27 percent chromium and are nickel-free; because of their low carbon content (less than 0.2 percent), they are not hardenable by heat treatment and have less critical anticorrosion applications, such as architectural and auto trim. Martensitic steels typically contain 11.5 to 18 percent chromium and up to 1.2 percent carbon with nickel sometimes added. They are hardenable by heat treatment, have

PIPING MATERIAL & METALURGY modest corrosion resistance, and are employed in cutlery, surgical instruments, wrenches, and turbines. Duplex stainless steels are a combination of austenitic and ferritic stainless steels in equal amounts; they contain 21 to 27 percent chromium, 1.35 to 8 percent nickel, 0.05 to 3 percent copper, and 0.05 to 5 percent molybdenum. Duplex stainless steels are stronger and more resistant to corrosion than austenitic and ferritic stainless steels, which makes them useful in storage-tank construction, chemical processing, and containers for transporting chemicals. Precipitationhardening stainless steel is characterized by its strength, which stems from the addition of aluminum, copper, and niobium to the alloy in amounts less than 0.5 percent of the alloy’s total mass. It is comparable to austenitic stainless steel with respect to its corrosion resistance, and it contains 15 to 17.5 percent chromium, 3 to 5 percent nickel, and 3 to 5 percent copper. Precipitation-hardening stainless steel is used in the construction of long shafts.

stainless steelThe nickel and chromium content of different types of stainless steel.Encyclopædia Britannica, Inc.