Mechanical properties – What Is Piping

Piping Materials are a very important part of Piping Engineering. The success of a project and the cost to a great extent depends on piping materials. Hence, proper selection of pipe materials plays an important role in the project economy and it is always preferred to choose piping materials that meet all technical requirements but are cheaper. There is a lot of variety of piping materials present in the market. Choosing the best and economic pipe material for a specific service is really challenging and here comes the role of piping material engineers. They in consultation with process engineers decide the right kind of pipe materials. In this article, we will explore the types of piping materials and their selection philosophy. Piping material is a broad term and is not limited to only the material of the pipe. It signifies the material of all piping components; pipes, fittings, valves, other items. So, in a sentence, we can say that piping material means the materials of all the components that are present in a specific pipe class. Parameters for Pipe Material Selection As pipes and pipelines carry various liquids and gases having different process conditions, the proper selection of pipe material is very important. The main process parameters that govern the piping material selection process are: Types of Fluid to be handled (Flammable, Explosive, Corrosive, Reactive, Volatile, or Hazardous to human skin).Design and operating temperatures. (Whether the temperature of fluid is low temperature, high temperature, or cryogenic)Design pressure of the fluid. In addition to the above parameters, some non-process parameters that are considered for pipe material selection are: Cost of the pipe materialAvailabilityManufacturability, Weldability, etc. Pipe Material Selection Basics Selection of basic Piping Metallurgy and Material (viz. CS, LTCS, AS, SS, etc.) for piping specification lies with Process/Metallurgy Engineer. The main selection is based on process, licensor, and/or intrinsic metallurgy requirements to suit process medium, like corrosion, high temperature, 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. Material Basics Metals are 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 the optimization and improvement of its mechanical properties. Alloying also helps in improving corrosion & oxidation characteristics, machinability, weldability, etc. Complex alloyed material is also being engineered for use in aerospace 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 more complex applications Mechanical Properties of Pipe Materials Certain mechanical properties are also considered while selecting pipe material for a specific service. They are Modulus of Elasticity (Young’s Modulus) – the ratio of stress to strain and measured using tension testsElastic range: Material returns to original shape afterload is releasedPlastic range: Material is permanently deformed even after the load is releasedYield Strength – It defines the transition from elastic to plastic phase and it establishes the limiting value at which this transition occursUltimate 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 the cross-sectional area before it’s a 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 testsToughness – Ability of a material to resist sudden and brittle fracture due to the rapid application of loads. Measured using the 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 Material Terms and definition Before diving into the actual piping material selection process, let's learn some of the important terms relevant to pipe materials. Creep Strength – Ability of a metal to withstand constant weight or force at elevated temperatures without yieldingBrittle fracture – Sudden & rapid failure of a metal due to application of energy with hardly any deformationStabilization – Addition of alloying elements to prevent carbon-chromium precipitation and formation of carbides, which reduces corrosion at higher tempsIntergranular Corrosion (IGC)– Corrosion occurring at grain boundaries of metals due to depletion of chromium by the 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 acidsReducing acids – In Chemistry reduction means loss of oxygen and gain of Hydrogen – examples are Hydrochloric acid, Hydrofluoric acid, and hydrobromic acidOxidizing acids – Oxidation (a chemical reaction between metal & Oxygen) means a gain of Oxygen and loss of Hydrogen – examples are Sulfuric acid, Nitric acid, and Chromic acidOrganic acids – are of carboxyl (COOH) group containing hydroxide (OH) – examples are Acetic acid, Formic acid, Citric acidStress Corrosion Cracking (SCC) – Failure of metal through a combined action of tensile stress and chemical corrosion. SCC also depends on service temp, solution environment, exposure duration, and metal propertiesHigh Temp Hydrogen Attack – Results in the 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 non-metallic inclusion, which builds up pressure and eventually bulges and blisters steel.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 the air, to form oxides is called oxidation. This process results in scaling. Effects of Alloying Elements The main alloying elements that influence the properties of pipe materials and hence their selection are Carbon (C) – More carbon means more strength and hardness but less ductility and toughnessPhosphorus (P) – High content decreases shock resistance & ductility making material brittleSilicon (Si) – Increases high temp properties making the metal more stable by increasing tensile strength without increasing brittleness when under 2%. It also resists oxidization & is used as a deoxidizing agentManganese (Mn) – It improves hot working characteristics by increasing hardening when combined with sulfurNickel (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 crackingChromium(Cr) – It is a hardening element & improves material strength at a higher temp. Improves high temp oxidation & corrosion resistance of steelMolybdenum (Mo) – It makes the steel harder and stable by increasing its creep resistance at a higher temp. 2% Mo in steel also reduces high temp oxidation rateColumbium/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 Pipe Material Selection Criteria Now selecting a proper pipe material is a difficult task as there is a wide variety of pipe materials available in the market. Piping materials can broadly be of three types; Metals, Non-metals, and Composites. Metallic pipe materials can again be sub-divided into two groups; Ferrous piping materials and non-ferrous piping materials.Composite pipe materials consist of both metals and non-metals.Types of pipe materials can easily be graphically represented as shown below: Type of Pipe Materials Cast Iron (CI) ASTM A126, A436 Pipe The usual features of CI pipe materials are given below: Cast Iron/Ductile Iron/Malleable Iron Pipes are brittle, low strength materials 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 shockDI & MI cannot be used at a temperature 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 Pipe Selection The usual features of Carbon Steel materials are 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 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 propertiesImpact properties/values at low temp are 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 Pipe Use limited to about 200° F or 93° C for basic utilities like water, air, nitrogenNormally piping connections are screwed to avoid damage to galvanizing due to welding Lined Piping Normally metallic, glass, non-metallic, cement-linedUsed for highly corrosive services like acids, caustic, process limited services, etc.CS Cement lined pipe normally used in seawater applications Alloy Steel (AS) – Also known as Cr-Mo steel Pipe Selection 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 bet -29°C up to 454°C design tempCr-1/2Mo steels can be used bet -29°C and up to 550 to 600°CPWHT or stress relieving is a must after welding Stainless Steel (SS) Pipe – Austenitic Grade Cr-Ni-Mo Used for high temp and process critical services and for cryogenic applicationsSelection governed by the process for specific service needsASTM A312 Gr TP 304 and 316 are normally used SS grades for pipesPresence of 2% Mo in SS316 gives better overall corrosion resistance properties than SS304SS316 has a higher resistance to pitting and crevice corrosion in chloride environmentsGrade L series has lower C (0.035%) which improves its use for higher temp up to 1100°F (600°C), has a 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, the diary industry, etc.Common applications of SS316 are food, pharma, marine, medical implant steel, etc.Grade 317 – use dictated by licensor/processGrades 321 and 347 are metallurgically very stable in high temp applications because of the addition of Columbium and/or TungstenImpact testing is not required if C < 0.1%Refer to ASTM 01.01 for impact test requirement for low temp/cryogenic services Exotic Grades – Not commonly used – High cost!!! Duplex SS: Grade 2205/2207 (UNS No. S31803/32760) Cr-Ni-Mo steelHas excellent strength & corrosion resistance, improved resistance to acids and chlorides and good weldability (ASTM A928) Nickel 200/201: (UNS No. N02200/N02201) Pure Nickel-very good mechanical propertiesExcellent resistance to corrosive mediaGood mechanical strength at a high temperatureGood ductility at low temp, good weldabilityNickel 201 has a 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) Ni-Cu Alloy - ASTM B165Good resistance to saline and acidic conditionsIdeal for high-velocity seawater/brackish water applicationsHigh resistance to cavitation and corrosionMonel 400 has oxidation resistance up to 550°CMonel 500 has higher tensile strength and hardness and resists oxidation up to 650°CIdeal for use with H2SO4 and other acidsIdeal material for valve trims Inconel Pipe Selection Inconel 600/601: (UNS No. N06600/06601) Ni-Cr-Fe Alloy - ASTM B167Excellent oxidation and scale resistance properties up to 1200°CExcellent resistance to corrosive mediaGood weldability and is resistant to chloride stress corrosion crackingInconel 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) Ni-Cr-Mo-Fe Alloy – ASTM B705Excellent strength and ductility between 700°C to 1100°CPresence of Mo further increases corrosion resistance at higher tempsIdeal for phosphorus acids, organic acids, seawater, boiler tubing, etc.Good weldability Inconel 800: (UNS No. N08800) Ni-Cr-Fe Alloy – ASTM B407 Inconel 800H: (UNS No. N08810) 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) Ni-Cr-Mo-Fe Alloy – ASTM B423High resistance to sulfuric acid, phosphoric acid, solvents, reducing acids and seawaterCupro Nickel: C70600, ASME B466/B467, B111Is an alloy of Cu & Ni, Cu 70-95% & Ni 30-95%Has very good corrosion resistance, especially to seawater applicationsGood mechanical strength & weldabilityB466 smls and B467 welded pipes used in marine applications like seawater piping and fittingsB111 used in exchanger/condenser tubesAlso used extensively in the automobile industry especially for brake tubing Hastalloy C276: (UNS No. N10276) Ni-Cr-Mo-W Alloy – ASTM B619/622Best alloy for extremely corrosive conditionsGood for reducing and oxidizing applicationsThe 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, seawater, saline solutions.Corrosion-resistant to wet HCL, hydrochloride solutions, etc. Alloy 20: (UNS No. N08020) Ni-Cr-Mo-Cu Alloy – ASTM B464Excellent mechanical properties, strength, and machinabilityExcellent stress corrosion resistance to boiling 20-40% Sulfuric acidWidely used in 98% Sulfuric acid service for valve trimsIdeal in food & pharma applications where product purity has to be guaranteed Aluminum and Al Alloy Pipe (ASTM B345) Excellent mechanical properties like strength, weldability, and formabilityGood surface finishHigh corrosion resistanceUsed in Gas & Oil transmission and distribution piping systemsAlso used in aircraft applications as it is light but strong Titanium: ASTM B861 – Seamless Pipe/ ASTM B862 – Welded Pipe These are practically pure grades of Titanium with about 6% NitrogenHas high strength, impact toughness, fabricability, formability, and weldabilityExceptional corrosion and erosion resistance which allowszero corrosion allowanceUsed 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 as Pipe Materials (PVC, UPVC, CPVC) ASTM D1784/1785Polyvinyl Chloride (PVC) is the most common material for plastic pipeUnplasticized (U-PVC) is the same as PVC but has resins added as additives to make it harderService temp up to 60°C (140°F)Joining methods are solvent welding, threading or flangingChemically inert, good corrosion and weather resistance, high strength, good electric and thermal insulatorUsed in chemical processing, chilled water distribution, hemical drains, etc.Not suitable for oxidizing agents like concentrated sulfuric acid, nitric acid, esters and aminesPolyvinyls will burn but does not support combustion because of its high chlorine content, will extinguish immediately upon flame removalChlorinated 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 liquidsUsed for hot process piping, corrosive liquids, hot and cold water lines, etc.Joining methods are solvent welding, threading or flanging Polyolefins (PP, CPP, PE) as Pipe Materials Polypropylene (PP) is the lightest thermoplastic 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, saltwater disposal, chilled water, and demineralized water lines.Copolymer Polypropylene (CPP) is a copolymer of propylene and polybutylene. 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 a wide range of common chemicalsMedium Density Polyethylene (MDPE) is a thermoplastic that has a lesser density than HDPE. It has good shock and drops resistance and better stress cracking resistance than HDPE. It has a lower hardness and rigidity when compared to HDPE. Used a lot in gas piping and fitting and in packaging films. Joining by hot gas weldingHigh-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 weldingCross-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 Fluoroplastics (PVDF, HALAR, PTFE, FEP, PFA) Pipe Materials 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. It also withstands most acids, bases, and organic solvents. PVDF is not recommended for strong caustics. The 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 chlorotrifluoroethylene. Excellent resistance to a 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 an excellent application for high purity hydrogen peroxide application. Joining by thermal butt fusion. Polytetrafluoroethylene (PTFE) PTFE offers the 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 an excellent sealant material. Normally an opaque white material. Fluorinated Ethylene Propylene (FEP) This fluoroplastic was invented by DuPont. It can be melt-extruded and fabricated by conventional methods which allow 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). It has a glossy surface and is transparent when in a 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 temperature. 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. The maximum temp limit is 120°C (250°F). EPDM - Is a terpolymer elastomer made from the 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. The maximum temp limit is 100°C (212°F). Nitrile BUNA-N- Nitrile rubber is a copolymer of butadiene and acrylonitrile. It has excellent elastomeric properties. It has excellent resistance to aliphatic hydrocarbons and aromatic solvents. Excellent material for valve seating. The 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. The maximum temp limit is 100°C (212°F). NEOPRENE – is a chlorinated synthetic rubber used primarily as a seating and sealing surface for valves. The maximum temp limit is 100°C (212°F). NATURAL RUBBER – Is polymer isoprene with the highest molecular weight. Derived from Hevea (Rubber) tree. Used as the diaphragm and sealing material because of its elastomeric properties and resistance to abrasion. The maximum temperature limit is 100°C (212°F). Thermoset Plastics as Piping Materials 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. The broad range of chemical resistance. High strength to weight ratio. Ease of fabrication and flexibility in design. Good electrical insulation properties. It can be used up to a temperature 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 seawater piping. Non-metallic piping material use is generally restricted to about 120°C (250°F). Check with vendor for specific applications for services and corrosion, sizes, fitting dimensions, jointing procedures, etc. Typical Pipe Material Service checks Caustic : Always check NaOH or KOH concentration before material selectionUse Baume’s scale (Caustic Cracking Curve RP0403) for referenceTypical material is CS and PWHT is requiredTypical valve trim is MonelIf PWHT is not mandatory, do not allow steam out Amine: Services include MEA (Mono Ethanolamine), DEA (Di Ethanolamine), DIPA (Di iso propanolamine), DGA (Di Glycolamine), etc.Typical material is CS and PWHT is requiredFresh amines do not cause SCC; PWHT not required, but if exposed to lean and rich amines, they require PWHTRestrict fluid velocity to wt% 50 to 75 ppm, it is susceptible to SCCPWHT required and material hardness to be restricted to 200 BHNNACE MR-0103 valve are not required but acceptable. Many use NACE valves as a normCS with 300 series SS trim is typical for valvesAlways consult client/process for specific metallurgy requirements Hydrogen (API 910/941): Hydrogen service defined as a combination of H2 partial pressure and temp above the curve for CS per API 941Use Nelson’s Curve for material selection limitsCS can be used up to 232°C232° to 330°C use 1.25Cr or 2.25Cr/Mo steel or Duplex steel330° to 400°C use SS 321400°C use SS 347 or Alloy 800For 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: Very critical and hazardous serviceTypical material is CSPWHT is normally required, restrict hardness to 200 BHNUse special precaution during purchase of piping components, shielding and color codingStrictly follow piping specifications Chlorine (Dry and Liquid): Follow Chlorine Institute guidelines for material selection and design. Leakage is hazardousStrictly follow pipe specification requirementsBall valves with soft seats are typically usedCS is normally used in liquid chlorine servicesPWHT is normally not requiredExercise high precaution in purchase spec for piping components w.r.t. cleanliness, packing and shipping requirements Ammonia (Aqueous and Gaseous application): In aqueous (Ammonia with water) CS is commonly used materialStrictly follow pipe specification requirementsIn gaseous service, low temp and cryogenic material and codes shall be usedDo not use CS if temp is 31%): CGA (Compressed Gas Association) standard shall be followedStrictly follow pipe specification requirements, special notes for design and material selectionCommonly used material is CS, SS, and MonelVelocity in a pipe is a critical factor for material selectionPTFE is preferred material as gasket filler and for valve stem packingAvoid threaded pipingCleanliness requirement very critical during material handling/shipping after fabrication