Most Common Steels used in Process Piping Industry: A LiteratureIntroduction: The most common material used in Hydrocarbon inductries are various grades of steel. This article will provide a short write up on common Steels.Carbon Steel (Temperature Range -29 degree centigrade(C) to 427 degree C): This is the most common and cheapest material used in process plants. Carbon steels are used in most general refinery applications. It is routinely used for most organic chemicals and neutral or basic aqueous solutions at moderate temperatures. Carbon steels are extensively used in temperature range of (-) 29 degee centigrade to 427 degee centigradeLow Temperature Carbon steel (LTCS-Temp range -45.5 degree C to 427 degree C) can  be used to a low temperature of (- 45.5)degree centigrade.Killed Carbon Steel are defined as those which are thoroughly deoxidized during melting process. Deoxidation is accomplished by use of silicon, manganeese and aluminium additions to combine with dissolved gases, usually oxygen, during steel making. This results in cleaner, better qualtity steel which has fewer gas pockets and inclusions. Killed carbon steel is specified for major equipment in the following services to minimise the possibility or extent of hydrogen blistering and  hydrogen embrittlement: where hydrogen is a major component in the process  stream. where hydrogen sulphied H2S is present with an  aqueous phase or where liquid water

containing H2S  is present. Process streams containing any amount of Hydro  flouoric acid (HF), boron trifluoride (BF3)

or  (BF) compounds; or

Monoethanolamine (MEA) and diethanolamine (DEA) in  solutions of greater than 5 weight percent.

Low Alloy Steel (Temperature range -29 degree C to 593 degree C):Low Alloy Steels  contain one or more alloying elements to improve mechanical  or corrosion resisting properties of carbon steel. Nickel increases toughness and improves low temperature properties & corrosion resistance. Chromium and silicon improve hardness, abrasion resistance, corrosion resistance and resistance to oxidation. Molybdnum provides strength at elevated temperatures. Some of the low alloy steels are listed below. Carbon   1/2% Moly and Manganese   1/2% Moly: These low alloy steels are used for

higher temperature  services and most frequently for intermediate   temperatures for its resistance to hydrogen attack. They have the same maximum temperature limitation as killed steel (ASME Code   1000 deg. F) but the strength above 700 deg.F is substantially greater.

1% chrome   1/2% Moly and 1 1/4% Chrome   1/2% Moly: These alloys are used for higher reistance to hydrogen attack and sulphur corrosion. They are also used for services where temperatures are above the rated temperature for C   1/2 Mo steel.

2 1/4 Chrome 1% Moly and 3% chrome   1% Moly: These alloys have the same uses as 1 1/4% Cr, but have greater resistance to hydrogen attack and higher strength at elevated temperature.

5% chrome   1/2% Moly: This alloy is used most   frequently for protection against combined sulphur attack at temperatures above 550 deg.F. Its resistance to hydrogen attack is better than 2 1/4% Cr_ 1% Moly.

9% Chrome   1% Moly: This alloy is generally limited to heater tubes. It has a higher reistance to high sulphur stocks at elevated temperatures. It also has a maximum allowable metal temperature in oxidising atmospheres.

Stainless Steel (Temperature range -257 degree C to 538 degree C): They are heat & corrosion resistant, noncontaminating and easily fabricated into complex shapes. There are three groups of Stainless steels. 1) Martensitic, 2) Ferritic & 3) Austenitic.

1. Martensitic stainless steel : Martensitic alloys contain 12-20 percent chromium with controlled amount of carbon and other additives.Type 410 is a typical member of this group. These alloys can be hardened by heat treatment, which can increase tensile strength. Corrosion resistance is inferior to Austenitc Stainless steels and these are generally used in mild corrosive environments.

2. Ferritic stainless steel: Ferritic steels contain 15-30 percent chromium with low carbon content( 0.1percent). The higher chromium content improves its corrosion resistance. A typical member of this group is Type 430. The strength of these can be increased by cold working but not by heat treatment. Type 430 is widely used in nitric acid plants. In addition, it is very resistant to scaling and high temp oxidation upto 800 degree cent.

3. Austenitic stainless steel: Austenitic steels are the most corrosion resistant of the three groups. These steels contain 16-26 percent chromium 6-22 percent nickel. Carbon is kept low(0.08 percent max) to minimize carbide precipitation. Welding may cause chromium carbide precipitation, which deplete the alloy of some chromium and lowers its corrosion resistance in some specific environments, notably nitric acid. The carbide precipitation can be eliminated by heat treatment(solution annealing). To avoid precipitation special steels stabilized with titanium, niobium, or tantalum have been developed(Types 321,347 & 348). Another approach to the problem is the use of low carbon stainless steel such as types 304L & 316L with .03 percent max carbon.

The addition of molybdenum to austenitic alloy(types 316, 316L) provides generally better corrosion resistance and improved resistance to pitting.The chromium-nickel steels, particularly the 18-8 alloys, perform best under oxidizing conditions, since the resistance depends on an oxide film on the surface of the alloy. Reducing conditions and chloride ions destroy this and bring on rapid attack. Chloride ions tend to cause pitting and crevice corrosion. When combined with high tensile stresses they can cause stress-corrosion cracking.The detailed list of commonly used steels in hydrocarbon industries is given in following  table:

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Common Non Ferrous Materials used in Process Piping IndustryIn comparison with metallic materials, the use of plastics is limited to relatively moderate

temperatures and pressures [230degC (450degF) is considered high for plastics]. Plastics are also less resistant to mechanical abuse and have high expansion rates, low strengths (thermoplastics) and only fair resistance to solvents. However, they are lightweight, are good thermal and electrical insulators, are easy to fabricate and install, and have low friction factors. Since plastics do not corrode in the electrochemical sense, they offer another advantage over metals. The

important thermoplastics used commercially are polyethylene, polyvinyl chloride (PVC), fluorocarbons (Teflon, Halar, Kel-F, Kynar) and polypropylene. Important thermosetting plastics are general-purpose polyester glass reinforced, bisphenol-based polyester glass, epoxy glass, vinyl ester glass, furan and phenolic glass, and asbestos reinforced. While using non-metallic piping, viz HDPE, PVC, FRP etc, the designer shall take care of the service, pressure & temperature. Manufacturer’s recommendation shall be taken into account.Thermoplastics: The most chemical-resistant plastic commercially available today is tetrafluoroethylene or TFE (Teflon). This thermoplastic is practically unaffected by all alkalies and acids except fluorine and chlorine gas at elevated temperatures and molten metals. It retains its properties up to 260 deg C (500 deg F). Perfluoroalkoxy, or PFA (Teflon), has the general properties and chemical resistance of FEP at a temperature approaching 300 deg C (600 deg F). Polyethylene is the lowest cost plastic commercially available. Mechanical properties are generally poor, particularly above 50 deg C (120 deg F), and pipe must be fully supported. Carbon-filled grades are resistant to sunlight and weathering. Polypropylene has a chemical resistance about the same as that of polyethylene, but it can be used at 120 deg C (250 deg F).Thermosetting plastics: Among the thermosetting materials are phenolic plastics filled with asbestos, carbon or graphite, glass, and silica. Relatively low cost, good mechanical properties, and chemical resistance (except against strong alkalies) make phenolics popular for chemical equipment. Furan plastics filled with asbestos and glass have much better alkali resistance than phenolic resins. Polyester resins reinforced with fibreglass, have good strength and good chemical resistance except to alkalies. Epoxies reinforced with fibreglass have very high strengths and resistance to heat. The chemical resistance of the epoxy resin is excellent in non-oxidizing and weak acids but not good against strong acids. Alkaline resistance is excellent in weak solutions.Rubber and elastomers: Rubber and elastomers are widely used as lining materials. The ability to bond natural rubber to itself and to steel makes it ideal for lining tanks. Natural rubber is resistant to dilute mineral acids, alkalies and salts, but oxidizing media, oils and most organic solvents will attack it. Hard rubber is made by adding 25 percent or more of sulphur to natural or synthetic rubber and, as such, is both hard and strong. Chloroprene or neoprene rubber is resistant to attack by ozone, sunlight, oils, gasoline and aromatic or hydrogenated solvents, but is easily permeated by water, thus limiting its use as a tank lining. Nitrile rubber is known for resistance to oils and solvents. Butyl rubber’s resistance to dilute mineral acids and alkalies is exceptional. Hypalon has outstanding resistance to ozone and oxidizing agents except fuming nitric and sulphuric acids.

Fluoroelastomers (Viton-A, Kel-F, Kalrez) combine excellent chemical and temperature resistance.Medium Alloys: A group of (mostly) proprietry alloys with somewhat better corrosion resistance than stainless steels are called medium alloys. A popular member of this group is 20alloy. Made by a number of companies under various trade names. Durimet 20, Carpenter 20 are a few names. This alloy was originally developed to fill the need for a material with sulphuric resistance superior to stainless steels. Other members of this group are Incoloy 825 and Hastelloy G-3 . These alloys have extensive applications in sulphuric acid systems. Because of their increased nickel and moly contents they are more tolerant of chloride-ion contamination than standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking and molybdenum improves resistance to crevice corrosion and pitting.High alloys: The group of materials called high alloys all contain relatively large percentage ofNickel. Hastelloy B2  contains 61% Nickel & 28% Mo. The alloy has unusually very high resistance to all concentrations of HCL at all temperatures in the absence of oxidizing agents. Other materials of this group are Chlorimet 2 & Hastelloy C-276.Nickel & Nickel alloys : The metal is widely used for handling alkalies particularly in handling and storing caustic soda. Neutral alkaline solutions, seawater and mild atmospheric conditions do not affect nickel. A large number of nickel based alloys are commercially available. One of the best known out of these is monel 400 with 67% Ni and 30 % Copper. This Ni-Cu alloy is ductile and tough. It’s corrosion resistance is better than it’s components, being more resistant than nickel in reducing environments and more resistant than copper in oxidizing environments.Copper and copper alloys: Copper and it’s alloys are widely used in chemical processing, particularly when heat and thermal conductivity is very important. Main copper alloys are brasses(Cu-Zn), Bronzes( Cu- Sn) and Cupronickels. Some of the bronzes are very popular in process industry , like Aluminium and silicon bronzes because they combine good strength with corrosion resistance. Cupronickels have 10-30% nickel and have become very popular because it has the highest corrosion resistance of all copper alloys.This finds it’s application in heat-exchanger tubing and it’s resistance to seawater is especially outstanding.

Titanium: Titanium has become increasingly important as a construction material. It is strong and of medium weight. Corrosion resistance is very superior in oxidizing and mild reducing media. Titanium is usually not bothered by impingement attack, crevice corrosion and pitting attack in sea water. Its general resistance to sea water is excellent.The detailed list of commonly used non ferrous materials in hydrocarbon industries is given in following table.

Codes and Standards extensively used in piping industryThe great expansion of piping industry where it is today if mainly for the available codes, standards and recommended practices. The main concern for designing any process plant is safety of personnel involved. Design of Piping systems complying these codes, standards or recommended practices ensures safety along with standardization of required items.  Every piping engineer should possess a basic knowledge of the extensively used codes and standards. The following write up will try to provide a sum up of common codes and standards which are extensively used in recent process piping industry.

Codes Vs Standards: Codes prescribes requirements for design, materials, fabrication, erection, examination,

assembly, test, and inspection of piping systems, whereas standards contain design and construction rules and requirements for individual piping components such as elbows, tees, returns, flanges, valves, and other in-line items.

Compliance to code is generally mandated by regulations imposed by regulatory and enforcement agencies. At times, the insurance carrier for the facility leaves hardly any choice for the owner but to comply with the requirements of a code or codes to ensure safety of the workers and the general public. Compliance to standards is normally required by the rules of the applicable code or the purchaser’s specification.

Recommended Practice:Recommended Practices, prepared by professional organisations or professional bodies are optional set of documents which can be used for good engineering practice.

Even though every country have their own codes and standards but still the American codes and standards are most widely used. The major codes and standards which are used in day to day piping application are listed below:

A. ASME CODES:

1.0 ASME B31: CODE FOR PRESSURE PIPING:-ASME B31.3 – Process Piping: This code normally provides rules for piping found in petroleum refineries, chemical,pharmaceutical,textile, paper, semiconductor, and cryogenic plants, and related processing plants and terminals including piping for fluids like raw, intermediate and finished chemicals, petroleum products, gas, steam, air and water, fluidized solids, refrigerants, cryogenic fluids etc.  For process piping professionals this code is of atmost importance.This Code does not provide information on the following:(a) piping systems designed for internal gage pressures at or above zero but less than 105 kPa (15

psi), provided the fluid handled is nonflammable, nontoxic, and not damaging to human tissues and its design temperature is from −29°C (−20°F) through 186°C (366°F).(b) power boilers and boiler external piping which is required to conform to B31.1.(c) tubes, tube headers, crossovers, and manifolds of fired heaters, which are internal to the heater enclosure(d) pressure vessels, heat exchangers, pumps, compressors, and other fluid handling or processing equipment, including internal piping and connections for external piping.(e) piping covered by ASME B31.4, B31.8, or B31.11, although located on the company property(f) plumbing, sanitary sewers, and storm sewers.(g) piping for fire-protection systems(h) piping covered by applicable governmental regulations

ASME B31.1 – Power Piping: This code provides requirements for piping typically found in electric power generating stations, in industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems. This code is mainly important for Power piping professionals. It does not apply to piping systems covered by other sections of the Code for Pressure Piping, and other piping which is specifically excluded from the scope of this code.

ASME B31.4 – Pipeline Transportation Systems for Liquids and Slurries: This code provides requirements for piping transporting liquids between production facilities, tank farm, natural gas processing plants, plants and terminals and within terminals, pumping, regulating, metering stations, and other delivery and receiving points.ASME B31.5 – Refrigeration Piping and Heat Transfer Components: This code prescribes requirements for piping for refrigerants, heat transfer components and secondary coolants for temperatures as low as -320 degree F (-196 degree C)ASME B31.8 – Gas Transmission and Distribution Piping Systems: This code covers the piping transporting products that are mostly gas (Liquefied Petroleum Gas) between sources and terminals. This code also covers safety aspects of the operation and maintenance of those facilities.ASME B31.9 – Building Services PipingAMSE B31.11 – Slurry Transportation Piping Systems.ASME B31.12 – Hydrogen Piping and Pipelines.

2.0 ASME BOILER AND PRESSURE VESSEL CODE:-It contains 11 sections as mentioned below:

Section I Power BoilersSection II Material SpecificationsSection III Rules for Construction of Nuclear Power Plant ComponentsSection IV Heating BoilersSection V Nondestructive ExaminationSection VI Recommended Rules for Care and Operation of Heating BoilersSection VII Recommended Rules for Care of Power BoilersSection VIII Pressure VesselsSection IX Welding and Brazing QualificationsSection X Fiber-Reinforced Plastic Pressure VesselsSection XI Rules for In-Service Inspection of Nuclear Power Plant Components

Out of this 11 sections Section VIII is very important for Process Piping engineers.

B. PIPING COMPONENT STANDARDS: The major piping component standards which are used frequently are listed below:ASME B36.10M: Welded and Seamless Wrought Steel PipeASME B36.19M: Stainless Steel PipeASME B16.9: Factory-Made Wrought Steel Buttwelding FittingsASME B16.5: Pipe Flanges and Flanged FittingsASME B16.11: Forged Fittings, Socket Welding and ThreadedASME B1.1: Unified Inch Screw ThreadsASME B16.20: Metallic Gaskets for Pipe Flanges.ASME B16.25: Buttwelding EndsASME B16.10: Face-to-Face and End-To-End Dimensions of ValvesMSS SP-58: Pipe Hangers and Supports — Materials, Design, and Manufacture.BS 6501, Part 1: Flexible Metal HoseNFPA 1963: Standard for Fire Hose ConnectionsRefer ASME code B 31.3 for more of the component standards

C. ASTM STANDARDS: The American Society for Testing and Materials (ASTM) is a scientific and technical organization that develops and publishes voluntary standards on the characteristics and performance of materials, products, systems, and services. The standards published by theASTM include test procedures for determining or verifying characteristics, such as chemical composition, and measuring performance, such as tensile strength and bending properties. The

standards cover refined materials, such as steel, and basic products, such as machinery and fabricated equipment. The standards are developed by committees drawn from a broad spectrum of professional, industrial, and commercial interests. Many of the standards are made mandatory by reference in applicable piping codes.The major ASTM standards are listed below:

A36: Carbon Structural SteelA105: Carbon Steel Forgings, for Piping ApplicationsA106: Seamless Carbon Steel Pipe for High-Temperature ServiceA312: Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel PipeA335: Seamless Ferritic Alloy Steel Pipe for High-Temperature ServiceA358: Electric-Fusion-Welded Austenitic Chromium-Nickel Alloy Stainless Steel Pipe for High-Temperature Service and General ApplicationsA516: Pressure Vessel Plates, Carbon Steel, for Moderate and Lower-Temperature ServiceA671: Electric-Fusion-Welded Steel Pipe for Atmospheric and Lower TemperaturesA672: Electric-Fusion-Welded Steel Pipe for High-Pressure Service at Moderate TemperaturesSuggested reading for more on ASTM standards: Refer ASME B 31.3 Specification index for Apeendix A.

D. API STANDARDS: The American Petroleum Institute (API) publishes specifications, bulletins, recommended practices, standards, and other publications as an aid to procurement of standardized equipment and materials.The major ones are listed below for your reference:

API RP 520: Recommended Practice for Sizing, Selection,and Installation of Pressure-Relieving Devices in Refineries.API 610: Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas IndustriesAPI 650: Welded Tanks for Oil StorageAPI 661: Air-Cooled Heat Exchangers for General Refinery ServiceAPI 560: Fired Heaters for General Refinery ServiceAPI 617: Axial and Centrifugal Compressors and Expander-compressors for Petroleum, Chemical and Gas Industry ServicesAPI 618: Reciprocating Compressors for Petroleum, Chemical, and Gas Industry ServicesAPI 612: Petroleum, Petrochemical and Natural Gas Industries-Steam Turbines-Special-purpose Applications

There are several other codes and standards which are used in piping industry like AMERICAN WATER WORKS ASSOCIATION, AMERICAN WELDING SOCIETY, AMERICAN

SOCIETY OF SANITARY ENGINEERS, AMERICAN SOCIETY OF CIVIL ENGINEERS, AMERICAN SOCIETY FOR NONDESTRUCTIVE TESTING, AMERICAN IRON AND STEEL INSTITUTE, EXPANSION JOINT MANUFACTURERS ASSOCIATION, MANUFACTURERS STANDARDIZATION SOCIETY OF THE VALVE AND FITTINGS INDUSTRY, NATIONAL FIRE PROTECTION ASSOCIATION, TUBULAR EXCHANGER MANUFACTURERS ASSOCIATION etc.

Also there are non american standards like BRITISH STANDARDS AND SPECIFICATIONS, DIN STANDARDS AND SPECIFICATIONS, JAPANESE STANDARDS AND SPECIFICATIONS, ISO STANDARDS AND SPECIFICATIONS etc.

Important questions & answers from ASME B 31.3 which a Piping stress engineer must knowASME B 31.3 is the bible of process piping engineering and every piping engineer should frequently use this code for his knowledge enhancement. But to study a code similar to B 31.3 is time consuming and also difficult because the contents are not at all interesting. Also every now and then it will say to refer to some other point of the code which will irritate you. But still every piping engineer should learn few basic points from it. The following literature will try to point out 11 basic and useful points from the code about which every piping engineer must be aware.

1. What is the scope of ASME B 31.3? What does it covers and what does not? Ans:  Refer to the ASME B 31.3-Process Piping section from my earlier post.Link:  http://www.whatispiping.com/?p=44Alternatively refer the below attached figure ( Figure 300.1.1 from code ASME B 31.3)

2. What are the disturbing parameters against which the piping system must be designed?Ans: The piping system must stand strong (should not fail) against the following major effects: Design Pressure and Temperature: Each component thickness must be sufficient to withstand

most severe combination of temperature and pressure.

Ambient effects like pressure reduction due to cooling, fluid expansion effect, possibility of moisture condensation and build up of ice due to atmospheric icing, low ambient temperature etc.

Dynamic effects like impact force due to external or internal unexpected conditions, Wind force, Earthquake force, Vibration and discharge (Relief valve) reaction forces, cyclic effects etc.

Component self weight including insulation, rigid body weights along with the medium it transport.

Thermal expansion and contraction effects due to resistance from free displacement or due to thermal gradients (thermal bowing effect) etc.

Movement of pipe supports or connected equipments etc.

3. How to calculate the allowable stress for a carbon steel pipe?Ans: The material allowable stress for any material other than bolting material, cast iron and malleable iron are the minimum of the following:1. one-third of tensile strength at maximum temperature.2. two-thirds of yield strength at maximum temperature.3. for austenitic stainless steels and nickel alloys having similar stress–strain behavior, the

lower of two thirds of yield strength and 90% of yield strength at temperature.4. 100% of the average stress for a creep rate of 0.01% per 1 000 h5. 67% of the average stress for rupture at the end of 100 000 h6. 80% of the minimum stress for rupture at the end of 100 000 h7. for structural grade materials, the basic allowable stress shall be 0.92 times the lowest value

determined (1) through (6) above.

4. What is the allowable for Sustained, Occasional and Expansion Stress as per ASME B 31.3?Ans:  Calculated sustained stress (SL)< Sh (Basic allowable stress at maximum temperature)Calculated occasional stress including sustained stress< 1.33 ShCalculated expansion stress< SA = f [ 1.25( Sc + Sh) − SL]Here f =stress range factor,   Sc =basic allowable stress at minimum metal temperature and SL=calculated sustained stress. The sustained stress (SL) is calculated using the following code formulas:

Here,Ii = sustained in-plane moment index. In the absence of more applicable data, Ii is taken asthe greater of 0.75ii or 1.00.Io = sustained out-plane moment index. In the absence of more applicable data, Io is taken as the greater of 0.75io or 1.00.Mi = in-plane moment due to sustained loads, e.g.,pressure and weightMo = out-plane moment due to sustained loads, e.g.,pressure and weightZ = sustained section modulusIt = sustained torsional moment index. In the absence of more applicable data, It is takenas 1.00.Mt = torsional moment due to sustained loads, e.g.,pressure and weightAp = cross-sectional area of the pipe, considering nominal pipe dimensions less allowances;Fa = longitudinal force due to sustained loads, e.g.,pressure and weightIa = sustained longitudinal force index. In the absence of more applicable data, Ia is taken as 1.00.

5. What are steps for calculating the pipe thickness for a 10 inch carbon steel (A 106-Grade B) pipe carrying a fluid with design pressure 15 bar and design temperatre of 250 degree centigrade?Ans: The pipe thickness (t) for internal design pressure (P) is calculated from the following equation.

Here, D=Outside diameter of pipe, obtain the diameter from pipe manufacturer standard.         S=stress value at design temperature from code Table A-1         E=quality factor from code Table A-1A or A-1B        W=weld joint strength reduction factor from code        Y=coefficient from code Table 304.1.1Using the above formula calculate the pressure design thickness, t.Now add the sum of the mechanical allowances (thread or groove depth) plus corrosion and erosion allowances if any with t to get minimum required thickness, tm.Next add the mill tolerance with this value to get calculated pipe thickness. For seamless pipe the mill tolerance is 12.5% under tolerance. So calculated pipe thickness will be tm/(1-0.125)=tm/0.875.Now accept the available pipe thickness (based on next nearest higher pipe schedule) just higher than the calculated value from manufacturer standard thickness tables.6. How many types of fluid services are available for process piping?Ans: In process piping industry following fluid services are available.. Category D Fluid Service: nonflammable, nontoxic, and not damaging to human tissues, the

design pressure does not exceed 150 psig, the design temperature is from -20 degree F to 366 degree F.

Category M Fluid Service: a fluid service in which the potential for personnel exposure is judged to be significant and in which a single exposure to a very small quantity of a toxic fluid, caused by leakage, can produce serious irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken.

Elavated Temperature Fluid service: a fluid service in which the piping metal temperature is sustained equal to or greater than Tcr (Tcr=temperature 25°C (50°F) below the temperature identifying the start of time-dependent properties).

Normal Fluid Service: a fluid service pertaining to most piping covered by this Code, i.e., not subject to the rules for Category D, Category M, Elevated Temperature, High Pressure, or High Purity Fluid Service.

High Pressure Fluid Service: a fluid service for which the owner specifies the use of Chapter IX for piping design and construction. High pressure is considered herein to be

pressure in excess of that allowed by the ASME B16.5 Class 2500 rating for the specified design temperature and material group.

High Purity Fluid Service: a fluid service that requires alternative methods of fabrication, inspection, examination, and testing not covered elsewhere in the Code, with the intent to produce a controlled level of cleanness. The term thus applies to piping systems defined for other purposes as high purity, ultra high purity, hygienic, or aseptic.

7. What do you mean by the term SIF?Ans: The stress intensification factor or SIF is an intensifier of bending or torsional stress local to a piping component such as tees, elbows and has a value great than or equal to 1.0. Its value depends on component geometry. Code B 31.3 Appendix D (shown in below figure) provides formulas to calculate the SIF values.

8.  When do you feel that a piping system is not required formal stress analysis?Ans: Formal pipe stress analysis will not be required if any of the following 3 mentioned criteria are satisfied:1. if the system duplicates, or replaces without significant change, a system operating with a

successful service record (operating successfully for more than 10 years without major failure).

2. if the system can readily be judged adequate by comparison with previously analyzed systems.

3. if the system is of uniform size, has no more than two points of fixation, no intermediate restraints, and falls within the limitations of empirical equation mentioned below:

Here,D = outside diameter of pipe, mm (in.)Ea = reference modulus of elasticity at 21°C (70°F),MPa (ksi)K1 = 208 000 SA/Ea, (mm/m)2 = 30 SA/Ea, (in./ft)2L = developed length of piping between anchors,m (ft)SA = allowable displacement stress rangeU = anchor distance, straight line between anchors,m (ft)y = resultant of total displacement strains, mm (in.), to be absorbed by the piping system9.  How will you calculate the displacement (Expansion) stress range for a piping system?Ans: Expansion stress range (SE) for a complex piping system is normally calculated using softwares like Caesar II or AutoPipe. However, the same can be calculated using the following code equations:

hereAp = cross-sectional area of pipeFa = range of axial forces due to displacement strains between any two conditions being evaluatedia = axial stress intensification factor. In the absence of more applicable data, ia p 1.0 for elbows, pipe bends, and miter bends (single, closely spaced, and widely spaced), and ia =io (or i when listed) in Appendix D for other components;it = torsional stress intensification factor. In the absence of more applicable data, it=1.0;Mt = torsional momentSa = axial stress range due to displacement strains= iaXFa/Ap

Sb = resultant bending stressSt = torsional stress= itXMt/2ZZ = section modulus of pipeii = in-plane stress intensification factor from Appendix Dio = out-plane stress intensification factor from Appendix DMi = in-plane bending momentMo = out-plane bending momentSb = resultant bending stress

10. What do you mean by the term “Cold Spring”?Ans: Cold spring is the intentional initial deformation applied to a piping system during assembly to produce a desired initial displacement and stress. Cold spring is beneficial in that it serves to balance the magnitude of stress under initial and extreme displacement conditions.

When cold spring is properly applied there is less likelihood of overstrain during initial operation; hence, it is recommended especially for piping materials of limited ductility. There is also less deviation from as installed dimensions during initial operation, so that hangers will not be displaced as far from their original settings.

However now a days most of the EPC organizations does not prefer the use of Cold Spring while analysis any system.

11. How to decide whether Reinforcement is required for a piping branch connection or not?Ans: When a branch connection is made in any parent pipe the pipe connection is weakened by the opening that is made in it. So it is required that the wall thickness after the opening must be sufficiently in excess of the required thickness to sustain the pressure. This requirement is checked by calculating the required reinforcement area (A1) and available reinforcement area (A2+A3+A4) and if available area is more than the required area then no reinforcement is required. Otherwise additional reinforcement need to be added. The equations for calculating the required and available area are listed below for your information from the code. Please refer the code for notations used:

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Piping insulation: Important Considerations for Piping EngineerWhy Insulation?:An insulation system serves three principal purposes: the significant reduction in heat transfer of thermal energy to and from the surface of the

piping system (Heat Conservation). the prevention of moisture formation and collection on the surface of the piping system due to

condensation (Cold Insulation). the prevention of potentially injurious personnel contact with the surface of the exposed

piping system (Personal Protection). sometimes Steam traced/Electric traced insulation, Regeneration insulation, jacketting etc are

used as per process / liscensor requirement. Fire proofing, fire protection and acoustic insulation (to absorb vibration) is provided based

on project specification/ ITB requirement.Insulation Material: Low temperature insulation is frequently made of expanded cellular plastic or foam rubber

material. Moderate temperature insulations are made from grass fibre products. High temperature insulation is made of preformed cementations or refractory materials or

blankets made from ceramic fibres. Insulation and accessory materials has to be 100% asbestos free. Normally mineral fibre, cellular glass, ceramic fibre, glass fibre, polyisocyanurate,

polyurethane foam etc are used as insulation material.Few important points related to insulation (May vary from project to project): Hot insulation or Heat conservation insulation is used when normal operating temperature

exceeds 65 degree centigrade unless heat loss is desirable. (Control process temperature) Personnel protection is used in accessible areas if temperature exceeds 65 degree centigrade. Steam traced, electric traced, jacketting etc are used for process temperature control. Insulation thickness is determined based on pipe size, normal operating temperature,

temperature controlling requirement (extent of heat loss/gain) etc. At a minimum 25 mm thickness is used.

If insulation thickness is more than 75 mm then insulation is provided in two or more layers (multi layer).

Insulation shall not be applied untill hydrostatic/ pneumatic testing. Insulation upto 12 inch NPS pipe shall be held with ss 304 tie wire and for >12 inch NPS ss

304 bands are used. All flanges will be insulated other than hydrogen service or high health hazard material

services.

All valves other than control valves and relief valves shall be insulated.

Piping Elbows and Bends: A useful detailed literature for piping engineersPiping Elbows and Bends are very important pipe fitting which are used very frequently for changing direction in piping system. Piping Elbow and Piping bend are not the same, even though sometimes these two terms are interchangeably used.A BEND is simply a generic term in piping for an “offset” – a change in direction of the piping. It signifies that there is a “bend” i.e, a change in direction of the piping (usually for some specific reason) – but it lacks specific, engineering definition as to direction and degree. Bends are usually made by using a bending machine (hot bending and cold bending) on site and suited for a specific need. Use of bends are economic as it reduces number of expensive fittings.An ELBOW, on the other hand, is a specific, standard, engineered bend pre-fabricated as a spool piece  (based on ASME B 16.9) and designed to either be screwed, flanged, or welded to the piping it is associated with. An elbow can be 45 degree or 90 degree. There can also be custom-designed elbows, although most are catagorized as either “short radius” or long radius”.In short “All bends are elbows but all elbows are not bend”

Whenever the term elbow is used, it must also carry the qualifiers of type (45 or 90 degree) and radius (short or long) – besides the nominal size.

Elbows can change direction to any angle as per requirement. An elbow angle can be defined as the angle by which the flow direction deviates from its original flowing direction (See Fig.1 below).Even though An elbow angle can be anything greater than 0 but less or equal to 90°But still a change in direction greater than 90° at a single point is not desirable. Normally, a 45° and a 90° elbow combinedly used while making piping layouts for such situations.

Fig.1 A typical elbow with elbow angle (phi)Elbow angle can be easily calculated using simple geometrical technique of mathematics. Lets give an example for you. Refer to Fig.2. Pipe direction is changing at point A with the help of an elbow and again the direction is changing at the point G using another elbow.

In order to find out the elbow angle at A, it is necessary to consider a plane which contains the arms of the elbow. If there had been no change in direction at point A, the pipe would have moved along line AD but pipe is moving along line AG. Plane AFGD contains lines AD and AG and elbow angle (phi) is marked which denotes the angle by which the flow is deviating from its original direction.

Fig.2 Example figure for elbow angle calculation

Considering right angle triangle AGD, tan(phi) = √( x2 + z2)/ySimilarly elbow angle at G is given by : tan (phi1)=√ (y2 +z2)/xElbow Radius: Elbows or bends are available in various radii for a smooth change in direction which are expressed in terms of pipe nominal size expressed in inches. Elbows or bends are available in three radii,a. Long radius elbows (Radius = 1.5D): used most frequently where there is a need to keep the frictional fluid pressure loss down to a minimum, there is ample space and volume to allow for a wider turn and generate less pressure drop.b. Long radius elbows (Radius > 1.5D): Used sometimes for specific applications for transporting high viscous fluids likes slurry, low polymer etc. For radius more than 1.5D pipe bends are usually used and these can be made to any radius.However, 3D & 5D pipe bends are most commonly usedb. Short radius elbows (Radius = 1.0D): to be used only in locations where space does not permit use of long radies elbow and there is a need to reduce the cost of elbows. In jacketed piping the short radius elbow is used for the core pipe.Here D is nominal pipe size in inches.There are three major parameters which dictates the radius selection for elbow. Space availability, cost and pressure drop.Pipe bends are preferred where pressure drop is of a major consideration.Use of short radius elbows should be avoided as far as possible due to abrupt change in direction causing high pressure drop.Minimum thickness requirement:

 Whether an elbow or bend is used the minimum thickness requirement from code must be met. Code ASME B 31.3 provides equation for calculating minimum thickness required (t) in finished form for a given internal design pressure (P) as shown below:

Fig.3: Code equation for minimum thickness requirement calculationHere,R1 = bend radius of welding elbow or pipe bendD = outside diameter of pipeW = weld joint strength reduction factorY = coefficient from Code Table 304.1.1S = stress value for material from Table A-1 at maximum temperatureE = quality factor from Table A-1A or A-1BAdd any corrosion, erosion, mechanical allowances with this calculated value to get the thickness required.End Connections:For connecting elbow/bend to pipe the following type of end connections are available Butt welded: Used alongwith large bore (>=2 inch) piping Socket welded: Used alongwith pipe size Screwed: Flanged:Butt welded Elbows: Pipe is connected to butt welded elbow as shown in Fig. 4 by having a butt-welding joint. Butt welded fittings are supplied with bevel ends suitable for welding to pipe. It is important

to indicate the connected pipe thickness /schedule while ordering. All edge preparations for butt welding should conform to ASME B16.25.

Dimensions of butt welded elbows are as per ASME B16.9. This standard is applicable for carbon steel & alloy steel butt weld fittings of NPS 1/2” through 48”.

Fig.4: A typical Butt-Welded Elbow Dimensions of stainless steel butt welded fittings are as per MSS-SP-43. Physical dimensions

for fittings are identical under ASME B16.9 and MSS-SP-43. It is implied that the scope of ASME B16.9 deals primarily with the wall thicknesses which are common to carbon and low alloy steel piping, whereas MSS-SP-43 deals specifically with schedule 5S & 10S in stainless steel piping.

Dimensions for short radius elbows are as per ASME B16.28 in case of carbon steel & low alloy steel and MSS-SP-59 for stainless steel.

Butt welded fittings are usually used for sizes 2” & above. However, for smaller sizes up to 1-1/2” on critical lines where use of socket welded joints is prohibited, pipe bends are normally used. These bends are usually of 5D radius and made at site by cold bending of pipe. Alternatively, butt welded elbows can be used in lieu of pipe bends but usually smaller dia lines are field routed and it is not possible to have the requirement known at initial stage of the project for procurement purpose. So pipe bends are preferred. However, pipe bends do occupy more space and particularly in pharmaceutical plants where major portion of piping is of small dia. and layout is congested, butt welded elbows are preferred.

Butt welded joints can be radiographed and hence preferred for all critical services. Material standards as applicable to butt welded fittings are as follows:ASTM A234:

This specification covers wrought carbon steel & alloy steel fittings of seamless and welded construction. Unless seamless or welded construction is specified in order, either may be furnished at the option of the supplier. All welded construction fittings as per this standard are supplied with 100% radiography. Under ASTM A234, several grades are available depending upon chemical composition. Selection would depend upon pipe material connected to these fittings.

Some of the grades available under this specification and corresponding connected pipe material specification are listed below:

 ASTM A403:

This specification covers two general classes, WP & CR, of wrought austenitic stainless steel fittings of seamless and welded construction.Class WP fittings are manufactured to the requirements of ASME B16.9 & ASME B16.28 and are subdivided into three subclasses as follows:WP – SManufactured from seamless product by a seamless method of manufacture.WP – W These fittings contain welds and all welds made by the fitting manufacturer including starting pipe weld if the pipe was welded with the addition of filler material are radiographed. However no radiography is done for the starting pipe weld if the pipe was welded without the addition of filler material.WP-WX These fittings contain welds and all welds whether made by the fitting manufacturer or by the starting material manufacturer are radiographed.Class CR fittings are manufactured to the requirements of MSS-SP-43 and do not require non-destructive examination.Under ASTM A403 several grades are available depending upon chemical composition. Selection would depend upon pipe material connected to these fittings. Some of the grades available under this specification and corresponding connected pipe material specification are listed below:

 ASTM A420: This specification covers wrought carbon steel and alloy steel fittings of seamless & welded

construction intended for use at low temperatures. It covers four grades WPL6, WPL9, WPL3 & WPL8 depending upon chemical composition. Fittings WPL6 are impact tested at temp – 50° C, WPL9 at -75° C, WPL3 at -100° C and WPL8 at -195° C temperature.

The allowable pressure ratings for fittings may be calculated as for straight seamless pipe in accordance with the rules established in the applicable section of ASME B31.3.

The pipe wall thickness and material type shall be that with which the fittings have been ordered to be used, their identity on the fittings is in lieu of pressure rating markings.

Reducers used in Piping Industry: A short literaturePipelines are not of uniform size and there is requirement of reducing or expanding the lines depending on process requirement or availability of material. Here comes the importance of a special pipe fitting called Reducers.

Reducers are most extensively used in piping industry to reduce or expand the straight part of run pipe. Basically, reducers are available in two styles:

Concentric reducers and Eccentric reducers.

Concentric Reducers:As shown in Fig. 1. In this type of reducers area reduction is concentric and center line of the pipe on bigger end and smaller end  remains same.  These styles are normally used for vertical lines.

Fig.1: Concentric Reducers

 

Eccentric Reducers:As shown in Fig. 2 , in this style of reducer there is an offset in between the center lines of bigger end and center line of smaller end. This offset or eccentricity will maintain a flat side either on top or on bottom side.

Fig.2: Eccentric Reducer

 This offset or eccentricity can easily be found out by the following equation:

Eccentricity=(Bigger end ID-Smaller end ID)/2

While using this type of reducer the user has the option of orienting the flat side. Usually for horizontal lines, eccentric reducers are oriented with either the flat side up or down and the same with deviation is mentioned in isometric.

Normally eccentric reducers with flat side down are preferred for following cases on horizontal lines:• On Sleepers and Piperacks• On lines requiring gravity flow• On pump suction line which handle slurry.

Eccentric reducers with flat side up are used for all pump suction lines (excluding pumps handling slurry) on horizontal lines. This way one can avoid air getting trapped inside the pipeline during initial venting through pump casing and will help in avoiding Cavitation.

Depending on end connections of this fitting with straight pipe, reducers are grouped as follows:

Butt Welding reducers:  The applicable pressure rating, dimensional and material standards for butt welding reducers are same as those applicable to butt welding elbows.Socket welding reducers: As shown in Fig.3. such reducers are available in concentric type only &  in the form of a coupling with one end socket to fit larger diameter pipe and other end socket to fit smaller diameter pipe.  Standards are same as those applicable to socket welding elbows.

Fig.3: Socket Welded Reducers

Screwed reducers: Available only in concentric type and are in the form of coupling having one end to fit bigger pipe and other end to fit smaller pipe.  ASME B16.11 is applicable dimensional standard. Material standards including pressure ratings are same as of screwed elbows.Flanged reducers: Their pressure rating, use, material and dimensional standards are same as those applicableto flanged elbows. Regardless of reduction their face to face dimensions are governed by the larger pipe size.

Functions of Gaskets for leak proof Flanged jointsGasket is one of the basic elements for flanged joints in piping system of process plants.

A gasket can be defined as a material or combination of materials clamped between two separable mechanical members of a mechanical joint (flanged joint) which produces the weakest link of the joint. Gaskets are used to create a static seal between two stationary members of a mechanical assembly (the flanged joint). The gasket material flows (interpose a semi-plastic material between the flange facings) into the imperfections between the mating surfaces by an external force (bolt tightening force) and maintain a tight seal (seals the minute surface irregularities to prevent leakage of the fluid) under operating conditions. The amount of flow (seal) of the gasket material that is required to produce a tight seal is dependent upon the roughness of the surface. The gasket must be able to maintain this seal under all the operating conditions of the system including extreme upsets of temperature and pressure. Therefore, it is important to ensure proper design and selection of the gaskets to prevent flange-leakage problems and avoid costly shutdowns of the process plants. The following article will try to explain the main points related to gaskets.

Working philosophy of a gasket to prevent leakage: Refer the above figure which shows the three major forces acting on the gasket. Normally the gasket is seated by tightening the bolts on the flanges before the application of the internal pressure. Upon the application of the internal pressure in the joint, an end force (Hydrostatic end force) tends to separate the flanges and to decrease the unit stress (Residual stress) on the gasket. Leakage will occur under pressure if the hydrostatic end force is sufficiently great and the difference between hydrostatic end force and the bolt-load reduces the gasket load below a critical value.  To explain it in more clear language we can say that there are three principal forces acting on any gasketed joint. They are:  Bolt Load which applies the initial compressive load that flows the gasket material into

surface imperfections to form a seal.

The hydrostatic end force, that tends to separate flanges when the system is pressurized.

Internal pressure acting on the portion of the gasket exposed to internal pressure, tending to blow the gasket out of the joint and/or to bypass the gasket under operating conditions. 

Even though there are other shock forces that may be created due to sudden changes in temperature and pressure. Creep relaxation is another factor that may come into the picture. The initial compression force applied to a joint must serve several purposes. It must be sufficient to initially seat the gasket and flow the gasket into the imperfections on

the gasket seating surfaces regardless of operating conditions.

Initial compression force must be great enough to compensate for the total hydrostatic end force that would be present during operating conditions.

It must be sufficient to maintain a residual load on the gasket/flange interface.

Now from a practical standpoint, residual load on the gasket must be “X” times internal pressure if a tight joint is required to be maintained. This unknown quantity “X” is what is specified as the “m” factor in the ASME Pressure Vessel Code and will vary depending upon the type of gasket being used. Actually the “m” value is the ratio of residual unit stress (bolt load minus hydrostatic

end force) on gasket to internal pressure of the system. The larger the value of “m”, the more assurance the designer has of obtaining a tight joint.Gasket Types: Gaskets can be grouped into three main categories as follows: Non-metallic Gaskets: Usually composite sheet materials are used with flat face flanges and

low pressure class applications. Non-metallic gaskets are manufactured non-asbestos material or Compressed Asbestos Fibre (CAF). Non-asbestos types include arimid fibre, glass fibre, elastomer, Teflon (PTFE) and flexible graphite gaskets. Full face gasket types are suitable for use with flat-face (FF) flanges and flat-ring gasket types are suitable for use with raised face (RF) flanges.

Semi-metallic Gaskets: Semi-metallic gaskets are composites of metal and non-metallic materials. The metal is intended to offer the strength and resiliency while the non-metallic portion of a gasket provides conformability and sealability. Commonly used semi-metallic gaskets are spiral wound, metal jacketed, Cam profile and a variety of metal-reinforced graphite gaskets. Semi metallic gaskets are designed for the widest range of operating conditions of temperature and pressure. Semi-metallic gaskets are used on raised face, male-and female and tongue and groove flanges.

Metallic Gaskets: Metallic gaskets are fabricated from one or a combination of metal to the desired shape and size. Common metallic gaskets are ring-joint gaskets and lens rings. They are suitable for high-pressure and temperature applications and require high bolt load to seal.

Common gasket configurations: Aside from the choice of gasket material, the structure or configuration of the gasket is also significant. Following are descriptions of four major types. Graphite foil: The physical and chemical properties of graphite foil make it suitable as a

sealing material for relatively arduous operating condition. In an oxidizing environment, graphite foil can be used in the temperature range of –200 to +500°C, and in a reducing atmosphere, it can be used at temperatures between –200 and 2,000°C. Because graphite foil has no binder materials, it has excellent chemical resistance, and is not affected by most of the commercially used common chemicals. It also has very good stress-relaxation properties.

Spiral-wound: As the name implies, the spiral-wound gasket is made by winding a preformed-metal strip and a filler on the periphery of a metal winding mandrel. All spiral-wound gaskets are furnished with a centering ring. In addition to controlling compression, these rings serve to locate the gasket centrally within the bolt circle. Inner rings are used where the material (such as a gasket with PTFE filler) has a tendency for inward buckling. The ring also prevents the buildup of solids between the inside diameter of the gasket and the bore of pipe. Under vacuum condition, the ring protects against damage that would occur if a pieces of a broken component were drawn into the the system. Spiral-wound gaskets can operate at temperatures from –250 to 1,000°C, and pressures from vacuum to 350 bar. Spiral-wound gaskets up to 1-in. diameter and up to class number 600 require a uniform bolt stress

of 25,000 psi to compress the gasket. Larger sizes and classes require 30,000 psi to compress the gasket.

Ring-joint: Ring-joint gaskets are commonly used in grooved flanges for high-pressure-piping systems and vessels. Their applicable pressure range is from 1,000 to 15,000 psi. These gaskets are designed to give very high gasket pressure with moderate bolt load. These joints are not generally pressure-actuated. The hardness must be less than that of the flange material so that proper flow of material occurs without damaging flange surfaces. The most widely used ring-joint gaskets are of the oval and octagonal type. Oval-type gaskets contact the flange face at the curved surface and provide a highly reliable seal. However, the curved shape makes it more difficult to achieve accurate dimensioning and surface finishing. Oval gaskets also have the disadvantage that they can only be used once, so they may not be the best choice for sealing flanges that have to be opened routinely. On the other hand, because they are constructed of only straight faces, octagonal-type gaskets are usually less expensive, they can be dimensioned more accurately, and are easier to surface finish than the oval-type gasket. However, a greater torque load is required to flow the gasket material into imperfections that may reside on the flange faces. Octagonal gaskets can be used more than once.

Corrugated-metal: This type of gasket is available in a wide range of metals, including brass, copper, coppernickel alloys, steel, monel, and aluminium. Corrugated metal gaskets can be manufactured to just about any shape and size required. The thickness of the metal is normally 0.25 or 0.3 mm, with corrugations having a pitch of 1.6, 3.2, and 6.4 mm. The sealing mechanism is based on point contact between the peaks of the corrugations and the mating flanges

Gasket Standards: Following standards are normally adopted for specifying gaskets. ASME B16.21 Non-metallic flat gaskets for pipe flanges.

ASME B16.20 Metallic Gaskets for steel pipe flanges, Ring Joint, Spiral Wound and Jacketed

IS2712 Specification for compressed Asbestos fibre jointing.

BS 3381 Sprial Wound Gaskets to suit BS 1560 Flanges

Selection of Gaskets: The gasket material selected should be one which is not adversely affected physically or

chemically by the service conditions.

The two types of gaskets most commonly known are ring gaskets and full face gaskets. The latter as the name implies, covers the entire flange face and are pierced by the bolt holes. They are intended for use with flat face flanges. Ring gaskets extend to the inside of the flange bolt holes and consequently are self centering. They are usually used with raised face or lap joint flanges but may also be used with flat-faced flanges.

Flat-ring gaskets are widely used wherever service condition permits because of the ease with which they may be cut from flat sheet and installed. They are commonly fabricated from such materials as rubber, paper, cloth, asbestos, plastics, copper, lead, aluminum, nickel, monel, and soft iron. The gaskets are usually made in thickness from 1/64 to 1/8 in. Paper, cloth and rubber gaskets are not recommended for use above 120° C. Asbestos-composition gaskets may be used up to 350° C or slightly higher, ferrous and nickel-alloy metal gaskets may be used up to the maximum temperature rating of the flanges.

Upon initial compression a gasket will flow both axially and radially. The axial flow is required to fill depressions in the flange facing and prevent leakage. Radial flow serves no useful purpose unless the gasket is confined. Where a flange joint is heated, a greater gasket pressure is produced due to the difference between the flange body and the bolts. This greater pressure coupled with the usual softening of the gasket material at elevated temperatures causes additional axial and radial gasket flow. To compensate for this, the flange bolts are usually re-tightened a second or third time after the joint is heated to the normal operating temperature. A thick gasket will flow radially to a far greater extent than a thin gasket. Some thin gaskets show practically no radial flow at extremely high unit pressures. Consequently, for high temperatures a thin gasket has the advantage of maintaining a permanent thickness while a thick gasket will continue to flow radially and may leak, in time, due to the resulting reduced gasket pressure. However in attempting utmost utilization of thin gasket advantage, one may find that gasket selected has insufficient thickness to seal the irregularities, in the commercial flange faces. The spiral wound asbestos-metallic gasket combines the advantages of both the thick and thin gasket. Although a relatively thick gasket (most common types are 0.175” thick) its spirally laminated construction confines the asbestos filler between axially flexible metal layers. This eliminates the radial flow characteristics of a thick gasket and provides the resiliency to adjust to vary service conditions. Spiral wound gaskets are available with different filler materials such as Teflon, grafoil etc. to suit fluid compatibility. Spiral wound gaskets used with raised face flanges usually have an inner metal ring and an outer centering ring.

Laminated gaskets are fabricated with a metal jacket and a soft filler, usually of asbestos. Such gaskets can be used up to temperatures of about 400° C to 450° C and require less bolt load to seat and keep tight than solid metal flat ring gaskets.

Serrated metal gaskets are fabricated of solid metal and have concentric grooves machined into the faces. This greatly reduces the contact area on initially tightening thereby reducing the bolt load. As the gasket is deformed, the contact surface area increases. Serrated gaskets are useful where soft gaskets or laminated gaskets are unsatisfactory and bolt load is excessive with a flat-ring metal gasket. Smooth-finished flange faces should be used with serrated gaskets.

Corrugated gaskets with asbestos filling are similar to laminated gaskets except that the surface is rigid with concentric rings as with the case of serrated gaskets. Corrugated gaskets require less seating force than laminated or serrated gaskets and are extensively used in low-pressure liquid and gas service. Corrugated metal gaskets without asbestos may be used to higher temperature than those with asbestos filling.

Two standard types of ring-joint gaskets are available for high-pressure service. One type has an oval cross section, and the other has an octagonal cross section. These rings are fabricated of solid metal, usually soft iron, soft steel, monel, 4-6% chrome, and stainless steels. The alloy-steel rings should be heat treated to soften them.

It is recommended that ring joint gasket be used for class 150 flanged joints. When the ring joint or spiral wound gasket is selected, it is recommended that line flanges be of the welding neck type.

Parameters affecting Gasket performance:The performance of the gasket is affected by a number of factors. All of these factors must be taken into consideration when selecting a gasket: The Flange Load: All gasket materials must have sufficient flange pressure to compress the

gasket enough to insure that a tight, unbroken seal occurs. The flange pressure, or minimum seating stress, necessary to accomplish this is known as the “y” factor. This flange pressure must be applied uniformly across the entire seating area to achieve perfect sealing. However, in actual service, the distribution around the gasket is not uniform. The greatest force is exerted on the area directly surrounding the bolts. The lowest force occurs mid-way between two bolts. This factor must be taken into account by the flange designer.

The Internal Pressure: In service, as soon as pressure is applied to the vessel, the initial gasket compression is reduced by the internal pressure acting against the gasket (blowout pressure) and the flanges (hydrostatic end force). To account for this, an additional preload must be placed on the gasket material. An “m” or maintenance factor has been established by ASME to account for this preload. The “m” factor defines how many times the residual load (original load minus the internal pressure) must exceed the internal pressure. In this calculation, the normal pressure and the test pressure should be taken into account.

Temperature: The effects of both ambient and process temperature on the gasket material, the flanges and the bolts must be taken into account. These effects include bolt elongation, creep relaxation of the gasket material or thermal degradation. This can result in a reduction of the flange load. The higher he operating temperature, the more care needs to be taken with the asket material selection. As the system is pressurized and heated, the joint deforms. Different coefficients of expansion between the bolts, the flanges and the pipe can result in forces which can affect the gasket. The relative stiffness of the bolted joint determines whether there is a net gain or loss in the bolt load. Generally, flexible joints lose bolt load.

Fluid: The media being sealed, usually a liquid or a gas with a gas being harder to seal than a liquid. The effect of temperature on many fluids causes them to become more aggressive. Therefore, a fluid that can be sealed at ambient temperature, may adversely affect the gasket at a higher temperature. The gasket material must be resistant to corrosive attack from the fluid. It should chemically resist the system fluid to prevent serious impairment of its physical properties.

Surface Finish of the Gasket: The surface finish of a gasket — which consists of grooves or channels pressed or machined onto the outer surface — governs the thickness and compressibility required by the gasket material to form a physical barrier in the clearance gap between the flanges. A finish that is too fine or shallow is undesirable, especially on hard gasket materials, because the smooth surface may lack the required grip, which will allow extrusion to occur. On the other hand, a finish that is too deep will yield a gasket that requires a higher bolt load, which may make it difficult to form a tight seal, especially when large flange surfaces are involved. Fine machining marks applied to the flange face, tangent to the direction of applied fluid pressure can also be helpful. Flange faces with non-slip grooves that are approximately 0.125 mm deep are recommended for gaskets more than 0.5 mm thick; and for thinner gaskets, grooves 0.065 mm deep are recommended. Under no circumstances should the flange-sealing surface be machined with tool marks extending radially across the gasket-sealing surface; such marks could allow leakage.

Gasket Thickness: For a given material, it is a general rule that a thinner gasket is able to handle a higher compressive stresses than thicker one. However, thinner materials require a higher surface finish quality. As a rule of thumb, the gasket should be at least four times thicker than the maximum surface roughness of the flange faces. The gasket must be thick enough to occupy the shape of the flange faces and still compress under the bolt load. In situations where vibration is unavoidable, a thicker gasket than the minimum required should be employed.

Gasket Width: In order to reduce the bolt load required to produce a particular gasket pressure, it is advisable not to have the gasket wider than is necessary. For a given gasket stress, a raised face flange with a narrow gasket will require less pre-load, and thus less flange strength than a full-face gasket. In general, high-pressure gaskets tend to be narrow.

Stress Relaxation: This factor is a measure of the material’s resiliency over a period of time, and is normally expressed as a percentage loss per unit of time. All gasket material will lose some resiliency over time, due to the flow or thinning of the material caused by the applied pressure. After some initial relaxation, the residual stress should remain constant for the gasket.

Gasket Outer Diameter: For two gaskets made of the same material and having the same width, the one with a larger outer diameter will withstand a higher pressure. Therefore, it is advisable to use a gasket with an external diameter that is as large as possible.

Heat Tracing of Piping SystemsHeat Tracing of Piping SystemsHeat Tracing is a generalized term relating to the application of radiant heat input to piping systems from tubing attached to the outside of the pipe.When Heat tracing is used to ensure that the system functions from a process standpoint regardless of climate conditions it is known asProcess Control TracingAgain when Heat tracing is used to prevent freeze up due to climatic conditions only it is known asWinterization Tracing.General Requirements General Steam tracing supply lines shall be taken from the top of the supply header to assure dry quality steam. Identify the locations for steam tracing supply manifolds and condensate manifolds early in design to

reserve space in plant layout. This applies to non-steam supply and return manifolds (hot oil, glycol, etc.).

Allow for increase in insulation sizing to allow for tracers.Instrument Application This specification is to be used by Piping for heat tracing of all in-line instruments. Piping will also

provide steam supply and condensate collection manifolds for all other instruments. The break between Piping  Traced Instruments and Control Systems traced instruments will match the drawing break between the two departments.

System Description Using various media such as steam, hot water, glycol, or hot oil heat tracing is installed to protect the

piping, equipment, and instruments against temperatures that would cause congealing or freezing of the process fluids, interfere with operation, or cause damage to the equipment.

Design Requirements  The daily average low temperature of the coldest month shall be used to select the low ambient design

temperature that then determines the degree of winterizing protection required. No winterizing is required for water service except where a sustained temperature below minus 1

degree C is often recorded for 24 hours or longer. Compressors, blowers, and other mechanical equipment shall be specified for operation at low

ambient design temperature.Methods of Heat Conservation  Where feasible, insulation shall be used for heat conservation. Heat tracing, plus insulation, is the alternative method for heat conservation. Heat transfer cement may be utilized when a process line requires a high heat input and common

methods of heat tracing are inadequate. Steam jacketing is utilized in specific cases where steam tracing with heat transfer cement is

inadequate. Electric tracing is utilized when precise temperature control is required or where steam tracing is not

practical. Thermostat setting for electric tracing should not be higher than fluid operating temperature.Methods for Winterization  Winterizing by circulation shall be provided where a sufficient power source is available to keep the

fluid circulating. Utility water and utility air lines in intermittent service shall be winterized by draining.

Winterizing by steam tracing is the preferred method when winterizing by circulation and draining is impracticable.

Winterizing by electric tracing is utilized when a precise temperature control is required or where steam tracing is not practical. Thermostat setting for electric tracing should not be higher than fluid operating temperature.

Minimum tracing steam pressure shall be 1 Bar; maximum required is 10.3 Bar. At minimum pressure, condensate shall be routed to the plant sewer system. If condensate is collected, the minimum usable pressure shall be 1.7 Bar.

Tracer Description Tracer Size and Length  Required tracer size shall be determined by piping heat loss and tracer steam pressure found in the

Heat Loss Chart (Fig. 1) Minimum tracer size shall be 3/8 of an inch OD tubing; maximum size shall be 1 inch OD tubing. For

economy, where Heat Loss Chart indicates the requirements for multiple tracers, a single tracer with heat transfer cement shall be considered.

When using heat transfer cement, tracers of 3/8 of an inch and 1/2 of an inch OD tubing are recommended. If more tracer area is required, multiple tracers of 3/8 of an inch and 1/2 of an inch shall be used.

Maximum tracer length shall be based on tracer size and steam pressure as follows:o    Steam pressure 1 Bar through 1.7 Bar

60m for 3/8 of an inch and 1/2 of an inch tracers 100m for 3/4 of an inch and 1 inch tracerso    Steam pressure 3.5 Bar through 13.8 Bar

60m for 3/8 of an inch and 1/2 of an inch tracers 120m for 3/4 of an inch and 1 inch tracerso    Tracer lengths for tracing with heat transfer cement shall be based on recommendation of manufacturer.

For stainless steel lines, the tracer material shall be low carbon steel. Stainless steel instrument leads shall be traced with copper tubing.

Each tracer shall have its own trap. Tracer traps shall discharge to sewer. If condensate must be collected, minimum usable pressure is 1.7 Bar.

Compression type fittings shall be installed outside of the insulation OD. Socket type fittings may be installed inside of the insulation. The steam tracers shall be pressure tested before the insulation is applied. Under emergency

conditions, the insulation may be applied but the fittings shall be left exposed until the testing is complete.

Tracer Pocket Depth  Pocket depth is the distance the tracer rises in the direction of flow from a low point to a high point.

The total pocket depth is the sum of all risers of the tracer. Maximum tracer total pocket depth shall be equal to 40 percent of tracing steam gage pressure

expressed in meters.

Example: Tracing steam 10.3 bar 30 m x 0.40 = 12 m feet total pocket depth

Products  Steam tracing tubing materials shall be in accordance with material specifications. Tracers shall be OD tubing. Soft annealed copper tubing shall be used where the temperature of the

product line or tracing steam does not exceed 204 °C. Above this temperature, dead soft annealed hydraulic quality, low carbon, seamless steel tubing shall be used where the temperature of the product line or tracing steam does not exceed 399 °C.

For aluminum pipe lines, carbon steel tracer material shall not be used. For aluminum pipe lines and all lines above 399 °C the tracer material shall be stainless steel. For conditions where the tracer could overheat lines containing acid, caustic, amine, phenolic water, or

other chemicals, insulation spacer blocks shall be installed between tracer and pipe.

Fig.1: Typical Heat Loss Chart