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Piping General Definitions

Piping General Definitions

Learn more about Piping General Definitions are essential for piping stress analysis, piping design and piping fabrication (include welding process). Therefor, Little P.Eng. for Engineers Training shares the the following piping definitions below:

  • Absolute Viscosity. Absolute viscosity or the coefficient of absolute viscosity is a measure of the internal resistance. In the centimeter, gram, second (cgs) or metric system, the unit of absolute viscosity is the poise (abbreviated P), which is equal to 100 centipoise (cP). The English units used to measure or express viscosity are slugs per foot-second or pound force seconds per square foot. Sometimes, the English units are also expressed as pound mass per foot-second or poundal seconds per square foot.

  • Adhesive Joint. A joint made in plastic piping by the use of an adhesive substance which forms a continuous bond between the mating surfaces without dissolving either one of them. Refer to Part D of this handbook.

  • Air-Hardened Steel. A steel that hardens during cooling in air from a temperature above its transformation range.1

  • Alloy Steel. A steel which owes its distinctive properties to elements other than carbon. Steel is considered to be alloy steel when the maximum of the range given for the content of alloying elements exceeds one or more of the following limits:

Manganese 1.65 percent

Silicon 0.60 percent

Copper 0.60 percent

or a definite range or a definite minimum quantity of any of the following elements is specified or required within the limits of the recognized field of constructional alloy steels:

Aluminum Nickel

Boron Titanium

Chromium (up to 3.99 percent) Tungsten Cobalt Vanadium

Columbium Zirconium


or any other alloying element added to obtain a desired alloying effect.

Small quantities of certain elements are unavoidably present in alloy steels. In many applications, these are not considered to be important and are not specified or required. When not specified or required, they should not exceed the following amounts:

Copper 0.35 percent

Chromium 0.20 percent

Nickel 0.25 percent

Molybdenum 0.06 percent

  • Ambient Temperature. The temperature of the surrounding medium, usually used to refer to the temperature of the air in which a structure is situated or a device operates.

  • Anchor. A rigid restraint providing substantially full fixation, permitting neither translatory nor rotational displacement of the pipe.

  • Annealing. Heating a metal to a temperature above a critical temperature and holding above that range for a proper period of time, followed by cooling at a suitable rate to below that range for such purposes as reducing hardness, improving machinability, facilitating cold working, producing a desired microstructure, or obtaining desired mechanical, physical, or other properties. (A softening treatment is often carried out just below the critical range which is referred to as a subcritical annealing.)

  • Arc Cutting. A group of cutting processes in which the severing or removing of metals is effected by melting with the heat of an arc between an electrode and the base metal (includes carbon, metal, gas metal, gas tungsten, plasma, and air carbon arc cutting). See also Oxygen Cutting.

  • Arc Welding. A group of welding processes in which coalescence is produced by heating with an electric arc or arcs, with or without the application of pressure and with or without the use of filler metal.

  • Assembly. The joining together of two or more piping components by bolting, welding, caulking, brazing, soldering, cementing, or threading into their installed location as specified by the engineering design.

  • Automatic Welding. Welding with equipment which performs the entire welding operation without constant observation and adjustment of the controls by an opera- tor. The equipment may or may not perform the loading and unloading of the work.

  • Backing Ring. Backing in the form of a ring that can be used in the welding of piping to prevent weld spatter from entering a pipe and to ensure full penetration of the weld to the inside of the pipe wall.

  • Ball Joint. A component which permits universal rotational movement in a piping system.

  • Base Metal. The metal to be welded, brazed, soldered, or cut. It is also referred to as parent metal.

  • Bell-Welded Pipe. Furnace-welded pipe produced in individual lengths from cut- length skelp, having its longitudinal butt joint forge-welded by the mechanical pressure developed in drawing the furnace-heating skelp through a cone-shaped die (commonly known as a welding bell), which serves as a combined forming and welding die.

  • Bevel. A type of edge or end preparation.

  • Bevel Angle. The angle formed between the prepared edge of a member and a plane perpendicular to the surface of the member.

  • Blank Flange. A flange that is not drilled but is otherwise complete.

  • Blind Flange. A flange used to close the end of a pipe. It produces a blind end which is also known as a dead end.

  • Bond. The junction of the weld metal and the base metal, or the junction of the base metal parts when weld metal is not present.

  • Branch Connection. The attachment of a branch pipe to the run of a main pipe with or without the use of fittings.

  • Braze Welding. A method of welding whereby a groove, fillet, plug, or slot weld is made using a nonferrous filler metal having a melting point below that of the base metals, but above 800°F. The filler metal is not distributed in the joint by capillary action. (Bronze welding, the term formerly used, is a misnomer.)

  • Brazing. A metal joining process in which coalescence is produced by use of a nonferrous filler metal having a melting point above 800°F but lower than that of the base metals joined. The filler metal is distributed between the closely fitted surfaces of the joint by capillary.

  • Butt Joint. A joint between two members lying approximately in the same plane.

  • Butt Weld. Weld along a seam that is butted edge to edge.

  • Bypass. A small passage around a large valve for warming up a line. An emergency connection around a reducing valve, trap, etc., to use in case it is out of commission.

  • Carbon Steel. A steel which owes its distinctive properties chiefly to the carbon (as distinguished from the other elements) which it contains. Steel is considered to be carbon steel when no minimum content is specified or required for aluminum, boron, chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, or zirconium or for any other element added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese, 1.65 percent; silicon, 0.60 percent; copper, 0.60 percent.

  • Cast Iron. A generic term for the family of high carbon-silicon-iron casting alloys including gray, white, malleable, and ductile iron.

  • Centrifugally Cast Pipe. Pipe formed from the solidification of molten metal in a rotating mold. Both metal and sand molds are used. After casting, if required the pipe is machined, to sound metal, on the internal and external diameters to the surface roughness and dimensional requirements of the applicable material specification.

  • Certificate of Compliance. A written statement that the materials, equipment, or services are in accordance with the specified requirements. It may have to be supported by documented evidence.

  • Certified Material Test Report (CMTR). A document attesting that the material is in accordance with specified requirements, including the actual results of all required chemical analyses, tests, and examinations.

  • Chamfering. The preparation of a contour, other than for a square groove weld, on the edge of a member for welding.

  • Cold Bending. The bending of pipe to a predetermined radius at any temperature below some specified phase change or transformation temperature but especially at or near room temperature. Frequently, pipe is bent to a radius of 5 times the nominal pipe diameter.

  • Cold Working. Deformation of a metal plastically. Although ordinarily done at room temperature, cold working may be done at the temperature and rate at which strain hardening occurs. Bending of steel piping at 1300°F (704°C) would be considered a cold-working operation.

  • Companion Flange. A pipe flange suited to connect with another flange or with a flanged valve or fitting. A loose flange which is attached to a pipe by threading, van stoning, welding, or similar method as distinguished from a flange which is cast integrally with a fitting or pipe.

  • Consumable Insert. Preplaced filler metal which is completely fused into the root of the joint and becomes part of the weld.

  • Continuous-Welded Pipe. Furnace- welded pipe produced in continuous lengths from coiled skelp and subsequently cut into individual lengths, having its longitudinal butt joint forge- welded by the mechanical pressure developed in rolling the hot-formed skelp through a set of round pass welding rolls.

  • Contractor. The entity responsible for consumable insert ring in- furnishing materials and services for fabserted in pipe joint eccentrically for welding in fabrication and installation of piping and horizontal position.

  • Control Piping. All piping, valves, and fittings used to interconnect air, gas, or hydraulically operated control apparatus or instrument transmitters and receivers.

  • Controlled Cooling. A process of cooling from an elevated temperature in a predetermined manner to avoid hardening, cracking, or internal damage or to produce a desired metallurgical micro-structure. This cooling usually follows the final hot-forming or post heating operation.

  • Corner Joint. A joint between two members located approximately at right angles to each other in the form of an L.

  • Coupling. A threaded sleeve used to connect two pipes. Commercial couplings have internal threads to fit external threads on pipe.

  • Covered Electrode. A filler metal electrode, used in arc welding, consisting of a metal core wire with a relatively thick covering which provides protection for the molten metal from the atmosphere, improves the properties of the weld metal, and stabilizes the arc. Covered electrodes are extensively used in shop fabrication and field erection of piping of carbon, alloy, and stainless steels.

  • Crack. A fracture-type imperfection characterized by a sharp tip and high ratio of length and depth to opening displacement.

  • Creep or Plastic Flow of Metals. At sufficiently high temperatures, all metals flow under stress. The higher the temperature and stress, the greater the tendency to plastic flow for any given metal.

  • Cutting Torch. A device used in oxygen, air, or powder cutting for controlling and directing the gases used for preheating and the oxygen or powder used for cutting the metal.

  • Defect. A flaw or an imperfection of such size, shape, orientation, location, or properties as to be rejectable per the applicable minimum acceptance standards.

  • Density. The density of a substance is the mass of the substance per unit volume. It may be expressed in a variety of units.

  • Deposited Metal. Filler metal that has been added during a welding operation.

  • Depth of Fusion. The distance that fusion extends into the base metal from the surface melted during welding.

  • Designer. Responsible for ensuring that the engineering design of piping complies with the requirements of the applicable code and standard and any additional requirements established by the owner.

  • Dew Point. The temperature at which the vapor condenses when it is cooled at constant pressure.

  • Dilatant Liquid. If the viscosity of a liquid increases as agitation is increased at constant temperature, the liquid is termed dilatant. Examples are clay slurries and candy compounds.

  • Discontinuity. A lack of continuity or cohesion; an interruption in the normal physical structure of material or a product.

  • Double Submerged Arc-Welded Pipe. Pipe having a longitudinal butt joint produced by at least two passes, one of which is on the inside of the pipe. Coalescence is produced by heating with an electric arc or arcs between the bare metal electrode or electrodes and the work. The welding is shielded by a blanket of granular, fusible material on the work. Pressure is not used, and filler metal for the inside and outside welds is obtained from the electrode or electrodes.

  • Ductile Iron. A cast ferrous material in which the free graphite is in a spheroidal form rather than a fluke form. The desirable properties of ductile iron are achieved by means of chemistry and a ferritizing heat treatment of the castings.

  • Eddy Current Testing. This is a non-destructive testing method in which eddy current flow is induced in the test object. Changes in the flow caused by variations in the object are reflected into a nearby coil or coils for subsequent analysis by suitable instrumentation and techniques.

  • Edge Joint. A joint between the edges of two or more parallel or nearly parallel members.

  • Edge Preparation. The contour prepared on the edge of a member for welding.

  • Electric Flash-Welded Pipe. Pipe having a longitudinal butt joint in which coalescence is produced simultaneously.

  • Edge preparation. over the entire area of abutting surfaces by the heat obtained from resistance to the flow of electric current between the two surfaces and by the application of pressure after heating is substantially completed. Flashing and upsetting are accompanied by expulsion of metal from the joint.

  • Electric Fusion-Welded Pipe. Pipe having a longitudinal or spiral butt joint in which coalescence is produced in the preformed tube by manual or automatic electric arc welding. The weld may be single or double and may be made with or without the use of filler metal.

  • Electric Resistance-Welded Pipe. Pipe produced in individual lengths or in continuous lengths from coiled skelp and subsequently cut into individual lengths having a longitudinal butt joint in which coalescence is produced by the heat obtained from resistance of the pipe to the flow of electric current in a circuit of which the pipe is a part and by the application of pressure.

  • End Preparation. The contour prepared on the end of a pipe, fitting, or nozzle for welding. The particular preparation is prescribed by the governing code.

  • Engineering Design. The detailed design developed from process requirements and conforming to established design criteria, including all necessary drawings and specifications, governing a piping installation.

  • Equipment Connection. An integral part of such equipment as pressure vessels, heat exchanges, pumps, etc., designed for attachment of pipe or piping components.

  • Erection. The complete installation of a piping system, including any field assembly, fabrication, testing, and inspection of the system.

  • Erosion. Destruction of materials by the abrasive action of moving fluids, usually accelerated by the presence of solid particles.

  • Examination. The procedures for all visual observation and non-destructive testing.

  • Expansion Joint. A flexible piping component which absorbs thermal and/or terminal movement.

  • Extruded Nozzles. The forming of nozzle (tee) outlets in pipe by pulling hemi-spherically or conically shaped dies through a circular hole from the inside of the pipe. Although some cold extruding is done, it is generally performed on steel after the area to be shaped has been heated to temperatures between 2000 and 1600°F (1093 and 871°C).

  • Extruded Pipe. Pipe produced from hollow or solid round forgings, usually in a hydraulic extrusion press. In this process, the forging is contained in a cylindrical die. Initially a punch at the end of the extrusion plunger pierces the forging. The extrusion plunger then forces the contained billet between the cylindrical die and the punch to form the pipe, the latter acting as a mandrel. One variation of this process utilizes autofrettage (hydraulic expansion) and heat treatment, above the re-crystallization temperature of the material, to produce a wrought structure.

  • Fabrication. Primarily, the joining of piping components into integral pieces ready for assembly. It includes bending, forming, threading, welding, or other operations upon these components, if not part of assembly. It may be done in a shop or in the field.

  • Face of Weld. The exposed surface of a weld on the side from which the welding was done.

  • Filler Metal. Metal to be added in welding, soldering, brazing, or braze welding.

  • Fillet Weld. A weld of an approximately triangular cross section joining two surfaces approximately at right angles to each other in a lap joint, tee joint, corner joint, or socket weld.

  • Fire Hazard. Situation in which a material of more than average combustibility or excludability exists in the presence of a potential ignition source.

  • Flat-Land Bevel. A square extended root face preparation extensively used in inert-gas, root-pass welding of piping.

  • Flat Position. The position of welding which is performed from the upper side of the joint, while the face of the weld is approximately horizontal.

  • Flaw. An imperfection of unintentional discontinuity which is detectable by a non-destructive examination.

  • Flux. Material used to dissolve, prevent accumulation of, or facilitate removal of oxides and other undesirable substances during welding, brazing, or soldering.

  • Flux-Cored Arc Welding (FCAW). An arc welding process that employs a continuous tubular filler metal (consumable) electrode having a core of flux for shielding. Adding shielding may or may not be obtained from an externally supplied gas or gas mixture.

  • Forge Weld. A method of manufacture similar to hammer welding. The term forge welded is applied more particularly to headers and large drums, while hammer welded usually refers to pipe.

  • Forged and Bored Pipe. Pipe produced by boring or trepanning of a forged billet.

  • Full-Fillet Weld. A fillet weld whose size is equal to the thickness of the thinner member joined.

  • Fusion. The melting together of filler and base metal, or of base metal only, which results in coalescence.

  • Fusion Zone. The area of base metal melted as determined on the cross section of a weld.

  • Galvanizing. A process by which the Fusion zone is the section of surface of iron or steel is covered with the parent metal which melts during the weld a layer of zinc coating process.

  • Gas Metal Arc Welding (GMAW). An arc welding process that employs a contin- uous solid filler metal (consumable) electrode. Shielding is obtained entirely from an externally supplied gas or gas mixture.4,8 (Some methods of this process have been called MIG or CO2 welding.)

  • Gas Tungsten Arc Welding (GTAW). An arc welding process that employs a tungsten (nonconsumable) electrode. Shielding is obtained from a gas or gas mixture. Pressure may or may not be used, and filler metal may or may not be used. (This process has sometimes been called TIG welding.) When shielding is obtained by the use of an inert gas such as helium or argon, this process is called inert-gas tungsten arc welding.

  • Gas Welding. Welding process in which coalescence is produced by heating with a gas flame or flames, with or without the application of pressure and with or without the use of filler metal.

  • Groove. The opening provided for a groove weld.

  • Groove Angle. The total included angle of the groove between parts to be joined by a groove weld.

  • Groove Face. That surface of a member included in the groove.

  • Groove Radius. The radius of a J or U groove.

  • Groove Weld. A weld made in the groove between two members to be joined. The standard type of groove welds are square, single-V, single-bevel, single-U, single-J, double-V, double-U, double-bevel, double-J, and flat-land single, and double-V groove welds.

  • Hammer Weld. Method of manufacturing large pipe (usually NPS 20 or DN 500 and larger) by bending a plate into circular form, heating the overlapped edges to a welding temperature, and welding the longitudinal seam with a power hammer applied to the outside of the weld while the inner side is supported on an over- hung anvil.

  • Hangers and Supports. Hangers and supports include elements which transfer the load from the pipe or structural attachment to the supporting structure or equipment. They include hanging-type fixtures such as hanger rods, spring hangers, sway braces, counterweights, turnbuckles, struts, chains, guides, and anchors and bearing-type fixtures such as saddles, bases, rollers, brackets, and sliding supports.

  • Header. A pipe or fitting to which a number of branch pipes are connected.

  • Heat-Affected Zone. That portion of the base metal which has not been melted but whose mechanical properties or micro-structure has been altered by the heat of welding or cutting.

  • Heat Fusion Joint. A joint made in thermoplastic piping by heating the parts sufficiently to permit fusion of the materials when the parts are pressed together.

  • Horizontal Fixed Position. In pipe welding, the position of a pipe joint in which the axis of the pipe is approximately horizontal and the pipe is not rotated during the operation.

  • Horizontal-Position Fillet Weld. Welding is performed on the upper side of an approximately horizontal surface and against an approximately vertical surface.

  • Horizontal-Position Groove Weld. The position of welding in which the weld axis lies in an approximately horizontal plane and the face of the weld lies in an approximately vertical plane.

  • Horizontal Rolled Position. The position of a pipe joint in which welding is performed in the flat position by rotating the pipe.

  • Hot Bending. Bending of piping to a predetermined radius after heating to a suitably high temperature for hot working. On many pipe sizes, the pipe is firmly packed with sand to avoid wrinkling and excessive out-of-roundness.

  • Hot Taps. Branch piping connections made to operating pipelines, mains, or other facilities while they are in operation.

  • Hot Working. The plastic deformation of metal at such a temperature and rate that strain hardening does not occur. Extruding or swaging of chromemoly piping at temperatures between 2000 and 1600°F (1093 and 871°C) would be considered hot-forming or hot-working operations.

  • Hydraulic Radius. The ratio of area of flowing fluid to the wetted perimeter.

  • Impact Test. A test to determine the behavior of materials when subjected to high rates of loading, usually in bending, tension, or torsion. The quantity measured is the energy absorbed in breaking the specimen by a single blow, as in Charpy or Izod tests.

  • Imperfection. A condition of being imperfect; a departure of a quality characteristic from its intended condition.

  • Incomplete Fusion. Fusion which is less than complete and which does not result in melting completely through the thickness of the joint.

  • Indication. The response or evidence from the application of a nondestructive examination.

  • Induction Heating. Heat treatment of completed welds in piping by means of placing induction coils around the piping. This type of heating is usually performed during field erection in those cases where stress relief of carbon and alloy-steel field welds is required by the applicable code.

  • Inspection. Activities performed by an authorized inspector to verify whether an item or activity conforms to specified requirements.

  • Instrument Piping. All piping, valves, and fittings used to connect instruments to main piping, to other instruments and apparatus, or to measuring equipment.

  • Interpass Temperature. In a multiple-pass weld, the minimum or maximum temperature of the deposited weld metal before the next pass is started.

  • Interrupted Welding. Interruption of welding and preheat by allowing the weld area to cool to room temperature as generally permitted on carbon-steel and on chromemoly alloy-steel piping after sufficient weld passes equal to at least one- third of the pipe wall thickness or two weld layers, whichever is greater, have been deposited.

  • Joint. A connection between two lengths of pipe or between a length of pipe and a fitting.

  • Joint Penetration. The minimum depth a groove weld extends from its face into a joint, exclusive of reinforcement.

  • Kinematic Viscosity. The ratio of the absolute viscosity to the mass density.

  • Weld joint penetration. In the metric system, kinematic viscosity is measured in strokes or square centimeters per second.

  • Laminar Flow. Fluid flow in a pipe is usually considered laminar if the Reynolds number is less than 2000. Depending upon many possible varying conditions, the flow may be laminar at a Reynolds number as low as 1200 or as high as 40,000; however, such conditions are not experienced in normal practice.

  • Lap Weld. Weld along a longitudinal seam in which one part is overlapped by the other. A term used to designate pipe made by this process.

  • Lapped Joint. A type of pipe joint made by using loose flanges on lengths of pipe whose ends are lapped over to give a bearing surface for a gasket or metal-to- metal joint.

  • Liquid Penetrant Examination or Inspection. This is a non-destructive examination method for finding discontinuities that are open to the surface of solid and essentially nonporous materials. This method is based on capillary action or capillary attraction by which the surface of a liquid in contact with a solid is elevated or depressed. A liquid penetrant, usually a red dye, is applied to the clean surface of the specimen. Time is allowed for the penetrant to seep into the opening. The excess penetrant is removed from the surface. A developer, normally white, is applied to aid in drawing the penetrant up or out to the surface. The red penetrant is drawn out of the discontinuity, which is located by the contrast and distinct appearance of the red penetrant against the white background of the developer.

  • Local Preheating. Preheating of a specific portion of a structure.

  • Local Stress-Relief Heat Treatment. Stress-relief heat treatment of a specific portion of a weldment. This is done extensively with induction coils, resistance coils, or propane torches in the field erection of steel piping.

  • Machine Welding. Welding with equipment which performs the welding operation under the observation and control of an operator. The equipment may or may not perform the loading and unloading of the work.

  • Magnetic Particle Examination or Inspection. This is a non-destructive examination method to locate surface and subsurface discontinuities in ferromagnetic materials. The presence of discontinuities is detected by the use of finely divided ferromagnetic particles applied over the surface. Some of these magnetic particles are gathered and held by the magnetic leakage field created by the discontinuity. The particles gathered at the surface form an outline of the discontinuity and generally indicate its location, size, shape, and extent.

  • Malleable Iron. Cast iron which has been heat-treated in an oven to relieve its brittleness. The process somewhat improves the tensile strength and enables the material to stretch to a limited extent without breaking.

  • Manual Welding. Welding wherein the entire welding operation is performed and controlled by hand.

  • Mean Velocity of Flow. Under steady state of flow, the mean velocity of flow at a given cross section of pipe is equal to the rate of flow Q divided by the area of cross section A. It is expressed in feet per second or meters per second.

where v = mean velocity of flow, in feet per second, ft/s (meters per second, m/s)

Q = rate of flow, in cubic feet per second, ft3 /s (cubic meters per second, m3 /s)

A = area of cross section, in square feet, ft2 (square meters, m2)

  • Mechanical Joint. A joint for the purpose of mechanical strength or leak resistance or both, where the mechanical strength is developed by threaded, grooved, rolled, flared, or flanged pipe ends or by bolts, pins, and compounds, gaskets, rolled ends, caulking, or machined and mated surfaces. These joints have particular application where ease of disassembly is desired.

  • Mill Length. Also, known as random length. The usual run-of-mill pipe is 16 to 20 ft (5 to 6 m) in length. Line pipe and pipe for power plant use are sometimes made in double lengths of 30 to 35 ft (10 to 12 m).

  • Miter. Two or more straight sections of pipe matched and joined on a line bisecting the angle of junction so as to produce a change in direction.4

  • Newtonian Liquid. A liquid is called newtonian if its viscosity is unaffected by the kind and magnitude of motion or agitation to which it may be subjected, as long as the temperature remains constant. Water and mineral oil are examples of newtonian liquids.

  • Nipple. A piece of pipe less than 12 in (0.3 m) long that may be threaded on both ends or on one end and provided with ends suitable for welding or a mechanical joint. Pipe over 12 in (0.3 m) long is regarded as cut pipe. Common types of nipples are close nipple, about twice the length of a standard pipe thread and without any shoulder; shoulder nipple, of any length and having a shoulder between the pipe threads; short nipple, a shoulder nipple slightly longer than a close nipple and of a definite length for each pipe size which conforms to manufacturer’ standard; long nipple, a shoulder nipple longer than a short nipple which is cut to a specific length.

  • Nominal Diameter (DN). A dimensionless designator of pipe in metric system. It indicates standard pipe size when followed by the specific size designation number without the millimeter symbol (for example, DN 40, DN 300).

  • Nominal Pipe Size (NPS). A dimensionless designator of pipe. It indicates stan- dard pipe size when followed by the specific size designation number without an inch symbol (for example, NPS 1¹⁄₂, NPS 12).2

  • Nominal Thickness. The thickness given in the product material specification or standard to which manufacturing tolerances are applied.5

  • Nondestructive Examination or Inspection. Inspection by methods that do not destroy the item, part, or component to determine its suitability for use.

  • Normalizing. A process in which a ferrous metal is heated to a suitable tempera- ture above the transformation range and is subsequently cooled in still air at room temperature.5

  • Nozzle. As applied to piping, this term usually refers to a flanged connection on a boiler, tank, or manifold consisting of a pipe flange, a short neck, and a welded attachment to the boiler or other vessel. A short length of pipe, one end of which is welded to the vessel with the other end chamfered for butt welding, is also referred to as a welding nozzle.

  • Overhead Position. The position of welding performed from the underside of the joint.

  • Oxidizing Flame. An oxyfuel gas flame having an oxidizing effect caused by excess oxygen.

  • Oxyacetylene Cutting. An oxygen-cutting process in which metals are severed by the chemical reaction of oxygen with the base metal at elevated temperatures. The necessary temperature is maintained by means of gas flames obtained from the combustion of acetylene with oxygen.

  • Oxyacetylene Welding. A gas welding process in which coalescence is produced by heating with a gas flame or flames obtained from the combustion of acetylene with oxygen, with or without the addition of filler metal.

  • Oxyfuel Gas Welding (OFGW). A group of welding processes in which coales- cence is produced by heating with a flame or flames obtained from the combustion of fuel gas with oxygen, with or without the application of pressure and with or without the use of filler metal.

  • Oxygen Cutting (OC). A group of cutting processes used to sever or remove metals by means of the reaction of oxygen with the base metal at elevated tempera- tures. In the case of oxidation-resistant metals, the reaction is facilitated by use of a chemical flux or metal powder.8

  • Oxygen Gouging. An application of oxygen cutting in which a chamfer or groove is formed.

  • Pass. A single progression of a welding or surfacing operation along a joint, weld deposit, or substrate. The result of a pass is a weld bead, layer, or spray deposit.

  • Peel Test. A destructive method of examination that mechanically separates a lap joint by peeling.

  • Peening. The mechanical working of metals by means of hammer blows.

  • Pickle. The chemical or electrochemical removal of surface oxides. Following welding operations, piping is frequently pickled in order to remove mill scale, oxides formed during storage, and the weld discolorations.

  • Pipe. A tube with a round cross section conforming to the dimensional requirements for nominal pipe size as tabulated in ASME B36.10M and ASME B36.19M. For special pipe having diameter not listed in the above-mentioned standards, the nominal diameter corresponds to the outside diameter.

  • Pipe Alignment Guide. A restraint in the form of a sleeve or frame that permits the pipeline to move freely only along the axis of the pipe.

  • Pipe Supporting Fixtures. Elements that transfer the load from the pipe or structural attachment to the support structure or equipment.

  • Pipeline or Transmission Line. A pipe installed for the purpose of transmitting gases, liquids, slurries, etc., from a source or sources of supply to one or more distribution centers or to one or more large-volume customers; a pipe installed to interconnect source or sources of supply to one or more distribution centers or to one or more large-volume customers; or a pipe installed to interconnect sources of supply.

  • Piping System. Interconnected piping subject to the same set or sets of design conditions.

  • Plasma Cutting. A group of cutting processes in which the severing or removal of metals is effected by melting with a stream of hot ionized gas.

  • Plastic. A material which contains as an essential ingredient an organic substance of high to ultrahigh molecular weight, is solid in its finished state, and at some stage of its manufacture or processing can be shaped by flow. The two general types of plastic are thermoplastic and thermosetting.

  • Polarity. The direction of flow of current with respect to the welding electrode and workpiece.

  • Porosity. Presence of gas pockets or voids in metal.

  • Positioning Weld. A weld made in a joint which has been so placed as to facilitate the making of the weld.

  • Postheating. The application of heat to a fabricated or welded section subsequent to a fabrication, welding, or cutting operation. Postheating may be done locally, as by induction heating; or the entire assembly may be postheated in a furnace.

  • Postweld Heat Treatment. Any heat treatment subsequent to welding.

  • Preheating. The application of heat to a base metal immediately prior to a welding or cutting operation.5

  • Pressure. The force per unit that is acting on a real or imaginary surface within a fluid is the pressure or intensity of pressure. It is expressed in pounds per square inch:

where p = absolute pressure at a point, psi (kg/cm2)

w = specific weight, lb/ft3 (kg/m3)

h = height of fluid column above the point, ft (m)

pa = atmospheric pressure, psi (kg/cm2)

The gauge pressure at a point is obtained by designating atmospheric pressure as zero:

where p = gauge pressure. To obtain absolute pressure from gauge pressure, add the atmospheric pressure to the gauge pressure.

  • Pressure Head. From the definition of pressure, the expression p/w is the pressure head. It can be defined as the height of the fluid above a point, and it is normally measured in feet.

  • Purging. The displacement during welding, by an inert or neutral gas, of the air inside the piping underneath the weld area in order to avoid oxidation or contamination of the underside of the weld. Gases most commonly used are argon, helium, and nitrogen (the last is principally limited to austenitic stainless steel). Purging can be done within a complete pipe section or by means of purging fixtures of a small area underneath the pipe weld.

  • Quenching. Rapid cooling of a heated metal.

  • Radiographic Examination or Inspection. Radiography is a non-destructive test method which makes use of short-wavelength radiations, such as X-rays or gamma rays, to penetrate objects for detecting the presence and nature of macroscopic defects or other structural discontinuities. The shadow image of defects or discontinuities is recorded either on a fluorescent screen or on photographic film.

  • Reinforcement. In branch connections, reinforcement is material around a branch opening that serves to strengthen it. The material is either integral in the branch components or added in the form of weld metal, a pad, a saddle, or a sleeve. In welding, reinforcement is weld metal in excess of the specified weld size.

  • Reinforcement Weld. Weld metal on the face of a groove weld in excess of the metal necessary for the specified weld size.

  • Repair. The process of physically restoring a non-conformance to a condition such that an item complies with the applicable requirements, including the code requirements.

  • Resistance Weld. Method of manufacturing pipe by bending a plate into circular form and passing electric current through the material to obtain a welding temperature.

  • Restraint. A structural attachment, device, or mechanism that limits movement of the pipe in one or more directions.

  • Reverse Polarity. The arrangement of direct-current arc welding leads with the work as the negative pole and the electrode as the positive pole of the welding arc; a synonym for direct-current electrode positive.

  • Reynolds Number. A dimensionless number. It is defined as the ratio of the dynamic forces of mass flow to the shear stress due to viscosity. It is expressed as

where R = Reynolds number

v = mean velocity of flow, ft/s (m/s)

p = weight density of fluid, lb/ft3 (kg/m3)

D = internal diameter of pipe, ft (m)

µ = absolute viscosity, in pound mass per foot second [lbm/ (ft · s)] or poundal seconds per square foot (centipoise)

  • Rolled Pipe. Pipe produced from a forged billet which is pierced by a conical mandrel between two diametrically opposed rolls. The pierced shell is subsequently rolled and expanded over mandrels of increasingly large diameter. Where closer dimensional tolerances are desired, the rolled pipe is cold- or hot-drawn through dies and then machined. One variation of this process produces the hollow shell by extrusion of the forged billet over a mandrel in a vertical, hydraulic piercing press.

  • Root Edge. A root face of zero width.

  • Root Face. That portion of the groove face adjacent to the root of the joint. This portion is also referred to as the root land.

  • Root of Joint. That portion of a joint to be welded where the members to be joined come closest to each other. In cross section, the root of a joint may be a point, a line, or an area.

  • Root Opening. The separation, between the members to be joined, at the root of the joint.

  • Root Penetration. The depth which a groove weld extends into the root of a joint as measured on the centerline of the root cross section. Sometimes welds are considered unacceptable if they show incomplete penetration.

  • Root Reinforcement. Weld reinforcement at the side other than that from which the welding was done.

  • Root Surface. The exposed surface of a weld on the side other than that from which the welding was done.

  • Run. The portion of a fitting having its end in line, or nearly so, as distinguished from branch connections, side outlets, etc.

  • Saddle Flange. Also, known as tank flange or boiler flange. A curved flange shaped to fit a boiler, tank, or other vessel and to receive a threaded pipe. A saddle flange is usually riveted or welded to the vessel.

  • Sample Piping. All piping, valves, and fittings used for the collection of samples of gas, steam, water, oil, etc.

  • Sargol. A special type of joint in which a lip is provided for welding to make the joint fluid tight, while mechanical strength is provided by bolted flanges. The Sargol joint is used with both Van Stone pipe and fittings.

  • Sarlun. An improved type of Sargol joint.

  • Schedule Numbers. Approximate values of the expression 1000P/S, where P is the service pressure and S is the allowable stress, both expressed in pounds per square inch.

  • Seal Weld. A fillet weld used on a pipe joint primarily to obtain fluid tightness as opposed to mechanical strength; usually used in conjunction with a threaded joint.8

  • Seamless Pipe. A wrought tubular product made without a welded seam. It is manufactured by hot-working steel or, if necessary, by subsequently cold-finishing the hot-worked tubular product to produce the desired shape, dimensions, and properties.

  • Semiautomatic Arc Welding. Arc welding with equipment which controls only the filler metal feed. The advance of the welding is manually controlled.3

  • Semisteel. A high grade of cast iron made by the addition of steel scrap to pip iron in a cupola or electric furnace. More correctly described as high-strength gray iron.

  • Service Fitting. A street ell or street tee having a male thread at one end.

  • Shielded Metal Arc Welding (SMAW). An arc welding process in which coalescence is produced by heating with an electric arc between a covered metal electrode and the work. Shielding is obtained from decomposition of the electrode covering. Pressure is not used, and filler metal is obtained from the electrode.

  • Shot Blasting. Mechanical removal of surface oxides and scale on the pipe inner and outer surfaces by the abrasive impingement of small steel pellets.

  • Single-Bevel, Single-J, Single-U, Single-V-Groove Welds. All are specific types of groove welds.

  • Size of Weld. For a groove weld, the joint penetration, which is the depth of chamfering plus the root penetration. For fillet welds, the leg length of the largest isosceles right triangle which can be inscribed within the fillet-weld cross section.

  • Skelp. A piece of plate prepared by forming and bending, ready for welding into pipe. Flat plates when used for butt-welded pipe are called skelp.

  • Slag Inclusion. Non-metallic solid material entrapped in weld metal or between weld metal.

  • Slurry. A two-phase mixture of solid particles in an aqueous phase.

  • Socket Weld. Fillet-type seal weld used to join pipe to valves and fittings or to other sections of pipe. Generally used for piping whose nominal diameter is NPS 2 (DN 50) or smaller.

  • Soldering. A metal-joining process in which coalescence is produced by heating to a suitable temperature and by using a nonferrous alloy fusible at temperatures below that of the base metals being joined. The filler metal is distributed between closely fitted surfaces of the joint by capillary action.

  • Solution Heat Treatment. Heating an alloy to a suitable temperature, holding at that temperature long enough to allow one or more constituents to enter into solid solution, and then cooling rapidly enough to hold the constituents in solution.

  • Solvent Cement Joint. A joint made in thermoplastic piping by the use of a solvent or solvent cement which forms a continuous bond between the mating surfaces.

  • Source Nipple. A short length of heavy-walled pipe between high-pressure mains and the first valve of bypass, drain, or instrument connections.

  • Spatter. In arc and gas welding, the metal particles expelled during welding that do not form part of the weld.8

  • Spatter Loss. Difference in weight between the amount of electrode consumed and the amount of electrode deposited.

  • Specific Gravity. The ratio of its weight to the weight of an equal volume of water at standard conditions.

  • Specific Volume. The volume of a unit mass of a fluid is its specific volume, and it is measured in cubic feet per pound mass (ft3 /lbm).

  • Specific Weight. The weight of a unit volume of a fluid is its specific weight. In English units, it is expressed in pounds per cubic foot (lb/ft3).

  • Spiral-Riveted. A method of manufacturing pipe by coiling a plate into a helix and riveting together the overlapped edges.

  • Spiral-Welded. A method of manufacturing pipe by coiling a plate into a helix and fusion-welding the overlapped or abutted edges.

  • Spiral-Welded Pipe. Pipe made by the electric-fusion-welded process with a butt joint, a lap joint, or a lock-seam joint.

  • Square-Groove Weld. A groove weld in which the pipe ends are not chamfered. Square-groove welds are generally used on piping and tubing of wall thickness no greater than ¹⁄₈ in (3 mm).

  • Stainless Steel. An alloy steel having unusual corrosion-resisting properties, usu- ally imparted by nickel and chromium.

  • Standard Dimension Ratio (SDR). The ratio of outside pipe diameter to wall thickness of thermoplastic pipe. It is calculated by dividing the specified outside diameter of the pipe by the specified wall thickness in inches.

  • Statically Cast Pipe. Pipe formed by the solidification of molten metal in a sand mold.

  • Straight Polarity. The arrangement of direct-current arc welding leads in which the work is the positive pole and the electrode is the negative pole of the welding arc; a synonym for direct-current electrode negative.

  • Stress Relieving. Uniform heating of a structure or portion thereof to a sufficient temperature to relieve the major portion of the residual stresses, followed by uni- form cooling.5

  • Stringer Bead. A type of weld bead made by moving the electrode in a direction essentially parallel to the axis of the bead. There is no appreciable transverse oscillation of the electrode. The deposition of a number of string beads is known as string beading and is used extensively in the welding of austenitic stainless-steel materials. See also Weave Bead.

  • Structural Attachments. Brackets, clips, lugs, or other elements welded, bolted, or clamped to the pipe support structures, such as stanchions, towers, building frames, and foundation. Equipment such as vessels, exchanges, and pumps is not considered to be pipe-supporting elements.

  • Submerged Arc Welding (SAW). An arc welding process that produces coalescence of metals by heating them with an arc or arcs drawn between a bare metal electrode or electrodes and the base metals. The arc is shielded by a blanket of granular fusible material. Pressure is not used, and filler metal is obtained from the electrode and sometimes from a supplementary welding rod, flux, or metal granules.

  • Supplemental Steel. Structural members that frame between existing building framing steel members and are significantly smaller than the existing steel.8

  • Swaging. Reducing the ends of pipe and tube sections with rotating dies which are pressed intermittently against the pipe or tube end.

  • Swivel Joint. A joint which permits single-plane rotational movement in a piping system.

  • Tack Weld. A small weld made to hold parts of a weldment in proper alignment until the final welds are made.

  • Tee Joint. A welded joint between two members located approximately at right angles to each other in the form of a T.

  • Tempering. A process of heating a normalized or quench-hardened steel to a temperature below the transformation range and, from there, cooling at any rate desired. This operation is also frequently called stress relieving.

  • Testing. An element of verification for the determination of the capability of an item to meet specified requirements by subjecting the item to a set of physical, chemical, environmental, or operating conditions.

  • Thermoplastic. A plastic which is capable of being repeatedly softened by increase of temperature and hardened by decrease of temperature.

  • Thermosetting Plastic. Plastic which is capable of being changed into a substantially infusible or insoluble product when cured under application of heat or chemical means.

  • Thixotropic Liquid. If the viscosity of a liquid decreases as agitation is increased at constant temperature, the liquid is called thixotropic. Examples include glues, greases, paints, etc.

  • Throat of a Weld. A term applied to fillet welds. It is the perpendicular distance from the beginning of the root of a joint to the hypotenuse of the largest right triangle that can be inscribed within the fillet-weld cross section.

  • Toe of Weld. The junction between the face of a weld and the base metal.

  • Transformation Range. A temperature range in which a phase change is initiated and completed.

  • Transformation Temperature. A temperature at which a phase change occurs.

  • Trepanning. The removal by destructive means of a small section of piping (usually containing a weld) for an evaluation of weld and base-metal soundness. The operation is frequently performed with a hole saw.

  • Tube. A hollow product of round or any other cross section having a continuous periphery. Round tube size may be specified with respect to any two, but not all three, of the following: outside diameter, inside diameter, and wall thickness. Dimensions and permissible variations (tolerances) are specified in the appropriate ASTM or ASME specifications.

  • Turbinizing. Mechanical removal of scale from the inside of the pipe by means of air-driven centrifugal rotating cleaners. The operation is performed on steel pipe bends after hot bending to remove loose scale and sand.

  • Turbulent Flow. Fluid flow in a pipe is usually considered turbulent if the Reynolds number is greater than 4000. Fluid flow with a Reynolds number between 2000 and 4000 is considered to be in ‘‘transition.’’

  • Ultrasonic Examination or Inspection. A non-destructive method in which beams of high-frequency sound waves that are introduced into the material being inspected are used to detect surface and subsurface flaws. The sound waves travel through the material with some attendant loss of energy and are reflected at interfaces. The reflected beam is detected and analyzed to define the presence and location of flaws.

  • Underbead Crack. A crack in the heat-affected zone or in previously deposited weld metal paralleling the underside contour of the deposited weld bead and usually not extending to the surface.

  • Undercut. A groove melted into the base material adjacent to the toe or root of a weld and left unfilled by weld material.

  • Van Stoning. Hot upsetting of lapping pipe ends to form integral lap flanges, the lap generally being of the same diameter as that of the raised face of standard flanges.

  • Vapor Pressure. The pressure exerted by the gaseous form, or vapor, of liquid. When the pressure above a liquid equals its vapor pressure, boiling occurs. If the pressure at any point in the flow of a liquid falls below the vapor pressure or becomes equal to the vapor pressure, the liquid flashes into vapor. This is called cavitation. The vapor thus formed travels with the liquid and collapses where the pressure is greater than vapor pressure. This could cause damage to pipe and other components.

  • Vertical Position. With respect to pipe welding, the position in which the axis of the pipe is vertical, with the welding being performed in the horizontal position. The pipe may or may not be rotated.

  • Viscosity. In flowing liquids, the internal friction or the internal resistance to relative motion of the fluid particles with respect to one another.

  • Weave Bead. A type of weld bead made with oscillation of the electrode transverse to the axis of the weld. Contrast to string bead.

  • Weld. A localized coalescence of material produced either by heating to suitable temperatures, with or without the application of pressure, or by application of pressure alone, with or without the use of filler material.

  • Weld Bead. A weld deposit resulting from a pass.

  • Weld Metal. That portion of a weld which has been melted during welding. The portion may be the filler metal or base metal or both.

  • Weld Metal Area. The area of the weld metal as measured on the cross section of a weld.

  • Weld-Prober Sawing. Removal of a boat-shaped sample from a pipe weld for examination of the weld and its adjacent base-metal area. This operation is usually performed in graphitization studies.

  • Weld Reinforcement. Weld material in excess of the specified weld size.

  • Weldability. The ability of a metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure and to perform satisfactorily in the intended service.

  • Welded Joint. A localized union of two or more members produced by the application of a welding process.

  • Welder. One who is capable of performing a manual or semiautomatic welding operation.

  • Welder Performance Qualification. Demonstration of a welder’s ability to produce welds in a manner described in a welding procedure specification that meets prescribed standards.

  • Welding Current. The current which flows through the electric welding circuit during the making of a weld.

  • Welding Fittings. Wrought- or forged-steel elbows, tees, reducers, and similar pieces for connection by welding to one another or to pipe. In small sizes, these fittings are available with counter bored ends for connection to pipe by fillet welding and are known as socket-weld fittings. In large sizes, the fittings are supplied with ends chamfered for connection to pipe by means of butt welding and are known as butt-welding fittings.

  • Welding Generator. The electric generator used for supplying welding current.

  • Welding Machine. Equipment used to perform the welding operation.

  • Welding Operator. One who operates a welding machine or automatic welding equipment.8

  • Welding Procedure. The detailed methods and practices involved in the production of a weldment.

  • Welding Procedure Qualification Record. Record of welding data and test results of the welding procedure qualifications, including essential variables of the process and the test results.

  • Welding Procedure Specification (WPS). The document which lists the parameters to be used in construction of weldments in accordance with the applicable code requirements.

  • Welding Rod. Filler metal, in wire or rod form, used in gas welding and brazing procedures and those arc welding processes where the electrode does not furnish the filler metal.

  • Welding Sequence. The order of making the welds in a weldment.

  • Weldment. An assembly whose component parts are to be joined by welding.

  • Wrought Iron. Iron refined in a plastic state in a puddling furnace. It is characterized by the presence of about 3 percent of slag irregularly mixed with pure iron and about 0.5 percent carbon and other elements in solution.

  • Wrought Pipe. The term wrought pipe refers to both wrought steel and wrought iron. Wrought in this sense means ‘‘worked,’’ as in the process of forming furnace- welded pipe from skelp or seamless pipe from plates or billets. The expression wrought pipe is thus used as a distinction from cast pipe. Wrought pipe in this sense should not be confused with wrought-iron pipe, which is only one variety of wrought pipe. When wrought-iron pipe is referred to, it should be designated by its complete name.


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