Pipe and Tubes
Pipes and Tubes
Pipes and tubes term is used to cover all hollow products. Although these products are normally produced in cylindrical form, they are often subsequently altered by various processing methods to produce square, oval, rectangular, and other symmetrical shapes. Such products have applications which are almost innumerable, but they are most commonly used as conveyors of fluids and as structural members. One of the major applications of the pipe is in the process and the metallurgical industries. The rolling mills for pipes are normally multi stand continuous mills.
Pipe is normally produced to several long-standing and broadly applicable industrial standards. While similar standards exist for specific industry application tubing, tube is frequently made to custom sizes and a broader range of diameters and tolerances. Many industrial and government standards exist for the production of pipes and tubes.
Tubes are frequently made to custom sizes and can often have a large number of specific sizes and tolerances than pipes. The term ‘tube’ is also normally applied to non-cylindrical sections (square or rectangular tube). Both pipes and tubes are associated with a level of rigidity and permanence while a hose is usually portable and flexible.
Difference between pipe and tube
In general practice the designations pipe and tube are almost interchangeable, but in the pipe fitting industry and engineering discipline the terms are uniquely defined. The main difference between a pipe and a tube is the way the diameter of the pipe or tube is designated. Pipe is normally designated by a ‘Nominal Pipe Size’ (NPS) based upon the ID (inside diameter) of the most common wall thickness while the tube is designated by the measured OD (outside diameter).
Pipe – Depending on the applicable standard to which it is manufactured, pipe is specified by the ID and a wall thickness, the ID called the nominal diameter may not exactly match the pipe size as it varies with the wall thickness. For example the ID of a 200 mm pipe varies from 213.54 mm ID for Schedule 5 pipe to 193.68 mm for Schedule 40 pipe.
Tube – Tube is most often defined by the outside diameter (OD) and a wall thickness. Hence a 200 mm tube has an outside diameter of 200 mm.
Markings on pipe and tube
Since there are many different types of pipe and tube and different standards which are applicable for each, it is not possible to give a definitive list of the exact information required to be marked on pipe and tube. However the common requirements which are to be continuously marked down its whole length are (i) NPS (nominal bore), (ii) Schedule (wall thickness), (iii) specification, (iv) grade, (iv) method of production (seamless or welded), (v) heat number, and (vi) the name or logo of the manufacturer.
Classification of pipe and tube
The two simplest and broadest commercial classifications of tubular products are pipe and tube. Although the application of the terms pipe and tube is not always consistent, the term pipe is normally used to describe cylindrical tubular products made to standard combinations of OD and wall thickness. These two broad classifications are subdivided into several named use groups. For example, the term tube covers three such groups namely (i) pressure tube, (ii) structural tube, and (iii) mechanical tube. The term pipe covers five such groups (i) standard pipe, (ii) line pipe, (iii) oil country tubular goods (OCTG), (iv) water well pipe, and (v) pressure pipe. There is also pipe for special applications, such as conduit pipe and tubular piling, which do not fit any of these classifications. Each of these use groups, in turn, has a number of uses or named-use subdivisions.
Pipes and tubes are frequently classified by the material of the pipes. As per this classification pipes and tubes can be classified (i) iron pipe such as cast iron pipe, and ductile iron pipe, (ii) steel pipe such as carbon steel pipe, special steel pipe, alloy steel pipe, and stainless steel pipe, (iii) type of coating such as galvanized pipe, aluminized pipe, and chromium coated pipe, (iv) copper and copper alloy (for example brass) pipe and tube, (v) aluminum and aluminum alloy pipe and tube, (vi) plastic pipe and such as PVC (polyvinyl chloride), CPVC (chlorinated polyvinyl chloride), FRP (fiber reinforced plastic), RPMP (reinforced polymer mortar pipe), PP (poly-propylene) (PP), PE (poly-ethylene), PEX (cross-linked high-density polyethylene), PB (poly-butylene), ABS (acrylo-nitrile butadiene styrene), HDPE (high density poly-ethylene), and GRP (glass reinforced plastic) pipe, (viii) cement, reinforced concrete and asbestos cement pipe, and (viii) pipes and tubes of other materials and they are normally designated by the material used for manufacture such as rubber, ceramic, lead, fiber glass, titanium and inconel etc.
Pipes and tubes are also designated by the method of manufacture such as (i) ERW (electric resistance welded) pipe, HFI (high frequency induction) welded and electric fusion welded (EFW) pipe, (ii) SAW (submerged arc welded) pipe such as LSAW (longitudinal submerged arc welded) and DSAW (double submerged arc welded) pipe, (iii) spiral welded pipe and SSAW (spiral submerged arc welded) pipe, (iv) pipe produced by UOE process, (v) seamless pipe produced by the various processes such as ‘pierce and pilger’ rolling process, ‘plug’ rolling process, ‘continuous mandrel’ rolling process, ‘push bench’ process, ‘pierce and draw’ process, ‘tube extrusion’ process, ‘cross rolling’ process, ‘Assel’ rolling process, and ‘Diecher, process, (vi) cold drawn pipe, (vii) cast pipe and centrifugally cast spun pipe, and (viii) fabricated pipe.
Pipes are often designated by the media they are conveying such as water pipe, steam pipe, air pipe, oil pipe, gas pipe, slurry pipe, sewer pipe, and exhaust pipe.
Pipes are also sometimes designated by the property of the media being carried such as gravity pipe, low pressure pipe, high pressure pipe and heat pipe etc.
Pipes are sometimes designated by the type of finish such as plain pipe or corrugated pipe.
Pipes are also differentiated by the method of their joining which can be welding, threading, mechanical coupling, flanged joining, soldering, brazing, compression fitting, push on gasket fitting, flaring and crimping.
Pipes are also identified by the type of lining used for corrosion and heat protection such as refractory lined, insulation lined, cement mortar lined, epoxy coated, rubber lined, bitumen mastic lined, poly-urethane lined, and PE lined pipe etc.
Production of pipe and tube
There are two main processes for metallic pipe manufacture. Seamless pipe is formed by drawing a solid billet over a piercing rod to create the hollow shell. The three main seamless processes are rotary piercing process, hot extrusion process, and cupping and drawing process.
Seamless pipe is normally more expensive to manufacture but provides higher pressure ratings. Welded pipe is formed by rolling plate and welding the seam. The weld seam is formed by ERW or EFW and is normally ground flush with the parent material as part of the production process. The weld zone can also be heat treated, so the seam is less visible. Welded pipe frequently has tighter dimensional tolerances than seamless, and can be cheaper if produced in large amounts. Large diameter pipe (around 200 mm or higher) can be ERW, or SAW pipe. Metal tubing due to the thinner wall thickness can be extruded but not always and many sanitary tubes such as hygienic stainless steel have a welded seam. Plastics are normally extruded due to the ease of handling the base materials.
Pipes in suitable sizes and most products classified as tubes, both seamless and welded, can be cold finished. The process can be used to increase or decrease the diameter, to produce shapes other than round, to produce a smoother surface or closer dimensional tolerances, or to modify mechanical properties. The process most commonly used is cold drawing, in which the descaled hot-worked tube is plastically deformed by drawing it through a die and over a mandrel (mandrel drawing) to work both exterior and interior surfaces. Cold drawing through the die only (without a mandrel) is called sink drawing or sinking. Fig 1 shows the different methods for the pipe manufacture.
Fig 1 Methods of manufacturing pipes and tubes
Finishing processes for pipe and tube
There are limitations to the hot manufacturing processes for pipe and tube such as (i) small diameters are impracticable, (ii) thin walls are difficult to get, (iii) tolerances are difficult to control, (iv) mechanical properties cannot be controlled adequately, (v) the surface finish on both the OD and the ID are rough, and (iv) sophisticated shapes are not possible. For these reasons pipe and tube are cold worked after extrusion or seam welding.
Non destructive tests for pipe and tube
Non-destructive tests (NDT) do not damage the pipe or tube being tested and so they are frequently incorporated into the end of the production line. The following describes the common types of NDT conducted on the pipes and tubes.
Ultrasonic testing – This test involves ultrasonic sound waves being aimed, through a coupling medium, at the material to be tested. A proportion is bounced back at the interface but the remainder enters the material and bounce from the internal surface, to the external surface, where a transducer converts them into electrical energy. This is then monitored on a cathode ray tube where results are compared with those from a calibration standard. Any deviations from the standard are visible, thus indicating cracks or internal defects.
Eddy-current testing – This involves inducing eddy currents into the material by exciting a coil which surmounts two narrow search coils surrounding the material. Any discontinuities in material are found by comparing the electrical conditions which exist in the two search coils. The fault signals are amplified and can be shown on a cathode ray tube or as an audible signal.
Hydrostatic testing – This is used to test the manufactured items under a pressure equivalent to or higher than the pressure to be encountered in service. It involves filling the pipe with water, which cannot be compressed, and increasing the pressure inside the pipe to that specified.
Magnetic particle testing – This method of testing is used when trying to detect discontinuities in material of ferromagnetic structure. The method is based on the principle that an imperfection causes a distortion in the magnetic field pattern of a magnetized component. The imperfection can be revealed by applying magnetic particles to the component during or after magnetization.
Radiographic (X-ray) testing – This is normally used to determine whether a weld is sound. It involves subjecting a weld or weld area to an X-ray source with an X-ray sensitive film plate on the under-side of the weld. The results are shown on the developed film (a photo-micrograph) and interpreted according to specification.
Dye-penetrant test – This is used to detect cracks and involves spraying a dye on the area to be tested. After allowing time for penetration, the surplus dye is removed and the area is then sprayed with a white developer. Any faults are revealed as coloured lines or spots caused by the developer absorbing the dye seeping from the cracks. If more sensitive results are needed, a fluorescent dye is used and the same process is followed. When viewed under ultra-violet light any defects show as a highly fluorescent line or spot.
Materials used for pipe and tube
Pipe can be made from a variety of materials. In the past, materials have included wood and lead (Latin for lead is plumbum; from this, the word plumbing has come). Nowadays several materials are used for the production of pipes. These materials are ceramics, fiberglass, concrete, plastics, and metals.
Concrete and ceramic pipes – Pipes can be made from concrete or ceramic materials. These pipes are normally used for low pressure applications such as gravity flow or drainage underground. Concrete pipes normally have a receiving bell or a stepped fitting, with various sealing methods applied at installation. Ceramic pipes are used for underground drainage which can be exposed to corrosive chemicals. These types of pipes are relatively inexpensive for the diameters in question and allow for ease of installation in rough site conditions.
Plastic pipes – Plastic pipe is widely used for its light weight, chemical resistance, non-corrosive properties, and ease of making connections. Plastic materials include PVC, CPVC, FRP, RPMP, PP, PE, PEX, PB, and ABS etc.
Metal pipes and tubes – Metallic pipes are commonly made from iron or steel with the metal chemistry and its finish being peculiar to the use fit and form. Typically metallic piping can be made of steel or iron, such as unfinished, black (lacquer) steel, carbon steel, stainless steel, or galvanized steel, brass, and ductile iron. Aluminum pipe or tube can be used where iron is incompatible with the service fluid or where weight is a concern. Aluminum is also used for heat transfer tubes such as in refrigerant systems. Copper tube is popular for domestic water (potable) plumbing systems. Copper can also be used where heat transfer is desirable (i.e. radiators or heat exchangers). Inconel, chrome molybdenum, and titanium steel alloys are used for high temperature and pressure piping in process systems where corrosion resistance is important.
Stainless steel pipes and tubes – Stainless steel pipes and tubes are used for a variety of reasons namely (i) to resist corrosion and oxidation, (ii) to resist high temperatures, (iii) for cleanliness and low maintenance costs, and (iv) to maintain the purity of materials which come in contact with pipe material. There are more than 60 grades of stainless steel available. The ability of stainless steel to resist corrosion is achieved by the addition of a minimum of 12 % chromium to the iron alloy. Additions of other elements affect other properties. The inherent characteristics of stainless steel permit the design of thin wall piping systems without fear of early failure due to corrosion. Because of the thinner wall thickness of stainless steel tube, it is not possible to thread tube and hence this is overcome by fusion welding to join such pipe and tube.
Type 304 stainless is the most widely used analysis for general corrosive resistant pipe and tube applications. It is used in chemical plants, refineries, paper mills, and food processing industries. Type 304 has a maximum carbon content of 0.08 %. It is not recommended for use in the temperature range between 400 deg C and 900 deg C due to the carbide precipitation at the grain boundaries which can result in inter-granular corrosion and early failure under certain conditions. Type 304L is the same as 304 except that a 0.03 % maximum carbon content is maintained which precludes carbon precipitation and permits the use of this analysis in welded assemblies under more severe corrosive conditions. Type 318 is much more resistant to pitting than other chromium nickel alloys due to the addition of 2 % to 3 % of molybdenum. It is particularly valuable wherever acids, brines, sulphur water, seawater or halogen salts are encountered. Type 316 is widely used in the sulphite paper industry and for manufacturing chemical plant equipment, photographic equipment, and plastics. Type 316L, like 304L, is held to a maximum carbon content of 0.03 %. This permits its use in welded assemblies without the need of final heat treatment. It is used extensively for pipe assemblies with welded fitting.
Classification of pipes sizes
Pipe sizes can be confusing since the terminology can relate to historical dimensions. For example, a half-inch iron pipe does not have any dimension which is a half inch. Initially, a half inch pipe did have an internal dimension of 0.5 inch but it also had thick walls. As technology improved, the wall thickness got thinner (saving material costs), but the outside diameter stayed the same so it can mate with existing older pipe. The history of copper pipe is similar. In the 1930s, the pipe was designated by its internal diameter and a 1/16 inch wall thickness. Hence, a 1 inch copper pipe has a 1-1/8 inch outside diameter. The outside diameter was the important dimension for mating with fittings. The wall thickness on modern copper is usually thinner than 1/16 inch, so the internal diameter is only “nominal” rather than a controlling dimension. Newer pipe technologies sometimes adopt a sizing system which is captive to the technology. PVC pipe uses the NPS.
Nominal pipe size and nominal diameter – NPS is a North American set of standard sizes for pipes used for high or low pressures and temperatures. Pipe size is specified with two non-dimensional numbers namely (i) a nominal pipe size based on inches, and (ii) a schedule (Sch.) which specifies the wall thickness. The European designation equivalent to NPS is DN (Diamètre Nominal / nominal diameter), in which sizes are measured in millimetres. The term NB (nominal bore) is also frequently used inter-changeably with NPS. Designating the OD allows pipes of the same size to be fit together no matter what the wall thickness. Pipe sizes are documented by many different international standards.
International standards for pipe sizes – For pipe sizes less than DN 350 (NPS 14 inch), both methods give a nominal value for the OD which is rounded off and is not the same as the actual OD. For example, NPS 2 inch and DN 50 is the same pipe, but the actual OD is 2.375 inch, or 60.325 mm. The only way to obtain the actual OD is to look it up in a reference table.
For pipe sizes of DN 350 (NPS 14 inch) and higher, the NPS size is the actual diameter in inches and the DN size is equal to NPS times 25 rounded to a convenient multiple of 50. For example, NPS 14 inch has an OD of 14 inch, or 355.6 mm, and is equivalent to DN 350. Since the OD is fixed for a given pipe size, the ID varies depending on the wall thickness of the pipe. For example, 2 inch Schedule 80 pipe has thicker walls and hence a smaller inside diameter than 2 inch Schedule 40 pipe.
Pipe sizes for other materials – While steel pipe has been produced for around 150 years, newer pipe materials such as PVC and galvanized pipe adopted the older steel pipe dimension conventions. Many different standards exist for pipe sizes, and their prevalence varies depending on industry and geographical area. The pipe size designation generally includes two numbers; one that indicates the outside or the nominal diameter, and the other that indicates the wall thickness. In the early twentieth century, American pipe was sized by ID. This practice was abandoned to improve compatibility with pipe fittings which normally fits the OD of the pipe, but it has had a lasting impact on modern standards around the world.
Sizes for copper tube – Copper tube was introduced in around 1900, but did not become popular until around 1950, depending on local building code adoption. Copper plumbing tube for residential plumbing follows an entirely different size system, often called ‘Copper Tube Size’ (CTS). Its nominal size is neither the inside nor outside diameter. Plastic tubing, such as PVC and CPVC, for plumbing applications also has different sizing standards. Normal wall-thicknesses of copper tubing are ‘Type K’, ‘Type L’, and ‘Type M’. Type K has the thickest wall section of the three types of pressure rated tubing and is normally used for deep underground burial such as under sidewalks and streets, with a suitable corrosion protection coating or continuous poly-ethylene sleeve as required by code. Type L has a thinner pipe wall section, and is used in residential and commercial water supply and pressure applications. Type M has the thinnest wall section, and is generally suitable for condensate and other drains, but sometimes prohibited for pressure applications, depending on local codes.
Types K and L are normally available in both hard drawn ‘sticks’ and in rolls of soft annealed tubing, whereas type M is normally available only in hard drawn ‘sticks’. Thin-walled types used to be relatively inexpensive, but since 2002 copper prices have risen considerably due to rising global demand and a stagnant supply.
In the plumbing field the size of copper tubing is measured by its nominal diameter (average ID). In some fields, heating and cooling technicians for example, use the OD to designate copper tube sizes. The HVAC field also use this different measurement to try and not confuse water pipe with copper pipe used for the HVAC field, as pipe used in the air conditioning (AC) field uses copper pipe which is made at the factory without processing oils which are incompatible with the oils used to lubricate the compressors in the AC system. The OD of copper tube is always 1/8 inch larger than its nominal size. Hence, 1 inch nominal copper tube and 1-1/8 inch ACR (air conditioning and refrigeration) tube is exactly the same tube with different size designations. The wall thickness of the tube never affects the sizing of the tube. Type K half inch nominal tube, is the same size as Type L half inch nominal tube (5/8 inch ACR).
Sizes for stainless steel tube – Stainless steel pipes, which came into more common use in the mid 20th century, permitted the use of thinner pipe walls with much less risk of failure due to corrosion. This led to the development of stainless steel tube. Stainless steel tube, because of its thinner wall, could not be threaded together according to the ASME code, and hence was fusion welded. This has led to the development of a range of hygienic stainless steel tube and fittings which can be used in applications requiring a clean and sanitary flow of liquids and where it is essential to avoid contamination of the products being carried. These applications cover the food processing, beverage, biotech and pharmaceutical industries including breweries and dairies.
In case of stainless steel tubes, the applications are low pressure with a maximum of 10 bars, the products are available in grades 304L and 316L, and the size range is from half inch to 6 inch OD. The tube and fittings are of welded construction with the internal bead rolled and polished to eliminate crevices, thus preventing interruptions to the flow and eliminating the risk of contamination or bug traps as well as facilitate easy cleaning. Hygienic tube and fittings are produced to various standards.
Pressure ratings for pipes and tubes
The production and installation of pressure piping is tightly regulated by the standards which are related to the boilers and the pressure vessels codes. Pressure piping is normally classified as pipe which carries pressures higher than 10 bars to 25 bars, although definitions vary. To ensure safe operation of the system, the production, storage, welding, testing, etc. of pressure piping is to stringent quality standards. In Europe, pressure piping uses the same pipe IDs and wall thicknesses as NPS, but labels them with a metric Diameter Nominal (DN) instead of the imperial NPS. For NPS larger than 14 inch, the DN is equal to the NPS multiplied by 25 (not 25.4). This is documented by EN 10255 and ISO 65, and it is often called DIN or ISO pipe. In North America and the UK, pressure piping is normally specified by NPS and Schedule. Pipe sizes are documented by a number of standards. Typically the pipe wall thickness is the controlled variable, and the ID is allowed to vary. The pipe wall thickness has a variance of around 12.5 %.
Wall thickness calculations for straight pipe under internal pressure – There are four equations which can be used to calculate the ‘pressure design wall thickness’ (t) of a straight pipe subject to internal pressure, two of which are (i) t = PD/2t(SE + PY), and (ii) t = PD/2SE. In these equations t is the pressure design thickness, d is the ID of the pipe (for pressure design calculation, the ID diameter of the pipe is the maximum value allowable under the specification), P is the internal design pressure, D is the OD of the pipe as listed in tables of standards or specifications or as measured, Y is the coefficient from ‘Values of coefficient Y for t less than D/6’ given in Tab 1, S is the stress value for material, and E is the quality factor. The equations assume t less than D/6. For pipe with t less than D/6 or P/SE higher than 0.385 additional factors need to be considered.
| Tab 1 Values of co-efficient Y for t less than D/6 | ||||||
| Sl. No. | Material | Temperature in degree centigrade | ||||
| 510 | 538 | 566 | 593 | Higher than 621 | ||
| 1 | Ferritic steels | 0.4 | 0.5 | 0.7 | 0.7 | 0.7 |
| 2 | Austenitic steels | 0.4 | 0.4 | 0.4 | 0.5 | 0.7 |
| 3 | Cast iron | 0.0 | ||||
Physical properties of piping materials
The reasons pipe and tube are made from different materials is because of the properties of different materials. Important properties are (i) malleability, (ii) ductility, (iii) brittleness, (iv) hardness, (v) elasticity, (vi) conductivity, and (vii) chemical resistance / resistance to corrosion.
Malleability can be defined as the property of a metal to be deformed by compression without cracking or rupturing. This property is very useful for copper tubing systems since it allows the tube to be bent to follow the required route quickly without the need for expensive and time consuming fittings.
Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically without fracture. Ductile metals lend themselves to be formed into desired cross sectional shapes easier and hence are cheaper to produce.
Brittleness is the tendency for a metal to crack or break with deformation. Metals displaying this property are not readily used for pipe or tube as this is a non desirable property of the material.
Hardness is the property of being rigid and resistant to pressure; not easily scratched. It is an advantage for high pressure systems but can be a disadvantage as it can increase machining, cutting, and fabrication times.
Elasticity is the property by virtue of which a material deformed under the load can regain its original dimensions when unloaded. This property is used in piping system designs where pipes can expand or contract due to temperature differences. The elasticity of piping materials can help the designer to cater for this requirement.
Conductivity is the ability of a material to conduct electrical current or heat. Some piping systems use high conductivity metals for high heat transfer while other piping systems use low conductivity plastic materials to prevent heat transfer.
Chemical resistance/ resistance to corrosion determine the degree to which a material resists the corrosive action of industrial chemicals. This is probably the most significant property which affects the choice of piping material and is the biggest contributor to the price of material. Specialist alloys of stainless steel such as Hastelloy can be 10 times to 20 times more expensive that standard stainless steel and can be slower to fit and weld which increase installation costs.
Common types of steel pipes
The common types of steel pipes are described below.
Standard pipe – It is standard weight, extra strong, and double extra strong welded or seamless pipe of ordinary finish and dimensional tolerances, produced in sizes upto 660 mm in nominal diameter, inclusive. This pipe is used for fluid conveyance and some structural purposes.
Conduit pipe – It is welded or seamless pipe intended especially for fabrication into rigid conduit, a product used for the protection of electrical wiring systems. Conduit pipe is not subjected to hydrostatic testing unless so specified. It can be galvanized or bare, as specified. It is furnished in standard weight pipe sizes from 6 mm to 150 mm in lengths of around 3 m to 6 m with plain ends or threaded ends.
Piling pipe – It is welded or seamless pipe for use as piles, with the cylinder section acting as a permanent load-carrying member or as a shell to form cast-in-place concrete piles. There are normally three grades, which have different minimum tensile strengths, a variety of diameters, ranging from 150 mm to 610 mm, and a variety of wall thicknesses. Ends can be plain or beveled for welding.
Pipe for nipples –It is standard weight, extra strong, or double extra strong welded or seamless pipe, produced for the production of pipe nipples. Pipe for nipples is normally produced in random lengths with plain ends and in nominal sizes from 3 mm to 300 mm. Close OD tolerances, sound welds, good threading properties, and surface cleanliness are essential in this product. Pipe for OCTG couplings are to be manufactured from seamless pipe. It is normally coated with oil or zinc and is well protected in shipment.
Transmission or line pipe – It is welded or seamless pipe presently produced in sizes ranging from 3 mm nominal to 1.2 m actual OD and is used principally for conveying gas or oil. Line pipe is produced with ends plain, threaded, beveled, grooved, flanged, or expanded, as required for various types of mechanical couplers or for welded joints. When threaded ends and couplings are needed, recessed couplings are used.
Water main pipe – It is welded or seamless steel pipe used for conveying water for municipal and industrial purposes. Pipe lines for such purposes are normally designated as flow mains, transmission mains, force mains, water mains, or distribution mains. The mains are generally laid underground. Sizes ranges from 40 mm to 2.45 m in nominal diameter in a variety of wall thicknesses. Pipe is produced with ends suitably prepared for mechanical couplers, with plain ends beveled for welding, or with bell and spigot joints for field connection. Pipe is produced in double random lengths of around 12 m, single random lengths of around 6 m, or in specified lengths. When required, it is produced with a specified coating or lining, or both.
Oil country tubular goods – OCTG is a collective term applied in the oil and gas industries to three kinds of pipe used in oil wells namely (i) drill pipe, (ii) casing, and (iii)tubing. The drill pipe is used to transmit power by rotary motion from ground level to a rotary drilling tool below the surface and to convey flushing media to the cutting face of the tool. Drill pipe is produced in sizes ranging from 60 mm to 170 mm in OD. Size designations refer to actual OD and weight per meter. Drill pipe is normally upset, either internally or externally, or both, and is prepared to accommodate welded-on types of joints.
Casing is used as a structural retainer for the walls of oil or gas wells, to exclude undesirable fluids and to confine and conduct oil or gas from productive sub-surface strata to ground level. Casing is produced in sizes from 115 mm to 500 mm in OD. Size designations refer to actual OD and weight per meter. Ends are normally threaded and furnished with couplings, but can be prepared to accommodate other types of joints. Tube is used within the casing of oil wells to conduct oil and gas to ground level. It is produced in sizes from 26 mm to 114 mm in OD, in several weights per meter. Ends are threaded for special integral-type joints or fitted with couplings and may or may not be upset externally.
Water well pipe – It is a collective term applied to four types of pipe which are used in water wells and these are (i) type I, drive pipe, (ii) type II, reamed and drifted pipe, (iii) type III, driven well pipe, and (iv) type IV, casing pipe. Drive pipe is used to transmit power from ground level to a rotary drill tool below the surface and to convey flushing media to the cutting face of the tool. The lengths of pipe have specially threaded ends that permit the lengths to butt inside the coupling. Drive pipe is produced in nominal sizes of 150 mm, 200 mm, 300 mm, 350 mm, and 400 mm OD. Driven well pipe is threaded pipe in short lengths used for the manual driving of a drill tool or for use with short rigs. It can be furnished in random lengths ranging from 0.9 m to 1.8 m or in random lengths ranging from 1.8 m to 3.0 m. Casing is used both to confine and conduct water to ground level and as a structural retainer for the walls of water wells. It is produced as threaded pipe in random lengths from 4.9 m to 6.7 m and in sizes from 90 mm to 220 mm in OD.
Pressure pipe – Pressure pipe as distinguished from pressure tube, is a commercial term for pipe which is used to convey fluids at high temperature or pressure, or both, but which is not subjected to the external application of heat. Pressure pipe ranges in size from 3 mm nominal to 660 mm actual OD in different wall thicknesses. Pressure pipe is furnished in random lengths, with threaded or plain ends, as needed.
Pressure tube – Pressure tube is given a separate classification. Pressure tube is distinguished from pressure pipe in that it is suitable for the application of external heat while conveying pressurized fluids. The pressure tube is produced to actual OD and minimum or average wall thickness and can be hot finished or cold finished.
Double-wall brazed tube – it is a specialty tube confined to small sizes. It is used in large amounts by the automotive industry for brake lines and fuel lines, and by the refrigeration industry for refrigerant lines. It is made by forming copper-coated strip into a tubular section with double walls, using either single-strip or double-strip construction. The tube is then heated in a reducing atmosphere to join all mating surfaces completely. The resulting product is thus copper coated both inside and outside. When required by the intended service, a tin coating can be done. Available sizes range from 3 mm to 15 mm in OD with wall thickness from 0.64 mm for 3 mm OD to 0.9 mm for 15 mm OD. The product is normally made for use with standard compression fittings. It can be sink-drawn for the improvement of surface finish and tolerances.
Structural tube – It is used for the welded, riveted, or bolted construction of bridges and buildings and for general structural purposes. It is available as round, square, rectangular, or special-shape tube, as well as tapered tube. Structural tube is produced with a maximum wall thickness of 13 mm and with maximum circumferences of 810 mm for seamless tubes and 1.22 m for welded tubes.
Mechanical tube – It includes welded and seamless tube used for a wide variety of mechanical purposes. It is normally produced to meet specific end-use requirements and hence is produced in many shapes, to a variety of chemical compositions and mechanical properties, and with hot-rolled or cold-finished surfaces. Mechanical tube is not produced to specified standard sizes. Instead, it is produced to specified dimensions, which can be anything the customer requires within the limitations of the production equipment or processes. Controlling tolerances are placed on OD and wall thickness for hot-finished tube and on OD, ID, and wall thickness for cold-finished tube. Specifications for size can include any two of the controlling dimensions namely OD, ID, and wall thickness but never all three. The chemical compositions normally available in steel mechanical tube cover a wide variety of standard grades. In addition to the standard grades, various high-strength low-alloy grades and unique chemistries are produced to customer specifications. When the steel is used, either carbon or alloy steel, needs normalizing or annealing after welding, such operations become a part of the specification. For example, a type of welded structural tube made from carbon steel with nominal carbon content of 0.50 % is normally normalized.
Welded mechanical tube – It is normally made by electric resistance welding, but some is made by the various fusion welding processes. In all instances, the exterior welding flash can be removed (if necessary) by cutting, grinding, or hammering. ERW mechanical tube is made from hot-rolled or cold-rolled carbon steel or from alloy steel strip. The welded tube can be made as-welded, hot finished, or cold finished. Hot-finishing operations normally consist of either a stretch reducing mill or a hot reducing mill (hot sinking). Micro-structural and hardness variations associated with the welding are modified by either seam annealing the weld zone or full body normalizing the entire tube. Sizes produced by ERW range in OD from 6.4 mm to 400 mm and in wall thickness from 1.65 mm to 17 mm for hot-rolled steel and 0.65 mm to 4.2 mm for cold rolled steel. Hydraulically or electrically driven stretch reducing mills accomplish tube elongation, reduction in diameter, and control of wall thickness on very long mill lengths in essentially a continuous process. Walls as thick as 18 mm are commercially available within limited OD ranges.
Continuous-welded cold-finished mechanical tube – As its name implies, continuous-welded cold-finished mechanical tube is hot formed by furnace butt welding and cold finished. It is furnished sink drawn or mandrel drawn and is available in OD upto 90 mm and wall thicknesses from 0.9 mm to 13 mm. The material is low-carbon steel, and the product is, in effect, a form of cold-drawn pipe. Although furnished in a narrower size range than ERW tube, it has two advantages namely (i) within the available size range, heavier walls are available, and (ii) there is no problem with flash.
Seamless mechanical tube – It is available both hot and cold finished, and in a wide variety of finishes and mechanical properties. It is made from carbon and alloy steels in sizes upto and including 325 mm in OD. Hot-finished seamless tube is produced by rotary piercing or extrusion processes. Hence, it has surfaces similar to the surface regularly produced on hot-rolled steel and, in general, cannot be held to dimensional tolerances as close as those of tube produced by cold finishing. It is produced in sizes as small as 38 mm in OD. Cold-finished mechanical tube can be produced by means of surface removal or by cold working. Surface removal includes turning, polishing, grinding, or machining. Cold working involves cold reducing to effect changes in cross-sectional dimensions. Drawing over a mandrel is the most common method of cold working mechanical tube.
Tube is prepared for drawing by first pointing it. The end of the tube is mechanically reduced in OD to allow the end to pass through the die for gripping. Tube is to be pickled and lubricated before drawing. Pickling is typically accomplished by hydrochloric or sulphuric acid immersion. A subsequent phosphate immersion coating on the steel surface acts as a binder for the soap like lubricant. Complete coverage of both the ID and OD is needed to prevent galling and chatter during drawing. Reductions in cross-sectional area of 10 % to 30 % are normal. Drawing is normally followed by a thermal treatment, straightening, and non-destructive inspection. Cold working and surface removal are used mainly for the purpose of obtaining smaller OD (down to 3.2 mm), better finishes, thinner walls, and closer dimensional accuracy than is possible in hot-finished tube. In addition, cold-worked tube offers improved mechanical properties and machinability. Cold working can also be used to produce tubes having cross-sectional shapes other than round. Cold-drawn tube can be made in the as-drawn, cold worked condition, or thermally treated to the desired combination of mechanical properties or micro-structure. Typical thermal treatments available are stress relief annealed, normalized, soft-annealed, or quenched and tempered.
Square, rectangular, and special-shape sections – These are produced in welded or seamless tube, starting with either round tube of the required diameter and wall thickness or square, unwelded tube. Squaring is done in a Turk’s head or by other cold-working methods. A Turk’s head consists of a frame in which are mounted four rolls with their axes at 90 degrees and adjusted so that the roll surfaces form an opening of the same shape as the section to be formed. The Turk’s head is mounted on a draw bench, and the round tubes are passed through the rolls in the same manner as they are passed through dies.
When Turk’s head shaping do not provide close enough tolerances on either the outside or inside of uniformly rounded corners, or close enough diagonal dimensions, the forming is normally accomplished by means of die-and-mandrel shaping. The corners of sections processed by this means are around 90 degree arcs and have greater uniformity through-out than is provided by Turk’s head shaping. Sections which can be processed in this manner are somewhat limited with respect to diameter, wall thickness, and outside and inside corner radii.
In addition to providing square and rectangular tube, several producers of welded or seamless tube make a variety of special sections, such as oval, streamline, hexagonal, octagonal, round inside and hexagonal or octagonal outside, ribbed inside or out, triangular, round-ended rectangular, and D-shape. Available production equipment limits the size range and sections available from the various producers. These special sections may be made by passing round tube through Turk’s head rolls or through a die with or without the use of a mandrel. Because the sections are special, dies and other tools are not kept available.
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