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What is a steel pipe?

What is a steel pipe?

Table of Contents

Steel pipe is a kind of steel with hollow circular section, whose length is far greater than the diameter or circumference. According to the material, it is divided into carbon steel pipe, alloy steel pipe and stainless steel pipe; according to the purpose, it is divided into transmission pipe, engineering structure, thermal equipment, petrochemical industry, machinery manufacturing, geological drilling, high-pressure equipment, etc.; according to the production process, it is divided into seamless steel pipe and welded steel pipe, of which seamless steel pipe is divided into hot rolling and cold rolling (drawing), and welded steel pipe is divided into straight seam welding Connect steel pipe and spiral seam welded steel pipe.
Steel pipe is not only used to transport fluid and powder solids, exchange heat energy, manufacture mechanical parts and containers, but also an economic steel. It can reduce the weight, save 20-40% metal and realize the factory mechanized construction. Making highway bridges with steel pipes can not only save steel and simplify construction, but also greatly reduce the area of protective coating and save investment and maintenance costs.

Classification of steel pipes

According to production method

20191220034045 27605 - What is a steel pipe?

According to the production method, the steel pipe can be divided into two categories: seamless steel pipe and welded steel pipe. The welded steel pipe is called welded pipe for short.

  • 1. Seamless steel pipes can be divided into hot-rolled seamless pipes, cold drawn pipes, precision steel pipes, hot expanded pipes, cold spun pipes and extruded pipes according to production methods.
  • Seamless steel pipe is made of high quality carbon steel or alloy steel, including hot rolling and cold rolling (drawing).
  • 2. Welded steel pipe is divided into furnace welded pipe, electric welded (resistance welded) pipe and automatic arc welded pipe due to different welding processes. Because of different welding forms, it is divided into straight welded pipe and spiral welded pipe. Because of its end shape, it is divided into circular welded pipe and special-shaped (square, flat, etc.) welded pipe.

Welded steel pipe is made of rolled steel plate welded by butt joint or spiral joint. In terms of manufacturing method, it is also divided into welded steel pipe for low-pressure fluid transmission, spiral welded steel pipe, direct rolled welded steel pipe, welded pipe, etc. Seamless steel pipe can be used in various industries of liquid gas pressure pipeline and gas pipeline. Welded pipes can be used for water pipes, gas pipes, heating pipes, electrical pipes, etc.

By material

The steel pipe can be divided into carbon pipe, alloy pipe and stainless steel pipe according to the material (i.e. steel type).
Carbon pipe can be divided into ordinary carbon steel pipe and high-quality carbon structural pipe.
Alloy pipe can be divided into: low alloy pipe, alloy structure pipe, high alloy pipe, high strength pipe. Bearing pipe, heat and acid resistant stainless pipe, precision alloy (such as kovar alloy) pipe and high temperature alloy pipe, etc.

Sort by connection

According to the connection mode of pipe end, steel pipe can be divided into smooth pipe (without thread at pipe end) and thread turning pipe (with thread at pipe end).
The threading pipe is divided into ordinary threading pipe and thickened threading pipe at the pipe end.
The thickened threading pipe can also be divided into: externally thickened (with external thread), internally thickened (with internal thread) and internally thickened (with internal and external thread), etc.
The threading pipe can also be divided into ordinary cylindrical or conical thread and special thread according to the thread type.
In addition, according to the user’s needs, the car’s wire pipes are generally equipped with pipe joints for delivery.

According to plating characteristics

According to the characteristics of surface coating, the steel pipe can be divided into black pipe (not coated) and coated pipe.
The coated pipe includes galvanized pipe, aluminized pipe, chrome plated pipe, aluminized pipe and other alloy coated steel pipes.
The coating pipe includes outer coating pipe, inner coating pipe and inner and outer coating pipe. The commonly used coating materials are plastic, epoxy resin, coal tar epoxy resin and various glass type anticorrosive coating materials. Galvanized pipe is divided into KBG pipe, JDG pipe, threaded pipe, etc

By use

  • 1. Pipe for pipeline. Such as: seamless pipe for water, gas pipe, steam pipe, oil transmission pipe, oil and natural gas trunk pipe. Agricultural irrigation faucet with pipe and sprinkler pipe.
  • 2. Pipes for thermal equipment. For example, boiling water pipe, superheated steam pipe for general boiler, superheated pipe, large smoke pipe, small smoke pipe, arch brick pipe and high temperature and high pressure boiler pipe for locomotive boiler, etc.
  • 3. Pipes for machinery industry. For example, aviation structural pipe (round pipe, elliptical pipe, flat elliptical pipe), automobile half axle pipe, axle pipe, automobile tractor structural pipe, tractor oil cooler pipe, agricultural machinery square pipe and rectangular pipe, transformer pipe and bearing pipe, etc.
  • 4. Pipes for petroleum geological drilling. Such as: oil drilling pipe, oil drill pipe (Kelly and hexagonal drill pipe), spud, oil tubing, oil casing and various pipe joints, geological drilling pipe (core pipe, casing, active drill pipe, spud, collar and pin joint, etc.).
  • 5. Pipes for chemical industry. Such as: oil cracking pipe, heat exchanger and pipe of chemical equipment, stainless acid resistant pipe, high pressure pipe for chemical fertilizer and pipe for chemical medium transmission, etc.
  • 6. Management of other departments. Such as: container pipe (high pressure cylinder pipe and general container pipe), instrument pipe, watch shell pipe, injection needle and medical device pipe, etc.

By section shape

There are many kinds and specifications of steel pipe products, and their performance requirements are also various. All these shall be distinguished according to the change of user requirements or working conditions. Generally, steel pipe products are classified according to section shape, production method, pipe material, connection method, plating characteristics and application, etc.
According to the cross-section shape, steel pipes can be divided into round steel pipes and special-shaped steel pipes.
Special shaped steel pipe refers to all kinds of steel pipes with non-circular circular cross-section. Among them, there are: square tube, rectangular tube, elliptical tube, flat elliptical tube, semicircle tube, hexagon tube, hexagon inner circle tube, unequal hexagon tube, equilateral triangle tube, pentagonal plum tube, octagonal tube, convex tube, double convex tube. Double concave tube, multiple concave tube, melon seed tube, flat tube, rhombus tube, star tube, parallelogram tube, ribbed tube, drop tube, inner fin tube, twisted tube, B-TUBE, d-tube and multilayer tube, etc.

According to the shape of vertical section, steel pipes are divided into equal section steel pipes and variable section steel pipes. Variable cross-section (or variable cross-section) steel pipe refers to the steel pipe with periodic or non periodic changes in cross-section shape, internal and external diameter and wall thickness along the pipe. It mainly includes: outer tapered pipe, inner tapered pipe, outer stepped pipe, inner stepped pipe, periodic section pipe, corrugated pipe, spiral pipe, steel pipe with heat sink and gun pipe with double line, etc.

Types of steel pipe

Welded steel pipe

The welded steel pipe refers to a steel pipe which is welded and formed into a shape of a circle or a square by bending a steel strip or a steel plate, and has a seam on the surface thereof. The raw material used for the welded steel pipe is a steel plate or a strip steel.
Since the 1930s, with the rapid development of high-quality strip continuous rolling production and the advancement of welding and inspection technology, the quality of welded joints has been continuously improved, the variety specifications of welded steel pipes have been increasing, and more and more fields have replaced Seam steel pipe. Welded steel pipes have lower cost and higher production efficiency than seamless steel pipes.

1. Pipe can be divided according to the basic shape of Longitudinal seam and spiral welded pipe are two:

(1) Straight seam welded pipe: simple production technology, high efficiency, low cost, rapid development. Longitudinal more general industrial use.

(2) Spiral welded steel pipe: Longitudinal strength generally higher than that, can narrow the larger diameter pipe billet production, you can also use the same width of the billet production of welded pipes of different diameter; but with the same lengthcompared to straight seam pipe, spiral welded steel pipe weld length of 30 to 100%, and lower production rate; Therefore, the smaller diameter pipe actually use straight seam welded, large diameter spiral welded pipe is mostly used.


  • (1) General welded steel pipe: welded steel pipe used to transport the general low-pressure fluid. With Q195A, Q215A, Q235A steel. Can be applied easily to other soft steel welding steel tube to the pressure, bending, flattening and other experiments, thethere is a certain surface quality requirements, delivery is usually the length of 4-10m, are often asked to cut to length (or double length) delivery.’s specifications with the nominal pipe diameter, said (mm or inches) nominal diameter and the actual difference, pipe requiredthick wall of ordinary steel and steel are two kinds of steel divided by the form of pipe with threaded end and without a thread two.

  • (2) Galvanized welded steel pipe: To improve the corrosion resistance of steel pipe, steel pipe for general (clarinet) for zinc galvanized steel and electrical steel hot-dip galvanized zinc are two kinds of hot dip galvanized zinc thickness, platingzinc and low cost.

  • (3) Oxygen welded steel pipe: pipe used for steel-making oxygen, usually with a small diameter welded steel pipe, size from 3 / 8 inch -2 inch eight. With 08,10,15,20 or Q195-Q235 steelband made of the corrosion, some of the aluminizing treatment.

  • (4) Wire tube: welded steel pipe is also a common carbon steel, concrete and various structures used in power distribution engineering, commonly used in nominal diameter from 13-76mm.

  • (5) Metric welded steel pipe: seamless form with the specifications, with expressed in mm diameter * wall thickness welded steel pipe, with ordinary carbon steel, high carbon steel or low alloy steel & P to tropical and cold with welding, orafter welding with a tropical method call made by the cold. Metric & P energy and thin-walled pipe points, common for structural parts, such as shaft or transmission fluid, used to produce thin-walled furniture, lamps, etc., to ensure that the strength of steeland bending test.

  • (6) Roller tube: welded steel pipe for roller conveyor, generally with the Q215, Q235A, B steel and 20 steel, diameter 63.5-219.0mm. On the tube bending, end with the center line of vertical, ovalthere is a certain degree requirements, general test for pressure and flattening.

How to Weld Steel Pipes

Welding pipes is one of the simplest and most durable ways to join piping together and steel pipes can simply be welded together with MIG (metal inert gas), TIG (tungsten inert gas or SMAW (stick metal arc) welding. Before welding two pipes together, avoid any gaps between the two pipes. While welding, take measures to ensure the continual alignment of the two pipes.

Align the two pipes and use clamps to hold them together, if necessary.

Fire your welding torch or strike your welding electrode on the steel to open an arc between the welding tool and pipes. Allow the torch to form a small welding puddle on the surface of the steel, called a tack weld.

Move the arc slightly and create another welding puddle elsewhere on the circumference of the pipes. Continue to move the arc and create more welding puddles until you have formed a series of tack welds at regular intervals along the circumference.

Move the welding tool thoroughly along the circumference of the pipes creating a welding puddle that extends fully around the circumference and connects the two pipes together.

Extinguish the arc and allow the welding puddle several minutes to dry.

Most common Weld steel pipes

Spiral welded pipe

Anti corrosion surface treatment process of spiral pipe - What is a steel pipe?

Hot expansion steel pipe mandrel surface pulsating spray device is a device to a thermal expansion pipe mandrel surface spray lubricant, which is characterized by injection device body ring parts with lubricant reservoir burglary, in Chu burglary ring wall open lubricationagents enter the hole, motor Chu burglary side installation, its spindle by driving wheel and the middle with a lubricant output hole pulsating ring phase, the lower part of the output hole placement beam radio regulatory group equipped with a beam of radio regulation.
Advantages are:using spray lubricant to the mandrel surface, can reduce the frictional resistance between the tube rolling mandrel and steel pipe to reduce the pressure load of the mill and the mill’s power consumption to extend the life of the mandrel, to improve the quality of the steel pipe rolling.
Hot expansion pipe with two-step push tube expanding machine set conical die expanding technology, digital intermediate frequency induction heating technology, hydraulic technology in a machine with a reasonable process, low energy consumption, lower capital investment, good product quality, a wide range of applicability of the raw material and product specifications, flexible variable cost of production volume, adaptability, replace the traditional steel industry, of Pullout expanding technology. difficult to resolve in the near future, the supply of large diameter steel pipe, thermal expansion pipe to become the source of the shortage of large-diameter steel pipe products. ease the tight supply situation of the large diameter pipe market.

erw pipe banner - What is a steel pipe?


ERW pipes means Electric Resistance Welded Pipes. It is cost-effective steel pipe with tighter Dimensional tolerances and less weight. The weld seam is heat treated after welding without untempered martensite remains. Also, the weld flash can be removed from both inner and outer surfaces.


ERW Galvanized Steel Pipe banner 1 - What is a steel pipe? 

For pipes or tubes size 4 inch (10.2mm) OD and below, strip is fed into a set of forming rolls which consists of horizontal and vertical rollers so placed as to gradually from the flat strip in to a tube which is then allowed to pass the welding electrodes. The electrodes are copper disks connected to the secondary of a revolving transformer assembly. The copper disk electrodes make contact on each side of the seam and temperature is raised to the welding point. Outside flash is removed by a cutting tool as the tube leaves the electrodes, inside flash is removed either by an air hammer or by passing a mandrel through the welded tube after the tube has been cooled. This is termed as Electric Resistance Welded or ERW tube/pipe.


Black ERW steel pipe banner - What is a steel pipe?
ERW pipe cold formed from a ribbon of steel pulled through a series of rollers and formed into a tube which is fused through a electric charge.

High Frequency Welded pipe banner - What is a steel pipe?


In High Frequency Induction (HFI) Welded Pipe production process, hot rolled steel coils are fed with a high capacity accumulator in order to achieve continuous welding.

HIGH-FREQUENCY WELDING is a welding process in which the heat source used to melt the joining surfaces is obtained from high-frequency (HF) alternating current (ac) resistance heating.

Coil edges are milled using carbide cutters in order to assure a high quality weld. Coils are then formed by cold forming method using a set of cage rolls and fin passes, and then joined using high frequency currents using induction or alternatively conduction methods.

Seamless steel pipe

A seamless steel pipe made of a single piece of metal and having no seams on its surface is called a seamless steel pipe. According to the production method, the seamless pipe is divided into a hot rolled pipe, a cold rolled pipe, a cold drawn pipe, an extruded pipe, a top pipe, and the like. According to the shape of the section, the seamless steel tube is divided into two types: a circular shape and a special shape. The shaped tube has a plurality of complicated shapes such as a square shape, an elliptical shape, a triangular shape, a hexagonal shape, a melon shape, a star shape, and a winged tube. The maximum diameter is 650mm and the minimum diameter is 0.3mm. Depending on the application, there are thick-walled tubes and thin-walled tubes. Seamless steel pipes are mainly used as petroleum geological drilling pipes, cracking pipes for petrochemicals, boiler tubes, bearing tubes, and high-precision structural steel tubes for automobiles, tractors, and aviation.

Precision steel pipe

Precision steel pipe is a kind of high-precision steel pipe material after cold drawing or hot rolling. Because the inner and outer walls of precision steel pipes have no oxidation layer, high pressure and no leakage, high precision, high smoothness, no deformation of cold bending, flaring, flattening without cracks, etc., they are mainly used to produce pneumatic or hydraulic components, such as cylinders or The cylinder can be a seamless tube or a welded tube.
Precision steel pipe production process
The production process of precision steel pipes is the same as the ordinary seamless pipe, which is the procedure of the final pickling and cold rolling.

Production process of  Precision steel pipe

Tube blank – inspection – stripping – inspection – heating – perforation – pickling passivation – grinding – lubrication drying – cold rolling – oil removal – cutting head – inspection – identification – -Product packaging

Galvanized steel pipes

Galvanized steel pipes are classified into cold-galvanized steel pipes and hot-dip galvanized steel pipes, and cold-galvanized steel pipes have been banned. The galvanized pipe is used as a water pipe. After a few years of use, a large amount of rust is generated in the pipe. The yellow water that flows out not only pollutes the sanitary ware, but also is mixed with bacteria that grow on the inner wall of the slick. The rust causes the heavy metal content in the water to be too high, which seriously endangers the health of the human body.

Production method of steel pipe

The steel pipe is divided into seamless steel pipe and welded steel pipe. The process of seamless steel tube production is to wear solid billets or ingots into hollow pipes and then roll them into steel pipes of the required size. The different methods of perforation and tube rolling make up different methods of producing seamless steel tubes. The production process of welded steel pipe is bending the tube billet (steel plate or strip) into tube and then welding the gap into steel pipe. Because of the different forming and welding methods adopted, different methods of producing welded steel pipes are formed.
Seamless steel tubes are mainly produced by hot rolling. Extrusion method is mainly used to produce low perforated low alloy high alloy steel pipes or special-shaped steel tubes and composite metal pipes. Cold rolling and cold drawing processes can be used to make hot rolled tubes into small diameter and thin walled steel tubes. The process of welding steel pipe is simple, the production efficiency is high, the cost is low, and the product variety is expanding day by day. The main production methods of seamless steel pipe and welded steel pipe are shown in Tables 1 and 2.
Table.1 Production methods of seamless steel pipe and welded steel pipe
Production method Basic process
perforation Tube rolling

On the automatic tube rolling mill.

Round tube billet in two roll piercer

Punch up
Two roll mill with roll back

Short plug rolling on pipe machine
On the cycle tube mill

Round steel ingots or billets in two rollers

Perforation on puncher;
Square or polygonal ingot in water
Punch on press
Cycle with variable cross section pass

Rolling with long mandrel on tube mill
Continuous rolling mill

Top rolling
Round tube billet in three or two rollers.

Perforation on puncher;
Continuous casting billet or square steel ingot at two
Perforation of roller type pressure piercer
On the 7~9 tube mill

Mandrel rolling;
Attached stretch reducing mill
On the three roll mill

Round tube billet in two roll piercer

Upper perforation;
Continuous casting billet in three roll piercer
Punch up
Belt length on three roller skew rolling mill

Mandrel rolling
On the extended tube mill.

Perforation on two roll piercer In the guide roller with disc shaped tension.

Long mandrel with two roll skew rolling mill
Planetary tube mill

Top rolling
Casting hollow tube blank Rolling on planetary tube mill
Table.2 Production methods of seamless steel pipe and welded steel pipe
Production method Basic process
perforation Tube rolling
Pipe jacking method A cup shaped hair on the hydraulic press.

Top up on pipe jacking machine
Extrusion method After heating, the tube is on the extruder.

Extrusion molding
It can continue rolling or drawing.
Cold rolling method Cold rolled tube mill with hot rolled tube

Top rolling
Cold drawing method Use hot or cold rolled tubes to cool

Pull-out system

Chain furnace welding Heated tube passes through welded pipe mold

Simultaneous welding
Continuous furnace welding Heated tube billets in roll forming

Bending forming on welding machine
Simultaneous welding

Resistance welding Continuous bending on roller forming machine

Koji forming
Welding on resistance welding pipe machine
arc welding Pressing or forming on a press.

Roll up on roll bending machine
Type (straight seam), connected to forming machine.
Continuous bending (spiral seam)
Welding on submerged arc welding machine

Inductance welding Continuous bending on roller forming machine

Koji forming
Welding on inductive welding pipe machine

Production method of Seamless Tube and Pipe

The main seamless tube manufacturing processes came into being toward the end of the nineteenth century. As patent and proprietary rights expired, the various parallel developments initially pursued became less distinct and their individual forming stages were merged into new processes. Today, the state of the art has developed to the point where preference is given to the following modern high-performance processes:

The continuous mandrel rolling process and the push bench process in the size range from approx. 21 to 178 mm outside diameter.

The multi-stand plug mill (MPM) with controlled (constrained) floating mandrel bar and the plug mill process in the size range from approx. 140 to 406 mm outside diameter.

The cross roll piercing and pilger rolling process in the size range from approx. 250 to 660 mm outside diameter.

Mandrel Mill Process

mandrel mill - What is a steel pipe?

In the Mandrel Mill Process, a solid round (billet) is used. It is heated in a rotary hearth heating furnace and then pierced by a piercer. The pierced billet or hollow shell is rolled by a mandrel mill to reduce the outside diameter and wall thickness which forms a multiple length mother tube. The mother tube is reheated and further reduced to specified dimensions by the stretch reducer. The tube is then cooled, cut, straightened and subjected to finishing and inspection processes befor shipment.

mandrel mill - What is a steel pipe?

* Note: Processes marked by an asterisk are conducted specification and/or customer requirements

Mannesmann plug mill process

plug mill - What is a steel pipe?

Plug Mill Process, a solid round (billet) is used. It is uniformly heated in the rotary hearth heating furnace and then pierced by a Mannesmann piercer. The pierced billet or hollow shell is rollreduced in outside diameter and wall thickness. The rolled tube simultaneously burnished inside and outside by a reeling machine. The reeled tube is then sized by a sizing mill to the specified dimensions. From this step the tube goes through the straightener. This process completes the hot working of the tube. The tube (referred to as a mother tube) after finishing and inspection, becomes a finished product.

plug mill - What is a steel pipe?

Production method of Welded Tube and Pipe

Ever since it became possible to manufacture strip and plate, people have constantly tried to bend the material and connect its edges in order to manufacture tube and pipe. This led to the development of the oldest welding process, that of forge-welding, which goes back over 150 years.

In 1825, the British ironware merchant James Whitehouse was granted a patent for the manufacture of welded pipe. The process consisted of forging individual metal plates over a mandrel to produce an open-seam pipe, and then heating the mating edges of the open seam and welding them by pressing them together mechanically in a draw bench.

The technology evolved to the point where strip could be formed and welded in one pass in a welding furnace. The development of this butt-welding concept culminated in 1931 in the Fretz-Moon process devised by J. Moon, an American, and his German colleague Fretz.

Welding lines employing this process are still operating successfully today in the manufacture of tube up to outside diameters of approx. 114 mm. Aside from this hot pressure welding technique, in which the strip is heated in a furnace to welding temperature, several other processes were devised by the American E. Thomson between the years 1886 and 1890 enabling metals to be electrically welded. The basis for this was the property discovered by James P. Joule whereby passing an electric current through a conductor causes it to heat up due to its electrical resistance.

In 1898, the Standard Tool Company, USA, was granted a patent covering the application of electric resistance welding for tube and pipe manufacture. The production of electric resistance welded tube and pipe received a considerable boost in the United States, and much later in Germany, following the establishment of continuous hot strip rolling mills for the production of the bulk starting material necessary for large-scale manufacture. During the Second World War, an argon arc welding process was invented – again in the United States – which enabled the efficient welding of magnesium in aircraft construction.

As a consequence of this development, various gas-shielded welding processes were developed, predominantly for the production of stainless steel tube.Following the far-reaching developments which have occurred in the energy sector in the last 30 years, and the resultant construction of large-capacity long-distance pipelines, the submerged-arc welding process has gained a position of pre-eminence for the welding of line pipe of diameters upward of approx. 500 mm.

Electric Weld Pipe Mill

Steel strip in coil, which has been slit into the required width from wide strip, is shaped by a series of forming rolls into a multiple length shell. The longitudinal edges are continously joined by high frequency resistance/induction welding.
The weld of multiple length shell is then head treated electrically, sized and cut to specified lengths by a flying cut-off machine. The cut pipe is straightened and squared at both ends.
These operations are followed by ultrasonic inspection or hydrostatic testing.

erw process - What is a steel pipe?

From: Werner Sölken

Class of steel pipe

Pipe class is a document that specifies the type of the components such as a type of pipe, schedule, material, flange ratings, branch types, valve types and valve trim material, gasket and all the other components specific requirements to be used for different fluids under different operating conditions in a plant.

Pipe class is developed considering Operating Pressure, temperature and corrosive environment. Different material specifications are segregated in separate “Piping Class”.  Pipe class is part of the Piping specification.

Shortcode is used to cover all the information applicable to the given pipe class. This pipe class is also included in the line number so that field construction engineer can easily identify the material required.

Pipe Class 879x1024 1 - What is a steel pipe?

Check the link for the example of a piping class specification.

Specifications of steel pipe

Pipe Specifications are technical documents that are generated by private companies to address additional requirements applicable to a specific product or application.
Piping Specs provides specific/additional requirements for the materials, components or services that are beyond the code and standard requirements. For Example, if you want A106 Gr B pipe with Maximum carbon of 0.23% against standard requirements of 0.3% Max, you must be specified this requirement in your purchase specification.

Why did Specification require?

It allows a purchaser to include special requirements as per design and service condition.
It also allows customization of the product.
Please note requirement in the specification is Must meet requirements.
Normally, oil & gas facilities generate their own specification during FEED stage that is front-end engineering and design or sometimes uses readily available specification from well-established company such as Shell, Bechtel, EIL and chevron.

What Is Piping System?

Piping System is an assembly of various components put together with a proper method of joints, functionally to transport fluid from its source to destination. The different components put together are defined as piping components.
A piping system is a network of pipes, fittings and valves intended to perform a specific job i.e. to carry or transfer fluids from one equipment to another. The plumbing network supplying water at your home is a common example of a piping system. Other more rigorous examples include steam piping in a power plant, milk piping in a dairy, paint piping in a paint manufacturing plant, oil piping in a refinery, so and so forth.

Components of a Piping System

The most common components of a piping system are pipespipe fittings and valves.
Pipes: Pipes are long cylinders used to carry or transfer fluids. The most common fluids are water, oil, steam, air, milk or finished products like paints, juices. Other uncommon examples include pulp, acids, alkalies, chemicals etc.
Pipe Fittings: Pipe fittings are used to connect lengths of pipes to construct a long piping system; commonly used fittings are flanges, elbows, tees, reducers, expansion bellows etc.
Valves: Valves are used to stop, divert or control fluid flow. Common valve types are gate valves, globe valves, butterfly valves, ball valves, control valves; the selection is based on intended function and application.

In addition, a number of devices like strainers, traps, expansion loops are necessary for keeping the fluid clean and in good condition, and to accommodate expansion/contraction due to temperature variations.

Dimension of steel pipe

Pipe dimensions are given in size and schedule number. Three different terms are commonly used to define the size of the pipes.

  • NPS – Nominal Pipe Size
  • NB – Nominal Bore
  • DN – Diameter Nominal

Pipe Size NPS 649x631 1 - What is a steel pipe?

Pipe Size NPS NB DN

You can check the complete guide on Pipe Size and Schedule (Click on the link) for in-depth understanding.

Dimensions of the pipes are covered in following Standard

  • ASME B36.10 – Welded and Seamless Wrought Steel Pipe (Carbon & Alloy Steel)
  • ASME B36.19 – Stainless Steel Pipe

What is Nominal Pipe Size?

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. The name NPS is based on the earlier “Iron Pipe Size” (IPS) system.

That IPS system was established to designate the pipe size. The size represented the approximate inside diameter of the pipe in inches. An IPS 6″ pipe is one whose inside diameter is approximately 6 inches. Users started to call the pipe as 2inch, 4inch, 6inch pipe and so on. To begin, each pipe size was produced to have one thickness, which later was termed as standard (STD) or standard weight (STD.WT.). The outside diameter of the pipe was standardized.

As the industrial requirements handling higher pressure fluids, pipes were manufactured with thicker walls, which has become known as an extra strong (XS) or extra heavy (XH). The higher pressure requirements increased further, with thicker wall pipes. Accordingly, pipes were made with double extra strong (XXS) or double extra heavy (XXH) walls, while the standardized outside diameters are unchanged. Note that on this website only terms XS & XXS are used.

Pipe Schedule

So, at the IPS time only three walltickness were in use. In March 1927, the American Standards Association surveyed industry and created a system that designated wall thicknesses based on smaller steps between sizes. The designation known as nominal pipe size replaced iron pipe size, and the term schedule (SCH) was invented to specify the nominal wall thickness of pipe. By adding schedule numbers to the IPS standards, today we know a range of wall thicknesses, namely:

SCH 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, STD, XS AND XXS.

Nominal pipe size (NPS) is a dimensionless designator of pipe size. It indicates standard pipe size when followed by the specific size designation number without an inch symbol. For example, NPS 6 indicates a pipe whose outside diameter is 168.3 mm.

The NPS is very loosely related to the inside diameter in inches, and NPS 12 and smaller pipe has outside diameter greater than the size designator. For NPS 14 and larger, the NPS is equal to 14inch.

pipes2 - What is a steel pipe?

For a given NPS, the outside diameter stays constant and the wall thickness increases with larger schedule number. The inside diameter will depend upon the pipe wall thickness specified by the schedule number.


Pipe size is specified with two non-dimensional numbers,

  • Nominal pipe size (NPS)
  • Schedule number (SCH)

and the relationship between these numbers determine the inside diameter of a pipe.

Stainless steel pipe dimensions determined by ASME B36.19 covering the outside diameter and the Schedule wall thickness. Note that stainless wall thicknesses to ASME B36.19 all have an “S” suffix. Sizes without an “S” suffix are to ASME B36.10 which is intended for carbon steel pipes.

The International Standards Organization (ISO) also employs a system with a dimensionless designator.
Diameter nominal (DN) is used in the metric unit system. It indicates standard pipe size when followed by the specific size designation number without a millimeter symbol. For example, DN 80 is the equivalent designation of NPS 3. Below a table with equivalents for NPS and DN pipe sizes.

NPS 1/2 3/4 1 1.1/4 1.1/2 2 2.1/2 3 3.1/2 4
DN 15 20 25 32 40 50 65 80 90 100

Note: For NPS ≥ 4, the related DN = 25 multiplied by the NPS number.

Do you now what is “ein zweihunderter Rohr”?. Germans means with that a pipe NPS 8 or DN 200. In this case, the Dutch talking about a “8 duimer”. I’m really curious how people in other countries indicates a pipe.

Examples of actual O.D. and I.D.


  • NPS 1 actual O.D. = 1.5/16″ (33.4 mm)
  • NPS 2 actual O.D. = 2.3/8″ (60.3 mm)
  • NPS 3 actual O.D. = 3.1/2″ (88.9 mm)
  • NPS 4 actual O.D. = 4.1/2″ (114.3 mm)
  • NPS 12 actual O.D. = 12.3/4″ (323.9 mm)
  • NPS 14 ACTUAL O.D. = 14″(355.6 MM)


  • NPS 1-SCH 40 = O.D.33,4 mm – WT. 3,38 mm – I.D. 26,64 mm
  • NPS 1-SCH 80 = O.D.33,4 mm – WT. 4,55 mm – I.D. 24,30 mm
  • NPS 1-SCH 160 = O.D.33,4 mm – WT. 6,35 mm – I.D. 20,70 mm

Such as above defined, no inside diameter corresponds to the truth 1″ (25,4 mm).
The inside diameter is determined by the wall thickness (WT).

Facts you need to know!

Schedule 40 and 80 approaching the STD and XS and are in many cases the same.
From NPS 12 and above the wall thickness between schedule 40 and STD are different, from NPS 10 and above the wall thickness between schedule 80 and XS are different.

Schedule 10, 40 and 80 are in many cases the same as schedule 10S, 40S and 80S.
But watch out, from NPS 12 – NPS 22 the wall thicknesses in some cases are different. Pipes with suffix “S” have in that range thinner wall ticknesses.

ASME B36.19 does not cover all pipe sizes. Therefore, the dimensional requirements of ASME B36.10 apply to stainless steel pipe of the sizes and schedules not covered by ASME B36.19.

Terminology relating to steel pipes


  • A. Nominal size It is the nominal size stipulated in the standard, which is the desired size for users and manufacturers, and is also the order size specified in the contract.
  • B. Actual size It is the actual size obtained in the production process, which is usually larger than or smaller than the nominal size. This phenomenon of greater than or less than nominal size is called deviation.
  • C. Rice weight: =0.02466* wall thickness per meter (external diameter wall thickness)

Deviation and tolerance

  • A. Deviation: in the production process, because the actual size is difficult to meet the nominal size requirements, that is, often greater than or less than the nominal size, so the standard specifies the difference between the actual size and the nominal size. Positive deviation is called positive deviation, and negative difference is called negative bias.
  • B. Tolerance: the sum of the absolute value of the positive and negative deviations specified in the standard is called. tolerance It is also known as the tolerance zone.

Deviation is directional, that is, “positive” or “negative”. Tolerance is not directional. Therefore, it is wrong to call the deviation value “positive tolerance” or “negative tolerance”.

Delivery length

Delivery length is also known as user request length or contract length. There are several requirements for delivery length in the standard:
A. Normal length (also known as non fixed length): any length which is within the standard specified length and without fixed length requirements is called the normal length. For example, the structural pipe standard: hot rolled (extruded and expanded) steel pipe 3000mm ~ 12000mm; cold drawn (rolled) steel pipe 2000mmm ~ 10500mm.
B. Fixed length: fixed length should be within the normal length range, which is a fixed length size required in the contract. But in practice, it is impossible to cut out the absolute length, so the standard positive deviation is specified in the standard.
The structural management standards are:
The production rate of fixed length pipe is larger than that of normal length pipe, and the requirement of production enterprise to raise the price is reasonable. The rate of increase is not the same for all enterprises, generally based on the base price increase of about 10%.
C. Double length The double length should be within the normal length range. The contract should indicate the length of the single ruler and the multiple of the total length (for example, 3000mm * 3, that is, the 3 times of 3000mm, and the total length is 9000mm). In practice, the allowable positive deviation 20mm should be added on the basis of the total length, and the allowance for each single length should be left. Take the structural tube as an example, specify the margin allowance: the outer diameter is less than 159mm, 5 to 10mm, and the outer diameter >159mm is 10 ~ 15mm.
If there is no double length deviation and cutting allowance stipulated in the standard, it should be negotiated between the supplier and the buyer and marked in the contract. The length of the double foot length is the same as the length of the fixed length, which will result in a substantial reduction in the yield of the production enterprises. Therefore, it is reasonable for the manufacturers to raise the price, and the rate of increase is basically the same as that of the fixed length.
D. Range length: the length of the range is within the normal length range. When the user requests a certain fixed range length, it needs to be specified in the contract.
For example, the normal length is 3000 to 12000mm, and the range is 6000 to 8000mm or 8000 to 10000mm.
It can be seen that the length of the scope is more relaxed than the length of the fixed length and the length of the ruler, but it is much more stringent than the normal length. Therefore, it is reasonable for the manufacturers to raise the price. The increase in the price will generally be increased by about 4% on the base price.

Uneven wall thickness

The thickness of the steel tube can not be the same everywhere, and there is an unequal wall thickness phenomenon on the cross section and longitudinal pipe body, that is, the wall thickness is uneven. In order to control this inhomogeneity, the allowable index of uneven wall thickness is stipulated in some steel pipe standards, and the general rule does not exceed 80% of the wall thickness tolerance.


On the cross section of the circular steel tube, there is a phenomenon of unequal outer diameter, that is, there is a maximum outer diameter and minimum outer diameter which are not always perpendicular to each other. The difference between the maximum outer diameter and the minimum outer diameter is ellipticity (or Roundness ) In order to control the ellipticity, some steel pipe standards specify the allowable index of ellipticity. The general rule is that it does not exceed 80% of the outer diameter tolerance.

Bending degree

The length of the steel pipe is curvilinear, and its curve is called bending degree by numbers. The flexure specified in the standard is generally divided into two categories:

  • A. Local curvature: with a meter long ruler, the chord height (mm) is measured at the maximum bending point of the steel pipe, that is, the local curvature value, whose unit is mm/m, indicating the method such as 2.5mm/m. This method is also suitable for pipe end bending.
  • B. Full length bending: with a string, tighten from both ends of the tube, measure the maximum chord height (mm) of the steel pipe bending, and then convert it into the percentage of length (meter), that is, the full-length bending of the length of the steel pipe.

For example, the length of the steel pipe is 8m and the maximum chord height is 30mm.

0.03 x 8m x 100%=0.375%

What is the difference between a steel pipe and a steel tube?

What is a steel tube?

Steel tube can be divided into seamless steel tube and welded steel tube. Generally speaking, the outer diameter is less than 89 mm but the steel tube larger than 4 mm is referred to as the small diameter seamless steel tube.

20171214122739 74909 - What is a steel pipe?
Seamless steel tube is a kind of hollow steel, with round, square and seamless rectangular shape. Seamless steel tube consists of steel ingot or solid tube, and then hot rolled, cold rolled or cold drawn seamless steel tube. Cross section, mainly used for conveying fluids. Compared with solid steel such as round steel, the bending and torsional strength of seamless steel tube are lighter. Seamless steel tube is widely used in manufacturing structural parts and mechanical parts, such as oil drill pipe, automobile transmission shaft, bicycle bracket and steel structure scaffold.
According to different production process, seamless steel tube can be divided into hot rolled steel tube, cold rolled steel tube, cold drawn steel tube and extruded steel pipe. Usually, the hot rolled seamless tube is produced by the automatic rolling mill.
Check the solid tube and process its surface defect, and cut the tube into the required length to make the center point on the perforated surface of the solid tube; The solid tube is fed into a heating furnace. Drilling holes in the solid tube with a perforating machine; The solid tube of the solid tube is formed by grinding roller and perforating plug. The solid tube with empty cavity will be machined by automatic tube grinder. Use the rewire machine to make the pipe wall thick and uniform; The pipe is processed by the sizing machine or reducer to meet the specifications. It is an advanced method to manufacture hot rolled seamless steel tube with continuous tube rolling mill.

If you want to get a smaller, more high-quality seamless steel tube, you must use cold rolling process, cold drawing or combination of two processes. The cold rolling process is carried out in the double – roll rolling mill. The cold drawing process is carried out in single – chain or double – chain cold drawing machine. Extrusion can produce smaller diameter steel tubes.

Manufacturing Steel Tubes

There are three main methods of manufacturing tubing that lend their names to tubing classifications. 
·     Seamless. Seamless steel tubes are produced through extrusion. Extruded tubes can be formed in a hot or cold process. Long sections of steel bar are forced through a die that blocks out the intended shape of the tube. 
·     As-welded or electric resistant welded (ERW). This method involves passing a rolled sheet of steel through two weld rollers. The weld rollers have a groove around their circumference, through which the steel roll passes. There is a contact at the roll seam that transmits electricity at a high enough current to weld the seam closed. The resulting weld is very small.
·     Drawn-over-mandrel (DOM). A mandrel is a small piece of metal inserted into the tube to define a shape. It gives the tube extra support to prevent unwanted wrinkling during drawing. The tube is passed through a die that has a smaller diameter than the current tube size. As the tube is drawn, it shrinks to match the size of the die’s diameter. This process allows for tight tolerances and specifications.
Each of these manufacturing processes allows manufacturers the ability to form varying tube shapes and sizes. Tubes are not always cylindrical, and can be made in triangle, square or other polygonal shapes. Steel tubes destined for certain applications require extra processes as well. Applications involving hydrogen must be factory pre-cleaned or certified as instrument grade due to hydrogen’s reactive characteristics that can cause metal embrittlement or even explosions. 
Generally, a steel tube manufacturing process is chosen for its interaction with the type of steel to be used. Certain types of steel react poorly to heat because of carbon content, so they can’t be easily welded. Steel types also play a role in decisions regarding use in volatile applications.

Steel Tube Applications

Steel tubing is used in high numbers in plumbing applications. The reasons are threefold. One, stainless steel is very sturdy, composed of 80-90 percent steel and 10-20 percent chromium. Steel tubing can handle the types of pressures exerted by water upon plumbing structures. Additionally, the surface of stainless steel does not allow for much adherence by particles or bacteria, so purification processes will not pass on unintended detritus. Finally, steel does not contaminate drinking water.

Steel tubes are also used in a wide variety of structural applications, such as industrial and residential construction. Examples include fences, gates, railings, playground and athletic equipment. Steel is often used for construction tubes over other metals like aluminum when extra stress resistance is necessary. Steel tubes can also be used in automotive applications and even as parts of furniture.

What is a steel pipe?

Steel pipe is long that are used for a variety of purposesThey are produced by two distinct methods which result in either a welded or seamless pipe. In both methods, raw steel is first cast into a more workable starting form. It is then made into a pipe by stretching the steel out into a seamless tube or forcing the edges together and sealing them with a weld. The first methods for producing steel pipe were introduced in the early 1800s, and they have steadily evolved into the modern processes we use today. Each year, millions of tons of steel pipe are produced. Its versatility makes it the most often used product produced by the steel industry.

Steel pipes are found in a variety of places. Since they are strong, they are used underground for transporting water and gas throughout cities and towns. They are also employed in construction to protect electrical wires. While steel pipes are strong, they can also be lightweight. This makes them perfect for use in bicycle frame manufacture. Other places they find utility is in automobiles, refrigeration units, heating and plumbing systems, flagpoles, street lamps, and medicine to name a few.

What is the difference between a steel pipe and a steel tube?

One of the most frequent questions we get from customers is “What’s the difference between pipe and tube”? Although pipes and tubes may look similar, they are in fact quite different in nomenclature and sizing. Remember that pipes and tubes are rarely interchangeable. Here are some of the main differences between the two products:
(Source: Network Arrangement – 
Pipes are always round in shape. Tubes can be square, rectangular or round, as shown in the image to the right.
Tube is stronger than pipe.
Metal Types
Tube is available in hot rolled steel and cold rolled steel. Pipe is typically black steel (hot rolled). Both items can be galvanized.
Although copper and brass tubes can be shaped relatively easily, tubes are are typically rigid. Pipes, on the other hand, are invariably rigid and cannot be shaped without special equipment.

Tubing is generally used for structural purposes and the OD is an important and exact number. Tubing size is specified by OD and the wall thickness (WT); and the measured OD and stated OD are generally within very close tolerances of each other. Tubing is usually more expensive than pipe due to tighter manufacturing tolerances.
Interestingly, while the stated and measured OD’s of tubing are almost exact most of the time, copper tubing generally has a measured OD that is 1/8” larger than stated OD. As such, maybe it should be called copper pipe. However, stainless steel, aluminum, and steel tubing all have measured and stated OD’s that are exact or very close.
Pipes are categorized as tubular vessels used in pipeline and piping systems, and commonly transport gases or fluids. They are specified by “Nominal Pipe Size” (NPS) and Schedule (wall thickness). NPS is a size standard established by the American National Standards Institute (ANSI), and should NOT be confused with the various thread standards such as NPT and NPSC.
The manufacturing of Nominal Pipe Sizes from 1/8” to 12” is based on a standardized nominal outside diameter (OD) that is different from the measured OD. NPS pipe 14” and up have measured OD’s that correspond to the nominal size.
The reason many people, including plumbers, engineers, and others, mistakenly believe NPS refers to the ID on smaller pipes is because of how the standard was originally defined. The standardized OD was originally defined so that a pipe with a standardized OD and wall thickness typical of the period, would have a pipe ID that was approximately equal to the nominal size of the pipe. For example, 3” Schedule 40 NPS has an outside diameter and wall thickness that very, very roughly gives it an inside diameter of 3”. Regardless of the wall thickness the nominal OD of a pipe will not change.
There you have the differences explained as simply as possible. Of course if something isn’t clear certainly let us know.

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Img from

Pipe is typically available in larger sizes than tube.

Only steel pipes are pressure rated and intended to be used for the transference of fluids or gas. Tubes, on the other hand, are used in structural applications.
Telescoping abilities
Tube can be telescoped. Remember to account for the flash weld inside the tube. Pipe, on the other hand, does not have a flash weld. DOM (Drawn over Mandrel) Tube is the best material for telescoping because the inside flash weld has been removed.
Pipes are only provided with an inside (nominal) diameter and a “schedule” (which means wall thickness). There’s an easy way to remember this: since pipe is used to transfer fluids or gas, the size of the opening through which the fluids or gas can pass through is probably more important to you than the outer dimensions of the pipe. Tube measurements, on the other hand, are provided as an outside diameter and set ranges of wall thickness.

Pipe making of steel pipe

  • Both skelp and billets are used to make pipes. Skelp is made into welded pipe. It is first placed on an unwinding machine. As the spool of steel is unwound, it is heated. The steel is then passed through a series of grooved rollers. As it passes by, the rollers cause the edges of the skelp to curl together. This forms an unwelded pipe.
  • The steel next passes by welding electrodes. These devices seal the two ends of the pipe together. The welded seam is then passed through a high pressure roller which helps create a tight weld. The pipe is then cut to a desired length and stacked for further processing. Welded steel pipe is a continuous process and depending on the size of the pipe, it can be made as fast as 1,100 ft (335.3 m) per minute.
  • When seamless pipe is needed, square billets are used for production. They are heated and molded to form a cylinder shape, also called a round. The round is then put in a furnace where it is heated white-hot. The heated round is then rolled with great pressure. This high pressure rolling causes the billet to stretch out and a hole to form in the center. Since this hole is irregularly shaped, a bullet shaped piercer point is pushed through the middle of the billet as it is being rolled. After the piercing stage, the pipe may still be of irregular thickness and shape. To correct this it is passed through another series of rolling mills.

Quality Control of steel pipe

A variety of measures are taken to ensure that the finished steel pipe meets specifications. For example, x-ray gauges are used to regulate the thickness of the steel. The gauges work by utilizing two x rays. One ray is directed at a steel of known thickness. The other is directed at the passing steel on the production line. If there is any variance between the two rays, the gauge will automatically trigger a resizing of the rollers to compensate.

Pipes are also inspected for defects at the end of the process. One method of testing a pipe is by using a special machine. This machine fills the pipe with water and then increases the pressure to see if it holds. Defective pipes are returned for scrap.

Source: Network Arrangement – China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Please notice that you might be interested in the other technical articles we’ve published:

How do you make a steel pipe?

They are produced by two distinct methods which result in either a welded or seamless pipe. In both methods, raw steel is first cast into a more workable starting form. It is then made into a pipe by stretching the steel out into a seamless tube or forcing the edges together and sealing them with a weld.

What is the difference between schedule 40 and 80 steel pipe?

Schedule 40 pipe has thinner walls, so it is best for applications involving relatively low water pressure. Schedule 80 pipe has thicker walls and is able to withstand higher PSI (pounds per square inch).

What does Schedule 40 mean?

The pipe schedule refers to the pipe wall thickness. The higher the schedule, the thicker is the pipe wall. For example: 2-inch nominal size steel pipe: schedule 40 has a wall thickness of 0.154 inches and schedule 80 has a wall thickness of 0.218 inches.

What is pressure rating pipe?

All Pipes and various pipe fittings are most commonly classified based on their pressure temperature rating or commonly known as P-T Rating. The most common way of specifying pipe pressure temperature rating is given by ASME 16.5, using pound ratings (or lb ratings): #150, #300, #400, #600, #900, #1500 and #2500.

what is a steel pipe - What is a steel pipe?
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What is a steel pipe?
Steel pipe is a kind of steel with hollow circular section, whose length is far greater than the diameter or circumference.
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