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# The difference between steel tube and steel pipe

## Foreign trade alloy steel pipe: difference between T92 and P92

With regard to the difference between T/P, T-tube generally refers to tube, that is, small tube, which is generally used as superheater or reheater in boiler, and p-tube is large pipe, which is generally used on main steam pipe. For example, there is no difference in composition between T and P in 92 steel, but the process is different, so the structure is not the same when used.
T92 is used to manufacture superheater or reheater of boiler in thermal power plant. The outer surface of tube is corroded by high temperature and flue gas generated by fuel combustion, and the inner surface is eroded by high temperature and high pressure steam.
P92 is used to manufacture header and steam pipe, mainly used to transport high temperature and high pressure steam, which is eroded by high temperature and high pressure steam.
The control range of the two components is the same, but the production process is different. At present, T92 is produced by piercing + cold drawing or cold rolling, while P92 is produced by forging, hot rolling or extrusion. Due to different production processes, the microstructure of P92 is thicker than that of T92.

## The difference between tube and pipe

Tube and pipe both mean “pipe” in Chinese, which can not be distinguished. However, some English specifications have different requirements. For example, there are two basic formulas in section I of ASME code.
For tubing, the wall thickness is calculated according to the formula pg-27.2.1

t=PD÷(2S+P)+0.05D+e

For piping, the wall thickness is calculated according to the formula pg-27.2.2

t=PD÷(2SE+2yP)+C

The calculation requirements and results are different, which inevitably involves the understanding and difference between Chinese tube and pipe.

• The first method is based on pipe diameter. Tube is 5-inch or less than 6-inch, which is accepted by most people.
• The second is based on the shape of pipe cross-section, round section belongs to pipe, other shapes belong to tube. This statement is inferred from the passage of ANSI b31.1, but it is not strict.

There are other distinguishing methods, such as from the point of view of technology and average wall thickness tolerance. Most of these methods are speculative and have no authentic source.
Mr. Steiner (member of the national inspection and Regulation Commission of the United States) thinks that the problem of how to distinguish steel tube and steel pipe has occurred in many other countries. Both German and Spanish can only translate tube and pipe into the same word “pipe”, which is difficult to distinguish. In the application of PG-27.2.1 and PG-27.2.2 formulas, the boundary is very vague. In fact, ASME did not focus on the size of pipe diameter or pipe section, but focused on the nature of pipe or pipe use when formulating this rule. In the scope of boilers and pressure vessels, tube mainly focuses on the internal and external heat transfer and heat exchange purposes, while pipe is mainly based on the function of conveying the pressure medium from one place to another. In this way, the pipes of pipe nature are generally connected with various accessories (tee, elbow) and valves.
From this point of view, the economizer gas pipe and water wall pipe must belong to the tube type, while the main steam pipe, downcomer pipe and other pipes connected with tee and valve must belong to the pipe class.
One question: is tubing and piping determined only by pipe diameter or some other indicator?
Question 2: under what circumstances does the tubing formula PG-27.2.1 be used, and when the piping formula PG-27.2.2 be used?
The answer of ASME is uniform
The wall thickness formula PG-27.2.1 in ASME section I is applicable to piping in furnace, superheater and reheater pipe, economizer pipe and so on. These pipe elements are arranged and arranged in a certain space to transfer heat from the outside to the pressure medium in the pipe. In order to carry out such heat exchange function, the pipe belongs to tubing, which generally does not need to be equipped with accessories (TEE, elbow) and valve on the pipe element.
If the function of the pipe is to transport the pressure medium from one position to another, instead of focusing on the heat exchange between the medium inside the pipe and the outside during the transportation, the PG-27.2.2 formula is used, and the pipe element for this purpose is called piping.
In order to guide and control the flow rate of the conveying medium, the pipe system is equipped with accessories (tee, elbow) and valve, and the standard size of the pipe matches the imported accessory device. The wall thickness formula is checked by pg-27.2.2 or ANSI B31.3 formula 104.1.2.
It seems that this is the original intention of ASME to distinguish tube from pipe, and it is also the authentic source of all methods for distinguishing tube and pipe. All other comprehensible methods must be implemented according to this fundamental interpretation.
The above official ASME documents can help us to clarify the various ambiguities about tube and pipe. According to the arrangement of heating surface and calculation of heat transfer, the tubes are tube, and the pipes with system stress analysis (flexibility and stiffness calculation) are pipe according to the power piping specification of ANSI B31.1 in ASME.

Difference Tube Pipe
Section shape Tube has square nozzles, rectangular nozzles and circular nozzles Pipes are all round
Toughness There are rigid tubes as well as flexible tubes made of copper and brass Pipes are rigid and bend resistant
Pipe diameter (commonly used for instruments) Tube is Small diameter The pipe diameter is generally between 1 / 8 “~ 5 / 8” (= 3-16 mm) Pipe is The pipe diameter is generally between 15-1500 mm (= 1 / 2-60 “). There are also pipes smaller than or larger than this range, but the usage is less
Size standard features For tube OD For example, 1 / 4 “(6.25 mm) od tube is a tube with an outer diameter of 1 / 4”. Tube pays attention to the accuracy of outer diameter, because tube is connected by ferrule, and pipes with the same outer diameter can be connected with ferrule For pipe Indicates the pipe diameter specification. If DN50 means 2 “pipe, it can be expressed as Φ 57 × 3.5mm Φ fifty-seven × three point five m m It can also be expressed asΦ 60 × 3mm Φ sixty × three m m 。 The pipe pays attention to the wall thickness, because the pipe mainly transports the fluid and has high requirements for the internal pressure capacity
Market abbreviation The wall thickness of tube is thin It is called thin-walled tube in the market. It is expressed by its actual thickness (inch or mm), generally 1-2mm Pipe’s The wall thickness is standard In general, it is indicated by the wall thickness serial number (abbreviated as Sch. No., schedule number), which is also known as the pressure resistance grade number. From Sch. No. 5 to sch. No. XXS, different pipe diameters or materials have their own standard wall thickness series; or the pipes with the same Sch. No. but different pipe diameters or materials have different actual wall thickness.
Connection mode The tube wall is thin, and it is not allowed to thread on the tube. After annealing, the tube is connected by ferrule, also known as Crimping connection The connection modes of pipe include flange connection, thread connection And welding connection three, most of the field use flange connection, low pressure occasions allow to use threaded connection
Connection features Tubes with the same od can be connected with ferrule, which is fast and can be disassembled for many times Flange and thread shall be clearly marked with standard number, such as 2 “- ansi-150-rf
The pipe is bent According to the bending angle, it can be bent directly according to the requirements (without the use of pipe joints), which can reduce the potential leakage points Elbow joint is usually used for connection when bending (elbow pipe joint is generally threaded, if it is necessary to bend pipe, it should be arched thread)
Install A 6m pipe can be cut at will according to the specific situation of site construction A 6m pipe can be cut at will according to the specific situation of site construction
Applications Tube is generally used for heat tracing and thermal insulation of measuring pipeline, pneumatic signal pipeline of instrument system and sample pipeline of online analyzer Pipe is widely used in process pipeline and utility pipeline
Connection requirements Be able to apply double ferrule and single ferrule Do not connect with double ferrule or single ferrule
Pipe characteristics The tubes are generally seamless stainless steel, copper and brass tubes Seamless steel pipe and seamless steel pipe are selected according to the application

## The difference between tube and pipe, experts say:

I’ve seen a lot of discussions about this. The author of an article has consulted the ASME society
In fact, the ASME B31.1 standard has a clear description:

• 1. Different shapes: the tube has square nozzle, rectangular nozzle and round pipe; the pipe is round;
• 2. Different toughness: tube has rigid, flexible tube made of red copper and brass; pipe is rigid and bending resistant;
• 3. Different classification methods: tube by outer diameter and wall thickness; pipe by wall thickness code pipe schedule and nominal diameter (European standard) = National pipe size (American Standard)
• 4. Different use environment: tube is used for small pipe diameter, 10 inch tube is rare; pipe is used for large pipe diameter, 10 inch pipe is very common, from half inch to several feet pipe;
• 5. Different emphases are required: tube pays attention to the accuracy of outer diameter, because it involves pressure bearing, which is used for cooler tube, heat exchanger tube and boiler tube; pipe pays attention to wall thickness, because pipe mainly transports fluid and requires high internal pressure capacity;
• 6. The relationship between the wall thickness after the wall thickness grade is different: when the wall thickness level of tube is increased by 1 level, the wall thickness increases by 1 mm or 2 mm, and the increase is fixed; while the wall thickness of pipe is expressed by schedule, and the numerical relationship of each grade is uncertain. For example, the wall thickness of sch.20 with 1 / 8 “(NPS) = 6 mm (DN) is about 1.245 mm, while that of sch.30 is about 1.448 mm, but that of sch.40 is 1.727 mm The connection method is different: the tube can be connected quickly, and it can be connected by expanding, brazing or coupling; the connection of pipe is labor-saving, which can be welded, threaded or flanged.

Therefore, the problem is simple:

• Pipe is used to transport fluid.
• The tube is used to conduct heat.
• The outer diameter of 1-inch pipe is 33.4mm. The outer diameter of 2-inch pipe is 60.3mm.
• 1 inch tube, 25.4 mm. 2 inch tube, 50.8 mm.
• Pipe according to ASME b36.10 b36.19 is pipe.
• The one that doesn’t conform is tube.
• The round one may be pipe.
• But it’s not circular. It must be tube.

The general technical conditions of steel pipe are A530 and steel tube are A450. The main differences are as follows:
A530: the wall thickness is expressed by nominal wall thickness, the allowable deviation of wall thickness is ± 12.5%, the allowable deviation of outer diameter is loose (the minimum deviation above is 0.4mm), and there are no requirements for flaring test, crimping test and hardness.
A450: the wall thickness is expressed by the minimum wall thickness. The allowable deviation of the wall thickness is zero with a negative tolerance. The allowable deviation of the outer diameter is strictly controlled (the maximum deviation above is 0.4mm). There are requirements for flaring test, crimping test and hardness.
In addition, the specification of each pipe and tube has its own technical requirements, which should be read carefully. It is difficult to give a clear boundary between steel tube or steel pipe in general pipeline. However, steel tube must be used for expanding heat exchanger tube, furnace tube and instrument ferrule. Tube tube and steel pipe can be used for welded pipe. Due to strict manufacturing and inspection requirements, steel tube can be used in industries and mines with more stringent pressure and temperature. Of course, the price is higher.

There are about 12 ASTM stainless steel pipe standards, including seamless steel pipe and welded steel pipe. The steel is mainly austenitic stainless steel, which has three austenitic ferrite duplex stainless steel standards. Through studying the differences between pipe and tube standards, it is found that although the two standards have their own systems, the main technical contents of the standards are basically the same, except for the size and specification of steel pipes, the chemical composition of steel pipes, the requirements of shape and weight of steel pipes, mechanical properties and process tests, thermal treatment requirements, hydraulic tests and non-destructive tests are basically the same. Therefore, this paper will not introduce these contents which are basically the same as the tube standard, but only introduce the contents about the size and specification of pipe.
（1） According to the American national standard ANSI / ASME B36.10M “steel pipe” (first formulated by American Society of mechanical engineering in 1934), steel The size and specification of pipe are unified, standardized and serialized in the United States. The word “pipe” is used to distinguish “pipe” from “tube” and is applicable to those tubular products used for pipelines and pipeline systems.

## The main differences between “steel pipe” and “steel tube” are as follows:

(1) The size and specification of steel pipe are regulated by national standards. When formulating specific product standards, they can only be selected from B36.10M standard, and the size series cannot be customized.
The size and specification of “steel tube” are defined in each product standard according to the use of the product and the user’s requirements when formulating each product standard.
(2) Steel pipe is mainly used for pipeline or piping system under high temperature, low temperature or pressure. In B36.10M, the nominal size of steel pipe is expressed by the specification number NPs (nominal pipe side) (in inches). There are 44 NPs from 1 / 8-80, which are 1 / 8, 1 / 4, 3 / 8, 1 / 2, 3 / 4, 1, 1-1 / 4, 1-1 / 2, 2, 2-1 / 2, 3, 3-1 / 2, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20, 22, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 52, 56:6o, 64, 68, 72, 76, 80. Each size contains different sequences of the species. The value of NPs is actually the nominal outer diameter (0d) of the steel pipe. However, the value of NPs 1 / 8 ~ npsl2 is different from the nominal outer diameter of the standardized steel pipe, and the pipes of these specifications are actually produced according to the standard nominal outer diameter.
（2） Are these specifications in ANSI / ASME B36.10M applicable to stainless steel pipe?
no, it isn’t. On the basis of ANSI / Ames B36.10M, the size standard of ANSI / Ames b36.19m stainless steel pipe is formulated. The dimensions and specifications specified in the standard are listed in Table 1 and the nominal weight in Table 2. The standard also stipulates that the allowable dimension deviation shall be specified by each standard when the specific product standard is formulated, and its size depends on the manufacturing method and specific requirements of the steel pipe. In addition, the calculation method of steel pipe weight and the selection principle of steel pipe wall thickness are also specified.
Note: the weight listed in this table is the weight of carbon steel pipe. The difference in weight of stainless steel pipe is that the weight of ferrite stainless steel pipe is reduced by 5%, and that of austenitic stainless steel pipe is increased by 2%.
（3） Through the introduction of ANSI / Ames B36.10M and b36.19m, we have a general understanding of the main differences between steel tube and steel pipe. This difference is mainly reflected in the service conditions of the pipe and the requirements for the size and specification. Looking back on the specific standard of steel pipe, it is found that the specification of steel pipe size in the standard is mainly to determine the NPs number and specify the allowable size deviation. They are as follows:
(1) When ordering, the user shall indicate the NPs number, serial number of steel pipe wall thickness or average nominal wall thickness or minimum wall thickness in the contract.
(2) In a999 / a999m “general technical requirements for stainless steel pipe”, it is stipulated that the general size of stainless steel pipe is nps1 / 8 ~ nps48, and the corresponding outer diameter is 0.405 ~ 1219.2mm.
(3) The allowable deviations of outer diameter, wall thickness, inner diameter, length and weight are specified.
① Allowable deviation of wall thickness

• a. When the user orders with the minimum wall thickness, the wall thickness deviation is not allowed.
• b. The deviation between the minimum wall thickness at any point of the steel pipe (including seamless steel pipe and welded pipe) and the nominal wall thickness shall not be greater than l2.5% of the specified nominal wall thickness. The list of inspection values for minimum wall thickness is also specified in the appendix of the standard.
• C. For cast and boring steel pipes, the wall thickness deviation shall not exceed 3.2mm, and the specified wall thickness shall not have negative deviation. The wall thickness deviation of cast pipe shall not exceed 1.6 mm, and the specified wall thickness shall not have negative deviation.

② Allowable deviation of outer diameter
③ Allowable deviation of inner diameter
The inner diameter deviation of cast, boring and cast stainless steel pipe (pipe) shall not exceed 1.6 mm, and the specified inner diameter shall not have positive deviation. In addition, the standard also specifies the allowable deviation of steel pipe delivery length.
Recently, I read an old book and found the difference between pipe and tube in ASME:
Pipe and tube are currently translated as pipes, but pipe is generally expressed by inner diameter and pipe wall thickness number (SCH), including welded pipe and seamless pipe. The main function of pipe is to transport fluid. While the pipe represented by outer diameter and wall thickness also includes welded pipe and seamless pipe. The main function of pipe is to transfer heat, such as heat exchanger tube and condenser tube. In welding procedure qualification, the focus is on The pipe and tube can be used in the process evaluation because there is no distinction between the pipe function and the pipe function. In ASME section II, pipe can be regarded as a fluid delivery pipe and tube as a heat transfer tube.

## Standard

### American Standard for steel pipe:

• ASTM A53 “uncoated and hot dip galvanized welded and seamless nominal pipe”; API 5L “Specification for pipeline steel”;
• ASTM A106 seamless carbon steel nominal pipe for high temperature service;
• ASTM A312 “seamless and welded austenitic stainless steel nominal pipe”;
• ASTM A333 “seamless and welded nominal steel tubes for low temperature service”;
• ASTM A335 Seamless Ferritic Alloy Steel nominal pipe for high temperature service;
• ASTM A358 “arc fusion welded austenitic chromium nickel alloy steel nominal pipe for high temperature service” – plate welded pipe;
• ASTM A671 “nominal pipe for electric fusion welding at room and low temperature” – plate welded pipe;
• ASTM A672 “nominal pipe for electric fusion welding at medium temperature and high pressure” – plate welded pipe;
• ASTM A691 “steel nominal tubes for electric fusion welding of carbon and alloy steels for high temperature and high pressure service” – plate welded pipe;
• ASTM A530 General requirements for carbon and alloy steel nominal pipes for special purposes;

### American Standard for steel tube

• ASTM A179 seamless cold drawn mild steel tubes for heat exchangers and condensers;
• ASTM A213 Seamless Ferritic and austenitic alloy steel tubes for boilers, superheaters and heat exchangers;
• ASTM A249 “welded austenitic steel tubes for boilers, superheaters, heat exchangers and condensers”;
• ASTM A334 seamless and welded carbon and alloy steel tubes for low temperature service;
• ASTM A450 General requirements for carbon steel, ferritic alloy steel and austenitic alloy steel pipes;

### Chinese national standard for pipe:

1. GB / T 3091-2001 welded steel pipe for low pressure fluid transportation;
2. GB / T 8163-1999 seamless steel tubes for fluid transportation;
3. GB / T 9711.1-1997 petroleum and natural gas industries – steel pipes for transmission – Technical delivery conditions – Part 1: Grade a steel pipes;
4. GB / T 12771-2008 stainless steel welded steel pipe for fluid transportation;
5. GB / T 14976-2002 stainless steel seamless steel tubes for fluid transportation;
6. GB / T 18984-2003 seamless steel pipe for low temperature pipeline;
7. Hg / T 20537.3-1992 “technical requirements for austenitic stainless steel welded steel pipe for chemical plant”;
8. Hg / T 20537.4-1992 “technical requirements for austenitic stainless steel large diameter welded steel pipe for chemical plant”;

In addition, although the following national standards do not distinguish pipe from tube, they can also be used for pipe.
For example:

• GB 3087 seamless steel tubes for low and medium pressure boilers;
• GB 5310 seamless steel tubes for high pressure boilers;
• GB 6479 seamless steel tubes for high pressure fertilizer equipment;
• GB 9948 seamless steel tubes for petroleum cracking;

However, the diameter expression of pipe components is not always nominal diameter, and pipe for pipe is not all pipe. For example, high temperature and high pressure pipes and clean pipes are often expressed in the form of outer diameter x wall thickness. Pipes can be divided into two types, pipe and tube. However, they can’t be separated when expressed in Chinese. They are all called Guanzi.
In ASTM and JIS steel pipe standard system, pipe and tube have obvious differences in technical requirements and dimensional tolerance. Before the 1990s, China’s steel pipe standard system did not distinguish between pipe and tube. In recent years, China’s steel pipe standards are gradually developing towards the direction of separating pipe and tube standards.

– Yaang Pipe Industry Co., Limited (www.epowermetals.com)

(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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