Introduction to Pipe
A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey products from one location to another. The products must be able to flow. These includes liquids and gases (fluids), slurries, powders and masses of small solids.
Difference between Pipe and Tube
In common usage the words pipe and tube are usually interchangeable, but in industry and engineering, the terms are uniquely defined.
In short: Tube is measured by outside diameter, the pipe is measured by nominal diameter.
Pipe is generally specified by a Nominal Pipe Size (NPS) indicating a constant Outside Diameter (OD) and a Schedule (SCH) that defines the pipe wall thickness. Nominal Pipe Size (NPS) and Outside Diameter (OD) values are not always equal.
- For NPS ⅛ to 12 – The NPS and OD values are different.
- For NPS 14 and above – The NPS and OD values are equal.
Pipes are usually rigid and have no flexibility.
Tube is most often specified by the Outside Diameter (OD) and wall thickness, but may be specified by any two of Outside Diameter (OD), Inside Diameter (ID), and Wall Thickness (WT). In tubing, OD is an important and exact number. The measured OD and stated OD are either exactly same or within very close tolerances of each other. Tubing is usually more expensive than pipe due to tighter manufacturing tolerances. Tube can be both rigid and flexible.
Pipes accommodate larger applications with sizes ranging from a half-inch to several feet. Tubes are generally used in applications that require smaller diameters. While 10-inch pipes are common, it’s rare that you will come across a 10-inch tube. The tolerances in various pipe dimensions are comparatively more relaxed as compared to that of tubes. Here the tolerance refers to diameter tolerance, wall thickness tolerance, straightness tolerance, roundness tolerance etc. Also generally the outer and inner surfaces of tubes are much more smoother than that of compared to pipes. Pipe assemblies are almost always constructed with the use of fittings such as elbows, tees, and so on, while tube may be formed or bent into custom configurations.
Pipes are made out of many types of materials including metals, alloys, ceramic, glass, fiberglass, concrete and plastic. Engineering companies have materials engineers to determine materials to be used in process piping systems. Material selection is mainly based upon the fluid service, design pressure, design temperature and cost with few more considerations. According to materials, majority of pipes used in oil and gas sector fall under following categories:
- Carbon Steel Pipes
- Stainless Steel Pipes
- Alloy Steel Pipes
- Galvanized Iron Pipes
For more details on steels, check out Most Common types of Steel in Process Piping Industry
Apart from above mentioned pipes Cast Iron Pipes, Cement Pipes, Plastic Pipes etc are also used in some engineering applications but the scope is much limited.
Pipes are usually produced by two distinct methods which result in either a welded pipe or a seamless pipe. In both methods, raw material is first cast into ingots. 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.
Seamless Pipe and Tube
Seamless pipe, as the name suggests, is a pipe without a seam or a weld-joint. Seamless Steel Pipe is made from a solid round steel ‘billet’ which is heated and pushed or pulled over a form until the steel is shaped into a hollow pipe. The hollow pipe is then extruded through a die and mandrel combination to reduce the outside diameter and to expand the inside diameter.
- For more on Seamless Pipe Manufacturing, check out Introduction to Seamless Pipe Manufacturing
Welded Pipe and Tube
Welded pipe is manufactured by rolling metal sheet and then welding it longitudinally across its length. The welded pipe manufacturing process begins with the rolling of a steel coil to the desired gauge thickness to form a flat strip. The flat strip is cut to the width that corresponds with the size of the pipe to be produced. The pipe is formed by moving the cut coil through a series of rollers, and the seam of the tube is then welded in a chemically neutral environment. In a welded pipe, the seam or the weld-joint is the weaker part of the pipe limiting the strength of the pipe to the strength of the weld-joint.
- For more on Welded Pipe Manufacturing, check out Introduction to Welded Pipe Manufacturing
Difference between Seamless and Welded Pipes and Tubes
- Seamless pipes don’t have a weld seam. Traditionally, the seam of welded pipes has been viewed as a weak spot, vulnerable to failure and corrosion. However, improvements in the manufacturing process for welded pipes have increased the strength and performance of the weld seam. Still seamless pipes prevent any possibility of a weak seam. In general, welded steel pipes can withhold 20% less working pressure than seamless ones.
- Seamless pipes has higher working pressures compared to welded pipes of the same material and size due to having no welded seam.
- Seamless pipes offers superior corrosion resistance because it has little potential for impurities, defects and differences in malleability that may be associated with the welded area in welded pipe.
- Seamless pipes do not require testing for weld integrity. This is a must for welded pipes.
- Seamless pipes harden as they are manufactured, so heat treatment after manufacturing is not required. Most welded pipes require heat treatment after manufacturing.
- The cost of welded pipes is less expensive than seamless pipes and is readily available in long continuous lengths.
- Seamless pipes can be substituted for welded pipe but welded pipes may not always be able to be substituted for seamless pipe due to the weld seam.
- The wall thickness of welded pipes is generally more consistent than that of seamless pipes. Its easier to get a uniform thick flat strip and produce a welded pipe than drawing a uniformly thick seamless pipe through extrusion. Also welded pipes surface finish quality is much better than seamless pipes.
- Longer procurement lead time is required for seamless pipes as compared to welded pipes.
Pipe Size and Schedule (Nominal Pipe Size and Schedule)
Pipe sizes are specified by a number of national and international standards. There are two common methods for designating pipe outside diameter (OD). The North American method is called Nominal Pipe Size (NPS ) and is based on inches (also frequently referred to as Nominal Bore (NB )). The European version is called Nominal Diameter (DN )and is based on millimeters.
Designating the outside diameter allows pipes of the same size to be fit together no matter what the wall thickness. Since the outside diameter is fixed for a given pipe size, the inside diameter will vary depending on the wall thickness of the pipe. For example, 2″ Schedule 80 pipe has thicker walls and therefore a smaller inside diameter than 2″ Schedule 40 pipe.
For more details on pipe sizes, check out Nominal Pipe Size and Schedule.
The pipe length measures the distance between the two ends of the pipe. Pipe length can be measured in meter or feet or any other unit of distance measurement. The most common terms used to designate the pipe lengths are
- Single Random Length (SRL)
- Double Random Length (DRL)
- Cut Length
Single Random Length (SRL)
Single Random Length (SRL) usually measure 5 to 7 meters in lengths. Pipe sizes below 2″ are generally manufactured in single random length.
Double Random Length (DRL)
Double Random Length (DRL) usually measure 11 to 13 meters in lengths. Pipe sizes above 2″ are generally manufactured in double random length.
Cut length pipes don’t have any specific defined length. They are cut into specific lengths according to the project requirements. Cut length pipes are generally more expensive but helps in reducing the pipe cutting time and resources on site and also reduces the small pipe lengths wastage arises after cutting SRL and DRL pipes to required lengths.
Pipe End Types
Pipe ends describe the configuration of pipe section end. Understanding pipe ends is important as it would be inappropriate to specify welded connection to a threaded pipe or threaded connection to a large diameter pipe etc. There are three main types of pipe ends.
- Plain Ends (PE)
- Threaded Ends (TE)
- Beveled Ends (BE)
Plain End Pipe
A Plain End (PE) pipe is a pipe that has been cut at 90° perpendicular to the pipe length run. The plain ends are generally used for the smaller diameters pipe systems and in combination with Slip On flanges and Socket Weld fittings and flanges.
Threaded End Pipe
A Threaded End (TE) pipe is a pipe that has tapered grooves cut into the ends of the pipe length run. Typically used on pipe size 3″ and smaller, threaded pipes and fittings are also referred to as screwed pipes and fittings.
Threaded pipe and Threaded fittings can easily be assembled without welding or other permanent means of connection. Threaded pipe and fittings have threads that are either male or female. Male threads are cut into the outer surface of a pipe or fitting, while female threads are cut into the inner surface of the fitting. As threaded pipe and fittings are assembled, two pieces are pulled together. The distance that is pulled together is called the thread engagement.
Beveled End Pipe
A Bevel End (BE) pipe is a pipe that has been cut at a bevel angle to the pipe length run. A bevel is a surface that is not at a right angle (perpendicular) to another surface. The standard angle on a pipe bevel is 37.5° but other non standard angles can also be produced. Beveling of pipe or tubing is done to prepare the ends for welding. The bevel ends are applied to all diameters of butt weld flanges or fittings, and will be directly welded to each other or to the pipe.
For more on Piping System, check out Introduction to Piping System.
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