Zagouan (Tunisia Aqueduc)

HISTORY OF PIPE

Since man invented agriculture and civilisation, he has sought to transport water from the nearby stream to irrigate his fields or to provide his home with potable water which took a lot of time and was laborious. His ingenuity gave birth to the invention and the pipe. Initially using available natural resources, early humans probably made the first bamboo pipe. Needing to move larger quantities of water, they next dug logs. The Egyptian and Aztec civilizations made clay pipes. The first metal pipes were made by the Greeks and Romans from lead and bronze. The use of iron as a material for making pipes originated with the invention of gunpowder. Gunpowder, of course, is not used to make iron, but gunpowder necessitated the invention of stronger cannons. Iron pipes soon followed. Eventually, exotic metals were developed and pipes became the highly specialized product they are today.

PIPING MATERIALS

The term pipe is used to refer to a hollow tubular body used to convey any product possessing flow characteristics such as those found in liquids, gases, vapors, liquefied solids and fine powders.

A complete list of materials used to make pipes would certainly be quite long. Some materials include:

• concrete,
• glass,
• the lead,
• brass,
• the copper,
• plastic,
• aluminum,
• cast iron,
• carbon steel,
• steel alloys,
• etc,

With such a wide range of materials available, selecting one suitable for a particular need can be confusing. It is for this reason that a thorough understanding of the hose's intended use is essential. Each material has limitations that may make it unsuitable for a given application. We will base our article on the most commonly used material in the piping industry which is carbon steel pipes.

MANUFACTURING METHODS

Carbon steel pipe can be manufactured using several different techniques, each of which produces a pipe with certain characteristics. These characteristics include strength, wall thickness, corrosion resistance, and temperature and pressure limitations. For example, pipes having the same wall thickness but manufactured by different methods may vary in strength and pressure limits.

The manufacturing methods we will mention include seamless, butt-welded, and spiral-welded pipe.

Seamless pipe is formed by piercing a solid, near-molten, steel rod, called a billet, with a mandrel to produce a pipe that has no seams or joints. The below Figure depicts the manufacturing process of seamless pipe.

Butt-welded pipe is formed by feeding hot steel plate through shapers that will roll it into a hollow circular shape. Forcibly squeezing the two ends of the plate together will produce a fused joint or seam. the below Figure shows the steel plate as it begins the process of forming butt-welded pipe.

Spiral-welded pipe is formed by twisting strips of metal into a spiral shape, similar to a barber’s pole, then welding where the edges join one another to form a seam.
This type of pipe is restricted to piping systems using low pressures due to its thin walls. Figure 2-3 shows spiral-welded pipe as it appears before welding.

The below Figure shows the three pipes previously described in their final form.

Each of the three methods for producing pipe has its advantages and disadvantages. Butt-welded pipe, for example, is formed from rolled plate that has a more uniform wall thickness and can be inspected for defects priorto forming and welding. This manufacturing method is particularly useful when thin walls and long lengths are needed. Because of the welded seam, however, there is always the possibility of defects that escape the numerous quality control checks performed during the manufacturing process.

As a result, The American National Standards Institute (ANSI) developed strict guidelines for the manufacture of pipe. Pressure Piping Code B 31 was written to govern the manufacture of pipe. In particular, code B31.1.0 assigns a strength factor of 85% for rolled pipe, 60% for spiral-welded and 100% efficiency for seamless pipe.

Generally, wider wall thicknesses are produced by the seamless method. However, for the many low-pressure uses of pipe, the continuous welded method is the most economical. Seamless pipe is produced in single and double random lengths. Single random lengths vary from 16
' to 20' long. Pipe 2" and below is found in double random lengths measuring 35' to 40
' long.

SIZING OF PIPE

As manufacturing methods differ, there are also different ways to categorize the size of a pipe. Pipe is identified by three different size categories:

• nominal pipe size,
• outside diameter,
• inside diameter,

(see Figure below).

Nominal pipe size (NPS) is used to describe a pipe by name only. In process piping, the term nominal refers to the name of the pipe, much like the name 2 × 4 given to a piece of lumber. The lumber does not actually measure 2" × 4", nor does a 6" pipe actually measure 6 in diameter. It’s just an easy way to identify lumber and pipe.

Outside diameter (OD) and inside diameter (ID), as their names imply, refer to pipe by their actual outside and inside measurements.

Pipe 1/8"  to 12" has an outside diameter greater than its nominal pipe size, while pipe 14" and above has an outside diameter equal to its nominal pipe size.

In process piping, the method of sizing pipe maintains a uniform outside diameter while varying the inside diameter. This method achieves the desired strength necessary for pipe to perform its intended function while operating under various temperatures and pressures.

WALL THICKNESS

Wall thickness is a term used to describe the thickness of the metal used to make a pipe. Wall thickness is also commonly referred to as a pipe’s weight. Originally manufactured in weights known as standard, extra strong, and double extra strong, pipe has since increased in complexity with the development of new chemical processes.

Commodities with ever-changing corrosive properties,high temperatures, and extreme pressures have necessitated the development of numerous additional selections of wall thicknesses for pipe. Now called schedules, these additional wall thicknesses allow a pipe to be selected to meet the exact requirements needed for safe operation.

An example of this variance in wall thickness is shown in Figure below.

As you can see in Table below, nominal size is not equal to either the actual OD or the ID for pipe 12" and smaller. It is simply a convenient method to use when referring to pipe.

As a piping drafter, you should be aware however, pipe 14" and larger is identified by its actual outside measurement. The chart in Table below shows typical pipe diameters and wall thicknesses. The following formula can be used to calculate a pipe’s inside diameter (ID):

ID = OD minus (2 × WALL THICKNESS)

Before selecting pipe, careful consideration must be given to its material, temperature and pressure allowances, corrosion resistance, and more. Buying and installing pipe that does not meet the minimum requirements can be dangerous and deadly. Using pipe that far exceeds what is required to do the job can result in tremendous cost overruns.

METHODS OF JOINING PIPE

There are several methods for joining pipe together. The three methods we will focus on are those most widely used in piping systems made of carbon steel, as shown in below  Figure. They are :

• butt-welded (BW),
• screwed (Scrd),
• socket-weld (SW).

Butt-Weld Connections

A butt-weld joint is made by welding the beveled ends of pipe together. Beveled ends (BE) indicate that the ends of the pipe are not cut square, but rather are cut or ground to have a tapered edge. In preparation for the welding process, a welder will separate two pieces of pipe by a 1/16"  space, known as a root gap. During the welding process, the two ends are drawn together and the 1/16"  gap disappears.

Screwed or Threaded Connections

Another common means of joining pipe is the threaded end (TE) connection. Typically used on pipe 3" and smaller, threaded connections are generally referred to as screwed pipe. With tapered grooves cut into the ends of a run of pipe, screwed pipe and screwed fittings can easily be assembled without welding or other permanent means of attachment. Screwed pipe and its mating fittings will have threads that are either male or female. Male threads are cut into the outside of a pipe or fitting, while female threads are cut into the inside of the fitting.

As screwed pipe and fittings are assembled, a short length of pipe is drawn into the fitting. This connection length is called a thread engagement. When drawing and dimensioning screwed pipe, a piping drafter must be aware of this lost length of pipe. As the diameter of the pipe increases, so will the length of the thread engagement. below Table provides a chart indicating the thread engagements for small bore pipe.

Socket-Weld Connections

The third method of joining carbon steel pipe is socket welding. When assembling pipe with socket-weld fittings, the pipe is inserted into the fitting before welding, unlike a butt-weld connection that has the pipe and fitting placed end-to-end. Inside the socket-weld fitting is a collar that prevents the pipe from being inserted too deeply into the fitting.

As with screwed connections, a short amount of pipe is lost when the socket-weld connections are made. Below Table provides the socket depths for pipe sizes through 3" in diameter. Before the weld is made, the pipe fitter will back the pipe off the collar approximately 1/8" to allow for heat expansion during the welding procedure. Pipe used for socket-weld connections will be prepared with a plain end. Plain end (PE) means the pipe is cut square, or perpendicular to, the long axis, unlike buttweld fittings that have beveled ends.

CAST IRON PIPE

Not all piping systems require pipe designed to withstand the extreme conditions found in process piping facilities. Cast iron pipe, which has been in use for centuries, is used primarily in gravity flow applications such as storm and sanitary sewers, and waste and vent piping installations. Residential, commercial, and industrial facilities routinely are built with some form of gravity flow systems. The corrosion resistance properties of cast iron pipe make it the ideal product for permanent below ground gravity flow installations.

The term cast iron refers to a large group of ferrous metals. Cast irons are primarily alloys of iron that contain more than 2% carbon and 1% or more silicon. Cast iron, like steel, does corrode. What makes cast iron different is its graphite content. As cast iron corrodes, an insoluble layer of graphite compounds is produced. The density and adherent strength of these compounds form a barrier around the pipe that prevents further corrosion. In steel this graphite content does not exist, and the compounds created during corrosion cannot bond together. Unable to adhere to the pipe, they flake off and expose an unprotected metal surface that perpetuates the corrosion cycle.

In tests of severely corroded cast iron pipe, the graphite compounds have withstood pressures of several hundred pounds per square inch, although corrosion had actually penetrated the pipe wall. Considering the low cost of raw manufacturing materials and the relative ease of manufacture, cast iron is the least expensive of the engineering metals. These benefits make cast iron the choice application in environments that demand good corrosion resistance.

Joining Cast Iron Pipe

Cast iron pipe is grouped into two basic categories:

• hub,
• spigot,
• hubless,
   

The hub, or bell, and spigot joint uses pipe with two different end types. The hub end of the pipe has an enlarged diameter, thus resembling a bell.

The spigot end of the adjoining pipe has a flat or plain-end shape. The spigot is inserted into the bell to establish a joint.

Two methods of preventing leaks on bell and spigot joints are compression and lead and oakum. The compression joint uses a one-piece rubber gasket to create a leak-proof seal. As shown in below Figure, when the spigot end of the pipe is placed into the hub containing a gasket, the joint is sealed by displacing and compressing the rubber gasket. Unlike welded pipe, this joint can absorb vibration and can be deflected up to 5° without leakage or failure.

The lead and oakum joint is made with oakum fiber and molten lead to create a strong, yet flexible, leak-proof and root-proof joint. When the molten lead is poured over the waterproof oakum fiber, which is a loose, oil laden, hemp-like packing material, the joint becomes completely sealed. Water will not leak out and, when used underground, roots cannot grow through the joints. See below Figure.

Hubless cast iron pipe uses pipe and fittings manufactured without a hub. The method of joining these pipe and fittings uses a hubless coupling that slips over the plain ends of the pipe and fittings and is tightened to seal the ends. Hubless cast iron pipe is made in only one wall thickness and ranges in diameter from 1½ to 10. Below Figure depicts the hubless cast iron pipe joint.

PLASTIC PIPE

The latest entry into the materials list for manufactur ing pipe is plastic. Not originally thought of as a product capable of performing in the environs of a piping process facility, plastic has emerged as a reliable, safe, and costeffective alternative material. There is a broad range of plastic compounds being developed today.
For piping systems, two categories are most effective:

• fluoroplastics,
• thermoplastics,

Fluoroplasticsare found in materials like PTFE, PVDF, ECTFE, CTFE, PFA, and FEP. As a group, fluoroplastics perform extremely well in aggressive chemical services at temperatures from –328 F° to +500 F°.

Thermoplastics are those that require melting during the manufacturing process.

These plastics can be welded or injection molded into shapes for machining into piping system components. For some piping systems, it is now inconceivable not to use plastics. Pipes made from plastic are replacing traditional, expensive materials like glass or ceramic-lined pipe. Some plastics such as UHMW PE, PVDF, CTFE, and nylon have such excellent wear resistance that they prove in Taber Abrasion Tests to be five to ten times better in this regard than 304 Stainless Steel. The Taber Abrasion Test cycles an abrasive wheel over the face of a
plate made of the material being tested. After 1,000 cycles of the wheel, the plate is measured to determine the amount of weight loss. below Table lists the results.

wall thicknesses are required, and leaks from high pressures and expansion and contraction are difficult to control. Joints made with solvent cement have proven more reliable. Though, once hardened, cemented joints cannot be disassembled. They offer good resistance to abrasive chemical and high-pressure commodities and are available in a large selection of fittings without the need of threads. Heat fusion must be performed on some plastic compounds that are resistant to chemical solvents. Pipe can either be butt-joined or socket-joined. Heat fusion can be used with thinner wall thicknesses and are pressure resistant beyond the burst pressure of the pipe. Socket fittings provide large surface contact between pipe and fittings and are resistant to separation. For this reason they cannot be disassembled. Though fabrication with plastic may sound simple, caution must be exercised when using plastic pipe.

The effectiveness of a particular grade of plastic must be tested before it is chosen for a particular service. Four important variables must be evaluated: chemical resistance, pressure limitations, temperature limitations, and stress. The various molecular components of plastics make them susceptible to chemical reactions with certain compounds. Hazardous mixtures must be avoided. Pressure and temperature limitations must be established for obvious reasons. Pipe that is overheated or pressurized beyond capacity can rupture, split, or burst. Stress, as applied to pipe, entails physical demands such as length of service, resistance to expansion and contraction, and fluctuations in pressure and temperature. Excessive stresses in the form of restricted expansion and contraction, and frequent or sudden changes in internal pressure and temperature must be avoided.

DRAWING PIPE

Pipe can be represented on drawings as either single line or double line. Pipe 12" and smaller is typically drawn single line and pipe 14" and larger is drawn double line. Single-line drawings are used to identify the center-line of the pipe. Double lines are used to represent thepipe’s nominal size diameter.

The standard scale used on piping drawings is 3/8"  = 1"
–0". Typically hand drawn, single-line pipe is drawn with a 0.9mm or a double wide 0.7mm fine-line lead holder.

When drawing single-line pipe with AutoCAD, a PLINE having a width of approximately .56" ( 9/16" ) is used on full-scale drawings or .0175" when drawing to 3/8" =1"
-0". Double-line pipe uses standard line widths to draw the pipe’s nominal size diameter. A centerline is used on all double pipe to allow for the placement of dimensions.

Below Figure provides several representations of pipe as it may appear on a drawing. When pipe is represented on a drawing, typically the pipe’s nominal size dimension is used to identify pipe size. One would find it difficult to draw a 4" pipe to its actual outside diameter of 4"
-0½" especially on such a small scale as 3/8"  = 1"
-0".

There are certain applications, however, when the pipe’s true outside diameter dimension is used to represent the pipe on a drawing. Drawings created with most software packages are an example. Piping softwareprograms draw with such accuracy that pipe is drawn using the actual outside diameter.
 NOTE: Pipe created by means other than a piping software program in this text will be drawn using nominal sizes. Be aware that drawings created with a piping software program use actual outside dimensions and will differ slightly from manual and AutoCAD generated drawings.