Ductile Iron Pipes
Ductile Iron Pipes
Ductile Iron pipes are pipes made of ductile iron. Ductile iron is a spheroidized graphite cast iron. The high degree of dependability of the ductile iron is primarily due to its high strength, durability, and impact and corrosion resistance. Ductile iron pipes are normally used for potable water distribution and the pumping of slurries, sewage and process chemicals. These iron pipes are a direct development of earlier cast iron pipes which it has now almost replaced. The high level of dependability of the ductile iron pipes is because of its various superior properties. These pipes are the most sought after pipes for several applications.
Ductile iron pipe is designed as a flexible conduit and there are distinct national standards available for these pipes. Casting processes are similar to those for gray cast iron pipe; however, ductile iron pipe needs significant refinement in casting, higher quality raw materials, treatment with special additives, and a higher level of quality control. It has ultimate tensile strength of 415 MPa, yield strength of 290 MPa with 10 % elongation. It is available with standard joints and many types of special joints. The high level of operational dependability of ductile iron pipe stems from its major advantages of machinability, impact and corrosion resistance, and high strength, thus making it a pipe material which is rugged and durable with high resistance to impact needing virtually no maintenance. For these reasons, it has completely replaced cast iron as the choice piping material of waterworks professionals.
Ductile iron pipes are designed and manufactured to the most stringent standards of the industry. These pipes resist damage during shipping and handling, and, once installed, withstand the most demanding operating conditions, including water hammer, frozen ground, deep trenches, areas of high water table and heavy traffic, river crossings, pipe-on-supports, rocky trenches, and areas of shifting, expansive, and unstable soils. The installation of these pipes is simple and requires no complex line-and grade drawings or laying schedules. These pipes can be installed in a wide variety of trench and bedding conditions and can be easily cut in the field. Direct tapping, even in straight lines, does not affect the integrity of the pipes.
Ductile iron pipe is produced in 5.5 meters or 6 meters nominal laying lengths and in size ranges of 80 mm to 2,000 mm (inside diameter). The nominal thickness of these pipes can vary from 6 mm to 22 mm depending on the size of the pipe and its pressure class. The normal pressure class of pipes are 150 psi, 200 psi, 250 psi, 300 psi, and 350 psi (1 MPa, 1.4 MPa, 1.7 MPa, 2.1 MPa, and 2.4 MPa respectively). The pipe is furnished with several different types of joints, and a wide variety of standard fittings are available without special order. Although the ductile iron pipe is normally furnished with cement-mortar lining, optional internal linings are also available for a wide range of special applications. Further, the pipe’s normally larger-than-nominal inside diameters, combined with its high flow coefficient, offer substantial savings on pumping and power costs over the life of the pipeline.
History of ductile iron pipes
American Cast Iron Pipe Company pioneered the development of ductile iron pipes. The first ductile iron pipe was cast experimentally in 1948. Years of metallurgical, casting, and quality control refinement followed. In 1955 ductile iron pipe was introduced into the market-place when the first shipment was made. Since then production of ductile iron pipes has grown steadily and it is now a predominant piping material for conveying water and other liquids.
The advent of ductile-iron pipe in 1948 was one of the most significant developments in the pressure pipe industry. Quickly recognized as a pipe material with all the good qualities of gray cast-iron pipe plus additional strength and ductility, it was first used for special and severe conditions of high pressure, water hammer, and excessive external loads. With an experience of a large number of years, it has proved to be virtually trouble and maintenance free as an underground pressure pipe material, and at present it is used in the transportation of raw and potable water, sewage, slurries, and process chemicals. The strength and rugged durability of ductile iron pipe result in a high level of operational reliability.
The phenomenal strength and impact resistance of the ductile iron pipe, along with many other advantages, created a rapid increase in demand for this pipe as engineers and utility officials realized that it can be transported, handled and installed with virtually no damage to the pipe. In service, ductile iron pipe has shown that expenditure of repair is practically eliminated. Its corrosion resistance exceeds that of gray cast iron, a pipe product with a reputation of centuries of service in the transmission and distribution of water and gas. Evidence of wide acceptance of ductile iron pipe is demonstrated by its adoption throughout the world as an accepted underground pressure pipe for the transportation of water, wastes, and many industrial materials.
Ductile iron pipe is manufactured in accordance with the national standards. The outstanding characteristics of ductile iron pipe result from its unique metallurgical properties achieved during the manufacturing process. Ductile iron pipe with dimensions which make it compatible with gray iron pipe and fittings is available with a wide variety of joints which equip it for specific types of service. As with gray cast iron pipe, protection from severe environmental factors is available. Ductile iron pipe provides versatility in design and while it can be designed in accordance with details of different standards, it is not confined to a specific system. Engineers can choose other use conditions observing the available strength and ductility of ductile iron pipe.
Because of its high strength and ability to carry great earth loads, ductile iron pipe has found wide acceptance in service as gravity flow waste lines and culvert pipes. In sewer service, ductile iron pipe with standard push-on joints has virtually eliminated infiltration as well as exfiltration. Its resistance to impact, convenient pipe lengths, and easy joint assembly have caused engineers and those responsible for construction to become aware of its many advantages, and its use in sewer service has increased rapidly in recent years.
Ductile iron pipe joints are ideally suited for gravity flow pipelines as shown by the test results of internal pressure of 6.9 MPa, external pressures of 3 MPa, and negative air pressure of 1 MPa with no leakage and no infiltration. Ductile iron gravity flow pipelines are required to be designed and specified in accordance with several national standards. Exceptional ring and beam strengths make ductile iron pipe an ideal structure for culvert pipes.
An advantage of ductile iron pipe in sewer service is the fact that it’s inside diameter is greater than nominal. This results in greater flow capacity for a given pipe size, and hence considerable savings can be achieved. Ductile iron pipe is available with standard shop linings or cement-mortar linings for normal domestic sewage. Special linings are available for more aggressive wastes. Another valuable feature of ductile iron pipe in sewer service is its ability to withstand great depths of earth cover under nominal laying conditions.
Ductile iron production process was developed by ‘The International Nickel Company’ in 1948. It is normally defined as cast iron with primary graphite in the nodular or spheroidal form. This change in the graphite form is accomplished by adding an inoculant, normally magnesium, to molten iron of appropriate composition. The matrix is predominantly ferritic for maximum impact resistance and ductility. The chemical composition of ductile iron is similar to gray cast iron except for the inoculant addition. The chemistry is adjusted to meet the physical test requirements of the standards.
Ductile iron was previously known as nodular iron or spheroidal-graphite iron (the international term is ductile iron). As the name ductile iron suggests this grade of cast iron has a high degree of ductility. The main characteristic of this material is the structure of the graphite, which is present as tiny spheres (nodules). In ductile iron, eutectic graphite separates from the liquid iron during solidification in a manner similar to that in which eutectic graphite separates in gray cast iron. However, because of additives introduced in the liquid iron before casting, the graphite grows as spheres, rather than as flakes the form characteristic of the gray iron. Since ductile iron contains spheroidal graphite, it is much stronger and has higher elongation than gray iron or malleable iron. It can be considered as a natural composite in which the spheroidal graphite imparts unique properties to ductile iron.
Ductile iron not only retains all of the attractive qualities of gray iron, such as machinability and corrosion resistance, but also provides additional strength, toughness, and ductility. Although its chemical properties are similar to those of gray Iron, ductile iron incorporates significant casting refinements, additional metallurgical processes, and superior quality control. Due to its spheroidal graphite form, ductile iron has around twice the strength of gray iron as determined by tensile, beam, ring bending, and bursting tests. Its impact strength and elongation are many times greater than the impact strength and elongation of gray iron.
Ductile iron is a family of cast graphitic irons which possess high strength, ductility and resistance to shock. Annealed cast ductile iron can be bent, twisted or deformed without fracturing. Its strength, toughness, and ductility duplicate many grades of steel and far exceed those of standard gray irons. Yet it possesses the advantages of design flexibility and low cost casting procedures similar to gray iron. The difference between ductile iron and gray iron is in the graphite formation. Ordinary gray iron is characterized by a random flake graphite pattern in the metal. In ductile iron the addition of a few hundredths of 1 % of magnesium or cerium causes the graphite to form in small spheroids rather than flakes. These create fewer discontinuities in the structure of the metal and produce a stronger, more ductile iron. This nodular graphite structure inhibits the creation of linear cracks hence the ability to withstand distortion.
Ductile iron is produced by treating molten low sulphur base iron with magnesium under closely controlled conditions. The startling change in the metal is characterized by the free graphite in ductile iron being deposited in spheroidal or nodular form instead flake form as gray cast iron. With the free graphite in nodular form, the continuity of the metal matrix is at a maximum, accounting for the formation of a far stronger and tougher ductile material greatly exceeding gray iron in strength, in ductility and in impact characteristics.
Ductile iron is acclaimed as one of the most significant developments of the century and has had an increasing impact on the industry. Besides the property of ductility, ductile iron has, in addition, strength and impact resistance much greater than that of gray iron. Ductile iron also retains the corrosion resistance property of gray iron hence it is an ideal material for pipes. Microstructure of ductile iron is shown in Fig 1.
Fig 1 Microstructure of ductile iron
Production process of ductile iron pipes
Ductile iron pipe is produced by a casting technique known as centrifugal casting. The art of centrifugal casting of hollow metal objects is quite old and has been practiced on a commercial scale since the beginning of the nineteenth century. The earliest English patent (Eckert) dates as far back as 1809. The earliest American patent was issued in 1848. At about the same time Andrew Shanks, in London, England, began to make cast-iron pipe 12 feet (3.66 meters) long and 3 inch (76 millimetres) in diameter by pouring molten metal into a spinning wrought-iron mould. His process was described in America in the Scientific American of December 1, 1849, and it is of interest to note that in its basic features of design the Shanks machine does not differ in any way from the great majority of machines working with a cold mould at the present day.
The centrifugal casting methods used in the manufacture of ductile iron pipe have been in the process of commercial development and refinement since 1925. In the centrifugal casting process, a controlled amount of molten metal having the proper characteristics is introduced into a rotating metal or sand-lined mould, fitted with a socket core, in such way as to distribute the metal over the interior of the mould surface by centrifugal force. This force holds the metal in place until solidification occurs. Pipe removed from the mould is furnace annealed to produce the prescribed physical and mechanical properties and eliminate any casting stresses which may have been present. After cleaning, hydrostatic testing, dimensional gauging, weighing, coating, lining and marking, the pipe is ready for shipment. Fig 2 shows the flowsheet of the manufacturing process for ductile iron pipe.
Fig 2 Flowsheet of the manufacturing process for ductile iron pipe
Ductile iron pipes are produced either from virgin metal produced in a blast furnace from iron ore or from metal produced by recyclinng of iron and steel scrap in a melting furnace which can be a cupola or electric furnace. The liquid iron is desulphurized and its composition and temperature is adjusted to precise levels in a coreless electric induction furnace before it is treated with magnesium. Magnesium addition is done under close control conditions since it is the most important step in the production of ductile iron pipes. The magnesium treated metal is introduced into a horizontal rotating mould with the quantity of the metal poured controlling the pipe thickness. The centrifugal force generated by rotation holds the metal against the mould wall and forces lighter, non-metallic impurities to the inside of the pipe to be removed in the cleaning process. After the iron has solidified, the casting machine is stopped and the pipe is stripped from the mould. The pipe is then annealed with precisely controlled time and temperature cycles to produce optimum physical properties. After annealing, the pipe is taken to the processing station where it is cleaned, machined, hydro-statistically tested, lined, and coated and then final inspection is done before dispatch.
Characteristics of ductile iron pipe
Ductile iron pipes have the combination of chemical analysis and heat treatment which give them the desirable combination of high strength and good ductility. These pipes also have the high impact strength and toughness to withstand shocks normally encountered in transportation, handling, and installation. These characteristics also provide added security against stresses by water hammering, highway traffic and unexpected adverse forces.
The pipe provides additional reliability and factor of safety for unusual conditions which are normally encountered when the pipes are buried in the earth due to extreme traffic loads, heavy backfill, or earth movements. Beam tests, free bend tests, and ring tests demonstrate the suitability of ductile iron pipes. Ductile iron pipe ability to bend under load before ultimate failure, greatly increases its resistance to the beam load.
Energy consumption and accompanying pumping costs are directly related to head losses which in turn are directly related to the inside diameter of the pipe. Ductile iron pipes having inside diameters greater than nominal diameters and high flow coefficient result into significant energy savings over the years.
Extensive field tests have shown that ductile iron pipes have very good soil corrosion resistance. Ductile iron pipes need no external corrosion protection. In most areas of highly corrosive soils, simple, economical polyethelene encasement provides good corrosion resistance for the pipes.
Ductile iron pipes has tremendous bursting strength which makes it ideally suited for high pressure applications. The bursting strength of the pipe also provides an additional safety factor against water hammer.
Ductile iron pipes are easy to install in the field since a wide variety of joints and standard fittings are available. The pipes can also be cut and direct tapped in the field. Ductile iron pipes once installed, requires little or no maintenance over the life of the pipeline.
Ductile Iron pipe can be welded successfully to produce welds which have mechanical properties comparable to those of the base iron. As with any base material, the success of welding ductile Iron pipe depends on suitable equipment, correct procedures, qualified welders, and effective quality control procedures.
Type of joints
In addition to the further development of ductile iron pipe and its production process and in order to adapt the pipe to the increasing operating pressures in the piping networks, improvements have also been made to the joining technology of the pipes. Presently, all joints available for use with ductile iron pipe are designed to be bottle-tight and can be easily assembled. Some of the most common joints normally used for ductile iron pipes are given below.
Packed socket joint – Until the introduction of rubber-sealed socket joints (around 1930) pipes and fittings in cast iron were mainly joined by means of packed sockets. These were not restrained joints. The packed socket joint is rigid and is not tight in case of movement.
The pipe joint is of immense importance for the reliability of a pipeline. In the area of iron pipes, the advantages of rubber-sealed socket joints were recognized very early on. After all, the rubber seal gives the pipeline flexibility which allows it to adapt to the stresses produced by traffic vibrations and ground movements as well as strain and compression forces in the pipe run without any adverse effects on tightness at the connection points.
Screwed socket joints – The screwed socket joint has been used since a long time. Structural design and dimensioning are determined in detail in standards. The inside of the socket and the outside of the screw ring are buttress-threaded according to the direction of the load. A screw ring axially compresses the elastic sealing element, which has hard rubber protective edges at the front and the back, into its seating through a sliding ring. This produces the seal between the socket and the spigot end. The protective edges prevent the soft rubber part under compression in the middle from being forced out into the sealing gap.
The necessary angular deflections – the joint allows deflections of upto 3 degrees from straightness – are only produced during installation after tightening the screw ring. These days the screwed socket joint is only used for fittings in the range from DN 40 to DN 400. A restrained joint can be produced by using additional elements.
Bolted gland joints – The bolted gland joint has been in use also since a long time. Its dimensional construction is covered in standards. Here, it is the gland which applies pressure through T-head bolts onto the wedge-shaped sealing element, which has a hard rubber protective edge on the front. The sealing principle is practically the same as with the screwed socket joint. The necessary angular deflections – the joint also allows deflections of upto 3 degrees from straightness – are only produced after the joint has been assembled. These days the bolted gland joint is only used in combination with certain types of fittings in the range from DN 500 to DN 1000.
Push on joint – This joint was developed in 1956. Nowadays, rubber-sealed push-on joints are predominantly used for the ductile iron pipelines. The sealing element is produced from a mixture of hard and soft rubber. The rubber gasketed push on joint is the fastest and easiest to assemble joint and hence it is the most widely used joint for water and waste water service today. Because the joint is bottle tight, push on joints can be used in wet trench conditions and in under water conditions. These joints are normally used for the underground ductile iron pipelines (pipes, fittings, valves). These are flexible, rubber-sealed joints which offer both technical and economic advantages for installation.
Mechanical joints – The mechanical joint was developed for gas industry use in the late 1920s but has since been used extensively in the water industry. This joint has standardized dimensions and uses the basic principle of the stuffing box and gland, with a rubber gasket being compressed by the gland. This joint has been replaced by the push on joint for most of the applications. However, the mechanical joint is still used in some underground installations, primarily on fittings.
Flanged joints – Flanged joint is one of the oldest types of joints. It is a rigid joint which is mainly used in the above ground installations such as open bays and pipe galleries. A flanged joint consists of two flanges, a sealing element, and a certain number of hexagon head bolts with nuts and washers. The material of the sealing element depends on the purpose of use in each case. The flanges of ductile iron pipes, fittings and valves are provided with raised sealing faces. It produces a water-tight seal.
Flanged joints are restrained ones, but not moveable joints, and they transfer longitudinal and bending stresses from pipe to pipe. The individual advantages of push-on joints and flanged joints can be combined by installing flanged sockets and flanged spigots on the flanges of fittings.
Flanged joints tend to be favoured for pipelines above ground, as are used for example in pumping houses, waterworks, or elevated tanks. As regards shut-off valves in urban water supply networks, for decades the flanged joint was also normally used in underground pipelines for reasons of maintenance and repair.
Restrained joints – These are special type of push on or mechanical joints designed to provide longitudinal restraint. The restrained joint is used in conjunction with or in lieu of thrust blocks to provide restraint against thrust forces due to the internal pressures. Various types of restrained joints are available for the entire range of the sizes of ductile iron pipes.
Ball and socket joint – Boltless configurations of ball and socket joints provide flexibility (maximum deflection of 15 degrees per joint) and restraint against joint separation. Theses joints are frequently specified in sub-aqueous crossings, locations needing large changes in alignment and grade, and in geologically hazardous (earthquake prone) areas.
Miscellaneous joints – These are a variety of miscellaneous joints most of which are modifications of the mechanical joint. Miscellaneous joints with stuffing box configuration have been developed for use with tapping
A variety of ductile iron pipe fittings equipped with mechanical, push-on, restrained, or flanged joints, or plain ends are readily available for installation at the job site. This wide variety of fittings, combined with the ability of ductile iron pipe to be cut in the field, enables installers to bypass unexpected obstacles during installation. This is a major advantage of ductile iron over other materials. Standard ductile iron fittings for water and other liquids are manufactured in accordance with the national standards.
The fittings can be normal fittings or compact fittings. Although shorter and lighter than normal fittings, compact fittings incorporate the high strength of the ductile iron while maintaining comparable pressure ratings. A variety of configurations are available in these two types of fittings. Special fittings such as long radius fittings, reducing elbows, reducing-on-the-run tees, side outlet fittings, eccentric reducers, wall pipe, welded-on bosses, dual purpose and transition sleeves, and lateral and true wyes are also available from some manufacturers.
Ductile iron pipe installed in water systems today is normally furnished with a cement-mortar lining. Cement mortar lining prevents tuberculation by creating a high pH condition at the pipe wall as well as providing a barrier between the water and the pipe wall. Additionally, cement linings create a smooth surface inside the pipe, thus maintaining a hydraulically smooth flow surface, which means less friction and thus less head loss. The Hazen-Williams coefficient, or ‘C’ value, is 140 for ductile iron pipe with cement-mortar lining.
The centrifugal process of applying cement-mortar linings is used in modern practice. By using this method, excellent quality control of the cement mortar and the centrifugal lining operation can be maintained. Centrifugal lining enables the pipe manufacturer to produce cement-lined pipe of the highest quality—smooth, free of defects and meeting the rigid requirements of the standards. The lined pipe are stored in a moist atmosphere during the curing period, or given a seal coating to prevent too rapid loss of moisture. The cement lining adheres to the wall of the pipe so that the pipe can be cut and tapped without damage to the lining.
Special linings are available for service requirements where cement-mortar linings are not applicable. These linings include polyethylene, epoxy, petroleum asphaltic coal tar epoxy, and others. The pipe manufacturers are to be consulted regarding the availability and proper use of these linings.