Ductile Iron Pipes
Ductile Iron Pipes
Ductile Iron pipes are pipes made of ductile iron. They are commonly used for potable water distribution and the pumping of slurries, sewage and process chemicals. Ductile iron is a spheroidized graphite cast iron. Ductile iron pipes are a direct development of earlier cast iron pipes which it has now almost replaced. The high level of dependability on ductile iron pipes is due to its various superior properties and it is a most sought after pipe for many applications.
History of ductile iron pipes
American Cast Iron Pipe Company pioneered the development of ductile iron pipes and produced experimental casts of these pipes as early as in 1948. The first shipment was made in 1955. Since then production of ductile iron pipes has grown steadily and it is now a predominant piping material for conveying water and other liquids.
As the name suggest, it is a cast iron having the property of ductility. This quality of cast iron long sought after by metallurgists, was realized in 1948. 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.
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. Tensile strength and yield strength of ductile iron is around 42 tons/Sq mm and 30 tons/Sq mm respectively and elongation is around 10 %. Microstructures of gray cast iron and ductile iron is shown in Fig. 1
Fig 1 Micro structure of gray iron (left) and ductile iron (right)
Ductile iron pipes production process
Ductile iron pipe is produced by a technique known as centrifugal casting, originally developed by Dimitr Sensaud deLavaud for the production of cast iron pipe in 1918. For the production of ductile iron pipe, the molten ductile iron is poured into a rapidly spinning water-cooled mold where centrifugal force causes an even spread of iron around the circumference.
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 molten iron is desulphurized and its composition and temperature is adjusted to precise levels in a coreless electric furnace before it is treated with magnesium. Magnesium addition is done under close control coditions 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, nonmetallic 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 controoled time and temperature cycles to produce optimum physical properties. After annealing pipe is taken to processing station where it is cleaned, machined, hydrostatistically tested, lined and coated and final inspection is done before despatch.
Ductile iron pipes are manufactued in size ranges of 75 mm to 1600 mm (inside diameter). The nominal thickness of these pipes can vary fron 6 mm to 22 mm depending on the size of the pipe and its pressure class. The normal pressure class of pipes are 150, 200, 250, 300 and 350 psi.
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.
- Ductile iron pipes also have the high impact strength and toughness to withstand shocks usually encountered in transportation, handling and installation. These characteristics also provlde added security against stresses by highway traffic and unexpected adverse forces.
- The pipe provides additional reliability and factor of safety for unusual conditions which are usually 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 beam load.
- Ductile iron pipes also have the high impact strength and toughness to withstand shocks usually encountered in transportation, handling and installation. These characteristics also provlde added security against stresses by water hammer, highway traffic and unexpected adverse forces.
- Energy consumption and accompanying pumping costs are directly related to head losses which in turn are directly related to inside diameters. 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 prvides good corrosion resistance for the pipes.
- Ductile iron pipes has tremendous bursting strength makes it ideally suited for high pressure applications. The pipe’s bursting strength 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.
Type of joints
The following are the types of joints usually used for ductile iron pipes
- Push on joint – This joint was developed in 1956. 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 they are bottle tight, push on joints can be used in wet trench conditions and in under water conditions.
- Mechanical joints – Although replaced by the push on joint for most applications, the mechanical joint is still used in some underground installations, primarily on fittings.
- Flanged joints – It is a rigid joint mainly used in aboveground installations. The flanged joint employs a gasket between two flanges that are joined with a series of nuts and bolts to produce a watertight seal.
- Restrained joints – It is a special type of push on or mechanical joint 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.
- Ball and socket joint – Boltless configurations of ball and socket joints provide flexibility (maximum deflection of 15 deg per joint) and restraint against joint separation. Theses joints are usually specified in subaqueous crossing, locations requiring large changes in alignment and grade, and in earth prone areas.
- Miscellaneous joints – A variety of joints most of which are modifications of the mechanical joint or stuffing box configuration have been developed for use with tapping sleeves, repair sleeves, couplings and connectors etc.