The Basics of Ductile Iron Pipe
Introduced into the U.S. marketplace in 1955, ductile iron pipe (DIP) is pressure pipe commonly used for pressurized water transmission and sewer force mains.
The predominant wall material is ductile iron, a spheroidized graphite cast iron, although an internal cement mortar lining usually serves to inhibit corrosion from the fluid being distributed. Various types of external coating are also used to inhibit corrosion from the environment.
Ductile iron pipe is a direct development of earlier cast iron pipe. Ductile iron has proven to be a better pipe material, being stronger and more fracture resistant; however, like most ferrous materials, it is susceptible to corrosion and retains some brittle characteristics.
Although ductile iron is commonly used in water systems, it has several advantages and disadvantages that owners should factor into its long-term management.
Advantages
- Greater ductility than cast iron
- Greater impact resistance than cast iron
- Greater strength than cast iron
- Lighter and easier to lay than cast iron
- Simplicity of joints
- Joints can accommodate some angular deflection
- Low pumping costs due to large nominal inside diameter
Disadvantages
- Similar corrosion rate to cast iron and steel pipe
- Prone to external and internal corrosion
- Internal and external protection systems required
- Limited number of protection systems available
- Polyethylene wrappings can be damaged
How Does DIP Fail?
Like every pipe material, DIP has specific failure modes and indicators that the pipe is nearing failure. Corrosion is the primary cause of failure for any metallic pipe material. There are three types of failures primarily seen in DIP.
Break Failure
- Internal pitting and graphitization corrosion can lead to ductile iron failure. Indicators that this is occurring are lining damage, wall loss from internal pitting, graphitization, leaks, excessive external loads, and pressure variations.
- External pitting and graphitization corrosion is another type of break failure in DIP. Indicators that this is occurring are damage to the pipeline coating, wall loss from external pits, graphitization (hard to detect), leaks, external loads, and pressure variations.
Structural Failure
- Pipe movements from thermal variations, seismic shifts, or external loading can lead to structural failure. The indicators that this type of problem may be occurring are joint leaks, poor bedding, or pipe movement.
- Thermal contraction, poor support leading to movement, or internal pressure can also result in structural failure. The indicators of this failure mode are circumferential cracks, frost regions, leaks, pipe movements, and expansive clays.
- Internal pressures, external loadings, and thermal stresses can be identified by longitudinal cracks, frost regions, and changing internal or external loads.
- Leadite joints can lead to eventual structural failure and are identified by cracking at the bell of the pipe.
Leaks
- Loss of soil support and bending can result in leaks on ductile iron. Leaks can be identified through acoustic surveys (inline on large-diameter pipe, external on small diameter pipe), wet areas, or pressure variations.