Introduced into the U.S. marketplace in 1955, ductile iron pipe (DIP) is pressure pipe commonly used for potable water and sewage distribution. 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, and various types of external coating are used to inhibit corrosion from the environment.
Ductile iron pipe is a direct development of earlier cast iron pipe which it has superseded. 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.
- Greater ductility than grey iron
- Greater impact resistance than grey iron
- Greater strength than grey iron
- Lighter and easier to lay than grey iron
- Simplicity of joints
- Joints can accommodate some angular deflection
- Low pumping cost due to larger nominal inside diameter
- Similar rate of corrosion to grey iron and steel
- Prone to external and internal corrosion
- Internal and external protection systems required
- Limited number of protection systems available in U.S.
- Polyethylene wrappings can be damaged
MAIN FORMS OF FAILURE IN DUCTILE IRON PIPES
|Form of Failure||Causes of Failure||Indicators of Failure|
|Internal pitting and graphitization corrosion||Damaged lining, wall loss from internal pitting, graphitization (hard to detect), leaks, external loads, and pressure variations|
|Break Failure||External pitting and graphitization corrosion||Damaged protection, wall loss from external pitting, graphitization (hard to detect), leaks, external loads, and pressure variations|
|Structural Failure||Movements from thermal, seismic, external loading||Joint leaks, poor bedding, and pipe movements|
|Thermal contraction, poor support leading to movement, internal pressure||Circumferential cracks, frost regions, leaks, pipe movements, and expansive clays|
|Internal pressures, external loadings, thermal stresses||Longitudinal cracks, frost regions, and changed internal/external loads|
|Leaks||Loss of soil support and bending failure||Leak noise and wet areas|