Cast Iron Pipe
The Basics of Cast Iron Pipe
Cast iron (also referred to as grey cast iron) is a historical pipe type manufactured in the 19th and 20th centuries that was commonly used for pressurized water transmission and sewer force mains.
Cast iron pipe was gradually superseded by ductile iron pipe, with most existing manufacturing plants transitioning to ductile iron during the 1970s and 1980s.
Most cast iron pipes that are in service today were manufactured by either pit casting or spin casting. The earliest cast iron pipes were vertically pit-cast grey iron.
In the early 19th century, the first pit-cast iron pipes in the U.S. were imported, but from 1830 onwards, local production became more widely established. Pit-cast iron pipes were manufactured and installed until the 1940s. There is currently almost no new manufacture of cast iron pipe.
Cast iron pipe is typically found in older water and sewer systems. It has several advantages and disadvantages that owners should factor into its long-term management.
- Thicker wall than ductile iron or steel
- Similar rate of corrosion to ductile iron and steel
- Most pipes after 1950 supplied with cement mortar lining or retrofitted
- No elastic behavior and lower mechanical strength
- Prone to external and internal corrosion in aggressive conditions
- Older pipes having caulked joints with little flexibility
- Often no external protection
- Most pipes unlined before 1960
- Manufacturing defects including variations in wall thickness
- Poor records
How Does Cast Iron Pipe Fail?
Like every pipe material, cast iron pipe 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 cast iron pipe.
- Internal pitting and graphitization corrosion can lead to cast 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 or graphitization corrosion is another type of break failure in cast iron pipe. 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.
- Manufacturing defects can also lead to break failure. This type of problem is typically identified through cracks in the body or bell of the cast iron pipe.
- 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.
- Loss of soil support and bending can result in leaks on cast iron. Leaks can be identified through acoustic surveys (inline on large-diameter pipe, external on small diameter pipe), wet areas, or pressure variations.
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