Ductile iron pipe is iron treated with magnesium so its graphite forms tiny spheres instead of flakes, which makes it strong and able to bend slightly without cracking. It is used mainly for municipal water mains and sewer force mains, and it is lined with cement mortar to stop internal corrosion and tuberculation. Pipelines like these are cleaned by flushing, swabbing and pigging rather than with drain augers.
- Ductile iron and the cast iron in house drains are both iron, but the shape of the graphite inside makes one tough and the other brittle.
- The cement mortar lining is what keeps a ductile iron water main from growing the rust nodules found in old cast iron drains.
- Ductile iron is a pressure pipe for municipal mains; you will rarely find it inside a house.
- Pipeline cleaning uses pigs, swabs and flushing that travel with the flow, because long mains cannot be reached with cables.
- Municipal mains belong to the utility; homeowners should never open hydrants or enter manholes.
01 /What is ductile iron pipe?
Ductile iron is a cast iron whose graphite forms small spheres, or nodules, rather than flakes. A small addition of magnesium to the molten iron makes the difference. In grey cast iron, the kind in house drains, graphite flakes act like tiny internal cracks, which is why grey iron is strong in compression but brittle. Spheres do not concentrate stress the same way, so ductile iron can stretch and deflect under load before it breaks. That toughness is why it largely replaced grey cast iron for pressure water mains in the second half of the 20th century.
02 /How is ductile iron pipe made?
Ductile iron pipe is made by treating molten iron with magnesium, spinning it into shape in a rotating mould, heat-treating it, and then lining and coating it. North American standards from the American Water Works Association are widely used in Canadian specifications for the pipe, its cement lining and its external protection.
- Melt and treat the iron
Molten iron is treated with magnesium so that the graphite will form spheres as it solidifies.
- Spin cast
The iron is poured into a rotating mould, which throws it outward to form a dense pipe wall of even thickness.
- Anneal
Heat treatment relieves casting stresses and develops the ductility the pipe is named for.
- Line the inside
Cement mortar is applied to the inside and spun smooth; a thin seal coat may follow.
- Coat the outside
A zinc coating with an asphaltic topcoat is common. In corrosive soils, a loose polyethylene sleeve may be added when the pipe is laid.
- Test and mark
Pipes are pressure tested and marked with their class and the standard they meet.
Can ductile iron pipe be welded? It can be, but only under controlled, specialized procedures, typically for items such as welded-on outlets or restraint rings, and usually following the manufacturer's procedures. Field welding is not the normal way to join or repair it; gasketed and mechanical joints are.
03 /Why is ductile iron pipe cement lined?
Ductile iron pipe is cement lined to keep water away from the iron. Bare iron in contact with water corrodes and grows tubercles, the same hard rust nodules that narrow old cast iron drains, which cut flow capacity and can discolour tap water. A cement mortar lining gives a smooth barrier, and the alkaline cement helps protect the iron surface beneath it. The result is a main that keeps most of its capacity for decades.
| Condition | Older unlined grey cast iron main | Cement-lined ductile iron main |
|---|---|---|
| Inside wall after decades | Tuberculation and a rough surface | Smooth lining, usually intact |
| Flow capacity | Falls as the bore narrows | Largely maintained |
| Water quality | Episodes of rusty or discoloured water | Fewer iron-related complaints |
| Typical remedy | Clean and line, or replace | Routine flushing |
The same mechanism explains why house drains need descaling and water mains mostly do not. Cast iron drains were never cement lined, so their walls roughen over time; see cast iron drain pipe cleaning.
04 /What is ductile iron pipe used for, and why?
Ductile iron is used where a pipe must carry pressure, resist heavy loads and last a long time. Its main uses are these:
- Water distribution and transmission mains, the pipes under streets that feed hydrants and service connections.
- Sewer force mains, which carry pumped sewage under pressure from a lift station. Sewage service can call for linings suited to sewer gas rather than plain cement.
- Fire protection lines for larger buildings.
- Gravity sewers in demanding locations, such as deep burial, shallow cover under traffic, or crossings.
- Industrial and plant piping that needs strength and pressure capacity.
Utilities choose it for strength, pressure rating, tolerance of ground movement, ease of tapping for connections and a long service record. PVC, polyethylene and concrete pressure pipe compete with it, and many Canadian cities use a mix.
Inside a house, you will rarely see ductile iron. The water service from the main is usually copper, polyethylene or PEX, and some older homes still have lead or galvanized service pipe. Some Canadian cities run lead service replacement programs; check with your utility if your house is older. Joints and sizes for the cast iron you are more likely to find in a basement are in cast iron pipe joints, sizes and cutting.
05 /What is pipeline cleaning, and what is a pipeline pig?
Pipeline cleaning is the removal of deposits, debris and water from long pressure pipelines, such as oil, gas, water and industrial lines, usually without digging them up or taking them apart. The characteristic tool is the pig, a device sized to the bore that travels with the flow and scrapes, sweeps or inspects as it goes.
| Pig type | What it is | Used for |
|---|---|---|
| Foam pig or swab | A bullet-shaped foam plug, sometimes with an abrasive coating | Water mains, light deposits, drying |
| Cup or disc pig | A steel body with rubber cups or discs that seal against the wall | Cleaning and separating products in oil and gas lines |
| Brush or scraper pig | Cups plus wire brushes or blades | Wax, scale and debris |
| Gel pig | A slug of thick gel | Carrying debris; separating products |
| Inline inspection pig | An instrumented pig that measures wall loss and cracks | Integrity checks on oil and gas pipelines |
Pigs are launched and received at special fittings or temporary connections, and they need a long run of constant bore with gentle bends. Some water utilities also use ice pigging, in which an ice slurry is pumped through a main to scour sediment, and air scouring. None of this transfers to a house drain, which is short, full of traps and tight fittings, and cleaned with the methods in pipe cleaning methods compared. The household tools that share the name are sorted out in drain pipe cleaning tools.
06 /How are municipal water and sewer mains cleaned?
Municipal mains are cleaned by the utility on its own programs, with methods matched to what the main carries.
| Main | Common cleaning methods | What residents may notice |
|---|---|---|
| Water distribution mains | Unidirectional flushing through hydrants; swabbing; air scouring; ice pigging; cleaning and relining | Notices on the street, brief low pressure, temporary discoloured water |
| Gravity sanitary sewer mains | Combination jet and vacuum trucks; root cutting; camera inspection | Crews at manholes; occasionally air or water moving in a low toilet |
| Storm sewers and culverts | Jetting and vacuuming; catch basin cleaning | Trucks at catch basins and ditches |
07 /What does this mean for the pipes in your house?
For a homeowner, ductile iron and pipeline cleaning mostly explain what happens outside the property line. The drain inside your house is a different system with different materials and different tools.
- Your drains are most likely ABS, PVC or grey cast iron, not ductile iron. Check with What Pipe Do I Have?.
- Your cast iron drain has no cement lining, which is why it builds tuberculation and why descaling exists.
- Your lateral runs to the municipal main; who owns which part varies by city. See pipe cleaning in Calgary for how to confirm it.
- The municipal main is the utility's job. If yours backs up at the same time as neighbours, call the city before paying a contractor.
For your own lines, the Pipe Method Matcher picks a cleaning method, and you can request quotes when you are ready.
FAQQuestions people ask
What is the difference between cast iron and ductile iron pipe?
Both are iron with graphite inside. In grey cast iron the graphite forms flakes, which make it brittle; in ductile iron, magnesium treatment makes the graphite form spheres, so the pipe can bend and deflect before breaking. Cast iron is used for house drains; ductile iron mainly for pressure mains.
Why is ductile iron pipe cement lined?
To keep water off the iron. The cement mortar lining stops corrosion and tuberculation, keeps the bore smooth so the main keeps its flow capacity, and reduces rusty-water problems.
Can ductile iron pipe be welded?
Only under controlled, specialized procedures, usually for attachments such as outlets or restraint rings. Ductile iron mains are normally joined and repaired with gasketed and mechanical fittings, not field welds.
What is ductile iron pipe used for?
Mainly municipal water mains, sewer force mains and fire lines, plus some gravity sewers in demanding locations. It is chosen for strength, pressure capacity and long service life.
What is pigging in a pipeline?
Pigging is sending a device called a pig through a pipeline, pushed by the fluid behind it, to clean deposits, remove water or inspect the pipe wall. It is used in oil, gas and water transmission lines, not house drains.
Why is my tap water brown after the city flushes the mains?
Flushing stirs up sediment and iron deposits in the mains. Running a cold tap until the water clears usually solves it; avoid laundry until then, and contact the utility if it persists.
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