A Brief Discussion on the Performance of Ductile Iron Pipes

Ductile iron pipes are widely used in urban water supply, drainage, and gas transmission projects. It has the corrosion resistance of ordinary cast iron pipes and the strength and toughness of steel pipes. Its weight is 1/2-1/3 lighter than cast iron pipes of the same diameter, close to steel pipes, but its corrosion resistance is several times higher than that of steel pipes. Therefore, in various environments, ductile iron pipes do not require special anti-corrosion treatment and can be directly buried underground. The cement mortar lining of ductile iron pipes should have sufficient sealing force in practical use. The sealing force between cement mortar and ductile iron pipe is 2MPa, which is four times the sealing force between cement mortar and steel pipe. At the same time, the roughness coefficient of the lining cement mortar is very low, and the convective resistance is small, which can be comparable to fiberglass pipes and plastic pipes. As a universal water supply pipe material, there are many types of interface methods, and the most commonly used is the sliding "T" type flexible (rubber) interface, which has low construction cost, easy installation, and good sealing performance. And allow the pipeline to have certain bends, greatly facilitating the construction of pipelines in some special areas.The performance, characteristics, and standards of ductile iron pipes are introduced as follows:

Composition and properties of ductile iron pipes

1. The main chemical components of ductile iron pipes and fittings are carbon, silicon, manganese, phosphorus, sulfur, and magnesium.

2. Mechanical properties of ductile iron pipes. It has high strength, high elongation, and low hardness, making it convenient for mechanical processing.

3. Corrosion resistance of ductile iron pipes. Its corrosion resistance is superior to steel pipes and comparable to ordinary cast iron pipes, which has been widely recognized.

4. Ductile iron pipes have resistance to electrical corrosion. Due to its material composition, it has a high resistance and is less prone to electrical corrosion. The resistance value of steel is (10-20) Ω. The resistance value of ductile iron is (50-70) Ω.

5. Ductile iron pipes have heat resistance. After the graphite in ductile iron becomes spherical, it improves the density of the material and prevents the invasion of oxidizing gases from the outside, thus possessing heat resistance and growth resistance.

6. Welding performance of ductile iron pipes. Under the guidance of professionals, cast iron welding rods can be used for welding.

7. The lining function of ductile iron pipes. One is to improve the corrosion resistance of ductile iron pipes, and the other is to protect water quality and prevent secondary pollution.

8. Anti corrosion coating on the outer surface of ductile iron pipes. Generally, zinc spraying combined with asphalt paint or epoxy resin paint is used to provide good protection for some soils with low electrical resistivity. The final coating of asphalt or resin paint can serve as a corrosion-resistant layer for polyethylene sleeves.

Characteristics of flexible interfaces for ductile iron pipes

1. Good sealing performance. Due to the tight compression of the rubber ring by the flange, it is tightly connected to the inner surface of the cast pipe socket and the outer surface of the socket, thus achieving sufficient airtightness and water sealing.

2. Has flexibility. Due to the elasticity of the rubber ring, the inner surface of the cast pipe socket and the pressure flange are conical, thus obtaining flexibility, allowing the pipeline to adapt well to slight settlement or vibration of the foundation.

3. Has good scalability. Due to temperature changes, the expansion and contraction of the cast pipe are easily absorbed by it, and no special expansion joints are required.

4. Construction is simple and fast. Quick and safe installation work can be carried out using simple tools such as ratchet wrench or screw steel. Additionally, after installation, backfill soil can be directly added.

5. The rubber ring is not easily aged, and it is almost completely embedded in the groove of the socket, with little exposed part and less contact with oxygen, so the risk of aging is low.

6. Can prevent the impact of electrochemical corrosion. The rubber ring of the interface isolates each cast pipe from each other, thus minimizing the impact of electrochemical corrosion.

7. The interface of the cast pipe is prone to detachment. Due to the characteristics of flexible interface construction, its detachment from friction force cannot be very large. Therefore, appropriate protective measures must be taken at the ends of pipes with pull-out force, bends, tees, and variable diameter parts.

Related standards for ductile iron pipes

1. The wall thickness standard of ductile iron pipes is calculated as a function of the nominal diameter DN, such as T=K (0.5+0.001DN), where K is the wall thickness grade coefficient, and the four grades of 9, 10, 11, and 12 are the standard wall thicknesses. DN is the nominal caliber. The unit is mm, and the diameter of the ductile iron pipe for DN (100-200) mm straight pipes is calculated using the following additional formula: T=5.8+0.003DN. The minimum wall thickness of the centrifugal ductile iron pipe is not less than 6mm, and the negative deviation of the wall thickness is 1.3+0.001DN. The most commonly used pipe wall thickness in water supply pipeline construction is K9 grade.

2. The weight standard for ductile iron pipes. The density of ductile iron pipes is 7050Kg/m3. Allowable weight deviation: For nominal diameters less than or equal to 200mm, the allowable weight deviation is -8%. For nominal diameters DN greater than 200mm, the allowable weight deviation is -5%.

3. The length of ductile iron pipes. The most commonly used standard is 6m, and the manufacturing length deviation of socket straight pipes is plus or minus 30mm. Ductile iron pipes should be straight, and their maximum deviation should not exceed 0.125% of the effective length of the cast pipe.

4. The material composition of ductile iron pipes. There is a certain amount of spherical graphite distributed on the ferrite and pearlite matrix. According to the different diameters and elongation rates, the composition ratio of ferrite and pearlite is different, and the roundness and size of graphite are also different.

5. Marking of ductile iron pipes with T-shaped sliding in flexible interfaces. There should be a white painted insertion mark on the outer surface of the socket.