I. What is the Cord Layer?

In dredging rubber hoses, the cord layer is a structural reinforcement layer embedded within the rubber tube wall, made of high-strength fibers or metal wires. It serves as the ‘pressure-bearing skeleton’ of the entire rubber hose, similar to steel bars in reinforced concrete. Its primary function is to withstand the internal working pressure, vacuum, and various external loads of the pipeline. Rubber hoses without a cord layer will expand indefinitely like a balloon under pressure until they rupture. The presence of the cord layer significantly enhances the tube’s strength, stiffness, and dimensional stability.

II. Structure and Arrangement of the Cord Layer

The structural design of the cord layer directly determines the performance of the rubber hose. There are mainly two basic structures:
1. Winding Type
– Description: Single or multiple combined cords are spirally wound around the hose blank at a specific angle (usually close to 54.7°, known as the ‘balanced angle’).
– Advantages: ■ Simple manufacturing process and high production efficiency. ■ Good flexibility of the hose body and small bending radius. ■ Excellent fatigue resistance, suitable for dynamic applications (e.g., stern swing pipes on ships).
– Disadvantages: ■ Relatively low pressure-bearing capacity. ■ Prone to ‘birdcage’ expansion deformation under high pressure.
– Application: Medium-low pressure dredging conditions with high flexibility requirements.

2. Braided Type
– Description: Cords are woven into a cylindrical mesh sleeve using a braiding machine, which is then placed over the hose blank. The braiding angle is also precisely calculated.
– Advantages: ■ Extremely high pressure-bearing capacity, stable structure, and good anti-expansion performance. ■ Accurate dimensions and good rigidity of the hose body.
– Disadvantages: ■ Complex manufacturing process and higher cost. ■ Slightly lower flexibility compared to the winding type.
– Application: High-pressure, large-diameter dredging projects with strict pressure requirements.

Modern high-performance dredging hoses often adopt a ‘multi-layer composite structure’, such as:
• Inner rubber layer: Wear-resistant and corrosion-resistant.
• Inner cord layer (braided): Main pressure-bearing layer.
• Middle rubber layer: Bonding and cushioning.
• Outer cord layer (wound or braided): Provides additional strength and stability.
• Outer rubber layer: Weather-resistant, wear-resistant, and tear-resistant.

III. Core Functions of the Cord Layer

1. Pressure Bearing and Expansion Limitation: Withstand the positive pressure of internal slurry and negative pressure during suction, while limiting radial expansion of the hose body to ensure conveying efficiency and safety.
2. Providing Stiffness and Shape Retention: Impart necessary rigidity to the pipe to prevent collapse under pressure and maintain a circular cross-section.
3. Resisting External Loads: Withstand mechanical stresses such as bending, torsion, and tension, especially in dynamic environments like ship movement and wave action.
4. Influencing Flexibility: The material, number of layers, and arrangement angle of the cords collectively determine the overall flexibility and minimum bending radius of the hose body.

IV. Lining Material
The selection of lining material is crucial for balancing strength, flexibility, corrosion resistance, and cost.
1. Synthetic Fibers
– Polyester: The most commonly used material, offering good overall performance, high strength, fatigue resistance, corrosion resistance, and moderate cost.
– Nylon: Excellent in strength and impact resistance, with good elasticity, but slightly lower modulus. Its strength is reduced in wet conditions.
– Aramid: A high-end choice. It has extremely high strength, is lightweight, has a high modulus (almost no elongation), and is heat-resistant. Used in special dredging pipes with ultra-high pressure and lightweight requirements, but it is expensive.
2. Metal Lining
– Steel Wire: Offers the highest strength and extremely high modulus (almost no extension), with strong pressure-bearing capacity.
– Disadvantages: Heavy weight, poor flexibility, prone to corrosion (requires strict rubber coating), and relatively shorter fatigue life compared to fibers.
– Applications: Mainly used in ultra-high pressure, static onshore pipeline sections, or as reinforcement in stress concentration areas such as pipe flange joints.

V. Key Points of Manufacturing Process
1. Bonding System: This is one of the core technologies in manufacturing. The surface of the lining must undergo special treatment (e.g., RFL impregnation) and match the rubber formula to ensure extremely high bonding strength between the lining and rubber. Bonding failure can lead to delamination of the lining layer, bulging of the pipe body, or even rupture.
2. Laying Angle: The winding or weaving angle of the lining is precisely calculated to achieve the optimal balance of pressure-bearing and tensile performance. The theoretical \”balanced angle\” (approximately 54°44′) allows the pipe body to maintain constant diameter and length under pressure.
3. Uniformity: The uniformity of tension and distribution of the lining in the pipe body is critical. Any unevenness will cause stress concentration, making it a weak point in the pipe.

VI. Common Failure Modes of Lining Layer and Analysis
1. Lining Breakage:
– Causes: Operating under overpressure, external mechanical damage (e.g., impact, crushing), fatigue fracture, stress concentration at joints.
– Manifestations: Local bulging or rupture of the pipe body.
2. Lining-Rubber Bonding Failure:
– Causes: Unqualified bonding system, aging during use, medium penetration corrosion, moisture or contamination of the lining during production.
– Manifestations: \”Interlayer separation\” of the pipe body, bulging, and a sharp drop in pressure-bearing capacity.
3. Corrosion/Degradation:
– Fiber Lining: Long-term use in acidic/alkaline slurry or internal water penetration may lead to reduced fiber strength.
– Metal Lining: After the rubber coating layer is damaged, steel wires contact water and air, causing rapid rusting and loss of strength.
4. Twisting Deformation:
– Causes: Abnormal torsional forces acting on the pipe body during installation or use, leading to changes in the lining angle and structural damage.

Summary
The lining layer of dredging rubber pipes is the \”heart\” of its mechanical performance. Its material selection (polyester/aramid/steel wire), structural design (winding/weaving/composite), and manufacturing process (bonding/angle/uniformity) directly determine the working pressure, service life, safety, and applicable scenarios of the pipe. In selecting and evaluating dredging hoses, in-depth analysis of the lining layer is a crucial aspect. For users, strictly using according to the design pressure, avoiding mechanical damage, and regularly checking for abnormalities such as bulging in the pipe body are key to ensuring the long-term safe operation of the lining layer.