What is a spiral steel pipe?

What is a spiral pipe?

A spiral steel pipe is a type of steel pipe formed by cold rolling strip steel coils into a spiral shape and welding them using the submerged arc welding process. The production process involves feeding strip steel into a pipe welding unit, where it undergoes multiple passes of rolling to gradually form a round pipe with an open gap. By adjusting the pressure of the squeezing rollers, the gap between weld seams can be controlled between 1 to 3 millimeters, ensuring that both ends of the weld are flush.

The characteristics of spiral steel pipes include simple production processes, low manufacturing costs, and the ability to produce large-diameter, long-length pipes as needed. They are widely used for transporting various media such as liquids, gases, and solid powders due to their lightweight, high strength, corrosion resistance, and easy installation. Spiral steel pipes have a spiral structure made from steel plates and can be made from different materials according to user requirements, such as steel, carbon steel, etc.

Spiral pipes are also known as spiral welded pipes, which are made by rolling low-carbon structural steel or low-alloy structural steel strips into pipe blanks at a certain forming angle and welding the seams. This process allows for the production of large-diameter pipes using narrow strips of steel.

The production process of spiral steel pipes involves several steps:

1.Raw materials, including strip coils, welding wires, and fluxes, undergo strict physical and chemical inspections before being used.
2.Strip ends are butt-welded using single or double submerged arc welding, and additional welding is done after pipe coiling.
3.Before forming, the strip undergoes straightening, edge trimming, edge planing, surface cleaning, conveying, and edge bending.
4.The pressure of the squeezing cylinders on both sides of the conveyor is controlled using an electric contact pressure gauge to ensure smooth strip feeding.

5.External or internal control roller forming is used.
6.Weld gap control devices are used to ensure that the gap between weld seams meets welding requirements, with strict control over pipe diameter, misalignment, and weld gap.
7.Both internal and external welding are performed using single or double submerged arc welding machines to achieve stable welding specifications.
8.Welded seams are inspected using online continuous ultrasonic flaw detectors to ensure 100% coverage of non-destructive testing of spiral welds. Any defects trigger automatic alarms and marking, prompting production workers to adjust process parameters and eliminate defects promptly.
9.Steel pipes are cut into individual lengths using plasma cutting machines.
10.Each batch of steel pipes undergoes strict initial inspection to check mechanical properties, chemical composition, fusion condition of welds, surface quality, and non-destructive testing to ensure compliance with manufacturing standards before being formally put into production.
11.Sections of welded seams with continuous ultrasonic flaw detection markings undergo manual ultrasonic and X-ray reexamination. If defects are found, they are repaired, retested non-destructively, and rechecked until defects are eliminated.
12.Sections of steel pipes containing welded seams and intersecting T-joints undergo X-ray television or radiographic inspection.
13.Each steel pipe undergoes hydrostatic testing with radial sealing, with test pressure and time strictly controlled by a steel pipe hydrostatic testing device. Test parameters are automatically printed and recorded.
14.Pipe ends are machined to ensure perpendicularity, bevel angle, and blunt edge accuracy.

Surface treatment methods for spiral steel pipes include:
1.Cleaning: Solvent or emulsion is applied to clean the surface of the steel to remove oil, grease, dust, smooth agents, and similar organic substances. However, it cannot remove rust, scale, flux, etc., and thus serves as an auxiliary measure in corrosion protection production.
2.Tool rust removal: Steel wire brushes and other tools are primarily used to grind the surface of the steel to remove loose or raised scale, rust, welding slag, etc. Manual tool rust removal can achieve Sa2 level, while power tool rust removal can achieve Sa3 level. If the steel surface has firmly adhered scale, tool rust removal may not be effective enough to achieve the required anchor pattern depth for corrosion protection construction.
3.Acid pickling: Chemical and electrolytic methods are commonly used for pickling treatment. Only chemical pickling is used for pipeline corrosion protection, which can remove scale, rust, old coatings, and can sometimes be used as a post-treatment after shot blasting rust removal. Although chemical cleaning can achieve a certain degree of cleanliness and roughness on the surface, the anchor pattern is shallow, and it can easily cause environmental pollution.
4.Shot (blast) rust removal: Shot (blast) rust removal involves high-speed rotation of shot (blast) blades driven by high-power motors, which causes abrasives such as steel shot, steel grit, wire segments, minerals, etc., to impact and abrade the surface of the steel under the centrifugal force effect. This method not only thoroughly removes rust, oxide, and dirt but also achieves the desired average roughness of the steel surface under the aggressive impact and frictional forces of abrasives. After shot (blast) rust removal, the physical adsorption effect on the steel surface is enhanced, and the mechanical adhesion effect between the anti-corrosion layer and the steel surface is strengthened. Therefore, shot (blast) rust removal is an ideal rust removal method for pipeline corrosion protection. Generally, shot blasting (sand) rust removal is mainly used for internal and external treatment of pipes, while shot peening (sand) rust removal is mainly used for surface treatment of pipes.
How to increase the stability of spiral pipes:
1.Small and medium-sized steel, coil bars, rebars, medium-caliber steel pipes, steel wires, and wire ropes can be stored in a well-ventilated shed, but they must be covered and padded.
2.Some small steel materials, thin steel plates, steel strips, silicon steel sheets, small-diameter or thin-walled steel pipes, various cold-rolled and cold-drawn steel materials, as well as high-priced and easily corroded metal products, can be stored indoors.
3.The site or warehouse for storing spiral steel pipe products should be located in a clean, dry area with good drainage, away from factories or mines producing harmful gases or dust. The site should be cleared of weeds and debris to keep the steel clean.
4.Large-scale steel, steel rails, thick steel plates, large-diameter steel pipes, forgings, etc., can be stored outdoors.
5.Acidic, alkaline, saline, cement, and other corrosive materials should not be stored together with steel materials. Different varieties of steel should be stored separately to prevent confusion and corrosion contact.
These steps and methods contribute to the production, surface treatment, and stability enhancement of spiral steel pipes.

Principles of Stacking Requirements:

1.The principle requirement for stacking spiral steel pipes is to stack them securely and safely, ensuring that they are stacked according to variety and specifications. Different materials should be stacked separately to prevent mixing and mutual erosion.

2.It is forbidden to store items with corrosive effects on steel around the stack of spiral steel pipes.

3.The bottom of the stack of spiral steel pipes should be elevated, firm, and flat to prevent moisture absorption or deformation of the materials.

4.Materials of the same kind should be stacked separately according to the order of receipt.

5.Spiral steel pipes stored in the open air must be placed on wooden pads or stone strips with a slightly inclined stack surface to facilitate drainage. Attention should be paid to the straight placement of materials to prevent bending and deformation.

6.The stacking height of spiral steel pipes should not exceed 1.2m for manual work and 1.5m for mechanical work, with a stack width not exceeding 2.5m.

7.There should be a certain aisle between stacks, with an inspection aisle generally being 0.5m wide. The width of entry and exit aisles depends on the size of the materials and transportation machinery, usually ranging from 1.5m to 2.0m.

8.Angle steel and channel steel should be placed flat with the mouth facing downwards when stored in the open air, while I-beams should be placed upright. The I-groove surface of the steel should not face upwards to prevent water accumulation and rusting.

9.The bottom of the stack should be elevated. If the warehouse has a sunny cement floor, an elevation of 0.1m is sufficient. If it is a dirt floor, an elevation of 0.2m to 0.5m is required. For outdoor sites, the elevation should be 0.3m to 0.5m on cement ground and 0.5m to 0.7m on sand and mud surfaces.

Quality Inspection

Before leaving the factory, spiral steel pipes should undergo mechanical performance tests, flattening tests, and expansion tests, meeting the requirements specified by the standards. The quality inspection methods for straight seam steel pipes are as follows:

1.Surface judgment, also known as visual inspection. Visual inspection of welded joints is a simple and widely used inspection method. It is an important part of finished product inspection, mainly used to detect defects on the surface of welds and dimensional deviations. It is generally conducted through visual observation, aided by standard templates, gauges, and magnifying glasses. If defects appear on the surface of the weld, there may be defects inside the weld.

2.Physical inspection methods: Physical inspection methods involve using physical phenomena for measurement or inspection. Non-destructive testing methods are generally used to inspect the internal defects of materials or workpieces. Non-destructive testing methods include ultrasonic testing, radiographic testing, penetrant testing, magnetic particle testing, etc.

3.Strength test of pressure vessels: In addition to conducting sealing tests, pressure vessels also need to undergo strength tests. There are two common types: hydrostatic test and pneumatic test. Both tests can inspect the tightness of welds of vessels and pipes under pressure. Pneumatic testing is more sensitive and faster than hydrostatic testing, and the tested products do not need drainage treatment afterwards, making it particularly suitable for products with drainage difficulties. However, pneumatic testing carries greater risks than hydrostatic testing. During testing, relevant safety measures must be followed to prevent accidents.

4.Tightness test: Welded containers for storing liquids or gases can be tested for non-tightness defects in welds, such as through-penetration cracks, pores, slag inclusions, incomplete penetration, and loose structures, using tightness tests. Tightness testing methods include kerosene testing, water loading testing, water flushing testing, etc.

5.Hydrostatic test: Each steel pipe should undergo a hydrostatic pressure test without leakage. The test pressure is calculated according to the following formula: P=2ST/D, where S is the test stress of hydrostatic pressure test (MPa), and the test stress of hydrostatic test is selected as 60% of the minimum yield strength specified by the corresponding steel strip standard (235Mpa for Q235). Holding time: Spiral welded seams of steel pipes for fluid transportation should be subjected to X-ray or ultrasonic inspection sampling (20%).

According to the quality inspection results of spiral steel pipes, they are usually classified into three categories: qualified products, rework products, and scrap products. Qualified products refer to spiral steel pipes whose appearance and internal quality meet the relevant standards or delivery acceptance technical conditions; rework products refer to spiral steel pipes whose appearance and internal quality do not fully meet the standards and acceptance criteria but are allowed to be reworked, and can meet the standards and acceptance criteria after rework; scrap products refer to spiral steel pipes whose appearance and internal quality are unqualified, and are not allowed to be reworked or cannot meet the standards and acceptance criteria even after rework.

Scrap products are further divided into internal scrap and external scrap. Internal scrap refers to scrap spiral steel pipes found in foundries or casting workshops; external scrap refers to scrap spiral steel pipes found after delivery, usually revealed during mechanical processing, heat treatment, or use, causing much greater economic losses than internal scrap. To reduce external scrap, it is best to conduct sample heat treatment and rough processing before leaving the factory for batch-produced spiral steel pipes, to discover potential defects in spiral steel pipes as early as possible in the spiral steel pipe factory, in order to take necessary remedial measures promptly.

Standard Classification

1.Submerged Arc Welded Spiral Seam Steel Pipe for Pressure Fluid Transportation SY5036-2000: Mainly used for transporting oil and natural gas pipelines.

2.Spiral Seam High-Frequency Welded Steel Pipe for Pressure Fluid Transportation SY5038-2000: Welded using high-frequency lap welding method, used for pressure fluid transportation. These pipes have strong pressure-bearing capacity, good plasticity, and are easy to weld and shape.

3.Submerged Arc Welded Spiral Seam Steel Pipe for General Low-Pressure Fluid Transportation SY5037-2000: Made using double-sided automatic submerged arc welding or single-sided welding methods, used for general low-pressure fluid transportation such as water, gas, air, and steam.

Common standards for spiral steel pipes generally include:

SY/T5037-2008: Spiral submerged arc welded steel pipe for ordinary fluid transportation.
GB/T9711.1-2008: Part 1 of Technical Delivery Conditions for Steel Pipes for Petroleum and Natural Gas Industries (Grade A pipes, with stricter requirements indicated by GB/T9711.2 Grade B pipes).
API-5L: American Petroleum Institute standards for pipeline steel pipes, divided into PSL1 and PSL2 levels.
SY/T5040-2008: Spiral welded steel pipe for piles.
SY/T5037-2008 is commonly referred to as the departmental standard set by the Petroleum Department.

GB/T9711.1-2008 specifies national spiral pipes for the petroleum and natural gas industries, categorized as Grade A.

Spiral Seam High-Frequency Welded Steel Pipe for General Low-Pressure Fluid Transportation (SY5039-2000): Made from hot-rolled steel strip coils, formed into pipes through constant temperature spiral forming, and welded using high-frequency lap welding method for general low-pressure fluid transportation.

Spiral Welded Seam Steel Pipe for Piling (SY5040-2000): Made from hot-rolled steel strip coils, formed into pipes through constant temperature spiral forming, and welded using double-sided submerged arc welding or high-frequency welding. Used for civil engineering structures, wharves, bridges, and other foundation piles.

Material of spiral steel pipe

Spiral steel pipe has many materials. Steel plates of different materials have different strengths and iron contents.General steel pipe materials are as follows:

Q235A，Q235B、10#、20#、Q345（16Mn）、

L245(B)、L290（X42）、L320（X46）、L360（X52）、L390(X56)、L415(X60)、L450(X65)、L485(X70)、L555(X80)

L290NB/MB（X42N/M）、L360NB/MB（X52N/M）、L390NB/MB(X56N/M)、L415NB/MB(X60N/M)、L450MB(X65)、L485MB(X70)、L555MB(X80)。

The most commonly used steel pipe material is Q235B, and the ones with higher hardness are L245M and L360M.

Spiral steel pipe deviations and tolerances

A. Spiral steel pipe deviation: During the production process, because the actual size is difficult to meet the nominal size requirements, that is, it is often larger or smaller than the nominal size, so the standard stipulates that a difference between the actual size and the nominal size is allowed. A positive difference is called a positive deviation, and a negative difference is called a negative deviation.
B. Tolerance of spiral steel pipe: The sum of the absolute values of the positive and negative deviation values stipulated in the standard is called tolerance, also called “tolerance zone”.

Deviation is directional, that is, expressed as “positive” or “negative”; tolerance has no direction. Therefore, it is wrong to call the deviation value “positive tolerance” or “negative tolerance”.

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