3LPE Coated Pipe


What is 3LPE Coated Pipe?

Three-layer polyethylene (3LPE) is a type of multilayer coating that is typically applied to the exterior surface of the pipe. It is made up of three functional components: a high-performance fusion bonded epoxy (FBE) primer, a copolymer adhesive, and a tough, durable polyethylene outer layer. 3LPE systems provide excellent pipeline protection in highly corrosive environments. They have superior mechanical and adhesive properties, as well as excellent resistance to cathodic disbondment. 3LPE coatings are typically fusion bonded to the steel surface and then cured in place to create an extremely strong and durable bond. This bond provides exceptional resistance to both internal and external corrosion for the life of the pipe. In addition, 3LPE coatings have excellent abrasion resistance, which protects the pipe from external damage during installation and operation.

Table of Contents

The Component of 3LPE Coated Pipe

Layer 1 of 3LPE Coating: This is the layer that protects against corrosion. This layer is composed of fusion-bonded epoxy and provides excellent corrosion protection. The fusion-bonded epoxy adheres extremely well to the blasted steel surface.

Layer 2 of 3LPE Coating: This is the copolymer sticky layer. The copolymer adhesive is a grafted polyethylene compound with maleic anhydride. This material adheres well to the fusion-bonded epoxy and the top layer polyethylene.

Layer 3 of 3LPE Coating: This layer is made of polyethylene and serves as a physical barrier. Due to the similarity between the copolymer adhesive and polyethylene, they adhere easily to one another.

Production Standards of 3LPE Coated Pipes

3LPE Coated pipes are manufactured according to the following standards:

Performance of 3LPE Coated Pipe

No. Item Performance Indicators
1 Tensile Strength Axial Direction, Mpa ≥20
Circumferential Direction, Mpa ≥20
Deviation, % ≤15
2 Elongation after Fracture, % ≥600
3 Indentation Hardness (mm) 23℃ ≤0.2
50℃ or 70℃ ≤0.3
4 Endurably Environmental Stress Cracking (F50),h ≥1000

Minimum Thickness of 3LPE Coated Pipe

Pipe Sizes (Specified O.D.) Minimum Coating Thickness Chart (mm)
≤ 10 3 /4” (273.1 mm) 2.5
> 12 3 /4” (323.9 mm) to ≤ 18” (457 mm) 2.8
> 20″ (508.0 mm) to ≤ 30″ (762 mm) 3
> 32″ (813.0 mm) 3.3

3LPE Coated Pipe Production Process

Pipeline 3LPE external anticorrosion is the use of a 3LPE production line that will be coated with three different types of anti-corrosion ingredients to produce a 3LPE anti-corrosion coating. Among them, the steel pipe surface should be pretreated with rust removal grade Sa2.5 or higher, the epoxy powder bottom layer coated to spiral operation after shot blasting derusting the steel pipe; the middle layer of adhesive and the outer layer of polyethylene are coated on the continuous running steel pipe coated with powder epoxy via extrusion winding or extrusion coating, respectively.

- Pipe Surface Preparation

  1. Surface treatment of steel pipe is critical in the 3LPE coating technique. The surface treatment quality of steel pipe has a direct impact on the adhesion and anti-corrosion performance of the anticorrosion coating.
  2. Prior to surface pretreatment of steel pipe, the pipe’s appearance should be inspected for oil contamination, welding slag, burrs, damage, and other flaws.
  3. Inspect the ellipticity, form of the weld, and height of the steel pipe. Steel pipe has an excessive ellipticity, which adversely affects the quality of the anti-corrosion coating for a period of time and makes bubbles at the pipe end easy to form. Second, it will have an effect on steel pipe field construction. If the welding seam of the steel pipe is triangular or rectangular, it is easy to introduce defects such as bubbles and coating thinning during the coating forming process, lowering the quality and increasing material consumption.

- Surface Pre-Treatment Cleanness Test

The degree of rust on an untreated steel surface is classified into four “Cleanness” categories: A\B\C\D. The uncoated steel surface and the steel surface cleaned after the original coating has been removed are classified according to their “Cleanness” values. Corrosion and derusting on steel surfaces are graded using a written description and images of representative examples. There are four types of corrosion:

  1. Completely coated steel surface with an oxide skin and nearly little rust;
  2. Corroded steel surface with a portion of the oxide layer removed;
  3. A steel surface that has been stripped by rust or is scrapable and has minor pitting;
  4. It is common to find steel surfaces where the oxide scale has been completely exfoliated by rust and pitting.

- AD&PE Application

Extruders are the primary equipment used in adhesive and polyethylene coating production. Its fundamental principle is to feed granular raw materials into the extruder and then into the heating cylinder via the rotating screw shaft, where they are heated to a molten state and extruded from the die head.

The coating procedure should take into account the powder epoxy spraying and adhesive coating time intervals and should guarantee that the epoxy powder is not in its curing condition after coating the adhesive, since the powder epoxy curing will lose its capacity to react with the adhesive. Similarly, coating polyethylene should be carried out in the adhesive’s melting stage to guarantee proper fusion between the two.

- Coating Thickness and Peeling Test

Measurement of thickness: a thickness gauge is used to determine the thickness of the anti-corrosion coating on the pipe body and weld bead. To guarantee that the coating thickness does not fall below the required level, the first five tubes of each shift should be examined, followed by every five tubes or even one by one. Each test included three tube head locations, a middle position, and an end position, as well as three, six, nine, and twelve points. Every two hours, the thickness gauge should be calibrated.

Peel strength test: the peel strength test is a critical performance criterion for the three-layer structure anticorrosion layer since adhesion directly impacts the coating’s anti-corrosion performance. At least one sample test every shift at two different temperatures. When measuring, the coating must be fully scratched and the effective width of the coating determined. 20°C, 100N/cm, and 50°C, 70N/cm are the industry standards.

- Dew Point Detection and Appearance Inspection

Dew point detection: A spark leak detector was used to test the cooled coated steel pipe 100 percent. The leak detection voltage was 25kV. If a leak occurs, the operator should mark the leak point for subsequent treatment. To achieve 100% detection, the effective width of the probe should be bigger than the pitch. Throughout the operation, pay close attention to the sensitivity of the leak detector at all times to avoid the non-alarm and false alarm phenomena.

Visual inspection of coating appearance: Inspect each coating visually. The surface of the polyethylene layer should be smooth, without bubbles, pitting, wrinkles, fractures, or warping, and the color should be uniform. Avoid dark bubbles and twisted edges at all costs.

- Flexibility and Impact Test

Bending strength: no cracking/peeling, no leakage point

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