Quality Control in 3PE Anti corrosion Construction of Steel Pipes

1. Preface

The long-term stable operation of crude oil export pipelines relies on high-quality anti-corrosion coatings to provide corrosion protection. In China, 3PE coatings are widely used in buried and subsea oil and gas pipelines for the comprehensive consideration of mechanical damage resistance and corrosion resistance. Although the construction process of 3PE coating is very mature, quality problems in 3PE anti-corrosion are not uncommon, such as coating peeling, corrosion perforation, and separation of epoxy powder and adhesive interface. The analysis of various coating quality issues is mainly due to the lack of quality control measures in the production process. This article combines production project experience to explore and analyze the key quality control links in 3PE anti-corrosion.

From the perspective of process flow, the 3PE anti-corrosion construction of steel pipes mainly includes four stages: steel pipe receiving, steel pipe surface treatment, coating construction, finished pipe inspection and treatment. The key to the steel pipe receiving process is to control the quality of the coated pipes and ensure that they meet the construction requirements; The key to the surface treatment of steel pipes is to control the cleanliness of the steel pipe surface and ensure the tight adhesion between the coating and the steel pipe substrate; The key to the coating construction process is to control the matching of process parameters such as gelation time and curing time with the layout of the production line, in order to achieve the unity between the layers of the 3PE coating; As the final stage of quality control in the painting workshop, the inspection and processing of finished pipes focuses on ensuring that the online quality inspection indicators and apparent quality of the coating meet the painting requirements.

2. Selection of shot blasting and rust removal abrasives

The selection of abrasive used in shot blasting rust removal plays a decisive role in the final rust removal effect. The selection of abrasive mainly considers particle size, hardness, and electrical conductivity of the abrasive.

It is generally believed that the higher the hardness of the abrasive, the better the rust removal effect. The general criterion for selecting steel abrasives in shot blasting is to use the hardest and smallest steel abrasive that can quickly destroy the oxide layer, remove surface pollutants, and treat the steel pipe surface to the required anchor depth. However, in practical applications, higher hardness abrasives are generally not used unless necessary. Although hard abrasives have higher cleaning efficiency compared to softer abrasives, the internal wear of the shot blasting machine is faster, and the abrasive consumption rate is higher, resulting in higher processing costs. Therefore, the selection of abrasive hardness should also comprehensively consider rust removal efficiency, rust removal effect, and unit processing cost, and make the final judgment based on the application effect of the production line. Before determining the choice of abrasive, the conductivity of the abrasive should be tested to avoid contamination of the pipe body by the abrasive, resulting in excessive salt contamination of the steel pipe. The ISO21809-1 standard generally requires that the conductivity of abrasives should not exceed 60 ms/cm. In typical anti-corrosion production, the conductivity of abrasives can be controlled within 70 μs/cm.

3. Gelation time

The gelation time is the process of spraying epoxy powder from the high-temperature curing reaction to the apparent solidification. The guiding significance of this indicator for the production line is that the distance between adhesive overlap and epoxy powder spraying should not exceed the minimum distance traveled by the steel pipe during the gelation time, in order to ensure that the adhesive adheres to the surface of the epoxy coating when it still has viscosity on the surface of the epoxy powder, and to achieve the best bonding effect between different coatings.

4. Curing time

The curing time is the process of spraying epoxy powder from the beginning of high-temperature curing reaction to the completion of curing. The guiding significance of this indicator for the production line is that the distance between the powder coating completion and the water cooling zone should not be less than the maximum distance traveled by the steel pipe during the curing time, to ensure that the epoxy powder coating can complete the curing process before the steel pipe cools.

5. Water cooling

After the outer polyethylene coating is completed, the steel pipe enters the water cooling zone. For the cooling of 3PE coatings, on the one hand, the force of the spray water hitting the coating surface should be controlled to avoid apparent defects caused by water impact, and on the other hand, the cooling rate should be ensured to prevent the coating from contacting rollers or other hard mechanisms at high temperatures and causing compression deformation.

6. Pipe end reservation

After completing the online detection of coating thickness, leakage points, etc., it is necessary to treat the coating on the pipe end to achieve the reserved length of the pipe end required by the design. In order to prevent the phenomenon of peeling of the 3PE anti-corrosion coating on the pipe end, the FBE coating will be 5-20 mm longer than the outer polyethylene layer.

The common method for reserving pipe ends is to use mechanical polishing to remove the coating in the pipe end area. The FBE coating reserved by this process may become thinner, making it difficult to achieve the expected protective effect. If the reserved processing method for the pipe end is mechanical cutting technology, it can avoid thinning the FBE coating during the processing and achieve better pipe end protection effect.