Exploration of 3PE External Corrosion Protection Technology for Steel Pipes

1. Preface

The general process for 3PE external anti-corrosion is: preheating → shot blasting → acid washing → anchor pattern degree → cleanliness inspection and pipe end wrapping paper → chromate treatment → medium frequency heating → powder spraying → glue wrapping → polyethylene wrapping → cooling → online detection → pipe end grinding → grass rope wrapping → finished product storage.

2. Preheating process

The preheating process is an essential step in the external anti-corrosion process of steel pipes. There are generally two types of equipment selection, namely gas heating type and medium frequency heating type. The comparison of the two preheating methods is as follows:

(1) Environmental protection: Gas heating generally uses green fuel natural gas; Medium frequency heating uses electrical energy, which generates harsh noise during operation. At the same time, if the medium frequency power supply is not well preprocessed, the odd harmonics of the medium frequency power supply can also cause pollution to the power grid.

(2) Energy saving: Gas heating type consumes less energy, with a maximum gas consumption of about 60 m³N per hour (when the flame directly contacts the steel pipe, if heat exchange is used, the gas consumption will increase).

(3) Pipe surface pretreatment: Gas type preheating furnace can burn off a small amount of oil stains that may adhere to the outer surface of steel pipes, playing a role in removing oil stains.

(4) Peeling: By utilizing the skin effect of intermediate frequency heating, the surface anti-corrosion layer of the scrap steel pipe can be quickly softened by passing it through an intermediate frequency coil, making peeling convenient and fast.

(5) Equipment cost: The equipment cost is not very comparable, but the overall cost of medium frequency heating furnaces is higher than that of gas heating furnaces.

Gas heating furnaces have many advantages compared to medium frequency heating furnaces, but it is still recommended to use medium frequency heating furnaces as preheating equipment for the following reasons: ① The selection of gas heating furnaces is limited by the source of gas; ② The proportion of steel pipes with oil stains is negligible, and manual pretreatment can be carried out; ③ It is very difficult to peel off waste pipes, but after being heated and softened by an intermediate frequency preheating furnace, it is simple and feasible. This is also the main reason for choosing intermediate frequency for pipe surface preheating.

3. Shot blasting process

The double shot blasting method can effectively improve the surface cleanliness and production speed of steel pipes. Steel pipes can pass through a first shot blasting and a second shot blasting at a relatively fast speed. Most of the oxide scale falls off during the first shot blasting. After the second shot blasting, the steel shot in the second shot blasting chamber is relatively clean, which can achieve a high level of cleanliness on the surface of the steel pipe and obtain the required anchor grain degree.

4. Acid washing and chromate treatment

Acid washing can improve the surface cleanliness of steel pipes, while chromate treatment can enhance powder adhesion. The combination of the two can comprehensively enhance the peel strength, adhesion, and cathodic peel strength of the anti-corrosion layer.

5. Spray room design

The advantages of this layout are: ① It can effectively extend the spray room and enhance the cooling capacity while keeping the footprint unchanged; ② The spray room can be effectively sealed, and steam will not overflow into the workshop. But this method requires the addition of necessary water control devices to pour out the water inside the steel pipe.