Pipeline pickling refers to a series of technical treatment processes on the inner surface of a pipe according to a specific process procedure using an acid or alkali solution with a small amount of corrosion inhibitor or active agent added, which will removes oxides and grease from the surface of the pipe by chemical action to obtain a shiny metal surface, ensuring that the inner wall of the pipe has a specified cleanliness.
The method of pickling the pipeline can be divided into a trough pickling method and a circulation pickling method, and the cyclic pickling can be further divided into on-line circulating pickling and off-line circulating pickling.
The trough pickling method removes the pipe or the installed pipe, and is immersed in the chemical pickling tank in sections. After passing the technical treatment, it is transported to the construction site to restore the original installation state. In-line circulation pickling method is to connect the pipes that have been installed in place to form a loop by hose, and use a corrosion-resistant pump to drive the chemical cleaning liquid into the circuit for circulating washing. This method can not remove the installed pipeline, but the circuit must avoid components such as hydraulic cylinders and valves. The off-line circulation pickling method is to remove some short pipe fittings or pipes which are not suitable to form a loop at the installation position, and connect the hoses and joints in a loop to perform cyclic pickling.
With the development of hydraulic technology and the increasing requirements for system cleanliness, the cyclic pickling method can show significant advantages. For example, the pickling speed is fast and the effect is good; no large acid tanks and special workshops are needed; the pipelines are repeatedly disassembled and transported, the piping cycle is shortened, and the possibility of re-contamination and re-rusting of the pipelines is reduced; Welded connections reduce flanges, joints, bolts, etc.; reduce engineering costs. However, the cycle pickling process is more complicated, and the pickling quality inspection is also more difficult. Once the operation error causes an acid leakage accident, the technical management requirements are stricter. Pipeline pickling is suitable for a large number of centralized, cleanliness and high cleanliness pipelines. For some pipe projects with large diameters and short lengths, tank pickling is more suitable.
Pickling quality requirements
Pipeline pickling usually uses inorganic acids such as sulfuric acid, hydrochloric acid and phosphoric acid. In order to ensure the quality, the pickling solution should have the correct formula, which can completely remove the oxide, rust and scale of the pipe wall by chemical action, and can not cause excessive corrosion and re-rusting of the pipe wall; a reasonable process for pickling The process and strict operating procedures ensure that all parts of the inner surface of the pipe are thoroughly and evenly cleaned and the residual acid, residual water and suspended solids on the inner wall of the pipe are exhausted.
The quality requirements for pipe pickling are: (1) After the pickling, the inner wall of the pipe is rusted and the oil is completely removed, and the cleanliness should meet the requirements of relevant standards. (2) Pickling parts must not be pickled. If the pipe is soaked in the pickling solution for too long, the surface of the steel will become rough and form a honeycomb surface. (3) Pickling parts shall not appear to be pickled. If the pipe is soaked in the pickling solution for too short or the concentration of the pickling solution is not enough, the steel will still have thin rust after the pickling or the scale is not removed. (4) Ensure that the thread and the acid-resistant seal are not corroded. Before pickling, apply yellow to oil on the thread. The seal that is not resistant to acid should be replaced by acid-resistant seal.
Europe, the United States and Japan have a variety of processes for pipe pickling. For example, Germany's MOGGE company advocates hydrochloric acid-type cyclic pickling. Some companies in the United States and France advocate the use of phosphoric acid type cyclic pickling. Some Japanese companies advocate the use of phosphoric acid or hydrochloric acid type tank pickling. After the 1950s in China, pipe pickling was usually acid-washed with mineral acid (hydrochloric acid or sulfuric acid) and then neutralized with lime water or sodium carbonate. After the 1980s, various new processes were also adopted.
The most commonly used pickling process programs are:
(1) Hydrochloric acid type tank type dry pickling: preparation before pickling → pickling → neutralization, passivation of finished product
(2) Phosphate type tank pickling: preparation before pickling → pickling finished product.
(3) Hydrochloric acid type cyclic pickling: composition loop → test leak → degreasing → pickling → water washing → secondary pickling → neutralization → passivation → drying → rust prevention → finished product