Carbon steel pipe pickling technology

Carbon steel pipe pickling technology

1. Before the speech

Since the 1950s, industrial production has been developing in the direction of large-scale and automation. The application of hydraulic transmission has spread throughout all fields of the national economy and become one of the fastest developing technologies in the machinery industry. For example, in pangang is widely used in the three periods of development of hydraulic transmission, because it has a series of advantages, such as hydraulic control force is small, in the process of work can be a larger range of stepless speed regulation, in reciprocating and rotating movement, can often quickly without the impact of variable speed and the reversing, easy access to all kinds of complicated movements, make the mechanical automation degree is greatly increased.

The installation of hydraulic transmission equipment is to connect the hydraulic equipment and valve components with pipes according to the requirements of the drawings and fix the positions. So in the hydraulic system of equipment installation in addition to the pumping station equipment, a lot of work is the installation of the pipe, the pipe installation process is the most important work, is the online circular pipe pickling work, especially the carbon steel pipe, because of the severe corrosion than the stainless steel pipe and a little touch water rust will return, carbon steel pipe derusting process is more complicated than the stainless steel pipe, But because the material is much cheaper than the stainless steel pipe, so there is more than 90% on engineering hydraulic system is the use of carbon steel seamless pipe, so carbon steel tube derusting pickling is the whole hydraulic system is very important part of the pipeline construction technology, the pros and cons of pickling effect is symbol of the primary measure of pipe quality, it will be directly related to the cleanliness of hydraulic system, The system runs normally.

2. Process characteristics and application field

In order to ensure the construction quality of hydraulic pipe, after the hydraulic pipe is blanking, making and welding, pickling treatment should be carried out before circulating washing, which is a crucial process, because the quality of pipe pickling will directly affect the construction quality and construction progress of the whole hydraulic system. Although the effect of hydrochloric acid pickling is good, it is easy to happen quality and safety accidents if you do not pay attention to it, because although the corrosion rate of hydrochloric acid on iron is slow, but with the dissolution of rust, more and more iron will be directly exposed to hydrogen ions, and the dissolution rate of iron will gradually accelerate. With the dissolution of iron, the carbon content of steel tube is relatively increased, and the steel tube becomes brittle. The hydrogen produced by the reaction will penetrate into the tube wall, resulting in hydrogen embrittlement. In addition, too much iron dissolution, thin tube wall, reduced pressure resistance. With the reduction of various performance of the pipeline, it will eventually affect the normal use and life, and even the accident of pipeline burst, and hydrochloric acid to the operator is particularly big harm, in the operation is very inconvenient, acid discharge has a great impact on the surrounding environment.

3. Prerequisite conditions and requirements for carbon steel pipe circulation pickling

3.1 Methods and general requirements for pipe pickling

Carbon steel pipe pickling will by chemical action, is the purpose of the container on the surface of the oxide, remove rust and oil, to obtain a shiny metal surface, guarantee the purity of the pipe wall, antirust treatment after pickling also makes the bottom of the tube wall metal protection, prevent further corrosion, pipeline pickling is a key link in the process of pipeline construction. There are many pickling methods for pipelines. The current pickling methods include tank pickling and circulation pickling, and circulation pickling can be divided into online pickling and off-line pickling. The circulating pickling method is to connect the pipe with a hose or a flexible joint to form a loop, and use an acid pump to drive acid liquid into the pipe for circulating pickling. It can not remove the installed pipeline, as long as the hydraulic components are disconnected and connected with a hose can be carried out, which is the so-called line circulation pickling. If some short pipe fittings or pipes that are not suitable to form a loop in the installation position are removed, the local hose or connector is connected into a loop for circulation pickling, which is called off-line circulation pickling.

The general requirements for pipe circulation pickling are as follows:

(1) There should be a correct formula, that is, the formula solution of pickling should be able to fully remove the oxide of the pipe wall through chemical action, and can not cause excessive corrosion and re-rust of the pipe wall.

(2) There should be a reasonable pickling circuit to ensure that all parts of the pipe wall can be thoroughly and evenly pickled.

(3) There should be reasonable process flow and strict operating procedures, and special work teams should be equipped to deal with the problems in the process of pickling in time to ensure the smooth progress of pickling work.

(4) After pickling and neutralization, the water should be thoroughly cleaned, and washed with hydraulic oil in time to ensure that all parts of the pipe are evenly oiling, and the suspended matter in the pipe generated during pickling should be washed away.


3.2 Facilities and construction requirements for circulating pickling

3. 2. 1 Requirements for facilities and personnel for circulating pickling

When each set of pickling device is pickling, 8-12 pickling workers should be equipped to form the construction operators of the pickling circuit, so as to ensure that the system can be normally and continuously pickling, and ensure that the pipes after pickling are reset in time, and the oil circulation flushing should be started in time.

3. 2.2 Selection of pickling site

The circulating pickling site is usually located on the top of the underground oil depot or near the oil station of the pickling pipeline system to be pickled, and should generally meet the following conditions:

(1) does not affect the site construction, in case of acid leakage is not easy to damage electrical, mechanical and other precision components.

(2) Ensure that the pipeline connection is simple, and the rubber hose for circulating pickling should be as short as possible.

(3) Close to the water source, power source and air source, all kinds of sewage discharge is convenient.

3.2.3 Requirements for external conditions

(1) Requirements for water for pickling

When pickling, the water consumption is large, the water quality requirements are higher, usually using tap water or industrial water. The amount of water used for pickling varies with the circuit of the system being pickled. Generally, it should be guaranteed that:
Pressure P = 0. 5 mpa
Flow rate 100-200 m 3 / h

(2) Requirements for compressed air

Compressed air is usually used in purging and drying circulation pickling pipelines, which requires dry compressed air, large flow rate and sufficient pressure. If there is no compressed air delivered centrally, each pickling device should be equipped with an air compressor and air bag, so as to timely clean the debris and water in the hydraulic pipeline after pickling.

3. 3 Short-circuit requirements for circulating pickling circuit

The short-circuit of pickling circuit is an important link in the whole pickling process. It is absolutely necessary to prevent leakage and acid cannot be recycled, and deal with the "dead spot". First of all, to on-site investigation of the pipeline layout of the whole hydraulic system, pay special attention to the high point, low point, far point situation, and then according to the principle of economic and reasonable pipeline of the whole system is divided into a number of circuits. Pickling circuit connection follows the principle of low into high, its purpose is to make the pipe filled with acid everywhere, to avoid the formation of air bags in the inner wall of the pipe without being pickling.

Usually, the director P, T, L as a loop, do disposable pickling, this loop is more important, but also more complex loop, especially when the system is relatively large, valve platform (rack) more, increased the difficulty of pickling, relatively speaking, P tube is more important. The quality of P pipe pickling directly affects the flushing of the system and the trouble-free operation of the valve platform after normal work, because the control oil of the hydraulic valve comes from the P pipe pressure oil.

It is worth mentioning that when considering the pickling circuit shorting, the flushing scheme of the hydraulic system should be taken into account, whether the system working pump is used to wash, or the independent flushing device is used, and whether the washing is brought into the valve platform (the servo valve is short-connected by the flushing plate). The method of circuit shorting and the materials used vary with the flushing scheme. If the system working pump is used for washing, and is brought into the valve platform when washing, then the main P, T and L will enter the main pipe of the system unchanged after circulating pickling, and the flushing pressure is usually 2-5mpa. So the bypass valve and valve seal used in the short bypass must withstand pressure of 5Mpa. Compared WITH THE scheme using independent washing device, THIS scheme has the advantages of short time limit, economical and practical, and is worthy of praise in engineering practice.

Pipes with the same pipe diameter or small diameter change are placed in the same loop for pickling. Pipes with larger diameter are arranged in the front of the loop, while pipes with smaller diameter are arranged in the back. A pickling loop starts from the pickling tank and ends in the pickling tank. Under the condition that the pickling pressure is not more than 0.4mpa, the loop can be longer, but the thin pipe connection should not be too long, which will affect the pickling effect. At the same time, slender steel pipe will cause the temperature rise too fast, the liquid temperature is too high, directly affect the effect of pickling passivation, especially the passivation temperature can not exceed 50℃, otherwise it will destroy passivation. Hydraulic pipe with small diameter (less than 60mm) should be connected directly by hose to the tank, so as to facilitate the inspection of the liquid return of the small pipe.

In the loop where the pipe diameter is greater than 60mm, the high point shall be equipped with an exhaust valve. If the high point of the pipe diameter is particularly large (greater than 219mm), because of its especially large exhaust volume, it is necessary to separately lead the hose back to the pickling box to check whether there is liquid flow, so as to test whether the air in the pipe at the high point is exhausted and full of acid. If the air source pressure is low (less than 0.3mpa), then the pipe diameter above 42mm must be added to the low point of the blowdown valve to prevent residual passivation liquid. The setting of exhaust valve and drain valve is usually carried out during pipeline installation, while the design of hydraulic system pipeline often ignores the setting of exhaust valve and drain valve. Therefore, once neglected in the pipeline installation, it will cause some points of the blowdown valve because of space limitations and difficult to fill. To replace the exhaust valve or drain valve, magnetic drill shall be used to prevent iron debris from falling into the pipe. In the connection of temporary flange or temporary reducer flange, full consideration should be given to the sewage and exhaust of the formal pipeline to prevent the formation of residual passivation liquid and air bag. However, it should be noted that the installation of temporary pipelines must be the same as that of regular pipelines. Arc welding and electric welding should be adopted.


3. 4 Quality standards for pickling

After the pickling of the pipeline, the quality inspection of the pickling pipeline shall be carried out. The quality standards are:

(1) The inner wall of the pipe is iron-gray white, and the rust and oil pollution are all removed. Tube inner wall cleanliness is Hf(equivalent to Sa3).

(2) Pickling parts shall not have pickling or less pickling phenomenon, the inner wall of the tube should be neutral (pickling refers to the steel soaked in acid washing solution for a long time, metal dissolved in acid, so that the steel surface becomes thick, honeycomb pockmarked phenomenon; Underpickling refers to the fact that the steel is soaked in the acid washing solution for a short time, or the acid concentration is not enough, so that there is still a thin rust or iron sheet of the steel after pickling, and black ash is wiped by hand. This phenomenon is called underpickling.

(3) The pipe after pickling is washed by oil circulation, and the inspection results should meet the cleanliness requirements of the drawing design.

3. 5 General provisions for pipe pickling

(1) The arrangement of pickling device: it should not affect the professional construction of the site as the premise, in case of acid leakage, water and so on, it is not easy to damage equipment and electrical appliances, easy to lay the temporary hose for pickling, and easy to carry the pipe nearby, good ventilation, close to the water source, air source and power source, convenient sewage and other principles.

(2) When there is obvious oil spot in the pipeline, the pipeline should be necessary degreased before pickling.

(3) The concentration, temperature and time of acid solution should be mastered according to the degree of corrosion of pipeline pickling to prevent over-pickling.

(4) When the acid mixture is prepared, the water must be put into the tank first, and then the acid is prepared to prevent the acid from exploding when it meets water. When the acid to wear rubber protective equipment, acid should be slow. When the pipe is put into or taken out of the acid tank, do not directly take it by hand. Use special tools to handle it gently to prevent acid water from hurting people.

(5) The temporary pipeline of pickling circuit must use argon arc welding and electric welding as the formal pipeline, without any carelessness.

(6) The crude washing oil should be the same as the oil of the corresponding hydraulic lubrication system or interchangeable.

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