Pipeline pickling before commissioning
Pickling of metals
Pickling of metals is a special technology for removing parts, billet, mill scale, rust or oxide from the surface. The process involves the use of various solutions of acids, alkalis and salts. By pickling, the product is prepared for coating or combining with other elements.
The effectiveness of the method is determined by the physical and chemical properties of the deposits, its density and structural features. The maximum result is achieved when the oxide scale contains a lot of ferrous oxide (FeO), which is well exposed to the ravages of the acid, and hematite (Fe2O3), which is an insoluble compound, is absent.
The pickling procedure is preceded by cleaning the inner surface of pipes and assemblies from scale, slag, corrosive scab and other contaminants. It is carried out mechanically. After that the pipe is thoroughly purged with compressed air. It is important that welding, gas cutting, or shaping a pipe needs to be shaped using heat before it is pickled. If the pipeline has already passed this procedure, then the second one is not needed. However, lubricating systems of fluid-friction bearings, pipelines of hydraulic systems, sliding bearings of electrical machines should be subjected to secondary pickling.
Direct pickling is performed using one of two methods:
- The first method involves the use of an acidic solution (sulfuric, hydrochloric or phosphoric), which is poured into the bath.
- The second method consists in pumping a solution of orthophosphoric acid through the pipeline using special equipment.
Pipeline parts with oil or grease coating must be degreased to speed up the pickling process and reduce the consumption of acidic composition. For this, a 2-3% alkali solution is used, in which the OP-7 emulsifier and trisodium phosphate are added.
This method is used to handle trunk pipelines, where there are slopes, nozzles or plugs through which the solution is drained. This method is necessarily agreed with the customer and the manufacturer.
If it is planned that the installed pipeline will be divided into several sectors, through which the pickling solution will be pumped, then it is important to take into account several factors:
- The capacity of the section should not exceed 80% of the solution volume, otherwise it is difficult to avoid foam penetration into the pipeline being processed (it inevitably appears on the surface of the liquid);
- In order to ensure a smooth discharge of the solution, it is necessary to give the section an inclined position;
- If the section consists of several pipes of different diameter, then its reduction should go in the direction of the drain;
- The flow rate of the acidic solution should be at 3 m / s;
- It is necessary to create conditions under which air plugs will not appear, while all bends and nozzles should be placed strictly horizontally.
The connection is done using inventory rolls, which are made of pipes, and hoses of acid-resistant rubber. At the same time they must withstand a pressure of 0.6 MPa. Plugs are installed on the fittings, and pipes with flanges are put in place of the removed fittings. Its length should be the same as the valve body.
In order to control the pickling process, branch pipes with detachable joints are fixed on the pipe sectors. The quality of the procedure can be judged by the inside state of the nozzle.
Before connecting the pipe section to the equipment, it must be checked for tightness with compressed air. All defects observed are eliminated, and only after that they make a connection to the unit and start pickling. It is carried out in two stages:
- The solution of orthophosphoric acid, heated to 50-60°C, is driven through a pipe. Based on the state of the inner surface, time varies. As a rule, the process lasts at least 2 hours. From time to time the direction of the flow changes to the opposite – for this purpose, the appropriate valves are used. After pumping the solution is poured into the tank, and the nozzles are removed from the pipe. When pickling, it is necessary to check the level of the solution concentration. This is done using chemical analysis, because the hydrometer readings may be inaccurate due to the presence of oxide and other compounds in the acid. At the very end, the looped sector of the pipe is blown with compressed air and filled with a solution of phosphoric acid.
- The second stage involves the immunization, that is, flushing of the inner surface with an acid solution heated to 60°C for 60 minutes. In order to assess the quality of washing, it is necessary to determine the presence and amount of visible impurities in the solution. At the end of the process, it merges into the tank. The pipe is blown again, plugs are removed from the pipes one by one.
Drying quality can be checked with filter paper. When flushing, it is put under a stream of air that comes out of the pipe. If it does not get wet, then it is concluded that the pipeline is dried. After that, the armature is returned to its place, the pipe section is split up and connected to the adjacent ones. Pipes and outlets are closed with plugs to prevent the penetration of dirt and moisture. Now the section can be filled with working medium.
One of the pickling methods used for pipes and pipeline assemblies involves the use of large vats. They are made of sheet or carbon steel. In the latter case, the inner surface is treated with a special rubber material or cement, 5-25 mm thick, resistant to acids. Outside the vat is covered with bakelite lacquer. Standard dimensions of the tank:
- Length: 8-15 m;
- Width: 1-1.2 m;
- Height: 1.5-1.7 m.
For further processing of the pipeline (neutralization of the acid solution, flushing, removal of grease) the same vats are used, but only from carbon steel trimmed from the inside with bakelite lacquer. Acid, alkali, water is heated in special tanks, equipped with coils, through which steam is supplied.
The pickling solution is prepared from water (2/3 of the vat volume), to which acid is added to a certain concentration in a thin stream and with constant stirring. The density of the composition is controlled with a hydrometer.
Pickling with phosphoric acid
Before direct pickling, preparatory work is performed. Screwed pipes are treated with bakelite or other acid resistant laquer. Sometimes use solid oil or other grease. If necessary, the parts are degreased in an alkaline solution heated to 70 degrees.
The pipes are placed in a vat with a 15-20% acid solution, the temperature of which is 50-60 degrees. Due to its action, scale and corrosive scab dissolve. This procedure takes 6-12 hours – time is determined by the state of the inner surface of the pipe. Next, the product is transferred into a vat filled with the same acid, but a lower concentration (2%). In 1-2 hours while it is in the tank, a phosphate film is formed on the surface, which creates protection against secondary corrosion. This effect lasts for almost 6 months.
After “vatting” the pipe is blown and thoroughly dried with compressed air, and its ends are closed with plugs. A sign of high-quality pickling is a dark gray, slightly rough surface with a faint greenish tint.
Pickling with sulfuric or hydrochloric acid
Special additives – inhibitory additives – are necessarily introduced into the solutions of these acids. A two-component additive of ChM brand is used for sulfuric acid. It includes pickling regulators and a frother. The PV-5 composition is used for hydrochloric acid. Catapine additive is suitable for both acids. Inhibitors protect the metal from corrosion and reduce the intensity of acid evaporation. This allows to maintain optimal operating conditions and reduce acid consumption.
Pickling technology involves:
- Immersion of pipes in a vat with a solution heated to 20-30 degrees. The time of the procedure varies from 10 to 24 hours depending on the degree of damage to the inner surface with a corrosive coating. If you want to reduce the time, the solution is heated and the parts are shaken periodically.
- Removing of pipes and assemblies from the vat, repeated flushing in running water.
- Elimination of acid residues: parts are transferred to a container into which soda or lime solution is poured, and left for 1 hour.
- Repeated flushing in hot water.
- Air drying.
Flushing of parts in cold water is allowed, after which they are dried with steam or compressed air. Dry pipes are treated with oil, dropping them into the tank, the inner surface is treated with a soft brush with a long handle or by pulling the oil napkin. Then the parts are stacked on the shelves for 2-3 hours. With the passage of time, ends of the pipes are closed with plugs.
Where to order the pickling of metals
One of the activities of ASGARD-Service Company is pickling of metals. To get a detailed consultation, order a service and find out the cost, please contact us by phone or e-mail indicated on our website. All work is carried out in compliance with pickling technology and safety rules, using high-quality consumables and equipment. This allows to guarantee the high quality of the processing of the metal surface.