Before anticorrosion treatment, the surface of pipeline substrate should be treated to remove water, oil, dirt, pollutants, rust and scale. There are many methods of surface treatment. Generally, manual derusting, sand blasting or chemical derusting are commonly used. When manually derusting, the quality standard should reach St3 level; When sandblasting or chemical derusting, the quality standard should reach Sa2.5 level, and the surface roughness should be 30 ~ 50? M is more suitable. The treated surface shall be primed within 4 hours.
External anticorrosion design of pipeline
Petroleum asphalt anticorrosive coating is the oldest anticorrosive coating, which is used well in most dry areas. With the development of technology, epoxy coal tar pitch and zinc-rich coating have replaced petroleum pitch as anticorrosion coatings for steel water pipelines. Epoxy coal tar pitch series coatings have always been the first choice for anticorrosion coatings of steel water pipelines because of their excellent anticorrosion performance and low price.
The temperature of the conveying medium of epoxy coal tar pitch anticorrosive coating is generally not higher than 1 10℃. In order to meet the requirements of different corrosive environments, it is divided into three levels: ordinary level, enhanced level and extra enhanced level. The ordinary structure is one primer and three topcoats, and the dry film thickness is ≥ 0.3 mm; The strengthening grade structure is primer → topcoat → topcoat, glass cloth, topcoat → topcoat, and the dry film thickness is ≥ 0.4 mm; The structure of special strengthening grade is primer → topcoat → topcoat, glass cloth, topcoat → topcoat, glass cloth, topcoat → topcoat, with dry film thickness ≥0.6mm, and topcoat, glass cloth and topcoat shall be applied continuously. In practical engineering, the grade should be selected according to the engineering conditions and conveying medium.
During the construction of anticorrosive coating, the ambient temperature should be higher than 5℃ and the air humidity should be lower than 80%. 5% diluent, 20 ~ 25mm glass cloth blank holder and 100 ~ 150mm lap length. Voltage inspection of coating: general level: 2000V reinforcement stage: 2500V special reinforcement grade: 3000 V. Coating inspection: every 10km, with leakage point ≯5, measure with low-voltage audio signal leak detector.
Pipeline anticorrosion design
The internal anticorrosion of steel pipes needs to choose different anticorrosion technologies according to different conveying media. Commonly used anti-corrosion technologies include cement mortar lining, anti-corrosion coating and plastic lining.
Cement mortar lining is mainly used for pipelines conveying water supply and drinking water. The lining shall be portland cement, ordinary portland cement or slag portland cement, numbered 425 or 525. Before lining construction, check the deformation of the pipeline, the maximum vertical deformation shall not be greater than 2% of the pipe diameter, and there shall be no condensation and water accumulation on the inner wall. The weight ratio of cement mortar is in the range of 1: 1 ~ 1: 2, the slump of cement mortar should be 60 ~ 80 mm, and the compressive strength should not be lower than 30Mpa.
Generally, three layers of epoxy coal tar pitch anti-corrosion coating can be used for anti-corrosion in pipelines conveying non-drinking water, especially sewage. One layer of primer and two layers of paint, or two layers of inorganic zinc-rich primer and epoxy resin coating, the dry film thickness is 0.2 ~ 0.4 mm ..
Cathodic protection design of pipeline
Cathodic protection belongs to electrochemical protection, which is an anti-corrosion technology that changes the corrosion potential of metal by applying current to reduce its corrosion rate. Usually, two methods are adopted, namely, sacrificial anode cathodic protection and impressed current cathodic protection. Buried steel pipelines are usually protected by sacrificial anode cathodic protection and anti-corrosion coating, which are widely used in anti-corrosion engineering and have good results.
In the design of cathodic protection system, we should first understand the length, diameter, wall thickness and coating type of the pipeline, as well as the soil geological structure and soil resistance of the pipeline's geographical location, and calculate and determine the type, specification, quantity and service life of sacrificial anodes. Alloys of aluminum, zinc and manganese are commonly used as sacrificial anodes. The open-circuit potential of anode is above 1.5v, and a group of anodes are generally buried in the range of 200 ~ 500m. The anode cable can be vv rubber steel core cable, and the filling material can be 50%CaSO4 and 50% bentonite. The general design service life can reach 15 ~ 30 years.