Pressure test of Petrochemical Pipeline
Petrochemical plant is a production plant that mainly produces all kinds of fuel oil and chemical raw materials based on petroleum cracking processing. Most of the process media in the unit are flammable, explosive and toxic substances. Therefore, during the construction of petrochemical plant, the installation quality of various petrochemical pipelines must be strictly controlled, and their leakage is strictly prohibited, otherwise serious consequences will be caused. In the process of pipeline installation, in order to check the quality of weld and the tightness of flange connection, pipeline pressure test is a very important and essential key process. This paper briefly describes the preparation, conditions, methods, steps and safety technical measures of pipeline pressure test, so as to provide reference for similar projects.
Preparations before pressure test
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There are many process pipeline systems and complex trends in large petrochemical plants. In order to make the pressure test work normally, sufficient technical preparations must be made in advance. Before pressure test, the pressure test scheme shall be prepared according to the process flow chart, the pressure test process shall be clarified, and the pressure test medium, methods, steps and various safety technical measures shall be determined as required.
Pipeline integrity inspection
Pipeline integrity inspection is a necessary work before pipeline pressure test. Pressure test shall not be carried out for systems that have not passed the integrity inspection. Integrity inspection is based on technical documents such as pipeline system diagram, pipeline plan, pipeline section, pipeline support diagram, pipeline simple pressure test system diagram, etc. The first method of integrity inspection is that the construction team shall inspect the pipelines constructed by itself according to the design drawings; the second is that the construction technicians shall recheck each pipeline of the pressure test system one by one; the third is that after all pipelines in the pressure test system are inspected as qualified according to the design drawings, they shall be reported to the quality supervisor and the owner for review and quality inspection. The content of integrity check is divided into two parts: hardware and software. Hardware inspection is to check whether the model, specification, material, coordinate, elevation, flow direction of valve, direction of handle, type and position of buttress, support and hanger, slope, verticality and levelness of pipeline installed on site meet the design requirements and specifications; The welding of pipes shall also be comprehensively inspected, mainly including whether the pipe welding has been completed, whether the NDT is qualified, whether the welded junction requiring heat treatment has been treated, whether the stainless steel welded junction has been pickled and passivated, etc. Software inspection is to check whether the pipeline installation records and welding records are correct and complete, and whether various record sheets have been signed and confirmed.
Pressure test is a dangerous work. Therefore, sufficient material preparation shall be carried out before this work. It mainly includes the maintenance, safety inspection and mobilization layout of pressure test equipment; Calibration, inspection and installation of various instruments and meters for pressure test; Installation and layout of temporary pressure test pipelines and accessories; Preparation of blind plate, bolt, nut, gasket and other materials for pressure test; Implementation of isolation measures for equipment, instruments, valves, pipe fittings, safety valves, flow meters, etc; Supply of materials required for various safety technical measures and site layout during pressure test.
Pressure test is a complex process. In order to complete this work, effective organization and coordination must be done in terms of personnel. A special pressure test team shall be established to be fully responsible for the pressure test of the pipeline, and the pressure test personnel shall be equipped with effective communication tools to make the whole pressure test proceed in an orderly manner.
Pressure test medium
Pipeline pressure test media are generally divided into two categories: one is gas and the other is liquid. The gas is generally air, dry oil-free air and nitrogen. Liquid generally adopts water, clean water and pure water. The medium used for pressure test is generally determined according to the specific requirements of process pipeline. Hydraulic pressure is safer than pneumatic pressure in pressure test. Therefore, if there are no special requirements for the pipeline, water is generally used as the pressure test medium. According to the requirements of pipeline system, there are strict regulations on the requirements of pressure test water. For clean water for pressure test, a suitable filter shall be set in the water filling pipeline. The requirement for test water for stainless steel pipeline is that the chloride ion content shall not exceed 25ppm. The pipes with sand blasting treatment inside the pipeline shall be tested with aqueous solution containing appropriate rust inhibitor after installation, and the rust inhibitor shall not affect the subsequent process medium.
The test pressure of the pipeline under internal pressure is 1.5 times of the design pressure of the pipeline; When the design temperature of the pipeline is higher than the test temperature, the test pressure shall comply with the following formula: Ps=1.5δ1/δ2 δ1/δ2>6.5, take the value of 6.5; When PS produces stress exceeding the yield strength at the test temperature, the test pressure shall be reduced to the maximum test pressure when the pipeline pressure does not exceed the yield strength. The test pressure of pneumatic test pipeline is 1.15 times of the design pressure; When the pressure of the pneumatic test pipeline is greater than 0.6MPa, it can be carried out only after it is specified in the design document or approved by the owner and safety measures are taken. Before the hydraulic test, the pressure test system shall be filled with water first, the exhaust valve shall be set at the highest point of the pipeline to exhaust the air, the pressure gauge shall be installed at the highest position and the position easy to observe on the ground, the measured pressure shall be subject to the reading of the pressure gauge at the highest position, and then the pressure shall be increased slowly. When the test pressure is reached, it shall be stabilized for 10 minutes. After comprehensive inspection, it shall be qualified if there is no leakage and no deformation through visual inspection. During the air pressure strength test, the pressure shall be increased slowly. When the pressure rises to 50% of the test pressure, it can continue to increase the pressure step by step according to 10% of the test pressure after confirming that there is no leakage and abnormality. Stabilize the pressure at each level for 3 minutes until the test pressure is maintained for 10 minutes, and then reduce the pressure to the design pressure. It is qualified if there is no leakage and no deformation through visual inspection. After the pipeline strength test is qualified, the air tightness test of the pipeline shall be carried out. For the air tightness test with liquid as medium, after the pressure of the pipeline system is reduced to the pressure specified in the design document, the direct observation method is used for comprehensive inspection, and it is qualified if there is no leakage. For pipelines with hydraulic pressure for strength test and pneumatic pressure for air tightness test, the pressure shall be increased slowly after the pipeline purging is qualified. When the pressure rises to 50% of the test pressure, check it. After confirming that there is no leakage and abnormality, continue to increase the pressure. After that, check every 10% of the test pressure until the test pressure, and then conduct a comprehensive inspection and stabilize the pressure for 30 minutes, It is qualified if there is no leakage and pressure drop. For the pipeline with air pressure strength test, when the strength test is qualified, directly reduce the test pressure to the pressure of air tightness test, stabilize the pressure for 30 minutes, and it is qualified if there is no leakage and pressure drop. The inspection adopts the method of brushing leakage detection fluid at the welded junction, flange and seal. (the most professional safety production management – risk world network)
Safety technical regulations for pressure test
Pipeline pressure test is very dangerous, and various safety technical measures shall be taken. Generally, the length of the hydraulic test pipe section shall not exceed 1000m. The temporary reinforcement measures used for the test shall be checked, confirmed to be safe and reliable, and marked. The pressure gauge for the test shall be within the verification qualification period, the accuracy shall not be lower than grade 1.5, the measuring range shall be 1.5 ~ 2 times of the measured pressure, and there shall be no less than 2 pressure gauges in the pressure test system. When injecting water into the hydraulic test system, the air shall be exhausted, preferably at an ambient temperature of more than 5 ℃, otherwise anti freezing measures must be taken. For alloy steel pipe system, the liquid temperature shall not be lower than 5 ℃. In case of leakage during the test, it shall not be repaired under pressure. After the defects are eliminated, the pressure test shall be carried out again. After the pressure test is qualified, the pressure shall be relieved in time, and the liquid in the pipe shall be drained in time during the liquid pressure test. After the system test, all temporary blind plates shall be removed in time and the pressure test records shall be filled in. During the pressure test, a warning line shall be set in the pressure test area. Irrelevant personnel shall not enter. The operator must obey the command and shall not open and close the valve at will.
Source: Network Arrangement – Piping System Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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