Step-by-step operation guide for pressure-sealing of pipelines: From construction preparation to safe withdrawal
In industrial fields such as petroleum, chemical engineering, urban gas and heat supply networks, pipelines are often tasked with transporting high-pressure, flammable, explosive or toxic media. When pipelines experience leaks, need to be renovated or additional valves are installed, traditional shutdown and purging operations not only cause significant economic losses but also may lead to environmental and safety issues due to the emission of the media. The pressure-bearing pipeline sealing technology has thus emerged: It can isolate the working pipeline section through mechanical sealing without shutting down the pipeline or reducing the pressure of the medium, thereby enabling safe and efficient on-site emergency repairs and renovations. A complete pressure sealing operation process can be systematically summarized into five steps. These five steps are closely interlinked, and each step involves precise equipment operation, strict safety control, and rich on-site experience. This article will break down the core content, technical points, and precautions of these five steps one by one, helping practitioners and engineering managers establish a clear construction framework. Step 1: Construction Preparation and On-site Survey Any successful pressure-sealing operation begins with meticulous pre-planning. Although this step does not directly affect the pipeline, it determines the safety boundaries for all subsequent operations. 1.1 Confirmation of Technical Data and Operating Conditions The operation team must first obtain the design drawings, operation records and previous maintenance files of the pipeline to be operated, and clearly define the following key parameters: pipeline material (carbon steel, stainless steel, ductile iron or PE), nominal diameter and wall thickness, type of transported medium (natural gas, crude oil, refined oil, water, steam or chemical products), normal operating pressure and temperature range, design pressure and maximum allowable operating pressure. At the same time, they need to understand the type of external anti-corrosion coating of the pipeline (3PE, epoxy coal tar pitch or sacrificial anode) and the distribution of underground facilities around the pipeline to avoid secondary damage during construction. 1.2 Safety Protection and Emergency Preparedness The operation area with pressure must be designated as a restricted zone, and clear safety signs and barriers should be set up. All personnel entering the site must wear anti-static work clothes, safety helmets, protective gloves and anti-shock shoes; when dealing with flammable and explosive media, portable gas detection alarms must also be equipped. Adequate dry powder fire extinguishers, fire sand and emergency leak sealing equipment should be provided on site, and escape routes should be planned in advance. Pressure gauges and thermometers should be installed upstream and downstream of the operation point for real-time monitoring. In case of necessity, a remote data collection and alarm system should be set up. 1.3 Equipment and Tool Inspection Before unloading, a comprehensive inspection of all pressure-sealing equipment is required: whether the cutter disc of the hole-opening machine is sharp, and whether the feed mechanism is flexible; whether the sealing components of the sealing device are intact and free of aging cracks; whether the bidirectional sealing of the clamp valve is reliable; whether the oil level, oil quality and pipe joint of the hydraulic power station are normal. All equipment should be placed in categories according to model and applicable pipe diameter, and sufficient spare parts for wear and tear (sealing rings, O-rings, blades, lubricating grease, etc.) should be prepared. For new equipment used for the first time or equipment after major repair, a trial run should be conducted on a simulated pipeline in advance. 1.4 Welding Process Qualification and Personnel Qualifications If the work involves welding flange sections or sealing elbows on in-service pipelines, a welding process qualification must be completed in advance to ensure that the welding parameters will not damage the mechanical properties of the pipeline base material. Welders must hold special equipment welding operation certificates for the corresponding projects. At the same time, personnel conducting pressure-bearing opening and sealing operations should have received professional training and possess the qualification certificates for practical operation assessment. The core value of the first step operation lies in "resolving the problem before the operation begins". Ignoring any aspect of preparation may lead to leaks, fires or even explosions during the subsequent high-pressure operations. Only by fully identifying risks in advance can a solid safety foundation be laid for the subsequent four steps of operation. Step 2: Welding connection pieces and conducting pressure test Once the initial step of confirming the availability for the operation is completed, the construction enters the substantive operation stage: welding sealing pipe fittings (such as flange sections, split three-way joints, or welded blocking plates) onto the pipeline to establish the connection interface between the equipment and the pipeline. 2.1 Pipe Fitting Positioning and Welding Based on the pre-designed opening positions, mark the center lines of the flange sections or blocking three-way joints on the pipeline. Use opening positioning fixtures or magnetic drills to assist in fixing the pipe fittings, ensuring that their axes are vertically intersecting with the pipeline axis, with an allowable deviation of no more than 1 millimeter. Before welding, remove the anti-corrosion layer, rust, and oil stains from the pipeline surface, exposing the metallic luster. Use low-hydrogen type welding rods or corresponding gas protection welding processes, and follow the qualified welding parameters for base welding, filling, and covering welding. For high-pressure or pipelines containing corrosive media such as hydrogen sulfide, local dehydrogenation treatment or heat treatment may be required after welding. 2.2 Non-destructive Testing of Welds After welding is completed, all circumferential welds and longitudinal welds in contact with the medium shall undergo non-destructive testing. Common methods include magnetic particle testing (MT) or penetrant testing (PT) to detect surface cracks; for pipelines with higher design pressures, ultrasonic testing (UT) or radiographic testing (RT) shall also be used to inspect internal defects. The test results must meet the qualified level of relevant standards (such as NB/T 47013). 2.3 Connection Component Pressure Test In order to verify the welding quality and sealing performance, a pressure test needs to be conducted on the connecting components and the cavity formed by them and the pipeline. The test medium usually uses clean water or nitrogen. The test pressure is 1.5 times the operating pressure of the pipeline and is not lower than the design pressure. The holding pressure time should be no less than 15 minutes. All weld seams and flange sealing surfaces should be checked for no leakage and no visible deformation to be considered qualified. After the test, the medium should be drained, and drying or replacement treatment should be carried out according to the requirements of the pipeline's transported medium. 2.4 Protection of Flange Sealing Surfaces After the pressure test is passed, the flange sealing surfaces should be cleaned immediately and coated with anti-rust oil. Protective blind plates or covers should be installed to prevent foreign objects from entering.
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