I. Pre-installation Inspection and Preparation 

Before installation, it is necessary to verify that the nominal pressure, nominal diameter and material of the flange are consistent with the design drawings. The focus should be on inspecting the sealing surface of the flange. It must be flat and smooth without burrs, radial grooves, scratches, etc. For the concave-convex flange, the height of the convex surface must not be lower than the depth of the concave surface, and the two should be able to fit naturally. At the same time, it is necessary to confirm that the flange has undergone ultrasonic testing and has no layering or other defects. The gasket should be soft in texture, not aged or deteriorated, without layering or cracks, and the gasket must not be reused. The sealing surface must be cleaned to expose the metallic luster using special tools before assembly, and rust and oil stains must be removed, but the sealing surface must not be damaged. 

II. Assembly and Positioning of Pipes and Flanges 

This is the most crucial step in the installation process. The center of the flange must be aligned with the center of the pipe in a straight line, and the sealing surface of the flange should be perpendicular to the centerline of the pipe. Specifically, for nominal diameters less than 300 millimeters, the deviation is allowed to be no more than 1 millimeter; for diameters greater than 300 millimeters, it is no more than 2 millimeters. 

There are several special requirements for butt-weld flanges: The gap between any point on the inner wall of the flange and the outer surface of the pipe must not exceed 2 millimeters; the total gap between two points in the radial direction must not exceed 3 millimeters (a 1 mm relaxation is allowed for large diameters); the misalignment should not exceed 10% of the pipe wall thickness and should not exceed 1 millimeter; the distance that the pipe extends into the flange is the weld leg size plus 1 millimeter, but it should not be less than 4 millimeters. 

During assembly, it is necessary to first clean the inner and outer sides of the pipe fittings. After inserting the flange into the pipe end, use a beveling tool or angle ruler to align it in two perpendicular directions, first up and down, then left and right. After confirming that there are no errors, perform symmetrical spot welding for fixation, with at least 4 points. Special attention should be paid that for accessories such as branch pipes and elbows, they should be welded first before assembling the flange to prevent welding deformation from affecting the position of the flange. 

III. Welding Process 

The welding quality of the butt-welded flange directly determines the connection strength. The weld seam must meet the first-level weld standard and undergo 100% ultrasonic testing. The performance of the welding material should match the base material, and the welding process qualification should be completed before construction. The thickness of the positioning weld seam should be no less than 3 millimeters and the length should be no less than 40 millimeters. The welding environment requires that the gas protection welding wind speed should not exceed 2 meters per second and the relative humidity should not exceed 80%. 

If it is a stainless steel butt-weld flange, the welding current should be reduced by approximately 20% compared to that for carbon steel. The narrow weld bead technique should be adopted to control the heat input, prevent hot cracks, and the welding rods must be kept dry. After welding, the slag and spatter should be thoroughly removed to ensure the smoothness of the inner hole, and there should be no residual spatter on the flange surface. 

IV. Flange Connection and Bolt Tightening 

The same specification of bolts should be used on the same pipeline, with the installation direction kept consistent and the nuts on the same side. On horizontal pipelines, the top two bolt holes should remain level, and on vertical pipelines, the two holes closest to the wall should be parallel to the wall surface. 

When tightening, it must be done in a symmetrical crosswise and gradual uniform manner. Never force a certain part to be fully tightened at once. After tightening, the length of the bolt protruding beyond the nut should not exceed 5 millimeters, which is approximately 2 to 3 turns. For bolts and nuts made of high-temperature, low-temperature, stainless steel or alloy steel, graphite engine oil or molybdenum disulfide grease should be applied for lubrication. It is strictly prohibited to use multiple layers of gaskets or inclined washers to eliminate deviations. For large-diameter gaskets, they should not be spliced in a flat joint manner; instead, inclined搭 or labyrinth forms should be used. At the same time， it is strictly forbidden to perform forceful alignment， heat the pipe， or stack gaskets to make up for the insufficient threads， or engage in any other illegal operations. 

V. Installation Location and Layout Requirements 

The distance between the flange and the support or the wall should be no less than 200 millimeters to facilitate future disassembly and assembly. The net distance between the branch pipe flange and the outer wall of the main pipe should be no less than 100 millimeters to ensure that bolts can be inserted. The flanges of buried pipelines should not be directly buried underground. If they must be buried, anti-corrosion measures should be taken, and inspection wells should be set at the connection flange joints. The parallelism deviation between two connection flanges should not exceed 2 millimeters. 

VI. Post-operation Thermal Tightening 

This step is often overlooked by many people, but it is extremely important. After the steam injection operation is completed, it should be maintained at the working temperature for 24 hours before performing the tightening. When the design pressure is less than 6 megapascals, the maximum internal pressure during the tightening should not exceed 0.3 megapascals; when the design pressure is greater than or equal to 6 megapascals, the maximum internal pressure should not exceed 0.5 megapascals. The tightening should be moderate and corresponding safety measures should be taken. 

Summary

The installation of butt-weld flanges can be summarized in eight words: check rigorously, align precisely, weld through completely, and tighten evenly. Before installation, strictly check the specifications and sealing surfaces. During assembly, ensure coaxiality, perpendicularity, gap and edge deviation meet the standards. During welding, guarantee the quality of the primary weld and conduct flaw detection. During tightening, apply force symmetrically and crosswise evenly, and perform hot tightening after operation. By strictly following these requirements, common problems such as leakage, loosening, and weld cracking can be effectively avoided, ensuring the long-term safe operation of the pipeline system.