Industrial Pipeline Installation Work Instruction

XII. Operating instructions for the installation of pressure pipelines (for reference only) 1 Scope This procedure applies to the installation of carbon steel pipelines in various industrial pipelines and utility pipelines. 2 Materials 2.1 Pipes, pipe fittings, valves, and coil tubing must be accompanied by a certificate of conformity from the manufacturer; otherwise, testing for the missing items shall be carried out to ensure that their specifications meet the current ** or industry standards. 2.1.1 Before use, pipes, pipe fittings, and valves shall undergo a visual inspection; their surfaces are required to be free from: 2.1.1.1 defects such as cracks, shrinkages, inclusions, folds, and double layers ； 2.1.1.2 Rust or dents not exceeding the negative wall thickness tolerance ； 3 Common Tools 3.1 Power tools: Electric groove cutters, grinding wheel cutters, threaders, electric hammers, pipe benders, hand drills, portable oxygen-acetylene pipe cutters, electric pressure testing pumps, lathes. 3.2 Measuring tools: steel tape measures, reel tapes, flange squares, steel rulers, spirit levels, magnetic plumb bobs, protractors, compasses, welding inspection gauges, levels, outside calipers, thickness gauges, etc. 3.3 Tools: pipe wrenches, adjustable wrenches, socket wrenches (regular socket wrenches, special socket wrenches), ring wrenches, sleeve wrenches, chain pliers, wire cutters, screwdrivers, scissors, hand saws, hammers, flat chisels, punches, pipe compressors, sledgehammers, jacks, ladders. 3.4 Lifting equipment: chain hand hoists, rigging (hemp ropes, nylon ropes, steel wires), snap hooks and thimbles, steel bars, crowbars, cranes, blocks and pulley systems, winches. 3.5 Welding/cutting equipment: welding machines (manual arc welders, AC welders, DC welders), oxygen cylinders, acetylene cylinders, pressure regulators, welding torches, cutting knives, pressure gauges, rubber hoses, etc. 4 Operating Conditions 4.1 The civil engineering works related to the pipelines have been completed and approved upon inspection, ensuring that the pipeline installation work can be carried out in full. 4.2 The equipment connected to the pipeline has been leveled, aligned, and secured, and secondary grouting has been carried out. 4.3 All required drawings, documents, technical files, etc., are now available, and have undergone drawing review as well as technical/safety briefings. 4.4 The work instruction sheets and start-up reports have been prepared and approved, and the necessary technical training has been completed. 4.5 The pipes, valves, pipe fittings, etc. have passed inspection, and have been rust removed, cleaned, degreased, and free of internal corrosion. 4.6 The tools and other items specified in the work instruction are ready. 4.7 The labor force, construction environment, etc., meet the requirements; the supply of water, electricity, gas, and other utilities required for construction can be ensured according to the schedule. 5 Operating Procedures 5.1 The operating sequence is shown in Figure 1. Figure 1 Process flow diagram 5.2 Installation of medium and low-pressure pipelines 5.2.1 Connection methods. The connection of medium- and low-pressure pipes generally involves methods such as welding, flange connection, and threaded connection. 5.2.2 Pipe Fitting Processing 5.2.2.1 Stamped elbows are divided into stamped seamless elbows and stamped welded elbows, and are generally manufactured by factories. 5.2.2.2 Welded Elbows Welded elbows are also known as shrimp-shell elbows, as shown in Figure 1. The shrimp waist is composed of two terminal segments and several intermediate segments, with the terminal segments being half the size of the intermediate segments. Let the dorsal and ventral heights of the middle segment be A and B, respectively; then the dorsal and ventral heights of the terminal segment are A/2 and B/2, respectively. The back height and belly height of the end section can be calculated using the following formulas: A/2 = (R + D/2) tan B/2 = (R + D/2) tan Where: A/2 – Back height of the end section (mm) D – Diameter of the pipe (mm) B/2 – Belly height of the end section (mm) α – Bending angle N – Number of sections in the middle of the elbow Based on the calculated values of the back height and belly height, an unwrapped diagram can be drawn as shown in Figure 2: 5.2.2.2.1 Draw a semicircle according to the pipe diameter, and divide it into 8 equal parts ； 5.2.2.2.2 The various division points on the upper semicircle are intersected with the diameters 1–9 using vertical lines, yielding the intersection points 2, 3, 4,…… ； 5.2.2.2.3 On the vertical lines at both ends of the semicircle, mark line segments such that 1–1’ = A/2 and 9–9’ = B/2. Then, connect points 1’ and 9’ with a straight line; this will give the desired intersection points 2’, 3’, 4’, etc ； 5.2.2.2.4 Consider another line segment L=πD; divide it into 16 equal parts, and draw several parallel lines perpendicular to L passing through the points 2, 3, 4, … ； 5.2.2.2.5 Use a set square to mark points 1–1’, 2–2’, 3–3’, … on A in Figure 2, place them on B, and then connect them with smooth curves; this will yield the expanded view of the end section ； 5.2.2.2.6 Rotating the developed view of the end section by another 180° yields the developed view of the middle section. Cut the drawn expansion diagram to size, wrap it around the outer wall of the pipe and mark lines; then, refer to Figure 3 to assemble the elbow and carry out welding. The number of segments in a welded elbow: when there are no specific requirements in the design, a 90° elbow should have no fewer than 4 segments, a 60° or 45° elbow should have no fewer than 3 segments, and a 30° elbow should have no fewer than 2 segments. 5.2.2.3 Bending of pipes 5.2.2.3.1 Cutting length: When bending pipes to different angles, the cutting length can be calculated based on the bending angle and radius of curvature of the pipe. The calculation formula is as follows: L = 2πR/n (mm), where L represents the unfolded length of the pipe (i.e., the cutting length in mm), and R represents the radius of curvature of the pipe (in mm) ； N —— number of minutes ； n=360°/α (α is the angle of the elbow). Substituting this into the formula gives L = 2πα/180°. 5.2.2.3.2 Requirements for the bending radius of pipes: For hot bending – it shall be no less than 3.5 times the outer diameter of the pipe ； Cold bending – shall be not less than 4 times the outer diameter of the tube ； Welded elbows – shall be no less than 1.5 times the outer diameter of the pipe ； Stamped elbow – should be not less than the outer diameter of the pipe. 5.2.2.3.3 d is the outer diameter of the pipe: the radius of curvature R refers to the dimension at the center of the pipe. In general estimates, if R is set to 4d, then for a 90° bend, L = (6~7)d ； For a 45° bend, L = (2.5~3)d. 5.2.2.3.4 Before bending, sand should be placed inside the pipe. The sand must not contain any dirt or combustible materials; it should be placed on a steel plate and heated to dry it. The particle size of sand is generally: 1.6 mm when the pipe diameter is 20–25 ； When the pipe diameter is 40–75, it is 3.2 mm ； When the pipe diameter is 100~125, it is 4.8mm ； The pipe diameter ranges from 150 to 300, which is 6.4 mm. 5.2.2.3.5 Pour the dried sand into the steel pipe, and while pouring, strike the pipe wall with a hammer to ensure that the sand particles are compacted evenly. After filling with sand, the tube opening must be tightly plugged with a wooden stopper. 5.2.2.3.6 For the areas of the steel pipe that need to be heated during bending, mark them with white lead oil. The minimum bending radius must be no less than 3.5 times the outer diameter of the pipe (generally, 4 times the pipe diameter). The length of the area to be heated is calculated using the following formulas: For a 90° elbow, S = 2πD = 6.3D; for a 135° elbow, S = 8/3πD = 8.4D. Here, S represents the length of the area to be heated ； D – Outer diameter of the steel pipe. 5.2.2.3.7 Place the section of the steel pipe to be heated on a floor furnace; surround the heated section with burning coke, and cover it with thin steel plates on top. Continue heating until the surface of the steel pipe turns golden red and starts to peel off (750–1000°C). During heating, flip the steel pipe at any time. 5.2.2.3.8 After the steel pipe has been heated to the required temperature, one end of the pipe is tied with white-brown rope, and the pipe is pulled onto the bending platform by manual force (it can also be pulled using a winch, pulleys, etc.). One person uses a bucket of cold water to pour it on the parts of the steel pipe that do not need to be bent, while several people push a bender to pull one end of the steel pipe, thereby bending it to the desired angle. 5.2.2.3.9 The outer wall of the bent steel pipe thins out, but it must not be less than 10% of the wall thickness of the connected pipe. When bending welded steel pipes, the welds should be placed in areas where they are subjected to minimal stress and can be easily inspected and maintained after installation. Generally, the weld forms a 45° angle with the flexural surface. “Type II compensators should preferably be formed by bending a single pipe. When the expansion bend itself requires a joint, the joint should be located in the middle of both arms. 5.2.2.3.10 After bending is complete, wait for the steel pipe to cool down, then remove the wooden plugs, clean away the sand, and rust-remove and coat it with oil. 5.2.2.3.11 When bending by heating, the following points should be noted: 5.2.2.3.11.1 The sand and gravel filler should be pre-heated and dried; 30% fine sand should be mixed into the coarse sand and pebbles to ensure a uniform and appropriate composition, thereby enhancing its compactness. While pouring the sand and gravel, it is necessary to tap on the surface continuously. To ensure the required ellipticity at the pipe bending section, it is essential to compact it thoroughly. When there is a large amount of bending work and conditions permit, a common sandblasting table can be assembled. 5.2.2.3.11.2 The heating of the pipe should be uniform; uneven heating must be avoided to prevent surface irregularities. Generally, the temperature should be maintained at around 800–900°C, and should not exceed 1000°C℃ ； 5.2.2.3.11.3 When bending, the pipe should be kept flat; when pulling the pipe, the force applied should be even and straight, and it is not advisable to apply force upward ； 5.2.2.3.11.4 To control the degree of bending within a specified range, cold water can be used; once the bending degree in a particular area has reached the desired level, cold water can be applied there to fix its position ； 5.2.2.3.11.5 Use a template for inspection at any time during bending; it is generally assumed that the pipe will rebound somewhat after bending, so the required angle can be 3°–5° greater than the angle specified for bending deformation ； 5.2.2.3.11.6 The sand on the back side after cooling should be clean to prevent it from sticking to the pipe wall. 5.2.2.4 Equal-diameter tee: The method for drawing the expansion diagram of an equal-diameter tee is the same as that for a tees with different diameters; the latter will be used as an example for clarity here. Based on the actual diameter of the pipes, a side view of the tee is drawn, as shown in Figure 4 ； Draw a semicircle at the end of the branch pipe, dividing the circle into 6 equal parts ； Draw lines perpendicular to each other with the respective division points as diameters, such that they intersect on the main pipe; thus, the line segments 1–1, 2–2, and 3–3 can be obtained…… ； Calculate the circumference of the branch pipe: L = πD (where D is the diameter of the branch pipe). Then draw a line segment of length L and divide it into 12 equal parts ； Draw lines perpendicular to L through each division point, and on each of these perpendicular lines, measure 1–1, 2–2, 3–3, 4–4, 3–3, 2–2, 1–1 from left to right ； Connecting the points 1, 2, 3, 4, 3, 2, 1 with smooth curves yields the left half of Figure 5 ； The right half is symmetric to the left half; by drawing the right half using the symmetry method, the expanded view of the tee branch is obtained. Make a template from the expansion diagram of the tee branch, and wrap it around the pipe’s outer surface to mark and cut the material. Fasten the prepared tee branch to the main pipe, and you can mark the oval hole that needs to be cut. 5.2.2.5 Oblique tee: Determining the joint line: Draw the front view and side view of the tee. Draw semicircles on the tube ends of the two projection planes respectively and divide them into 6 equal parts. In the side view, draw vertical lines from the equally spaced points on the semicircle to the circumference of the main pipe. Then draw horizontal lines to the right; these lines intersect at points corresponding to those where parallel lines drawn from the equally spaced points on the semicircle of line AB in the front view meet. Connecting these intersection points forms the desired joining curve, as shown in Figure 6 ； Unfolded diagram of the branch pipe: In the front view, extend the line 1–1 from the end face AB of the small pipe; its length is equal to the circumference of the small pipe. This line is divided into 12 equal segments. Perpendicular lines drawn from each of these segments to the line 1–1 intersect the parallel lines drawn from various points on the joint line, and the points of intersection form a curve, which represents the unfolded diagram of the branch pipe, as shown in Figure 6. Development drawing of the main surface: Draw a vertical line from point C; its length should be equal to the circumference of the large pipe (or half of that circumference). Then, using the midpoint 1 as a reference, mark all points along the arc in side view ‘a’. Draw lines parallel to DC to the left, and draw vertical lines from each of these points downward. The intersections of these lines form a curve, which constitutes the actual developed shape of the opening. An equal-diameter inclined tee can be drawn in the same manner. 5.2.2.6 Reducers These are fittings used to change the diameter of pipes during welding; they are generally available in both concentric and eccentric types, allowing for selection based on construction and design requirements. 5.2.2.6.1 Elbows made of steel plate coils. Template creation: 5.2.2.6.1.1 Draw the elevation views of the large and small ends, as shown in Figure 7 ； 5.2.2.6.1.2 Extend the hypotenuses ab and cd; they intersect at point O ； 5.2.2.6.1.3 With oa and ob as radii, draw arcs to points aE and bF; this yields the development diagram of the large and small ends. 5.2.2.6.2 It is difficult to obtain vertices with large size differences; the difference in diameter is very small, and the intersection points of the diagonal sides are far apart, making it inconvenient to use the aforementioned method for expansion. An approximate approach can be used to draw a template: 5.2.2.6.2.1 Draw a elevation view, as shown in Figure 8 ； 5.2.2.6.2.2 Draw semicircles with AB and CD as diameters respectively, and divide them into 6 equal parts ； 5.2.2.6.2.3 Construct a trapezoidal template with the length of string a as the top, the length of string b as the bottom, and the length AC as the height ； 5.2.2.6.2.4 Using small trapezoidal templates, after joining 12 pieces together, the total length must be rechecked against the circumference to prevent any errors in marking. After repair and trimming, it forms the large and small end development drawing. 5.2.2.6.3 Layout of eccentric reducers 5.2.2.6.3.1 Draw the elevation view of the eccentric reducer, as shown in Figure 9 I ； 5.2.2.6.3.2 Extend line 7—A and line 1—B to intersect at point 0 ； 5.2.2.6.3.3 Draw a semicircle with lines 1–7 as its diameter and divide it into 6 equal parts ； 5.2.2.6.3.4 Draw concentric arcs with 7 as the center and the points that divide the semicircle into equal parts as the radii; these arcs intersect the lines 1–7 to form the points 6’, 5’, 4’, 3’, and 2’ respectively ； 5.2.2.6.3.5 The lines connecting 0 to 06’, 05’, 04’, …… 01 intersect line AB at points 6”, 5”, 4”, 3”, and 2” respectively ； 5.2.2.6.3.6 Draw concentric arc segments with 0 as the center and radii of 0–7, 0–6’, 0–5’, … 0–1 respectively ； 5.2.2.6.3.7 Choose any point on the arc with a radius of 0–7 as the center; using the radius of half of that arc, draw arcs sequentially until points 6’, 5’, 4’, …, 1 are obtained (Figure 9 II) ； 5.2.2.6.3.8 With 0 as the center, arcs are drawn with radii of 0–A, 0–6”, …, 0–B; these arcs intersect sequentially at points on the radii 0–7’, 0–6’, …, 0–1’. Connecting these points with curves yields the development diagram of the eccentric reducer. 5.2.2.6.4 Rolling: Lines are drawn on the steel plate according to the template, and it is cut; then it is rolled into a circular shape using cold bending or hot bending methods. A 1/4 circle arc template is used to check whether the curvature of the inner circle is correct. After adjustments are made to meet the requirements, welding can proceed. 5.2.2.6.5 Forging of large and small ends from steel pipes: When the pipe diameter is small and it is difficult to roll the steel plate, the pipe forging method is often used to create large and small ends. Heating the pipe by dropping it can be done using a blacksmith’s furnace or an oxy-acetylene torch, with a heating temperature of around 800–950°C. When bending the pipe to achieve a consistent diameter, strike it while rotating it, from larger to smaller diameter, ensuring a smooth gradual transition in the pipe’s surface curvature. The hammer surface must be kept flat to prevent pitting on the pipe wall; if it cannot be flattened in one attempt, it can be reheated until it is flattened. When bending the eccentric large and small ends, the lower wall of the tube should not be heated; if heating is used with an oven, the lower wall of the tube can be cooled with water before bending it. When throwing it, rotate it left and right to ensure a smooth transition. To facilitate assessing the roundness of the tube, one tube can be rolled to form two ends of different sizes and then cut in the middle. The length of the diameter-changing transition section depends on the tube diameter ； To reduce local resistance, it should generally not be less than the outer diameter of the larger pipe. 5.2.2.6.6 Welded steel pipe elbows Welded steel pipe elbows are commonly used at construction sites; they are easy to manufacture and do not require bending equipment. The shapes of the male and female ends of the Zhengxin component, along with the layout drawing for cutting, are shown in 10Ⅰ. The dimensions of A, B, and L in the figure can be determined using the following formulas: A = πDH/n, B = πdH/n, L = (3–4)(DH – dH). Here, DH represents the outer diameter of the larger tube (in mm) ； dH — outer diameter of the tube. N is the number of segments; for pipes with a diameter of 50–100 mm, n=4–6 ； For pipes of 100~400 mm, n=6~8. The eccentric stub-end form and the cutting development are shown in Book 10 II. The dimensions of A, B, C, D, and E in the figure are determined by the following formulas: A = πDH/8, B = 3ΔL/12, C = 2ΔL/12, D = ΔL/12, E = 2(DH – dH). Here, ΔL represents the difference in the circumferences of the larger and smaller tubes. Figure 10: Ⅲ represents butt welding. Draw the cutting line according to the template and make the cut; heat the base of the remaining portion with a welding torch, then gently tap it with a hand hammer to make its end have the same diameter as that of the small pipe. The intervalve gap is sealed by welding. 5.2.2.7 Points to note when cutting with templates: Choose an appropriate material for the template: The material used to make the template should not be too thick; 1–3 mm is an appropriate thickness. It should not curl or deform, and it is best to use a material that is moderately hard, such as kraft paper, asphalt paper, thin plastic sheets, or thin iron sheets. Calculate the appropriate unrolled length of the template; for a circular tube template, this length is equal to the outer diameter of the pipe plus the thickness of the template material, multiplied by π. However, due to seasonal and material variations, the calculated unfolded length may differ from the actual circumference of the pipe. For example, with templates made of asphalt paper or similar materials, they harden in winter, which results in them not fitting tightly around the outer wall of the pipe; as a result, such templates appear to be too short ； When it softens in summer, the template is easily stretched. Therefore, corresponding measures must be taken to appropriately increase or decrease the unwound length of the sample. It should be noted that neither extension nor shortening can be done after the expansion curve has been drawn. Conduct inspections and verifications; after the template is prepared, its shape must be checked and its dimensions verified. It should then be wrapped around the outer wall of the pipe to determine the surrounding dimension. The template should fit tightly against the pipe wall, with no gaps or overlaps between its ends. 5.2.3 Alignment and Welding 5.2.3.1 General Provisions The welding of medium and low pressure pipelines shall be carried out in accordance with the design requirements; when no such provisions are given in the design, the following clauses may be referred to. 5.2.3.1.2 For carbon steel pipes with an outer diameter ≤ 57 mm and a wall thickness ≤ 3.5 mm, oxy-acetylene welding can be used; for the rest, shielded metal arc welding is recommended. 5.2.3.2 Groove shape: When the design does not specify, the groove shape, dimensions, and alignment gap for butt-welded pipes shall be selected in accordance with the requirements of Appendix 1. 5.2.3.4 Pipe Alignment 5.2.3.4.1 For butt welding of pipes and fittings with equal wall thicknesses, their inner walls should be aligned with one another. When the design does not specify otherwise, the misalignment between the inner walls shall meet the following requirements: 5.2.3.4.1.1 For welds of grade I and II, the misalignment shall be ≤10% of the wall thickness, and also ≤1 mm ； 5.2.3.4.1.2 For Class III and IV welds, it shall be ≤20% of the wall thickness and ≤2 mm. 5.2.3.4.2 When the design does not specify otherwise, the misalignment amount for butt welding of pipes and fittings with different wall thicknesses shall meet the following requirements: For the inner wall misalignment, if it exceeds the value specified in item ① above, it shall be processed according to Figure 10(a) ； Outer wall misalignment amount: when the thickness of the thin layer ≤ 10 mm and the thickness difference > 3 mm ； When the thickness of the thin sheet is >10 mm, and the thickness difference is >30% of the thickness of the thin sheet or >5 mm, it shall be processed according to Figure 10(b). 5.2.3.5 Bevel cleaning: Before aligning the pipe ends, use a hand-held grinder or sandpaper and files to clean the surface of the bevel as well as its surrounding areas, removing burrs and contaminants such as oil, paint, and rust. The cleaning area should cover at least 10 mm. Upon post-inspection, no defects such as cracks or delaminations shall be found. Welding work should be carried out promptly on the aligned pipe ends that have been cleaned and found to be in good condition. 5.2.3.6 Alignment and Fixing 5.2.3.6.1 For alignment, specialized aligners with bolt connections should be used (aligners with internal openings shall be used for pipes with an outer diameter > 273 mm) ； For pipes with an outer diameter < 273 mm, an external alignment tool is used. 5.2.3.6.2 If welding alignment fixtures are to be used, the welding process and welding materials shall meet the requirements for pipeline welding. 5.2.3.6.3 The removal of welding fixtures should be carried out using an oxy-acetylene flame, and any remaining weld scars should be removed with a hand-held grinder. 5.2.3.6.4 After being aligned and fixed using clamps, the centerlines of the two pipe ends should lie on the same straight line; the deviation in straightness shall not exceed 1 mm/m, with the maximum total deviation across the entire length not exceeding 10 mm. 5.2.3.6.5 Forced alignment methods shall not be used to reduce the amount of misalignment or the deviation in concentricity, nor shall heating be employed to decrease the gap between the joints. 5.2.3.7 Spot Welding 5.2.3.7.1 The site for spot welding should be free from the effects of wind, rain, snow, etc. 5.2.3.7.2 The process and welding materials for spot welding shall be consistent with the welding requirements of such pipelines. 5.2.3.7.3 The length of the spot weld seam shall be 10~15 mm, the spot weld height shall be 2~4 mm, and it shall not exceed 2/3 of the tube wall thickness. 5.2.3.7.4 The spot welding spacing depends on the diameter of the tube; generally, 50–300 mm is appropriate, and each weld joint should have no fewer than three spots. 5.2.3.8 Welding Process 5.2.3.8.1 The welding area should be protected from adverse weather conditions during welding. If appropriate measures are taken (such as preheating, shading, and heating) to ensure that the weldment maintains a sufficient temperature for welding without affecting the welder’s skills, welding can be carried out at any external temperature. 5.2.3.8.2 When aligning the welded components, for the welding of spot welds and welds using fixing clamps, the welding materials and process measures selected shall be the same as those required for regular welding. When using a fixture set to align and remove the gap fixture, the base material should not be damaged ； After removal, any remaining traces should be polished and trimmed, and a thorough inspection should be carried out. When using root spot welding, the welds must be carefully inspected; any defects found should be addressed promptly. 5.2.3.8.3 Arcing on the surface of the welded parts and test current are not permitted. 5.2.3.8.4 To reduce stress and deformation, reasonable welding methods and sequences should be adopted. 5.2.3.8.5 For submerged arc welding and gas shielded welding, trial welding should be carried out on test plates prior to actual welding; the welding parameters must be adjusted properly before proceeding with the actual welding process. 5.2.3.8.6 During welding, attention should be paid to the welding quality at the start and end of the weld; the weld pool should be filled completely when ending the weld. The joints between layers in multi-layer welding should be offset. During submerged arc welding, arc starting plates and arc extinguishing plates should be installed at both ends of the longitudinal weld. 5.2.3.8.7 When welding pipes, prevent backflow of air inside the pipe. 5.2.3.8.8 Unless there are special process requirements, each weld shall be completed in one continuous pass. If the welding process has to be interrupted for some reason, measures should be taken in accordance with the process requirements to prevent cracks. Before resuming welding, it is necessary to check to ensure that there are no cracks; only then can welding proceed as per the original process requirements. 5.2.3.8.9 The tools used for aligning pipe cold-drawn weld joints must not be removed until the entire weld joint has been welded and heat-treated. 5.2.3.8.10 For defective welds, a quality analysis shall be conducted, and measures shall be established before rework can be carried out. Rework on the same area should not exceed three times. 5.2.4 Threaded Connections 5.2.4.1 Galvanized steel pipes shall be connected by threading; welding shall not be used. Table 1: Selection Table for Gaskets at Pipe Threaded Connections. Name of Gasket, Applicable Media: White putty – water, gas, compressed air; White putty + hemp fibers – water, compressed air; Yellow powder (lead monoxide) + glycerin – gas, compressed air, acetylene, ammonia; Yellow powder (lead monoxide) + distilled water – oxygen and ethylene; Polytetrafluoroethylene tape – steam at <250°C, gas, compressed air, oxygen, acetylene, ammonia; can also be used with corrosive media. 5.2.4.2 When the design does not specify otherwise, the sealing gaskets for threaded connections shall be selected in accordance with the provisions listed in Table 1. 5.2.4.2 Methods and requirements 5.2.4.2.1 Tapping of pipe ends 5.2.4.2.1.1 Manual tapping: Suitable for processing the pipe threads of low-pressure liquid transport steel pipes with a nominal diameter of 15–100 mm. The method is to clamp the tube on a tube press, and then thread it using a tube threading die. 5.2.4.2.1.2 Mechanical threading: Suitable for threading seamless steel pipes used for fluid transport and steel pipes used for transporting low-pressure fluids. The machining must be carried out by a skilled turner. 5.2.4.2.1.3 Lathe threading: Suitable for threading seamless steel pipes used for transporting fluids and steel pipes used for transporting low-pressure fluids. The machining must be carried out by a skilled turner. 5.2.4.2.1.4 Thread type: The connection threads for medium and low pressure steel pipes shall be tapered pipe threads of Chengdu type, with a taper ratio of 1:16, an inclination angle of 1°47’24”, and a thread profile angle of 55°. 5.2.4.2.1.5 The threads shall be smooth and intact, without burrs or stray threads; the total length of broken or missing threads shall not exceed 10% of the total length of the thread, and there shall be no connected broken threads in the longitudinal direction. 5.2.4.2.2 Wrap hemp thread or sealing tape around the threaded pipe end or fitting in order to screw it into a fitting with internal threads by 2–3 turns. 5.2.4.2.3 Flexible joints in threaded connections – Oil seals: An oil seal consists of three separate components, namely the male part, the female part, and the sleeve. One end of the male connector features a plug, which mates with the socket on the female connector; the other end has internal threads that form a short-thread connection with the external threads of the pipe ； One end of the female connector features a receiving mouth that mates with the male connector’s plug, while the other end has internal threads that form a short-thread connection with the external threads of the pipe ； The outer surface of the female sleeve is hexagonal, and its inner surface is equipped with internal threads that mesh with the external threads on the male thread. When connecting, a gasket should be placed on the male end; for steam pipes, an asbestos rubber gasket should be used ； Rubber gaskets can be added to upper-water or low-temperature plumbing pipes. The sleeve should be attached to the male end, with the side of the sleeve that has the internal threads facing the female end. If the sleeve is forgotten to be installed or its orientation is reversed, it is often necessary to remove the male end in order to redo the process. Before locking, the male and female parts must be aligned and leveled; otherwise, leakage is likely to occur. 5.2.4.2.4 Use a pipe wrench to turn the pipe (or fitting) until it is tightened. For pipe fittings such as tees and elbows, a greater torque can be applied; for valves, however, the torque should be kept to a minimum. 5.2.4.2.5 The jaw size of the pipe wrench used should be suitable for the pipe specification. 5.2.4.2.6 After the threads are tightened, no sealing compound should be forced into the pipe. It is preferable to leave 1–2 threads exposed at the end; any excess sealing compound that has been extruded must be thoroughly removed. 5.2.5 Flange Connections 5.2.5.1 Welding of Flanges to Pipes 5.2.5.1.1 Butt Welded Flanges 5.2.5.1.1.1 Fit the flange over the pipe end such that the pipe opening extends 1.5 times the pipe wall thickness into the flange’s sealing surface, and mark four equal points evenly around the inner circumference of the flange. 5.2.5.1.1.2 First, spot-weld the flange to the pipe at points above the circumference; then use a 90° angle gauge to adjust the position of the flange in the vertical direction, so that its sealing surface is perpendicular to the pipe’s centerline. 5.2.5.1.1.3 Weld a second spot below it, use a 90° square to adjust the position horizontally, and after it is correct, weld the third and fourth spots on the left and right sides. 5.2.5.1.1.4 For spot welding of paired flanges, the bolt holes must be aligned accurately; the plumb line method shall be used for this purpose. 5.2.5.1.1.5 Only after two inspections confirm that the spot welding is satisfactory may the fillet welding connection between the flange and the pipe be carried out; thereafter, the welds on the inside and outside of the pipe must be cleaned thoroughly, with no debris remaining on the flange’s sealing surface. 5.2.5.1.2 Butt weld flanges. The connection between butt-weld flanges and pipes is made by butt welding. The inspection of the perpendicularity between the flange sealing surface and the pipe centerline, as well as the alignment methods and screw hole positioning, are basically the same for flat welding flanges. The welding process and operations are consistent with those for this pipeline. 5.2.5.1.3 Loose flange. Butt-weld slip-on flanges are suitable for piping connections in applications involving high temperatures, high pressures, and corrosive media. Sometimes, it is also used in the connection of ordinary pipes so that the bolt holes can be easily aligned during flange connections. This flange consists of a male-female shoulder ring and a flange. During connection, the flange is fitted into the shoulder ring, which is then welded to the pipe along the same centerline. When the bolt is tightened, the protrusions and recesses on the shoulder ring are subjected to force, thereby compressing the washer tightly for a seal. During combined installation, first fit the flange over the pipe, then place the welding ring at the pipe end, and subsequently carry out spot welding, position adjustment, and welding. For flanged laps, put on a loose flange first and then perform the flanging. 5.2.5.2 Soft gaskets 5.2.5.2.1 Selection of soft gaskets When the design does not specify, the material for the gasket can be selected based on the pressure, temperature, and properties of the medium, as indicated in the table below. Production of soft gaskets 5.2.5.2.2 For homemade soft gaskets, use scissors to cut them into gaskets with handles; when a large quantity is needed, it is advisable to use a gasket cutter. The perimeter of the cut or trimmed gasket should be neat, and its dimensions should match those of the flange sealing surface; the deviation shall not exceed the values specified in Tables 2 and 3. When assembling large-diameter soft gaskets, they should be cut so that beveled joints can be made; flush butt joints are not permitted. Installation of soft gaskets: 5.2.5.2.3 The soft gasket shall be placed between the sealing surfaces of the two flanges, and must be concentric with them without any offset. Before installing the gasket, both sides thereof shall be coated with graphite powder, graphite oil, or molybdenum disulfide grease as required by the design (no coating is needed if not specified in the design). Table 2 Flange gaskets – Gasket material, applicable media, maximum working pressure, maximum operating temperature
Rubber sheet: Water, compressed air, inert gases; 0.6 Mpa; 60°C
Flat rubber sheet: 1 Mpa; 60°C
Low-pressure rubber-ash sheet: Water, compressed air, inert gases, steam, gas; 1.6 Mpa; 200°C
Medium-pressure rubber-ash sheet: Water, compressed air, inert gases, steam, gas, oxidizing gases (SO2, NO, CL), acids, dilute alkali solutions, ammonia; 4 Mpa; 350°C
High-pressure rubber-ash sheet: Steam, compressed air, gas, inert gases; 10 Mpa; 450°C
Acid-resistant ash sheet: Organic solvents, hydrocarbons, concentrated inorganic acids (nitric acid, sulfuric acid, hydrochloric acid), strongly oxidizing hydrochloric acid solutions; 0.6 Mpa; 300°C
Acid-resistant rubber-ash sheet: Oil products, liquefied gas, solvents, hydrogen, sulfiding catalysts; 4 Mpa; 350°C
Soft polyvinyl chloride sheet: Water, compressed air, acids, dilute alkali solutions, oxidizing gases; 0.6 Mpa; 50°C
Impregnated white ash: Oxidizing gases; 0.6 Mpa; 300°C
5.2.5.3 Flange fastening
5.2.5.3.1 General provisions
5.2.5.3.1.1 The same type of bolts should be used for fastening each pair of flanges, and the installation direction should be consistent. Table 3 Allowable tolerances for soft gaskets (mm) Nominal diameter DN Flange seal surface type Flat Grooved Ribbed Allowable tolerances Inner diameter Outer diameter Inner diameter Outer diameter Inner diameter Outer diameter <125 ≥125 +2.5 +3.5 -2 -3.5 +2 +3 -1.5 -3 +1 +1.5 -1 -1.5 5.2.5.3.1.2 When washers are required, only one washer shall be used per bolt. 5.2.5.3.1.3 In the following situations, and when the design does not specify otherwise, bolts and nuts should be coated with molybdenum disulfide grease, graphite oil, or graphite powder:  Alloy steel bolts and nuts ；  The pipeline temperature is designed to be above 100°C or below 0℃ ；  Flange connections for outdoor installations ；  Areas with corrosive media or exposure to atmospheric corrosion. 5.2.5.3.2 Method of tightening bolts 5.2.5.3.2.1 First, use an appropriate wrench to tighten the bolts in a symmetrical cross pattern, in two or three steps; the force applied should be even, without being too forceful, so that about two turns of the thread of the nut are exposed (appropriate bolts of the right length should be selected in advance). 5.2.5.3.2.2 When tightening the bolts, the operator should stand firmly and avoid any shaking or uneven force application ； When working at heights, in addition to wearing a safety belt, wrenches should also be tied to the safety rope to prevent them from falling and causing injury. 5.2.5.3.2.3 Thermal or cold tightening of bolts. When the design does not specify otherwise, the flange bolts of high-temperature or low-temperature pipelines should be thermally tightened or cold tightened during operation in accordance with the requirements listed in Table 4. Table 4: Thermal and cold tightening temperatures for flange bolts, pipeline operating temperature, first thermal and cold tightening temperatures, second thermal and cold tightening temperatures. 250–350: Operating temperature – –; >350: Operating temperature 350, operating temperature –20 to –70: Operating temperature – –; <–70: Operating temperature –70