In accordance with pressure piping specifications, forced alignment shall not be adopted for connections between pipes, and between pipes and equipment. Before painting carbon steel compressed air pipelines, rust, welding slag, burrs, oil, water and other contaminants on the surface shall be removed. Welds shall not be painted prior to pressure testing. For welding of carbon steel compressed air pipelines, butt welds shall adopt GTAW (Gas Tungsten Arc Welding) or GTAW root pass plus SMAW (Shielded Metal Arc Welding) fill.
1. Installation Requirements for Compressed Air Pipelines
Welded steel pipes or seamless steel pipes can be used for compressed air pipelines. Piping elbows shall preferably be bent elbows, with a bending radius not less than 3 times the nominal diameter and ovality not exceeding 8%. After cutting, drilling and welding of pipelines, the interior shall be thoroughly cleaned without residual molten metal, debris or other contaminants.
All supports and hangers of the piping system shall be installed firmly at correct positions without inclination or loosening. Vertically installed pipelines shall be plumb. The allowable deviation is 12 mm for pipes longer than 4 m, and 4 mm for pipes within 4 m. Horizontally installed pipelines shall have a certain gradient, and the gradient deviation shall not exceed ±0.0005. After pipeline installation, compressed air purging shall be carried out to remove internal contaminants. Generally, the outer surface of pipelines shall be coated with 1~2 coats of anti-rust paint, followed by one coat of light blue finish paint. Buried pipelines shall be coated with asphalt paint.
Pipe Cutting
The actual length of pipe sections for installation is determined according to drawing dimensions, and pipe sections are cut from raw materials to such installation lengths, which is defined as pipe cutting. Drawings generally do not mark installation lengths but only provide relative dimensions for components, turning points and support positioning; therefore, cutting lengths can only be calculated based on drawing dimensions.
Cutting length for threaded pipe sections: Drawing dimension minus the length of pipe sections required inside fittings; alternatively, calculated as the length between connecting edges of two fittings plus the effective thread length.
Cutting length for welded pipe sections: Drawing dimension minus fitting length and weld gap; weld shrinkage allowance shall be added when necessary.
Cutting length for flanged pipe sections: Drawing dimension minus flange thickness; if only one end is fitted with a flange, half of the flange thickness shall be subtracted.
Pipe sections are cut from raw pipes using sawing, oxy-acetylene flame cutting, mechanical cutting and other methods. Pipe ends shall be flat and free of burrs and defects. Any defects shall be trimmed with files, pipe end scrapers, grinding wheels and other tools.
Thread Machining
Threaded pipe sections can be processed manually or mechanically. Manual threading uses pipe threaders, while mechanical threading generally adopts threading machines. Lathes can be used for large-diameter or long threads. Machined threads shall have sufficient effective length and 1~2 incomplete thread leads at the thread end. Pipe threads must be complete and smooth without burrs or crossed threads. The total length of broken or missing threads shall not exceed 10% of the total thread length, and adjacent broken sections shall not be aligned longitudinally.
After preparing pipe sections, whether to machine bevels and the bevel dimensions shall be determined according to pipe wall thickness and diameter. Bevels shall be machined when the wall thickness exceeds 3 mm. Double-sided bevels shall be prepared if double-sided welding is required. Bevels for welds with high quality requirements shall be mechanically machined; oxy-acetylene flame cutting is acceptable for bevels without special requirements.
Threaded Connection
Appropriate sealing filler such as PTFE sealing tape shall be applied between threads during threaded connection. Traditional sealing materials such as hemp yarn and red lead paint are gradually replaced by PTFE tape. PTFE sealing tape resists corrosion by various media, withstands pressure up to 70 MPa, and works within a temperature range of -180°C ~ 260°C.
Suitable pipe wrenches shall be used to tighten pipe threads. After tightening, 1~2 threads shall be exposed. If too few threads are exposed or the joint still feels loose after full tightening, the pipe thread shall be re-machined.
2. Pipe Bending
Cold bending is preferred for pipe bends. The minimum bending radius of cold bent pipes shall not be less than 3 times the outer diameter of the pipe. If stamped elbows are adopted, the bending radius shall not be less than 1 time the pipe outer diameter.
The difference between the maximum outer diameter and minimum outer diameter of the bent pipe shall not exceed 8% of the pipe diameter.
The bent section of the pipe shall be free from wrinkles, peeling and other defects.
Pipe thread machining shall comply with the provisions of design technical documents. After thread machining, the surface shall be free of cracks, depressions, burrs and other defects. For threads with minor mechanical damage or incomplete profile, the cumulative damaged length over the full length shall not exceed 1/3 turn, and the reduction of thread height shall not exceed 1/5 of the original thread height.
3. Laying of Carbon Steel Compressed Air Pipelines
For butt welds of carbon steel compressed air pipelines, surface defects including cracks, blowholes, slag inclusions, spatter and incomplete penetration are not allowed. The depth of undercut shall be less than 0.5 mm, and the total undercut length on both sides of the weld shall be less than 10% of the total weld length. Weld reinforcement height shall be ≤1+0.1b (b = weld width) and shall not exceed 3 mm.
Pipe cutting and bevel preparation shall adopt mechanical processing. The cut end face shall be flat and perpendicular to the pipe axis. The allowable deviation is 1% of the pipe diameter and shall not exceed 2.0 mm. The form and dimensions of pipe welding bevels shall comply with the welding procedure specification. Iron filings, burrs and other debris shall be thoroughly removed after bevel machining.
Pipeline routing shall comply with design technical documents. The allowable straightness deviation of horizontal pipelines is 2/1000 and shall not exceed 30 mm; the allowable verticality deviation of risers is 3/1000 and shall not exceed 20 mm. Pipelines shall be installed at coordinates and elevations specified in design documents. The allowable coordinate deviation is 15 mm, and elevation deviation is ±15 mm.
The clearance between the outer wall of a pipe and adjacent pipes or fitting edges shall not be less than 10 mm. Flanges or unions on parallel pipelines shall be staggered by no less than 100 mm. Sleeves shall be installed for pipes passing through walls, and the distance between joints and wall surfaces shall preferably be greater than 800 mm.
The gradient of carbon steel compressed air pipelines shall comply with technical documents. If not specified, the gradient shall be 12.5/1000 ~ 25/1000.
Flanged connections shall keep pipes concentric. Connecting bolts shall pass through freely. The mating surfaces of two flanges shall be parallel, and the parallelism deviation shall not exceed 1.5/1000 of the flange diameter.
Installation of pipe supports and clamps shall meet the following requirements:
(1) Supports shall be positioned correctly and fixed firmly. Pipes shall be in tight contact with clamps. No pipe welds shall be located at supports and clamps.
(2) Support spacing for straight pipe sections shall comply with specifications; additional supports shall be arranged near tangent points for bent sections.
(3) Direct contact between different pipe supports is prohibited.
Hose installation shall comply with the following provisions:
(1) For hoses with outer diameter greater than 30 mm, the minimum bending radius shall not be less than 7 times the hose outer diameter.
(2) Hoses shall be free of twisting and deformation.
(3) Friction between hoses, and between hoses and other objects is prohibited. If hoses are close to heat sources, thermal insulation measures must be provided.
The coating thickness and number of paint coats for pipelines shall meet design technical requirements. Coatings shall be uniform with consistent color, free of missing coating, sagging, bubbles and other defects.
Carbon steel compressed air pipelines must be tapped and purged in accordance with procedures. After purging, the inner surface of pipelines shall comply with design technical documents. If no requirements are specified, no rust, scale or other foreign objects shall remain inside.
4. Pressure Test of Pneumatic Piping System
Installation sequence of compressed air pipelines: Material arrival → Material inspection → Support fabrication → Support installation → Pipe cutting → Bevel preparation → Welding / thread machining → Fitting assembly → Pre-assembly on ground → Positioning → Connection → Fixing → Preparation for purging and test facilities → Purging → Strength test → Tightness test → Pressure relief → Anti-corrosion construction → Official air supply.
Pressure tests of pneumatic piping systems shall comply with design technical documents. The test pressure shall be 1.5 times the working pressure. Stabilize pressure under test pressure for 10 minutes, then reduce the pressure to working pressure for system inspection. No leakage shall occur at pipe welds and connections, and no permanent deformation of pipelines is permitted.