What are the installation and construction requirements of stainless steel welded ventilation pipes?

Detailed requirements for installation and construction of stainless steel welded ventilation pipes

Stainless steel welded ventilation pipes are widely used in chemical, food, medicine, electronics and other industries with strict requirements on ventilation environment because of their advantages of corrosion resistance, strong sealing and long service life. Its installation and construction should strictly follow the specifications and standards, covering the whole process of preliminary preparation, on-site installation, welding technology, quality inspection, etc., so as to ensure the stable operation of the system, reliable sealing and meeting the design requirements. The following are detailed construction requirements:

A, the construction preparation requirements

(1) Technical preparation

Before construction, it is necessary to be fully familiar with the design drawings, technical disclosure documents and relevant specifications (such as Code for Acceptance of Construction Quality of Ventilation and Air Conditioning Engineering (GB 50243) and Code for Construction and Acceptance of Stainless Steel Welded Pipes), and make clear the pipeline direction, pipe diameter specification, wall thickness, connection mode, slope requirements and cross-avoidance scheme with other pipelines (water supply and drainage, electricity).

According to the material characteristics of stainless steel (for example, austenitic stainless steel is prone to intergranular corrosion and large thermal deformation), a special construction scheme is formulated, focusing on the welding process parameters, anti-corrosion measures, hoisting scheme and quality control points, and welding process qualification tests are carried out when necessary to ensure the welding quality meets the requirements.

Organize construction personnel to carry out technical training and safety disclosure, focusing on stainless steel welding operation, hoisting safety, pollution prevention measures, etc. Operators need to hold valid special operation certificates (welder certificates, rigger certificates, etc.) for their posts.

(2) Material preparation and inspection

Stainless steel pipes, pipe fittings, flanges, covered electrode and other materials should meet the design requirements and national standards. When entering the site, documents such as quality certificate, certificate of approval and material list should be provided to check whether the material specifications, models and materials (such as 304 and 316L) are consistent with the design.

The appearance quality of pipes shall meet the following requirements: the surface shall be free from obvious scratches, pits, corrosion and deformation, and the weld shall be smooth and free from defects such as cracks, blowholes and slag inclusion; The deviation of pipe wall thickness should be within the allowable range (generally 5%), and the ultrasonic thickness gauge should be used for random sampling.

Pipe fittings (elbows, tees, large and small heads) shall be consistent with the material of the pipe, with the required dimensional accuracy and no burr or flash at the interface; The sealing surface of flange shall be flat and smooth, free from scratches and corrosion, and the deviation of bolt hole position shall not exceed 1 mm..

Welding materials should match the pipe materials (for example, E308L covered electrode is used for welding 304 stainless steel, and E316L covered electrode is used for welding 316L stainless steel). covered electrode should be stored in a dry and ventilated warehouse, and dried as required before use (generally, the drying temperature of E308L covered electrode is 200-250℃, and the heat preservation time is 1-2 hours) to avoid the influence of moisture on welding quality.

All materials should be classified and stored after entering the site to avoid contact with other materials such as carbon steel and prevent electrochemical corrosion; The storage site should be flat and dry, and the pipes should be padded with wood (≥100mm or higher from the ground) to prevent rusting due to damp.

(3) Site preparation

The construction site shall be cleaned, leveled and compacted, and sufficient material stacking area, processing area and working space shall be reserved; The construction site shall be equipped with ventilation facilities, fire fighting equipment and welding protective articles (such as welding caps, protective gloves and ventilation masks).

According to the design requirements, the center line and elevation control line of pipeline installation should be set up, and the position and size of reserved holes and embedded parts should be checked for accuracy. If there is any deviation, it should be handled in time (such as expanding holes and repairing embedded parts).

Set up necessary operating platforms (such as scaffolding and mobile platforms), which should be firm and stable to meet the needs of construction personnel for operation and hoisting of pipes and equipment; Hoisting equipment (such as cranes and winches) shall be qualified through inspection, and hoisting ropes shall be made of stainless steel or anticorrosive materials to avoid polluting stainless steel pipes.

Prepare tools and equipment needed for construction, such as welding machine (argon arc welding machine, manual arc welding machine), cutting machine, beveling machine, grinding machine, ultrasonic flaw detector, pressure test equipment, etc. Tools need to be checked and debugged in advance to ensure good performance.

Second, the pipeline processing and production requirements

(1) Pipe cutting

Mechanical cutting (such as plasma cutting machine and grinding wheel cutting machine) or laser cutting should be used for cutting stainless steel pipes, so as to avoid oxyacetylene flame cutting (high temperature may easily lead to surface oxidation of pipes, burning loss of alloy elements and intergranular corrosion).

Before cutting, it is necessary to draw a cutting line on the pipe to ensure the accurate cutting size, and the cutting surface should be flat and perpendicular to the axis of the pipe, with a deviation of less than 1 mm/m; After cutting, the burr, flash and oxide scale at the incision should be removed in time, and polished with an angle grinder to avoid affecting the welding quality and pipeline circulation.

For pipes with a wall thickness of 6 mm or more, after cutting, groove processing is required. The groove is generally V-shaped (angle is 60-70, blunt edge is 1-2mm, and gap is 2-3mm). Groove processing can be carried out by using a groove machine or an angle grinder. After processing, impurities such as oil, rust and oxide scale on the groove surface should be cleaned to ensure that the groove is clean and dry.

(2) Pipe bending and forming

Stainless steel pipes should be bent by cold bending process to avoid hot bending (high temperature can easily lead to pipe deformation, grain growth and reduce corrosion resistance); The bending radius should meet the design requirements, generally not less than 3 times of the outer diameter of the pipe (for thin-walled pipes, the bending radius can be appropriately increased) to prevent cracks and folds from appearing at the bend.

When bending with a pipe bender, it is necessary to adjust the pressure and speed of the pipe bender according to the specifications and wall thickness of the pipe to avoid the pipe from flattening due to excessive pressure; After bending, check the ellipticity (allowable deviation ≤3%) and angle deviation (allowable deviation+/-1) of the bent pipe, and correct or replace it if it exceeds the standard.

T-joints, large and small fittings need to be integrally formed or welded, and the welded fittings need to ensure the weld quality to avoid defects such as weld overlap and incomplete penetration; The inner wall of pipe fittings shall be smooth, without obvious bulge and burr, so as to ensure smooth air flow.

(3) Pipeline assembly

Before pipeline assembly, it is necessary to check the specifications, dimensions and surface quality of pipes and fittings again, and clean up the oil stains and impurities at the interface to ensure that the interface is clean; When assembling, ensure that the pipeline axis is aligned and the interface clearance is even, so as to avoid misalignment and deflection (the misalignment shall not exceed 10% of the wall thickness and not more than 1mm).

For flanged pipes, the flange surface should be perpendicular to the pipeline axis, and the sealing gasket between flanges should be stainless steel wound gasket or PTFE gasket (avoid using rubber gasket to prevent aging pollution), and the gasket should be placed in the center of the flange sealing surface without deviation; The fastening of bolts shall be symmetrical and uniform, and the fastening torque shall meet the requirements (generally, the fastening torque of M16 bolts is 40-60 nm), so as to ensure that the flange connection is tightly sealed.

In the process of pipeline assembly, it is necessary to avoid direct contact between the pipe surface and carbon steel tools and scaffolding, and rubber pads and stainless steel pads can be used for isolation to prevent scratching the surface or causing electrochemical corrosion.

Third, the pipeline installation and construction requirements

(1) Installation sequence and slope

Pipeline installation should follow the principle of "main pipe first, branch pipe later, large pipe first, small pipe later, high altitude first, and ground later", and the construction sequence should be arranged reasonably to avoid mutual interference of cross operations; For complex pipelines, it is necessary to draw the installation flow chart to clarify the installation sequence and connection mode of each section of pipelines.

The slope of the ventilation pipeline should meet the design requirements (generally 1‰-3‰), and the slope should be directed to the drainage point to ensure the smooth discharge of condensate in the pipeline; When installing, use a level meter to detect the slope of the pipeline, and adjust it in time to avoid the phenomenon of inverted slope.

Sufficient expansion compensation should be reserved during pipeline installation. For straight pipe sections with a length of more than 20m, expansion joints (such as stainless steel corrugated expansion joints) should be set to compensate the thermal expansion and cold contraction of the pipeline due to temperature changes, so as to prevent the pipeline from being deformed or damaged. The installation position of expansion joint should be close to the fixed bracket to ensure uniform stress.

(2) bracket installation

Stainless steel pipe supports shall be made of stainless steel (such as 304 and 316L) to avoid using carbon steel supports (contact with stainless steel will cause electrochemical corrosion); The form of support (such as sliding supports, fixed support and hanger) should be determined according to the pipeline direction, weight and stress, and meet the design requirements.

The bracket installation spacing should strictly follow the specification requirements: for pipes with DN≤100mm, the bracket spacing should not exceed 2.5 m; For pipes with DN > 100mm, the bracket spacing shall not exceed 3.5m；; Hanger spacing is consistent with the bracket spacing, and the hanger shall be installed vertically and shall not be tilted.

The installation position of the support shall be away from the pipeline welds, valves, flanges and other parts, and the distance from the weld edge shall be not less than 50mm；; Rubber pad or stainless steel pad should be set between the bracket and the pipeline to prevent the bracket from scratching the surface of the pipeline and reduce the noise caused by pipeline vibration.

The fixed bracket shall be installed firmly, and the connection with the wall and floor shall be reliable (such as fixed by expansion bolts) to ensure that it can bear the weight of the pipeline and the thrust generated by thermal expansion and contraction; Sliding supports needs to ensure that the pipeline can slide freely without jamming, and the sliding surface should be flat and smooth.

(3) Welding construction requirements

Selection of welding method: argon arc welding (TIG welding) is preferred for stainless steel pipes. For pipes with wall thickness ≥8mm, argon arc welding can be used as backing and manual arc welding can be used to fill the cover to ensure the penetration of the weld root and good surface formation.

Welding process parameters: Reasonable process parameters need to be determined according to the pipe material, wall thickness and welding method. For example, when argon arc welding 304 stainless steel pipe (wall thickness is 3mm), the welding current is 80-120A, the argon flow rate is 8-12L/min, and the welding speed is 5-10cm/min; In the welding process, the current, voltage and welding speed should be strictly controlled to avoid overheating, incomplete penetration and porosity.

Welding environment requirements: the ambient temperature during welding shall not be lower than 5℃ and the relative humidity shall not exceed 80%; If welding is carried out outdoors or in humid environment, wind, rain and moisture-proof measures (such as building temporary protective shed and using dehumidifier) should be taken to avoid affecting the welding quality.

Welding operation requirements: Welders should hold relevant certificates, and before welding, they should clean the groove and impurities within 20mm on both sides, such as oil stain, rust, scale, etc., to ensure that the welding area is clean; When welding, short arc welding is needed to avoid the burning loss of alloy elements caused by too long arc; During multi-layer welding, each layer of weld should be cleaned (polished by angle grinder to remove welding slag and air holes) before the next layer of welding, and the interlayer temperature should be controlled below 60℃ to prevent intergranular corrosion caused by excessive heat affected zone.

Weld quality requirements: the weld surface should be smooth and smooth, without cracks, pores, slag inclusion, incomplete penetration, undercut and other defects; The residual height of weld is generally 0-3mm, the undercut depth is not more than 0.5mm, and the length is not more than 10% of the total length of weld; The weld width is uniform, and the deviation is less than 3mm.

(4) Connection of valves and equipment

Before valve installation, strength test and leak test shall be carried out, the pressure of strength test is 1.5 times of the nominal pressure of the valve, and the pressure of leak test is 1.1 times of the nominal pressure. Only after passing the test can the valve be installed. The installation position and direction of the valve shall meet the design requirements, and the handle or operating mechanism shall be easy to operate and shall not affect the pipeline maintenance.

Flexible short pipes (such as stainless steel corrugated short pipes and PTFE short pipes) should be used when pipes are connected with fans, air conditioning units and other equipment. The length of flexible short pipes is generally 150-200mm, and both ends should be kept flat and not twisted during installation. The connection between flexible short pipes and pipes and equipment should be firmly sealed to avoid air leakage.

The pipeline at the equipment interface needs to be aligned to ensure that the axis of the pipeline is consistent with the axis of the equipment interface, and the deviation is less than 2mm, so as to prevent the equipment from vibration or damage caused by the deviation of pipeline installation.

Four, quality inspection and test requirements

(1) Appearance inspection

After the pipeline installation is completed, it is necessary to conduct a comprehensive inspection of the pipeline appearance: the pipeline surface is free of scratches, pits, corrosion and deformation, and the weld surface is smooth and smooth without obvious defects; The slope and direction of the pipeline meet the design requirements, and the supports are installed firmly and evenly spaced; Flanges shall be tightly connected, bolts shall be tightened evenly, and sealing gaskets shall be free from leakage.

Check the straightness and levelness of the pipeline with a ruler and a feeler gauge. The straightness deviation shall not exceed 1mm/m and the total length deviation shall not exceed 20mm；; The levelness deviation shall not exceed 1mm/m, and the total length deviation shall not exceed 15 mm..

(II) Non-destructive testing of welds

For welds under pressure or in important parts (such as the main pipe of pipeline and the welds connected with equipment), nondestructive testing is required. The detection method can be ultrasonic flaw detection or radiographic flaw detection, and the detection ratio should meet the design requirements (generally not less than 10% of the total number of welds).

Ultrasonic flaw detection shall conform to the requirements of GB/T 11345, Ultrasonic Detection Technology, Detection Grade and Evaluation of Weld Nondestructive Testing, and the internal quality grade of weld shall not be lower than Grade II; X-ray flaw detection shall conform to the requirements of X-ray detection technology, detection grade and evaluation of weld nondestructive testing GB/T 3323, and the internal quality grade of weld shall not be lower than Grade II.

If the weld is found to be defective, it should be repaired in time, and nondestructive testing should be carried out again after repair until it is qualified; The number of repairs for the same weld shall not exceed 2 times. If it exceeds 2 times, a special repair plan shall be formulated, which can be implemented only after it is approved by the technical director.

(3) Air tightness test

After the installation of stainless steel ventilation pipeline is completed, air tightness test shall be conducted to check the sealing performance of the pipeline system, and the test pressure shall meet the design requirements (generally 0.5-1.0kPa).

Before the test, it is necessary to close all valves, plug the openings at both ends of the pipeline, inject air into the pipeline with an air compressor, slowly raise the pressure to the test pressure, keep the pressure stable for 30 minutes, and then lower it to the design pressure. Use soapy water or air tightness testing instruments to check whether there is air leakage at the pipeline joints, welds, flanges and other parts.

Qualified standard of air tightness test: the pressure drop shall not exceed 10% of the design pressure, and there shall be no obvious air leakage point; If the test is unqualified, it is necessary to find the leaking part and repair it. After the repair, the test is repeated until it is qualified.

(4) Cleanliness inspection

For stainless steel ventilation pipes used in food, medicine, electronics and other industries, cleanliness inspection is required after installation to ensure that there are no pollutants such as oil, dust and impurities inside the pipes.

Cleaning methods can be water washing and air purging: neutral detergent is needed for water washing, washed with clear water after washing, and then dried with compressed air; During air purging, the pressure of compressed air shall not be less than 0.5MPa, and the purging time shall not be less than 30 minutes until the air discharged from the pipeline outlet is free of dust and impurities.

After the cleanliness inspection is qualified, the openings at both ends of the pipeline should be blocked in time to prevent pollutants from subsequent construction from entering the pipeline.

V. Anticorrosion and Protection Requirements

After the installation of stainless steel pipeline is completed, it is necessary to clean the surface of the pipeline to remove impurities such as oil, dust and welding slag, and then wipe it with stainless steel special cleaner to keep the surface clean and dry.

For stainless steel pipes installed outdoors or in corrosive environment, additional anti-corrosion measures (such as painting special anti-corrosion coatings for stainless steel and wrapping anti-corrosion insulation layer) should be taken to prevent the pipes from being corroded; Anticorrosive coatings should be made of products compatible with stainless steel. Before painting, it is necessary to ensure that the pipeline surface is free of rust and oil stain, and the painting thickness is uniform, without brush leakage and sagging.

During transportation and installation of pipelines, protective measures should be taken to avoid collision with sharp objects and surface scratches; After the installation is completed, the construction site should be cleaned in time to avoid debris accumulation around the pipeline, which will affect the heat dissipation of the pipeline or cause corrosion.