Brief introduction: the processing and installation method of stainless steel welded air duct in environmental protection project of painting workshop

Processing and installation method of stainless steel welded air duct for environmental protection engineering in painting workshop

Stainless steel welded air duct in painting workshop is suitable for working conditions such as waste gas collection, paint mist treatment, environmental protection exhaust, etc., and needs to give consideration to welding tightness, corrosion resistance and installation tightness (to prevent waste gas leakage). At the same time, it is suitable for the layout of environmental protection pipeline in painting workshop. Machining is mainly based on mechanical forming+argon arc welding, and installation follows the principle of "prefabrication before site, main pipe before branch pipe". The core processing and installation methods are as follows, and it is suitable for the technological requirements of environmental protection engineering in painting workshop throughout.

First, the core processing requirements and methods (mainly workshop prefabrication)

304 stainless steel (suitable for slightly corrosive coating waste gas, 316L for high-demand scenes) is preferred as the base material. The thickness of the plate is determined according to the diameter/side length of the air duct: 0.8~1.2mm for diameter ≤800mm, 1.2~1.5mm for diameter ≤1500mm, and 1.5~2.0mm for diameter > 1500mm. The roundness of the air duct shall be ensured throughout the processing.

Blanking and molding: CNC plasma/laser blanking is used to ensure that the incision is smooth and burr-free (burr will lead to paint and dust accumulation, which will affect environmental protection and exhaust); The circular air duct is rolled by a rolling machine, with the butt gap ≤0.5mm and the ellipticity deviation ≤ 1% of the diameter; Rectangular air duct is formed by flanging machine, the flanging angle is 90, the edge line is straight, and there is no warping at the splicing place.

Welding operation (core process): Argon arc welding (one-sided welding and double-sided forming) is adopted uniformly to ensure the sealing of weld and prevent waste gas leakage; Before welding, clean the groove and the oil and dust within 20mm on both sides (oil is forbidden in the painting workshop to avoid oil mist pollution caused by welding), and the groove angle is 30 ~ 45; The weld seam is continuous, without slag inclusion, pores and cracks. After welding, the welding slag shall be knocked out in time, and the weld seam shall be passivated by pickling (to improve corrosion resistance and adapt to the humid environment with slight paint mist in the painting workshop).

Flange fabrication and connection: stainless steel flanges are welded at both ends of the air duct, and the flange faces are flat and perpendicular to the axis of the air duct (deviation ≤ 1 mm/m); The flange bolt holes are evenly distributed, the hole spacing deviation is ≤2mm, and the distance between the center of the bolt hole and the flange edge is consistent, so as to ensure the accurate on-site docking; The sealing surface of flange should be polished and smooth, and the installation position of gasket should be reserved.

Prefabrication inspection: after each section of prefabricated air duct is completed, make an initial air tightness inspection (simple method: plug both ends of the air duct, introduce compressed air, apply soap solution to the weld/splice, and it will be qualified if there are no bubbles); At the same time, check the size, roundness/straightness of the air duct and number it (according to the installation order) to facilitate on-site docking.

Second, the on-site installation method (adapted to the on-site working conditions of the painting workshop)

Before installation, it is necessary to check the axis, elevation and bracket position of the site, and accurately connect with the environmental protection equipment (paint mist processor, activated carbon adsorption box and fan) in the painting workshop to avoid the deformation of the air duct and ensure the installation tightness.

Fabrication and installation of bracket: the bracket is made of stainless steel with the same material to avoid electrochemical corrosion caused by contact of different materials; Determine the bracket spacing according to the weight and span of the air duct: the diameter of circular air duct is ≤1000mm, the spacing is 3~4m, the diameter is > 1000 mm, and the spacing is 2 ~ 3 m; The bracket shall be installed firmly and connected with the wall/beam-column reliably, and a rubber pad shall be added between the air duct and the bracket (damping, preventing the air duct from being scratched, and avoiding vibration and air leakage caused by hard contact). It is forbidden to directly press the bracket on the weld.

Duct splicing and installation: follow the principle of "main pipe first, branch pipe later, upper and lower first, inner and outer first" and splice according to prefabricated number; When the air ducts are butted, the flange faces are closely attached, and an oil-resistant and temperature-resistant rubber gasket (suitable for coating waste gas, with a thickness of 3~5mm, and the gasket is not spliced or damaged) is installed in the middle, and the bolts are fastened diagonally evenly to prevent air leakage caused by gasket deviation; Only flange connection is allowed for on-site splicing of welded air ducts, and on-site welding is prohibited (to avoid smoke and dust generated by welding in the painting workshop and to prevent leakage caused by poor on-site welding quality).

Docking with environmental protection equipment: when the air duct is connected with the fan, paint mist processor, adsorption box and other equipment, it adopts flexible soft joint (stainless steel canvas soft joint) with a length of 200~300mm, so as to avoid the weld cracking caused by the transmission of equipment vibration to the air duct; The two ends of the soft joint are fastened with clamps, which are tightly sealed without distortion or fold.

Installation of special-shaped pipes and fittings: elbows, tees, reducing fittings and other special-shaped fittings are prefabricated in the factory and directly docked on site; The radius of curvature of the elbow shall be ≥1.5 times the diameter of the air duct, and the included angle between the tee branch pipe and the main pipe shall be determined according to the site layout, so as to ensure smooth exhaust and no dead angle of paint accumulation; All fittings and straight pipes are connected by flanges, and the sealing requirements are consistent with those of straight pipes.

Iii. Post-installation inspection and closing (to ensure that the environmental protection project reaches the standard)

After the installation, the overall air tightness test and appearance acceptance are required to ensure that the environmental protection requirements of waste gas collection and discharge in the painting workshop are met, and there is no leakage, no vibration and smooth exhaust.

Air tightness test: the combination of light leakage method and air leakage test is adopted; Light leakage method: light source is placed in the air duct, and it is qualified if there is no light leakage outside (suitable for low-pressure air duct); The air leakage test shall be conducted in high-demand working conditions, and the air leakage shall meet the requirements of Code for Acceptance of Construction Quality of Ventilation and Air Conditioning Engineering (GB50243), so as to ensure no leakage of waste gas.

Vibration and noise inspection: after starting the fan, check that there is no obvious vibration and abnormal sound in the air duct, no looseness in the bracket, no excessive stretching/compression in the soft joint, and add a shock absorber bracket if necessary.

Finishing treatment: clean the surface of the air duct as a whole to remove oil and dust during installation; The exposed parts of flange bolts shall be rusted; Mark the interfaces and fastening points installed on site for later maintenance; Commissioning with the environmental protection system of the painting workshop to ensure that the exhaust flow and pressure of the air duct match the design requirements.

Iv. Matters needing attention in painting workshop

Oil-forbidden operation in the whole process: avoid contact with oil pollution during processing and installation, and clean tools and supports in advance to prevent oil pollution from entering the air duct and mixing with paint mist to form potential safety hazards.

Avoid dead angle of paint accumulation: the inner wall of the air duct is smooth, without concave-convex, burr and special-shaped fittings without dead zone, so as to prevent paint mist from accumulating and affect the exhaust efficiency and later cleaning.

Adapting workshop layout: There are many pipelines and limited space in the painting workshop. The installation of air ducts should avoid production equipment and running tracks, and sufficient maintenance space (≥500mm) should be reserved to facilitate the cleaning of paint mist on the inner wall of air ducts and the maintenance of environmental protection equipment.