What are the installation standards for installing stainless steel welded ventilation pipes in subway?

The installation of stainless steel welded ventilation pipes in subway follows GB 50243 Code for Acceptance of Construction Quality of Ventilation and Air-conditioning Engineering, JGJ/T 141 Technical Specification for Ventilation Pipes and subway special specifications, focusing on eight links: material control, welding, fabrication, installation, sealing, support and hanger, anti-corrosion and acceptance, so as to ensure that the system is airtight, corrosion-resistant, stable and safe.

First, the material standard (core premise)

Material requirements

SUS304/316L stainless steel plate is commonly used in subway ventilation, and the original mechanical properties, intergranular corrosion and chemical composition test report should be provided.

Plate thickness: 0.8–1.5 mm is commonly used in low-voltage systems; High pressure/smoke exhaust system ≥ 1.0mm; The negative deviation of thickness is ≤0.02mm, and the surface roughness Ra is ≤ 0.4 μ m..

It is forbidden to use 201 stainless steel (the corrosion resistance is insufficient, and the underground wet environment of the subway is easy to rust).

Welding material matching

ER308/ER316L welding wire is selected, which is made of the same material as the base material; E308-16/E316L-16 is used in covered electrode, and carbon steel welding materials are strictly prohibited.

Protective gas: pure argon (Ar≥99.99%), with argon filling on the back for protection, with flow rate of 8–15l/min.

Auxiliary material standard

Flange: stainless steel of the same material, or chrome-plated/galvanized carbon steel flange.

Gasket: EPDM, PTFE and silicone rubber (nonflammable/nonflammable), with a thickness of 3–5 mm and a compression ratio of 30%.

Bolt: SUS304 stainless steel, with elastic pad and flat pad, with a torque of 20N m..

Second, the welding process standard (quality core)

welding process

Thickness **≤1.2mm**: manual TIG welding.

Thickness * * > 1.2 mm * *: pulsed argon arc welding to reduce thermal deformation.

Splicing and flange connection must be fully welded continuously. Spot welding, missing welding and gas welding are strictly prohibited.

Process parameters (reference)

0.8mm plate: current 30–50a, argon 8–10l/min.

1.0mm plate: current 50–80a, argon 10–12l/min.

1.2mm plate: current 80–110a, argon 10–12l/min.

Weld quality requirements

Appearance: silvery white/golden yellow, without cracks, pores, slag inclusion and flash; The undercut depth is ≤0.5mm, and the continuous length is ≤100mm.

Inside: the back is protected by argon, and there is no incomplete penetration and concave; The height of the weld is flush with the base metal, and the residual height is 0–1 mm..

Inner wall: the weld is polished smoothly with roughness Ra≤3.2μm, which reduces wind resistance and prevents dust accumulation.

postweld treatment

Remove welding slag and splash, and passivate by pickling (form Cr₂O₃ protective film to improve corrosion resistance).

100% visual inspection of welds, high pressure/key parts sampling nondestructive testing (PT/MT).

Third, the duct production standards

dimensional accuracy

Rectangular duct: side length deviation ≤±1mm, diagonal deviation ≤ 2 mm.

Circular duct: diameter deviation ≤±0.5mm, ellipticity ≤ 1 mm/m.

Flanging: verticality deviation ≤1mm/m and angle deviation ≤ 1.

Connection and reinforcement

Thickness **≤1mm**: it can be snapped/riveted; > 1 mm: argon arc welding, gas welding is strictly prohibited.

Flange connection: flange flatness deviation ≤ 2mm; Bolt hole spacing: low pressure **≤150mm**, high pressure **≤100mm**.

Reinforcement: The rectangular air duct has a large side of * * > 630mm * * and a circular diameter of * * > 800mm * *, and is provided with reinforcement ribs/angle steel frames with even spacing and regular arrangement.

Cleaning and protection

After production: compressed air is used to purge the inside and outside to remove oil stains and sundries.

Port: sealed with plastic film to prevent pollution and scratches during transportation/installation.

Fourth, the site installation standards

Preparation before installation

Check the size, number and direction of air duct, and check the integrity of weld, flange and gasket.

Crossing the firewall/floor: a steel sleeve with a thickness of **≥1.6mm shall be embedded, and the gap shall be filled with * * incombustible material (mineral wool) * *.

Installation of supports and hangers (key)

Spacing: installed horizontally, side length/diameter **≤400mm**, spacing * * ≤ 4m * *; > ＞400mm, spacing * * 3m * * or less; Vertical installation, spacing **≤4m**, single straight pipe ≥2 fixed points.

Material: rubber/PTFE spacer is added at the contact between stainless steel air duct and carbon steel hanger to block electrochemical corrosion.

Location: Avoid tuyeres, valves, inspection doors and automatic control mechanisms; Rubber gaskets shall be added at the contact parts between supports and hangers and air ducts to prevent cold bridges.

Duct connection and sealing

Flange butt joint: coaxial and parallel, the gasket is continuous and without breakpoints, and the joint is chamfered at 45, and the dislocation is ≤2mm.

Bolts: Tighten diagonally and evenly, with nuts on the same side; PTFE spacer is added between flanges to prevent corrosion.

Air leakage rate: low-pressure system **≤3%, high-pressure/smoke exhaust system ≤ 1%; Sectional air tightness test after installation **(0.5kPa compressed air, soapy water for leak detection, no bubbles).

System layout

Direction: straight, less elbows, reducing wind resistance; Standard elbow for turning (R≥1.5D).

Slope: condensate drainage slope **≥0.5%, and drain valve * * is set at the low point.

Distance: **≥150mm** from other pipelines for easy maintenance.

Five, anti-corrosion and thermal insulation standards

antiseptic

Stainless steel air ducts are generally not anticorrosive, but welds and scratches need to be passivated.

Carbon steel hanger: derusting+antirust paint+topcoat, or galvanized.

thermal insulation

Material: rubber, plastic and centrifugal glass wool (Class A combustion performance), with the thickness as designed.

Construction: the thermal insulation layer is dense, without gaps, and the flat-fell joints are staggered and sealed; Anti-corrosion wood cushion blocks (the same as the insulation layer thickness) are set at the supports and hangers to prevent cold bridges.

Protective layer: aluminum foil/stainless steel plate, with overlapping * * ≥ 30–50 mm * *, and the seam is sealed, so as not to puncture the moisture-proof layer.

VI. Acceptance Criteria (Final Judgment)

dominant item

The materials and welding materials are qualified and the test report is complete.

The weld has no cracks, incomplete penetration, air holes and slag inclusion, and the appearance and internal quality are up to standard.

Air leakage rate and air tightness test are qualified.

The spacing, material and anticorrosion of supports and hangers conform to the specifications.

General project

Dimensional accuracy, flatness and straightness are within the allowable deviation.

Specification for flange, sealing, heat preservation and marking.

Data acceptance

Material certificate, test report, welding procedure qualification, construction record, concealed acceptance and test report are complete.

Seven, the subway special special requirements

Environmental adaptation

In humid underground and Cl-containing environment, 316L stainless steel is preferred to improve the corrosion-resistant life.

Smoke exhaust system: high temperature resistant (800℃/30min), complete structure, no deformation and no leakage.

Fire prevention and safety

Crossing the fire zone: a fire damper (fused at 70℃/280℃) is provided, and the air duct is wrapped with fire protection (the thickness is as designed).

Anti-seismic: According to GB 50981 Code for Seismic Design of Building Electromechanical Engineering, anti-seismic supports and hangers are provided.

Noise and vibration

The air duct is reinforced in place, and the hanger is supported with damping pads to reduce the running noise and vibration.