What is the reason for air leakage in the installed stainless steel welded ventilation pipeline?

All causes of air leakage in stainless steel welded air duct+corresponding phenomenon

A, welding body defects (the main air leakage point)

Incomplete weld, root gap of one-sided welding

Thin-plate stainless steel duct is often welded only on one side, and the back is not fused to form a through gap, so the wind pressure will leak as soon as it comes up; Stainless steel with wall thickness ≤1.2mm is most likely to appear.

Pore, sand hole and slag inclusion

Argon protection is not in place, welding wire oil stain, plate scale, high wind speed blow away shielding gas, weld is covered with fine pinholes, and there is slight air leakage. The higher the static pressure, the greater the leakage.

Crack (cold crack/hot crack)

Stainless steel expands when heated and contracts when cooled, and the welding current is too large, the cooling is too fast, the weld filling is insufficient, and fine through cracks appear in the weld; Corners and flange roots are high.

Gap of virtual welding, broken welding and intermittent welding

The welder's gun walking speed is unstable, the arc stops halfway, and one section of the weld is fused and the other section is suspended, forming a long leak.

Flash, undercut and perforation

Excessive current burns through the pipe wall, forming holes; The undercut depth exceeds the standard, and the thin wall directly penetrates the air leakage.

Second, the plate and blanking production problems

Plate bending cracking, hem tearing

The hard material and bending R of stainless steel are too small, and invisible microcracks appear at the bending angle, resulting in air leakage during pressure test.

Staggered splicing and excessive gap.

The butt joint of the pipe joint is more than 1 mm, and the groove gap exceeds 2mm, so the welding cannot be completely fused, leaving a seam on the inside.

Damage of base metal of pipe wall

Scratches, holes, small holes with naked eyes but continuous air leakage during hoisting and transshipment.

Three, flange, connection sealing parts air leakage (second only to weld)

Flange welding gap

Leakage welding and slag inclusion of fillet weld between flange and air duct cylinder; Weld fracture at flange flanging.

Failure of flange gasket

The gasket is damaged, broken and the flat-fell seam is not staggered;

The gasket is too thin/thick, resulting in insufficient compression after compaction;

Rubber pad aging, oil corrosion deformation;

The flange is uneven, the gasket is partially suspended, and the seal is invalid.

Flange bolts are not tightened evenly.

Unilateral tightening, diagonal not tightened step by step, flange local warping, gasket pressure is not tight and cracks appear.

Flange deformation and warping

When welding flanges, they are deformed at high temperature, and the two flanges are not tightly attached, and there is a gap between them.

Four, installation, lifting, reinforcement of leakage points

Air leakage in welding openings of supports and hangers.

The bracket is directly welded on the outer wall of the air duct, and the welding burns through the pipe wall; Air duct opening reinforcement and incomplete welding plugging.

Duct deformation tensile crack weld

The distance between lifting points is too large, there is no anti-sway bracket, and the fan vibrates to pull the air duct, and the original weld is torn.

The welding at both ends of the soft joint/the flange is not tightly sealed.

Canvas soft joint, flange gaskets at both ends of stainless steel compensator are missing, or soft joint flanging weld is perforated.

Weld defects of elbow, reducer and tee special-shaped parts

Special-shaped parts have many welding seams and corners are difficult to weld, which easily leads to incomplete penetration and porosity.

Five, fan, valve fittings interface air leakage

Air leakage of air valve, fire valve, muffler flange weld and gasket;

The short pipe at the inlet and outlet of the fan is welded and perforated, and the flange is not tightly sealed;

The sealing rubber strip of the cover plate of the inspection port and the test port falls off and the screws are loose.

Six, construction technology and environmental additional reasons

Argon welding protection is not in place

The back of the pipe is not filled with argon, the back of the weld is oxidized to form honeycomb holes, and there is a lot of air leakage inside; The most common hidden air leakage causes of stainless steel air ducts.

Spot welding without full welding or cutting corners.

The design requires full welding, and only spot welding is fixed on site, and the remaining long gaps directly leak air.

No leak detection or repair welding after welding.

No light transmission, light leakage and wind pressure light leakage tests were done before completion, and tiny sand holes were not repaired in advance.

Temperature difference stress cracking

Long-term alternation of hot and cold in high-temperature exhaust pipeline, fatigue cracking of stainless steel weld, and gradual air leakage in the later stage.

Fast distinguishing air leakage position and simple judgment

Leakage along the weld: welding process defects (blowhole, incomplete penetration, no back argon)

Leakage between two flanges: gasket, flange deformation, uneven tightness of bolts.

Corner/tee leakage: the welding operation of special-shaped parts is difficult and the fusion is poor.

Leakage after a period of use: vibration tearing, cold and hot stress cracking.

Micro-uniform micro-leakage in many places: there are a lot of pores in the weld, and argon is not filled for protection.

Simple governance scheme

Micro sand holes and air holes: argon arc welding for repair welding after polishing;

Incomplete large gap: full welding after grooving and root cleaning;

Flange leakage: replace the complete rubber gasket without splicing, tighten the bolts diagonally evenly, and correct the deformed flange;

Vibration tearing and leakage: increase supports and hangers and add compensators to eliminate stress;

Preventive measures: argon filling protection in stainless steel welded pipe, full welding of weld seam and light leakage detection after completion.