What should be paid attention to when stainless steel welded air duct is applied to waste gas treatment project?

Here, we will introduce a notice for stainless steel welded air duct which is suitable for waste gas treatment project and directly used for scheme/construction/acceptance. The content is professional and conforms to the reality of environmental protection projects. It is not wordy but has complete points.

Matters needing attention of stainless steel welded air duct in waste gas treatment project

I. Material selection (the most critical waste gas project)

Selecting materials according to the corrosiveness of waste gas

Ordinary acid and alkali, moisture and normal temperature flue gas: 304 stainless steel is commonly used.

Chlorine, sulfur, high humidity and strong condensation acidity: 316L stainless steel must be used.

Strong corrosive and high temperature flue gas: 2205 dual-phase steel/310S high temperature resistant stainless steel can be upgraded.

Plate thickness

Duct diameter < 600 mm: δ≥ 0.8 ~ 1.0 mm.

Diameter 600 ~ 1200mm: δ≥ 1.0 ~ 1.2mm.

Diameter > ＞1200mm, large negative pressure and high altitude installation: δ≥ 1.2 ~ 1.5mm.

Long-term negative pressure/vibration of exhaust gas system is strictly prohibited.

Second, the welding process requirements

Argon arc welding (TIG) or argon arc backing+electric welding of the cover must be adopted.

Direct full welding by ordinary manual arc welding is prohibited, which is prone to porosity, smoke leakage and corrosion.

Full weld and air tightness are preferred.

Air leakage of exhaust air duct is strictly prohibited, and longitudinal and transverse joints are continuously full welded, and spot welding and missing welding are not allowed.

postweld treatment

Remove welding slag and splash

In strong corrosive environment, pickling passivation/polishing must be done to restore corrosion resistance.

Kerosene penetrant inspection or smoke leak detection shall be conducted for the weld to prevent leakage.

Third, the structure and production considerations

flanged connection

Flange and air duct are fully welded and reinforced, and triangular support ribs are added to prevent negative pressure deformation.

The gasket is made of heat-resistant and corrosion-resistant materials: silica gel gasket, fluororubber gasket and EPDM.

Ordinary rubber pads are strictly prohibited.

Stiffening rib setting

Angle steel/flat steel reinforcing rings must be added to large-diameter and high negative pressure air ducts to avoid vibration, deformation and howling during operation.

Avoid dead angle of effusion

The waste gas often carries condensed water, and the air duct should have a gradient of 1% ~ 2%, and the low point should be provided with a drain valve/outlet to prevent water corrosion.

Four, the system design and installation points

Wind speed control

General waste gas: 8 ~ 12m/s

Dust and particulate matter: 12 ~ 16m/s to prevent deposition and blockage.

Too low wind speed is easy to deposit dust and scale, which accelerates corrosion.

Spacing between supports and hangers

Horizontal pipe: ≤ 2.5 ~ 3m

Vertical pipe: ≤3m

Rubber pad/anti-corrosion isolation pad is added to the bracket hoop to avoid galvanic corrosion caused by direct contact between carbon steel bracket and stainless steel.

Avoid hard pulling.

Long-distance air duct is equipped with stainless steel expansion joint/soft connection to eliminate thermal expansion and cold contraction and equipment vibration.

Interface with equipment

The interfaces such as fan, spray tower, activated carbon box and UV light and oxygen must be sealed and reliable.

Anti-corrosion soft joints shall be used for vibration parts to avoid weld cracking.

V. Key points of anti-corrosion and leakage prevention

Direct contact between carbon steel and stainless steel is strictly prohibited.

Brackets, bolts and hangers must be made of 304/316 stainless steel or insulated gaskets.

Outdoor/outdoor air duct

Rain protection, sun protection and heat preservation (if necessary) are needed to prevent the temperature difference between day and night from generating a large amount of condensed water.

Welds are the hardest hit areas for corrosion.

If pickling passivation is not in place, the weld will be rusted and perforated, and the waste gas project should be paid special attention to.

Vi. Matters needing attention in operation and maintenance

Check flange seal, weld, hanger and drain regularly.

Dust waste gas should be cleaned and dredged regularly to prevent scaling and weight gain from causing deformation.

The condensate of acid waste gas must be discharged in time to avoid soaking the bottom for a long time.

If local corrosion is found, repair welding and passivation shall be carried out immediately to prevent expanding perforation.

Seven, acceptance points

Appearance: the weld is smooth and free of air holes and sand holes.

Air tightness: positive pressure/negative pressure leak detection without leakage.

Slope, drainage port, stiffener and support are complete and standard.

Material certificate, welding material certificate and passivation record are complete.