Interpreting Shell and Nozzle Weld Symbols on a Stainless Tank
A workshop is fabricating a 1,000 mm × 600 mm × 800 mm rectangular tank from 6 mm 316L stainless steel plate. The longitudinal shell seam is detailed as a single-V butt weld on the arrow side, 60° included angle, 2 mm root gap and 1.5 mm root land; the tail of the symbol reads 'GTAW root, SMAW fill'. A 100 mm diameter nozzle in the side plate carries a 6 mm fillet weld with the all-around symbol at the reference-line/arrow junction; the arrow points to the nozzle side. The drawing note beside a lifting lug says 'weld both sides where accessible', but the lug symbol shows a 5 mm fillet on the arrow side only. The fitter has cut the shell plates with a plasma cutter and proposes a 4 mm root gap because the edges are rough. The welder asks whether the nozzle can be welded after the tank is completed and whether the lifting-lug note changes the symbol.
1. Interpret the shell-seam symbol and the nozzle symbol: what preparation, size, side and extent of welding does each require?
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Shell seam: the symbol is on the arrow side and is a single-V butt weld. The arrow-side plate is bevelled to a 60° included angle; the other plate is square. The 2 mm root gap and 1.5 mm root land are fit-up requirements, not weld sizes. The tail 'GTAW root, SMAW fill' means the root pass is gas tungsten arc welding and the fill and cap are shielded metal arc welding. Because it is a single-V on the arrow side, preparation is on that side and the weld is made from that side. For 316L, the root must be protected by an inert purge.
Nozzle: the 6 mm dimension is the fillet leg length, not the throat. The arrow points to the nozzle side, so the fillet is placed in the corner between the nozzle and the side plate on the arrow side. The all-around circle means the fillet continues around the entire circumference of the nozzle; it is not a spot or intermittent weld.
2. Explain how the proposed 4 mm root gap and plasma-cut edges would affect the shell seam, and what fit-up checks you would make before welding.
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A 4 mm gap is double the specified 2 mm. It increases the risk of burn-through, excessive root penetration and shrinkage, and it changes the weld cross-section. The GTAW root may not bridge the gap without backing or a consumable insert. Plasma-cut edges are rough and may carry dross, oxides and a hard cut zone; on 316L they must be ground clean to bright metal and the bevel checked.
Before welding, check the bevel angle (60° included), root land (1.5 mm), root gap (2 mm, using a gap gauge), alignment and tack welds, and cleanliness-no oil, paint, oxide or moisture. Check that the nozzle opening and shell seam will not interfere. If the gap cannot be closed, do not simply fill it; re-fit the plates or use an approved procedure with backing. Rough edges cause variable penetration and can trap slag, so the fitter should correct the preparation before welding starts.
3. Resolve the apparent conflict between the lifting-lug note and its 5 mm arrow-side fillet symbol, and advise on welding the nozzle after tank completion.
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The specific weld symbol normally governs over a general note, but the two may not actually conflict. The symbol specifies a 5 mm arrow-side fillet; the note 'weld both sides where accessible' could mean an additional 5 mm fillet on the other side where access permits. The correct action is to query the designer before welding, because adding a second side changes the weld size, heat input and distortion. Do not guess.
For the nozzle, welding after the tank is complete is possible only if access and purge are available. The shell seam root is GTAW, so the tank must be purged before the root is welded. Once the tank is closed, purging the nozzle opening may be difficult, and the stainless steel can be contaminated. It is better to weld the nozzle before final closure so the inside can be purged and the fillet can be made in a more favourable position. If it must be done later, use an approved purge dam and inspect for sugaring or oxidation.