Open-Top Coolant Tank: Sheet Area, Floor Loading and Thickness Choice
A farm-equipment repair unit needs an open-top rectangular water tank for a coolant sump. Internal dimensions are 900 mm × 600 mm × 500 mm deep. It will be fabricated from 3 mm mild-steel sheet (density 7850 kg/m³) with a 20 mm flange along the bottom edge of each side plate and 15 mm overlap at each of the four vertical corners; all seams are continuously welded. The shop has two sheets of 2000 mm × 1000 mm × 3 mm in stock and normally adds 12% for nesting waste and offcuts. The tank will stand on a workshop mezzanine floor rated at 300 kg/m². The customer wants the tank filled to 80% of its depth in normal use. A supervisor suggests dropping to 2 mm sheet to save material and weight.
1. Calculate the net sheet area for the tank (bottom plus four sides), add the stated bottom flange and corner overlaps, then apply 12% waste. Show whether the two 2000 × 1000 mm sheets in stock are sufficient.
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Bottom: 0.900 × 0.600 = 0.540 m². Two long sides: 2 × (0.900 × 0.500) = 0.900 m². Two short sides: 2 × (0.600 × 0.500) = 0.600 m². Net area = 0.540 + 0.900 + 0.600 = 2.040 m². Bottom flange: bottom perimeter = 2(0.900 + 0.600) = 3.000 m; 20 mm wide → 3.000 × 0.020 = 0.060 m². Corner overlaps: 4 × 0.015 × 0.500 = 0.030 m². Gross before waste = 2.040 + 0.060 + 0.030 = 2.130 m². With 12% waste: 2.130 × 1.12 = 2.386 m². Stock: 2 × (2.000 × 1.000) = 4.000 m², so the area is sufficient with 1.614 m² spare. Judgement: area alone does not prove the plates nest. The bottom is 900 × 600 and each side is 900 × 500 plus flanges, all within 2000 × 1000, so two sheets are enough if the grain direction is kept consistent. Confirm that the 12% is a purchasing allowance and is not added twice by the store.
2. Calculate the tank capacity, the volume and mass of water at 80% fill, and the resulting floor loading in kg/m². Compare this with the 300 kg/m² rating and recommend an action.
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Internal volume = 0.900 × 0.600 × 0.500 = 0.270 m³ = 270 L. At 80% of depth: 270 × 0.80 = 216 L, so water mass ≈ 216 kg (1 kg/L). Floor contact area = 0.900 × 0.600 = 0.540 m². Loading = 216/0.540 = 400 kg/m², which exceeds the 300 kg/m² rating by 100 kg/m², or 33%. Allowable water mass at the rating = 300 × 0.540 = 162 kg = 162 L, i.e. 162/270 = 60% of depth. Recommendation: do not run at 80% on that mezzanine without a load-spreading plate or an engineer's check. A 1.2 m × 0.9 m × 6 mm steel spreader would reduce pressure to 216/1.08 = 200 kg/m², but the floor's total load capacity, not only local pressure, must also be confirmed. If 80% fill is essential, relocate the tank to ground level or build a properly designed support frame.
3. The supervisor proposes 2 mm sheet. Calculate the mass of the 2.386 m² gross order at 2 mm and 3 mm (7850 kg/m³) and the percentage saving. Give a workshop judgement on the proposal.
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Mass per m²: 3 mm → 0.003 × 7850 = 23.55 kg/m²; 2 mm → 0.002 × 7850 = 15.70 kg/m². Gross area 2.386 m²: 3 mm = 2.386 × 23.55 = 56.2 kg; 2 mm = 2.386 × 15.70 = 37.5 kg. Saving = 18.7 kg, or 33.3%. Capacity is unchanged because internal dimensions are fixed, so the material and handling saving is real. Judgement: 2 mm is a false economy for a welded open tank. Wall stiffness depends on thickness cubed, so a 2 mm wall has only about (2/3)³ = 30% of the 3 mm stiffness; the sides will bow and weld distortion will be worse, especially with continuous seams. There is also no corrosion allowance: a 3 mm wall losing 1 mm still has two-thirds of its section, while a 2 mm wall loses half. Keep 3 mm for the bottom and sides; if 2 mm is mandatory, add top-edge stiffeners and a 40 × 40 × 4 mm angle frame, and shorten the inspection interval.