Build a compressed-air receiver from ductile steel plate with dished or hemispherical ends, size the wall from the hoop-stress formula using a strength and safety factor you can name, and prove it with a water test at 1.5 times working pressure before it ever holds air. Using the corpus’s own figures, a 400 mm vessel at 8 bar gauge comes out at about 5.3 mm of mild steel, but the number is only as good as the steel, and the rating on a salvaged tank must come from its own marking.
Part of Pneumatic Systems
A receiver is the pressure vessel that holds compressed air. Its wall, ends, welds and fittings decide whether a failure is a leak or an explosion.
Why Air in a Tank Is So Much More Dangerous Than Water in One
Water barely compresses and air does. Take a 100 L (0.1 m³) tank at 8 bar gauge, which is 9 bar absolute. If it burst, the energy released, counted three ways:
- Pressure x volume = 800,000 Pa x 0.1 m³ = 80,000 J, the convention used by Tire Basics
- Adiabatic expansion = 912,000 x 0.1 / 0.4 x (1 - 9^-0.2857) = 106,000 J
- Isothermal expansion = 912,000 x 0.1 x ln 9 = 200,000 J
Ranges of 80-200 kJ equal a 10 kg mass moving at 126 to 200 m/s. Now fill the same tank with water and pressurise it to the 12 bar gauge test pressure (1.2 MPa). Water’s bulk modulus is about 2.2 GPa (recalled, not from the corpus), so its volume shrinks by 1.2 x 10⁶ / 2.2 x 10⁹ x 0.1 m³ = 5.5 x 10⁻⁵ m³, and the energy stored is ½ x 1.2 x 10⁶ x 5.5 x 10⁻⁵ = 33 J. That is why the water test is safe: a failure under water is a sprayed leak where a failure under air is a blast. The test is only that safe if every bubble of air is bled out first, since any trapped air stores energy of its own. The boiler article makes the same point about testing with water (Boiler Construction).
What You Need
- Ductile steel plate (mild steel or wrought iron). Never cast iron: it is brittle and shatters without warning, as the boiler article states.
- Dished or hemispherical ends. The carrier says flat ends are much weaker. A sphere carries half the hoop stress of a cylinder of the same radius and wall (force balance on a hemisphere: the pressure force P x π r² is carried by a wall of length 2π r, so the stress is P x r / 2t against P x r / t for a cylinder), so a hemispherical end is the thinnest safe form.
- Full-penetration welds, ground smooth and inspected, per the carrier, with a dye penetrant check if you can.
- Fittings: relief valve, gauge, drain at the lowest point, inlet from the compressor with a check valve, and an outlet.
Sizing the Wall From Sourced Inputs
The corpus’s compressed-air article gives the wall formula
t = P x r / (S x E)
with P the gauge pressure, r the inner radius, S the allowable stress and E the weld efficiency factor (0.7-0.85 for manual welding). Derivation: a cylinder of wall t and radius r holds a hoop stress σ = P x r / t; setting σ equal to S x E gives the formula. It is for thin walls (t well under a tenth of r).
The inputs below all come from the corpus. Tensile strength: mild steel 50,000-70,000 psi and wrought iron 35,000-50,000 psi (Tensile Strength). Safety factor: 5-8 for boilers and pressure vessels (Safety Factors). Use the weakest end of each, because a salvaged plate’s grade is unknown.
Worked example: 400 mm diameter, 8 bar gauge.
- P = 0.8 MPa, r = 0.2 m
- Lowest mild-steel tensile strength: 50,000 psi x 0.006895 MPa/psi = 345 MPa
- Allowable stress with the safety factor of 8: 345 / 8 = 43.1 MPa
- With a weld efficiency of 0.7: t = 0.8 x 0.2 / (43.1 x 0.7) = 5.3 mm
- Wrought iron at its lowest strength (35,000 psi = 241 MPa): S = 241 / 8 = 30.2 MPa, so t = 0.16 / (30.2 x 0.7) = 7.6 mm
- A weld efficiency of 0.85 with mild steel gives 4.4 mm
The carrier says 3-6 mm for plate thickness. The compressed-air article reaches 2 mm for a 300 mm vessel at 1 MPa only by using a safety factor of 4 on a stronger steel; with the factors above the same vessel needs 5.0 mm (0.3 m, 1.0 MPa, mild steel, E = 0.7). These results contain no allowance for corrosion, for a wall thinned by forming, for pitting or for fittings; no corrosion figure is sourced here, so add margin and drain religiously. Treat the calculated thickness as a minimum for a known steel, never as a rating for a tank of unknown steel.
Method: Test Before It Holds Air
Step 1: Fill it completely with water Open a valve at the highest point and let water run out steadily so no air is trapped.
Step 2: Pressurise slowly with a hand pump Raise the pressure to 1.5 x working pressure: 12 bar gauge for an 8 bar vessel. A hand pump is the corpus’s method (Hand Pumps for the pump idea, though the pump needs a gauge and a valve to hold pressure).
Step 3: Hold 30 minutes and inspect Look for weeping seams, bulges and drops on the gauge. Any leak or visible deformation means reject.
Step 4: Mark it Record the test pressure and date on a stamped plate or the log. Re-test annually and after any repair.
The Relief Setting Is Anchored to the Vessel, Not the Compressor
The carrier sets the relief valve at 110% of working pressure. For an 8 bar vessel that is 8.8 bar gauge, which is 1.1 / 1.5 = 73% of the 12 bar test pressure, so the valve opens well inside what the tank proved it can hold. That anchoring works only if the tank was actually proved. If the vessel is salvaged and its rating is unknown, set working pressure from a test you perform, never from what the compressor can reach, and take the relief pressure from the vessel’s own marking or a real pressure-vessel reference. This corpus cannot give that figure for an unmarked tank.
Common Mistakes
- Welding on a used fuel or gas tank. Fuel residue explodes; fill the tank with water and drain it completely before cutting or welding, and do not use a tank that carried something you cannot identify.
- No drain at the lowest point, or a drain you never open.
- A relief valve with a shut-off valve in front of it.
- Testing with air first. Do the water test, then the air.
Safety
A corroded or over-pressured receiver fails by bursting
A receiver fails by splitting or by throwing off an end, and the stored energy is 80-200 kJ at 100 L and 8 bar gauge. Condensate sits in the bottom of the tank and corrodes the wall from the inside where nobody looks, so drain it daily and re-inspect the inside whenever you can see it. Fit a relief valve with nothing between it and the tank, point its outlet away from people, and never raise its setting to stop it lifting. Never weld, drill or grind a tank that has held air under pressure or fuel. Stand clear while it is filled for the first time.
What’s Next
- Learn next: Safety Devices — how the relief valve, gauge and drain are built
- Combine with: Moisture Removal — keeping the wall dry