A reaction vessel must resist what it holds, the temperature it reaches and the pressure it carries, and the carrier’s table matches material to chemical: mild steel for ammonia and general work, stainless for mild acids, glass-lined steel for most acids and alkalis, lead-lined steel for dilute-to-moderate sulfuric acid, and stoneware for low-pressure acid work. Weld seams are tested with water at 1.5 times working pressure because water, being nearly incompressible, stores thousands of times less energy than gas at the same pressure.
Part of Industrial Chemistry
Materials, pressure rules and seals for vessels in which chemical reactions run.
The Carrier’s Material Table
| Material | Resists | Fails against | Best for |
|---|---|---|---|
| Mild steel | Moderate heat, moderate pressure | Strong acids, chlorine | General reactions, ammonia |
| Stainless steel | Heat, mild acids, corrosion | Strong HCl, fluorine | Versatile, preferred if available |
| Cast iron | Heat, moderate pressure | Concentrated acids | Soda ash, general |
| Lead-lined steel | Sulfuric acid (dilute to moderate) | Heat above 300 °C | Lead chamber process |
| Glass-lined steel | Most acids and alkalis | Thermal shock, impact | Acid reactions |
| Stoneware or ceramic | Most chemicals | Mechanical shock, pressure | Low-pressure acid work |
Related corpus pages: Chemical Vessels (porcelain and glazed ceramics, which resist most acids but not hydrofluoric acid or hot strong alkali), Stainless Steel, and for reagent storage Container Selection.
Pressure Rules From The Carrier
- Never exceed the rated pressure, and fit a relief valve set to 90% of maximum.
- Test all welded seams by pressurising with water to 1.5 times working pressure; water is incompressible, so failures are less violent than with gas.
- Inspect for corrosion regularly, because pitting weakens vessel walls.
- Flanges and gaskets are the commonest leak points: check them often.
Pressure-vessel failure modes are in Pressure Vessel Theory and Failure Modes, and the wall-thickness arithmetic in Pressure and Temperature. For gaskets see Gaskets and Seals; gasket material must be checked against the chemical as carefully as the vessel.
What You Need
- A material chosen from the table for every surface the contents touch, including the lid and gaskets.
- A hydraulic test setup: pump, calibrated gauge, a vent at the top for air, a safe place to test (behind a barrier or in a pit).
- A relief valve that discharges to a safe place.
- Agitation with a seal that suits the vessel.
- An inspection schedule.
Method: Testing A New Vessel
Step 1: Fill with water and bleed all the air from the top.
Step 2: Raise the pressure slowly to the test pressure, 1.5 times the working pressure.
Step 3: Hold and inspect seams and flanges from behind a barrier.
Step 4: Reduce the pressure slowly, drain and dry.
Step 5: Mark the working pressure on the vessel and record the test.
Worked Example: Why Water Is The Safer Test Fluid
Compare the energy stored in 100 litres (0.1 m³).
Gas at 10 atmospheres. The work available from isothermal expansion is P1 × V1 × ln(P1 ÷ P0). P1 = 10 × 101,325 Pa = 1.013 MPa, so P1 × V1 = 1.013 × 0.1 = 0.1013 MJ = 101 kJ. ln(10) = 2.30. Energy = 101 × 2.30 = 233 kJ.
Water at the test pressure, 15 atmospheres. Stored elastic energy ≈ P² × V ÷ (2 × K), with the bulk modulus of water K ≈ 2.2 GPa (a recalled value). P = 15 × 101,325 = 1.52 MPa. Energy = (1.52 × 10⁶)² × 0.1 ÷ (2 × 2.2 × 10⁹) = 2.31 × 10¹¹ ÷ 4.4 × 10⁹ = 52 J.
Ratio: 233,000 ÷ 52 ≈ 4,500. A water-filled vessel that fails releases a few tens of joules of compression energy plus a spray, where the same vessel full of gas releases a blast. The figures are order-of-magnitude. For hot water, which flashes to steam on rupture, the stored energy is far larger; see the boiler and pressure-vessel pages.
Common Mistakes
- Using mild steel for chlorine or strong acids.
- Pressure testing with air to save time.
- Skipping the top vent while filling, so trapped air remains compressible and stores energy.
- Ignoring corrosion under linings.
- Choosing the gasket after the vessel.
Safety
A failed vessel is a bomb, and corrosion makes it fail without warning
Test with water, from behind a barrier, with everyone else clear; never exceed the rated pressure; never plug a relief valve. Lead linings are toxic and fail above 300 °C; glass linings shatter on shock or sudden heating. See Safety and Waste.
What’s Next
- Learn next: Heat Exchangers — moving heat in and out
- Combine with: Pressure and Temperature — the stress arithmetic