Chemical equipment is designed by answering four questions in order: what does it contain, how hot will it get, how much pressure will it carry, and how will it fail. The material follows from the first, the wall thickness from the second and third, and the safety devices from the fourth. As the carrier’s rules put it, test every vessel with water at 1.5 times its working pressure and set the relief valve at 90% of the maximum.
Part of Industrial Chemistry
Selecting vessel and heat-exchanger materials, sizes and safety devices for chemical processes.
Four Questions, In Order
- What does it contain? This decides the material, using the carrier’s table in Reaction Vessels.
- How hot? Strength falls with temperature, and heat accelerates corrosion.
- What pressure? Pressure sets wall thickness (see the hoop-stress working in Pressure and Temperature) and which devices are required.
- How will it fail, and what happens then? Leak, crack, rupture, corrosion, blocked outlet, cooling failure. A relief path, a way to stop the feed and a place for the contents to go are part of the design.
The two main families of equipment are vessels, the subject of Reaction Vessels, and heat exchangers, in Heat Exchangers. Reagent containers (as opposed to process vessels) are covered by Container Selection for acids and alkalis and Storage Containers for petroleum and solvents. Pressure in particular has dedicated corpus treatments: Pressure Vessel Theory and, for sterilisers, Autoclave Construction.
What You Need
- A written process description: contents, temperatures, pressures, flows.
- Material data for each wetted part; consult Chemical Vessels and Stainless Steel.
- Test equipment: a hydraulic test pump, a calibrated pressure gauge, a relief valve.
- A fire plan for the site; for solvent stores see Fire Prevention, which is about petroleum vapour and is the right reference for a solvent store, not for a reactor hall.
Method: A Design Review Before Building
Step 1: List every substance that touches each surface, including cleaning agents.
Step 2: Choose a material that resists all of them at the maximum temperature.
Step 3: Set the maximum working pressure and the test pressure at 1.5 times it.
Step 4: Set the relief valve at 90% of the vessel’s maximum, per the carrier.
Step 5: Walk the failure list, adding a safeguard for each.
Worked Example: Pressure Numbers For A Vessel
Take a vessel rated for a maximum working pressure of 10 atmospheres (illustrative).
- Hydrostatic (water) test pressure: 1.5 × 10 = 15 atmospheres, per the carrier.
- Relief valve set point at 90% of the maximum: 0.9 × 10 = 9 atmospheres, so the valve opens before the maximum is reached.
- Normal operating pressure must then sit below 9 atmospheres with margin, or the valve will lift in normal use; operate at no more than, say, 7 to 8 atmospheres.
The test-pressure margin applies to the vessel’s maximum rating, not its operating level. Where a vessel will carry hot water or steam, remember its stored energy; the corpus’s boiler article notes just 30 psi can flash to roughly 1,700 times its liquid volume on rupture (Boiler Construction).
Common Mistakes
- Choosing material by what is at hand rather than by what it holds.
- Setting the relief valve above the vessel’s maximum.
- Using gas for the pressure test where water would do. Gas stores far more energy.
- No relief path for the contents, only for the pressure.
- Reusing a food or fuel container for acid.
Safety
A vessel is a stored-energy device
Never exceed the rated pressure, never plug or tamper with a relief valve, test with water not gas, and place vessels so a failure harms no one. For a hall that houses reactors, write your own safety rules rather than borrowing an explosives-manufacturing code: the corpus’s defence safety-protocols page is about explosives and does not fit. See Safety and Waste.
What’s Next
- Learn next: Reaction Vessels — materials, pressure and seals
- Combine with: Heat Exchangers — the other main equipment family