Process Safety · Lesson 5 of 9 · 12 min read
Stoessel criticality classes
Four temperatures, five classes, and the safety measures that go with each.
Educational overview. The principles here are general and simplified, and the figures and rules of thumb are typical values, not limits for your plant. Decisions need process-specific data, the applicable codes and standards, and a qualified assessment.
Four temperatures
| Level | Meaning | Where it comes from |
|---|---|---|
| Tp | Process temperature when control is lost | The process design |
| MTSR | Maximum temperature the synthesis reaction can reach after a cooling failure | Tp + Xacc·ΔTad (reaction calorimetry) |
| MTT | Maximum technical temperature: boiling point (open system) or the temperature at the relief set pressure (closed system) | Solvent, reactor design |
| TD24 | Highest temperature at which the mass is thermally stable for practical purposes (TMRad = 24 h) | DSC / ARC kinetics |
Five classes
| Class | Order | What happens after a cooling failure | Typical measures |
|---|---|---|---|
| 1 | Tp < MTSR < MTT < TD24 | Under the screening assumptions: no boiling and no decomposition expected. The mass would sit at MTSR. | Normal process control. Lower thermal-escalation concern (screening result; verify for your process). |
| 2 | Tp < MTSR < TD24 < MTT | Low concern at MTSR, but if the mass is held hot for long it could slowly heat up to the decomposition, and boiling would not stop it. | Avoid long hold times at high temperature; alarm on temperature. |
| 3 | Tp < MTT < MTSR < TD24 | The batch boils. Evaporative cooling stops the temperature at MTT, if the condenser / vent can take the vapour at that rate. | Check reflux condenser and vapour line capacity; reliable condenser cooling; emergency cooling as an option. |
| 4 | Tp < MTT < TD24 < MTSR | Boiling stops the temperature before TD24, but only if it works. If the condenser fails or is too small, the decomposition follows. | Technical measures to keep the boiling barrier reliable; emergency relief sized for the case. |
| 5 | TD24 < MTSR, with TD24 < MTT | The decomposition is triggered before boiling can help. | Redesign: dosing control, lower accumulation, different temperature or concentration. Otherwise reliable emergency measures (quench, dump, crash cooling) plus relief for the decomposition. |
Classes 1–2 indicate a lower thermal-escalation concern; 3–4 rely on equipment; 5 relies on emergency measures. The class is a screening result, not proof of safety.
The best answer to a high class is almost always to change the process so the class drops, typically by reducing accumulation (feed-controlled dosing at a sufficiently high Tp), diluting, or choosing a higher-boiling solvent.
🧮 Stoessel class calculator
Enter your data; the class and severity update as you type.
Educational screening tool Results are illustrative and depend on the stated assumptions. They are not a substitute for a qualified process-safety assessment, the applicable design code, vendor data, laboratory testing or engineering review.
Want to feel the classes rather than calculate them? Play Runaway!: you design four reactions and the cooling always fails at the worst moment.
✍️ Check yourself: Tp = 30 °C, MTSR = 95 °C, boiling point 66 °C (THF), TD24 = 180 °C. Which class, and what protects you?
MTT (66) < MTSR (95) < TD24 (180): Class 3. The boiling barrier works as long as the reflux condenser and vapour line can take the boil-up rate at 66 °C.
✍️ Check yourself: Which change usually lowers the class of a semi-batch reaction most effectively?
MTSR depends on accumulation. Feed-controlled operation keeps Xacc small, so MTSR stays close to Tp. Lower Tp often increases accumulation, and a lower-boiling solvent lowers MTT.
Key takeaways
- Four levels: Tp, MTSR, MTT, TD24. Their order gives the class.
- Class 1–2: lower concern (screening). Class 3–4: the boiling barrier must be reliable and sized. Class 5: redesign or emergency measures.
- The usual lever is accumulation: feed-controlled dosing, dilution, solvent choice.