Thermal runaway study
Process engineering
A HAZOP (Hazard and Operability Study) followed by a LOPA (Layers of Protection Analysis) highlighted one critical case in a batch reaction: thermal runaway with rising temperature and pressure. The aim of the study was to determine whether the existing layers were sufficient and which additional measure was required.
About the case
Context:
In a batch reactor, loss or delay of temperature control can lead to uncontrolled heat generation. Possible initiating causes include faulty measurement, reduced heat removal or incorrect dosing. The potential consequences include exceeding the design limits for temperature and pressure.
- Approach:
Selection from HAZOP to LOPA: the runaway scenario was selected because of its high severity. The initiating event was quantified and the existing independent protection layers (IPLs) were assessed for independence, reliability and detectability. - Existing layers (generic):
- Process control with alarms and operator intervention within defined response times.
- Periodic cleaning/calibration and preventive maintenance to limit degradation of measurement and control functions.
- Passive/mitigating provisions (e.g. controlled pressure relief or emergency cooling), where present.
- LOPA result: the sum of the effective IPLs fell short of the risk reduction needed to meet the target value. A Risk Reduction Factor (RRF) gap remained, corresponding to an instrumented measure at SIL 1 level.
- Recommended measure: implementation of a Safety Instrumented Function (SIF) for automatic quench (a controlled, automatic intervention to moderate the reaction), triggered by an independent detection signal (e.g. pressure or temperature monitoring). The SIF is substantiated with SIL verification, a proof test interval and a Site Acceptance Test (SAT) to safeguard functional performance at commissioning and throughout the lifecycle.
Result:
With the “automatic quench” SIF (SIL 1), supplemented by the existing generic layers (operator response, maintenance programmes and any passive/mitigating provisions), the RRF gap is closed and the likelihood of the runaway consequence drops to the target TEL (Tolerable Event Likelihood). The proposed set of measures is independent, verifiable and aimed at interrupting or slowing down a thermal runaway in time.