In this module
Objectives covered: LO 2.7.1. By the end you will be able to:
- Describe What-If, Checklist, HAZOP, FMEA and Bow-tie methods in simple terms.
- Choose a suitable technique for a process or piece of plant.
- Explain when OSHA Process Safety Management requires a process hazard analysis.
7.1 Why process plants need special methods
In plants that handle large amounts of flammable, toxic or high-pressure material, one failure can harm many people at once. Ordinary JHA looks at a worker's task. A Process Hazard Analysis (PHA) looks at the whole process: pipes, vessels, pumps, controls and what happens when something goes wrong.
7.2 The main techniques
| Technique | How it works | Best for |
|---|---|---|
| Checklist | Compare the plant with a list of known good practices | Standard equipment, quick reviews |
| What-If | The team asks "What if...?" questions, e.g. "What if the cooling water fails?" | Simple processes, early design |
| HAZOP | Go line by line using guide words (No, More, Less, Reverse, Other than) on each parameter (flow, pressure, temperature, level) | Complex continuous processes: refineries, fertiliser, gas plants |
| FMEA | For each component, list how it can fail, the effect, and how it is detected | Machines, equipment, electrical systems |
| Fault tree | Start from a top event and work back to the causes with AND/OR logic | Analysing major events in depth |
| Bow-tie | Causes on the left, consequences on the right, the event in the middle, barriers on both sides | Explaining major hazards and barriers to everyone |
7.3 A HAZOP line in plain words
| Guide word + parameter | Cause | Consequence | Safeguard | Action |
|---|---|---|---|---|
| MORE pressure in ammonia storage tank | Refrigeration compressor trips on a hot day | Tank over-pressure, ammonia release | Pressure relief valve to flare | Add high-pressure alarm and automatic standby compressor start |
7.4 Bow-tie in one picture
Think of a bow tie. In the middle is the top event, for example "loss of containment of gas". On the left are the threats (corrosion, overpressure, impact) with prevention barriers (inspection, relief valves, crash barriers). On the right are the consequences (fire, explosion, toxic exposure) with mitigation barriers (gas detection, emergency shutdown, firewater, evacuation).
- OSHA 29 CFR 1910.119 Process Safety Management: a PHA is required for covered processes, revalidated at least every 5 years.
- IEC 61882: HAZOP studies. IEC 60812: FMEA. IEC 31010: risk assessment techniques.
- CCPS (Center for Chemical Process Safety) guidelines for hazard evaluation procedures.
In August 2016 an ammonia storage tank at the DAP fertiliser factory in Anwara, Chattogram, ruptured and released ammonia into the surrounding area, forcing people to be treated in hospital. Ammonia storage is a classic HAZOP topic: what if the pressure is more, the temperature is more, the level is more? Fertiliser, gas processing and LPG plants in Bangladesh now commonly use HAZOP before new projects and major modifications.
Learning Check
Answer these two questions to check your understanding. You can try again as many times as you like.