🎓 Lesson 23
D5
HAZOP for Separation Units: Key Guidewords & Deviations
HAZOP is a structured team-based method to find potential hazards and operability problems in a separation process by systematically asking 'what if' questions using simple words like 'no', 'more', or 'less'.
🎯 Learning Objectives
- ✓ Explain the purpose and scope of HAZOP in separation units using appropriate guidewords and deviation logic
- ✓ Apply standard HAZOP guidewords (e.g., NO, MORE, LESS, AS WELL AS) to identify deviations in a distillation column node
- ✓ Analyze a completed HAZOP worksheet to evaluate adequacy of safeguards and prioritize recommendations
- ✓ Design a simplified HAZOP study plan for a solvent extraction unit including node selection, team composition, and documentation requirements
📖 Why This Matters
In separation units—like distillation columns, extractors, or centrifuges—even small deviations (e.g., 'no reflux', 'high pressure', 'reverse flow') can trigger runaway reactions, toxic releases, or equipment rupture. HAZOP is not optional: it’s legally required for Process Safety Management (PSM) compliance in mining and mineral processing facilities handling flammables, toxics, or high-pressure systems. Skipping HAZOP costs lives—like the 2013 Texas fertilizer plant explosion, where undetected deviation in ammonium nitrate storage led to catastrophic failure.
📘 Core Principles
HAZOP relies on two foundational elements: (1) Guidewords—simple, generic modifiers (e.g., NO, MORE, LESS, PART OF, AS WELL AS, REVERSE, OTHER THAN) that stimulate creative deviation thinking; and (2) Process parameters—measurable variables tied to unit operation intent (e.g., FLOW, PRESSURE, LEVEL, TEMPERATURE, PHASE, COMPOSITION). Each node (a discrete section of piping or equipment with consistent function) is examined by applying each guideword to each parameter. For example, applying 'NO' to 'FLOW' in a condenser cooling water line yields the deviation 'NO COOLING WATER FLOW', prompting analysis of causes (pump failure), consequences (overpressure, tube rupture), safeguards (flow alarm, relief valve), and actions (install redundant pump). The method is iterative, team-driven, and documented in standardized worksheets traceable to P&IDs.
📐 HAZOP Deviation Severity–Likelihood Risk Ranking
While HAZOP itself is qualitative, risk ranking uses a semi-quantitative matrix to prioritize deviations. A 5×5 matrix multiplies Likelihood (1–5) and Consequence Severity (1–5) to yield a Risk Priority Number (RPN = 1–25). RPN ≥ 12 typically triggers mandatory action; RPN ≥ 16 requires immediate mitigation before startup.
Risk Priority Number (RPN)
RPN = Severity × LikelihoodSemi-quantitative risk score used to prioritize HAZOP recommendations based on consequence impact and probability.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| Severity | Consequence Severity Rating | dimensionless (1–5 scale) | Rated per CCPS severity criteria: 1 = negligible, 5 = multiple fatalities / major environmental release |
| Likelihood | Frequency/Probability Rating | dimensionless (1–5 scale) | Rated per CCPS likelihood bands: 1 = extremely unlikely (<10⁻⁶/yr), 5 = frequent (>1/yr) |
Typical Ranges:
Acceptable for startup: 1–9
Requires action: 10–16
Stop work required: ≥17
💡 Worked Example
Problem: A HAZOP team identifies 'NO FLOW' in the raffinate stream of a mixer-settler solvent extraction unit. Consequence severity is rated 4 (major injury, $2M+ loss); likelihood is rated 3 (credible—historical pump seal failures every 18 months). Calculate RPN and determine required action.
1.
Step 1: Assign Severity = 4 (based on consequence matrix per CCPS Guidelines)
2.
Step 2: Assign Likelihood = 3 (per frequency data: 0.05–0.1 events/year → Level 3)
3.
Step 3: Compute RPN = 4 × 3 = 12
4.
Step 4: Consult risk matrix: RPN = 12 falls in 'High Risk' zone — requires engineered safeguard (e.g., flow switch interlock + alarm) before commissioning.
Answer:
The result is RPN = 12, which falls within the High Risk range (10–16) requiring formal mitigation within 30 days per CCPS Risk-Based Process Safety.
🏗️ Real-World Application
At the BHP Olympic Dam copper–uranium solvent extraction plant (South Australia), a HAZOP on the loaded electrolyte stripping column identified the deviation 'MORE TEMPERATURE' due to failed reboiler steam control valve. Causes included lack of position feedback and no high-temperature trip. Consequences were thermal decomposition of organic extractant (kerosene-based), generating hydrogen cyanide gas. The recommendation—installing redundant temperature sensors with auto-trip at 95°C—was implemented, preventing potential acute toxicity exposure during future upsets. This case appears in AusIMM’s 2021 Process Safety Casebook.
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