2.2 Hazard Identification (HAZID) & HAZOP Studies

Hazard identification is the critical foundation of process safety management. Within the process industries, qualitative risk assessment techniques are deployed across different lifecycle stages to systematically uncover process hazards, design deficiencies, and operational deviations. The two most prominent qualitative methodologies mandated by international standards (such as IEC 61882) are Hazard Identification (HAZID) and Hazard and Operability (HAZOP) studies.

1. Comparing HAZID and HAZOP

While both HAZID and HAZOP share the core objective of identifying safety and operational risks, they differ fundamentally in scope, timing, technical granularity, and methodology.

Hazard Identification (HAZID)

HAZID is an early-stage, high-level screening tool used to identify major hazards associated with plant location, layout, process chemistry, external environment, and logistics. It evaluates:

• External Hazards: Seismic activity, flooding, lightning, extreme weather, aircraft crash impact, nearby industrial hazards (domino risks).
• Facility Layout Hazards: Spacing between process units, control room positioning, toxic/flammable gas dispersion paths, emergency access routes.
• Process & Chemical Hazards: Large inventory toxic gas storage, high-pressure reactivity, runaway reaction potential.

---

2. HAZOP Methodology & Node Selection

A Hazard and Operability (HAZOP) study is a structured, highly systematic technique designed to identify process hazards and operational problems by evaluating deviations from the design intent.

The Concept of Design Intent

Every process line, vessel, or instrument loop is engineered to operate within defined parameters (e.g., transfer 50 m³/hr of hexane from Tank T-101 to Reactor R-201 at 25°C and 2.0 barg). This specified baseline is the design intent.

Node Selection Protocol

To make a HAZOP manageable, the P&IDs are divided into discrete sections called nodes.

• Definition of a Node: A section of piping, vessel, or process equipment with a specific, uniform design intent.
• Node Boundaries: Typical nodes include a pump suction line, a heat exchanger tube-side circuit, a distillation column overhead system, or a chemical reactor vessel.
• Selecting Node Sizes: Nodes must be neither too large (risking missed detail) nor too small (leading to excessive repetition and team fatigue).

[ Tank T-101 ] ===> ( Node 1: Pump Suction Line ) ===> [ Pump P-102 ] ===> ( Node 2: Pump Discharge Line ) ===> [ Reactor R-201 ]

---

3. Guide Words and Process Parameters

The core mechanism of a HAZOP study involves applying standard Guide Words to specific Process Parameters to generate hypothetical process deviations:

$ext{Deviation} = ext{Guide Word} + ext{Process Parameter}$

Standard HAZOP Parameters

• Flow: Fluid transfer rate through piping or equipment.
• Pressure: Internal system force exerted on walls or vessels.
• Temperature: Thermal state of process fluid or equipment.
• Level: Height of liquid phase inside vessels or columns.
• Viscosity / Composition: Chemical makeup, phase behavior, concentration.
• Addition / Phase: Mixing sequence, batch charging, vapor/liquid fraction.
• Service / Utility: Supply of cooling water, steam, instrument air, electricity, nitrogen purge.

Standard HAZOP Guide Words (IEC 61882)

Deviation Matrix Examples

+------------------+--------------------+-----------------------------------------------------+
| Guide Word       | Parameter          | Process Deviation & Physical Cause                  |
+------------------+--------------------+-----------------------------------------------------+
| NO               | FLOW               | No flow due to closed XV-101 or pump trip           |
| MORE             | PRESSURE           | More pressure due to tube rupture in heat exchanger |
| REVERSE          | FLOW               | Reverse flow due to check valve CV-202 failure      |
| AS WELL AS       | COMPOSITION        | Air ingress creating explosive atmosphere in tank   |
+------------------+--------------------+-----------------------------------------------------+

---

4. Conducting the HAZOP Meeting & Review Workflow

A HAZOP study follows a rigid, repeating analytical sequence for every node under evaluation:

[ Select Node & Define Intent ]
               │
               ▼
   [ Select Process Parameter ]
               │
               ▼
     [ Apply Guide Word ] ──► (Generates Deviation)
               │
               ▼
    [ Identify Causes ] ──► (Is cause credible?)
               │ (Yes)
               ▼
  [ Evaluate Consequences ] ──► (Assuming NO safeguards work)
               │
               ▼
  [ List Existing Safeguards ] ──► (Hardware, Alarms, Interlocks)
               │
               ▼
[ Determine Risk & Recommendations ] ──► (Assign Action Owner)

Detailed Steps in the Node Assessment:

1. Define Design Intent: The Chair explains the node boundaries, fluid properties, operating pressure, temperature, and flow direction.
2. Apply Deviation: Combine a parameter and guide word (e.g., More Pressure).
3. Identify Causes: Brainstorm all credible engineering and operational causes (e.g., control valve failed full open, downstream manual block valve closed, cooling water pump failed).
4. Identify Consequences: Trace the worst-case unmitigated sequence of events resulting from the cause (e.g., vessel overpressurization $ ightarrow$ mechanical loss of containment $ ightarrow$ toxic cloud release $ ightarrow$ off-site fatalities). Crucial rule: Consequences must be evaluated assuming all existing safeguards fail.
5. Identify Safeguards: Record engineered and procedural safeguards currently shown on the P&ID or specified in procedures (e.g., Pressure Safety Relief Valve PSV-104, high-pressure trip PSHH-102 closing SDV-101, independent pressure alarm).
6. Formulate Recommendations: If existing safeguards are deemed inadequate to achieve tolerability, the team logs an explicit HAZOP Recommendation (e.g., "Install SIL 2 high-pressure interlock on feed line").

---

5. Multidisciplinary Team Composition & Roles

A HAZOP study cannot be conducted by a single engineer; it relies on the collective intelligence and dynamic cross-examination of a multidisciplinary team.

                           ┌─────────────────────────┐
                           │   Independent Chair     │
                           └────────────┬────────────┘
                                        │
        ┌───────────────────────────────┼───────────────────────────────┐
        │                               │                               │
┌───────┴─────────┐           ┌─────────┴─────────┐           ┌─────────┴─────────┐
│ Process Engineer│           │ Operations Lead   │           │ Instrument Eng.   │
└─────────────────┘           └───────────────────┘           └───────────────────┘
        │                               │                               │
┌───────┴─────────┐           ┌─────────┴─────────┐           ┌─────────┴─────────┐
│ Maintenance/Mech│           │ Scribe / Secretary│           │ HSE Specialist    │
└─────────────────┘           └───────────────────┘           └───────────────────┘

Roles and Responsibilities

• HAZOP Leader / Chair: An independent, trained expert in HAZOP methodology. Controls meeting flow, ensures systematic application of guide words, prevents rabbit-hole discussions, and maintains objectivity. Must not be the direct designer of the system.
• HAZOP Scribe / Secretary: Dedicated professional recording deviations, causes, consequences, safeguards, and actions in specialized HAZOP software (e.g., PHA-Pro, HazopJet).
• Process Design Engineer: Explains the process design intent, heat/material balances, equipment sizing, and operating envelopes.
• Operations Representative: Provides real-world practical insight on how operators run the plant, manual valve operations, startup/shutdown sequences, and alarm response capabilities.
• Instrument / Control Engineer: Explains control loops, Programmable Logic Controller (PLC) logic, trip settings, interlocks, cause-and-effect matrices, and valve fail positions.
• Maintenance / Mechanical Engineer: Assesses mechanical integrity, corrosion allowances, pump curves, relief valve sizing, and piping specifications.
• HSE Specialist: Provides regulatory context (COMAH, DSEAR), toxicological data, dispersion modelling insights, and corporate risk matrix standards.

---

6. P&ID Review & Action Tracking

The primary working document for a HAZOP is the Piping & Instrumentation Diagram (P&ID). During the study:

• Every line and vessel examined is physically highlighted (color-coded) on master P&ID prints as the node evaluation completes.
• Action Item Quality: Recommendations must follow the SMART framework (Specific, Measurable, Achievable, Relevant, Time-bound).
• Closing the Loop: Every logged recommendation must be assigned to a named action owner. Recommendations cannot be closed out without documented technical justification and formal sign-off by the engineering authority.