Prof. Enrico Zio

Hazard Operability Analysis

Prof. Enrico Zio

Politecnico di Milano

Dipartimento di Energia

Prof. Enrico Zio

HAZOP

• Qualitative

• Deductive (search for causes)

• Inductive (consequence analysis)

AIM:

Identification of possible process anomalies and

their associated causes and consequences

Prof. Enrico Zio

HAZOP

Initially developed to analyze chemical process

systems; later extended to complex operations and

other types of systems (e.g., software)

It is a qualitative, structured and systematic

examination of a planned or existing process or

operation in order to identify and evaluate problems

that may represent risks to personnel or equipment, or

prevent efficient operation

Deductive (search for causes)

Inductive (consequence analysis)

Prof. Enrico Zio

HAZOP: When?

Design

Phase

Completeness

of HAZOP

study

Impact of

results on

Design

HAZOP as a final check

upon detailed design

HAZOP study: better later than never. It may also be

performed on an existing facility (improvement of the

operability, risk reduction)

Prof. Enrico Zio

HAZOP: Who?

HAZOP team members (multidisciplinary):

Team Leader (HAZOP experienced)

Project engineers

Process engineers

Instrument/electrical engineers

Safety engineers

Maintenance Engineers

….

Prof. Enrico Zio

HAZOP: How?

1. Clearly identify the study nodes, i.e., the locations

(e.g., on piping) at which the process parameters are

investigated

2. Identify the functionally independent process units

(pumps, vessels, heat exchangers, etc.) that are

between the nodes (which cause changes in the

parameters between nodes)

3. For each node identify its operation modes (start-up,regime, shut-down, maintenance, etc.) and thecorresponding plant configurations (valves open orclosed, pumps on or off, etc.)

4. For each node in each of its operation modes,identify the possible deviations from nominalbehavior, by compiling an HAZOP table

Prof. Enrico Zio

HAZOP: Procedure steps

1. Decompose the system in functionally independent process units

2. For each process unit identify its operation modes (start-up, regime, shut-down, maintenance, etc.) and configurations (valves open or closed, pumps on or off, etc.)

3. For each process unit in each of its operation modes, identify the possible deviations from nominal behavior, by compiling an HAZOP table

Prof. Enrico Zio

HAZOP: Procedure steps

specify unit in/out fluxes (energy, mass, control signals, etc.) and process variables (temperature, flow rate, pressure, concentrations, etc.)

write down unit functions (heating, cooling, pumping, filtering, etc.)

apply keywords (low, high, no, reverse etc.) to the unit process variables and functions => process deviations

for each process deviation (qualitatively) identify its possible causes and consequences

Prof. Enrico Zio

HAZOP TABLE

Source: IEC 61882

Prof. Enrico Zio

HAZOP: A synoptic

Taken from Rausand, M. and Høyland, A.: "System Reliability Theory: Models,

Statistical methods, and Applications" (2nd ed.), Wiley, Hoboken, 2004

Prof. Enrico Zio

HAZOP: Details of procedure steps

Specify elements: in/out fluxes (e.g., energy, mass, controlsignals, etc.), process variables (e.g., temperature, flow rate,pressure, concentrations, etc.), etc.

Flow Composition pH

Pressure Addition Sequence

Temperature Separation Signal

Mixing Time Start/stop Stirring Phase

Operate Transfer Speed

Level Maintain Particle size

Services Viscosity Measure

Communication Reaction Control

Prof. Enrico Zio

HAZOP: Details of procedure steps

Specify elements: in/out fluxes (e.g., energy, mass, controlsignals, etc.), process variables (e.g., temperature, flow rate,pressure, concentrations, etc.), etc.

Apply Guide-Words to the unit process variables and in/outfluxes => process deviations

Guide-word Meaning Example

No (not, none) None of the design intent is achieved No flow when production is expected

More (more of, higher) Quantitative increase in a parameter Higher temperature than designed

Less (less of, lower) Quantitative decrease in a parameter Lower pressure than normal

As well as (more than) An additional activity occurs Other valves closed at the same time

(logic fault or human error)

Part of Only some of the design intention is

Achieved

Only part of the system is shut down

Reverse Logical opposite of the design intention

occurs

Back-flow when the system shuts

down

Other than

(other)

Complete substitution - another activity

takes place

Liquids in the gas piping

Early /late The timing is different from the intention

Prof. Enrico Zio

HAZOP: Details of procedure steps

Specify elements: in/out fluxes (e.g., energy, mass, controlsignals, etc.), process variables (e.g., temperature, flow rate,pressure, concentrations, etc.), etc.

Apply keywords (low, high, no, reverse etc.) to the unit processvariables and in/out fluxes => process deviations

For each process deviation (qualitatively) identify its possiblecauses and consequences

Prof. Enrico Zio

HAZOP TABLE

UNIT :

OPERATION MODE:

Keyword Deviation Cause Consequence Hazard Actions needed

More More Temperature

Additional Thermal Resistance

Higher pressurein tank

Release due to Overpressure

Install high temperature warning and pressurerelief valve

Prof. Enrico Zio

HAZOP TABLE

Keyword Deviation Cause Consequence Hazard Actions needed

More More Temperature

Additional Thermal Resistance

Higher pressurein tank

Release due to Overpressure

Install high temperature warning and pressurerelief valve

1. Identify the deviation (install an alarm)

2. Compensate for the deviation (automatic control system)

3. Prevent the deviation from occurring

4. Prevent further escalation of the deviation (plant shut-down)

5. Relieve the process from the hazardous deviation (pressure safety valve)

UNIT :

OPERATION MODE:

Prof. Enrico Zio

EXAMPLE: SMALL EXTERNAL POOL

The water of the pool is in re-circulation through pumps that aspire the water of the

compensation tank making it pass through the treatment organs before throw it

again in the pool.

SYSTEM TO

CORRECT WATER

PH

SYSTEM TO

DISINFECT THE WATER

SYSTEM TO

FILTER

THE WATER

Prof. Enrico Zio

HAZOP TABLEThe objective of the HAZOP analysis is to identify the possible deviations to the

normal operation that can contribute to the reduction of the quality of the water.

Prof. Enrico Zio

SYSTEM: shell & tube heat exchanger

Study Node: 1

Operational Mode: Nominal Conditions

Design Intent: P= 2bar, T=20°C, Flow=1l/sec

Process fluid

Cooling water

Hazop: example

12

3

4

Prof. Enrico Zio

Solution

Guide

WordElement Deviation Causes Consequences Action

LESS FLOW Less flow of cooling

water

Pipe blockage Temperature of

process fluid remains

constant

High Temperature

Alarm

NONE FLOW No cooling water flow Failure of inlet cooling

water valve to open

Process fluid

temperature is not

lowered accordingly

Install

Temperature

indicator before

and after the

process fluid line

Install TAH

MORE FLOW More cooling flow Failure of cooling water

valve

Temperature of

process fluid decrease

Low Temperature

Alarm

REVERSE FLOW Reverse s cooling fluid

flow

Not credible Not credible Not credible

MORE PRESSURE More pressure on tube

side

Failure of process fluid

valve

Bursting of tube Install high

pressure alarm

.... ... .... …. …. ....

Prof. Enrico Zio

HAZOP results

•Improvement of system or operations

– Reduced risk and better contingency

– More efficient operations

•Improvement of procedures

– Logical order

– Completeness

•General awareness among involved parties

Prof. Enrico Zio

HAZOP: Strength

1. Simple and systematic (computer tools available)

2. Include consequence effects also on other units: domino effects.

3. Covers human errors

4. Covers safety as well as operational aspects

5. It gives good identification of cause and excellent identification of critical deviations.

6. HAZOP is an excellent well-proven method for studying large plant in a specific manner.

Prof. Enrico Zio

HAZOP: weakness

1. Very time consuming and laborious (boredom for

analysts)

2. Tends to generate many failure events with

insignificance consequences and generate many

failure events which have the same consequences

3. Does not identify all causes of deviations (it may

omit some scenarios)

4. Does not allow to consider with multiple-combination

events

5. Gives little account to the probabilities of events or

consequences (meaningfulness of deviations are

expert judgment based)

Prof. Enrico Zio

HAZOP: comments

1. Include consequence effects also on other units: domino effects.

2. Simple and systematic (computer tools available)

3. Subjective (relies on analyst’s expertise)

4. Often used in support to the construction of FT and for RCM

Prof. Enrico Zio

Questions:

1. What is the different between FMEA and HAZOP?

2. Who should take the job of making FMEA and HAZOP?

3. How to improve the FMEA and HAZOP?