Process Hazard

Evaluation

Techniques Overview

Introduction

� What is our traditional safety approach?

� When to identify hazard? Design stage or

operation stage?

� Good hazard evaluations do not just happen

then what special is required?

� Ensure safety during lifecycle of the plant

� Do I need to evaluate hazard for same capacity,

same licensor plant even at same site?

� Scale and complexity: Is it important?

Evaluation need?

� Process hazard evaluation is organised effort.

� Qualitative plus quantitative analysis.

� Investigating effects of raw material impurities

� Using an alternative feedstock

� Devising improved catalysts

� Effect of change in op. data

� Change in op. sequence

� Change in control scheme

All these parameters call for a systematic analysis

Evaluation Techniques

� What – if analysis

� Checklist method

� Hazard and operability study (Hazop)

� Failure modes and effects analysis (FMEA)

� Fault tree analysis (FTA)

� Each method has advantage and disadvantage.

� Hazop is most popular method.

HAZOP study

� Most commonly used and recommended for complex plants

� Used to explore activities such as changing feedstocks, operating conditions and procedures

� Hazop analysis investigates all possible deviations from the way a design is expected to work and the hazards associated with with these deviations.

Hazop Study

� Study beyond designer’s normal plant operating intentions and identify hazard due to offset condition.

� Detail review of process design, operating conditions, safety and operability of plant

� Hazop study covers entire process safety examination based on certain guidewords

Hazop study

� Hazop is based on keyword methodology as presented by AICHE and it reviews each p & I

Diagram

� The process is divided into functional segments termed as nodes

� Process equipment (Reactor, column, vessel)

with associated piping, control loop forms a node.

� Nodes are chosen in logical order by process function

Hazop

� How to select and identify a node? eg.

� Function of the node under study is stated and normal design bases with respect to materials, action, origin, destination, design conditions, etc. are clarified by designer

� For each study node, possible deviations from intended design condition are identified by applying standard guidewords

Guideword table

� Guideword table

� Parameters

� Cause and consequences

� Safeguards

� Risk assessment

� Recommendation

� Sample Hazop worksheet

Hazop team

� Who should attend Hazop?

� Who should chair Hazop? Why chairman?

� Plan nodes in advance

� Hazop of vendor package items

� Different stages of Hazop

� Hazop documentation

� Action taken report

Sample problem

� Assignment for one sample problem to fill Hazop worksheet

What if checklist analysis

method

� Most applications and for repeat plants

� Based on raising ‘what if’ questions around

selected equipment, valve, instrument, etc.

� Questions focus on identifying hazards from

raw material receipt to product dispatch

� Answers make for design basis of the facility

� “What if” part is unstructured but checklist part

is structured

What if / Checklist method

� What if part raises concerns about what can go wrong if process goes forward as then conceived

� Checklist part of the review fills gap by following one or more checklists to ensure listed topics are covered

� Combination of two methods accommodates creative thoughts and structure

What if / Checklist

� Method not only limited to P & IDs but includes piping layout, equipment layouts

� Method goes beyond P & IDs to address effect of operations and actions of personnel (Ergonomics)

� Access for valves, maintenance, SOP to be reviewed and updated based on answers

What if / Checklist

� Sample are� What is the use of a local instrument

provided to avoid an explosion unless it is mounted in a area accessible for operator to take readings?

� How does operator gain safe access to the top of the tank truck for sampling?

� What if the operator charges excess catalyst into the reactor?

� Analyze all possible human errors

What if /checklist

� Success of this technique lies and depends on quality of questions

� Powerful method but generally used for small plants and for repeat plants

� Show Typical process hazard checklist

Other methods- FMEA

� Methodology is equipment based rather than process oriented, specialized technique

� Fundamental analysis of specific concern

� If a local pressure indicator reading is not correct and leading operator to make mistake, FMEA would not cover because a localized failure does not directly cause a system failure

FMEA method

� Best suited to “Go-No Go” processes where operations are either on or off, equipment either works or fails, steps are done or not done

� Examples are loading, unloading mechanical handling systems, etc

� FMEA generally focuses on hardware failure and can be used as an aid for improving equipment design.

Fault tree analysis (FTA)

� This is merely an event review

� Determine and analyse logical combinations of initiating events and contributing conditions that result in a specific final event

� Example, complex control loop, reactor runaway, etc.

� Needs to be supported with additional method such as Hazop