Study 3: Detailed Design Hazards

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Process Safety Guide:
GBHE-PSG-HST-040
Study 3: Detailed Design Hazards
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Process Safety Guide: Study 3: Detailed Design Hazards
CONTENTS
3.0 PURPOSE
3.0.1 Team
3.0.2 Timing
3.0.3 Preparation
3.0.4 Documentation
HAZARD STUDY 3: APPLICATION
3.1 Continuous Processes
3.2 Batch Processes
3.3 Mechanical Handling Operations
3.4 Maintenance and Operating Procedures
3.5 Programmable Electronic Systems
3.6 Failure Modes and Effects Analysis (FMEA) for Programmable Electronic
Systems
3.7 Electrical Systems
3.8 Buildings
3.9 Other Studies
3.10 Other Related Tools
3.11 Human Factors
3.12 Review of Hazard Study 3
APPENDICES
A Continuous Processes
B Batch Processes
C Mechanical Handling Operations Guide Diagram
D Maintenance / Operating Procedure
E Programmable Electronic Systems
F DCS FMEA Method
G Electrical Systems Guide Diagram
H Building Design and Operability

3.0 PURPOSE
Hazard Study 3 involves a detailed review of a firm design aimed at the
identification of hazard and operability problems. Relief and blowdown studies,
area classification, personal protection and manual handling may, if appropriate,
be included at this stage.
This study is a conventional hazard and operability study or ’HAZOP’ carried out
using guidewords. In the case of process plant, this study will be based on a firm
Piping and Instrument Line Diagram (P&ID). Outline operating procedures and
outline commissioning procedures should if at all possible be available for the
study. The consequences of deviations are identified and, where necessary,
appropriate corrective actions initiated. The study also provides an opportunity to
review potential maintenance and product quality issues.
The GBHE Hazard Study 3 is the ’HAZOP’ technique as referred to in US
Federal Legislation on Major Hazard Plants, OSHA 29CFR Part 1910.119
Process Safety Management of Highly Hazardous Chemicals and specified in the
AIChE, Centre for Chemical Process Safety ’Guidelines for Hazard Evaluation
Procedures’.
This document should be used in conjunction with a process safety publication
such as ’HAZOP: Guide to best practice’ (available from IChemE) or ‘Guidelines
for Hazard Evaluation Procedures’ (available from CCPS)
3.0.1 Team
The 'HAZOP: Guide to best practice' book gives advice on appointing a Hazard
Study Team Leader as well as assistance in selecting the team size and
membership. The team composition should be agreed between the Hazard Study
Leader and the Project Manager.
3.0.2 Timing
Process Hazard Study 3 is best carried out when a firm reviewed Piping and
Instrumentation Diagram or Engineering Line Diagram together with outline
operating, commissioning, maintenance and test procedures is available. Hazard
Study 2 for the relevant section with its actions/recommendations should be
complete as far as is practicable.

If required for Programmable Electronic Systems (PES), Hazard Study 3 is best
carried out when the PES system design is at an advanced stage but not
necessarily complete (see Hazard Study 2, section 2.2).
Most major design decisions should have been taken.

3.0.3 Preparation
The ’HAZOP: Guide to best practice’ book, chapter 5.3, gives details of the
preparation to be carried out and the information required for a study to begin.
Additional information is given below.
For the study of batch and continuous processes the following should be
available (if applicable):
(a) A firm reviewed Piping and Instrument Diagram (P&ID) or Engineering
Line Diagram (ELD).
(b) Outline operating, commissioning, maintenance and test procedures in so
far as these are not obvious from the design.
(c) Actions/recommendations from Hazard Study 2 should be completed as
far as is practicable.).
(d) Classification of the ’type’ and ’grading’ of alarm and trip systems as
described in GBHE_PSG_EP_3. It would be advantageous to have
preliminary trip logic diagrams available prior to Hazard study 3. Additional
critical safety instrumented systems may be identified in the study.
(e) Area electrical classification drawings where zoned areas have been
identified.
(f) Relief systems philosophy.
(g) A list of vessels and pipework to be registered as requiring Periodic
Inspection (see GBHE_PSG_EP_4) may be provided, but this list is not
essential. In any case this list will be updated as a result of the Hazard
Study 3.
(h) A list of Critical Machine Systems (see GBHE_PSG_EP_5) may be
provided, but this list is not essential. In any case this list will be updated
as a result of the Hazard Study 3.
(j) DCS input/output allocation philosophy if applicable.

In addition, for Batch processes a full sequence description is required.
It is important to check that the documentation, e.g. line diagrams, to be studied
is acceptable to the Site.
For Hazard Study 3 of Programmable Electronic Systems the following should be
available:
(1) Specifications.
(2) Configuration diagrams.
(3) Block diagram representation.
(4) Input/output card signal allocation.
System manuals will be useful. The responsibility for input/output card allocation
checks and line for line software (sequence) checks lies with the design engineer
and not with the study 3 team. The Hazard Study 3 should verify that these
responsibilities have been accepted and performed.
Outline operating and maintenance procedures/instructions are important
requirements for the study. These may be refined during the study. The retrieval
of hazard study notes and drawings, relevant at the time of study, should be
facilitated by appropriate referencing and filing because future modifications may
well necessitate recovery of the original detailed design philosophy.
There needs to be a plan for review meetings (for actions and recommendations
raised).
3.0.4 Documentation
Documentation, in the form of the record of the Hazard Study meetings, and
supporting documents together with evidence of the completion of all actions
should be filed in the Project SSHE Dossier (worksheet j). It is important that
drawings and records of the equipment studied, marked up P&IDs used in the
study, are also retained in the Project SHE Dossier together with the Hazard
Study records.
Meeting records for circulation should include copies of the P&ID(s).

The Project Manager should ensure that project documentation, area
classification, relief philosophy, vessel or piping and critical machine registration
are updated after the study.

HAZARD STUDY 3: APPLICATION
The following sections 3.1 to 3.12 give further detailed information on the use of
the various Hazard Study 3 checklists and worksheets.

3.1 CONTINUOUS PROCESSES
It is generally obvious when the Continuous Process Guide
Diagram (appendix A) should be used (i.e. for a steadily
operating continuous process that is on line for a significant
period of time). In some cases the choice is not so clear,
e.g. the feeding of chlorine to a batch reactor over an
extended period from a 1 ton drum or the offloading of a
tanker (tank truck). In such cases the Hazard Study Leader
shall decide if a more productive study will result from the
continuous or the batch checklists. Typical of the things to consider include which
is more important – the rate at which the substance is added or the total quantity
of the substance added.
Studies of continuous chemical processes are carried out in a series of meetings
where P&IDs are examined, line by line, vessel by vessel, using a list of
guidewords to stimulate the Hazard Study team's consideration of all conceivable
deviations from design intent.
The detailed examination of cause and effect of deviations in both normal and
abnormal plant operation is designed to minimize problems at commissioning
and start-up, and to ensure continued safe and reliable operation of the plant.
This systematic study of design detail should identify areas of concern which can,
if necessary, be resolved outside the hazard study meeting.
The guidewords listed in appendix A are considered systematically by the team
of mixed disciplines, led by the trained Hazard Study Leader. The process lines
and vessels examined are marked on the P&ID and listed on the record
worksheet (worksheet n). The Hazard Study Leader is responsible for ensuring
the Hazard Study records are of a satisfactory quality. Should the cause and
effect of a deviation (e.g. low flow) cause no hazard, environmental, health,
operability or quality problems then no comment may be needed in the summary
on the worksheet. It will be assumed that 'deviations' excluded from the standard
list of guidewords have been considered but dismissed and their exclusion from
the worksheet summary is not an oversight, but in the interest of brevity and
team efficiency. In the USA however, full recording is recommended.
Continuous processes also entail discontinuous operations (e.g. start-up,
controlled shutdown, emergency shutdown). These should be treated in a similar
fashion as batch processes.

Should potential problems be identified, then a record of the preventative or
corrective measures designed to minimize the likelihood and consequences
should be specified. Any further action should be noted and progressed outside
the meeting. Where extensive discussions are held, these should also be
recorded even if they have not lead to identification of a hazard.
Should the Hazard Study 3 call into question the fundamental rationale of the
hazard control measures agreed at Hazard Study 2, then it is the responsibility of
the Project Manager (or nominee) to ensure that the Hazard Study 2 report is
updated. In some countries this may be a legal requirement (e.g. SEVESO
directive).
3.2 BATCH PROCESSES
It is generally clear when the batch process method and
guide diagram (appendix B) should be used, but grey areas
shall be handled in accordance with the Hazard Study
Leader’s judgment.
The general characteristics of batch plants as compared with
continuous plants are as follows:
(a) The status of the various parts of the plants are changing cyclically with
respect to time, and therefore a line diagram alone gives a very
incomplete picture.
(b) The processes are usually multistage and the individual units
multipurpose. For example, in a chemical reactor the process steps could
involve:
(1) Charge solvent;
(2) Charge reactants;
(3) Heat to reaction temperature;
(4) Control at reaction temperature for the required period;
(5) Add final components at controlled rate;
(6) Cool down products to discharge temperature;

(7) Discharge.
Batch reactions are prone to special problems if the process is exothermic or can
produce gas. Excessive temperature and/or pressure may be developed without
careful control of the progress of reaction.
Two methods of control are commonly used:
(i) In ’all-in’ reactions, all reactants are charged and reaction is
completed by subjecting the mixture to an appropriate
temperature/pressure program. Control may be lost if cooling fails
during an exothermic phase or if the effects of scale-up are not
taken into account.
(ii) In ’progressive addition’ reactions, a key reactant is charged under
conditions that will ensure rapid consumption. Accumulation due to
inappropriate temperature, poor mixing or other reasons is a
common cause of hazard.
The latter technique is preferred, where practicable, for potentially
hazardous batch reactions.
(c) Batch plants are often multi-product, and reaction units usually have to be
cleaned out and modified when changing from one product to another.
(d) From the comments above it will be clear that there can be several ’norms’
for batch plants. At the very least there will be two:
(1) an ’active’ state when the item is in use; and
(2) an ’inactive’ state when the item is not in use.
This is in contrast to a continuous plant where, when in steady state
operation, a fixed ’norm’ in terms of flow, pressure, temperature etc. can
be defined for each and every part of the plant.
(e) Operators may take part in some of the process activities such as
charging material from drums or removing product from filters. Even well
trained and well-motivated operators will make occasional mistakes.
During the study the question should be asked "How often will an operator make
a mistake?" and not "If an operator makes a mistake ...". If the consequences are
serious the possibility of error should be designed out.

For the purpose of the hazard study it will be necessary to know the sequence of
process operations in addition to having a P&ID, which describe the plant. The
sequence can be in a variety of formats, usually a process summary (such as a
batch master print-out), but could be a logic diagram, dot chart or sequence
flow chart. With complicated or proprietary items of equipment a considerable
amount of preparatory work may be necessary before the study.
The detailed sequence of the examination is shown in appendix B.1.
The approach usually adopted in a hazard and operability study of a batch
process is to apply the guidewords initially (see appendices B.2 & B.3) to each
step of the process. Applied to a vessel such as a reactor this would lead to the
examination of various lines which could then be marked off on the line
diagram as having been examined.
Other lines not identified with a normal process step (e.g., relief lines, vents,
etc.), would then be examined before moving on to the next major item of
equipment.
Significant points to bear in mind in the study of a batch process are:
(1) Multipurpose lines will have more than one ’normal’ state and each should
be examined.
(2) Services, e.g. heating/cooling systems, can be examined in detail at the
heating/cooling step or can be ’mopped up’ before the process sequence
moves on to another vessel.
(3) The omission of one or more steps in the process is not uncommon and
the consequences of such possible mal-operations need to be examined.
(4) In batch processes Quantity is a critical parameter. More of quantity
should always address the possibility of a double charge, which is a
common error.
(5) For many process steps only the first guideword (NO/NOT) will be
relevant. If the operation is to ’check vessel empty’, then it either is or isn’t
(though various causes may be identified for the not empty condition).
(6) In multiproduct plants, the first and last batches of a campaign are
different and need to be treated as such.

Problems can include:
(i) The need to check that vessels are empty.
(ii) Water contamination of the first batch of a campaign.
(iii) The handling of recycled materials and heels.
(iv) The defeat of instrumented trips to start the first batch of a campaign.
The checklist given in appendix B.2 handles quantities of substances well and is
analogous to the flow guidewords (high flow, low flow, no flow, reverse flow, etc.).
However, there are other considerations such as changes in physical conditions,
start-up and shutdown conditions, registered equipment (critical and controlled),
mechanical integrity, effluents and emergencies. Of particular concern to a batch
process are the effects of agitation failure and temperature deviations.
These could be handled by applying the batch checklist to them. However, use of
the relevant sections of the continuous checklist in appendix A is probably more
convenient.
Lastly, it is worth stressing that a deviation in one part of a batch process
sequence, or at a particular time, may not necessarily result in a hazard at that
time or place, but may manifest itself elsewhere or later.
With batch processes it may be more appropriate to examine the various stages
in the process, from approval of the recipe through the batch cycle to discharge
and decontamination.
3.3 MECHANICAL HANDLING OPERATIONS
In Hazard Study 2, significant hazards of Mechanical
Handling relate to moving objects/equipment, where
inadequate control can lead to serious injury by virtue of
stored potential or kinetic energy (e.g. mixing equipment,
palletizers, conveyors, rail shunting, etc.) should have been
identified.
For the more detailed Hazard Study 3, the guide diagram in appendix C should
be used.

Packaged units are a common feature in mechanical handling equipment and
’loss of control’ is a foreseeable failure mechanism. The majority of packaged
units are delivered with control units fitted as standard off-the-shelf items - not
always compatible with business purchasing specifications and increasingly
involving programmable microprocessors.
In most cases, it is advantageous to study the interface between such units and
the chemical plant because of the hostile environment, site specific service
constraints and significance of unit failure for the chemical process (e.g. chemical
may solidify if the unit stops). In particular, hazards can arise when electrical
isolation of drives is not independent from associated control/interlock circuits.
One of the main problems in using packaged units is the lack of readily available
information on control circuits and operating function. However, it is suggested
that appendix C will promote useful dialogue with the supplier before the units
arrive on site unsuitable for demanding duties in often novel chemical processes
where routine maintenance and testing have not been addressed.
Guidance on packing lines is included in GBHE-PGP-005 (Specialist
Techniques).
3.4 MAINTENANCE AND OPERATING PROCEDURES
A selective detailed examination of maintenance, operating, start-up
and shutdown procedures may be valuable for moderate to high
hazard operations, to which any of the following apply:
(a) They are largely manual.
(b) Many of the preventative measures are procedural.
(c) Human error has been identified as the cause of a significant number of
undesirable deviations at Hazard Study 2 or 3,
The guide diagram in appendix D.1 may be used for the examination.
Consideration should be given to the initial state of the plant. Should the state be
different from that expected, the action required and/or consequences expected
should be considered and recorded. Where potential events are caused by
operator error then the cause should be stated as operator error.
In selected cases, the hazard study team may wish to use a more detailed
process given in appendix D.2 to examine operating/maintenance procedures
(e.g. reactor start-up). Consideration should be given to the initial state of the
plant.

Should the state be different from that expected, the action required and/or
consequences expected should be considered and recorded. Where potential
events are caused by operator error then the cause should be stated as operator
error.
3.5 PROGRAMMABLE ELECTRONIC SYSTEMS
Programmable electronic systems should be studied when:
(a) It is a regulatory requirement.
(b) It is part of a safety instrumented system (see
GBHE_PSG_EP_3, where a failure of a critical
system could lead to an unacceptable situation).
Other programmable electronic systems need not be studied.
A programmable electronic system is different from the process units or other
activities in that the PES, itself, has only limited ability to be a direct hazard, e.g.
damage to an operator’s health due to poor ergonomics, electrocution etc. The
main concern is the ability to initiate hazards in the plant or its inability to correct
and avert dangerous process events. The effect of partial or complete PES
failure should be considered when studying the process (i.e. P&IDs). The PES
hazard study is thus directed to understanding, reducing or eliminating PES
failures.
There are some basic problems in applying a conventional Hazard Study 3 line
by line deviation analysis to PES. The complexities of systems involving software
means that rigorous studies are not possible. Any analytical study of a system
involving software will take an impossibly long time. It may be practical and
important to study some of the critical logic flows using the deviation guideword
system (see appendix E.1).
Studies of PESs are best approached with block diagram representations (see
appendix E.2) of the equipment within defined cut points. The interfaces between
each item of equipment can be systematically examined. Identify the hardware
and software features of the system and conduct a Failure Modes and Effects
Analysis (FMEA). This type of analysis is better suited to the nature of a PES and
to the knowledge that the design team will have of the PES.

3.6 FAILURE MODES AND EFFECTS ANALYSIS (FMEA)
FOR PROGRAMMABLE ELECTRONIC SYSTEMS
Failure Mode and Effect Analysis (FMEA) is most effective for
mechanical and electrical systems such as packing lines,
electrical distribution systems and programmable electronic
systems (PES) (see appendix F.1).
An FMEA considers each part of the PES and asks how can it
fail (mode) and what will the effect be (effect). It then goes on
to ask ’how is the operator made aware?’ and ’What diagnostic or corrective
measures are present?’
The failure mode describes how the equipment fails (open, closed, on, off, leaks,
etc.). The effect of the failure mode is determined by the system’s response to
the equipment failure. An FMEA identifies single failure modes that either directly
result in or contribute to a hazardous situation. Human errors are not usually
examined directly in an FMEA; however the effects of a mis-operation as a result
of human error are usually indicated by an equipment failure mode.
An FMEA is not efficient for identifying an exhaustive list of combinations of
equipment failures that lead to accidents.
The best approach is to start by looking at each type of input signal starting with
the measuring device. In addition to failures, intermittent failures, partial failures
and recovery from failure should be considered. Then consider each type of
output signal and then the identifiable functions of the PES.
Some advice:
(a) Duplicated hardware is a common approach by the suppliers to a
requirement for increased reliability. Their calculations of reliability rates
for duplicate systems are of little value as they usually ignore dependency
(i.e. common cause and common mode errors). The operation and
resetting of duplicate systems should be studied.
(b) Multi-channel cards have a number of different failure modes that should
all be identified. The allocation of signals to cards should be a joint
Control/Electrical and Process activity.

(c) The DCS system software is beyond the scope of this study. It is
recommended that a statement about the supplier’s quality design system
and their internal or external auditing is obtained.
(d) The features of one supplier’s DCS or PLC may be similar so information
should be sought from previous studies. This can mean that there is no
need to invite a representative from the manufacturer to the study.
(e) There will be many power supplies, most of which will be duplicated for
reliability. All power and utilities failures should be alarmed in a way that
the operator will be aware.
(f) All identified system failures will probably be alarmed to the operator by a
message. These messages are likely to be infrequent and not immediately
understandable. How will the operator respond to these error messages,
who will he contact, and what level of training will they have been given?
(g) Connection to a network or any form of remote communications
introduces security problems due to inadvertent or deliberate contact. The
protection against unwanted access will probably be software based, the
protection means needs to be identified and a periodic test needs to be
defined to demonstrate that the protection is still effective.
(h) PES systems can easily produce an overload of alarms. In the event of a
plant upset, the operator may be faced with hundreds of alarms and miss
a critical alarm in a sea of trivia. Alarms should be prioritized.
Recording may be on a conventional Hazard Study 3 worksheet (worksheet n),
or on the specific FMEA Action List (worksheet p).
Advanced Control Systems can be studied using these techniques but such a
study will not address the control actions that an ACS is capable of taking.
The failure mode guidewords in appendix F.2 have been used on some projects
to prompt detailed consideration of the failure modes of modern PLC type control
systems and, whilst capable of further refinement, the approach does encourage
a structured examination of each key unit in the control loop (e.g. DP cell, P/I,
controller/computer, I/P, control valve). Many new instruments contain PLCs (DP
cells, density meters, controllers etc.) and their failure modes can be very
different from conventional instruments (e.g. loss of input can default, such that
automatic control reverts to manual without any audible alarm).

Such novel failure mechanisms can only be revealed by lateral consideration of
cause/effect deviations in input/output circuitry and software programs. In
particular, the wider implications of dependency or common mode failure should
be addressed.
The statement on the initial state of the plant implies an inspection, against a
checklist, by the operator. It seems prudent to enquire what may happen if the
operator finds any part of the plant in other than the required state and takes
steps to correct the state, for example, he opens a closed valve which should
have been open before starting the procedure detailed.
3.7 ELECTRICAL SYSTEMS
Consideration should be given to the hazard studying of Electrical
Distribution Systems whose failure will give rise to a hazardous
situation as identified in the emergencies section of a process or
other Hazard Study 3 checklist, as or when higher than normal
reliability is required for other safety or economic reasons.
On electrical systems, a one-line diagram or a block diagram
representation should be examined systematically to identify novel failure
mechanisms only revealed by creative thinking about cause/effect deviations in
input/output circuitry and software programming.
An electrical systems guide diagram is given in appendix G.
3.8 BUILDINGS
In Hazard Study 2, consideration will have been given to the
physical layout of buildings and to the containment of noxious and
harmful substances and the ’top events’ such as fire, explosion,
pollution, etc. At this later stage in the project it is often useful to use
the Hazard Study 3 techniques (see appendix H) to ensure that
there is also a clear understanding of non-SHE items (e.g. the
detailed operating and maintenance aspects) which will be
fundamental to the satisfactory performance of the building.
This is of particular importance when considering novel techniques and/or
systems to be incorporated in the project.

Factors worthy of further consideration should be highlighted, judged on the
degree of novelty or uncertainty attached to them, and/or on the impact their non-
conformance will have on the final operation of the project.
These factors should be listed and the Hazard Study Leader should then select
the most appropriate form of study normally selected from the previous methods
to examine, for example drains, ventilation systems, etc.
HAZCON and HAZDEM have been developed for construction and demolition
activities.
3.9 OTHER STUDIES
The engineering function should produce or update electrical
area classification drawings (see Hazard Study 1, appendix C,
Additional Assessments) where zoned areas have been
identified during the Hazard Study 2 and 3 stages of a project.
It is the Project Manager’s responsibility to ensure that the
project conforms with local fire codes and to GBHE
Responsible Care Management System (RCMS).
3.10 OTHER RELATED TOOLS
The use of Fault Trees is a specialized technique for hazard
evaluation rather than hazard identification. It is a whole new topic
requiring special skills, so further information on it is given in the
documentation on Quantified Risk Assessment.
3.11 HUMAN FACTORS
A Human Factors Study is a requirement under the US Process
Safety Management legislation 29 CFR 1910.119 for the
Manufacture of Highly Hazardous Chemicals and is recommended
for use elsewhere. It focuses on the factors that are likely to cause
or avoid human error in process operations. (worksheet q).

3.12 REVIEW OF HAZARD STUDY 3
• Check actions have been assessed, reviewed and closed out.
 Have the actions/recommendations from the Hazard
Study 3 been implemented in the way expected
without introducing new hazards?
 Have the solutions been adequately thought out –
does the solution deal with the concern, does it
introduce new concerns?
 Do the solutions represent significant changes which themselves
require hazard studying.
• Has the Project Manager notified the Hazard Study Leader of significant
changes to the design or operation made after the Hazard Study 3? Have
they been subjected to a Management of Change Procedure approved by
the Project Manager and the Commissioning Manager and referenced in
the Project SHE Dossier? If so, is it necessary to hold a further Hazard
Study meeting to review the changes.
• Have all modifications made to the P&IDs (or ELDs) been formally
recorded? They shall be formally reviewed at the Hazard Study 4 Stage.
• Draft QRAs/Fault Trees prepared before Hazard Study 3 should be
updated to incorporate any additional demands, changes to the P&IDs or
anything else which might affect the logic.
• Any other QRAs/Fault Trees that have not yet been started as a result of a
decision made earlier or due to insufficient information (e.g. trip
quantifications) should be completed.

.

Study 3: Detailed Design Hazards

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Study 3: Detailed Design Hazards