Transcript HAZARD AND OPERABILITY STUDY

HAZARD AND
OPERABILITY STUDY
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Brainstorming, Multidisciplinary Team Approach
Structured Using Guide Words
Problem Identifying
Cost Effective
When to Use: Optimal from a cost viewpoint when applied to new plants at the point where the design is nearly firm
and documented or to existing plants where a major redesign is planned. It can also be used for existing facilities.
Type of Results: The results are the team findings. Which include: (1) identification of hazards and operating
problems, (2) recommended changes in design, procedure, etc., to improve safety; and (3) recommendations for
follow-on studies where no conclusion was possible due to lack of information.
Nature of Results: Qualitative.
Data Requirements: The HazOp requires detailed plant descriptions, such as drawings, procedures, and flow charts.
A HazOp also requires considerable knowledge of the process, instrumentation, and operation, and this information
is usually provided by team members who are experts in these areas.
Staff Requirements: The HazOp team is ideally made up of 5 to 7 professionals, with support for recording and
reporting. For a small plant, a team as small as two or three could be effective.
Time and Cost: The time and cost of a HazOp are directly related to the size and complexity of the plant being
analyzed. In general, the team must spend about three hours for each major hardware item. Where the system
analyzed is similar to one investigated previously, the time is usually small. Additional time must be allowed for
planning, team coordination, and documentation. This additional time can be as much as two three times the team
effort as estimated above.
HAZOP STUDY - TEAM COMPOSITION
A Team Leader, an Expert in the HAZOP Technique
Technical Members, for Example
New Design
Existing Plant
Design or Project Engineer
Plant Superintendent
Process Engineer
(Foreman)
Process Supervisor
Commissioning Manager
Maintenance Engineer
Instrument Design Engineer
Instrument Engineer
Chemist
Technical Engineer
Principles of HAZOP
Concept
•Systems work well when operating under design conditions.
•Problems arise when deviations from design conditions occur.
Basis
•a word model, a process flow sheet (PFD) or a piping and
instrumentation diagram (P&ID)
Method
•use guide words to question every part of process to discover what deviations
from the intention of design can occur and what are their causes and
consequences may be.
PRINCIPLES OF HAZOPS
GUIDE WORDS*
NONE
MORE OF
LESS OF
PART OF
MORE THAN
OTHER
CAUSE
DEVIATION
(From standard
condition
or intention)
CONSEQUENCES
(Trivial, important,
catastrophic)
-hazard
-operating difficulties
*COVERING EVERY PARAMETER RELEVANT TO THE SYSTEM
UNDER REVIEW:
i.e. Flow Rate. Flow Quantity, Pressure, Temperature, Viscosity, Components
STUDY NODES
The locations (on P&ID or procedures) at which the process parameters are investigated
for deviations. These nodes are points where the process parameters (P, T, F etc.) have
an identified design intent.
INTENTION
The intention defines how the plant is expected to operate in the absence of deviations at
the study nodes.
DEVIATIONS
These are departures from the intension which can be discovered by systematically
applying the guide words.
•Process conditions
•activities
•substances
•time
•place
HAZOP STUDYGUIDE WORDS
Guide Words
Meaning
No, None
Negation of Intention
More Of
Quantitative Increase
Less Of
Quantitative Decrease
As Well As(More Than)
Qualitative Increase
Part Of
Qualitative Decrease
Reverse
Logical Opposite of Intention
Other Than
Complete Substitution
Deviations Generated by Each Guide Word
Guide word
NONE
MORE OF
LESS OF
PART OF
MORE THAN
OTHER THAN
Deviations
No forward flow when there should be, i.e. no flow.
More of any relevant physical property than there should
be, e.g. higher flow (rate or total quantity), higher
temperature, higher pressure, higher viscosity, etc.
Less of any relevant physical property than there should be,
e.g. lower flow (rate or total quantity), lower temperature,
lower pressure, etc.
Composition of system different from what it should be,
e.g. change in ratio of components, component missing, ect.
More components present in the system than there should
be, e.g. extra phase present (vapour, solid), impurities (air.
Water, acids, corrosion products), etc.
What else can happen apart from normal operation, e.g.
start-up, shutdown, uprating, low rate running, alternative
operation mode, failure of plant services, maintenance,
catalyst change, etc.
REVERSE: reverse flow
B
A
B
EXAMPLE
C
The flowsheet shows that raw material streams A and B are transferred by
pump to a reactor, where they react to form product C. Assume that the
flowrate of B should not exceed that of A. Otherwise, an explosion may
occur. Let’s consider the flow in line 1:
FB  FA
NONE
MORE
LESS
AS WELL AS
PART OF
REVERSE
OTHER THAN
No flow of A
Flow of A greater than design flow
Flow of A less than design flow
Transfer of some component additional to A
Failure to transfer a component of A
Flow of A in a direction opposite to design direction
Tansfer of some material other than A
HAZOP DISPLAY
Guide Word Deviation Possible Causes
Consequences Action Required
No
System Over- Shutdown
Heated
System
No Flow Pump Fail
Line Blockage
Operator
Stops Pump
More
More
Flow
Excessive
Over-Cooled
Pump Speed
Product
(Control System) (Incomplete
Reaction)
Product
Unacceptable;
Dump
Beginning
End
1
Select a vessel
2
Explain the general intention of the vessel and its lines
3
Select a line
4
Explain the intentin of the line
5
Apply the first guide words
6
Develop a meaningful deviation
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Examine possible causes
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Examine consequences
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Detect hazards
10
Make suitable record
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Repeat 6-10 for all meaningful deviations derived from first guide words
12
Repeat 5-11 for all the guide words
13
Mark line as having been examined
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Repeat 3-13 for each line
15
Select an auxiliary (e.g. Heating system)
16
Explain the intention of the auxiliary
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Repeat 5-12 for auxiliary
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Mark auxiliary as having been examined
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Repeat 15-18 for all auxiliaries
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Explain intention of the vessel
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Repeat 5-12
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Mark vessel as completed
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Repeat 1-22 for all vessels on flow sheet
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Mark flow sheet as completed
25
Repeat 1-24 for all flow sheets
Figure 8.9 Hazard and operability studies : detailed sequence of examination
(Chemical Industry Safety and Health Council, 1977 Item 6)
EXAMPLE
An alkene/alkane fraction containing small amounts of
suspended water is continuously pumped from a bulk
intermediate storage tank via a half-mile pipeline into a
buffer/settling tank where the residual water is settled out prior
to passing via a feed/product heat exchanger and preheater to the
reaction, is run off manually from the settling tank at intervals.
Residence time in the reaction section must be held within
closely defined limits to ensure adequate conversion of the
alkene and to avoid excessive formation of polymer.
Results of hazard and operability atudy of proposed olefine
dimerization unit: results for line section from intermediate storage to buffer/settling tank
Guide word
NONE
Deviation
No flow
Possible causes
(1)No hydrocarbon available
at intermediate storage.
Consequences
Loss of feed to reaction section
and reduced output. Polymer
formed in heat exchanger under
no flow conditions.
Action required
(a)Ensure good
counications with
intermediate storage
operator
(b)Install low level alarm
on settling tank LIC.
(2)J1 pump fails (motor
fault, loss of drive,
impeller corroded away
etc.)
As for (1)
(3)Line blockage, isolation
valve closed in error, or
lCV fails shut.
As for (1)
J1 pump overhears.
Covered by (b)
(c)Install kickback on J1
pump.
(d)Check design of J1
pump strainers.
(4)Line fracture
As for (1)
Hydrocarbon discharged into
area adjacent to public highway.
(1)
Covered by (b)
(e)Institute regular
patrolling & inspection
of transfer line.
Results of hazard and operability atudy of proposed olefine
dimerization unit: results for line section from intermediate storage to buffer/settling tank
Guide word
MORE OF
Deviation
More flow
Possible causes
(5)LCV fails open or LCV
bypass open in error.
Consequences
Settling tank overfills.
Action required
(f)Install high level alarm
on LIC and check
sizing of relief opposite
liquid overfilling.
(g)Institute locking off
procedure for LCV
bypass when not in use.
More pressure
More
temperature
(6)Isolation valve closed in
error or LCV closes, with
J1 pump running.
Incomplete separation of water
Phase in tank, leading to
Problems on reaction section.
(h)Extend J2 pump suction
line to 12’’ above tank
base.
Transfer line subjected to full
Pump delivery or surge pressure.
(j)Covered by (c) except
when kickback blocked
or isolated. Check line.
FQ and flange ratings
and reduce stroking
speed of LCV if
necessary. Install a PG
upstream of LCV and
an independent PG on
settling tank.
(7)Thermal expansion in an
Line fracture or flange leak.
isolated valved section due
to fire or strong sunlight.
(k)Install thermal expansion
relief on valved section
(relief discharge route to
be decided later in study).
(8)High intermediate storage
temperature.
(l)Check whether there is
adequate warning of
high temperature at
intermediate storage. If
not, install.
Higher pressure in transfer line
And settling tank.
(2)
Results of hazard and operability atudy of proposed olefine
dimerization unit: results for line section from intermediate storage to buffer/settling tank
Guide word
LESS OF
Deviation
Less flow
Possible causes
(9)Leaking flange of valved
stub not blanked and
leaking.
Consequences
Material loss adjacent to public
highway.
Action required
Covered by (e) and the
checks in (j).
Less
temperature
(10)Winter conditions.
Water sump and drain line
freeze up.
(m)Lag water sump down
to drain valve and steam
trace drain valve and
drain line downstream.
High water
concentration
in stream.
(11)High water level in
intermediate storage
tank.
Water sump fills up more quickly.
Increased chance of water phase
passing to reaction section.
(n)Arrange for frequent
draining off of water from
intermediate storage tank.
Install high interface level
alarm on sump.
High concentration of lower
alkanes or
alkenes in stream.
(12)Disturbance on distillation
columns upstream of
intermediate storage.
Higher system pressure.
(p)Check that design of
settling tank and associated
pipework, including relief
valve sizing, will cope with
sudden ingress of more
volatile hydrocarbons.
MORE
THAN
Organic acids
present
(13)As for (12)
Increased rate of corrosion of
tank base, sump and drain line.
(q)Check suitability of
materials of construction.
OTHER
Maintenance
(14)Equipment failure, flange
leak, etc.
Line cannot be completely
drained or purged.
(r)Install low-point drain and
N2 purge point downStream of LCV. Also
N2 vent on settling tank.
PART OF
(3)
C
HAZOP PREPLANNING ISSUES
Preplanning issues addressed in a typical refinery unit HAZOP include the
following:
• Verification of as-built conditions shown on the P&IDs
• Line segment boundaries set; markup of P&IDs
• List of support documents compiled
• P&IDs (base study document)
• Process flow diagrams (PFDs)
• Process description
• Operating manuals/procedures
• Processing materials information
• Equipment and material specifications
• Tentative schedules of time to be spent per P&IDs sheet
• Recording technique (computer program or data sheet) determination
• List of standard abbreviations and acronyms compiled
• Criticality rankings devised
• HAZOP training given to all team members (one day)
• Arrange for system or process briefings for team before work begins.
HAZOP STUDY LOGISTICS
Logistical development of this refinery unit HAZOP included the
following:
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Preplanning issues were addressed the prior week.
The team include three core team members and four part-time members.
The study included 16 moderately busy P&Ids.
The study took three and one-half weeks.
The team met 4 hours per day in morning review sessions and spent 2 hours
per day on individual efforts for reviews, follow-ups, and field checks.
Dedicated space was required for storing the large number of documents.
The study resulted in 170 data sheets.
The team recorder used a personal computer to record, sort, and retrieve data.
The Stone & Webster proprietary program PCHAZOPa was used.
The plant operator was the key contribution plant member of the team.
Key operating procedures were reviewed relative to the P&Ids and safe
engineering practices.