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INHERENTLY SAFE DESIGN
•
PROCESS RISK MANAGEMENT METHODS USEDDURING THE DESIGN PHASE CAN BE PUTINTO 4 CATEGORIES: – Inherent
–
Passive – Active
– Procedural
• TARGET IS A FAIL-SAFE INSTALLATION
• FROM: Dennis C. Hendershot and Kathy Pearson-Dafft, Safety ThroughDesign in the Chemical Process Industry: Inherently SaferProcess Design , AIChE Process Plant Safety Symposium,27OCT98
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INHERENT SAFETY DESIGN
• Inherent — Eliminating the hazard by usingmaterials and process conditions which are non-hazardous.
– Minimize — Reduce quantities of hazardous substances
– Substitute — Use less hazardous substances
– Moderate — Use less hazardous process conditions, lesshazardous forms of materials, or configure facilities tominimize impact from hazardous material releases or
uncontrolled energy release – Simplify — Configure facilities to simplify operation
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PASSIVE SAFE DESIGN
• Passive — Minimizing the hazard by processand equipment design features which reduceeither the frequency or consequence of the
hazard without the active functioning of anydevice.
– Location of facilities – separation of ignitionsources and fuels from other facilities
– Design equipment for design pressure in excess of the adiabatic pressure from a reaction.
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ACTIVE SAFE DESIGN
• Active — Using facilities to detect and correctprocess conditions:
– controls
– safety interlocks
– monitoring systems for hazards that develop overa long term
– and emergency shutdown systems to detect andcorrect process deviations.
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PROCEDURAL SAFE DESIGN
• Procedural — Prevention or minimization of incident impacts using:
• Safe operating procedures and operator
training• Administrative safety checks
• Management of Change
• Planned emergency response
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DESIGN IN OVERALL SAFETY MANAGEMENTArt M. Dowell, III, Layer of Protection Analysis, 1998 PROCESS PLANT SAFETYSYMPOSIUM, October 27, 1998 Houston, TX
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DESIGN OF SAFETY INSTRUMENTED SYSTEMS
• ACTIVE INHERENTLY SAFE DESIGNPROCEDURE (Separate instrumentationand control component in CHE 165
Design)• First Level – Alarm systems for out of
range situations and operator action
•
Second Level – Interlock systems toautomatically activate safety devices
• Third Level – Devices to minimize impact
of out of control conditions
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USE OF HAZAN AND HAZOP
• PHA’s (Process Hazards Analysis) Areused to define areas of concern
• HAZAN and HAZOP provide a summary
of the type of risk associated withvarious process locations and operations
– Frequency should be determined
–
Intensity should be determined
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OVERPRESSURIZATION EXAMPLE
• OVERPRESSURIZATION IS THE SUBJECT OFNUMEROUS CODES & REGULATIONS
– AIChE Design Institute for Emergency Relief Systems (DIERS)
– OSHA 29 CFR 1910.119 – Process SafetyManagement of Highly Hazardous Chemicals
– NFPA 30 – Flammable & Combustible Liquids
– API RP 520 and API RP 521 – Pressure Relieving
Devices and Depressurization Systems – ASME Boiler & Pressure Vessel Code
– ASME Performance Test Code 25, Safety & Relief Valves
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SOURCES OF OVERPRESSURIZATION
• API 521 LISTS THE FOLLOWINGCATEGORIES OF SOURCES
API RP521 Item
No.
Overpressure Cause API RP521 Item
No.
Overpressure Cause
1 Closed outlets on vessels 10 Abnormal heat or vapor input
2 Cooling water failure to condenser 11 Split exchanger tube
3 Top-tower reflux failure 12 Internal explosions
4 Side stream reflux failure 13 Chemical Reaction
5 Lean oil failure to absorber 14 Hydraulic expansion
6 Accumulation of noncondensables 15 Exterior fire
7 Entrance of highly volatile material 16 Power failure (steam, electric, or other)
8 Overfilling Storage or Surge Vessel Other
9 Failure of automatic control
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FIRST LEVEL DESIGN
• HOW ARE SOURCES ADDRESSED FOR ASTORAGE TANK?
• Item 1 in previous list - Closed outlets on vessels
– Would be a concern for a nozzle used for pressure control
in the tank, during filling operations.• Perhaps a temporary blind flange would have been left in place after a
maintenance operation.
• A pressure relief valve may malfunction.
–A PAH pressure switch (
ΔP) could be installed if there wasmeasurable difference between the Normal Operating
Pressure and the Maximum Allowable Working Pressure.
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SECOND LEVEL DESIGN
•
HOW ARE SOURCES ADDRESSED FOR ASTORAGE TANK?• Item 1 in previous list - Closed outlets on vessels
• Add a pressure relief valve to allow gas to leave thetank and be directed to an appropriate flare orscrubber.
• Set point needs to be at or slightly above theMaximum Allowable Working Pressure
• Need an interlock to: – Alarm to indicate valve has been activated and receiving
unit (flare or scrubber) is activated. – Shut down a valve in the tank fill line and/or shut off a
pump used for filling.
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THIRD LEVEL DESIGN
• HOW ARE SOURCES ADDRESSED FOR ASTORAGE TANK?
• Item 1 in previous list - Closed outlets on vessels
• Add a rupture disc to relieve to either a flare orscrubber.
• This level is to protect the equipment from failureon a major scale
• Need to have an indication that the rupture dischas opened – typically a wire across the disc
• Need to determine actions necessary when the
disc opens –
stop filling, start flare, etc.
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OTHER DESIGN CONSIDERATIONS
• A large storage tank is filled manually byan operator opening and closing a valve.Once a year, the tank overfills as the
operator is distracted by other activities. A high pressure alarm is added to thetank. After the alarm is added, the tank is typically overfilled twice a year.
• Why?
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EXAMPLE 1
• After the alarm was installed, theoperator relied on it to indicate a highlevel and did not supervise the filling
closely. The alarm loop turned out tohave a failure rate of twice per year, sothe system was not as reliable as themanual operation.
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OTHER CONSIDERATIONS – EXAMPLE 2
•
Fail-safe valves are either Air-to-Open or Air-to-Close, which equate to Fail Closed and FailOpen, respectively. Recommend the correctvalve for the following processes:
1. Flammable solvent heated by steam in a heatexchanger. Valve is on the steam supply line.
2. Exothermic reaction. Valve is on the reactantfeed line.
3. Endothermic reaction. Valve is on thereactant feed line.
4. Gas-fired utility furnace. Valve is on the gassupply line.
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EXAMPLE 2 - CONTINUED
• SPECIFY EITHER FAIL-CLOSED OR FAIL-OPEN FOR THE VALVES IN THESE SYSTEMS
5. Remote-operated valve on the drain for astorage tank.
6. Remote-operated valve on the fill line to astorage tank.
7. Gas-fired Combustion furnace. Valve is on
the air supply line.8. Steam supply line. Valve controls the
downstream steam pressure from the boiler.
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EXAMPLE 2 – SOLUTIONS 1
1. Valve to FAIL-CLOSED to preventoverheating the solvent
2. Valve to FAIL-CLOSED to avoid a
runaway reaction3. Valve to FAIL-CLOSED to avoid reactor
thermal stresses.
4. Valve to FAIL-CLOSED to stop gas flowto uncontrolled combustion.
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EXAMPLE 2 – SOLUTIONS 2
5. Valve to FAIL-CLOSED to preventdraining material from tank
6. Valve to FAIL-CLOSED to prevent
overfilling tank 7. Valve to FAIL-OPEN to maximize air
flow to furnace
8. Valve to FAIL-OPEN to avoid localizedoverpressure of line
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EXAMPLE 3
• 4 kg of water is trapped in between inletand discharge block valves in a pump.The pump continues to operate at 1 hp.
–
What is the rate of temperature increase inC/hr if the cP for the water is constant at 1kcal/(kg C)?
– What will happen if the pump continues to
operate?
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EXAMPLE 3 SOLUTION - 1
• Assume adiabatic conditions for thecalculations: Set up a heat balance:Q m Cp T Tref
Take the derivative with respect to time and
rearrange to getdQ
dtm C
p
dT
dt . And
resolving to getdT
dt
1
m Cp
dQ
dt
Using conversions:1 hp 0.178kcal
sec
m 4 kg dQ/dt 0.178kcal
sec Cp 1
kcal
kg C
dT/dt1
m Cp
dQ/dt dT/dt 160.2C
hr
3 SO O 2
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EXAMPLE 3 SOLUTION - 2
• Allowing the pump to continue to runwill eventually result in high pressuresteam formation. This could result in the
pump exploding.• Adding a thermal switch or a high
pressure switch to shut down the pumpcan prevent this from occurring.