INSTRUMENTATION AND CONTROLS FOR SAFETY

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INSTRUMENTATION AND CONTROLS FOR SAFETY

• M. B. Jennings
• CHE 185

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INHERENTLY SAFE DESIGN

• PROCESS RISK MANAGEMENT METHODS USED DURING THE
  DESIGN PHASE CAN BE PUT INTO 4 CATEGORIES
• Inherent
• Passive
• Active
• Procedural
• TARGET IS A FAIL-SAFE INSTALLATION
• FROM Dennis C. Hendershot and Kathy
  Pearson-Dafft, Safety Through Design in the
  Chemical Process Industry Inherently Safer
  Process Design , AIChE Process Plant Safety
  Symposium, 27OCT98

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INHERENT SAFETY DESIGN

• Inherent Eliminating the hazard by using
  materials and process conditions which are
  non-hazardous.
• Minimize Reduce quantities of hazardous
  substances
• Substitute Use less hazardous substances
• Moderate Use less hazardous process conditions,
  less hazardous forms of materials, or configure
  facilities to minimize 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 process and
  equipment design features which reduce either the
  frequency or consequence of the hazard without
  the active functioning of any device.
• Location of facilities separation of ignition
  sources 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 correct
  process conditions
• controls
• safety interlocks
• monitoring systems for hazards that develop over
  a long term
• and emergency shutdown systems to detect and
  correct 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 MANAGEMENT
Art M. Dowell, III, Layer of Protection Analysis,
1998 PROCESS PLANT SAFETY SYMPOSIUM, October 27,
1998 Houston, TX
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DESIGN OF SAFETY INSTRUMENTED SYSTEMS

• ACTIVE INHERENTLY SAFE DESIGN PROCEDURE (Separate
  instrumentation and control component in CHE 165
  Design)
• First Level Alarm systems for out of range
  situations and operator action
• Second Level Interlock systems to automatically
  activate safety devices
• Third Level Devices to minimize impact of out
  of control conditions

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USE OF HAZAN AND HAZOP

• PHAs (Process Hazards Analysis) Are used to
  define areas of concern
• HAZAN and HAZOP provide a summary of the type of
  risk associated with various process locations
  and operations
• Frequency should be determined
• Intensity should be determined

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OVERPRESSURIZATION EXAMPLE

• OVERPRESSURIZATION IS THE SUBJECT OF NUMEROUS
  CODES REGULATIONS
• AIChE Design Institute for Emergency Relief
  Systems (DIERS)
• OSHA 29 CFR 1910.119 Process Safety Management
  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 FOLLOWING CATEGORIES OF SOURCES

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FIRST LEVEL DESIGN

• HOW ARE SOURCES ADDRESSED FOR A STORAGE 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 (?P) could be installed if
  there was measurable difference between the
  Normal Operating Pressure and the Maximum
  Allowable Working Pressure.

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SECOND LEVEL DESIGN

• HOW ARE SOURCES ADDRESSED FOR A STORAGE TANK?
• Item 1 in previous list - Closed outlets on
  vessels
• Add a pressure relief valve to allow gas to leave
  the tank and be directed to an appropriate flare
  or scrubber.
• Set point needs to be at or slightly above the
  Maximum 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 A STORAGE TANK?
• Item 1 in previous list - Closed outlets on
  vessels
• Add a rupture disc to relieve to either a flare
  or scrubber.
• This level is to protect the equipment from
  failure on a major scale
• Need to have an indication that the rupture disc
  has 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 by an
  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 the tank. 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, the operator
  relied on it to indicate a high level and did not
  supervise the filling closely. The alarm loop
  turned out to have a failure rate of twice per
  year, so the system was not as reliable as the
  manual 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
  Fail Open, respectively. Recommend the correct
  valve for the following processes
• Flammable solvent heated by steam in a heat
  exchanger. Valve is on the steam supply line.
• Exothermic reaction. Valve is on the reactant
  feed line.
• Endothermic reaction. Valve is on the reactant
  feed line.
• Gas-fired utility furnace. Valve is on the gas
  supply line.

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EXAMPLE 2 - CONTINUED

• SPECIFY EITHER FAIL-CLOSED OR FAIL- OPEN FOR THE
  VALVES IN THESE SYSTEMS
• Remote-operated valve on the drain for a storage
  tank.
• Remote-operated valve on the fill line to a
  storage tank.
• Gas-fired Combustion furnace. Valve is on the air
  supply line.
• Steam supply line. Valve controls the downstream
  steam pressure from the boiler.

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EXAMPLE 2 SOLUTIONS 1

• Valve to FAIL-CLOSED to prevent overheating the
  solvent
• Valve to FAIL-CLOSED to avoid a runaway reaction
• Valve to FAIL-CLOSED to avoid reactor thermal
  stresses.
• Valve to FAIL-CLOSED to stop gas flow to
  uncontrolled combustion.

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EXAMPLE 2 SOLUTIONS 2

• Valve to FAIL-CLOSED to prevent draining material
  from tank
• Valve to FAIL-CLOSED to prevent overfilling tank
• Valve to FAIL-OPEN to maximize air flow to
  furnace
• Valve to FAIL-OPEN to avoid localized
  overpressure of line

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EXAMPLE 3

• 4 kg of water is trapped in between inlet and
  discharge block valves in a pump. The pump
  continues to operate at 1 hp.
• What is the rate of temperature increase in C/hr
  if the cP for the water is constant at 1 kcal/(kg
  C)?
• What will happen if the pump continues to operate?

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EXAMPLE 3 SOLUTION - 1

• Assume adiabatic conditions for the calculations

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EXAMPLE 3 SOLUTION - 2

• Allowing the pump to continue to run will
  eventually result in high pressure steam
  formation. This could result in the pump
  exploding.
• Adding a thermal switch or a high pressure switch
  to shut down the pump can prevent this from
  occurring.