Process Safety and Fire Protection
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Questions and Answers

What are the two fundamental constraints that must be considered when establishing a safety valve set pressure?

  1. The set pressure must be low enough to ensure that the relieving pressure never exceeds MAAP of the system. 2. The set pressure must be high enough to ensure that there is sufficient margin above NWP to allow the safety valve to close.

In which case can the set pressure of the safety valve be made to equal the MAWP?

  • When the percentage overpressure is less than or equal to the percentage MAAP of the system (correct)
  • When the percentage overpressure is greater than the percentage MAAP of the system

    • What is the label used for the relief installed on the reactor in the polymerization process?

    PSV-1

    Match the spring-operated valve type with its description:

    <p>Conventional = Provides full opening with minimum overpressure Balanced Bellows = Has a bellows for balancing the valve disc to minimize effects of backpressure</p> Signup and view all the answers

    Rupture discs open and remain open once they reach the specified relief set pressure.

    <p>True</p> Signup and view all the answers

    What is Layer of Protection Analysis (LOPA) used for?

    <p>To reduce risk in a process</p> Signup and view all the answers

    Inherently Safe Process Design targets reducing risks after an accident.

    <p>False</p> Signup and view all the answers

    What are the five categories of inherently safer design choices?

    <ol> <li>Intensification, 2. Substitution, 3. Attenuation, 4. Limitation of effects, 5. Simplification</li> </ol> Signup and view all the answers

    Passive fire protection means _________ fire and preventing it from spreading further.

    <p>containing</p> Signup and view all the answers

    Match the pressure terminology with their definitions:

    <p>Set pressure = The pressure at which the relief device begins to activate Backpressure = The pressure at the outlet of the relief device during the relief process Accumulation = The pressure increase over the MAWP of a vessel during the relief process Relieving pressure = The pressure at which the full capacity of the safety valve is achieved</p> Signup and view all the answers

    What is the major cause of chatter in relief valves?

    <p>The rapid opening and closing of a relief valve</p> Signup and view all the answers

    When should a rupture disc/pin be used?

    <p>To avoid losing entire contents</p> Signup and view all the answers

    Relief systems are rarely vented to the atmosphere.

    <p>True</p> Signup and view all the answers

    What is the purpose of a total containment system in relief systems?

    <p>To collect liquid and treat vapor using devices like condensers, scrubbers, incinerators, or flares</p> Signup and view all the answers

    Why would you use both pilot-operated relief and rupture disc/pin devices?

    <p>To protect against corrosion</p> Signup and view all the answers

    Study Notes

    Reliefs

    • A relief system is an emergency system for discharging gas during abnormal conditions.
    • It protects personnel from dangers of over-pressurizing equipment, minimizes chemical losses, prevents damage to equipment and adjoining property, and reduces insurance premiums.

    Potential Lines of Defense

    • Inherent safe process design prevents accidents in the first place.
    • Active control uses sensors to measure and stop accidents, while passive control does not use sensors.

    Layer of Protection Analysis (LOPA)

    • Reduces the risk of a process by identifying multiple layers of protection.
    • Each layer must be independent, effective, reliable, and auditable.

    Inherently Safe Process Design

    • Eliminates or minimizes hazards rather than controlling them.
    • Can be grouped into five categories: intensification, substitution, attenuation, limitation of effects, and simplification.

    Active and Passive Fire Protection

    • Active fire protection detects, stops, and escapes fires, while passive fire protection contains and prevents fires from spreading.
    • Both are crucial for protecting premises, staff, and business assets.

    Relief Systems

    • Include relief devices, collection piping, flashback protection, and gas outlets.
    • Protect process equipment from over-pressurization and ensure safe operation.

    Pressure Terminology

    • Set pressure: the pressure at which a relief device begins to activate.
    • Maximum allowable working pressure (MAWP): the maximum gauge pressure permissible at the top of a vessel.
    • Operating pressure: the gauge pressure during normal service, usually 10% below MAWP.
    • Accumulation: the pressure increase over MAWP during the relief process.

    Safety Valve Selection

    • Consider cost, type of disposal system, valve construction, operating characteristics, and approval.
    • Review every unit operation and process step to anticipate potential problems.

    Relief Types

    • Spring-operated valves: conventional and balanced-bellows types.
    • Rupture discs: specially designed to rupture at a specified relief set pressure.
    • Buckling-pin reliefs: used for high-pressure applications.
    • Pilot-operated reliefs: used for high-flow and high-pressure applications.

    When to Use Each Type

    • Spring-operated valves for liquid, gas, and corrosive services.
    • Rupture discs for high-pressure and high-flow applications.
    • Buckling-pin reliefs for high-pressure applications.
    • Pilot-operated reliefs for high-flow and high-pressure applications.

    Relief Systems

    • Include the relief device, collection piping, flashback protection, and gas outlets.
    • Protect process equipment from over-pressurization and ensure safe operation.### Balanced Bellows Spring-Operated Valves
    • Incorporates a bellows to minimize the effects of backpressure on valve performance
    • Advantages:
      • Relieving pressure not affected by backpressure
      • Can handle higher built-up back pressure
      • Protects spring and guiding surface from corrosion
    • Disadvantages:
      • Bellows susceptible to fatigue/rupture
      • May release flammables/toxics to atmosphere
      • Requires separate venting system

    Selection Criteria

    • Where total backpressure does not exceed approximately 50% of set pressure

    Spring-Loaded Pressure Reliefs

    • Types:
      • Relief valve: for liquid service, reaches full capacity at 25% overpressure, closes as pressure returns to set pressure
      • Safety valve: for gas service
      • Safety relief valve: for liquid and gas service

    Rupture Discs

    • Specially designed to rupture at a specified relief set pressure
    • Flexing of metal might occur as process pressure changes
    • Once opened, remains open, may cause complete discharge of material or allow air to enter
    • Available in larger sizes, cost less
    • Materials: metal, graphite, composite, and others

    Rupture Disc Applications

    • Protect expensive spring-loaded devices
    • Give absolute isolation when handling toxic chemicals and flammable gases
    • Relieve slurries that may plug spring-loaded devices
    • Protect complex parts of spring-loaded devices from reactive monomers

    Buckling-Pin Reliefs

    • Similar to rupture discs
    • Pin buckles at a precise pressure, valve stays open
    • Can be operated up to 95% of minimum set pressure

    Pilot-Operated Reliefs

    • Main valve controlled by smaller pilot valve, a spring-operated relief valve
    • Advantages:
      • Set pressure not affected by back pressure
      • Can be set very near to operating pressure
      • Less costly for larger size valves
    • Disadvantages:
      • More complex than spring-type
      • System is subjected to overpressure
      • Chance of back flow, piston susceptible to corrosion

    When to Use Each Type

    • Spring-Operated Valves:
      • Losing entire contents is unacceptable
      • Fluids above normal boiling point, toxic fluids
      • Need to avoid failing low, return to normal operations quickly
      • Withstand process pressure changes, including vacuum
    • Rupture Discs/Pin:
      • Capital and maintenance savings
      • Losing contents is not an issue
      • Need for fast-acting device
      • Handling flammable gases or toxic chemicals, high viscosity liquids or slurries
    • Both Types:
      • Need a positive seal
      • Protect safety valve from corrosion
      • System contains solids

    Chatter

    • Rapid opening and closing of a relief valve, causing mechanical failure of internals
    • Causes: oversized relief valve, excessive inlet pressure drop, high back pressures
    • Prevention methods: adding different size valves, larger inlet pipe sizes, increasing exit line size

    Relief Systems

    • Rarely vented to atmosphere, discharged to knockout system
    • Liquid collected, vapor discharged to treatment unit (condenser, scrubber, incinerator, flare, or combination)
    • Total containment system

    Example

    • Specify types of relief devices needed for polymerization reactor

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    Description

    This quiz covers the concepts of process safety, including potential lines of defense, inherently safe process design, and active and passive fire protection. It explores the differences between active and passive fire protection and how they can be used to mitigate risks in chemical plants.

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