Process Safety and Fire Protection

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