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TxOGA Flare Operation Discussion
April 28, 2009


Discussion Objectives
Define the need for plant flares Describe the components of a typical flare system Discuss the limitations and hazards associated with operating flare systems Discuss maintenance requirements of flare systems


Flares – What are they? Why do we need them?
“The primary function of a flare is to use combustionto
convert flammable, toxic or corrosive vapors to less objectionable compounds.” (API 521 paragraph 6.4.1) Safely during both… emergency and routine operations.


Flare Systems - Purpose
A system of Process Safety Management (PSM) critical mechanical equipment that gathers and safely burns hydrocarbons from pressure-relieving and vapordepressurizing systems Must be continuously availableMust be reliable for years Capable of performing through all operating-plant emergency conditions, including a site-wide general power failure or a weather event including a hurricane Difficult to add equipment or modify because they are very infrequently out of service as they serve many units that are infrequently out of service.
(API 537 paragraph 4.1)

Typical Flows to Flare SystemsEmergency Pressure relief flows Emergency depressurization Episodic Venting required for maintenance Venting required for regeneration Shutdown/Start-up operations (de-inventorying) Continuous – 99+% of Typical Operation Sweep gas through the flare system piping Process venting (continuous analyzer flows, gas seals, certain types of pressure control) Pressure Relief Valve leakage

TypicalFlare System


Flare System Major Components
Every flare system is unique and different Every flare system contains differential components Series of collection headers from sources Knockout drum Stack Flare Tip Liquid seal Purge reduction device (stack seal) Integrated pilot system Monitoring and operation instrumentation Flare hydrocarbon recovery systems


Typical Flare HeaderSystem
PRV 1 PRV 2 Legend: PRV – Pressure Relief Valve FIC – Flow Control Valve

Purge Gas

Process Unit Header




Purge Gas

Process Unit Header

Main Flare Header Purge Gas

To Flare Liquid Knock Out Drum 8

Hydraulic Design
Flare header is sized to limit the back pressure of each pressure relief device during various emergencyevents. The hydraulic design is a line sizing / rating problem Design minimizes the differential pressure to ensure each pressure relief device functions properly Design is based on specific line size, line length and maximum expected relief load for each relief event


Hydraulic Issues
Hydraulic issues specific to flare header design: Different relief events govern the size of various sectionsof the collection header A variety of materials discharge to the flare system Potential pressure discontinuities where pipe flows meet Volume expansion throughout header piping High velocity and significant acceleration effects


Knock-Out Drum

Pressure Vessels


Knock-Out Drum
Separates liquid from gas (2 phase separation) Prevents liquids from being discharged to atmospherePrevents or reduces smoke because of reduced liquid in the flame Increases flare tip life Allows draining of liquid prior to flare For reprocessing For disposal

Typical Knock-Out Drum

Light Liquid (oil) Heavy Liquid (water) Heavy Liquid (Outlet) Light Liquid (Outlet)

Liquid Seal
Flashback prevention Prevent air infiltration Helps maintain positive header pressure Staging deviceSome systems include an additional flame arrestor

Courtesy: John Zink Company


Flare System Purge Requirements
Purge gas composition Non-condensing – typically natural gas or nitrogen Non-corrosive Non-reactive to other gasses in the system Purge gas locations Upstream or downstream of a liquid seal End of major header or branch Purge must be continuous and measurable Purge gas...