Flare System

Typical flow diagram

Aker Kvaerner Powergas Pvt. Ltd.

System protectionPrimary protection-Control systemSecondary protection -Hard wired trip protectionTertiary protection -Safety valves, rupture disk


Reliving Scenarios-Fire-Power failureSteam failureC.W. failure-Inst. Air failureReflux failureSide reflux failure-Air cooler fan failureControl valve failure

-Refrigeration failure-Vaporisation in exchangers-Overfilling-Tube rupture-Entrance of volatile liquid-Chemical reaction (Run away reaction)Closed outletLiquid expansion


Reliving devicesSafety Relief valve.Rupture disk.Emergency de-pressurisation.


i) Safety valvesTypes-Conventional.-Balance bellows.-Pilot operated.Set pressure equals to design pressure/MAWP of equipment. Allowable Back pressure depends upon the type of safety valve.P = 3% of set pressure of safety valve.


ii) Rupture disk Rupture Disk is most preferred, fool prove, safety device, opens instantaneously to protect the system against over pressurisation. It is designed to burst when differential pressure across the disk exceeds a predetermined value at predetermined temperature. Rupture disks are used in gas, liquid, highly corrosive, highly viscous and congealing services. It is installed in various combinations, sole safety device, with safety valve, either inlet or in parallel depending upon the criticality of system. ---Continue--


Rupture diskAPPLICATIONS:The use of rupture disk as pressure-relieving device is preferred in the following cases: -Where pressure rise is so rapid that the inertia of a safety valve would be a disadvantage.Where minute leakage of process fluid (Toxic, highly inflammable, safety valve opens to atmosphere) cannot be tolerated under operating conditions.Service conditions led to deposition, which can make safety valve inoperative.Cold ambient condition could prevent a safety valve to operate.Solid containing gas makes safety valve inoperable. Corrosive gases spoil the safety valve internals. ---Continue---


Rupture diskRupture disk is used as sole safety device in following cases:

--Instantaneous relief is required.--As an additional safeguard for pressure relieve.

Following are the applications, where rupture disc is used in series, of safety valve at up-stream.

--To protect the safety valve against corrosion. --To prevent leakage of corrosive, toxic and valuable material. --To prevent total loss of material from the system following rupturing of rupture disc. --To protect the safety valve becoming inoperable due to congealing of material.


iii) Emergency Vapor De-pressurisation De-pressurisation during fire minimises the risk of failure of equipment even at lower than its design pressure, due to reduction in Yield Strength of its material of construction with increase of wall temperature during fire.

De-pressurisation also reduces the risk of increasing the internal pressure of equipment exceeding the Rupture pressure during external fire, thus preventing the occurrence of major leak of hazardous chemicals, which leads to safety and environment hazards.

De-pressurisation helps in reducing the wall temperature thus reduces the risk of pre-mature failure of equipment.

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Conditions for requirement of Emergency Vapor De-pressurisationAll the process equipment, as part of a system, should have atleast total cumulative inventory of 3 to 5 cubic meters of Butane or more volatile liquid under normal operating conditions, shall be provided with remotely operated depressurisation assembly.

As per the API-521 (Clause 3.19.1), the release capacity should be such that the continuous vapour depressurisation should reduce the equipment pressure to 50% of its design pressure within 15 minutes.

The API-521 (Clause 3.19.1), further states that when fire is controlling contingency, it may be appropriate to limit the provision of depressurisation facility only for the equipment and sections which operate 250 lbs/square inch and above, where the size and volume of a system is significant. The depressurisation is limited to 100 lbs/square inch or 50% of the design pressure, whichever is lower, within 15 minutes time (for vessels with wall thickness 1 or more, thinner vessel requires higher depressurisation rate, means lower than 15 minutes). ---Continue-


Emergency Vapor De-pressurisation Type of vapor de-pressurisation:

Controlled de-pressurisation.Uncontrolled de-pressurisation.

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Emergency Vapor De-pressurisation


Controlled de-pressurisation


Uncontrolled de-pressurisation


Flare load summaryFlare load summary : 1) Normal 2) Emergency*IT IS NOT NECESSARY THAT IF MASS FLOW IS HIGHER, THE FLARE LOAD WILL BE GOVERNING.


Sheet1

SR. NO.PLANTLOAD T/HRTOTAL

C.W. FAILUREFIREPROCESS UPSETDepressurisation

Unit no.T/HRMWTEMPT/HRMWTEMPT/HRMWTEMPT/HRMWTEMP

1UNIT A

2UNIT B

3UNIT C

Sheet2

Sheet3

Streams requiring disposalVapor and liquid relief from safety valve and rupture disk. De-pressurised vapors.Any operational inflammable waste stream does not have suitable outlet.

Venting Vs. Flaring


Venting considered only, otherwise flaringRelease occurs only in extreme emergency conditions.Release vapor / gas is lighter than Air (< 0.9 times the air).No risk and consequences of accidental flame ignition.Concentration of toxic / corrosive components in dispersed clouds do not reach in harmful level.Condensation of corrosive vapor should not occur.Stream should be free of liquid.Hot gas venting should not ignite.


Flaring Vs. Venting Considerations are to be made, based on the followings, whether to vent or flare the disposed streams.

Impact of environment. Safety and integrity of disposal system (Products are not combustible). Local regulations.Economic evaluations.


Common Vs. segregated flare systemsSegregated flare system may be required in following cases:

Release at various pressure levels, economic reasons to reduce the size of flare headers, sub-headers and stack.Widely differing potentials for liquid release.Corrosive and non-corrosive streams.Meet plant layouts / economics.Cold dry gas with wet gas.Contents of one stream may react with contents of other stream led to chemical reactions, producing heat.


Disposal of H2S rich gas Gas stream rich in H2S should not be combined with gas streams free of H2S. Because off:-H2S spread in entire flare network, shall call for NACE materials for entire system. H2S attack on the CS will make Pyrophoric substance that can lead to fire hazard in presence of explosive mixture (Auto ignition temperature is very low for Pyrophoric substance).


Disposal of Oxygen rich gasStream rich in oxygen should not be put into the common flare header.

Oxygen forms explosive mixture with hydrocarbons, leads to explosion, if fire source is available.Oxygen converts H2S in elemental sulfur that plugs, obstruct free flow of gas. As well,H2S produces Pyrophoric substance by reacting with pipe material. If Oxygen laden H/C gas ,already in explosive range, pass can create fire hazards either due to pyrophoric substance or by Static electricity.


Conventional flare system


Component of Flare networkFlare tip with pilots.Dry gas seal.Flare stack.Liquid seal.Flame front generator.Knock-out drum.Flare header and sub-headers.


Problem encounter in flare systemGlaring: Causes inconvenience to the surrounding population and feeling of unnecessary waste of resources.

Noise: Causes inconvenience to the surrounding population and prolonged exposure to excessive noise may cause mental irritation, fatigue and even deafness.

Radiation: Effects the surrounding population, properties and vegetation, also effects the working of operating staff in near vicinity.

Ground level concentration (GLC) : Long term effects of toxic, combustion products on population, structures and plant and animals.

Smoke: Causes considerable amount of environmental pollution.


Daily noise exposure limit OSHA Occupational Safety and Health Administration.NIOSH National Institute of Occupational Safety and Health.


Recommended flare radiation levels


Recommended GLC levelsThese levels of pollutants are as per OSHA standards

PollutantsPeriod of measurementMax. permissible level, PPM.CO8 hrs average1 hr average9.035.0SO2Annual arithmetic avg. mean24 hrs. avg.0.030.14Particulate matterAnnual geometric mean24 hrs avg.75 g/m3260 g/m3H2S 1hr avg.0.03NO2Annual arithmetic mean0.05Hydrocarbon3 hrs avg.0.24


Types of flare system (Tip controls efficient burning) Conventional : Conventional flare consists of pipe stack with flare tip and pilot at top. The kinetic energy of the gas does not aid to air mixing for improving the burning efficiency.Coanda effect flare:- High pressure gas injected from a narrow slot follows a profile of curved surface. The tulip of coanda profile aids in formation of hollow cylinder of gas entrains air up to 20 times of gas volume helps in efficient burning. This creates both turbulence and sucking of air.Jet Mix flare :- These are multiple nozzles and arms. The flare gas is spread out to the ambient air at high velocity promoted rapid turbulence mixing with air and thus produces efficient burning.Air Assisted flare:- These are provided with special mixing heads. Air is blow into the stack through a concentric pipe results in turbulence mixing of air, ensuring efficient burning. --Continue---


Flare tips


Types of flare system (Tip controls efficient burning) Steam assisted flare :- This flare achieves the smokeless capability by mixing steam with the effluent. The steam is introduced at various points to suck secondary air and turbulence to improve mixing. The steam also reduces flame temperature by dilution and thus reduces the thermal radiation.

Reaction takes place ;

CnH2n+2 +2nH2O ==== nCO2 + (6n+2) H2


Flare tips


Dry gas sealDry gas seal controls the Oxygen ingress in the flare system and also reduces the quantity of gas required for the purging. There are basically four types of Dry gas seals.Labyrinth seal.Arrestor seal.Fluidic seal @.Molecular seal @.


Dry gas seals


Flare header sizing Sub headers are sized for 0.9 mach number, main flare header for 0.7 to 0.8 mach number, stack is sized for 0.5 mach at peak flaring and 0.2 Mach for normal flaring.Most of times main flare header/sub-headers sizes are controlled by allowable back pressure rather than maximum allowable velocity as stated above.**All connections to main flare header are from the top in order to avoid back flow of condensate.


Prevents the flash back from travelling up-stream. Dry gas seal cannot prevent flash back.Protect system against air ingress during vacuum pulling.Provides positive sealing.Avoids pulsation in flame flare tip.Protect system against explosion. Liquid seal drum


API RP-521 recommends sizing should be based on gas flow area 3 Times inlet pipe cross sectional area.Liquid seal drum design( Normally vertical)D -Drum diameter, Md -Inlet pipe diameter, M --- Continue---


Disengaging height 2 to 3 Times the diameter.Minimum seal height: 4 w.c.Pipe height = 10 feet.Slotted / V-Notches dip tube controls surging flow.Seal height : 1.75 Times the back pressure.Design pressure = 7.0 kg/cm2g,Design temperature = As per header design.Minimum water flow = 1.25 dm3/sec.


Knock-out drum Liquid from the flare gas is required to be separated out before it is routed for flaring because:

Liquid particles 90 burn in flare unnoticed except marginally change in flame colour.Liquid particles 90 to 400 burning droplet visible in day light. Liquid particles 400 gives fire balls, Therefore, liquid requires separation before it enters flare stack for burning.


Knock- out drum designNormally Horizontal.-Diameter : i) Single flow

- Diameter :ii) Double flow0.7 times diameter calculated above. ---- Continue----


Length : ---Decided on liquid hold up basis. ---Normally 10 minutes Hold-up for liquid, removed by pump, on/off operations.Design pressure :7.0 kg/cm2g, withstanding internal explosion.Design temperature :20oC above the maximum temperature of any release.


Pilot ignition SystemFor pilot ignitions various devices are available, most common are:

Flame front generatorPiezometerBallistic spark ignition


Flame front generator


Flare gas purging system All the dead ends of flare sub-headers / main header requires gas purging. The dry gas seal also requires continuous purging to avoid air ingress in the flare header due to chimney effect and during vacuum pulling. Normally gas for purging should be heavier than the air.

The rate of purge flow should be sufficient to overcome any tendency of Flash back (1.0 fps min with H/C and up to 10.0 fps for hydrogen. Higher purge is required for the hydrogen because it has highest Flame propagation velocity). ----- Continue---


Flare gas purging systemIn order to control the purge rate, a sample point is provided below the dry gas seal to monitor the oxygen content. As per API-521, oxygen should be less than 6%V. Any increase in oxygen content calls for increase in purging rate.Purge gas lighter than air is more effective than heavier gas.In case of heavier than air purge gas, there is no buoyancy mechanism causing air entry into the stack, and there is thus no incentive to include a dry gas seal.


Zero Flaring/ flare gas recovery ConceptLocal environmental regulations Substantial reduction of CO2 and NOx emissionsReduction in CO2 Taxes (in some countries)Increase flare tip lifetimeNo loss in fuel gas purgingNo need of pilot gasIncreased profit due to the sales value of flare gas Less noise and light disturbancesAchieving status of clean environment.


Instantaneous Ignition SystemRequirement of an instantaneous ignition system, otherwise the pilot burner needs to be kept on.

Ballistic spark ignition technology

Continuous sparking ignition system


PPA possible optionLPHP0.8 MMSCFD1 MMSCFDPSHPSHRecovery Comp.1st Stage FGC2nd Stage FGCFlash Gas29 psigATM VENT A 0.05 MMSCFD0.2 MMSCFDATM VENT B


Tapti possible optionLPHP?? MMSCFD?? MMSCFDPSHPSH1st stage Recovery Comp.2nd stage Recovery CompTo suction of BoosterCompressorWaste gas to pilot ?? MMSCFD


Concept of Box flare Concept of box flare has been derived for the plants (Refinery and gas processing units) where substantial quantity of gas is required to be flared for quite some time. The concept of Box flare attends all the problems normally encountered with conventional flare. In Box flare, controlled combustion of flare gas is carried out inside the enclosed chamber and during peak flaring there is auto diversion of flare gas to conventional elevated flare stack.Glaring: Controlled combustion inside the enclosed chamber, invisible from outside. --Continue--


Concept of Box flareNoise: - Enclosed chamber is provided with noise insulation from outside, which arrests noise emission from the burning.Radiation :-Enclosed chamber provided with refractory inside, acts as the barrier, does not allow heat radiation at outside working area.Ground Level Concentration:- Chimney at the top of box flare and controlled complete combustion helps in controlling the GLC rising above the dangerous level.Smoke :- Combustion is controlled by stages to ensure that gas should burn completely. Besides, there is check to divert the excess gas above its capacity towards elevated flare, thus ensures no smoke to emit from box flare.


Box flare


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Flare System

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