chemical & process technology_ inert gas or fuel gas for flare purge

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Inert Gas or Fuel Gas For Flare Purge ?Saturday, August 22, 2009

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Flare is commonly installed in oil and gas process plant to burnhydrocarbon and/or toxic gas to avoid formation of combustiblemixture, to minimize green house effect (GHE), to minimize healthhazards to personnel on site, etc. There are several earlier postsrelated to Flare :

Quick Estimate Flare Stack DiameterEstimate Subsonic Flare Tip Pressure Drop With GraphDerived CorrelationQuick Estimate Flare Tip Pressure DropFlare Tip Type Quick Selection ChartGuideline on Quick Determination of Flare Stack Support TypeFlare Smokeless Ringlemann ChartFLARE combustion efficiencyQuick Estimate Ground Level Unburnt Flammable Gas For Vent Pipe orFlame-out Flare Stack

Flare collection header is normally "NO flow" as most (if not all) devices connected to flareheader are non-discharging fluid into flare system. Among all are pressure relief valve(PRV), blowdown valve (BDV), overpressure dump valve (PCV), etc. All these devices arekept as close position during normal plant operation and will only open in the eventof overpressure, emergency situation i.e. fire, runaway reaction, plantshutdown/blowdown for maintenance.

On the flare tip end, it is open to atmosphere. it is very likely that atmosphere air containoxygen ingress and stay into flare collection header. PRV/BDV/PCV passing and open ondemand will discharge large quantity of hydrocarbon gas into flare collector header filled withair and create combustible mixture, as this combustible mixture travel along flare headerand reach flare tip which equipped with flare pilot, combustible mixture will be ignited andpotentially created flash back to the flare header and flare knock-out drum (KOD). Subject toflare header capacity and mechanical integrity, large flash back lead to severe internalpressure act on the piping & vessel and vapor wave results severe vibration andmovement of structure, this potential lead to catastrophe failure of flare collection system.Therefore a flare header is sweep or purge with fuel gas or inert gas i.e. Nitrogen.

Advantages using inert gas compare to fuel gas as purge gas

i) Environment & Green House Effect (GHE)IG : Inert gas has NO impact to environmentFG : Burn fuel gas in atmosphere generate Carbon Dioxide (CO2) which contributes toincrease of Co2 content in atmosphere and increases Green House Effect (GHE)

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Useful Documents Related to Pressure Relief Valve(PRV) - Part 1Pressure Relief Valve (PRV) has been widely use in

ENVIRONEMENT", OCT 15, 2007 is the Blog ActionDay and the theme of the day is ENVIRONMENT. Havewe ever ask ourself how much damage we have made toour environment ? I m sure many of us.......

Chemical & Process Technology: Inert Gas or Fuel Gas For Flare Purge ? http://webwormcpt.blogspot.com/2009/08/use-of-nitrogen-for-flare-purge...

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ii) Burn back damage flare tip - reduce life span of flare tipIG : Inert gas do not burn. NO burn back and potential damage of flare tip.FG : Potential FG burn back damage flare and shorten flare tip life span.

iii) High OPEX avoid Burn backIG : NO burn back. Minimum purge rate and low OPEX.FG : Potential burn back lead to high purge rate (potential 10 times higher than purge rate ofIG) and high OPEX

iv) Visible FlameIG : Inert gas do not burn. No flame present.FG : FG continuous burn and continuous visible flare at flare tip. Potential create uneasysituation in environment sensitive area.

v) Smoke FlaringIG : Inert gas do not burn. No smoke flaring issue.FG : Burning heavy FG lead to smoke flaring. Potential create uneasy situation inenvironment sensitive area. Increase likelihood of unburnt component and impact onenvironment.

vi) Steam injection for smokeless flaringIG : Inert gas do not burn. No smoke flaring issue.FG : Burning heavy FG lead to smoke flaring. Steam injection to reduce/eliminate smokeflaring. This increases CAPEX (additional steam injection facilities) and OPEX (steam loss).

vii) RadiationIG : Inert gas do not burn. No additional radiation.FG : Fuel gas burn lead to increase of radiation level (on top of solar radiation) to personnelworking near flare stack.

Disadvantages using inert gas compare to fuel gas as purge gas

a) Fuel Gas Readily Available in PlantIG : Required Nitrogen generator or use of Liquid Nitrogen and evaporator. AdditionalCAPEX and OPEX.FG : Fuel gas readily available in plant. Minimum CAPEX and OPEX. Some plant generatehydrocarbon gas which shall be disposed off. This gas is readily serve as purge gas andinccur NO cost.

b) Inert Gas CloudIG : Flare system purge with inert gas, entire flare system is filled with IG gas (whichpotential heavier than air). Once any PSV/BDV open and release large amount of gas intoflare header, it will "push" IG release through the flare tip. Heavy IG (compare to air) will sinkcreate a IG gas cloud near plant. This is potential fatal thread (suffocation) to personnel onsite.FG : Continuous flaring lead to no or nearly no potential of gas present in atmosphere

c) Unburnt hydrocarbon gas emissionIG : PRV/BDV/PCV leak or passing lead to low heating value mixure (less than 200 btu/ft3)which is non-combustible. Release of hydrocarbon gas into atmosphere directly has moreGHE impact than burning it. For example 1 mol of methane create 1 mol of CO2 if it is burnt.1 mol of methane create 20-21 mol of equivalent CO2 if it unburnt.FG : Continuous flaring lead to no or nearly no unburnt gas in atmosphere

d) Combustible Cloud lead to Instant IgnitionIG : Slowly hydrocarbon gas emission to atmosphere and built-up of combustible mixture inthe plant, once the heating value for auto-ignition is reached, the combustible mixturepotentially ignited. Its impact is just like a explosion and potential thread to personnel andsurrounding facilities.FG : Continuous flaring lead to no or nearly no unburnt gas in atmosphere

Concluding remarkInert gas purging is normally understood as clean, low CAPEX, low OPEX, etc and regardsas most likely candidate for flare purging. However, the associated SAFETY related issuemay needs additional attention and focus. All...use inert gas wisely...

Related TopicFlare related topics...CO2 related topics...Personnel Exposure Time For Heat RadiationHeat Radiation For Pain & Blistering ThresholdSimplified Equation for Wind Speed Estimation At Different HeightEstimate Wind Speed At Flare Tip At Different HeightAssess AIV with "D/t-method" with Polynomial PWL Limit LineModel Fix Pressure Drop Device in FLARENET

Labels: CO2, Environment, Fire, Flare

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