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Flare
FCCU Flare KOD Sludge: O2 reacts with unsaturates and forms polymer. O2 via RVs or PCVs. Merox unit vent gas has O2. Add RDs upstream of passing RVs. Consider (a) Routing vent stream with O2 to a furnace firebox via a few floor nozzles or a special burner with an additional nozzle (b) Diesel circulation to make the sludge flow and route to oil recovery (c) Electrical heater in KOD or pump suction to keep sludge in solution and (d) Route sludge to delayed coking or incinerator. Rather than get rid of or live with the sludge, isolate air or O2 reaching KOD to avoid the sludge formation. A small weir with a manhole upstream will help to scoop out. Projecting pump suction nozzles above vessel bottom may avoid muck to KOD Pumps
Header LT: HP RV/ BDV tail pipe temperature may be low - not all the way to flare tip due to heat gain from header metal + ambient. BDV release goes down quick. RV release doesn’t continue as operators respond. Transient heat transfer analysis or Hysys Imperial College Blowdown Model. SS Tail pipe with LTCS/ CS headers
Header Liquid Pockets: Review and avoid. Hydrate blockage can reverse - rupture RDs sending gas/ condensate/ oil to utility system. See Safety Alert
Subsea U tube header to Flare: Provided to a far off flare to avoid expensive tripod supported bridges to the flare. They get filled with condensing liquid HC + seawater entry due to corrosion. Cause obstruction in relief path + high backpressure on RVs. Results in burning liquid flow via flare tip whenever there is a relief + floating burning oil drifts towards platform. Avoid. Results in high purge rate + periodical HP gas sweep to clear the U tube
Radiation Calcs: Many factors get factored or combined conservatively in the equations used (1) Wind blows towards observer at the highest speed at the same time as peak rate. Wind is free to blow in any direction and there may be no wind in a given instant or during peak flow (2) Peak or noon solar radiation at all times. Line-of-sight coincidence of sun and flame only during early morning or late evening with a weak solar component (3) Ignoring 15-20% attenuation due to humid air (4) 100% heat release. Up to 50% may go as unburnt and soot. Unburnt gets accounted in F, Fraction Transmitted (5) Sonic flares aerate better leading to higher combustion. Interestingly with lower F (6) F = 0.1-0.15 for Sonic; 0.2 to 0.3 for LP. More C1, less is F. One may assume F = 0.15 for a wellhead sonic tip but in reality, it is high due to oil carry over - soot and unaccounted additional heat release. Read "environment.gov.ab.ca/info/library/6694.pdf" on observed variations - up and downwind; far and near. “Furnace Operations", Dr RD Reed - 2 points (i) As H/C ratio declines from 0.25 to 0.13 (Paraffins to Acetylenes), unburnt increases from 5 to 50%. Unburnt reduces heat output. We always take 100% combustion (ii) LP flares at peak loads and < H/C 0.25 smoke more, reducing heat release. Soot absorbs heat, reducing radiation. Dr Reed suggests F = 0.11 for H/C 0.33 (methane); 0.12 for H/C 0.25; 0.07 for H/C 0.17 (7) Papers at flare supplier websites: F measured vary (8) Loads: Flare header line packing reduces net output to flare tip (9) During a blowdown or cumulative RV loads, all sources do not start at the highest pressure nor release at the same instant (10) 20% variations in load estimate by different process engineers. Not simple as Current = Voltage/ Resistance. Empirical co-relations with ± 15% accuracy. Some process engineers enjoy doing it to fourth order accuracy not realizing these are estimates. Note: Use LHV (Lower Heating Value) to calculate Q. Not HHV (Higher Heating Value). HHV applies when water in flue gas condenses and releases its latent heat
Radiation - Wind Speed: Use 30 kmph as in API 521 sample. Some wrongly go for higher wind speeds, even gale speed specified for flare stack structure or flare pilots to stay lift. High speed winds cool and reduce surface temperature - high air heat transfer coefft. Ref: ‘Flare Radiation Flux Eqm Temperature Chart’ in Sizing. At 6.3 kW/sq.m, steel surface temperature is 175°C at 30 kmph and 270°C at 0 kmph. Flux with 30 kmph has to double to match still air temperature. Higher wind speeds may flatten flame but cool surfaces. Gale winds AND flare full blast AND an operator in open - zero chance
Stack Height: Decided by tip velocity for radiation and dispersion. Minimum statutory heights in different countries, regardless of exit velocity. High tip velocity = High tip ΔP = Smaller tip = Less money on tip. High tip ΔP = high back pressure that reduces flow volume/ stack/ header diameters. Do not give all ΔP to tip or tail pipes. Equal ΔP/ meter run. In 2-3 iterations find best split - tip Vs header size. (1) Start with 30% ΔP for tip. In some plants, headers are longer. Factor accordingly. (2) ΔP/m is based on straight length and not equivalent length. For existing plant, eye-balled run lengths and correct count of bends + fittings will do. Bends + fittings take bulk of ΔP
Sonic Flare: API 521 method and flame lengths are for sub-sonic pipe flares. There is no published or open literature correlation to estimate the length of well aerated sonic flame. Since flame envelope is an indication of air: HC mixture in LEL-HEL range, some estimate flame length based on dispersion calcs. Vendor literature shows good variation between measured and estimated lengths. Field feedback suggests that actual radiation is higher than predicted (“Tie the engineer who predicted to a pole at the foot of the flare bridge”) due to liquid carry over. Use API/ Flaresim to check supplier recommendations. API predicts longer flame; flame centre closer to observer. In shorter sonic flames, flame centre is away from observer. API flux = F*Heat output/Surface of sphere of diameter D. Do not buy supplier’s suggestion that in a multi-tip sonic flare, flames far away is shielded by the flames near. Radiation is from a hot gas cloud
Blowdown: ESD isolation to limit HC inventory to 50 m³. BDVs based on PVgas > 100 bar. m³ or 4 m³ of LPG= inventory
Blowdown: Vessels with HP gas or Oil don’t cooldown. Vessels with light ends and condensate may reach MDMT
Blowdown: API - pressure brought down to 50%DP with thicker walls or to 100 psig in 15 minutes. Compressors with seal oil may require a faster rate. Safety may want it faster. Analyzing prevailing pressure + allowable stress at prevailing metal temperature, one can extend blowdown time
Blowdown Rate Reduction: If peak blowdown > inflow, don’t waste money providing a bigger flare. Consider extended/ zoned/ staggered blowdown or via dynamic simulation or HIPPS to reduce load. See ‘Relief Systems - Sizing’ in Training
Blowdown: BDV ROs selected on next standard size. Vessel may blow down faster with higher initial rates than 15 minutes assumed. All vessels are not at BDV rated pressure when blowdown is initiated. Considering ± errors in correlations/ calculations, it does not matter if a vessel is blown in 13 or 17 minutes. Due to header packing, tip gets less than total load. Exact 15 minutes precision run is not called for except to harass a junior engineer performing the calcs! Don’t sweat the small stuff. It takes attention away from key issues. Relief calcs were done in pre-Hysys and Flarenet days too. Plants were designed on Grote’s simple blowdown equation. Availability of tools instead of saving calculation time shouldn’t exponentially increase it. During reviews, where there is an obsessive attention to minor issues, usually there is always a big hole. Unlike downstream industries with known feed/ products/ properties, in oil & gas there are too many unknowns - in flow; composition etc. The wellfluid that flows in, is different from the ones on which multiple simulations for many future years were done. Seen cases where 15-year analyses were done but the field was dry after installation. One platform got shrunk to half capacity halfway thru the project and finally got mothballed. Spend time on transient and start-up issues that are not covered in steady state simulation
BDV: 50% back pressure assumed for choked flow that gives the smallest orifice size and lowest flow. Blowdown flow rapidly declines and backpressure falls as square of the declining flow
BDV RO: Single RO will do for this short duration service. No need for multiple ROs
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