Flame arresters are another way of preventing flashback. These are not very commonly used but could be effective against flashback. Their disadvantage comes from the fact that they can easily become blocked by dust, carryover, corrosion products, materials liable to polymerisation, etc. A flame arrester should be considered only if there is no other viable or economic alternative.
a. Flame arresters shall be used only in clean systems, where plugging, scale buildup, dust, or other accumulation cannot occur during any phase of operation (startup, normal, shutdown, emergency, etc.) and if there are no practical alternatives. Their use shall be subject to BP approval.
b. Provision shall be made for periodically checking condition of flame arresters and maintaining flame arresters.
c. It shall be possible to maintain or replace flame arresters without shutting down plant. 4.6.1.5 Flame radiation
Modify to Read 4.6.1.5.1 Height of flares
Height of flare shall be determined by the following considerations: a. Maximum allowable thermal radiation levels as specified in 4.6.1.5.3.
b. Adequate dispersion of flammable and/or toxic gases, even with flare extinguished, such that their concentration shall not cause significant impact to personnel and property and shall be acceptable to local regulations.
c. Dispersion analysis should not only consider flare design flowrate but also other scenarios representing other conditions that may increase potential severity of impact (e.g., higher molecular weight vapour at lower velocity may reach grade level, buildings, process, and breathing air intakes or platforms while flammable).
d. CIRRUS, PHAST, or other BP approved dispersion model shall be used to determine concentration versus distance and possible consequences if flare is extinguished. e. Calculations of ground level concentrations shall be subject to BP approval. Acceptable
concentrations shall be based on period over which conditions leading to release can be sustained and health hazard that they represent.
f. Local or national height restrictions, e.g., for aircraft movements, shall be considered. 4.6.1.5.2 Calculation methods for flare thermal radiation
a. ISO 23251 or API RP 521 or another similar, BP approved method can be used to estimate preliminary thermal radiation levels at grade level, elevated platforms, and buildings.
ISO 23251 or API RP 521 method can be used for initial rough calculations but it is significantly inaccurate at fewer than 2 flame lengths. Most Flare Vendors have developed proprietary programs that are empirically based for their specific flare tips. The fraction of heat radiated used in their models may not be interchangeable with that used in ISO and API. Flare Vendor thermal radiation model is generally preferred .
b. If Flare Vendor proprietary program is used, Vendor shall indicate basis for calculations, including flame emissivity and shall supply calculated results for flame length, flame shape, and emission. BP will specify points where flare radiation calculations are required and environmental and operating conditions.
c. Positions critical to flare radiation calculations, particularly offshore, are: 1. Base of flare boom.
2. Nearest edge of platform.
3. Helideck.
4. Crane cabs.
5. Monkey board (drilling derrick). 6. Radio mast (includes fittings). 7. Drillers pipe rack.
d. Environmental conditions that should be used in thermal radiation calculations are:
1. No wind.
2. 32 km/h to 50 km/h (20 mph to 30 mph) wind.
Higher wind speed can be evaluated as required but the user should recognise that there is better cooling at higher wind speed that would mitigate the heat radiation.
e. Plant or process areas containing high thermal radiation levels (fired heaters, exothermic reactors, etc.) shall be considered in relation to and shall be additive to expected thermal radiation rates from both operational and emergency flaring events.
f. Duration of and additive effect from radiation of any other elevated flare(s) located onsite that would flare simultaneously with flare under design shall also be considered.
g. If flaring continuously, effects of solar radiation should be included in overall radiation rates. 4.6.1.5.3 Thermal radiation levels
a. Maximum permissible design level of radiation for exposure of personnel at maximum emergency flaring shall be based on the following:
1. Continuous full shift - 1,6 kW/m2
exposure (offsite public location, (500 Btu/ft2h)
outside plant boundary where public can be present)
2. Operational blowdown - 3,2 kW/m2
(maximum 30 min) (1000 Btu/ft2h)
3. 60 s peak exposure - 4,7 kW/m2
(escape time to safe haven) (1500 Btu/ft2h)
4. 20 s peak exposure - 6,3 kW/m2
(escape time to safe haven) (2000 Btu/ft2h)
b. In general, 3,2 kW/m2 (1000 Btu/ft2h) threshold is used where personnel can be normally
working and 4,7 kW/m2 (1500 Btu/ft2h) is used where personnel are not normally working.
c. Average peak solar radiation value of 1,0 kw/m2 (312 Btu/ft2-hr) should be used for solar
radiation allowance unless specific measured values are available for site.
Notes
• The figures given assume at least single layer whole body working clothing and hard hat.
• For flaring where the peak radiation load is intermittent, solar radiation can be excluded. For flare where the peak radiation load is continuous, solar radiation should be included. An appropriate allowance, dependent on latitude, should be made when determining permissible flare radiation.
• Metal surfaces irradiated at any of the time/level ratios given may produce burns on contact with bare skin.
• For offshore flares, it may not be possible to satisfy some of the requirements. Access to some areas may therefore have to be restricted, e.g., the flare structure, the bridge for a linked flare, and the drilling tower. It should be possible for any vital work in these areas to be performed under specified and controlled conditions.
• If necessary, these design levels may be achieved by the use of displacement or shielding. The requirements for any shielding system, and the type of system to be employed shall be agreed with BP at an early stage.
• On towers or other elevated structures where rapid escape is not possible, ladders shall be provided on the side away from the flare such that the tower or structure can provide some degree of shielding where necessary.
• In tower supported multiple flare systems, all access requirements shall be considered. Shielding shall be provided if specified by BP.
• A maximum ground level radiation will be specified by BP, either where access across a restricted access zone without shielding is required or where the ground covering may be ignited, e.g., grass or peat.
• The effect of flaring on equipment in the vicinity shall be considered, using the same design level above, from the following aspects:
• High temperature from radiation.
• Large temperature gradients between exposed and non exposed surfaces.
• Corrosive action of pollutants.
• Possibility of burning of unignited droplets.
• Effect of hot gases.