Producing site flares for offshore and onshore are also designed in accordance with API RP 521. Sections 1253 through 1256 discuss design concepts for both locations. Additional factors must be considered in the design of offshore flares.
1. Offshore Flare Location
The offshore flare often must be located adjacent to a manned production platform. API RP 521 recommends that the flare be located so that maximum radiation level does not exceed 500 Btu/hr/ft2 (excluding solar radiation) for areas where personnel are working continuously. This recommendation is consistent with experience.
2. Smokeless Burning
Most offshore platforms do not have steam for flare operation. Therefore, they use flares that do not require additional turbulence generation or that use an air blower or a water spray from a pumping unit. Air blower flares have a greater turndown than water spray flares.
3. Continuous Flaring
It may be necessary to continuously flare tail gas or excess produced gas which cannot be economically utilized. It may also be necessary to flare gases vented during process upsets until corrective action can be taken.
4. Offshore Flare Designs
Offshore platforms typically use the following kinds of flares:
– Nonsmokeless utility flares, which are used for continuous flaring of natural gas with a molecular weight of 20 or less, or for short-term emergency flaring of paraffinic hydrocarbon vapors with a molecular weight greater than 20. These flares incorporate a proprietary flame retention device to provide a more stable flame which helps to prevent liftoff (lifting of the flame from the top of the flare) or blowoff (complete extinguishment). They also include a flare windshield which prevents the flame from licking down the outside of the nozzle. See Figure 1200-28.
– Smokeless air blower flares, which are used for continuously flaring paraffinic hydrocarbon vapors with molecular weights greater than 20. These also incorporate a flame retention device and a windshield, but the configuration is quite different from the nonsmokeless flare. Very little forced air is required for smokeless burning. Secondary air is entrained by the flame itself. The flare has a flow sensor to start the air blower when needed. See Figure 1200-29.
– Multipoint flares, which are used where smokeless flaring, low radiation, and a large turndown are required. The tip requires no seals and no assist gas or blown air. These flares require high relief gas pressures—5 to 75 psig.
– Coanda-profile flares, which are used where smokeless flaring and low radiation are required. These flares entrain high volumes of air and create a stiff flame which resists wind effects. They require high relief gas pressures 5 to 75 psig. See Figure 1200-30.