Stack design flares

The flare stack design shall be in accordance with Specifications ME-0-JBOOI and ME-O-JSOIO. 

Figure 7-77. Diagrams for alternate flare stack designs of Straitz. By permission, Straitz, J. F. iii and Aitube, R. J., NAO, inc. [62].

The key factor in flare stack design is personnel escape time from the stack base at maximum discharge. Therefore, the selection of height and flare stack location should be made on the basis of safety for operating personnel as well as equipment. 

Lawler, J.B. Cut Costs on Flare Stack Design. Hydrocarbon Processing, September 1967, p. 139. 

One further point to be considered in any flare stack design is the stack location and height. Hajek and Ludwig recommend use of the following equation developed by a flare stack manufacturer ... 

Flare Systems. There is a good chance that the operating company will not have anyone experienced in flare system design. For feasibility cost estimates, rough estimates can be made by comparison with existing plants or a vendor can be contacted for budget cost estimates for the flare stacks and associated knockout drum, burner tip, igniter, and molecular seal. 

The main flare header should not be designed for critical flow at the entrance to the flare stack, or else noise and vibration will result. 

Flare stack sizing and pressure drop is included with considerations of pressure drop through the safety valve headers, blowdown drums, flare headers, seal drum, etc. Elevated flare tips incorporating various steam injection nozzle configurations are normally sized for a velocity of 120 m/s at maximum flow, as limited by excessive noise and the ability of manufacturers to design tips which will insure flame stability. This velocity is based on the inclusion of steam flow if injected internally, but the steam is not included if added through jets external to the main tip. 

The Seal Drum - A typical flare seal drum for an elevated flare stack is illustrated in Figure 7. A baffle maintains the normal water level, and the vapor inlet is submerged 75 mm to 100 mm. Drum dimensions are designed such that a 3 m slug of water is pressured back into the vertical inlet piping in the event of... 

Preferably, the HjS flare system should consist of a segregated header and separate line routed up the side of a conventional elevated flare stack, sharing the same structure, pilots and igniters. However, the HjS header may be tied into the regular flare seal drum if there are special mechanical design problems associated with the separate stack e.g., in the case of a flare which is to be dismantled for overhaul. Flare elevation must be sufficient to meet atmospheric pollution and ground level concentration requirements for the sulfur dioxide produced. 

API RP 521 (1997) disensses the design of hydranlic flame arresters (liqnid seal dmms) for flares. Fignre 5-5 shows a typical flare stack seal dmm. There are some nncertainties abont the effectiveness of the API... 

The seal tank/pot is not a separator but a physical liquid seal (Figure 7-70) to prevent the possibilities of backflash from the flare from backing into the process manifolds. It is essential for every stack design. 

Figure 7-70. Suggested seal pot/drum for flare stack system. (See API RP-521, Fig. B-1, 3rd Ed., 1990.) Design adapted with permission by this author from API RP-521, 3rd Ed. (1990) American Petroleum Institute [33].

Depending on the relief stream components (with or without noncondensable gases) and quenching efficiency, this arrangement often obviates the need for a subsequent scrubber and/or flare stack. The design of the quencher arm is critical to efficient condensation and avoidance of water hammer. Figure 23-55 is the more conventional passive type quench pool used in the chemical and nuclear industry. 

Velocity seals are more recent developments in air seal design. They use conical baffles to redirect and focus the purge gas flow field just below the flare tip to sweep air from the flare stack. Some velocity seal designs can reduce the purge gas flow rate requirement to about 1/10 of the rate needed without the seal. Also, some velocity seal designs reportedly require only about 25 to 33 percent of the purge gas used in diffusion seals (AICliE-CCPS, 1998). More details about air (purge reduction) seals may be found in API RP 521 (2007). 

Liquid seal drums Emergency vent streams are usually passed through a liquid seal, commonly water, before going to the flare stack. The liquid seal drum is usually located downstream of the knockout drum, and some vendors designs include them in the base of the flare stack. A liquid seal drum is used to maintain a positive pressure in the vent header system and upstream system. It also reduces the possibility of flame flashbacks, caused when air is inadvertently introduced into the flare system and the flame front pulls down into the stack it also acts as a mechanical damper on any explosive shock wave in the flare stack. Figure 23-58 is a schematic of a typical flare stack liquid seal drum, designed per API RP 521 criteria. 

If the plant safety shutdown is not rapid enough and an overpressure situation develops, then the pressure relief system is activated. Pressure vessel design codes such as the ASME Boiler and Pressure Vessel Code require relief devices to be fitted on all pressure vessels (see Section 13.17). If the relief system has been properly designed and maintained, then in the event of an overpressure incident, the plant contents will be vented via relief valves or bursting disks into the relief system, where liquids are recovered for treatment and vapors are sent to flare stacks or discharged to the atmosphere if it is safe to do so. The pressure relief system should allow the plant to be relieved of any source of overpressure before damage to process equipment (leaks, bursting, or explosion) can occur. 

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