Flare stacks

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. 

Venting to a knock-out vessel, to remove non-gaseous substances. (This may be followed by a scrubbing unit for gases or a flare-stack.)... 

SIMPLE TERRAIN INPUTS SOURCE TYPE EMISSION RATE (G/S) FLARE STACK HEIGHT (H) TOT HEAT RLS (CAL/S) RECEPTOR HEIGHT (M) UR6AN/RURAL OPTION EFF RELEASE HEIGHT (H) BUILDING HEIGHT (M)... 

With the flare tip and flare seal pressure drop and flare elevation fixed, the flare stack, headers and laterials are sized for the largest release, while not exceeding the maximum allowable operating pressure on the associated blowdown drums and water disengaging drums. These maximum allowable operating pressures are in turn determined by ... 

The purpose of a blowdown drum is to disengage closed safety valve releases and various drainage, blowdown and diverted materials into liquid and vapor streams which can be safely disposed of to appropriate storage and flaring facilities, respectively. Entrainment of liquid hydrocarbons into a flare stack is not acceptable, since the potential exists for burning liquid falling onto the ground or adjacent facilities. For this reason, a blowdown drum is required. 

It is important to note that even if the blowdown is effective in disengaging liquid and vapor, further condensation could occur downstream especially if the vented vapor exits the drum at a temperature above ambient conditions. A proportion of such condensible materials in the blowdown drum vapor release may condense as a result of cooling in the flare header and contact with seal water, and then disengage in the flare seal drum while condensible vapors which are not condensed out at this stage may condense in the flare stack or its inlet line, thus creating the potential for hazardous fallout of burning liquid from the flare. Condensed hydrocarbon in the seal drum can be entrained out with the... 

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. 

Any source of ignitable hydrocarbons, such as separators or floating roof tanks, should be at least 60 m from the base of the flare stack. 

Flares should be located to limit the maximum ground level, heat density to 1.6 kW/m at any property line. The minimum distance from the base of the flare stack to the property line should be 60 m. 

Clearance from structures higher than the flare stack 60 m. In addition, no structure where personnel access may be required while the multijet flare is in operation shall exceed in height of a projected diagonal line from the base of the flare stack to the top of the stack wall diametrically opposite. 

Clearance from structures lower than the flare stack 45 m. 

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... 

A secondary seal loop is provided for water withdrawal during major blows when turbulence at the downstream overflow connection to the primary seal loop interferes with normal drainage. Extending the base of the flare stack 3 diameters below the sloped inlet line provides vapor disengaging for the secondary seal leg. The bottom of the stack and inlet line up to 1.5 m above the seal water level are gunite lined for corrosion protection. 

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. 

Flare systems must be protected against any possibility of partial or complete blockage by ice, hydrates, solidification, etc. Seal Drums and Y-seals requiring winterizing should be provided with temperature-controlled steam injection to maintain the seal water temperature at 4 to 10 C. This limits the quantity of water vapor entering the flare stack. 

When winterizing is required, the steam tracing and insulation should include the first 7.5 m of the flare stack above the vapor inlet and also in the case of a drum seal, the vapor line from the seal drum to the flare. Where steam injection to one of the smokeless tips, as shown in Figures 11 B and C is used, then the steam ring should remain outside the top of the flare tip (i.e., not internal). Where severe ambient conditions are encountered then it is recommended that the entire seal drum and flare be insulated in addition to steam tracing and open steam injection at base of flare. 

The following calculation procedure is used to predict the heat flux K incident on a surface normal to the direction of radiation, at any distance from the flare stack and any elevation above grade. The information required for this procedure is as follows ... 

Xj = Downwind distance from flare stack to flame center, m... 

If provided, thermocouples shall be protected by inconel sheathing and shall terminate in a steel or cast iron juntion box located four (4) feet above grade at the base of the flare stack. 

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

Termination points for the flare stack shall be the flare / juction boxes at flare bottom (ground level) and on the ignition / control board. 

All other items which may be required for operation of the flare stacks shall be included into the supplier s scope. 

Relieving vapors from various pressure-relief and depressuring valves in the system must be collected in individual flare headers that should be appropriately located near each process area. Subheaders must be interconnected to a main flare header which feeds to a knock- out drum and disposal system. Condensates that are carried over by vapors are separated in the knock-out drum. The vapors that exit the vessel go to the flare stack where they are burned. 

Pressure levels at the flare header depend largely on the type and nature of pressure relieving devices that have been specified to protect upstream equipment, as well as the pressure levels of all equipment connected directly to the flare stack. 

The equivalent length of the main flare header is then calculated from the flare stack to the last safety valve, taking into consideration the straight length of the pipe and approximate equivalent lengths for bends, etc. If the achial location of the flare stack is not known by that time, it maybe assumed to be 500 ft from the last piece of equipment. Later on, even if it varies from 500 ft, it will not affect the pressure drop calculation at all compared with the entire length of the pipe. 

A second trial is required for two main flare headers, one collecting the low-pressure flares (usually 5 to 10 psig) and the other collecting relatively high-pressure flares (usually 15 to 20 psig). The two headers are connected to their individual knock-out drums. The vapor lines from the knock-out drums are combined into a single header connected to the flare stack. 

The subheaders in each process area similarly will have two levels of flare headers. The line sizing of each level of subheader in an individual area will depend upon the maximum simultaneous flow in that particular area. Thus the line sizing criterion of a subheader may be the largest single flow due to a blocked outlet condition. This flow may not necessarily be the controlling load for the flare stack. 

Of the three, corrosive vapors are usually piped up in a separate header right up to the flare stack since such lines are normally very small and if combined with other streams may run the risk of corroding the much larger and more expensive pipelines. 

---