Flare stacks Height

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)... 

Determine flare stack height above ground (grade) Refer to Figure 7-73. Based on the mach velocity of the vapor/gases lea ing the top dp of the flare stack (see Equation 7-76), determine the mach number, e.g., 0.2, then from Figure 7-73 ... 

Calculate the effective flare stack height, H Use the formula... 

Figure 6-3. Circles of 1,500 and 3,000 Btu/hr. sq. ft. heat intensity for one 48-inch flare stack height, 200 feet flare load. 970,000 Ib./hr. molecular weight. 44.

Spacing of elevated flares from process equipment or facility boundary depends on the flare stack height, flare load, and the allowable radiant heat intensity for personnel, public, and equipment. 

Heal release Flame length. L Flare stack diameter, d Flare tip exit vebcily Flame dstortion Factor. U Concentration parameter, CL Thru perameter, dlR Minimum distance. D Flare stack height, H... 

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. 

We shall now provide a second example to illustrate step-by-step calculations. In this example a flare stack is estimated to be 80% efficient in combusting HjS off-gas. The total off-gas through the stack is 400,000 kg/hr, of which 7.0 weight percent is H2S. The physieal stack height is 250 m, the stack diameter is 5.5 m, and the stack emission velocity is 18 m/s. The stack emission temperature is 15°C. The meteorological conditions may be described as a bright sunny day with a mean wind speed of 3 m/s. 

Determine the stack height required to give a heat intensity of 1500 Btu/hr/ft2 at a distance of 410 ft from the base of the flare. The flare diameter is 4 ft, the flare load is 970,000 lb/hr, and the molecular weight of the vapor is 44. 

The supervisor turned on the switch for the exhaust gas scrubber in order to detoxify the MIC, but the scrubber had been closed for a maintenance inspection. The flare stack to detoxify the exhaust gas by combustion had also been removed for a maintenance inspection and was of no use. About 23t of MIC vapor tore out the fracture plate, broke the safety valve, and flew out of the exhaust-gas scrubber to a height of 33m for two hours without being detoxified by the scrubber or the flare stack. After erupting, the vapor crept downwind over the residents, supposedly because of the stabilized atmospheric conditions and because the vapor was heavier than air. 

When personnel and equipment are exposed to flare radiation, chances are they are also being exposed to the radiation from the sun referred to as solar radiation. Flare radiation and solar radiation levels can be cumulative. For example, suppose the flare radiation level at a point of interest in 500 Btu/hr-fh on a cloudy day. On a clear day, however, when the sun is in full view, the radiation level could increase to over 800 Btu/hr-fh. Therefore, when estimating flare radiation, it is important that design engineers take into account the contribution from solar radiation because it can impact flare boom lengths and stack heights. 

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. 

With a heat intensity of 2,000 Btu/hr./sq.ft. (six times the intensity of solar radiation) the pain threshold is 8 seconds. Therefore, to allow a man time to run to safety, he should not be subjected to a heat intensity higher than approximately 1,500 Btu/hr./sq.ft. in the event of a major refinery or chemical plant failure which sends large amounts of combustible gas to the flare stack. A stack of sufficient height can be selected to satisfy this condition. But if this is impractical, adequate protective measures should be taken to ensure his safety. 

Nomographs can be used to solve typical stack height and heat intensity problems. To find the heat intensity q in Btu per hour square foot if y is known, first obtain X from Figure 6-5. Then with the value of X and the values of the flare load (in pounds per hour) and molecular weight, read q in Figure 6-6a. 

Flare stacks are oxygen-free along their fuU height with an operating flare. In the event of flame-out, gas supplied at the minimum purge rate will maintain safe conditions. This purge rate is more than adequate for an acceptable flame. 

The effectiveness of steam purging depends on heat loss from the stack. Thus, weather conditions are a factor. With purge steam in the flare stack, isothermal conditions exist from the bottom flange of the stack upward to an elevation determined by the weather and purge rate. Above this elevation, the stack wall temperature decreases with height. 

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 ... 

Because the flare stack gets hot and grows in height, the cable has to incorporate sag to acconunodate stretching under tension. Figure 3.10 illustrates the general flare scheme with the guy wires. 

A program to estimate the height of flare stack, Flarestack.exe, has been developed for simple and Brzustowski and Sommer approaches. One sketch has been added to the main form to understand the dimensions and parameters. The calculation can be performed by using either SI or English units... 

If more realistic stack parameters can be determined, then the estimate could alternatively be made with the point source option of SCREEN. In doing so, care should be taken to account for the vertical height of the flame in specifying the release height. Figure 12 shows an example for a flare release. 

Flares are an attempt to deliberately burn the flammable safety relief and/or process vents from a plant. The height of the stack is important to the safety of the surroundings and personnel, and the diameter is important to provide sufficient flow velocity to allow the vapors/ gases to leave the top of the stack at sufficient velocities to pro ide good mixing and dilution after ignition at the flare tip by pilot flames. 

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