Flare stack estimation

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. 

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. 

W = required vapor capacity in pounds per hour, or any flow rate in pounds per hour, vapor relief rate to flare stack, Ibs/hr W(. = charge weight of explosive, lb Wj. = effective charge weight, pounds of TNT for estimating surface burst effects in free air W, = required steam capacity flow or rate in pounds per hour, or other flow rate, Ib/hr Whe = hydrocarbon to be flared, Ibs/hr Wtnt equivalent charge weight of TNT, lb Wl = liquid flow rate, gal per min (gpm)... 

A drop in pressure was noticed in the control room and also at a pipeline pumping station. An 8-inch (20-cm) pipe between a sphere and a series of horizontal bullets had ruptured. Unfortunately, the operators could not identify the cause of the pressure drop. The release of LPG continued for —5-10 minutes when the gas cloud, estimated at 656 ft x 492 ft x 6.6 ft (200 m x 150 m x 2 m) high, drifted to a flare stack. It ignited, causing violent ground shock. 

Branan (2005) has given a simplified method for quick estimation of manifold pressure drops. Flare knockout drum sizing needs to be checked for proper vapour-liquid separation and hence prevention of carryover of liquid to the flare stack. Steam injection to the flare stack also needs to be reviewed to prevent potential black smoke and noise. Relief headers ... 

Flare stack diameter depends on the Mach number and is estimated by using the following equation ... 

A different approach is used to estimate the vertical and horizontal distances of the flame center from the top of the flare stack. 

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... 

These effective stack parameters are somewhat arbitrary, but the resulting buoyancy flux estimate is expected to give reasonable final plume rise estimates for flares. However, since building downwash estimates depend on transitional momentum plume rise and transitional buoyant plume rise calculations, the selection of effective stack parameters could influence the estimates. Therefore, building downwash estimates should be used with extra caution for flare releases. 

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. 

The use of the methods of Briggs to estimate plume rise are relied on in the SCREEN model. Stack tip downwash is estimated following Briggs (1973, p.4) for all sources except those employing the Schulman-Scire downwash algorithm. Buoyancy flux for non-flare point sources is calculated from ... 

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. 

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