Elevated stacks

CRSTER estimates ground-level concentrations resulting from up to 19 colocated elevated stack emissions. 

Elevated stacks to aid atmospheric dispersion of the vapor releases. 

When material is emitted from a single elevated stack, the resulting ground-level concentration exhibits maxima with respect to both downwind distance and windspeed. Both directly below the stack, where the plume has not yet touched the ground, and far downwind, where the plume has become very dilute, the concentrations approach zero therefore a maximum ground-level concentration occurs at some intermediate distance. Both at very high wind speeds, when the plume is rapidly diluted, and at very low... 

The different population densities in Europe and the USA has also brought about differences in accident emergency plans in the USA, the provision of a complete evacuation of the population within 16 km of the plant in a few hours is adopted, while in Europe the maximum comparable distance is equal to 10 km. It is indeed difficult to assure the evacuation of population centres with tens, hundreds or thousands of inhabitants. Here too, the countries differences in demographic conditions has to be compensated by additional plant features (generally, the use of double containment provided with intermediate filtration systems and the use of elevated stacks). 

Elevated stacks and spill pads available for realistic industrial release scenarios... 

Corrosion Resistant Fiber-Reinforced Plastic (FRP). Fiber glass reinforcement bonded with furfuryl alcohol thermosetting resias provides plastics with unique properties. Excellent resistance to corrosion and heat distortion coupled with low flame spread and low smoke emission are characteristics that make them valuable as laminating resins with fiber glass (75,76). Another valuable property of furan FRP is its strength at elevated temperature. Hand-layup, spray-up, and filament-win ding techniques are employed to produce an array of corrosion-resistant equipment, pipes, tanks, vats, ducts, scmbbers, stacks, and reaction vessels for industrial appHcations throughout the world. 

Bucket elevators, skip hoists, and cranes are used for top feeding of the furnace. Retention and downward flow are controlled by timing of the bottom discharge. Gases are propelled by a blower or by induced draft from a stack or discharge fan. In normal operation, the downward flow of sohds and upward flow of gas are constant with time, maintaining ideal steady-state conditions. 

The Hp also can exist with the pump running or off VVe can represent this value with an O or oval on the vertical line of the below graph. The Hp is added to and. stacked on top of the Hs. Let s say that our system is pumping cold water and requires 50 ft of elevation change and 10 psi of pressure change across the system. Now, our pump not only has to lift the liquid 50 ft, but it must also conquer 23 ft of Hp. Remember that 10 psi is 23.1 ft of Hp ... 

One of the effects of wind speed is to dilute continuously released pollutants at the point of emission. Whether a source is at the surface or elevated, this dilution takes place in the direction of plume transport. Figure 19-2 shows this effect of wind speed for an elevated source with an emission of 6 mass units per second. For a wind speed of 6 m s", there is 1 unit between the vertical parallel planes 1 m apart. When the wind is slowed to 2 m s there are 3 units between those same vertical parallel planes 1 m apart. Note that this dilution by the wind takes place at the point of emission. Because of this, wind speeds used in estimating plume dispersion are generally estimated at stack top. 

Fig. 20-2. Ratio of second highest 24-hr estimated concentrations from the CRSTER model (47) to measured concentrations as a function of the excess of receptor elevation over stack base evaluation A relative to the stack height h. Names with each symbol are power plants. Source From Turner and Irwin (48).

Martin, R. R.. Manning, F. S., and Reed, E. D., Watch for Elevated Dew Points in SO ,-Bearing Stack Gases, Hydrocarbon Processing, June, 1974. 

It is known that the vertical distribution of diffusing particles from an elevated point source is a function of the standard deviation of the vertical wind direction at the release point. The standard deviations of the vertical and horizontal wind directions are related to the standard deviations of particle concentrations in the vertical and horizontal directions within the plume itself. This is equivalent to saying that fluctuations in stack top conditions control the distribution of pollutant in the plume. Furthermore, it is known that the plume pollutant distributions follow a familiar Gaussian diffusion equation. 

In order to obtain results from SCREEN corresponding to the procedures for the simple elevated or flat terrain option, the user should select the full meteorology option, the automated distance array option, and, if applicable for the source, the simple elevated terrain option. The simple elevated terrain option would be used if the terrain rises above the stack base elevation but is less than the height of the physical stack. These, as well as the other options listed in the sidebar, are explained in more detail below. 

For relatively uniform elevated terrain, or as a "first cut" conservative estimate of terrain effects, the user should input the maximum terrain elevation (above stack base) within 50 km of the source, and exercise the automated distance array option out to 50 km. For isolated terrain features a separate calculation... 

The values of u and A0/AZ are based on assumed conditions of stability class F and stack height wind speed of 2.5 m/s for the stable layer above the inversion. The value of hj incorporates the effect of buoyancy induced dispersion on a/, however, elevated terrain effects are igndred. The equation above is solved by iteration, starting from an initial guess of x , = 5,000 m. The maximum ground-level concentration due to inversion break-up fumigation, Xf, is calculated from ... 

Elevated Flares See Flares for a general definition. The elevated flare, by the use of steam injection and effective tip design, operates as a smokeless combustion device. Flaring generally is of low luminosity up to about 20 % of maximum flaring load. Steam injection tends to introduce a source of noise to the operation, and a compromise between smoke elimination and noise is usually necessary. When adequately elevated (by means of a stack) this type of flare displays the best dispersion characteristics for malodorous and toxic combustion products. Visual and noise pollution often creates nuisance problems. Capital and operating costs tend to be high, and an appreciable plant area can be rendered unavailable for plant operations and equipment because of excessive radiant heat. 

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 ... 

Three types of stack for elevated flares are used ... 

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. 

For stack diameters up to 7.5 m the stack height is normally 15 m above grade. The bottom of the stack is elevated to allow air for combustion to enter. The minimum clearance between the bottom of the stack and grade is either 1.8 m or 0.3 D, whichever is greater. 

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