Flare stacks Sizing

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. 

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 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. 

Figure 7-73. Dimensional references for sizing a flare stack. Reprinted by permission, American Petroleum Institute, API RP-521, Guide for Pressure Relieving and Depressuring Systems, 3rd Ed., Nov. 1990 [33].

Effects of performance changes, 201-203 Head curve for single pump, 198 Relations between head, horsepower, capacity and speed, 200 Temperature rise 207-209 Viscosity corrections, 203-207 Purging, flare stack systems, 535 Reciprocating pumps, 215—219 Flow patterns, 219 Specification form, 219 Relief areas, 437 External fires, 451, 453 Sizing, 434, 436... 

Sprays and Mists Medium sized releases of moderate frequencies that are mixed immediately in to air upon release. Typically pipe gasket, pump seal and valve stem failures under high pressure. On occasion releases occur from flare stacks. 

Although, it may be impractical to keep the flow in high pressure subheaders below sonic, Mak [12] suggests that the main flare header should not be sized for critical flow at the outlet of the flare stack. This would obviate the undesirable noise and vibration resulting from sonic flow. Crocker s [13] equation for critical pressure can be expressed as ... 

It is necessary to size a flare stack for the largest probable simultaneous discharge from several safety valves as listed in Table 6-1. 

Experiments show that flame blowout occurs when vapor exit velocities are as high as 20 to 30% of the sonic velocity of the stack vapors. These results were obtained with small diameter pipes up to 0.152 inches. There is evidence that higher blowout velocities are attainable with pipes of larger diameter such as flare stacks but in the absence of data on blowout velocities for flare stacks, it is good practice to size flare stacks on a basis of 20% of the sonic velocity as the exit velocity. 

The flare header, which collects the vapors from the safety valves for safe discharge to the knockout drum and the flare stack, is sized for the largest vapor load caused by a single failure. This vapor load is obtained from a tabulation of relief loads from safety valves connected to the flare system. The loads which may occur simultaneously as a result of fire, cooling water failure, etc., are summed up. From these summations the largest load is determined. 

Replace flare stack with a bigger diameter or taller stack, increase size of relief header install a parallel relief header... 

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 ... 

Many emergency relief vents are therefore connected to a disposal system which may include knock-out dmms. quench tanks, flare stacks and scrubbers. The size and capacity of such systems needs to be checked once the relief area has been calculated, particularly where a new reaction is being sited in an existing reactor unit. In addition, consideration has to be given to the possibility of the reaction continuing in the disposal system. 

Sizing of a flare stack simple approach Calculation of stack diameter... 

Sizing of flare stack Brzustowski and Sommer approach [20] Calculation of flare stack diameter... 

A third emergency release is provided in the center of the stack, bypassing the multijet burners. The water seal to this release will blow at flaring rates higher than the design capacity of the flare. When the over-capacity seal has been blown, the flare is both luminous and smoky. But the unit is usually sized so that an overcapacity blow would be a rare occurrence. The over-capacity line may also discharge to an elevated flare rather than to the center of the multijet stack. 

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