Flare stacks Purging

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... 

Air (purge reduction) seals Air seals (also called purge reduction or gas seals) are often installed near the top of flare stacks. Others are located near the base of the flare. They are used to prevent air from entering the stack and are often recommended to prevent flashbacks and explosions. They also greatly reduce the amount of purge gas required. There are two basic types of air seals primarily used diffusion (sometimes called molecular or labyrinth seals) and velocity types. 

Velocity seals are more recent developments in air seal design. They use conical baffles to redirect and focus the purge gas flow field just below the flare tip to sweep air from the flare stack. Some velocity seal designs can reduce the purge gas flow rate requirement to about 1/10 of the rate needed without the seal. Also, some velocity seal designs reportedly require only about 25 to 33 percent of the purge gas used in diffusion seals (AICliE-CCPS, 1998). More details about air (purge reduction) seals may be found in API RP 521 (2007). 

In start-up it is desirable to purge the headers with fuel gas to remove the air prior to igniting the pilots at the flare tip. The purge time should be at least V/cfh hrs, where V is the total volume of headers (cubic feet), including knockout drum and flare stack, and cfh is the flow rate of fuel gas (cubic feet per hour). 

Flare stacks and connecting piping emd drums can be kept nonflammable by a small flow of oxygen-free gas adequate to prevent air from backing in. Hydrocarbon gas (fuel gas) and steam are the purge gases most often used because of their low cost and ready availability. Inert gases may be used but are more expensive and normally require special facilities to make them available in reasonable quantities. 

The operating cost of a 2-ft. diameter flare stack, when purged at the minimum rate, is nearly 1,500 a year (about 4 a day) if fuel gas is valued at 30 per million Btu. 

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. 

In conclusion, please do not use both hydrocarbon purges and the flare stack water seal together. One or the other Using both does not enhance plant safety and makes no engineering sense. I tried to explain all this to the refining VP referred to above, but to no avail. 

In the case of heavier-than-air purge gas, there is no buoyancy mechanism causing air entry into the stack, and there is thus no incentive to include a dry seal. Unlike a water seal, a dry seal cannot prevent a flashback from traveling upstream if a combustible mixture has been formed by the entry of air into the safety valve or flare headers. It only protects against internal burning flashback... 

Figure 6-19. Recommended minimum flammable gas purge for tall flare and blowdown stacks.

Reduction in purge rates to API 521 levels at wet and dry gas flare headers and use of existing steam-assisted tips and stack steam for bumback prevention rather than fuel gas. 

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