Stacks blowdown

The plant released 900 tons of substances covered by the TRI—or 2.4 times more than the plant reported in 1989. Blowdown stacks, whose contribution to releases was previously unknown, accounted for 430 tons of TRI releases barge loading accounted for 165 tons, although they are not required to be reported under TRI regulations. 

The refinery s 1989 TRI report showed 370 tons of reportable chemicals released from all sources to all media. Based on measurements and modeling conducted for this project, releases of TRI chemicals were 900 tons, about 2.4 times higher than reported. This difference reflects (1) the identification of blowdown stacks as a significant source (430 tons) whose contribution was previously unknown and unrecognized, (2) the addition of marine loading losses (160 tons) which are not reportable for TRI (Oge, 1988), and (3) lower emissions from the oil/water separator (—90 tons). Emissions from the inactive landfarm, a coker pond, and sewer vents were also identified as new sources. Figure 7 illustrates these changes. 

IK. Convert Blowdown Stacks Replace existing atmospheric blowdown stacks with flares. This reduces untreated hydrocarbon losses to the atmosphere, but creates criteria pollutants. 

Upgrade Blowdown Stacks Engineering complexity Regulatory requirements No emission banking Benzene exposure reduction... 

Hi) Emissions Reduction Banking. As previously discussed, there is no credit for emissions removed from blowdown stacks against future potential emissions that may occur from facility modifications. 

One of the project findings was the need for good emissions data for development of both environmental regulations and facility-specific pollution prevention plans. Since testing of blowdown stack emissions proved very difficult during the... 

Tall blowdown stacks are safe with no more than 6% oxygen 25 feet from the top. This is roughly half the oxygen required for flammable mixtures with hydrocarbons. Hence, flammable conditions would by limited to less than the top 25 feet of stack. 

The graphical method simplifies calculation of purge rates for blowdown stacks. However, it is not directly applicable to stacks of length-to iameter ratio less than 50 or to stacks shorter than 50 feet. With short stacks, the level of safe oxygen concentration should probably be closer to the top of the stack than 25 feet. 

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

The vent should drain to the manhole or box and be free of pockets, which might be subject to clogging or other obstructions. On crude oH units the blow-down stacks will frequently drain to the gas-trap manhole at the battery limit. Under these conditions the blowdown stack is usually equipped with an internal spray nozzle to prevent the emission of condensibles through the stack. The vent from the manhole is sized to handle the larger quantities of vapor and is connected to the blowdown stack. 

In the years prior to the March 2005 incident, eight serious flammable material releases occurred at that ISOM blowdown stack. Most ISOM startups had to cope with high levels in the splitter column. It was reported that neither Amoco nor BP investigated these as near miss upsets [14]. 

Blowdown stacks have been recognized as potentially hazardous for this type of service, and the industry has moved more towards closed relief systems to flare. Opportunities to tie the Splitter relief lines into a flare system were not taken, and the site continued to use F-20 as part of the relief and venting system for the Raffinate Splitter. The use of a flare. system would have reduced the severity of the incident [20]. 

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