Factors and Criteria for Vent Stack Design

Vent and flare stack are used dispose waste gas in safe and enviroment friendly manner. Flare stack is probably preferred as compare to vent stack if the release fluid contain HC from environmental point of view e.g. impact to greenhouse effect for 1 mole of C1 is about 20 times of 1 mole of CO2. However, flare stack may be costly.

For a vent stack, at least 3 main criteria shall be met :

i) Toxic / flammable component well dispersed and meeting health and safe level during Normal operation (Nor. Op.) and emergency situation (Emer. Op.). very much subject to components / species in vent gas. Local regulations will probably indicate the limits of various components. In additional, Material Safety Datasheet (MSDS) will probably indicate the limits. PHAST, FRED, etc can be used for dispersion study.

ii) Radiation due to ignition of lightning (Emer. Op.). In the event, vent gas is HC and potentially ignited by lightning, radiation study shall be conducted to ensure radiation at the receptor (human) point is meeting criteria e.g. 6.31 kW/m2. In addition to the radiation level, the temperature of receptor (equipment) shall not damage the equipment. FLARESIM, FRED, etc can be used for radiation study.

iii) Noise level at receptor meeting noise criteria during Normal operation (Nor. Op.) and emergency situation (Emer. Op.). Noise level at receptor (human) is meeting criteria e.g. 115 dBA with background noise. Keeping in mind, some plant owner may impose more stringent requirement i.e. 90 dBA @ receptor (human). FLARESIM, FRED, etc can be used for noise study.

If some spill over control PCV tie to vent header and release continuously into vent stack, fluid exit temperature may be a concern in some environment sensitive area.

API 521 has recommended Mach = 0.5 and this value is probably a good start for vent diameter determination. Vent stack height will subject to above criteria, location of vent stack, plant layout, wind speed, wind direction, etc. Modeling vent stack with inclusion of vent diameter and height using software as mentioned will be conducted in order to ensure meeting all criteria as mentioned earlier.

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