Tuesday, January 8, 2008
Flare and venting facilities is commonly provided in Oil and gas plant for proper disposal of hydrocarbon gas during plant black start-up, normal start up, operation, emergency relief and plant depressurization. A proper design would have elevated flare header sitting on pipe support and slopping down to flare / vent knock out drum (KOD). No low pocket is expected along the header.
There was a question raised.
Advantages
The advantages of burying flare header are :
- Flare header is normally large in diameter and it needs a large and strengthened pipe support to elevate and maintain proper slopping of header to KOD. It is costly to provide a large support. Burying flare header would save cost on providing pipe support.
- Post hot gas relief to header follow by quick cooling by cold ambient would lead to shrinkage and potential air ingress into the flare header. By burying flare header, soil would act as an insulation layer to minimize heat lost and to minimize the potential of air ingress.
Problems
However, there are many problems associated with buried flare header. One of the problems is creation of low pocket between sources e.g. Pressure relief devices, Blowdown valves, etc and flare stack. Low pocket would promote liquid accumulation and it derives other problems with liquid accumulation.
Flame-Out & Gas Cloud
Whenever a large relief, high velocity vapor may push the liquid in the form of liquid slug towards flare tip. As flare tip is not designed to accommodate liquid slug, it potential put-off the flare and result flame-out condition. It may create gas cloud and potentially lead to severe explosion if it ignited.
“Fire Rain”
Flame-out is expected in the event large liquid slug passing flare tip. On the other hand, there is still possibility of ignition of liquid slug. Liquid on fire came out from flare tip would drop to flare stack support structure and lead to flare stack structure failure prematurely.
Surge and severe vibration
High velocity vapor from relief pushing accumulated liquid may cause severe slugging flow in the flare header. In addition, there is potential of generation of slug knocking at elbow and result severe surging to the flare header. Both will creates severe flare header vibration and potentially lead to header and support failure.
Internal corrosion
Accumulated liquid in flare header would promote internal corrosion and accelerate metal loss.
External corrosion
Normally a buried would be protected by corrosion protection layer. Damage of this protective layer is difficult or non-detectable and potential risk of external corrosion. Internal corrosion couple with external corrosion would even accelerate the corrosion and leak hole is easily formed. It will create hazard in the event gas leak during large relief.
HSE issue
Gas leak will result health, safety and environmental problem.
There was a question raised.
Should a flare header buried in the soil ?
Advantages
The advantages of burying flare header are :
- Flare header is normally large in diameter and it needs a large and strengthened pipe support to elevate and maintain proper slopping of header to KOD. It is costly to provide a large support. Burying flare header would save cost on providing pipe support.
- Post hot gas relief to header follow by quick cooling by cold ambient would lead to shrinkage and potential air ingress into the flare header. By burying flare header, soil would act as an insulation layer to minimize heat lost and to minimize the potential of air ingress.
Problems
However, there are many problems associated with buried flare header. One of the problems is creation of low pocket between sources e.g. Pressure relief devices, Blowdown valves, etc and flare stack. Low pocket would promote liquid accumulation and it derives other problems with liquid accumulation.
Flame-Out & Gas Cloud
Whenever a large relief, high velocity vapor may push the liquid in the form of liquid slug towards flare tip. As flare tip is not designed to accommodate liquid slug, it potential put-off the flare and result flame-out condition. It may create gas cloud and potentially lead to severe explosion if it ignited.
“Fire Rain”
Flame-out is expected in the event large liquid slug passing flare tip. On the other hand, there is still possibility of ignition of liquid slug. Liquid on fire came out from flare tip would drop to flare stack support structure and lead to flare stack structure failure prematurely.
Surge and severe vibration
High velocity vapor from relief pushing accumulated liquid may cause severe slugging flow in the flare header. In addition, there is potential of generation of slug knocking at elbow and result severe surging to the flare header. Both will creates severe flare header vibration and potentially lead to header and support failure.
Internal corrosion
Accumulated liquid in flare header would promote internal corrosion and accelerate metal loss.
External corrosion
Normally a buried would be protected by corrosion protection layer. Damage of this protective layer is difficult or non-detectable and potential risk of external corrosion. Internal corrosion couple with external corrosion would even accelerate the corrosion and leak hole is easily formed. It will create hazard in the event gas leak during large relief.
HSE issue
Gas leak will result health, safety and environmental problem.
Related Post
Labels: Corrosion, Environment, Flare, Safety, Stack, Vent
Wednesday, July 11, 2007
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.
Labels: Environment, Flare, Stack, Vent
