Chemical & Process Technology

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.

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.

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Recently I have worked on a gas field development. This field contains very high Carbon Dioxide (~ 40% CO2), reasonably high Hydrogen Sulphide (>120ppm h2S), high mercury level (> 5000 microgram/Nm3 at 25 barg) and high sand production (> 2 kg/h). As compare to other sweet field, it gave a lot of challenges to the project team from technical and commercial perspectives. This post i would focus on what are the concerns and attention to be taken on for a high CO2 gas field development.

SEVERE CORROSION
CO2 present in water saturated gas results a very corrosive environment to the material containing it. Carbon steel normally used in oil and gas expose to ~40% CO2 will experience extremely high corrosion rate and corrosion resistance material such as Stainless steel, duplex stainless steel, etc shall be used. In marine laden environment, duplex stainless steel would be required to avoid Chloride Stress Corrosion Cracking.

FLARE COMBUSTIBLE
CO2 in gas will lower the heating value of sale. It shall be removed in order to increase the value of the heating value and subsequently increase the commercial value. The CO2 gas shall be removed in Acid Gas Removal unit such as Amine absorption system, membrane separation, etc. In this plant, we have adopted the membrane separation system. The CO2 removed from gas will be disposed off via a permeate flare. The lower heating value (LHV) of permeate flare gas shall be maintained at about 200-250 BTU/ft3 (rule of thumb for normal flare tip) for proper flaring.

Apart from normal flaring, a high CO2 gas is expected relief to flare system in the event of emergency depressurization and Opening of Pressure Relief Valve with high CO2 content. Similarly, the Lower heating value (LHV) shall be more than 200-250 BTU/ft3 as rule of thumb for normal flare tip. In some cases, sonic tip is employed to shorten flare boom length and requirement of dedicated flare platform. The lower heating value (LHV) required may be as high as 800-850 BTU/ft3 as rule of thumb.

LOW LHV FUEL
Fuel gas is normal extracted from the sales gas and supply to gas turbine, furnace, flare, etc as fuel. One of the major challenges with low LHV fuel gas would be proper selection of gas turbine. Some gas turbines may not work efficiently in low LHV (high Co2) gas and selection of gas turbine is getting very critical. This especially crucial during black start-up and restart-up as very high CO2 gas (without treatment) will only be available during this period. This may leads to adoption of dual fuel (gas & diesel) gas turbine and high capital investing is required.

DISPERSION ISSUE
High CO2 gas is burnt in flare system. In the event of flare flame-off, CO2 with high molecular weight (44) is heavier than air (29) and very high potential of felt on the platform and result the instantaneous high CO2 concentration which endanger operator from suffocation. Flare boom and tip design shall ensure proper dispersion of CO2 in the event of flare flame-off.

HYDRATE FORMATION TENDENCIES
Gas with high CO2 will have higher tendencies of hydrate formation. Increases of CO2 in gas will increase the hydrate formation temperature at same pressure. Special attention shall be paid at compressor anti-surge recycle valve, air cooler, etc. Hydrate management study shall be conducted to address hydrate formation for any operating scenarios.

CO2 GAS DISPOSAL
Presently not many countries imposed CO2 capture and re-injection. This includes the project I have handled before. With increasing awareness on global warming and greenhouse effect, there are / will be many efforts like researches, seminars, conferences and general protocol i.e. Kyoto protocol to reduce above mentioned environment impacts. One of the efforts would be CO2 re-injection back to the well. Develop a high CO2 field shall take into account of capital investment of new gas injection platform and associate facilities.

Other than 6 major technical concerns that i mentioned above related to high Co2 field development, i believe there are many other issue could be important and worth for discussion. Why not you share yours ?

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