Page 63 - Industrial Plant 2020

enable Ai Group to propose safe and optimized
solutions.

Gas recompression systems

and flare/vent network
The LNG is stored in the tanks in its liquefied form at
-162°C.
In the tank, a small quantity of LNG evaporates
continuously (“Boil Off Gas”).
This evaporation is mainly caused by external heat
input and/or fluctuations in liquid levels. Thus, this
evaporation increases pressure inside the tank.
The gas’ evaporation out of the tank is recovered
through a compression and treatment system called
“BOG system”.
In the event of significant evaporation (or from BOG
system’s failure), the tank’s pressure increases and
emergency channels are provided to evacuate the
overpressure according to the pressure threshold levels
reached. Fig. 2 Impact of the thermal radiation of the flame
Usually, pressure relief systems can be defined on three (with the courtesy of ENTREPOSE)
levels:
• Regular Run Level - Level 1: collection and
treatment of the evaporation gas by the BOG huge impact on the structure (metal or even concrete),
system. generating dramatic effect (serious damage and lethal
• Alert Level - Level 2: collection of gas from the effects start at 3 kW/m²) on staff work nearby.
valves to the flare to be burned (if there is a low- Thus, the time to extinguish the flame represents safety
pressure flare on site). issues Fig. 2.
• Critical Level - Level 3: Release of the valves
directly to the atmosphere. Notwithstanding these serious implications, existing
international standards do not regulate this type of risk
The requirements on the network concern the need to nor recommend any solution and design. Thus,
collect the gases (from the tank’s valves and gas commonly, standards do not take into account the
atmosphere) at the flare/venting network, while avoiding specificities relating to valve vents.
continuous flaring (cf. EN 1473). In general, their sizing recommendations are based
The position of the discharge points (as well as the solely on “pool” LNG fires or three-dimensional fires.
relevant dimensioning) is also mentioned in the different The case of flaring is never mentioned.
standards. Hence, in the event of ignition at the outlet of the valve,
As a last resort (if the event becomes uncontrolled), a flame detector triggers the rapid and massive
exceeding of the pressure at the last protection injection (for about 1 minute) of a large quantity of
threshold (290 mbarg, see in the nearby Fig.1), extinguishing agent into the pipe, which quenched the
evaporation gas is released directly into the atmosphere. flame.
Thus, as recommended by NFPA 59A (Appendix A
§14.8.1.1), if gas must be evacuated intermittently or in The Ai Group company, a UL listed manufacturer of BC
an emergency, the evacuation must be done vertically fire dry chemical for GRANITO™ extinguishing powder,
upwards, at high speed, to ensure safe dispersion. designs and markets a fire extinguishing system by
It is preferable the venting of the relief valves be carried GRANITO™ injection. Although this system has proven
out at separate points and not in a common manifold. its efficiency, the Ai Group is wondering the important
parameters in such extinction that explain its operation,
It is during these accidental conditions of sudden and and the possibilities of optimization, particularly relating
extensive discharge (up to one metric ton of methane to the quantity of powder. This study will fill the lacks
goes through the Pressure Safety Valve per minute) the and improve Ai Group’s expertise on extinguishing with
ignitions may occur at the vent openings. fire extinguishing powders by proposing the most
Flame of forty-meter-long occurs and can lead to a appropriate safe solutions.

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