Study on the Influence of Offshore Elevated Flare on the Layout of LNG Terminal

In order to save land, the elevated flare of the LNG terminal is usually considered to be arranged on the sea. The thermal radiation and leakage evaporation diffusion of elevated flare are the key issues to be considered in the plane layout of the wharf. In this paper, the physical process of the flare thermal radiation and the evaporation and diffusion of the terminal sump are analyzed by using engineering knowledge. Combined with the theory of thermal radiation and vapor diffusion and the actual project scenario, the corresponding mathematical model is given and applied to the engineering practice for a large number of calculations. The following conclusions are drawn: (1) The thermal radiation received by the LNG terminal is weakly related to the flare height, that is, the effect of increasing the flare height to control the thermal radiation at the header is not obvious; There is a linear positive correlation with the flare treatment capacity; It is strongly inversely proportional to the distance from the wharf to the flare. It is preferred to reduce the thermal radiation intensity at the wharf by adjusting the distance between the wharf and the flare. When the distance between the wharf and the flare is greater than 100m, the thermal radiation value of the wharf is always less than the specified limit. (2) Within the scope of conventional design, the safe distance between the wharf working platform and the flare is about 20∼90m; The optimal safe distance between the wharf fire control room, bridge approach pipe gallery and the flare should be greater than 61m and 41m respectively. (3) The combustible vapor concentration at the flare is linearly and positively correlated with the flare height, so the flare height should be reduced as much as possible during design. (4) The combustible vapor concentration at the flare is negatively correlated with the distance from the flare to the terminal sump. It is found that the distance should not be less than 180m through calculation. When the distance is more than 300m, the combustible vapor concentration at the flare has little relationship with the flare height and the ambient wind speed. When the distance is more than 340m, the combustible vapor concentration at the flare is always less than the specified limit.


Introduction
The wharf and elevated flare are important components of the LNG terminal project, and their layout location is not only related to the project investment, but also related to the feasibility of the project sea area use and project construction.In order to save the land area, the elevated flare is usually arranged at sea.Elevated flare is an open flame and spark-emitting facility.Its combustion not only brings thermal radiation hazards, but also may ignite flammable gases in the environment and cause explosion accidents.Therefore, the layout of offshore elevated flare and wharf should not only consider the impact of thermal radiation of elevated flare, but also consider the impact of leakage, evaporation and diffusion of LNG wharf.
According to the relevant requirements of the Code for Design of Combustible Gas Emission System in Petrochemical Industry (SH3009-2013), the thermal radiation intensity at the working platform, the fire control room and the pipe gallery of the approach bridge of the LNG terminal, where daily operation and inspection are required, shall not exceed 1.58kW/m2 (thermal radiation limit for outdoor activities of public personnel), 2.33kW/m 2 (thermal radiation limit for administrative areas of industrial and mining enterprises) and 3.20kW/m 2 (thermal radiation limit for production devices) [1].According to the requirements of Code for Fire Protection Design of Petroleum and Natural Gas Engineering (GB50183-2004), the concentration of flammable gas vapor leaked from LNG terminal to the elevated flare shall not exceed the lower explosive limit of natural gas.Therefore, it is necessary to deeply study the thermal radiation of elevated flare and the leakage and diffusion mechanism of LNG terminal, so as to provide theoretical guidance for the layout design of LNG terminal and offshore elevated flare and the planning of sea area use.

Thermal radiation of elevated flare and physical process of evaporation and diffusion of LNG terminal
As a safeguard measure for the pressure relief of LNG storage tanks, the elevated flare is not only characterized by large relief volume and high pressure, but also sudden and uncontrollable.Therefore, three permanent lights with an angle of 120° shall be set at the flare head to deal with the emergency discharge at any time due to the heat leakage, the liquid rollover , and the change of air pressure of the tank.Once the overpressure released vaporized gas (hereinafter referred to as BOG) enters the flare system, it first passes through the knockout drum in front of the flare [2].The liquid droplets in the gas precipitate, and the density of the gas is reduced after being heated by the electric heater in the knockout drum.It floats up to the flare head in the flare barrel, and is ignited by the burning lamp after spraying.Therefore, the combustion of elevated flare is a point source vertical jet flame with power and no phase change.The heat radiation intensity at the LNG terminal is related to the treatment capacity of the flare and the distance from the terminal to the flare.
During the loading and unloading process of LNG terminal, leakage is easy to occur at the ship interface, the valve behind the oil transfer arm and the emergency cut-off valve at the root of the approach bridge.The leaked LNG falls into the lower liquid collecting tray, and then flows into the terminal sump through the liquid guide ditch or liquid collecting pipe.The low temperature detector in the sump will give an audible and visual alarm after detecting the low temperature liquefied natural gas, and interlock to start the high expansion foam fire extinguishing system.LNG in the sump will gradually volatilize under the cover of foam.Volatile natural gas floats up in the atmosphere and diffuses in the air.When the concentration of combustible gas at the flare reaches the explosion limit, it will be ignited by the permanent light and cause an explosion accident.The steam concentration at the flare is related to the opening area of the sump, wind speed, and the distance from the sump to the flare [3].
Therefore, it is necessary to give a mathematical model suitable for the thermal radiation of elevated flare and the evaporation and diffusion of the sump of the wharf, and carry out quantitative calculation and analysis based on theoretical research and engineering practice.

Mathematical model and analysis of thermal radiation calculation of elevated flare
Fire thermal radiation model includes fireball thermal radiation model, pool fire thermal radiation model and jet fire thermal radiation model [4].According to the above analysis, the thermal radiation of elevated flare belongs to point source injection.In the research of jet fire experiment, the famous ones are the small-scale wind tunnel experiment carried out by Kalghatgi et al. [5], the natural convection laboratory-level flame experiment carried out by Hawthorne et al. [6], and the small-scale jet fire experiment carried out by Chamberlain et al. [7].Among the numerical calculation models of jet fire, the common ones are the stratified flame model, CE flame model, Lindstedt flue gas model, DTRM thermal radiation model, and jet fire heat flux model [8].In view of the shortcomings of long experimental research period and poor generalization of the model, this study selects the classical jet heat flux model [9,10] which has been tested in the field of LNG, and establishes the following model in combination with the actual physical scene of the elevated flare at the LNG terminal: In the above formula, q(r) is the heat flux received by the receiver (wharf) at the distance r from the flare, kW/m 2 ; τa is the atmospheric transmission rate; η is the thermal radiation coefficient; Qm is the mass flow rate of fuel (flare treatment capacity), kg/s; ΔHc is combustion heat, kJ/kg; Fp is the angle factor.
τa and Fp in formula 1 can be calculated as follows: τa=2.02×(PwXs) -0.09 (2) In the above formula,   is the partial pressure of water vapor in the atmosphere, Pa;   is the distance from the receptor to the flame surface, m.
can be calculated according to formula 4.   is approximately equal to the distance r from the receptor to the flame center in a large open environment.
Where, RH is the ambient relative humidity, %;   is the ambient temperature, K.
In addition, the elevated flare platform and the LNG terminal platform usually have the same elevation, so the distance r from the receiver to the flame center can be expressed as: Where, r is the distance from the receiver (wharf) to the flame center, m;  ℎ is the flare height, m;  ℎ is the horizontal distance from the flare to the receiver (wharf), m.
Substitute Formula 2 to Formula 5 into Formula 1 to get:

Mathematical model and analysis of evaporation and diffusion of the sump
The famous LNG leakage diffusion experiments at home and abroad include Burro experiment [11], Falcon experiment [12], Maplin Sands experiment [13], Gadila Jettision experiment [14], etc. Due to the large investment, long cycle and high risk of experimental research, large field experiments were no longer carried out by the end of the 20th century.At the same time, many mathematical models have been proposed, including Gaussian model, BM model, box plate model, etc. [15].In this paper, the Gaussian puff model, which is more suitable for the vapor diffusion of low-density natural gas, is selected and combined with the actual situation of the vapor diffusion in the sump, the expression of the vapor concentration at any point (x, y, z) in the downwind direction is given: Where, C (x, y, z) is the gas concentration at (x, y, z), kg/m 3 ; Q is the evaporation capacity in the sump, kg;   、  、  is the diffusion parameter in x, y and z directions, m; H is the effective height of the leakage source, m.
Considering that the location of LNG terminal sump is relatively open and flat, the relationship between the diffusion parameters   、  、  and downwind distance x is as follows: = 0.60 0.75 With the terminal sump as the center point of the coordinate system, and the line between the sump and the flare as the x-axis, let y=0, z=Hh, x=Lch, the steam concentration at the flare head is: Where, ( ℎ ，0， ℎ ) steam concentration at flare head, kg/m 3 ;  ℎ is the flare height, m;  ℎ is the horizontal distance from the terminal sump to the flare, m.
The effective height H of the leakage source is equal to the leakage source elevation plus the steam elevation.The center line point of the constructed coordinate system is located in the liquid collecting tank, so the leakage source elevation is 0. The effective height H of the leakage source is equal to the steam lift height.There are many mathematical models for calculating the steam lift height [16,17].According to the Technical Guidelines for Environmental Impact Assessment (HJ/T2.2-93)and in combination with the actual situation of the project, the following calculation formula is given in this paper: Where,   is the steam discharge rate of the terminal sump, m/s; D is the equivalent diameter of the sump, m;  is the ambient wind speed, m/s.

Numerical calculation results and quantitative analysis of thermal radiation of elevated flare
The thermal radiation model mentioned above is used for the following analysis: (1) Quantitative study of the relationship between thermal radiation at the wharf and its influencing factors provides an improvement direction for the control of thermal radiation intensity in engineering design; (2) In order to meet the requirements of thermal radiation limit of the wharf, the minimum safe distance between each part of the wharf and the flare is calculated quantitatively.The research conclusions with engineering application value are analyzed.In the calculation, the thermal radiation coefficient is 0.18 [18], and the mass flow rate of fuel is considered according to the flare treatment capacity of 90t/h, 100t/h, 110t/h, 120t/h, 130t/h and 140t/h commonly used in LNG terminal.The combustion heat is 4.91×10 4 kJ/kg。 The relative humidity in the sea air is 80%.The ambient temperature is 293K.The flare height is considered as 65m, 70m, 75m, 80m, 85m and 90m commonly used in LNG terminal.The calculation results of the flare thermal radiation numerical model are as follows:  It can be seen from Figure 1 that the thermal radiation intensity at the wharf increases with the increase of the flare treatment capacity.When the distance between the wharf and the flare is more than 100m, regardless of the flare height and treatment capacity, the thermal radiation value at the wharf is less than 1.58kW/m 2 (the thermal radiation limit for outdoor activities of public personnel).Therefore, in the design, when the distance between the wharf and the elevated flare is more than 100m, the thermal radiation effect on the various areas of the wharf may not be considered.
It can be seen from Figure 2 that the thermal radiation intensity at the wharf decreases monotonically with the increase of the flare height, but the monotonic gradient is less, indicating that the flare height is not the main factor affecting the thermal radiation at the wharf.In the design, it is not suitable to reduce the thermal radiation intensity at the wharf by increasing the flare height.
It can be seen from Figure 3 that the thermal radiation intensity at the wharf decreases with the increase of its distance from the flare; And with the increase of the distance between the wharf and the flare, the influence of the flare treatment capacity and the flare height on the thermal radiation intensity of the wharf is smaller, which shows that the distance between the wharf and the flare is the main factor affecting the thermal radiation intensity at the wharf.When considering the layout of the wharf and elevated flare, the thermal radiation intensity at the wharf can be reduced by adjusting the distance between the wharf and the flare.Further, according to the thermal radiation limits of the working platform of the wharf, the fire control room of the wharf, and the pipe gallery of the approach bridge, the safe distances between these three places and the flare are calculated as follows:  From Figure 4 to Figure 6, it can be seen that with the reduction of flare height and the enhancement of flare treatment capacity, the greater the required safety distance between the wharf working platform, fire control room, approach bridge pipe gallery and the flare is.The flare treatment capacity and flare height are within the conventional design range, and the safe distance between the wharf working platform and the flare is about 20~90m.When the distance between the fire control room of the wharf and the approach pipe gallery and the flare are more than 61m and 41m respectively, no matter how the flare treatment capacity and flare height are designed, the heat radiation at these two places will not exceed the standard.

Calculation results and quantitative analysis of evaporation and diffusion in the sump
The evaporation diffusion model mentioned above is used for the following analysis: (1) Quantitative study of the relationship between the steam concentration at the flare and its influencing factors provides an improvement direction for the control of steam concentration in engineering design; (2) In order to meet the requirements of steam concentration limit at the flare, the minimum safe distance the wharf and the flare is calculated quantitatively.The research conclusions with engineering application value are analyzed.
In the calculation, the steam discharge rate of the sump is 1 m/s, the equivalent diameter of the sump is 5 m, the liquid mass in the sump is 2.4 t, and the LNG density is 460 kg/m3.The calculation results of combustible gas concentration at the flare are shown in Figure 7~Figure 8 below.
On the premise that the vapor concentration limit (lower explosion limit) at the flare is known, the above evaporation diffusion model is used to calculate the minimum distance (safety distance) between the flare and the terminal sump.At this time, the model is transformed into an implicit equation about the distance between the flare and the terminal sump.In this paper, Visio Basic software is used to program and the explicit enumeration method is used to solve the problem iteratively.The calculation results are shown in Figure 9 and Figure 10: It can be seen from Figure 7 that the greater the distance between the flare and the terminal sump, the smaller the steam concentration at the flare; The distance between the flare and the sump should not be less than 180m, otherwise the steam concentration will increase significantly and the safety will decrease significantly; When the distance between the flare and the terminal sump is more than 300m, the steam concentration at the flare has little relationship with the flare height and the ambient wind speed.Figure shows that the higher the flare height, the greater the steam concentration at the flare; It shows that after LNG vaporization in the sump, it keeps floating upward, and the vapor cloud is at a higher position.The higher the flare is, the closer it is to the center of the vapor cloud, and the higher the vapor concentration is; In the design, the steam concentration at the flare can be reduced by reducing the flare height.
It can be seen from Figure 9 that the higher the flare height is, the larger the required safety distance (between the flare and the terminal sump) is; There is a linear positive correlation between the two.It can be seen from Figure 10 that the larger the ambient wind speed is, the smaller the safe distance between the flare and the terminal sump is; It shows that the dilution effect of strong wind on steam is greater than the negative effect of expanding the influence range of steam.When the distance between the flare and the terminal sump is more than 340m, the steam concentration at the flare always meets the requirements of the specified limit.Therefore, 340m is the minimum absolute safe distance between the flare and the terminal.

Conclusion
The main conclusions of this study are as follows: (1) The heat radiation received by the wharf is weakly related to the flare height, so it is not appropriate to control the heat radiation at the wharf by increasing the flare height; There is a linear positive correlation with the flare treatment capacity; It is strongly inversely proportional to the distance from the wharf to the flare.It is preferred to reduce the thermal radiation intensity at the wharf by adjusting the distance between the wharf and the flare.When the distance between the wharf and the flare is greater than 100m, the thermal radiation value of the wharf is always less than the specified limit.
(2) Within the scope of conventional design, the safe distance between the working platform of the wharf and the flare is about 20~90m; The best safe distance between the wharf fire control room, approach bridge pipe gallery and the flare should be greater than 61m and 41m respectively.
(3) The concentration of flammable vapor at the flare is linearly and positively correlated with the flare height.The density of natural gas is less than that of air.After leakage, evaporation rises, and the concentration of vapor above the flare gradually increases.The flare height should be reduced as much as possible during design.
(4) The combustible vapor concentration at the flare is negatively related to the distance from the flare to the terminal sump.It is found that the distance should not be less than 180m through calculation.When the distance is more than 300m, the combustible vapor concentration at the flare has little relationship with the flare height and the ambient wind speed.When the distance is more than 340m, the combustible vapor concentration at the flare is always less than the specified limit.

Figure 1 . 2 .
Figure 1.Influence of flare treatment capacity Figure 2. Influence of flare height on thermal radiation at wharf.on thermal radiation at wharf.

Figure 3 .
Figure 3. Influence of horizontal distance between wharf and flare on thermal radiation at wharf.

2 ) 2 )
Flare treatment capacity (t/h)Flare height 70m,Distance from wharf to flare 50m Flare height 80m,Distance from wharf to flare 50m Flare height 70m,Distance from wharf to flare 100m Flare height 80m,Distance from wharf to flare 100m 90t/h,Horizontal distance from wharf to flare 50mFlare treatment capacity 120t/h,Horizontal distance from wharf to flare 50m Flare treatment capacity 90t/h,Horizontal distance from wharf to flare 100m Flare treatment capacity 120t/h,Horizontal distance from wharf to flare 100mHorizontal distance from wharf to flare (m)

Figure 4 .
Figure 4. Calculation results of safe distance between Figure 5. Calculation results of safe distance wharf working platform and flare.betweenterminal fire control room and flare.

Figure 6 .
Figure 6.Calculation results of safe distance between approach bridge pipe gallery and flare.

Figure 7 . 8 .Figure 9 .Figure 10 .
Figure 7. Influence of distance between flare and the Figure 8. Influence of flare height on steam sump on vapor concentration at the flare concentration at the flare

3 )
Distance between flare and sump (m) Flare height 70m，Wind speed 2m/s Flare height 80m，Wind speed 2m/s Flare height 70m，Wind speed 7m/s Flare height 80m，Wind speed 7m/s between flare and sump 150m，Wind speed 2m/s Distance between flare and sump 250m，Wind speed 2m/s Distance between flare and sump 150m，Wind speed 7m/s Distance between flare and sump 250m，Wind speed 7m/s