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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)

Applied Chemistry for Engineering Vol.33 No.6 pp.653-660
DOI : https://doi.org/10.14478/ace.2022.1115

Diffusion Range and Pool Formation in the Leakage of Liquid Hydrogen Storage Tank Using CFD Tools

Soohyeon Kim, Minkyung Lee, Junghwan Kim, Jaehun Lee†

Institute of Gas R & D, Korea Gas Safety Corporation, Eumseong 27738, Korea

^† Corresponding Author: Korea Gas Safety Corporation Institute of Gas R & D, Eumseong 27738, Korea Tel: +82-043-750-1450 e-mail: sasimi@kgs.or.kr

Received October 27, 2022 ; Review November 9, 2022 ; Accepted November 10, 2022

Abstract

In liquid hydrogen storage tanks, tank damage or leakage in the surrounding pipes possess a major risk. Since these tanks store huge amounts of the fluid among all the liquid hydrogen process facilities, there is a high risk of leakage-related accidents. Therefore, in this study, we conducted a risk assessment of liquid hydrogen leakage for a grid-type liquid hydrogen storage tank (lattice-type pressure vessel (LPV): 18 m³) that overcame the low space efficiency of the existing pressure vessel shape. Through a commercially developed three-dimensional computational fluid dynamics program, the geometry of the site, where the liquid hydrogen storage tank will be installed, was obtained and simulations of the leakage scenarios for each situation were performed. From the computational flow analysis results, the pool formation behavior in the event of liquid hydrogen leakage was identified, and the resulting damage range was predicted.

Key Words : Liquid hydrogen tank , CFD(Computational fluid dynamics) , Risk analysis , LPV(Lattice-type pressure vessel)

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1. Introduction

Hydrogen has attracted significant attention as a new clean energy source for achieving carbon neutrality. Korea is aiming to reduce greenhouse gas emissions by 40% compared with those in 2018, as per the agreement of the United Nations Climate Change General Assembly in 2021. Therefore, research and development for establishing a hydrogen economy has attracted attention. To adopt hydrogen as an energy source, infrastructure for hydrogen production, storage, transportation, and utilization should be built.

According to the planning report released by Korea Agency for Infrastructure Technology Advancement(KAIA) in 2019, Liquid hydrogen is stored by liquefying gaseous hydrogen at a cryogenic temperature (20 K), which reduces its volume by approximately 800 times. Considering both the cost of the sites in urban areas and storage safety, a liquefied hydrogen-based infrastructure is preferred. Notably, low-pressure storage has the following advantages: 1) it is safer than high-pressure gas storage methods, 2) possesses lower explosion risk than the conventional high-pressure gas hydrogen due to low temperature, and 3) possesses higher purity storage than chemical methods. Moreover, its energy storage density per unit volume and unit weight is the highest among other hydrogen storage methods. Further, it can be used immediately without applying other processes, only with simple vaporization.

Storage tank technology that can withstand cryogenic temperatures is essential to store liquefied hydrogen. Generally, pressure vessels are used for various purposes such as nuclear reactions, pneumatic storage, and liquefied gas storage. As pressure vessels have to withstand the pressure load inside the vessel, cylindrical and circular vessels have been employed. However, existing pressure vessel shapes have low volumetric efficiency. To address this limitation, Senjanović et al. (2005)[1] proposed a tank wherein two parallel cylinders overlap with each other. Further, Regu Ramoo et al. (2011; 2012)[2,3] proposed a cube-shaped pressure vessel wherein 12 identical cylinders were superimposed. These methods aim to increase the volumetric efficiency while maintaining the basic shape of the existing pressure vessel. However, the applications of these methods are limited due to the complexity of the design and the manufacturing process[4]. To overcome these challenges, a grid-type pressure vessel (LPV) has been proposed. Unlike the existing cylindrical pressure vessel that withstands pressure with an outer tank, this vessel is configured to withstand pressure using an internal grid-type support structure. In addition, it has a higher volume ratio than a cylindrical- or spherical-shaped pressure vessel and can be adjusted to fit a given space[5]. The proposed tank with storage of 0.6 m³ will be fabricated in 2019 and used for hydrogen fuel cell trucks.

To build a liquefied hydrogen-based infrastructure, major facilities such as a cryogenic turbo expander, heat exchanger, and cold box must be included in addition to the storage tank. This is because during a hydrogen-liquefaction-plant leakage, a fire breakout or explosion may occur from the ignition source. Moreover, the risk in liquid hydrogen storage tanks can primarily be caused by storage tank leakage or surrounding piping. This leakage can be divided into two cases: 1) liquefied hydrogen damages the material, which subsequently damages the storage tank and piping, and 2) overpressure due to the rapid temperature rise of liquefied hydrogen damages the tank[6].

In this study, we conducted a risk assessment for a grid-type liquid hydrogen storage tank (LPV, 18 m³, 1.2 tons when 90% filled) that overcomes the limitation associated with existing pressure vessel shape (cylindrical and spherical), i.e., low space efficiency. Figure 1 shows the top and side views of a liquid hydrogen storage tank applied for computational flow analysis. A commercialized computational fluid dynamics (CFD) tool was used as the analysis program. The reliability of this program was confirmed by obtaining results similar to the actual experimental data obtained by performing computational flow analysis based on the experimental data of a liquefied hydrogen pool [7]. In addition, a quantitative risk assessment study to improve the safety of hydrogen charging systems by applying the CFD tool is actively conducted in Korea[8]. Therefore, computational flow analysis is performed based on the worst-case scenario that can occur around a liquid hydrogen storage tank. Further, it is intended to understand the maximum diffusion distance and the behavior of the liquefied hydrogen pool in case of hydrogen leakage.

1.1. Liquid hydrogen leak scenario and input variable setting

To select a suitable leak scenario for computational flow analysis, hazard and operability analysis (HAZOP), one of the qualitative risk assessment techniques, was performed; the location of the leak was selected based on this technique. Table 1 lists the HAZOP results for the liquid hydrogen storage tank, while Figure 2 shows the expected leakage area from the HAZOP result. Through qualitative risk assessment, an operating situation was assumed and the causes of liquefied hydrogen leak accidents and their risk results were derived by working parameters such as risk assessment, design, manufacture, and tank-related standards. Leakage from the welding part of the supply pipe and valve joint corresponding to No. 5 in Table 1 showed the highest risk level, which is the product of the frequency and strength of accidents.

Accordingly, assuming that leakage occurred at the first joint of the liquid hydrogen supply pipe, the computational flow analysis was performed on the leaked liquid hydrogen.

The leakage flow rate of liquid hydrogen is expressed as follows:

W = C_{d} S \sqrt{2 ρ Δ P} (kg/s)

(1)

where the main variables are the outflow coefficient (C_d ), cross-sectional area of the opening through which the liquid leaks (S ), liquid density (ρ), and pressure difference between the two ends of the leakage opening (ΔP ). The outflow coefficient (C_d ) was 0.99, considering the cases where reliable evidence could not be presented in the liquefied gas leakage flow rate formula[9]. 100% of the pipe cross-sectional area was applied to the cross-sectional area (S ) through which liquefied hydrogen leaks[10]. In addition, to derive diverse analysis according to the leakage pipe size, 80, 50, and 30% of the pipe cross-sectional area was applied. The density of liquid hydrogen (ρ) was applied with 74.48 kg/m³, at 3 bar 20 K, and was applied with 2 bar considering the internal pressure of 3 bar and the atmospheric pressure of 1 bar. Concerning the leakage time and wind speed, the liquid was set to leak for 600 s and the wind speed was set at 1.5 m/s(at reference height 10 m)[11]. Concerning the radiant heat related to evaporation after the liquid pool formation, 215 W/m² was applied by relating to the average value of the hot season (July-August) among the data from the Korea Meteorological Administration for the southern region, where the liquid hydrogen plant will be installed. The outside temperature was set at 313 K, and the atmospheric stability (Pasquill class) was applied with F, which is an extremely stable state.

Table 2 presents the variable and leakage flow rate calculation values. The scenarios were divided into scenarios A to D based on the leakage cross-sectional area (the leakage flow rate). The leakage flow rate was calculated at 2.782 kg/s when 100% of the pipe cross-sectional area was applied and at 0.250 kg/s when 30% was applied. CFLC is Courant-Friedrich-Levy number based on sound velocity, and CFLV is Courant-Friedrich-Levy number based on fluid flow velocity. For constants required for computational flow analysis, default values of 20 and 2 were applied.

2. Computational Flow Analysis Method

2.1. Pool Analysis Model

The pool model uses a two-dimensional (2D) pool equation to calculate the behavior and describes the diffusion model from the spill area to the ground. The pool model solves the conservation equations for mass, momentum, and enthalpy, and the equations for the leakage height, momentum equation, and gravitational term equation can be expressed as follows[12,13]:

\frac{\partial_{h}}{\partial_{t}} + \frac{\partial h_{u_{i}}}{\partial x_{i}} = \frac{{\dot{m}}_{L} - {\dot{m}}_{V}}{ρ_{l}}

(2)

\frac{\partial_{h_{u_{i}}}}{\partial t_{i}} + u_{j} \frac{\partial h_{u_{i}}}{\partial x_{i}} = F_{g, i} + F_{τ, i}

(3)

F_{g, i} = h g Δ \frac{\partial (h + z)}{\partial x_{i}}

(4)

The height of the ground z is included so that it can leak into the slopped terrain and calculate the impact of obstacles and structures. The shear stress between the pool and the ground is given by the following formula[12,13]:

F_{(τ, i)} = \frac{1}{8} f_{f} u_{i} | u_{i} |

(5)

The friction factors are given as follows depending on the laminar and turbulent flow[12,13].

f_{f, l a m} = \frac{64}{4 R e_{h}}

(6)

f_{f, t u r b} = {- 1.8 log (\frac{1.72}{R e_{h}} + {(\frac{\in_{g}}{12 h})}^{1.11})}^{- 2} if \frac{\in_{g}}{h} < 0.2

(7)

f_{f, t u r b} = {0.125 {(\frac{\in_{g}}{h})}^{1 / 3}} if \frac{\in_{g}}{h} \geq 0.2

(8)

The transport equation for the specific enthalpy of the pool model is given as follows[12,13].

\frac{\partial h θ}{\partial t} + u_{i} \frac{\partial h θ}{\partial x_{i}} = \frac{{\dot{m}}_{L}}{ρ_{l}} (θ_{L} - θ) + \frac{1}{ρ_{l}} ({\dot{q}}_{c} + {\dot{q}}_{r a d} + {\dot{q}}_{g} + {\dot{q}}_{e v a p})

(9)

The pool model has shown considerable reliability through demonstration and verification and is used as a tool for good analytical results in the hydrogen analysis field by many researchers[14,15]. Figure 3(a) shows the equation of the pool model as an algorithm. The leakage height, momentum, and gravity terms affect the shear stress between the ground and the liquid hydrogen pool, and the enthalpy transport equation is decided according to the fluidized bed formation. Figure 3(b) shows the procedure for geometric and scenario selections for confirming the pool results.

2.2. Geometry and Grid Configuration

Figure 4 shows the area where the proposed liquid hydrogen storage tank will be installed. Figure 4(a) indicates the domestic liquid hydrogen plant construction site. Figure 4(b) shows the geometry that serves as the basis of the computational flow analysis using computer-aided scenario design. In this analysis, 229,824 grids were applied, and the ratio of grid size variation was set to be less than 10%. To increase the reliability of the analysis results the grids were arranged tightly at the positions where the liquid hydrogen pool was expected to be formed.

3. Analysis of Liquid Hydrogen Leakage Results

To calculate the maximum risk distance for liquid hydrogen leakage, the maximum distance was calculated based on 100% (4 vol%) of the lower explosive limit concentration of hydrogen. To predict the maximum diffusion distance of hydrogen, the results of the leakage flow over time according to scenario A, which exhibits the largest leakage flow, are presented in Figure 5. Figure 5 shows that a constant flow rate (kg/s) was verified after approximately 100 s of hydrogen leakage.

Moreover, from approximately 100 s to when the leakage slowed down (500 s), the leakage flow results were similar. Based on these results, 500 s was selected as the reference time to check the hydrogen leakage results for a certain time.

3.1. Hydrogen Diffusion Distance Prediction

Table 3 and Figure 6 show the maximum diffusion distance (XY-axis) for each scenario under the same condition (500 s). The analysis results showed that the maximum diffusion distance increased when the leakage flow rate was higher. The result of the computational flow analysis of the hydrogen leakage accident demonstrated that the leakage was the largest in the X-axis direction compared with the Y-axis direction. Therefore, the diffusion distance may seem different depending on both the wind speed and direction even for leakages in the same sites.

To predict the maximum diffusion distance for hydrogen due to the leakage, a 2D computational flow analysis based on the XY-axis for each scenario under the same conditions (500 s) was conducted. Figure 7 shows the results. In the case of scenario A with the largest leakage flow, the range is approximately 14 m and 11 m in the X- and Y-axis directions, respectively. Conversely, in the case of scenario D with the smallest leakage flow forming an explosion range, the range is approximately 10 m and 6 m in the X- and Y-axis directions, respectively.

Figure 8 and 9 shows the 3D computational flow analysis results displaying the equipment and facilities around the liquid hydrogen storage tank for each scenario. By implementing the Flowvis 3D plot tool in the program, the behavior of the actual explosion range in case of leakage in the surrounding area where the liquid hydrogen storage tank will be installed could be identified. Moreover, the effect of the liquid hydrogen leak on surrounding buildings could be comprehended, which could not be identified using a 2D plot. Figure 8 shows the explosion range behavior centered on the location of the liquid hydrogen leakage. The lower and upper explosive limits of hydrogen were at 4 vol% and 75 vol% as standard, respectively. Moreover, the diffusion distance increased as the leakage flow rate increased. In addition, spreading to surrounding buildings such as a cold box and liquid hydrogen supply system was confirmed by the hydrogen leakage result shown in Figure 9.

3.2. Leaked Liquid Hydrogen Pool Prediction

Based on the results of the computational flow analysis, the 2D and 3D plots were used to analyze the behavior of the liquid hydrogen pool, and the temperature at the liquid hydrogen leakage site is shown in Figure 10. The result of the analysis showed that there was no temperature at which liquid hydrogen was formed. Thus, the behavior of direct evaporation and diffusion into the gas phase could be predicted without forming a liquid pool when liquid hydrogen leaks. Figure 11 present the pool evaporation rate (kg/s) for each scenario. Notably, the results obtained when the leakage flow rate for each scenario in Table 2 was compared with the evaporation rate in Figure 11 were similar. This is because all the leaked flow evaporated immediately after the liquid hydrogen leakage. To form a liquid hydrogen pool, the liquid hydrogen must remain on the ground to form a pool even when a certain part is evaporated by the outside temperature (313 K). However, as a result of the analysis, considering the temperature and evaporation rate of the liquid hydrogen pool, the actual liquid hydrogen pool was not formed.

4. Conclusion

This study designed a leakage scenario from the surroundings of a liquid hydrogen tank, and a computational flow analysis result was derived for the hydrogen diffusion distance prediction and liquid pool analysis using a commercialized CFD tool. Among the suggested scenarios, HAZOP was performed for the leakage location assuming that the highest leakage risk occurred in the supply and valve joint pipes. Therefore, computational flow analysis was performed considering factors, such as the actual operating conditions, surrounding buildings, atmospheric conditions, and radiant heat for the site where the liquid hydrogen plant will be installed, and the results were presented. According to the computational flow analysis result, the diffusion distances of the hydrogen explosion range affected up to 14-m (X-axis) and 11-m (Y-axis) points at the highest when the leakage flow rate was maximum under the same condition (500 s). Conversely, the explosion range was formed up to 10 m (X-axis) and 6 m (Y-axis) when the leakage flow rate was minimum. Consequently, the explosion range for each leakage flow rate was considered significant for calculating the safe distance of the liquid hydrogen plant site. In addition, the computational flow analysis was performed to predict the liquid hydrogen pool formation. However, the leakage flow rate and the evaporation rate were similar. Moreover, the temperature at which liquid hydrogen was formed did not exist. Therefore, the entire leakage flow was evaporated immediately after the leakage according to each scenario. In other words, the liquid hydrogen pool was not formed considering the evaporation rate and temperature.

Acknowledgment

This work is supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure, and Transport (Grant 19IHTP-B153609-03).

Figures

Figure 1.Liquid hydrogen storage tank (a) top view, (b) side view.

Figure 2.Liquid hydrogen storage tank (floor plan).

Figure 3.(a) Pool model algorithm, (b) CFD tool model algorithm.

Figure 4.(a) Location of plant, (b) Grid and location of plant.

Figure 5.Mass flow rate of scenario A.

Figure 6.Maximum diffusion distance by scenario (XY axis).

Figure 7.2D Hydrogen diffusion range according to leakage flow rate at 500 s (a) Scenario A, (b) Scenario B, (c) Scenario C, (d) Scenario D.

Figure 8.3D Hydrogen diffusion range according to leakage flow rate at 500 s (1). (a). Scenario A, (b) Scenario B, (c) Scenario C, (d) Scenario D.

Figure 9.3D Hydrogen diffusion range according to leakage flow rate at 500 s (2). (a) Scenario A, (b) Scenario B, (c) Scenario C, (d) Scenario D.

Figure 10.3D hydrogen temperature according to leakage flow rate at 500 s (a) Scenario A, (b) Scenario B, (c) Scenario C, (d) Scenario D.

Figure 11.Pool evaporation rate (kg/s) by scenario.

Tables

Table 1.Part of the Risk Assessment Result about Storage Tank

Table 2.Leakage Scenario of Liquid Hydrogen Tank

Table 3.Maximum Diffusion Distance by Scenario

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