Numerical Analysis for Performance and the Combustion Reactants of the Crankcase Explosion Relief Valve

Abstract

A crankcase explosion relief valve (CERV) is installed to minimize the effects of explosions occurring in the crankcase of marine engines. According to the regulations of the International Association of Classification Societies (IACS), installation of CERV is mandatory and it must be designed and manufactured based on the engine size. As there are various types of engines, the CERV must be developed accordingly. A high cost is required for the explosion test in the development process, so the test is performed for verification purposes in the final development stage. However, if the developed CERV causes an inappropriate result that is different from the expected performance in the explosion test, it needs to return to the development process so conducting the explosion test in the final stage may be unreasonable. In this study, to predict CERV performance, the chamber and CERV were modeled in 2D, and numerical analysis was performed assuming that the DISC of the CERV was opened by causing an explosion inside the chamber. The results of the numerical analysis were verified by comparing the results with the pressure rising during the explosion. In addition, the reasonable results were obtained for temperature, pressure, and combustion products through comparison to the theoretical calculation results. If the numerical analysis method used in this study is applied, it is expected to be able to predict the performance and reactants at the stage before the explosion test when developing the crankcase explosion relief valve.

1. Introduction

According to D. Woodyard’s book, there were 143 crankcase explosions between 1990 and 2001 [1]. Such an explosion gives fatal damage to ship engines, causing casualties and ship sinking. As a countermeasure against the crankcase explosion accident of ship engines, IACS (International Association of Classification Societies, London, UK) revised IACS UR M9 (Unified Requirements 37 for Machinery installations nr 9; Crankcase explosion relief valves for crankcases of internal combustion engines) in 2005 and made the crankcase explosion relief valve (CERV) mandatory for internal combustion engines with a cylinder diameter of 200 mm or more and a crankcase volume of 0.6 m³ or more [2]. In addition, IACS UR M66 was enacted for the CERV test method and now the test is being performed according to the revised version of Rev.3 in 2008 [3].

To produce CERV, not only the regulations of IACS but also the regulations of engine manufacturers should be satisfied. Therefore, it is manufactured by a limited number of companies. The most famous manufacturers include Hoerbiger in Switzerland and Prosave in Korea [4]. In addition, FTZU (Physical-Technical Testing Institute) in the Czech Republic is an institution that can perform the test according to IACS UR M66. The engine manufacturer MAN-ES [5] and the classification society, etc., also attend the test. After passing the test, the engine manufacturer MAN-ES and the classification society issue certificates. A CERV can be commercially available in the market only when the certificates are available. The commercially available CERVs are manufactured in various sizes as the sizes of marine engines vary, and models are classified according to the diameter of their valves.

Since it is unreasonable to conduct an experiment on an actual engine, the CERV is attached to a cylindrical container and tested. In this experiment, methane was put directly into the vessel to cause an explosion.

Many researchers performed explosion experiments similar to the CERV experiment, from a container with a capacity of 0.005 m³ to a relatively large container such as 0.02 or 0.012 m³ [6,7,8,9]. In the explosion experiment, it is difficult to test several models under the same conditions because the vessel is damaged due to the high pressure generated during an explosion. In addition, numerical analysis is used for analysis since it is difficult to observe the combustion process in the explosion experiment. Wang et al. studied the related rules on time, pressure, temperature, and concentration of combustion products according to the change of size, pipe length, and ignition point of a connected vessel using a numerical analysis for linked vessels [10]. Prodan et al. performed an experiment and numerical analysis on the ignition and flame propagation occurring in Methane-Air Mixtures in a small vessel. Explosion parameters such as minimum ignition energy, maximum pressure rise rate, maximum explosion pressure, and normal combustion rate were analyzed and the results were found to be consistent with the literature data [11]. Maremonti et al. [12] investigated gas explosions in two linked vessels by using numerical simulation. Their numerical results were compared to the experimental data reported by Phylaktou and Andrews [13] and they obtain a good agreement. Deng et al. performed experimental and simulation studies with a methane and carbon dioxide mixture in a 0.02 m³ nearly-spherical tank [14]. Ferrara et al. modeled gas explosions vented through ducts by using a two-dimensional axisymmetric model based on the unsteady Reynolds-averaged Navier–Stokes approach [15]. Di Sarli et al. [16] utilized a validated large-eddy simulation model of unsteady premixed flame propagation to study the vented gas explosions in the presence of obstacles. They investigated the phenomena of various conditions considering Methane–air mixtures with different composition ratios, variously shaped obstacles, and area block ratios. Research using numerical analysis can obtain reliable results, so it is used to analyze various valves. Kang et al. studied a nozzle-flapper servo valve using numerical analysis and experiments and confirmed that simulation using numerical analysis can obtain valid results for flow force and pressure stability [17].

In order to develop the CERV of this study, studies have been conducted previously. To analyze the performance of CERV that can be applied to large ships, explosion tests were performed, and it was designed to reduce the manufacturing cost [18]. In addition, to confirm the validity of the numerical analysis performed under the same conditions as the experiment, the results related to the explosion of methane were analyzed in a closed space [19]. This study attempts to numerically analyze the pressure, temperature, and combustion products generated during explosion by modeling under the same conditions as the experimental conditions of CERV [20]. In a prior study, the author performed an experimental study on ERV-735, the largest size among CERV models, according to IACS UR M66 regulations. To reduce part cost, a new CERV was developed by changing the structure of a flame arrester and the results of the explosion test were verified to satisfy the regulations of IACS and the engine manufacturer MAN-ES. In addition, this study was performed to identify whether it is possible to obtain reasonable results by using a numerical analysis method for the explosion occurring in an enclosed space. Comparing the numerical analysis results for each different methane concentration with the theoretical calculations and results of other prior studies, it was confirmed that valid results can be obtained with a difference of less than 5% [21].

The CERV prevents a flame from escaping to the outside when the explosion occurs and discharges the increased pressure inside the crankcase. Before an experiment, if the performance of pressure relief and temperature leakage, etc., can be estimated using numerical analysis at the manufacturing stage of CERV, it is expected that the time and cost required for the experiment can be saved and the development process can be shortened. In this study, not only pressure and temperature were analyzed to predict the performance of CERV, but also combustion reactants were analyzed to evaluate secondary explosion accidents that may occur due to combustion reactants remaining after the explosion.

2. Methodology

The CERV should be tested in a chamber with a size specified by IACS and the ratio of the free area of a valve to the total volume of a crankcase should be at least 115 cm²/m³. The CERV specifications are shown in

Table 1. Specification of the crankcase explosion relief valve.

Test Condition	Value
Explosion Relief Valve Type	ERV-735
Chamber Size	10 m3
Free area for Explosion Relief Valve	3905 cm2
Open pressure of Explosion Relief Valve	0.05 bar ± 20%
Ignition Energy	Less than 100 J

, i.e., the free area is 3905 cm² and the disk opening pressure is 0.05 bar ± 20%. The chamber size is set to 10 m³ according to the IACS regulations and the ignition source that causes an explosion must be ignited with an energy of 100 J or less.

Figure 1 shows the numerical analysis modeling of a chamber where CERV is installed. Since it is axially symmetrical, the number of grids was reduced by modeling half of it, thereby saving the time required for calculation. The chamber was cylindrical, and the ignition point was opposite the point where the CERV was installed. The pressure was measured at the center of the chamber (P1) and at the neck of the valve (P2).

The gas flow of CERV was analyzed using the commercial CFD (Computational Fluid Dynamics) code, i.e., Ansys Fluent 2020 R2, and Figure 2 shows the grid modeling. The grid dependence was confirmed for more than 130,000 grids and numerical analysis was performed using about 140,000 grids.

Table 2. Numerical Analysis Conditions.

Numerical Analysis Conditions	Value
Node	141,530
Element	140,408
Viscous Model	k-epsilon standard
Species Model	Species Transport
Mixture Properties	Methane-air-2step
Spark ignition Energy	100 J
Dynamic Mesh Pressure	0.05 bar
CH4 Mass Fraction	0.05503
O2 Mass Fraction	0.22018
Initial Temperature	15 °C
Initial Pressure	Atmosphere
Time Step Size	0.00005 s

shows the numerical analysis conditions. The mesh and element are 141,530 and 140,408 each and the k-epsilon standard is used for viscous model. Species transport was selected in the Species Model to configure flame turbulence and the internal combustion material was set to Methane-air-2step in Mixture Properties. In accordance with the IACS regulations, which limit the ignition energy to a maximum of 100 J, the ignition energy was set to 100 J. To realize the disc rising and falling by spring identical to an actual valve movement, the operating pressure was set to 0.05 bar using the Dynamic Mesh method. Considering the theoretical mixing ratio, the mass fraction of CH₄ was set to 0.05503 and O₂ to 0.22018 and the initial temperature was set to 15 °C and the pressure was atmospheric pressure. The conditions for numerical analysis are identical to the experimental conditions.

3. Results

Figure 3 shows the temperature distribution that changes over time due to ignition occurring inside the chamber where methane and air are mixed. The explosion relief valve opens because the pressure inside the chamber rises, but the temperature of the gas raised by the explosion did not escape out of the valve. This is because the flame arrester installed at the outlet of the valve is set to be porous to prevent the flame from releasing to the outside. In conclusion, the flame arrester of CERV minimizes human casualties by preventing flames from escaping to the outside even in the event of an explosion.

Figure 4 shows the pressure distribution over time that changes due to ignition. The crankcase explosion relief valve changes its open state according to the pressure inside the chamber and the pressure inside the chamber rises to its maximum after 0.30 s from the explosion.

Figure 5 shows the change in the CH₄ mass fraction over time after ignition. The explosion occurs and the CH₄ inside the chamber is reduced by the combustion reaction, the valve opens, and some CH₄ escapes to the outside. After 0.50 s, all of the CH₄ inside the chamber was combusted. If the valve opens excessively and a lot of CH₄ escapes to the outside, it is expected to cause an additional explosion.

Figure 6 shows the change of CO₂ mass fraction over time after ignition. As a combustion product, CO₂ is initially not present in the chamber but starts to increase after ignition. After 0.5 s when CH₄ is all combusted, the chamber is filled with CO_2, and a part of the CO₂ escapes to the outside through the open valve.

Figure 7 shows the temperature over time at the two measurement points, and it presents that the temperature rises rapidly at the moment when methane is combusted. The combustion process of methane starts and the temperature rises to 2337 K at P1 and the highest temperature rises to 2636 K. At P2, the combustion process begins, and the temperature rises to 2383 K and its maximum is 2452 K. When calculating the combustion temperature of methane theoretically, the maximum rising temperature was 2823 K [20] and the temperature in this study was lower than the theoretical calculation. The distance between the two points where the temperature was measured was 1.9 m and the time for the temperature to rise after the explosion was 0.16 s for P1 and 0.27 s for P2. Flame speed is calculated by dividing the distance (1.9 m) between the two points by the difference in arrival time (0.27–0.16 s). From this, it was confirmed that the speed of flame propagation was 17.27 m/s.

Figure 8 is a graph showing the mass fraction of combustion reactants and combustion products at the measurement point P1. The mass fraction represents the ratio of substance to the total mass of a mixture. Before the combustion reaction, the mass fraction of CH₄ and O₂ was 0.05503 and 0.22018, respectively, and the products CO₂ and H₂O after the combustion reaction have mass fractions of 0.155 and 0.127, respectively.

Since the combustion reaction has a theoretical mixing ratio of methane, the mass fraction of combustion products can be calculated using the chemical reaction formula and molar mass. Equation (1) is the chemical reaction formula and Table 3 is the molar mass of combustion products. The total amount of produced carbon dioxide, water, and nitrogen is 290.67 kg/mol and it becomes 0.151 and 0.123, respectively, when the value is divided again by the molar mass of water and carbon dioxide. It is almost similar to the mass fraction generated in the explosion analysis.

CH₄ + 2(O₂ + 3.76N₂) → CO₂ + 2H₂O + 7.52N₂

(1)

Table 3. Molar Mass of Combustion products.

Combustion Products	Value
CO2	44.01 kg/mol
H2O	18.01 kg/mol
N2	28.01 kg/mol

Table 3. Molar Mass of Combustion products.

Combustion Products	Value
CO2	44.01 kg/mol
H2O	18.01 kg/mol
N2	28.01 kg/mol

CO₂ + 2H₂O + 7.52N₂ = 44.01 + 18.01 + 7.52 × 28.01 = 290.67 kg/mol

CO₂ = 44.01/290.61 = 0.151

H₂O = 2 × 18.01/290.67 = 0.123

To verify the validity of the analysis results, the pressure obtained from the numerical analysis and the pressure measured experimentally were compared [20]. Figure 9 shows the configuration of the explosion test apparatus identical to the analysis. The pressure vessel was filled with a methane-air mixture of 9.5 vol% methane, and the explosion inside the pressure vessel detonated with ignition energy of 100 J from the opposite side of the CERV, and the pressure at points 1 (P1) and point 2 (P2). was measured.

Figure 10 compares the pressures at point P1 and Figure 11 compares the pressures at point P2. Comparing the maximum pressures at point P1, the numerical analysis result was 1.87 bar and the experiment was 1.75 bar, showing a difference of 6.4%. The time elapsed for the maximum pressure to appear was 0.31 s for the numerical analysis and 0.37 s for the experiment, which was 0.06 s faster with the numerical analysis. When checking the maximum pressure rise rate using the maximum pressure and rise time, the numerical analysis showed 13.0 bar·m/s and the experiment showed 10.2 bar·m/s, i.e., a difference of 2.8 bar·m/s.

At the same time, the maximum pressure at point P1 was higher in the numerical analysis results, and the maximum pressure at point P2 was higher in the experimental results. When comparing the maximum pressure at point P2, the numerical analysis gave 1.66 bar and the experiment gave 1.75 bar, showing a difference of 4.0%. The time for the maximum pressure to appear was 0.27 s in the numerical analysis and 0.36 s in the experiment, which was 0.09 s faster with the numerical analysis. The maximum pressure rise rate was 13.3 bar·m/s in the numerical analysis and 10.4 bar·m/s in the experiment. The difference was 2.9 bar·m/s. The reason for the difference in the maximum pressure rise rate at points P1 and P2 is thought to be that the disc of a crankcase explosion relief valve rises, the pressure is discharged from the flame arrester, and the flow rate vented through the hole is different.

The focus of experiments to develop a crankcase explosion relief is how low the pressure is maintained in the event of an explosion. Therefore, this study identified the validity of numerical analysis results with pressure as a variable. When comparing the results of numerical analysis and experiment based on the maximum pressure inside the chamber, the difference rate occurred within 6.4% and the results of numerical analysis are thought to be reasonable.

4. Conclusions

Numerical analysis was used to analyze the performance and combustion reactants of a crankcase explosion relief valve and the results are summarized as follows:

(1)

The results of numerical analysis for temperature, pressure, CH₄ mass fraction, and combustion products inside the chamber after the explosion met the standards of IACS and the engine manufacturer MAN-ES.

(2)

After the combustion reaction of methane, the temperature was 2337 K and 2383 K at the measurement points P1 and P2, respectively. The maximum temperature inside the chamber was 2636 K. It was lower than the combustion temperature of 2823 K from the theoretical calculation, and the flame speed of 17.27 m/s could be calculated using the combustion temperature of the two points.

(3)

The mass fractions of CO₂ and H₂O, i.e., combustion products, were 0.151 and 0.127, respectively, which were similar to the theoretical calculation using the chemical equation.

(4)

The amount of CH₄ remaining inside the chamber after the explosion was not enough to cause a secondary explosion, so the stability of CERV was verified.

(5)

From the numerical analysis results, the pressures at points P1 and P2 were 1.87 bar and 1.66 bar, with differences of 6.4% and 4.0% compared to the experimental results. Therefore, it is judged that the numerical analysis results have accuracy and reliability compared to the experimental results.

The temperature, combustion reactants, and mass fractions of combustion products were analyzed using numerical analysis for a crankcase explosion relief valve and the results were found similar to the theoretical calculation results. When comparing the results to the experimental results, the experimental pressure was within 5% of the analysis result. If the numerical analysis method used in this study is adopted, it is expected to be used for predicting the performance, combustion reactants, and maximum pressure before the explosion test when developing a crankcase explosion relief valve.