Analysis of hazard area associated with hydrogen gas transmission. pipelines

Transcription

1 Analysis of hazard area associated with hydrogen gas transmission pipelines Young-Do Jo a, Kyo-Shik Park a, Jae Wook Ko b, and Bum Jong Ahn c a Institute of Gas Safety Technology, Korea Gas Safety Corporation, 33-, Daeya-dong, Shihung-shi, Kyunggi-do, 49-7, Korea b Department of Chemical Engineering, Kwangwoon University, Korea c Graduate School of Energy, Korea Polytechnic University, Korea Abstract - Hydrogen is considered to be the most important future energy carrier in many applications, reducing greenhouse gas emissions significantly. To be applicable as energy carrier, the safety issues associated with hydrogen applications need to be investigated and fully understood. The transmission of hydrogen gas via pipeline has been considered as most economical. The rupture of a hydrogen gas transmission pipeline can lead to outcomes that can pose a significant threat to people and property in the immediate vicinity of the failure location. The dominant hazards are thermal radiation from sustained fire and shock pressure from gas cloud explosion. In case of the hydrogen gas transmission pipeline, the fire hazard is the slightly stronger threat. Therefore, in our study we presented a simplified equation to calculate the size of the affected area related to the diameter, the operating pressure and the length of the pipeline when the rupture happens in the hydrogen transmission pipeline. The equation is based on the gas release rate and the heat flux from fire to estimate the hazard area. This is directly proportional to the operating pressure raised to a half power, and to the

2 pipeline diameter. The simplified equation will be a useful tool for safety management of hydrogen gas transmission pipelines.. Introduction The worldwide demand for energy has been steadily on the increase along with economic development and increase of population. Until now, fossil fuel energy plays a significant role but it induces global climate change. Hydrogen, however, is considered to be the most important future energy carrier in many applications, reducing greenhouse gas emissions significantly [Shinnar, 003]. There are a lot of advantages for considering hydrogen as energy carrier. First, hydrogen can be produced from natural gas, biomass, ethanol, clean coal, solar, and wind/nuclear energy. Not only the sources of hydrogen are abundant but also the hydrogen power dramatically reduces greenhouse gas emissions. To be applicable hydrogen as energy carrier, the safety issues associated with hydrogen applications need to be investigated and fully understood. Hydrogen would be distributed for use in a direct heating fuel for energy consumers, in a raw material for various chemical processes, and in a source of energy for the local generation of electricity. The hazards associated with the pipeline carrying hydrogen gas have to be fully understood to maximize the safety of the pipeline. Failure of the pipeline can lead to various outcomes, some of which can pose a significant threat to people and properties in the immediate vicinity of the failure location. The hazardous area based on the full bore rupture of a hydrogen gas pipeline may be important to determine the minimum proximity of neighboring buildings to the pipeline also in view of land use planning.

3 This study proposes a simple, dependable approach to sizing the ground area potentially affected by the failure of hydrogen gas transmission pipelines.. Accident scenarios Hydrogen is a colorless, odorless, highly flammable material. The flammability range is from 4% to 75% by volume with air at atmospheric pressure. Given good mixing, even with a very low energy ignition source, i.e. 0.0mJ [Steven & John, 990], it can be detonated over the range from 0 to 60%. The possibility of a significant flash fire or unconfined vapor cloud explosion resulting from delayed remote ignition is extremely low due to the buoyant nature of hydrogen, which generally precludes the formation of a persistent vapor cloud at ground level. But within a few seconds after the start of the release, a large flammable gas cloud could be formed due to the turbulent mixing between hydrogen and ambient air. This cloud has a potential to explode due to the nature of hydrogen gas. Hydrogen has a very high burning rate compared to any other flammable gases. In the event of rupture, a mushroom-shaped gas cloud would form and then gradually grow in size. It will rise due to the momentum and buoyancy of the discharged gas. This cloud would, however, disperse rapidly, and a quasi-steady gas jet would establish itself. If the released gas ignites immediately with the rupture of a pipeline, it produces a jet fire just after a short-lived fireball. But if the mushroom-shaped gas cloud is ignited by delayed ignition at a near-by source, it makes a devastating gas cloud explosion. Dominant hazards could be, therefore, the overpressure effect of an unconfined vapor cloud explosion by ignition within a few seconds after leak, and the heat effect of thermal radiation from a sustained jet fire that may be preceded by a short-lived fireball or an explosion. 3

4 3. Hazard area analysis The credible worst-case accident is the full bore rupture of a high-pressured pipeline resulting in explosion and fire. In this work, the hazard model is based on the consequence model which consists of three parts; ) an effective release rate model for a hole of a high pressured pipeline, ) a fire model that relates the effective release rate of gas to the heat intensity of the fire, and 3) an unconfined vapor cloud explosion model that relates the maximum gas cloud to the overpressure of the explosion as a function of distance from the leak point. The detailed description and underlying assumption of each model are described in the following section. Thermal effect from jet fire The gas release rate through a hole on the pipeline varies with time. Within seconds of failure, the release rate will have dropped to a fraction of the peak initial value. It will decay even further over time until steady state. The effective release rate associated with the death probability of a person from fire and explosion would depend on the exact time of ignition. The probability of a fatality by the jet fire can be estimated by approximating the transient jet fire as a steady-state fire that is fed by the gas released at the effective rate. The effective rate of hydrogen release ( Q eff, shown in Equation ) from a hole on the pipeline is given as the following equation [Jo et al., 003]: Q eff = A α p P 0 max 0.3, α ( L) / d () 4

5 where α is the dimensionless hole size; p 0, the stagnation pressure at operating conditions; L, the pipe length from the gas supply station to the release point; d, pipeline diameter. From equation, if the dimensionless group, α L d, is greater than about 400, the effective release rate will be constant. It means that if the location of the accident is remote, i.e. several kilometers from a hydrogen gas supply station or a pump station, in the analysis of the hazard area of the fire, the effective hydrogen release rate from the pipeline can be considered as constant. The duration of exposure depends on so many circumstances that it would not be possible in fact to establish any specific rule to evaluate the degree of harm. Rausch recommends a value of 30 seconds as exposure time for the people in an urban area [Rausch et al., 977]. Therefore, the Probit equation (Pr, shown in Equation ) for being killed at a specified location from the jet flame of hydrogen gas can be written as the following equation, with the heat of combustion of hydrogen gas at room temperature, 8 H c =.49 0 J/kg, and the atmospheric transmissivity assumed conservatively as the unity, τ a = [Jo & Ahn, 003]. Pr = ln( Q eff / r ) () where r is the distance from a specified location to the fire; Q eff, the effective rate of hydrogen release. Pressure effect from unconfined gas cloud explosion The hydrogen released into the ambient environment is mixed with surrounding air, 5

6 makes a huge gas cloud with a flammable concentration, and subsequently ignites to produce a vapor cloud explosion. The positive buoyancy and the rapid molecular diffusion of gaseous hydrogen cause any release to quickly mix with the surrounding gases. The buoyant velocity of hydrogen at normal temperature and pressure is. m/s to 9 m/s [NASA, 997]. The flammable volume of hydrogen-air mixture, i.e. where hydrogen volume concentration is between the lower and the upper flammable limit, depends on release rate and release time. It was known that the flammable volume of the mixture is maximized at about 0 seconds after the start of the release due to the turbulent mixing between hydrogen gas and ambient air. If the turbulent mixing did not exist, then the maximum would be at the end of the release period or at the steady-state [Venetsanos et al., 003]. The total mass of released hydrogen gas (M, shown in Equation 3) at 0 seconds after failure is estimated very conservatively by assuming constant release rate without the friction loss of the pipeline. M = 0Q peak (3) where Q peak is peak release rate of hydrogen. If the hydrogen gas cloud is ignited and detonation occurs, the reacting zone is a shock wave and the accompanying blast wave has much greater potential for causing personnel injury or equipment damage. By assuming that the explosion of the hydrogen gas cloud behaves like the explosion of TNT on an equivalent energy basis, the explosion pressure with distance can be simply estimated from characteristics of the blast from the explosion of an equivalent mass of TNT. The equivalent mass of TNT is estimated by using the heat of combustion of hydrogen, the total mass of released hydrogen, the explosion efficiency of a vapor cloud explosion for hydrogen gas, and the 6

7 energy of explosion of TNT. A typical value for the energy of explosion of TNT is 4686 kj/kg. The empirical explosion efficiency of a vapor cloud explosion for hydrogen gas cloud is given as 0.03% in CCPS guideline [AIChE/CCPS, 994]. Therefore, the maximum equivalent mass of TNT ( following equation. m TNT, shown in Equation 4) is estimated by the m TNT = πd α γρ p 0 0 γ + γ + γ (4) where γ is specific heat ratio and ρ 0 is density of hydrogen at operating condition. Range of hazard distance The distance for the structural demolish is much higher than that for % fatalities from the overpressure of the explosion. By considering the death of a person inside a building due to the demolition of the building, the hazard distance of the explosion may be defined as the distance for reaching.3 kpa of the explosion overpressure, which is corresponding to the structure demolish. Therefore, the hazard distance associated with structural damage from unconfined gas cloud explosion can be estimated by using the Equation 5 and the blast characteristics of TNT rexp l =. 58α d p0 (5) If a high consequence distance from thermal radiation is defined as the area within 7

8 which both the extent of property damage and the chances of serious or fatal injury would be expected to be significant, the critical radiation intensity can be setting as 5 kw / m, which corresponds to a wood structure catching fire with about % of fatalities [TNO, 989]. The hazard distance is given from the failure point to the location where heat flux is equal to the threshold value of 5 00]. kw / m [Jo & Ahn, r fire 0.4 max 0.3, d α p0 (6) α ( L) / d = + For a transmission pipelines, i.e. high operation pressure with the large diameter of a pipeline, the hazard distance from the fire is longer than the other events as shown in Figure. Therefore, the worst-case event is the fire for hydrogen gas transmission pipelines. The hazard distance from hydrogen gas transmission pipelines is directly proportional to operating pressure raised to a half power, and to the pipeline diameter. It would be helpful to set a safety guideline specifying how far a building, which is difficult to evacuate, should be located from the high-pressure hydrogen gas transmission pipelines. 4. Conclusions Failure of a hydrogen gas transmission pipeline can lead to outcomes that can pose a serious damage in the immediate vicinity of the failure location. To estimation the hazard area it will be therefore needed to prevent such potential losses. The simplified equation has been derived to predict the inflicted range by fire and an 8

9 unconfined gas cloud explosion usually following the release of hydrogen gas. By comparing the hazard distance with the explosion and the fire, the hazard distance from a jet fire turns out to reach farther when the operating pressure is high enough. It is directly proportional to the operating pressure raised to a half power, and to the pipeline diameter. References AIChE/CCPS, Guidelines for evaluating the characteristics of vapor cloud explosions, flash fire, and BLEVEs, The Center for Chemical Process Safety of American Institute of Chemical Engineers; ISBN X, 994. Young-Do Jo, Kyo-Shik Park and Bum Jong Ahn, Risk Assessment for High-Pressured Natural Gas Pipeline in Urban Area The Sustainable City III, WIT press, UK, p54-547, 004. Jo YD, Ahn BJ, A simple model for the release rate of hazardous gas from a hole on high pressure pipeline, J. of Hazardous Materials 003; 97(-3 ): Young-Do Jo, Bum Jong Ahn, "Analysis of Hazard Area Associated with High Pressure Natural Gas Pipeline", Journal of Loss Prevention in the Process Industries, 5, p79-88, 00. National Aeronautics and Space Administration, Safety Standard for Hydrogen and Hydrogen Systems: Guideline for Hydrogen System Design, Materials Selection, Operations, Storage, and Transportation, Office of Safety and Mission Assurance, Washington, 997: -. Shinnar R, The hydrogen economy, fuel cells, and electric cars, Technology in Society 003; 5:

10 Rausch AH, Eisenberg NA, Lynch CJ., Continuing development of the Vulnerability model(vm), Department of Transportation. United States Coast Guard, Washington D.C., Report No. CG-53-77, Feb., 977. Steven RE, John AM., Safety recommendations for liquid and gas bulk hydrogen systems, Professional Safety 990;35(7):3-38. TNO Green Book, Methods for the determination of possible damage, Chapter, TNO, Rijswijk, The Netherlands, 989. Venetsanos AG, Huld T, Adams P, Bartzis JG., Source, dispersion and combustion modeling of an accidental release of hydrogen in an urban environment, Journal of Hazardous Material 003; A05: -5. Ratio of hazard distance(explosion/fire) bar-m² 50 bar-m² 00 bar-m² Dimensionless hole siae Figure. The ratio of hazard distance of explosion to that of fire from hydrogen pipeline. 0