A brief discussion on determining the fire duration of natural gas storage tanks and whether a sprinkler system is necessary

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I. Raising of the issue In recent years, with the extensive discovery and exploitation of natural gas fields in our country, the number of cities that use natural gas has been increasing rapidly. As the entry point to urban gas systems, more and more natural gas storage and distribution stations are being built. In actual construction, various opinions often arise regarding fire protection facilities, and the determination of the fire duration in natural gas storage tanks, as well as whether a sprinkler system is necessary, frequently become topics of discussion. These include different interpretations and references to relevant standards and specifications, as well as varying understandings of the properties of natural gas and liquefied petroleum gas. The author offers some views on these two issues merely for the reference of colleagues. II. Explanation and Understanding of Standards and Specifications 1. Excerpts from Relevant Specifications For the construction of urban gas facilities, GB50028-93 \"Code for Design of Urban Gas Systems\" (hereinafter referred to as the \"Gas Code\") should be used as the primary design guideline. 1) Article 1.0.2 of the Gas Code: These specifications apply to the design of new, expanded, or renovated urban gas projects. 　　As for the design of storage and distribution stations, corresponding provisions are given in Section 4 of Chapter 5. The regulations regarding fire protection facilities within such stations are as follows: 2) Article 5.4.3 of the Fire Protection Code: The fire separation distances, fire protection facilities, and fire access roads at storage and distribution stations shall be in accordance with the current standard GBJ16-87, the Code for Fire Protection Design of Buildings. 　　Thus, this leads us to the GBJ16-87 \"Code for Fire Protection Design of Buildings\" (hereinafter referred to as the \"Building Code\"). Upon reviewing all the provisions, this standard does not set specific requirements regarding the fire endurance time for storage tanks used to store natural gas as a particular medium; however, it does establish clear requirements for media belonging to the category of \"combustible gases\". At the same time, specific regulations have been established for the storage tanks of liquefied petroleum gas as a particular medium. 3) Article 8.3.4 of the Building Code: The fire duration for combustible gas storage tanks and open-air stockpiles of coal and coke shall be calculated as 3 hours. 　　For combustible gases, the code does not require the installation of a spray system. 　　There are the following specific requirements for liquefied petroleum gas storage tanks: 4) Article 8.2.7 of the Building Code: Storage areas with a total volume exceeding 50 m3, as well as individual tanks with a volume exceeding 20 m3, must be equipped with fixed sprinkler systems. 　　The fire duration for liquefied petroleum gas shall be calculated as 6.00 hours. 2. Understanding of the above standards 　　Since the penetration rate of natural gas in our country is much lower than that of liquefied petroleum gas, and its large-scale development has only begun in recent years, the relevant standards and regulations are not as clear as those for liquefied petroleum gas. Moreover, there are no separate regulations for the transmission and distribution of natural gas, as is the case with liquefied petroleum gas. Both types of gases are flammable and explosive, the equipment used for handling them is similar, and their target users are essentially the same. During the engineering discussions, arguments arose frequently, even leading the discussions down dead ends. The author believes that: 1) LPG storage tanks are already specified separately in Article 8.2.7 of the Building Code, and the provisions are very clear. Therefore, the other provisions do not apply. Similarly, these two provisions are specifically applicable to liquefied petroleum gas as a particular medium, and should not be applied to other types of media. 2) A combustible gas storage tank refers to a tank in which the medium is a pure gas (usually a compressed gas) that is flammable and explosive, and natural gas is precisely such a medium. Therefore, for Article 8.3.4 of the Building Code, natural gas is applicable. III. Differences between fires and explosions in natural gas storage tanks and liquefied petroleum gas storage tanks 1. Once natural gas leaks, it expands rapidly as a gas in a swift (even instantaneous) manner; once the gas supply is cut off promptly, the entire process is shorter compared to that of liquefied petroleum gas ； Once LPG leaks, a process of conversion from liquid to gas occurs, followed by gradual evaporation and dispersion; even if the gas supply is cut off, this entire process takes a relatively long time. 2. Due to its low density, only 59% that of air, natural gas spreads rapidly upward into the atmosphere once it leaks. After a liquefied petroleum gas leak, due to its higher density than air – 1.5 times that of air – it remains accumulated in low-lying areas on the ground for a long time after transitioning from a liquid to a gas state. As a result, fires caused by liquefied petroleum gas last much longer compared to those caused by natural gas. 3. Since the natural gas storage tank contains a single compressible gas, the pressure changes in the tanks adjacent to the ignited tank due to thermal radiation are not significant. LPG storage tanks are different in this regard: for every 1°C increase in temperature, the pressure rises by about 0.02–0.03 MPa. Moreover, since the leaked LPG remains in the tank area for an extended period of time, a fire there can generate extremely intense radiant heat that can even directly scorch adjacent tanks, easily leading to a sharp rise in pressure in those adjacent tanks, a decrease in their yield strength, and thus a secondary explosion. 4. Liquid liquefied petroleum gas has a high coefficient of thermal expansion; for example, the volume expansion coefficient of propane (in liquid form) in the range of 0°C to 5°C is about 13 times that of water. In a tank filled to its maximum capacity (85% of its volume), a temperature increase of just a few dozen degrees Celsius will cause the liquid to expand until it reaches the top of the tank (commonly known as the \"overflow point\"). At this point, if the temperature continues to rise, the pressure inside the tank will shift from the saturated vapor pressure of the medium itself to the expansion pressure. This expansion pressure increases sharply as the temperature rises further; for every 1°C increase in temperature, the expansion pressure rises by about 3 MPa. A temperature increase of just around 4°C can cause the pressure inside the tank to exceed 12 MPa, leading to the tank yielding and resulting in more serious consequences. 　　Therefore, it is highly necessary to install a spray water system in LPG storage tanks; it will help to reduce temperature and pressure in the event of a fire in the tank area or during hot summer weather. The pressure in natural gas storage tanks is set manually and does not change with temperature; therefore, there is no need for cooling via spraying during operation. In the event of a fire or explosion, it is usually a sudden and intense pure-gas explosion, and at such times, water spraying for cooling has little effect either on the tank in question or on adjacent tanks. IV. Conclusions: 1. Due to the differences in the properties of the substances stored and the circumstances of accidents, the fire protection facilities for natural gas storage tanks and liquefied petroleum gas storage tanks should vary accordingly. 2. It is not necessary to install a spray cooling system for natural gas storage tanks. 3. Natural gas should be classified as a flammable gas, and the fire duration should be considered to be 3 hours; extending this time further is meaningless. 4. Given the widespread use of natural gas in our country, it is necessary to address it separately, just like liquefied petroleum gas, when revising **standards and specifications. To provide clearer guidance for engineering design and construction.