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Issue 5, 2002 Guangdong Chemical Industry: Selection of Process Equipment for Liquefied Petroleum Gas Systems in Factories by Wu Hailu (Guangdong Petrochemical Special Equipment Company, Guangzhou, 510034) Abstract: This article provides a brief analysis of the selection issues related to storage tanks or cylinder sets, compressors, vaporizers, pressure regulators, and flow meters in the design of process equipment for liquefied petroleum gas systems in factories. Based on practical operational experience, reasonable selection plans are proposed. Keywords: Liquefied Petroleum Gas (LPG); Equipment; Selection. With societal progress and the rising standard of living, liquefied petroleum gas has been widely used not only in people’s daily lives but also increasingly in industrial production. Enterprises in industries such as lighting, ceramics, electronics, automotive, and building materials have adopted LPG as an energy source with high calorific value, high efficiency, and environmental friendliness, replacing traditional fuels like heavy oil and diesel. Factories that use LP gas are characterized by high gas consumption and multiple points of usage, with varying flow rates and pressures at each point. Based on these characteristics, it is very important to select appropriate process equipment in the design. Below, I will briefly describe it based on my many years of practical design experience. 1 Basic knowledge of LPG: LPG is a type of hydrocarbon that is obtained as a by-product during the extraction and refining of oil. Its main components are: propane (H8), propylene (H6), n-(iso)butane (QH10), n-(iso)butylene, and trans(cis)butylene (QH8), etc. According to China’s product standards, LPG is mainly a mixture of C and other components. It is in a gaseous state at standard conditions; it becomes liquid when the temperature drops below the dew point or when the pressure rises to a certain value. LPG has a high vapor pressure, which increases as the temperature rises; for example, at an outdoor temperature of 20°C, the saturated vapor pressure of propane is () 0.81 MPa ； When the outdoor temperature is 5()°C, the saturated vapor pressure is 1.71 MPa. LPG possesses the characteristics of both gaseous and liquid fuels; it can be liquefied at low pressures and vaporized at room temperature. When it transitions from a gaseous state to a liquid state, its volume is reduced by 250 to 300 times, which makes it easier to transport and store in liquid form. When it is used, it returns to a gaseous state for the user’s use. LPG has a high calorific value, ranging from approximately 20,000 to 27,000 kcal/Nm (that is, 83,736 kJ/Nm3 to 113,043.6 kJ/Nm), which is 14.59 to 19.69 times that of furnace gas with a calorific value of 5,740 kJ/Nm ； 8.07 to 10.89 times the calorific value of water gas (10,380 kJ/Nm3) ； It is 2.02 to 10.38 times the calorific value of oil-based gas (10,890–41,530 kJ/n). It is one of the ideal fuels for industrial and domestic use, highly favored by the public and thus widely utilized. 2 Equipment Selection 2.1 Storage Tanks or Cylinder Banks Generally, what determines whether a factory should use cylinder banks or storage tanks for gas supply is the factory’s daily gas consumption, denoted as Qd. Then Qd = Qh × h, where Qh is the gas consumption per hour, and h is the number of working hours per day. The total LPG storage capacity V should meet at least the amount required for two days of use by the enterprise, that is, V ≥ V (where V = Qr × 2). Referring to Tables 6.6.8 and 6.5.5 of the \"Code for Design of Urban Gas Supply\" GB50028—93 (1998 edition) (hereinafter referred to as the \"Code\"), when V ≤ 4 m, it is more common to use cylinder banks for gas supply; this is because 4 m indicates that the number of cylinders in storage exceeds 32 50-kg liquefied petroleum gas cylinders, and frequent handling of such a large number of cylinders reduces safety levels. But bottle arrays require much less space for gas supply than storage tanks ； The bottle bank system is simpler than the tank system, as it does not require a truck loading/unloading system (compressors, etc.) ； The gas cylinders can be used once they are delivered; it is only necessary to separate them into two groups. When one group is used up, an automatic switching device will switch the system to use the pipes connected to the other group of cylinders, allowing the operator to replace the exhausted cylinders. The bottle bank system offers advantages such as low investment costs, minimal space requirements, a short construction period, and simple operation, which is why many factories wish to adopt it. However, when a large amount of gas is required, the gas demand can only be met by frequently replacing the gas cylinders. Some enterprises have to change the bottles once or even twice a day, which poses risks to safe production in such companies. VIP Information: http://www.cqvip.com 42 – Selection of process equipment for liquefied petroleum gas systems in factories Therefore, when a company has high LPG consumption and the conditions permit, storage in tanks should be used as much as possible. Some large enterprises such as Foshan Kohler Sanitary Ware and Guangzhou Honda Motor do the same. Usually, due to objective factors such as limited factory land, companies mostly opt for buried storage tanks. According to Table 6.3.8 of the current Code, the safety distance requirements for buried storage tanks are 50% less than those for above-ground tanks, thereby significantly saving land required for gasification stations. When designing the total storage capacity V of LPG tanks, in addition to considering the amount of storage required by the enterprise and the actual available space, factors such as the gas supply source, transportation method, and distance must also be taken into account. If there is a limited supply of gas, or if the transportation vehicles have large capacity and long distances to cover, the storage volume should be relatively large; otherwise, it should be smaller. For example, when considering transporting LPG using a 9.5-ton tank truck, V should be set such that while ensuring the complete unloading of the 9.5-ton tank truck, there is enough LPG in another storage tank to keep production unaffected. According to Article 6.7.9 of the Standards, the maximum allowable filling mass G of the storage tank is given by G = 0.9pVh, where p is the density of LPG at 40°C; typically, the density of pure propane, which has the lowest density, is used, so G = 0.422Vh, with Vh being the volume of the storage tank. Therefore, the actual maximum filling mass for a 50m3 tank is 21.1 tons, for a 25m3 tank it is 10.55 tons, and for a 20m3 tank it is 8.44 tons. In accordance with Article 6.4.4 of the Standards (LPG storage tanks at vaporization stations should be no fewer than 2), to unload 9.5 tons of LPG from a tank truck without disrupting production, 2 tanks of 50 m3, 2 tanks of 25 m3, or 3 tanks of 20 m3 are all ideal options. 2.2 Compressors: Since most factories use underground storage tanks for gas supply, according to Article 6.3.20 of the Standards, many LPG pumps are not suitable for loading and unloading from tank trucks. Currently, most factories use compressors for unloading and transferring liquid between storage tanks; this process is a fluid dynamics-thermodynamics process. First, the gas and liquid phases of the storage tank are connected to those of the tank truck. The compressor extracts the gas from the storage tank and pushes it into the tank truck without discarding it, which results in an increase in the density, pressure, and temperature of the liquid inside the tank truck. By utilizing the pressure difference between the tank truck and the LPG storage tank, the liquid LPG in the tank truck is forced into the storage tank. Article 6.3.27 of the Standards provides a formula for calculating the exhaust volume required for a compressor: Lm = a(5–4y)Q(100/r). Where: Lm is the piston exhaust volume of the liquefied petroleum gas compressor (m3/h) ； Volume percentage composition of C2 and C3 in the vapor phase of liquefied petroleum gas at calculated temperature ； QL — Liquid liquefied petroleum gas, unloading capacity (m3/h) ； To calculate the temperature, use the outdoor calculated temperature (K) for winter ventilation ； The a, b, and wide-condition coefficients shall be taken according to the table below. Now, using this formula, we can calculate inversely the unloading capacity Q1 required for a compressor with a speed of 0.95 m/min to unload 9.5 tons (22.4 m³) from a tank truck. Taking the liquefied petroleum gas most commonly used in Guangdong as an example, its liquid-phase weight composition is 30% propane and 70% butane (hereafter referred to as conventional LPG). The calculated temperatures are 5°C for the coldest conditions in Guangdong region and 25°C for average temperatures. Referring to Chapter 1 of the Gas Design Manual (hereinafter referred to as the \"Manual\"), we convert its liquid weight components into gas volume components. The process will not be described in detail; when the result is 5, propane gas accounts for 71.3% of the liquid in a saturated state, and at 25°C it accounts for 69.1%. When the y-value is 5°C, Y=71.3t}^6 ； At 25°C, Y=69.1 t}6. By substituting this into the formula, it can be determined that at 5°C, Q = 23rn3/11; it takes 52 minutes to empty a tank truck weighing 9.5 tons. At 25°C, with a flow rate of 40 m/h, it takes 30 minutes to empty a tank truck holding 9.5 tons. It can be seen that at different temperatures, the same compressor exhibits significant differences in liquid discharge volume and unloading speed. Local areas should adapt measures to local conditions and determine the compressor model based on objective climate and economic factors. Generally, in factories, as long as ventilation is not frequent and it does not affect normal operations, the loading capacity does not need to be very high. Compressors with an exhaust volume of 0.95 m3/rain can meet the general unloading requirements. 2.3 Vaporizer: A vaporizer is used to force the liquid LPG to vaporize, so as to obtain a large flow rate of LPG gas to meet the production requirements. In accordance with Article 6.4.8 of the Standards, the total vaporization capacity of the vaporizers should be 1.5 times the plant’s peak hourly gas consumption; the number of vaporizers should be no less than 2, with at least one of them serving as a backup. There are many types of vaporizers, including those with hot water circulation, steam, and electric heating. Electric heating types are used in areas where electricity is readily available and in enterprises with low gas consumption ； The steam type is used in factories that have steam boilers ； The hot water circulation type is used in factories with a gas consumption of more than 1000 kg/h; in such cases, a hot water circulation type vaporizer is more economical, as its operating costs are much lower than those of electric heating types. The gasifier manufacturers have provided the amount of gas that can be produced by the gasifiers, so there is no need to calculate their heat transfer area or anything similar. In addition to its structure needing to comply with standards, it is also necessary to take into account the decline in heat transfer efficiency of the heat exchangers in the gasifier due to fouling that occurs over time as a result of prolonged use. It should be noted that at different ambient temperatures, the minimum operating temperature required for the vaporization of LPG components varies. Under normal circumstances, the temperatures provided by steam and electric heating are well above this minimum temperature; however, when hot water is used as a heat transfer medium, it is necessary to ensure that its temperature meets the requirements for the operation of the vaporizer. Furthermore, to prevent the formation of hydrates in the pipes, the transportation temperature should be 5 degrees higher than the dew point at the transportation pressure. Thus, the operating temperature of the vaporizer should equal T + 5, where T is the dew point at the LPG transportation pressure. Based on standard LPC calculations, the liquid-phase molecular components are 36% propane and 64% butane. At an ambient temperature of 25°C, in accordance with Manual formula ll-139: P = ΣXviP / 100, where P is the total vapor pressure of the mixed liquid (MPa) ； x —— Percentage of any liquid phase molecular component in the mixed liquid ； P —— the saturated vapor pressure of any component in the mixed liquid (MPa). From table 11-16, the P value for propane is 0.95 MPa (absolute), while that for butane is 0.240 MPa (absolute). Substituting into the formula gives P = 0.496 MPa (absolute). From the dew point chart, it can be seen that for common LPG, the dew point T at 0.5 MPa (absolute pressure) is 30°C; therefore, the operating temperature of the vaporizer is T = T + 5 = 35°C. The temperature difference with the atmosphere is △T = 35 – 25 = 10°C. To meet the requirements for vaporization and temperature increase, it is generally assumed that, with the circulating hot water flow rate remaining constant, the hot water temperature also decreases by 10~i ； . Therefore, the temperature of the hot water entering the vaporizer must be at least 45°C or higher (the temperature difference between the inlet and outlet of the hot water in the vaporizer is 45–35 = 10°C) to meet the requirements. Similarly, if calculated under the condition of 40V at the highest temperature, the operating temperature rr of the carburetor equals 55. AT=55— 40=15℃. The water temperature at the hot water inlet of the vaporizer must be at least 70°C or higher (70–55=15°C). If the temperature difference (P2/P1) kD at the hot water inlet and outlet of the vaporizer is 44, then the process equipment for industrial liquefied petroleum gas systems operates in a subcritical state. Using the relevant formula, Q = 96.3 Nin’/h. It can be seen from this that there is a significant difference between flow rates in supercritical and subcritical states. When making a selection, special attention should be paid to the following two factors: a) the pressure difference at the inlet and outlet. A low pressure difference results in a low flow rate, while a high pressure difference leads to a high flow rate ； b. Different components result in different flow rates, with pure butane having the lowest flow rate. 2.5 Flow Meters: The gas consumption of industrial equipment is much higher than that of ordinary residential users. For internal cost accounting, factories often require flow meters to be installed before such equipment. When making a selection, the following three points should be taken into account: a) Since the LPG components used in some factories are not entirely fixed, a volumetric flow meter should be used; the measured volumetric flow rate can be converted into a mass flow rate more accurately based on the different components ； b. Since the gas consumption of each device during peak hours can differ significantly from its minimum consumption, when selecting a flow meter, its measurement range must be able to cover the peak gas consumption, accurately reflect the normal levels of consumption, and also enable detection of the minimum flow rate through the device, in order to provide the most accurate representation of the actual conditions ； C. There are various types of flow meters, including diaphragm meters, turbine meters, and gear meters. The flow rates specified in their product samples are mostly those under operating conditions, and they are independent of the medium ； For certain types of gases, the standard cubic flow rate associated with a certain force F is applicable; however, since different gases have different flow rates, it is necessary to convert this value into the actual gas flow rate required ； Similarly to the principle of pressure relief valves, I usually use pure butane as the flow standard. Conclusion: As highly dangerous flammable and explosive substances, liquefied oil and gas require strict selection criteria for their storage and transportation equipment. In process selection, not only extensive experience is required, but thorough calculations are also necessary to meet the requirements of safety and reliability. In addition, there are many special valves and equipment in the LI}G process pipelines used at _Ding Plant; the selection of these items comes with certain requirements, which will not be detailed one by one here. Reference 1 ( ； B50028 93, Code for Design of Town Gas, 1998; Gas Installation Manual, China Architecture & Building Press, 1986 (received in September 2002). About the author: Wu Hailu, male, graduated in 1982 from Jiangsu Petroleum and Chemical Institute with a degree in chemical engineering machinery. He then worked at the design department of Guangdong Petrochemical Special Equipment Company, where he was involved in the design of process pipelines for liquefied petroleum gas and oil depots, holding the position of engineer and accumulating considerable experience in design work. (Continuing from page 53) Guangdong Petrochemicals lowers PS export prices. On the 26th, Guangdong Petrochemicals adjusted the export prices of PS again; the price for GPPS was set at 7,800 yuan per ton, a decrease of 200 yuan per ton, while the export price for HIPS was 8,000 yuan per ton, a decrease of 100 yuan per ton. It is understood that Guangdong Petrochemicals is still not producing PPS, and the production volume of HIPS is also low. Guangdong Province has banned the use of phosphorus-containing laundry detergents. According to relevant authorities, as of October 30, 2003, Guangdong completely prohibited the production, sale, and use of any products containing phosphorus in detergents. Currently, the Provincial Bureau of Technical Supervision, the Environmental Protection Bureau, the Administration for Industry and Commerce, and the Economic and Trade Commission are working on formulating specific implementation measures. Testing data shows that, thanks to years of efforts to improve the urban environment, the discharge of industrial wastewater and other major pollutants in Guangzhou’s urban areas has shown a downward trend. However, domestic sewage continues to increase, becoming a significant source of pollution and undermining the effectiveness of efforts to treat industrial wastewater. At present, the infrastructure for treating domestic sewage in Guangzhou is inadequate; 19 rivers in the urban area have essentially become channels for urban sewage. The total phosphorus level in the Guangzhou section of the Pearl River exceeds the allowed limit by 14 times. A survey team composed of members of the political consultative conferences from Guangdong Province and Guangzhou City found that phosphorus-containing detergents are the main cause of pollution in Guangzhou. The team recommended that Guangzhou ban the production and sale of such detergents in order to prevent water source contamination. At the end of August this year, the Guangdong Provincial Environmental Protection Bureau issued clear regulations: in order to reduce the damage caused by phosphorus pollution to drinking water sources and the water quality in coastal areas, and to prevent eutrophication of waters in rivers and coastal zones, starting from October 30, 2003, the production, sale, and use of phosphorus-containing detergents were prohibited throughout the province. The main products of phosphorus-containing detergents are synthetic laundry powders. Compared with phosphate-free detergents, phosphorus-containing detergents have a lower production cost and better cleaning performance, which gives them a large market share in China. VIP Information: http://www.cqvip.com