Maximum fill volume in propane, butane and LPG pressurized storage tanks

M. SAFAMIRZAEI, Pasargad Energy Development Co., Tehran, Iran 

Large volumes of flammable hydrocarbons are stored inside different storage tanks in gas, oil and petrochemical plants. In addition to maximizing profitability, the optimum design of process units and storage facilities must consider safety concerns to avoid catastrophic accidents. Due to the vast accumulation of hydrocarbons in the storage areas throughout the industry, even a minor error in storage facility design may lead to major casualties, million-dollar property loss and several days of production interruption.¹ Fire cases are believed to cause more than 85% of major accidents in storage areas. 

Propane, butane and liquified petroleum gas (LPG) produced in oil and gas refineries are usually stored in pressurized storage tanks (i.e., spheres, bullets and spheroids) and/or cryogenic tanks. Cryogenic tanks are used for huge parcels, and pressurized storage is implemented for medium and small storage capacities. Pressurized storage tanks are filled with pressurized liquid (i.e., propane, butane and LPG) that is in equilibrium with the vapor phase inside the storage tank. During the fire case, the stored pressurized liquid is vaporized and relieved to the flare system via a dedicated pressure safety valve (PSV). A PSV on a pressurized storage tank is sized only for vapor flow, and the maximum fill volume of the tank must be calculated so that the expanded liquid (due to temperature rise caused by fire and prior relieving) does not exceed the total tank volume. Usually, some additional margins (2%–5%) are also considered.² 

Typical recommendations. TABLE 1 shows the figures generally recommended for butane and propane pressurized storage tanks. Although the aforementioned recommendations usually result in safe design, each case must be studied based on the specific conditions (i.e., site condition, fluid composition, etc.).

 

BASIC DATA AND EXAMPLES 

As explained above, fluids in pressurized storage tanks are in equilibrium with relevant vapor. TABLE 2 shows the required basic data related to propane, iso-butane and n-butane at equilibrium conditions. 

FIG. 1 is the graphical illustration of the data provided in TABLE 1 for propane. Similar figures can be generated for n-butane and iso-butane. FIG. 1 and TABLE 2 show how temperature increases during the fire case results in higher pressure and lower liquid density (meaning, the specific volume of liquid is increased during the fire case). 

FIG. 1. The saturation pressure and saturated liquid density of propane. 

TABLE 3 was generated by calculating the specific volume of saturated liquid (l/kg). This table can be easily used to evaluate the maximum allowable fill volume. Examples for the pressurized storage of propane are discussed in the following sections. 

Example 1. The lowest recorded temperature in Tehran (Iran) is –15°C, and at this temperature, saturated liquid propane has a specific volume of 1.82 l/kg (TABLE 3). The highest recorded temperature in Tehran is 44°C, which shows the maximum expected working pressure (based on TABLE 2) is approximately 15.1 bar. It is obvious that the PSV should not be opened due to daily temperature changes based on the applicable standard [e.g., Iranian Petroleum Standard (IPS)] design pressure of 17.1 bar being the minimum for pressurized storage tanks of propane.³ When the PSV starts relieving excess pressure at 17.1 bar, propane measures approximately 50°C (TABLE 2). This means that saturated liquid propane has a specific volume of 2.23 l/kg when relieving starts (TABLE 3). It can be concluded that the maximum liquid expansion (due to the fire case in the winter) is estimated to be (Eq. 1):   

(Liquid volume when releiving starts / Liquid volume at lowest temperature) = (2.23 l⁄kg / 1.82 l⁄kg) ≈ 1.22 (1) 

A 22% liquid expansion is the maximum anticipated liquid volume change before relieving. Consequently, the maximum fill volume for this storage tank is 78% (100% – 22% = 78%). 

It should be noted that if a higher design pressure is selected and relieving occurs at the higher pressure and temperature, a lower allowable maximum fill volume will be calculated. The calculation for propane pressurized storage in Tehran shows consistency with the proposed values in TABLE 1.   

Example 2. In Ahvaz (southwest of Iran in the Khuzestan province), the lowest recorded temperature in the winter is –7°C. At this temperature, liquid propane has a saturated specific volume of 1.85 l/kg (TABLE 3). The highest recorded temperature in Ahvaz is 55°C, which shows the maximum expected working pressure (based on TABLE 2) as approximately 19.2 bar. PSVs cannot be opened due to daily temperature changes, and based on the IPS, a design pressure of 21.2 bar should be selected as the minimum for the pressurized propane storage tank in Ahvaz. When the PSV relieves excess pressure at 21.2 bar, the propane is measured at approximately 60ºC (TABLE 2). This means liquid propane has a saturated specific volume of 2.33 l/kg when relieving starts (TABLE 3).⁴ It can be concluded that liquid expansion (due to the fire case in the winter) is estimated to be (Eq. 2): 

(Liquid volume when relieving starts / Liquid volume at lowest temperature) = (2.33 l⁄kg / 1.85 l⁄kg) ≈ 1.26 (2) 

A 26% liquid expansion is the maximum anticipated liquid volume change before relieving. Consequently, the maximum fill volume for this storage tank is 74% (100% – 26% = 74%), which is lower than the minimum reported value in TABLE 1 for the maximum allowable fill volume of the propane pressurized storage tank. The design based on TABLE 1 is an unsafe design for this example.  

Example 3. In Abuja (Nigeria), the lowest recorded temperature in the winter is approximately 15°C, and at this temperature, liquid propane has a saturated specific volume of 1.97 l/kg (TABLE 3). The highest recorded temperature in Abuja is 40°C, which shows the maximum expected working pressure (based on TABLE 2) as approximately 13.7 bar. PSVs cannot be opened due to daily temperature changes, and based on the IPS, a design pressure of 15.7 bar is the minimum for a pressurized propane storage tank in Abuja. When the PSV starts relieving excess pressure at 15.7 bar, the propane is measured at approximately 46°C (TABLE 2). This means liquid propane has a saturated specific volume of 2.19 l/kg when relieving starts (TABLE 3). It can be concluded that liquid expansion (due to the fire case in the winter) is estimated to be (Eq. 3): 

Liquid volume when relieving starts / Liquid volume at lowest temperature = (2.19 l⁄kg / 1.97 l⁄kg) ≈ 1.11 (3) 

An 11% liquid expansion is the maximum anticipated liquid volume change before relieving. Consequently, the maximum fill volume for this storage tank is 89% (100% – 11% = 89%), significantly higher than the maximum reported value in TABLE 1 for the maximum allowable fill volume of the pressurized propane storage tank. Any design based on TABLE 1 is safe for this example, but it is not optimum. 

These examples show that the process engineer must calculate a specific saturated liquid volume at a minimum temperature and compare it with a specific volume at a relieving temperature to safely estimate the maximum allowable fill volume in a pressurized storage tank. Although TABLE 1 and similar data from engineering procedures are useful, they should not be trusted entirely, and performing more detailed calculations is recommended for a safe and optimum design. 

LPG storage. LPG is mostly comprised of propane, iso-butane and n-butane. Ethane (C2) and pentane (C5) may also be present in LPG, but C2 and C5 contents are usually negligible. Eq. 4 calculates the specific volume of LPG: 

V_mix = 1 / (∑i=1ⁿ ϑ_iρ_i)          (4) 

where: 

V_mix: Specific volume of the mixture 

v_i: Volume fraction of the component i 

p_i: Saturated liquid mass density of the component i 

This formula can estimate the specific volume of LPGs with different compositions and the maximum allowable fill volume for pressurized storage, as explained in previous sections. Using the formula for LPG mixtures results in an average relative deviation of 0.6% and a maximum relative deviation of 1.1%. TABLE 4 showcases the specific volume estimation error for the LPG mixtures by Eq. 4.⁵ 

Although Eq. 4 can be easily used, the LPG composition is sometimes unclear or may change during different seasons. For such cases, different LPG compositions must be examined. 

FIG. 2 represents saturated liquid volume at actual temperature compared to saturated liquid volume at 0°C for propane and n-butane. For example, at 30°C, saturated liquid propane has a saturated liquid volume of 109.1% compared to the saturated propane liquid volume at 0°C; and at 30°C, saturated liquid n-butane has a saturated liquid volume of 106% compared to the saturated n-butane liquid volume at 0°C. This means that the LPG specific volume at 30°C is 106%–109% of the LPG specific volume at 0°C. This figure can only be used for rapid estimations. 

FIG. 2. The volume of saturated liquid compared to the saturated liquid volume at 0°C (%). 

Takeaway. Although some recommendations are useful for the maximum allowable fill volume of pressurized storage tanks in engineering procedures, checking the maximum expected expansion of liquid is recommended to avoid unsafe conditions or non-optimal design. 

It should be noted that the worst conditions are the fire cases during the winter (at the lowest temperature) and when the tank is full. For this case, the designer must estimate a specific saturated liquid volume at the relieving and minimum site temperatures to estimate the maximum expected liquid expansion before relieving via a PSV. Process engineers may use the minimum average temperature (instead of the minimum recorded temperature) when sufficient justifications are available.  

LITERATURE CITED 

¹ Changa, J. I. and C. C. Lin, “A study of storage tank accidents,” Journal of Loss Prevention in the Process Industries, Vol. 19, January 2006. 

² Atkinson, G., S. Coldrick, S. Gant and L. Cusco, “Flammable vapor cloud generation from overfilling tanks: Learning the lessons from Buncefield,” Journal of Loss Prevention in the Process Industries, Vol. 35, May 2015. 

³ National Iranian Oil Refining Distribution Co., “Product specification,” 2003. 

⁴ Ministry of Petroleum, “IPS-E-PR-850 (1): Engineering standard for process requirements of vessels and separators,” 2009. 

⁵ Gas Processors Association Europe, “GPSA engineering data book, Section 6: Storage,” 13th Ed., 2013. 

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