A. Tamimi
Jordan University of Science & Technology
Irbid, Jordan
Explosive conditions in tanks that store crude oil and light petroleum distillates have been assessed in order to promote safe product storage.
Safe storage can be achieved by knowing the explosion limits for light distillates and crude oil, and understanding how those limits can be exceeded.
Light petroleum distillates include: LPG, gasoline, light and heavy naphtha, kerosine, and light gas oil (LGO).
Usually, LPG is stored under pressure in cylindrical or spherical tanks where the whole volume of the tank is full of LPG. There is no space above the product surface in the tank, and therefore, explosion and fire hazards caused by vapor space flammability are minimized.
Other light distillates, however, are stored in atmospheric floating-roof and domed tanks. It is not practical to fill these tanks completely, therefore, a space is left above the surface of the stored product. This space provides the potential for flammable mixtures and the possibility of explosion or fire.1
Very light petroleum distillates are a mixture of light and heavy hydrocarbon compounds. Part of the light compounds may vaporize under ambient storage conditions.
The vapor will gather in the vapor space above the product surface and form a vapor cloud where it is mixed with air that infiltrates into the tank. If the composition of the vapor cloud contains the proper proportions of air and light hydrocarbons, an ignitable or explosive mixture exists.2
The explosion or fire danger becomes critical when the storage temperature is near the flash point of the fraction or fractions being stored. Therefore, knowledge of the flash points and explosion limits of the petroleum fractions, the size of the vapor space in the tank, and the ambient temperature range that the stored product can be exposed to is important to assess the explosive potential and assure safe product storage.
OPERATING DATA GATHERED
Operating data were gathered to determine the actual operating conditions that could potentially cause an explosion or fire in storage tanks at the Zarka refinery in Jordan. Temperature data were taken for tanks storing crude oil, gasoline, naphtha, kerosine, and LGO.
The temperatures included ambient temperature, vapor space temperature, shade temperature, and temperatures in areas exposed to direct sunlight. The data were averaged throughout the year in two parts: September through April and May through August (Figs. 1 and 2).
Flash points and vapor pressure data were obtained from direct experimentation on the stored petroleum distillates in accordance with American Society for Testing and Materials (ASTM) testing methods. Data were collected for several samples which covered all ranges of vapor pressure for every product throughout the year.
The average local atmospheric pressure throughout the year at the Zarka refinery is about 97.4 kPa.
ANALYSIS
The vapor cloud in the vapor space of a tank is composed of light hydrocarbon and infiltrated air. The amount of hydrocarbon vapor depends on the volatility of the petroleum fraction under consideration.
Volatility is a strong function of vapor pressure and ambient storage temperature. Thus, the evaporation rate is mainly controlled by ambient weather conditions and the vapor pressure of the stored product.
Because the total pressure in the vapor space is atmospheric, and the vapor mixture obeys Dalton's law, the total pressure can be written as:
Patm = Ph + Pa (1)
where:
Ph = Vapor pressure of the light petroleum distillate
Pa = Partial pressure of air in the vapor space
Under atmospheric conditions, the ideal gas law can be applied on both air and hydrocarbon vapor as follows:
For hydrocarbons:
PhVsp = NhRT (2)
For air:
PaVsp = NaRT (3)
where:
Vsp = Total volume of the vapor space in the tank
Nh = Number of existing moles of hydrocarbon in the vapor space
R = Universal gas constant
T = Absolute storage temperature
Na = Number of existing moles of air in the vapor space
When Equation 2 is divided by Equation 3:
Ph/Pa = Nh/Na =Moles of hydrocarbon/Moles of air (4)
or:
Ph/(Ph + Pa) =Nh/(Na + Nh) =
Moles of hydrocarbon/Moles of mixture (5)
When Equation 1 is substituted into Equation 5:
Ph/Patm = Nh/(Na + Nh) (6)The right side of Equation 6 represents the mole fraction of hydrocarbon vapor in the vapor cloud mixture. If this mole fraction reaches the lower flammability limit of that stored product at an ambient temperature that is equal to or above the product's flash point, a real danger of fire or explosion exists.
The lower and upper flammability limits for most hydrocarbon compounds are related to the stoichiometric concentration, Cst, on a volume basis, at complete combustion conditions:3
Lower flammability limit (LFL) = 0.55Cst
Upper flammability limit (UFL) = 3.5Cst
The vapor pressure, initial boiling point, API gravity, flash points, and fire points of various petroleum distillates were determined by ASTM standard methods. The vapor pressure at standard conditions is the Reid vapor pressure (Rvp).
True vapor pressure values were calculated from Rvp values according to the ASTM correction method.
The number of moles of hydrocarbon vapor can be easily calculated from Equation 2 if the volume and the temperature of the vapor space are known. The temperatures of the vapor space throughout the year are shown in Figs. 1 and 2.
The air partial pressure is known from Equation 1 because the atmospheric pressure at the Zarka refinery is about 97.4 kPa. Thus, the number of moles of air for a specific vapor space can be determined at the ambient temperature.
Consequently, the molar (volumetric) concentration of hydrocarbon vapors in the vapor space is directly calculated from Equation 6. The results for both the LFL and UFL were made for all distillates over a range of Rvps of various fractions throughout the year (Fig. 3).
There is a strong relationship between the initial boiling point of a fraction and its volatility. The volatility of a fraction is a strong measure of flammability and explosion.
The flash point provides a warning limit of danger, while the fire point is the point where ignition or an explosion can occur.
The ambient temperature vs. the initial boiling point of various light petroleum distillates is shown in Fig. 4. The initial boiling point of a fraction covers a range that includes all cuts of that fraction throughout the year.
CONCLUSIONS
Explosion and fire hazards of light petroleum distillates are related directly to the volatility of those fractions. The more volatile the fraction, the higher the risk of fire or explosion.
The volatility of any petroleum product is characterized by its Rvp and by the slope of the distillation curve of that product at 10% evaporated (the temperature at which 10% of a measured sample of product will evaporate). Also, the initial boiling point of the product indicates the minimum temperature at which hydrocarbon vapor will begin to form in the vapor space. Fig. 3 shows Rvp values for various light petroleum distillates vs. ambient storage temperatures at the LFL and UFL.
At low ambient storage temperatures (below the freezing point of water) crude oil and gasoline tanks represent a potential source of fire and explosion because the vapor space contains enough light material to reach the LFL. Most vapors in the vapor space of crude oil tanks consist of C1-C3s. Gasoline tank vapors are Mostly C4 with some C5.
Although floating-roof tanks are used for gasoline storage to prevent evaporation by minimizing the vapor space available, the size of the vapor space below the roof can vary significantly depending on the roof seal and tank design.
All temperatures of the shade and vapor space converge to the same temperature at 5 a.m. and 11 p.m., but as the day progresses, the roof receives more heat from absorption of solar radiation. Thus, the vapor space temperature is higher than the bulk liquid temperature (Figs. 1 and 2).
The roof temperature is much higher than both liquid and vapor-space temperatures. The peak occurs at mid-day during solar noon.
At an ambient temperature range of 0-30 C., naphtha storage tanks are the source of danger. In this temperature range, the vapor space contains enough light materials to reach the LFL.
Naphtha storage tanks are usually pressurized (blanketed) with an inert gas, such as nitrogen, to prevent air infiltration into the vapor space. Blanketing prevents vapors from reaching the LFL.
Kerosine storage tanks become potentially hazardous when the ambient temperature reaches 40 C. If the ambient temperature reaches the flash point of kerosine, the vapor concentration can reach the LFL.
Ambient temperatures above 40 C. were experienced at the Zarka refinery during mid-July.
At these conditions, kerosine storage tanks were cooled with water sprays to reduce the vapor space temperature to below the flash point and vapor concentration to below the LFL.
Light gas oil storage tanks, and tanks that store heavier products, are not very susceptible to explosion or fire unless they are heated to the flash point of LGO. This is normally not possible unless the tank is heated by a fire in an adjacent tank.
Generally, the vapor space of crude oil and gasoline tanks is too rich (vapor concentration in the vapor space is above the UFL) when ambient temperatures are above the freezing point of water. Therefore, there is minimal explosion and fire danger during warmer periods.
The vapor space of kerosine and LGO storage tanks is usually too lean (vapor concentration in the vapor space below the LFL) at temperatures below 40 C.
Naphtha storage tanks present the greatest fire and explosion danger for most installations because the average ambient temperature for many countries is in the range of 25 30 C. In that temperature range, the concentration of vapor in the vapor space exceeds the LFL but doesn't rise above the UFL, and the tank temperature is often above the flash point of naphtha.
REFERENCES
- Nelson, W.L., Petroleum Refinery Engineering, McGraw Hill Book Co., New York, 1986.
- Jonker, P.E., Porter, W.J., and Scott, B.C., "Control Floating Roof Tank Emissions," Hydrocarbon Processing, Vol. 56, No. 5, 1977, p. 151.
- Bodurtha, F.T., industrial Explosion Prevention and Protection, McGraw Hill Book Co., New York, 1980.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
