
Keywords 

Storage tank; Crude oil; Evaporation loss; Rim seal;
Khark Island 

Introduction 

The Khark Island is main Iran oil export terminal which contains
about 40 crude oil storage tanks with capacity 1 million barrels. The
tanks mainly have flouting roof designs. These tanks are used to hold
oil for brief periods in order to stabilize flow between production
wells and transporting ships. Due to location of the Island which is
in hot climate, during storage, at ambient temperature and pressure,
light hydrocarbons of crude oil vaporize and assemble in the space
between the liquid and roof of the tank, these vapors are often vented
to the atmosphere. This process causes to pollute the environment
and also has effects on crude oil quality. Any reduction in the loss
will also have financial benefit. This makes the phenomena, crude
oil evaporation loss, an important issue which should be carefully
investigated and effects of various parameters be studied. 

Design of storage tanks depends on various parameters such as
the vapor pressure, storage temperature and pressure, and the toxicity
of liquid [1]. The fixingroof tanks are mainly used for petroleum
materials with a vapor pressure less than 1.5 psia [2], while floatingroof
tanks are used for petroleum materials with a vapor pressure of
1.12–11.5 psia [1]. An external floating roof tank typically consists
of an open topped cylindrical steel shell equipped with a roof that
floats on the surface of the storage liquid, which rises and falls as the
liquid level changes. Floating roof tanks are equipped with a sealing
system, which is attached to the roof perimeter and covers the gap
between the roof and the tank wall [3]. The basic designs available
for external floating roof rim seals are mechanical shoe seals, liquid mounted
seals, and vapormounted that called primary seals [4]. A
secondary seal is often used for covering the entire primary seal. The
floating roof structure and the sealing system are designed to reduce
evaporative losses of the petroleum materials. Evaporative losses
from the external floating roof tanks are limited to the losses from the
sealing system and roof fittings and any remaining liquids on the tank walls, while the floating roof falls down. There have been very limited
studies related to the storage tank evaporative losses. Wongwises
et al. [5] evaluated the gasoline evaporation losses from Thailand
storage sites and service stations during refueling and loading. They
estimated the total gasoline evaporative losses of about 21,000 tons/
year throughout the Thailand. Ramachandran [3] also investigated
the underlying causes of storage thank emissions and analyzed the
options of reducing them. 

Asharif and Zorgani [6] calculated evaporative losses from
existing large crude oil storage tanks located in a Libyan oil field
and investigated the operating variables including the number of
separation stages, operating temperature and pressure of each
separator. They concluded that the operation variables of the existing
process facilities can be adjusted in order to minimize the losses
from storage tanks. Digrado and Thorp [7] compared the evaporation
losses between the internal and external floating roofs. They also
determined the losses associated with different sealing arrangements
based on the American Petroleum Institute (API) standards [8,9]. 

Zareie et al. [10] experimentally determined the amount of the
volatile organic compounds emitted from an industrial external
floating roof tank by monitoring the level of the liquid in the tank and its temperature for a period of 35 days. They also compared
their findings with the values computed based on the API standards
and found out that the API predictions are slightly lower than the
experimental data. 

This brief review of the related literature indicates the shortage
of information in the field of storage tank evaporative losses. In the
present paper a numerical method has been developed for solving
the energy equation to predict the storage tank temperature and to
estimate the evaporative losses. More importantly, the effect of seal
type on the evaporative loss has been investigated. The numerical
predictions have been compared with the experimental data provided
from a storage tank with the capacity of one million barrels of light
crude oil located in Khark Island the main Iran oil export terminal. 

The Case Study 

The problem under consideration is a typical storage tank in
Khark Island shown in (Figure 1). As seen in the picture, the exterior
surface paint of the tank is white with two small rings of blue and
yellow color indicating that the tank is suitable for storing both heavy
and light crude oil. 

Two fitting types of the tank are shown in (Figure 2 and Figure 3).
(Figure 2) show deck leg of tank, the exiting of crude oil vapor from
gasketed area case to blacked the near area of gasketed, figure also
show exposed liquid on the tank internal walls that vaporize as time
goes on. (Figure 3) show vacuum breaker. The evaporation loss
from this part lead to dirty around it, also the exiting of vapor could
see in shadow of vacuum breaker. 

In the study, light crude oil with API of 33.36 has been stored
in the tank, where its chemical composition is given in Table 2. The experiment is carried out by Iran Oil Terminal company laboratory
located in Khark Island. This data has been used to calculate thermal
properties of the crude oil employing the commercial software HYSYS
version 3.1 (Table 1). 

Climate condition of the Khark Island 

Climate conditions such as ambient temperature, wind speed and
solar irradiation are directly related to the evaporative losses as will
be discussed later. Climate conditions have big effects on temperature
of crude oil within the storage tank, this temperature have big effects
on evaporative losses in crude oil storage tanks. The climatic data has
been extracted from the Iran Weather Institute information for 2007.
The average ambient temperature and wind speed for the 5th day of
each month are shown in (Figure 4). 

The solar radiation is the main cause of the evaporative losses in
the floating roof tanks. For estimating solar radiation on the earth
surface several engineering models have been proposed. In all of the
models the weather condition and geographic location are important
factors [11]. Kamali and Moradi [12] have examined various models
including Angstrom, Bristow and Campbell, Hargreaves and Reddy
for locations and weather conditions relevant to the present problem
and compared their finding with the experimental data. It was
suggested that Angstrom model with some modifications is more
suitable for Khark Island conditions, and thus has also been adopted
for the present study. 

Based on the Angstrom model, solar radiation, H, can be
estimated using the following equation: 

(1) 

Where a and b are coefficients that must be chosen according to
the location and weather conditions, S and S0, are average sunshine
duration and cloudless sunshine duration, respectively. Following
Kamali and Moradi, [12] a and b for Khark Island shown in (Table 3). 

The cloudless hourly global irradiation received can be calculated
using the following equation: 

(2) 

Where I_{sc} is set to 1367 W/m^{2} according to the world radiation center
[12] and ω is given by the following equation: 

(3) 

With using earlier equations, solar radiation for the Khark Island has
been calculated and is shown in (Figure 4). It can be realized that
solar radiation is highest during June where ambient temperature is
highest during July & August. 

The Numerical Method 

Energy balance of the tank 

A schematic diagram of the crude oil storage tank with all
incoming and outgoing forms of energy is shown in (Figure 5). 

In developing the energy balance of the tank, the oil temperature
variation inside the tank is neglected and a lumped system with
uniform temperature is considered. 


Considering the tank as an open system, the energy equation can
be expressed as: 

(4) 

Where includes all incoming and outgoing heat fluxes expressed
as: 

(5) 

Where q_{s} is the absorbed solar energy by the tank surface with
absorption coefficient, α and irradiating surface area of A_{S} with solar
radiation, H, defined as: 

(6) 

q_{cond} is the amount of heat conducted to the foundation ground
evaluated by Fourier’s law of heat conduction: 

(7) 

Where Δx, k and T_{soil} are thickness, conductivity coefficient and temperature of foundation base with the area of A_{b}, respectively. 

q_{conv} evaluates the convective exchange of energy between the
tank and the ambient: 

(8) 

In this study for calculating the convective heat transfer
coefficient, h, the correlation proposed by Churchill and Bernstein [13] has been employed, which is valid for vertical cylinders, when
Re.Pr > 0.2 related to the present case and expressed as: 

(9) 

Radiation heat exchange between the sky and the tank can be
obtained according to: 

(10) 

Where T_{sky} is the sky temperature evaluated following Kamali and
Moradi [12] as: 

(11) 

Exchange of energy has due to mass exchange including inlet and
outlet mass transfer are the others term. As the displacement of the
tank roof is negligible, the following assumption could be made: 

(12) 

Exchange of energy by ambient causes change in internal energy
which for crude oil could be expressed as below: 

(13) 

Specific heat capacity is assumed function of oil (tank) temperature
as follow 

(14) 

Replacing equation [14] in [13], the below equation could be obtained: 

(15) 

For simplicity, the quasi steady state condition has been assumed for
the temperature time variation, therefore: 

(16) 

By substituting mentioned relations for each term and considering
the assumptions, the energy equation will be formed as: 

(17) 

By substituting equation 14 into energy equation, the following
equation could be obtained: 

(18) 

Supposing T_{soil}=T_{∞} and arranging above relation, final form of energy
equation could be written as follow: 

(19) 

There is one unknown in above equation, which is tank temperature
that obtains from equation. 

It worth mentioning that in API method [14,15] a simple
correlation has been proposed for computing monthly averaged
tank temperature, which can also be used for estimating the monthly
averaged evaporative losses: 

(20) 

Estimating evaporation rate 

According to the API standards [14, 15] the total rates of
evaporative losses from external floating roof tanks are equal to the
sum of the rim seal losses, withdrawal losses, and deck fitting losses: 

(21) 

Rim seal loss from floating roof tanks can be estimated using the
following equation: 

(22) 

Where K_{C} is product factor and for crude oil is 0.4. k_{Ra}, k_{Rb}, n depended
to kind of seal that being used. These parameters for khark island
tanks are 0.7, 0.3 and 1.2 respectively. The vapor pressure, P* , is
evaluated according to: 

(23) 

Where the true vapor pressure, P_{VA}, for selected petroleum at the
stored liquid surface temperature can be determined using the
following equation: 

(24) 

The constants A and B can be calculated from the following equations: 

(25) 

Deck fitting losses from floating roof tanks can be estimated by the
following equation: 

(26) 

The value of F_{F} is calculated using the actual tankspecific data for the number of each fit type (N_{Fi}) multiplying by the fitting loss factor for
each fitting (K_{Fi}). 

(27) 

The deck fitting loss factor, K_{Fi} for a particular type of fitting, can be
estimated by the following equation: 

(28) 

For external floating roof tanks, the fitting wind correction factor, K_{V} is equal to 0.7. 

The withdrawal losses from floating roof storage tanks can be
estimated using the following: 

(29) 

Where NC is zero for the external floating roof. 

Discussion 

Average crude oil temperature 

The experimental temperatures have been measured using
infrared thermometer from roof surface of the storage tank for three
days (on 7 December 2008, 25 February 2009 and 2 June 2009). The
surface temperatures have been measured at various positions on the roof and the value which reported here is average one. The results
of numerical method and experimental data for tank temperature are
shown in (Table 4). As the surface is imposing to ambient condition
(and solar radiation), the surface temperature is expected to be
higher than average tank temperature especially at later of the day.
This could be seen in results of (Table 4). But generally, the numerical
results are in good agreement with measured values. 

In addition of comparing measured and numerical values of transient temperature, the numerical results of average monthly
tank temperatures have been also compared with similar value
calculated from equation [20]. As stated previously, the equation
[20] is proposed by API [14, 15]. The comparison has been presented
in (Figure 6). It could be seen that the numerical method predicts
slightly higher and lower temperature during first and last 6 months
of the year comparing to API proposed equation respectively. This is
probably due to simplicity of equation [20] that did not consider the
effects of other parameters such as wind speed in this equation. 

Evaporative loss 

Having examined the storage tank temperature, the evaporative
losses from storage tanks can now be determined by the API method
discussed earlier. (Figure 7) show monthly averaged evaporation loss
for the tank under investigation in 2007. A comparison has been
made between results obtained from numerical method and API [14, 15] method. Note from figure, the highest evaporations are occurred
during June, July and August. This is due to the fact that these are
hottest months in Khark Island. The peak in evaporation during
November are due to high wind speed during period of time. Note
from the figure, there are good agreement between the numerical
results and API method. 

Total losses from external floating roof tanks are summation of
the rim seal loss, withdrawal loss and deck fitting loss. Each individual
loss has been calculated and is shown in (Figure 8). Note from figure,
the withdrawal loss in all month was equal, therefore this part of
evaporation loss not changed with change of weather conditions,
also this result could be obtain from equation [29]. (Figure 8) show
that rim seal and deck fitting losses depended to climate conditions. 

(Figure 9) show the percentage of annual evaporation loss from
rim seal, deck fitting and withdrawal. Results of (Figure 9) show that
maximum of evaporation loss happen from the deck fitting equal 67% and minimum loss belong withdrawal loss equal 7%, Therefore
for decreasing total loss, deck fitting loss should be decreased, one
solution for this problem is the replace poor gaskets of fitting with
proper gaskets. 

Effect of rim seal on evaporation loss 

In equation [22] has been shown that evaporation loss has function of k_{Ra}, k_{Rb} and n, that these parameters depended to kind of seal that
being used. These parameters for khark island tanks are 0.7, 0.3
and 1.2 respectively, because the primary seal that used for storage
tanks is liquid mounted and the secondary seal is weather shield. The
following nine type tank seal has been studied evaporation loss for
each case has been calculated. In (Table 5) show, Rim seal loss factors
for floating roof tanks [14,15]. 

Firstly, it is assumed that the mechanical shoe seal is primary seal
and three types of secondary seal as: (1) no secondary seal (2) the
shoe mounted secondary seal (3) the rim mounted secondary seal.
Results of three cases are shown in (Figure 10). 

Secondly, it is assumed that the liquidmounted seal and three
types of secondary seal as: (1) no secondary seal (2) the weather
shield secondary seal (3) the rim mounted secondary seal. Results of
three cases are shown in (Figure 11). 

Finally, is assumed that the vapormounted seal and three types
of secondary seal as: (1) no secondary seal (2) the weather shield
secondary seal (3) the rim mounted secondary seal. Results of three
cases are shown in (Figure 12). 

As the type of sealing has a huge effect of the evaporation loss,
the annual evaporation loss from various seal types are compared and
presented in (Figure 13). 

The above results show that the losses are larger when the
used vapor mounted seal for storage tank. In addition, combination
primary and secondary seal drops the evaporation loss comparing
with using primary seal only. 

Liquid mounted as primary and rimmounted as secondary is best
combination in case of reducing evaporation lost. 

Conclusion 

One of the major difficulties related to crude oil storage tank, is
evaporation loss. Light hydrocarbons vaporize in the space between
the crude oil and the tank roof. This process affects the quality of the
crude oil and causes environmental pollution. 

In this study, a numerical scheme has been developed for
estimating the time variations of the storage tank temperature and
evaporative losses. The scheme is validated against the measured
values of the storage tank temperature at different times during a
day, where reasonable agreements are observed. 

The numerical results of tank temperature have been compared
with available experimental values and show a good agreement.
The numerical value of monthly averaged evaporation loss which
compared with the value of API AP42 standard shows a good
agreement too. These agreements are proved that the proposed
numerical method is able to predict the tank temperature and the
evaporation loss accurately. 

The results show that maximum losses are occurred in June,
July and August. This is due to high solar radiation and wind speed
respectively. Considering these facts, for reducing the losses,
the effects of wind speed and solar radiation on the tanks should
be reduced. These could be done by using wind obstruct wall and
thermal insulations. 

The withdrawal loss not changed with change of weather
conditions, but rim seal and deck fitting losses depended to climate
conditions. Maximum of evaporation loss happen from the deck
fitting equal 131.3 bbl/yr and minimum loss belong withdrawal loss
equal 13.7 bbl/yr. 

Comparing the results for primary seal show that liquid mounted
seal has lowest the evaporation loss among all three primary seals
which are available. 

The results show that the losses are larger when the used vapor
mounted seal/primary only for storage tank with 2292 bbl/yr and
liquid mounted as primary and rimmounted as secondary is best
combination have lowest evaporation loss with 14 bbl/yr. 

In addition, combination primary and secondary seal drops the
evaporation loss comparing with using primary seal only. 

Finally, it should be noted that selecting better seal for storage
tanks lead to decreasing evaporation loss, reducing environment
pollution and has economical advantages. 

A_{s} Area (m^{2})
A constant in the vapor pressure equation, (dimensionless)
B constant in the vapor pressure equation ( °K)
c_{p} Special heat capacity (J/kgK)
C_{s} shell factor, (m)
D tank diameter, (m)
F_{c} effective column diameter, (m)
F_{F} total deck fitting loss factor, (kgmole/yr)
H_{0} cloudless daily global irradiation received, (MJ/m2.hour)
H daily global irradiation, (MJ/m^{2}.hour)
h convection coefficient (w/m2 k)
h_{fg} evaporate enthalpy (KJ/kg)
h_{in} inlet enthalpy (KJ/kg)
h_{out} Outlet enthalpy (KJ/kg)
K conductivity (W/m K)
K_{Ra} zero wind speed rim seal loss factor, ( kgmole/m@ yr)
K_{Rb} wind speed dependent rim seal loss factor, (kg mole/(m/s)^{n} m @yr)
M_{V} vapor molecular weight, (kg/kgmole)
m Mass (kg)
total loss, (kg/s)
m_{F} deck fitting loss, ( kg/s)
m_{R} rim seal loss, (kg/s)
m_{WD} withdrawal loss, (kg/s)
_{in} Inlet mass (kg/s)
_{out} Outlet mass (kg/s)
N Number of day in year
NC number of fixed roof support columns, (dimensionless)
Nu Nusselt number (dimensionless)
PA atmospheric pressure, (kpa)
P^{*} vapor pressure function, (dimensionless)
Pr Perantel Number (dimensionless)
PVA true vapor pressure, (kpa)
Q annual throughput , (m^{3}/yr)
q Heat transfer energy (W/m^{2})
q_{s} Absorbed solar energy (W/m^{2})
q_{conv} Convection Heat transfer energy (W/m^{2})
q_{cond} Conduction Heat transfer energy (W/m^{2})
q_{sky} Radiation to sky(W/m^{2})
Ra Rayleigh number (dimensionless)
S average sunshine duration, (hour)
S_{0} cloudless sunshine duration, (hour)
T∞ ambient temperature (°K)
TS tank surface temperature, (°K)
T_{sky} Sky temperature ( K)
T_{soil} Soil temperature ( K)
U Internal energy (j)
V average ambient wind speed , (m/s)
W
Rate of work (W) WL average organic liquid density, (kg/m^{3})
Greek Symbols
α Absorption coefficient, (dimensionless)
δ declination angle, (degree)
φ Longitude, (degree)
ω hour angle, (degree)
ε emissivity (dimensionless)
σ Stefan–Boltzmann constant (W/m^{2} K^{4}) 


References 
 AbdelAal HK, Aggour M, Fahim M (2003) Petroleum and gas field processing.
In: Chapter 8 Storage Tanks and Other Field Facilities. New York: Marcel
Dekker Inc: p 1.
 Laverman RJ (1992) Emission Reduction Options for Floating Roof Tanks.
Second International Symposium on Aboveground Storage Tanks, Houston.
 Ramachandran S (2000) Reducing (controlling) vapour losses from storage
tanks. 7th annual India Oil & Gas Review, Symposium & International Exhibition,
Bombay, (IORS).
 Michael C (2006) Emission Factor Documentation for AP42 Section 7.1
Organic Liquid Storage Tanks. In: Chapter 2 Storage Tank Descriptions, U S
Environmental Protection Agency; p. 15.
 Wongwises S, Rattanaprayura I, Chanchaona S (1997) An Evaluation of
Evaporative Emissions of Gasoline from Storage Sites and Service Stations.
Thailand Thammasat Int. J Sc Tech. Vol 2 No 2.
 Asharif H, Zorgani E (2007) Adjustment of Process Variables to Reduce
Evaporation Losses from High pour pointcrude oil storage tanks. The 8th
international conference on petroleum phase behavior and fouling, France.
 Digrado BD, Gregory AT (2004) The aboveground steel storage tank handbook.
In: chapter 13: new fielderected aboveground storage tank products. New
Jersey: John Wiley & Sons Inc; p. 127.
 API (1989) Evaporative Loss from External Floating Roof Tanks. Bulletin No
2517, Third Edition, American Petroleum Institute, Washington, DC.
 API (1990) Evaporative Loss from Internal Floating Roof Tanks.API Publication
2519, American Petroleum Institute, Washington, DC.
 Zareie S, Mowla D, Fathi J (2007) Practical Study of VOCs Emission
from External Floating Roof Tanks. 5th international congress of chemical
engineering, Shiraz, Iran.
 Sen Z (2008) Solar Energy Fundamentals and Modeling Techniques. Londen:
Springer Veriag. p. 47140.
 Kamali GA, Moradi E (2005) Solar radiation fundamentals and application in
farms and new energy. Tehran: publication 21 century.
 Churchill SW, Bernstein MA (1977) Correlating Equation for Forced Convection
from Gases and Liquids to a Circular Cylinder in Cross Flow. J Heat Transfer
p.300306.
 API (1994) Manual of Petroleum Measurement Standards. In: Chapter 19
Evaporative Loss Measurement, Section 2 Evaporative Loss From Floating
Roof Tanks. Preliminary Draft, American Petroleum Institute, Washington DC.
 Michael C (2006) Emission Factor Documentation for AP42 Section 7.1
Organic Liquid Storage Tanks. In: Chapter 3Emission Estimation Procedure. U
S Environmental Protection Agency. p 911: 1518.


