SPRING 1999
VOLUME 15
NUMBER 2
Density Stratification in LNG Storage
by David Sheats and Michael Capers,
Scientific Instruments, Inc.
The use of LNG (Liquid Natural Gas) as an
important energy source around the world is increasing
steadily. It currently represents about 6% of the world’s
natural gas and is readily available at numerous sites
worldwide.
Factors contributing to this increased usage include
the discovery of large resources, development of costeffective methods of production, transportation and
storage and the fact that LNG is a very clean-burning
fuel, making it an attractive option where
environmental concerns are high.
LNG is stored at cryogenic temperatures, which
present some unique safety concerns and require
specialized instrumentation to be able to detect
potentially unsafe conditions. In the following
discussion, these concerns are identified and the
instrumentation techniques for addressing them are
explained. The end result is a solution that gives
confidence to the storage facility personnel that any
potentially hazardous condition will be detected in a
timely manner so that the proper corrective action may
be taken.
Production of LNG
LNG is produced through a liquefaction process
that lowers the temperature of the multi-component
mixture of hydrocarbons (mainly methane with lower
concentrations of higher hydrocarbons) to –160°C, a 600
to 1 reduction of volume from gas to liquid. In this state,
the product is economically transported via specialized
tanker ships from the production site typically quite
remote) to the customer. There are three primary types
of sites where LNG is stored, namely liquefaction/
export facilities, import / distribution facilities and peak
shaving facilities.
Liquefaction/Export
The liquefaction/export facility is located at the gas
source and performs the ʺcleaning,ʺ liquefaction and
initial storage of the finished product. Such long-term
supply countries as Indonesia, Algeria, Australia,
Malaysia, Brunei and the United Arab Emirates
represent this part of the LNG production train. New
liquefaction facilities are or soon will be online in
Nigeria, Trinidad, Oman and Qatar. New fields have
been discovered and remain to be developed in
Indonesia, Russia and off the coast of South America. A
ʺtypicalʺ LNG exporting facility will operate up to 2-10
production trains and may have up to 10 LNG storage
tanks. These tanks are located at a deep-water port and
are used to load the tankers for the oceanic
transportation of the product to customers.
Typical LNG storage tank
Import/Distribution
An import/distribution facility capable of
accommodating these LNG tankers is located at a deepwater port in countries that are using the LNG as an
energy source. From these storage facilities, typically
3-10 storage tanks, natural gas is re-gasified and fed into
a national pipeline system or trucked to remote storage
locations. Such facilities are located in Spain, Japan,
Taiwan, Korea and the United States.
Peak Shaving Facilities
A peak shaving facility is actually a production/
distribution plant combined into one. The purpose is to
produce LNG during low usage periods, and store that
gas for use during peak demands. The end result is the
ability to ʺshave the peakʺ demand requirement off the
gas supply lines, since that demand is normally only for
a short period, such as a very cold day. It is typically
owned and operated by a local gas company or pipeline
distributor. While the process is very similar to the
larger facilities, the production/ gasification rate is much
lower. The very nature of this type of facility dictates
that the gas will likely be stored for a much longer
period, which has significant implications pertaining to
possible density stratification as discussed below. The
storage tanks used at all three of the above types of
facilities may be above or below ground, but they are
fundamentally the same. They have an inner tank, an
insulation space and a concrete or high carbon steel
outer tank. They are typically 150ʹ in diameter, 160ʹ in
height, and have a storage volume of 15-18 million
gallons of product. There are presently plans in
development for tanks with significantly greater
capacity than those previously built and in service.
SPRING 1999
VOLUME 15
NUMBER 2
Density Stratification in LNG Storage
Rollover
A significant safety concern in the storage of LNG is
a phenomenon known as ʺrollover.ʺ If this occurs,
pressures inside the storage tank may rise to excessive
levels. The tanks are equipped with safety vent valves
that are designed to keep the pressures from rising to
levels that could cause structural damage. However,
when these valves operate, LNG is vented to the
atmosphere at an uncontrolled rate, which is an
additional safety concern.
LNG is heavier than air and could settle in pockets
of explosive mixtures. Perhaps the most well known
case of rollover is that which occurred in La Spezia,
Italy, in 1971. The actual pressures realized are
unknown since they exceeded available instrumentation
and gas vented uncontrolled in a highly populated area
for several hours. Fortunately, there was no explosion
and no major damage was done to the tanks. The
possibility of both potential problems was very evident,
and much attention to detecting and preventing a
similar occurrence has taken place since that time.
Rollover occurs under certain conditions as
stratified LNG comes to equilibrium. Stratification
occurs when the product in the tank forms in layers
with different densities and different temperatures.
Sudden mixing of LNG in any storage tank occurs as
two or more layer densities approach equality. Any heat
trapped in the system is released rapidly during mixing,
generating a vapor which may exceed the venting
capability of the tank.
The exact conditions that lead to rollover are
somewhat complex. If layering occurs (discussed
below), each layer is initially uniform, with the upper
layer lighter than the lower. Heat entering the top layer
comes through the walls of the tank. The liquid next to
the tank walls warms slightly, becomes less dense and
rises to the top, where it evaporates. Since light gases
evaporate first, the liquid in the top layer tends to
become denser.
Fluid circulation in a stratified LNG storage tank
At the same time, the liquid in the lower layer gains
heat through the floor and walls of the tank with a
convection flow similar to the top layer. But because of
the hydrostatic pressure of the upper layer, the rising
liquid does not evaporate but superheats. Thus the
lower layer becomes warmer and less dense. When the
two layers approach the same density, the interface
between the two becomes unstable and mixes rapidly.
When this occurs, the liquid from the lower layer that is
superheated gives off a large amount of vapor that rises
to the surface. It is this phenomenon that is known as
rollover.
Causes of Layering
What are the causes of layering? There are basically
two ways that layering is produced. The first, and most
obvious, occurs when liquid of a different composition
than that which is already in the tank is introduced into
the tank. This could be from a different gas field, or an
altered process that results in a different composition.
This is most likely to occur at a receiving terminal.
The second cause of layering comes from a process
referred to as ʺautostratification.ʺ In this mechanism,
unstratified liquid gains heat at the walls of the tank
and rises by convection. It flashes upon reaching the
surface. If sufficient nitrogen is present, the remaining
liquid will be less dense, creating a layer at the top. As
this layer increases, it seals in the lower layer and can
cause it to superheat, creating conditions leading to
rollover.
Layer Detection
The most effective method of preventing rollover is
to detect layering by means of instrumentation, and if
layering is present, take action to mix the layers under
controlled conditions before the final conditions leading
to rollover develop. If the size and composition of the
layers can be detected, the potential danger can be
accurately determined and the most cost-effective
approach can be chosen. This action may be the
operation of recirculation pumps (if available), or by
mixing with liquid in other tanks. Some installations
that have the capability of recirculation within a tank
choose to operate these pumps on a regular basis as a
preventive measure. Operation of these pumps is
expensive, however, and some facilities do not have a
simple method of recirculation.
Layer detection is best accomplished by means of a
temperature sensor and density sensor that are run
through the entire depth of the liquid.
SPRING 1999
VOLUME 15
NUMBER 2
Density Stratification in LNG Storage
In the absence of this, multiple temperature sensors
located at fixed intervals will also give an indication of
layering. The disadvantage to multiple sensors is the
difficulty of accurate calibration. The cost of multiple
density sensors is prohibitive.
Instrumentation
One instrument currently available for layer detection
and installed at a number of facilities around the world
is the M6280 LTD series from Scientific Instruments, Inc.
It is specifically designed to detect temperature and
density layering in LNG tanks. To accomplish this, a
single probe with installed temperature, density and
liquid level sensors is driven over the entire depth of the
liquid in the tank. The data is collected at fixed intervals
as specified by the customer.
Simple data analysis is performed to alert the
operator of apparent temperature or density layering. If
certain areas of the tank are suspect, a feature known as
ʺTop Scanʺ is available to examine a potential layer in
more detail, providing an accurate picture of the
suspected layer. As mentioned previously, because a
single sensor is used for data collection, calibration
errors are eliminated and an accurate relative profile is
obtained. With an accurate temperature and density
profile, detailed calculations can be performed
Typical display from M6280LTD System showing stratification
to properly determine the extent of the danger, and to
enable cost-effective action in a timely manner to
prevent rollover.
Summary
Although we are not aware of any rollover incidents
that have resulted in fires, explosions or major
structural tank damage, there have been many rollover
incidents throughout the LNG industry. An increasing
awareness of the potential problem has resulted in a
greater emphasis on installing instrumentation. Proper
instrumentation, such as Scientific Instruments, Inc.
M6280 LTD, is the most secure way to obtain
information so that preventive action can be quickly
implemented to avert a potential problem.
Scientific Instruments, Inc (SII) products are distributed by FlowQuip Group with offices in China,
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