The Compressed Natural Gas (CNG) Cylinder Pressure Storage
Technology in Natural Gas Vehicles (NGV) Research Trends
Rosli A. Bakar, Mohamad F. Othman, Semin, Abdul R. Ismail
Faculty of Mechanical Engineering, University Malaysia Pahang
m_fatasya@yahoo.com.my
ABSTRACT:
The compressed natural gas (CNG) cylinder pressure vessel become important in NGV fuel driving system because
demand in CNG base as increase. CNG pressure vessel suitable for gas operated vehicles can be made of fully metal,
hoop wrapped with metal liner, fully wrapped with metal liner or fully composite. Fully metal CNG pressure vessel is
cheap but it’s the heaviest pressure vessel compare to others. For weight sensitive application such as trucks, buses,
and taxis fully composite tanks is the most suitable but it comes with high price. So, many researchers had been done
to find the best solution to stored high volume of CNG with lightweight and cheap material. Even though the aim is
clear but the development of CNG pressure tank must follow the standard requirement to prevent failure. In this
paper, the trends of research by manufacturers, research organizations and universities have been analyzed to find the
problems and difficulties regarding CNG pressure vessel. The result of the overview showed that all type of tanks had
advantages and disadvantages, it depends on the sector and industry require which type of CNG tanks suitable for their
usage.
Keywords: Alternative fuel, CNG tanks, Compressed Natural Gas (CNG), cylinder pressure vessel,
1. INTRODUCTION
Natural gas is becoming an increasingly alternative
and attractive fuel for many transportation uses. Fuel
costs are significantly less compare gasoline and the
most suitable fuel for bridge to hydrogen.
Table 1: Characteristics Comparison Between
Gasoline and CNG [4].
GASOLINE
CNG
PROPERTIES
Vapour density
Ignition
Octane rating
Boiling point (Atm.Press)
Air-Fuel Ratio (Weight)
Chemical Reaction With
Rubber
Storage Pressure
3.5
430˚C
96
27˚C
14.5
Yes
0.68
700˚C
130
-162˚C
17.24
No
Atm. Pressure
20.6Mpa
Fuel Air Mixture Quality
Pollution CO-HC-Nox
Flame Speed m per sec
Combust. ability with air
Poor
High
0.83
1-16%
Good
Very Low
0.63
4-14%
The octane rating of natural gas is about 130, meaning
that engines could operate at compression ration of up
to 16:1 without “knock” or detonation. Many of the
automotive makers already built transportation with a
natural gas fuelling system and consumer does not
have to pay for the cost of conversion kits and required
accessories.
Most
importantly,
natural
gas
significantly reduces CO2 emissions by 20-25%
compare to gasoline because simple chemical
structures of natural gas (primarily methane – CH4)
contain one Carbon compare to diesel (C15H32) and
gasoline (C8H18) [1]. Like methane and hydrogen is a
lighter than air type of gas and can be blended to reduce
vehicle emission by an extra 50%. Natural gas
composition varies considerably over time and from
location to location [1]. Methane content is typically 7090% with the reminder primarily ethane, propane and
carbon dioxide [2].
At atmospheric pressure and temperature, natural gas
exists as a gas and has low density. Since the volumetric
energy density (joules/m3) is so low, natural gas is often
stored in a compressed state (CNG) at high pressure
stored in pressure vessels. To store in liquefied state
(LNG), cryogenic tank is used and temperature at -162ºC
must be maintain to avoid natural gas from evaporated. It
became the drawbacks for cryogenic tank because the boil
off of LNG can cause excessive pressure built up in
cryogenic tanks and boil off natural gas will be vent to the
atmosphere to maintain the pressure inside the tank [3].
Table 2: Typical Composition of Natural Gas [22]
CNG contents
Chemical
formula
% CNG
content
Methane
Ethane
Propane
Butane
Carbon Dioxide
Oxygen
Nitrogen
Hydrogen Sulphide
Rare Gas
CH4
C2H6
C3H8
C4H10
CO2
O2
N2
H2S
70-90%
0-20%
0-20%
0-20%
0-8%
0-0.2%
0-5%
0-5%
Trace
A,He,Ne,Xe
Cryogenic tanks also required large space compare to
pressure vessels. It shows that, LNG more suitable for
heavy duty vehicles such as trucks and buses and not
suitable for cars. In Malaysia, CNG were widely used
as fuel driving system in public transportation
especially for buses and taxis. To store CNG, pressure
vessels must be used to hold and store high pressure of
compressed natural gas (CNG) at pressure of 200 Bar
(2.06 x 107 Pa)[4].
Pressure vessels are manufactured all over the world
under various international standards such as ISO
11439:2000, ANSI/IAS NGV 2-1998, ISO 98091/1999, ASME Section VIII Div (1) and so on. These
standards do specify the requirements to be met during
the manufacture of pressure vessels to stored CNG.
However, the means used to meet these requirements
are not specified in any of these standards. For
example, base on ISO 11439:2000, it does not provide
design formulae nor list permissible stresses or strains,
but requires the adequacy of the design to be
established by appropriate calculations and
demonstrated by testing to show that pressure vessels
are capable of consistently passing the materials,
design qualification, production and batch tests
specified in this International Standard. The design
shall ensure a “leakage-before-break” failure mode
under feasible degradation of pressure parts during
normal service. If leakage of the metal cylinder occurs,
it shall be only by the growth of a fatigue crack [6].
All pressure vessels manufacturer must developed
their own methodology to check each essential
parameter specified in the relevant standards to meet
the requirement. For example, if attempt is made based
on ASME Section VIII Div (1), the same methodology
could also be extended to pressure vessels
manufactured under different standards. The only
difference would be in the acceptability norms that
obviously could be gathered from the applicable
standard [5].
2. CNG PRESSURE VESSELS TYPE:
Base on ISO 11439:2000, pressure vessels can be
made of any steel, aluminium or non-metallic material
construction but it does not cover pressure vessels
made of stainless steel or welded construction [6].
Pressure vessels can be classified into four types and
designated as follow:
i.
CNG-1 (metal) cylinder was the most
widely used pressure vessel is made of metal
and had a long history in NGV service. This
type pressure vessel was similar to the flatbottomed (DOT 3AA) cylinder used to stored
acetylene gas for welding. Unlike flat bottom
cylinder, CNG-1 cylinder has hemispherical
or dome shape end and made from low alloy
steel and are typically inexpensive and
durable such as AISI 4130 low alloy steel, AA
6061-T6 aluminum alloy [6][7].
Metal
Figure1: Cross-section of CNG-1 (Metal)
cylinder.
ii.
CNG-2 (Hoop-Wrapped) cylinder includes a
metal liner and a composite wrap. This pressure
vessel also called hoop-wrapped since the
composites wrap is only wound around the
cylinder sidewall in the hoop (circumferential)
directions. This pressure vessel is designed so
that the liner, without the wrap, can contain the
maximum fill pressure (1.25 times service
pressure). The metal liner will support up to 55%
of the internal gas pressure and composite will
support 45%. The liners are manufactured using
AISI 4130 low alloy steel or AA 6061-T6
aluminum alloy and the composite wrap is
manufactured from glass or glass fibers in epoxy
or polyester resin [6][7]. It made CNG-2 cylinder
lighter than CNG-1 cylinder.
Metal liner
Composites
Figure 2: Cross-section of CNG-2 (Hoopwrapped) cylinder.
iii.
CNG-3 (Full-Wrapped) cylinder includes a
metal liner wrapped with composite over the
entire sidewall and dome ends using the filament
winding machine [8]. In this type of pressure
vessel, the wrap provides the majority of the
containment strength about 80%. The main
purpose of the liner is to contain the natural gas
and prevent the CNG from contact with the
composites wrapped. This pressure vessel has
long services record as air breathing cylinders for
fire fighters because of its lightweight. This
cylinder lighter compare to CNG-1 and CNG-2
cylinders because of thin metal liner (AA 6061T6 aluminum alloy) and composite wrap is
manufactured from glass or carbon fibers (or
hybrid mix of glass and carbon) in an epoxy or
polyester resin [6][7].
Metal liner
Composites
Figure 3: Cross-section of CNG-3 (Fully-wrapped)
cylinder.
iv.
CNG-4 (All-Composite) cylinder includes a
plastic liner wrapped with composites over
the entire sidewall and dome ends by filament
winding [8]. In this type of pressure vessel
the entire CNG pressure load is carried by
composite. The plastics liner is sorely used as
a gas barrier. The cylinder has been used in
NGV services since 1991. Plastics liner is
manufactured from polyethylene or nylon.
The composite wrap is manufactured from
glass, carbon or mix of them in an epoxy or
polyester resin [6][7].
Composites liner
VSM = π (ro^2 - ri^2) L + 4π/3 (ro^3 - ri^3)
Pressure Vessel capacity is:
V = π (4/3ri^3 + ri^2L)
To study the principal stress of CNG pressure vessel, tank
can be divided into cylindrical and spherical components.
When thin wall cylindrical part is subjected to an internal
pressure, three mutually perpendicular principal stresses hoop stress, longitudinal stress, and radial stress are
developed in the cylinder material (figure 6).
Composites
Figure 4: Cross-section of CNG-1 (Fully
composites) cylinder.
3. DESIGN OF CNG PRESSURE VESSEL
CNG pressure vessels normally consist of three
components as shown in figure 1 based on standard
has been follow. These methods must be strictly in
accordance with the provisions of the prevailing code.
The methods described here are in line with the
requirements of ASME Section VIII Div (1) and hence
shall meet almost all the requirements of other codes
for pressure vessel manufacture.
Figure 5: CNG Pressure Vessel: 1. Shell 2. Head or
Dished End 3. Nozzle [9]
In practice, CNG pressure vessel is classified as thin
wall cylinder. So, the wall thickness, t is small in
comparison with the circumferential radius of
curvature, r. If the ratio r/t > 10, the cylinder is
consider to be thin shell [10]. In designing pressure
vessel, not only the code standard and shape of CNG
pressure vessel is important but there are other factors
must be considered such as selection of material based
on a working knowledge of the properties of material
and determination of the elasticity stability.
Before the actual design calculation of a pressure
vessel is done, the relationship between the optimum
height and the diameter should be determined. The
volume of the sheet metal needed to make a pressure
vessel (figure 5) is determined by the following
formula:
Figure 6: The normal stress σ1 (hoop stress), σ2
(longitudinal stress). [11]
If the ratio of thickness (t) and the inside diameter of the
cylinder (di) is less than 1:20, membrane theory may be
applied and the hoop and longitudinal stresses were
assume approximately constant across the wall thickness
(t). The magnitude of radial stress is negligibly small and
can be ignored. It is to be understood that this simplified
approximation is used extensively for the design of thin
cylindrical pressure vessels. However, in reality, radial
stress varies from zero at the outside surface to a value
equal to the internal pressure at the inside surface [10].
The ends of the cylinder are assumed closed. Hoop stress
is set up in resisting the bursting effect of the applied
pressure and is treated by taking the equilibrium of half of
the cylindrical part by looking at y-axis. Total force acting
on the half cylinder is:
Fh = Pi di L
Where: di = internal diameter of cylinder, and
L = length of cylinder.
The resisting force due to hoop stress σ1 acting on the
cylinder wall, for equilibrium, must equal the force Fh.
Thus:
Fh = 2σ1t L
Then:
Hoop stress, σ1 = Pi ri / t
Despite its simplicity, hoop stress can also be expressed in
terms of the radius of the circle passing through the
midpoint of the thickness. Then:
Then:
Hoop stress, σ1 = Pi (ri + 0.5t) / t
Eq. (3) gives the relevant maximum stress in a spherical
shell. The expressions for hoop stress for a thin
cylindrical shell and for a thin spherical shell are similar.
This simple deduction is great importance in the design of
pressure vessels because the thickness requirement for a
spherical pressure vessel of the same material strength
and thickness-to-diameter ratio is only one half that
required for a cylindrical shell. The change in internal
volume (∆V) of a spherical shell can be evaluated easily
from consideration of volumetric strain and the original
volume. Volumetric strain is equal to the sum of three
equal and mutually perpendicular strains. The change in
internal volume due to internal pressure is given by:
To determined the longitudinal stress σ2, by looking a
section perpendicular to the X-axis and considered the
free body diagram consisting of the portion of the
vessel and its contents located to the left of the section.
F1 = Pi π ri^2
The resisting force due to longitudinal stress σ2 acting
on the cylinder wall, for equilibrium, must equal the
force F1. Thus:
F1 = 2 σ2 r t
σh = Pi ri / 2t
∆V = (3 Pi di / 4Et)(1-V) Vo
Then:
Longitudinal stress, σ2 = Pi ri / 2t
From Eq. (1) and Eq. (2) noted that the hoop stress σ1
is twice as large as the longitudinal stress σ2:
σ1 = 2σ2
A sphere is a symmetrical body. The internal pressure
in a thin spherical part of CNG pressure vessels will
set up two mutually perpendicular hoop stresses of
equal magnitude and a radial stress.
When the thickness-to-diameter ratio is less than 1:20,
membrane theory permits us to ignore the radial stress
component. The stress system then reduces to one of
equal biaxial hoop or circumferential stresses.
4. TREND OF RESEARCH
(2)
Most of the researches done by manufacturers,
universities or research organizations stressed on
developing CNG pressure vessels to store high capacity of
compressed natural gas with less weight and small in
sizes.
Company like Tenaris Co. was produced seemless steel
tube pressure vessels as CNG storage tank. Tenaris is one
of the manufacturer developed the process for seemless
tube pressure vessels. To produce CNG pressure vessel,
seemless tube will be heated both part. At bottom, the
seemless tube will be totally closed and form a dished end
by forming process but on top of seemless tube, the
forming process were little bit complicated to form a
nozzle part [12].
Figure 8: Tenaris CNG seemless steel tube pressure
vessel (Dalmine S. p. A.) [13].
Figure 7: Spherical shell under internal pressure [10].
Considering the equilibrium of the half sphere, it can
be seen that the force on the half sphere due to internal
pressure Pi is:
F = (π/4) Pi di^2σ2 r t
The resisting force due to hoop stress is given by:
Fi = π di σh t
According to ISO 11439:2000, this type of pressure
vessel is considered under CNG-1 class because its using
100% of metal [6]. This type of pressure vessels was very
heavy. One way to manufacture light pressure vessel is by
changing the material from metal to composite but the
problem is porosity in composite material [14].
Refer to L. Varga, he designed CNG tank made of
aluminum and reinforced plastics [15]. He used aluminum
alloy (AlMgSi1) as a liner to protect and prevent the CNG
in contact with a composite (plastic). So, it solved the
problem of composite porosity. This hybrid structure
also reduces the weight of the pressure vessels.
Figure 9: Various layer of CNG pressure vessel made
by NK Co. Ltd. (CNG2) [16].
Figure 10: Process trial for wet-filament winding of large
tow-size carbon fiber composite [19].
This type of pressure vessels had been manufactured
by many of company such as NK Co. Ltd. (Korea) and
NGVI. Inc. (Italy). NK. Co. Ltd company for example,
used AISI 4130 as a liner and fiber glass and resin as
composite by hoop wrapping of resin embedded with
continuous fibers. The layer continues with axial fiber
and resin rich layer [16]. The external layer gives an
extra protection to the CNG pressure vessel.
Lincoln come out with new pressure vessel
(TUFFSHELL) design consist of a durable plastic liner
fully wrapped with epoxy impregnated carbon and fiber
glass. The liner is made from High Density Polyethylene
(HDPE) and has two aluminum end bosses, which
provides the structural interface to the tank. Figure 11
showed clearly where aluminum boss located.
Comdyne Inc. had taken another step by producing
CNG-3 (type3) fully wrapped with fiber glass and
aluminum alloy as a liner [17]. Even though Comdyne
CNG pressure vessel had failed once, it is not because
of CNG tank but acid from inside Dodge Van leak
through access door in van floor causing corrosion
cracking of CNG cylinder but no injury reported [18].
In conjunction with producing full composite (CNG-4)
pressure vessel, high cost of composite increase almost
40% of manufacture cost compare to CNG-1.
Composite CNG pressure vessel is typically fabricated
using the wet-filament winding process. This is a
process where the fiber tow is passed through a resin
bath to impregnate the tow and then wrapped around a
mandrel prior to curing in an oven at elevated
temperature. According To J. Michael Starbuck, using
large tow-size (50,000 filaments) carbon fiber will
reduce the cost of conventional tow-size carbon fiber
approximately 1⁄2 the cost and had a potential
improvement [19]. The Akzo Fortafil 3(C) large towsize carbon fiber was selected for evaluation based on
the vendor reported impregnated strand tensile
strength and the fiber’s processing characteristics. It
was determined from process trials that modifications
to the manufacturing method were required when
using the Akzo large tow-size carbon fiber. A typical
process trial fabrication set-up is shown in Figure 10.
The modifications consisted of increasing the fiber
tension, pulley diameters, and bandwidth, and
modifying the type of compaction used.
The
bandwidth is the amount of tow advance per one
revolution of the mandrel. Lincoln Composite Inc.
had been providing Natural Gas Vehicle (NGV) fuel
storage tanks to the industry for over 11 years, and
currently has over 50,000 containers in services [20].
Figure 11: Typical cross section of the TUFFSHELL allcomposite tank [21].
5. CONCLUSION
Various scenarios of development and manufacture CNG
pressure vessels have been discussed and it seem all the
CNG pressure vessels (CNG-1 TO CNG-4) had their own
advantages and disadvantages. The cost of manufactured
and material will increased from type-1 to type-4. New
solution, manufacturing process or material should be
used to reduce the cost but must follow the international
standard characteristics. Not only the cost but reliability
and life span of tank also must be considered to developed
economical and reliable tanks.
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