experimental simulation of airbag deployment for pipeline closing
TRANSCRIPT
Experimental simulation of airbag deployment for pipeline closing
F. Seiler*, H. Ende, D. Hensel, J. Srulijes
Aerothermodynamics ATC, French-German Research Institute of Saint-Louis (ISL), 5 rue du General Cassagnou, F-68301 Saint-Louis, France
Received 9 June 2005; received in revised form 14 June 2005; accepted 17 June 2005
Abstract
Small scale tests were carried out at ISL’s shock tube facility STA (100 mm inner diameter) to study the problem of closing a pipeline by
means of an airbag in case of explosions or gas leakages. Experiments were carried out to simulate the flow in a pipeline at velocities and gas
pressures as present in pipeline flows. In this study the gas used was nitrogen at static pressures of 0.2 up to 5 MPa and at flow velocities of
25 m/s up to 170 m/s. A special Nylon airbag, deployed from the tube wall into the pipe, was used to simulate the airbag inflation in a real
pipeline. For this purpose a special gas filling system consisting of a gas generator with a reservoir volume of up to 500 cm3 which permits air
pressures up to 17 MPa to be generated inside the airbag was developed at ISL. With a fast pyrotechnically opened valve the reservoir gas
was released for airbag filling. The airbag inflation was triggered in such a way that it opened in nearly 3 ms into the pipe flow generated by
the shock tube and continued for about 10 ms. For this application a special measuring chamber was designed and constructed with 20
measuring ports. Through two window ports, located one in front of the other, the airbag inflation could be visualized with up to 50
successive flash sparks illuminating a fast rotating film inside a drum camera. Pressure measurements using commercially available PCB
pressure gauges at 9 measuring ports placed along the inner tube surface gave some hints on the behaviour of the wall pressure during airbag
deployment. As a result from the experiments performed it is to conclude, that, with the Nylon airbag samples available, the pipe flow cannot
be blocked by the inflating airbag. The flow forces acting on the airbag during deployment are in the shock tube experiments of the order of
about 1000 N, which are not balanced by the airbags’ neck, fixing it to the shock tube wall. This outcome suggests that a mechanical support
is required to fix the airbag in its place during inflation.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Pipeline closing; Airbag; Shock tube simulation
1. Introduction
In industrial plants flammable/explosive powders, liquids
or gases are transported in small or large diameter pipelines.
Pipelines are also used to transport flammable gases, e.g.
methane, from a city or a country to another city or another
country over thousands of kilometres. There are some main
sources of danger, e.g. a pipeline leakage that contaminates
the environment and may become a risk for human beings.
Furthermore, if flammable materials are transported through
pipelines, accidental explosions may cause severe damage
to the facility and the surroundings.
In case of a pipeline failure, the pipe must be closed as
fast and as close as possible to the location of the accident to
0950-4230/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jlp.2005.06.034
* Corresponding author. Tel.: C33 3 8969 5042; fax: C33 3 8969 5048.
E-mail address: [email protected] (F. Seiler).
prevent damage to equipment and people. It is the state of
the art to use quick sliding spherical valves, which act
mechanically or pneumatically. They require big masses to
be moved for pipeline closing. For diameters of more than
0.6 m there is no solution available for the quick movement
of these big masses and therefore reasonable mechanical
valves cannot be manufactured for such pipelines. Conse-
quently, there is a need for a new and low cost technology
by which pipes up to diameters of more than one meter can
be closed very quickly. The airbag, well-known in cars for
more than 10 years, could provide a solution to this problem.
In automobiles the airbag is opened in ambient pressure
(about 100 kPa). But, in pipelines the gas is in motion and at
a pressure far in excess of ambient pressure, why
consequently the car airbag technique cannot be simply
applied as a pipeline closing system.
The modelling of an unfolding airbag into compressed
flow inside of a tube or duct has not yet been, to our best
knowledge, described in literature. In order to substitute the
well-known sliding valves, commonly used for pipeline
Journal of Loss Prevention in the Process Industries 19 (2005) 292–297
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Fig. 2. Shock tube STA for airbag investigations.
F. Seiler et al. / Journal of Loss Prevention in the Process Industries 19 (2005) 292–297 293
closing, by an airbag, the latter must be inflated by a high
pressure gas generator, be able to withstand high inner and
outer gas pressures without bursting, and stop the pipe flow
completely without being sheared off by the flow forces. To
fulfil these requirements, basic research has to be done to
obtain information on the interaction of bag inflation with
pipe flow, which was obtained experimentally and
theoretically by Seiler, Ende, & Hensel (2002), Seiler,
Ende, Hensel, & Srulijes (2003a,b), Seiler, Ende, Hensel, &
Srulijes (2004). This information must involve data on the
pressure distributions at the inner surface of the pipeline
tube as well as on the pressure profiles at the outer and inner
surface of the airbag during its opening cycle. Airbag
textiles used for car bags seem to not be adequate for this
purpose; see Seiler et al. (2004). New materials, which can
be inflated as required in milliseconds up to seconds and
which can sustain high pressure differences, especially at
the beginning of the bag opening phase, have to be found.
Furthermore, they eventually will even have to withstand
high temperatures in case of stopping moving flame fronts
as well as deflagration or explosion waves.
2. Shock tube equipment for airbag deployment
2.1. Shock tube laboratory
The main task of ISL on this project is to carry out small
scale tests to investigate the interaction of the pipe flow and
the inflating airbag. Experiments in the Shock Tube
Laboratory have been performed with ISL’s shock tube
STA. Fig. 1 shows the inside of the Shock Tube Laboratory
with the shock tube STA on the left hand side. Fig. 2 gives a
closer view of shock tube STA used for the airbag
deployment experiments.
2.2. Shock tube operation
The principle of shock tube operation is described with
the path-time-diagram in Fig. 3. As shown schematically,
shock tube STA consists of two tubes attached to each other,
Fig. 1. ISL Shock Tube Laboratory.
a driver and a driven section, with constant inner diameter of
100 mm. Both tubes are separated by a diaphragm.
The driver section contains the driver gas (nitrogen) at
pressure p5. The driven section contains the test gas, in this
study nitrogen, at pressure p1. For pipeline flow modelling a
thin copper membrane with a thickness of up to 1 mm is
used. This membrane bursts at the driver gas pressure p5,
resulting in a shock wave which propagates downstream
inside the driven section towards the closed end of the
driven tube. The shock wave sets the test gas in motion to
the gas velocity u2 and compresses it to the static pressure p2
heating it slightly to the gas temperature T2. The duration of
the uniform flow produced between the shock wave and
contact zone is on the order of 10 ms for the conditions used
in this study.
Shock tube STA was operated at different pipe flow
conditions as extensively described by Seiler, Ende, &
Hensel (2002), Seiler et al. (2003a,b), Seiler et al. (2004). In
the current work two conditions are presented; with
velocities and pressures as: (a) Low pressure condition
(LPC): u2z170 m/s, p2z0.2 MPa, (b) High pressure
condition (HPC): u2z25 m/s, p2z5 MPa. Usually, for
similar shock tube conditions, the flow data differ slightly
from experiment to experiment. This deviation mainly
depends on the burst pressure at which the membrane
Fig. 3. Principle sketch of shock tube operation.
Fig. 4. Shock tube measuring chamber for pipe flow simulation.
Fig. 5. Shock tube equipment.
F. Seiler et al. / Journal of Loss Prevention in the Process Industries 19 (2005) 292–297294
releases the driver gas. The variations in the flow data
studied are in the range of less than G5%.
Fig. 6. Measuring chamber equipped with nine pressure gauges, with airbag
housing and gas generator.
2.3. Measuring chamber
A special measuring chamber was designed and
constructed for the experiments performed. This test section
is equipped with 20 openings, 50 mm diameter each one,
five at each part along the four outer sides of the chamber.
The inner diameter of the chamber corresponds to the shock
tube inner diameter (100 mm). The tube set-up is given with
two photos in Fig. 4. The arrangement of the measuring
chamber installed between the shock tube and the dump
tank as well as the optics used are shown on the photographs
in Fig. 5.
For the shock tube experiments the airbag deployment
was visualized with a spark source and a refurbished
‘Strobodrum’ drum camera (right hand photo in Fig. 5). The
‘Nanolite-Ministrobokin’ light source (see on the left photo
in Fig. 5) produces 50 successive air sparks for illumination.
The airbag inflation could be visualized through the two
32 mm diameter window ports located above the airbag
housing (see Fig. 6). The image sequence was photographed
on a film rotating inside the ‘Strobodrum’ camera, using the
50 air sparks as the light source. The time interval from
spark to spark in the studies reported herein is 300 ms.
Pressure measurements made, using commercially-avail-
able PCB pressure gauges at nine measuring ports, gave
information about the time-dependent behaviour of the wall
pressure during airbag deployment. Five pressure gauges
were fixed on the opposite side of the airbag holder at
the upper tube part. Four sensors were available at the lower
side. The distance between the gauges is 115 mm. On Fig. 6
the airbag housing is seen fixed at the lower part of the
measuring chamber just under the window openings used
for visualizing the airbag inflation. For airbag filling, a gas
reservoir with a gas volume of 500 cm3 is placed under the
airbag housing.
2.4. Airbag deployment simulation
A cylindrical type C air bag manufactured by Auto-
motive Safety Components International Limited (ASCIL)
in the UK (see Fig. 7) was made airtight by inserting a party
balloon; see Seiler et al. (2004). The balloon is shown in
Fig. 8 together with the non-compressed airbag shape. The
airbags employed in the shock tube experiments were
Fig. 7. Type C airbag.
Table 1
LPC condition
Flow velocity u [m/s] z170
Static flow pressure pstat [MPa] z0.2
Gas generator reservoir pressure pres [MPa] 3
Deployed airbag inner pressure pairbag [MPa] z1.1
Pressure increase CDpfront [kPa] z100
Pressure decrease KDpback [kPa] z50
Force [N] 1178
F. Seiler et al. / Journal of Loss Prevention in the Process Industries 19 (2005) 292–297 295
woven by ASCIL with a special Nylon material. The airbag
was folded before deployment and placed inside of the
airbag housing. To fill the airbag, a fast opening valve
released the airbag gas contained inside the gas reservoir.
The pressure reservoir was filled with compressed air which
was produced by a special high-pressure gas compressor
designed for 100 MPa maximum pressures. The filling of
the airbag began by opening the valve between the pressure
reservoir and the airbag. The valve is operated pyrotechni-
cally and has been developed at ISL for this application. The
opening characteristics were adapted to the duration of the
shock tube flow, requiring an airbag deployment in about
3 ms. At the time the shock tube-generated pipe flow passed
the airbag housing, the airbag, which is mounted flush with
the tube surface before flow onset, was deployed from the
airbag holder by the reservoir pressure release.
3. Pipe flow simulations
3.1. Static airbag deployment
Tests of airbag filling in stationary flow were first carried
out to analyse the tightness of the airbags used. Visualiza-
tions showed that the airbag deployed nearly symmetrically
in its cylindrical shape inside of the measuring chamber.
Statically, the airbags used (equipped with the party
balloon) could withhold inner pressures up to approximately
Fig. 8. Airbag C and rubber inlet.
1 MPa before the seams burst. In contrast to former
experiments by Seiler et al. (2003a,b), performed without
inner rubber shell, in this case the airbag was completely
airtight up to the burst pressure.
3.2. Pipe flow duration
As discussed by means of the time-path-diagram in
Fig. 3, the pipe flow is generated by the shock wave front
which moves downstream the shock tube towards its end.
As soon as the shock enters the measuring chamber the pipe
flow starts with condition 2, which depends on the shock
tube operation, i.e. the driver and driven gases as well as
their fill-in gas pressures. For the conditions available
herein, see Section 2.2, the flow lasts as mentioned for about
10 ms and provides constant flow velocity u2, constant static
gas pressure p2, constant gas density r2 as well as a constant
gas temperature T2.
3.3. Dynamic airbag experiments
3.3.1. Low pressure condition
The airbag is initially placed, before deployment, inside
of the airbag housing. Once the shock generated flow passes
the airbag housing, the filling of the airbag starts with the
pyrotechnically opening of the valve placed between the
pressure reservoir and the airbag.
The experimental outcome for the LPC flow (see
Table 1) is depicted in Fig. 9 for the upper five pressure
gauges. The lower four pressure recordings show a similar
behaviour, see Seiler et al. (2004). Additionally, the image
Fig. 9. Pressure signals for the LPC condition.
Fig. 10. Torn off airbag C.
Fig. 11. Pressure signals for the HPC condition.
F. Seiler et al. / Journal of Loss Prevention in the Process Industries 19 (2005) 292–297296
series taken show the deployment of the airbag visualized
via the observation windows. The time points for film
illumination are marked in the pressure plot as a peak series
along the time coordinate. The pipe flow starts with the 6th
image of a total of 50 images taken. During the 17th and
18th images the airbag inflates quite symmetrically. Up to
image no. 21 the window is totally covered by the airbag
and appears as though it could close the pipe for
approximately 1 ms. The airbag is then displaced down-
stream as can be observed in the images 22 and 23.
As shown in Fig. 9, the five sharp pressure rise signals at
approximately 1–2 ms correspond one after the other to the
shock wave arrival time. Unfortunately, some electrical
disturbances caused by the trigger signal necessary to open
the airbag filling valve are also evident at this time. At the
moment when the airbag closes the pipe flow, images 19 up
to 21, the pressure raises rapidly in front of the airbag by
about CDpfrontz100 kPa (gauges no. 1 and 2) and drops
behind it by KDpbackz50 kPa (gauges no. 4 and 5). Gauge
3 is placed in opposite of the airbag housing showing a steep
pressure peak. The acting force on the airbag due to the
pressure difference is in the range of about 1200 N. This
forces the airbag in the downstream direction. This can be
seen in the photograph of the airbag in Fig. 10, taken after
the airbag’s neck, securing it to the tube wall, was sheared
off due to the strong flow forces exerted on the airbag body.
Because of this downstream motion of the airbag, the
downstream pressure gauges no. 4 and 5, covered by the
opened airbag, are at that time liberated and the pressure
boosts up abruptly to the condition in front of the bag, see
pressure-time-diagram in Fig. 9.
Table 2
HPC condition
Flow velocity u [m/s] z25
Static flow pressure pstat [MPa] z 5
Gas generator reservoir pressure pres [MPa] 17
Deployed airbag inner pressure pairbag [MPa] z6.1
Pressure increase CDpfront [kPa] z50
Pressure decrease KDpback [kPa] z50
Force [N] 785
3.3.2. High pressure condition
Similar results as for the LPC flow have been found for
the HPC flow (Table 2) performed with a static pressure of
5 MPa. An arrangement of the image series taken is
presented in Fig. 11. The sequence starts with image no.
14. In image no. 18 the airbag is seen to deploy, it is
deployed in image 21 and it moves downstream from image
24 up to 39. The downward movement is the result of the
small friction forces, which prevent the airbag from being
fixed in a stable manner inside the measuring chamber. The
neck was torn off because it could not withstand the high
shear forces due to the pressure difference between front and
back of the airbag. The front pressure increases by CDpfrontz50 kPa and diminishes by KDpbackz50 kPa at the
back. The pressure difference is of the same order as already
found for the LPC flow. The HPC data are listed in Table 2.
4. Discussion
4.1. Airbag dynamics
The dynamics throughout the airbag inflation period and
for the pipe-flow closing phase is outlined in Fig. 12. The
airbag deployment induces a pressure wave, which moves
upstream causing pressure intensification in front of the
airbag. Downstream the pressure is reduced by flow
expansion.
Fig. 12. Flow pattern during airbag deployment.
Fig. 13. Time-dependent front and back pressure formation.
F. Seiler et al. / Journal of Loss Prevention in the Process Industries 19 (2005) 292–297 297
The amount of pressure increase CDpfront and the drop
to KDpback as well as the temporal pressure change
dpfront/dt and dpback/dt depend on several variables, e.g.
the speed of airbag inflation, the flow speed, the static gas
pressure, the length of the pipeline and other factors. By the
rapid airbag deployment, the pressure increase by CDpfront
and is expanded by KDpback (Fig. 13). In absence of a
permanent lasting gas leakage, with ongoing time, the
pressure grow (CDpfront) and drop (KDpback) vanishes, as
qualitatively shown in Fig. 13. This behaviour is compar-
able to the ‘water hammer’ phenomena well-known in fast
closing water pipes.
According to the LPC and HPC shock tube experiments
the pressure difference, occurring during airbag deployment
develops practically independent of the static pressure level
present in the pipe flow to a maximum Dpz150 kPa.
Regarding this pressure difference between front and back
side, one may conclude that in the shock tube experiments
the airbag could not be auto-stabilized inside of the pipe in
order to resist the flow forces acting on the airbag’s face of
about 1000 N and more. This force must be balanced by
surface friction at the pipe wall. This demand cannot be
accomplished with the currently available airbag material
and design as the airbag neck is torn off by the strong
shearing forces present.
In the presence of a gas leakage in a gas pipeline in case
of damage, the forces acting on the airbag can be more
severe. For example, during air bag deployment the pressure
drops at the bag’s rear to 100 kPa ambient pressure, whereas
a pressure in excess of 5 MPa remains on the front.
Therefore, the acting force on the air bag becomes immense.
4.2. Airbag performance
The ASCIL type C airbags are more shear resistant than
those from former experiments; see Seiler et al. (2003a,b).
Nevertheless, from the current shock tube experiments it
was determined that the airbag design used cannot support
the flow forces exerted to the airbag’s front surface. In all of
the experiments performed, the airbags were torn off near
the neck (Fig. 10) which is used to fix the airbag to the gas
supply.
5. Summary
The experimental results using ISL’s shock tube facility
STA for pipe flow simulation showed that the ASCIL type C
airbags, improved with an inner rubber party balloon shell,
could statically withstand about 1 MPa inner pressure.
Nevertheless, the airbag material could not be stabilized
inside the pipe to balance the pressure forces present during
airbag inflation. In all experiments the neck was torn off
from the airbag body which sheared downwards. The
airbags available at the present time cannot be fixed inside
the pipe. During its inflation, no support balances the flow
forces and consequently the airbag is displaced downstream.
Future airbag concepts must be adapted to the high
pressure forces expected during airbag deployment. They
should be manufactured with stronger, airtight balloon
materials, which can last the required periods of time. Also,
airbag support remains a concern. A promising solution
could be to support the airbag mechanically during inflation.
Thus, first shock tube experiments have been carried out by
loading an airbag, deployed inside the measuring chamber,
with a pressure shock of about 200 kPa without damage and
displacement. This result makes it clear that for successfully
closing a pipeline with an inflating airbag an efficient
support should be considered
Acknowledgements
The work of ISL is financially supported by the EC
Contract No. G1RD-CT-2001-00661, entitled ‘Airbag
for Closing of Pipelines on Explosions and Leakages
(AIRPIPE)’.
References
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