Liquid butane filled load for a liner driven Pegasus experiment. Page: 4 of 5
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V. BUTANE SAFETY
At the conclusion of the RTMIX-5 experiment about 5
g of heated butane vapor was released to the Pegasus
vacuum chamber. Prior to conducting the experiment
butane safety issues were evaluated. Since the partial
pressure of the heated, flammable gas would be much
lower than the ambient air pressure, a target chamber
breach from shot damage would transport air from an
infinite supply into the vicinity of the leak within the
chamber. The initial velocity of debris from z-pinch
experiments is rarely in any direction other than along the
cylindrical liner axis, vertical in the case of Pegasus. If a
vacuum leak were to occur, it would be expected to occur
at the top or bottom of the vacuum chamber. The potential
for developing an explosive butane/air mixture then is
most probably at the vacuum chamber ends.
The primary butane safety plan was to first prevent a
vacuum breach, and then to flood the chamber with
sufficient CO2 immediately after the experiment so that
any subsequent mixing with air would always result in a
butane/CO2/air mixture that is too lean to burn. Protection
against vacuum breaks was accomplished by putting
debris catchers inside the vacuum chamber at the top and
bottom
A. Safe Pegasus Vacuum Chamber Venting
A remotely operated valve was opened shortly after the
experiment to flood the chamber with CO2. An input CO2
flow-rate of 300 liters/minute will give about 5 vacuum
chamber gas exchanges in 15 minutes. A directional valve
on the chamber allowed the CO2/butane mixture to be
vented out of the building when the chamber pressure
exceeded one Atm pressure. The one-way valve, which
supports vacuum in the chamber was set to open with
only a slight over-pressure in the chamber
As an extra safety precaution, the exhaust line gas was
further diluted with CO2 before reaching the end of the
vent line where the exhausted gas could mix with air. The
exhaust-gas-line diluent was turned on a few minutes
before the experiment. The exhaust line was calculated to
be able to handle flow-rates of twice the chamber input
CO2 flow rate without a large pressure differential. The
exhaust line diluent flow-rate was set to about 10% of the
chamber input CO2 flow-rate.
B. Venting After Shot Damage, Small Leak
In the event the debris catchers fail and vacuum is
broken during the experiment, it will most likely happen
at the top or bottom of the chamber for the reasons
previously stated. The top and bottom of the vacuum
chamber were each covered with CO2 filled balloons
containing enough total volume of CO2 to dilute butane in
the vacuum chamber to a safe mixture before dangerous
levels of 02 from air could enter the chamber. For small
leaks, chamber venting would still proceed primarily as
described above. Any gas mixture that diffused back
through the vacuum leak into the Pegasus building after
the vessel reached ambient pressure would be much toolean to burn and would be removed from the building
with existing exhaust fans. Since the mixture would not
be flammable, standard ventilation systems are adequate.
In the event that a vacuum breach occurred at a place
other than the top or bottom of the chamber, where no
CO2 filled balloons were present, air would enter the
chamber. For a small leak, the CO2 flood would still
dilute the mixture enough to ensure that flammable
conditions were avoided, and flushing would proceed as
described above.
C. Venting After Shot Damage, Large Leak
In the event of a catastrophic failure at a point not
protected by CO2 balloons, the chamber would rapidly fill
with room air. If the chamber were to reach ambient
pressure with the air and flammable gas isotropically
mixed, assuming no dilution with CO2, the resulting
mixture would still be too lean to burn. However, during
filling, an improbable transient flammable condition
could occur. If ignition also occurred, pressure from
combustion would drive gases back into the room through
the catastrophic failure point in the vacuum chamber.
Once in the room, the mixture would immediately
become too lean to support combustion. Inside the vessel,
we expect the combustion would quickly exhaust the fuel
or the 02 supply from air, much like the "poof" that
happens when lighting a propane grill after letting the gas
build up a little too long. The maximum energy release in
this case is calculated to be 250 kJ. The calculation
assumes 100% burn efficiency which requires all of the
butane to be simultaneously mixed with air in locally
flammable concentrations at the time of ignition; not a
likely scenario! Since there are likely to be many hot
sources for ignition inside the chamber after an
experiment, our expectation is that as soon the local
mixture at any one of the hot spots became flammable,
ignition would occur. Then, only the small quantity of
butane locally mixed with air in flammable concentrations
at the time of ignition would burn.
The chance that ignition would be delayed until
conditions for detonation could be established, and that
ignition would also occur, both before uniform mixing
and pressure equilibration with the room resulted in a
"too-lean" non-flammable mixture, is considered to be
vanishingly small. If a stoichiometric mixture of air and
butane did volume detonate to completion inside the
vessel, releasing 250 kJ of combustion energy, we
estimate that the maximum over-pressure produced
outside the chamber wall would be only 0.33 psi,
considered a safe level.
VI. SUMMARY AND CONCLUSIONS
Cylindrical hydrodynamic experiment loads can be
designed and fabricated to take advantage of the material
properties of flammable liquids. Safety in handling
flammable liquids and gasses is of paramount importance.
The techniques to handle and fill such loads are
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Salazar, Mike A.; Armijo, Elfino V.; Anderson, Wallace E.; Atchison, Walter L.; Bartos, Jacob J.; Garcia, Fermin et al. Liquid butane filled load for a liner driven Pegasus experiment., article, January 1, 2001; United States. (https://digital.library.unt.edu/ark:/67531/metadc929163/m1/4/?rotate=270: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.