Petroleum engineering
research offers clue
to TWA explosion
BY DAVID F. SALISBURY
A year after TWA 800's
central fuel tank exploded shortly after takeoff,
investigators still don't know why. But Stanford
Professor Sullivan S. Marsden has an idea: flow
electrification. A professor emeritus of petroleum
engineering, Marsden bases his theory on the similarities
between the conditions surrounding the mysterious
explosion that downed the Boeing 747 jumbo jet on July
17, 1996, and the factors that caused a series of
explosions that wracked petroleum plants around the world
in the 1950s. Those explosions finally were traced to the
electrostatic properties of hydrocarbon liquids,
including gasoline and jet fuel.
From the investigations of
those explosions, petroleum engineers learned that the
rapid movement of hydrocarbon liquids through metal pipes
can build up large electrostatic charges. The effect is
called "flow electrification," or streaming
potential. Marsden and his students studied various
aspects of this phenomenon in the 1960s.
After flow electrification
was determined as the cause of the refinery explosions,
the petroleum industry began to take a number of
precautions, such as never pumping liquid hydrocarbons
faster than one meter per second (2.2 miles per hour) and
filling tanks from the bottom, rather than the top, to
avoid splashing.
Based on the information
about the TWA 800 crash that has been made public,
Marsden says that there is a good chance that the
explosion in the aircraft's central fuel tank was touched
off by a discharge of static electricity generated by
flow electrification. Large electrostatic charges could
have resulted from pumping the fuel too rapidly between
wing tanks to maintain the aircraft's balance. Fuel
spraying from a leaking gasket or "o" ring in a
joint between pipes running through the central tank is
another possible cause. Or the charge could have been
created by the flow of fuel through dirty filters, he
says.
If Marsden's theory is
correct, a number of measures could be taken to make
certain that such an explosion does not happen again. One
procedural step, which the Federal Aviation
Administration has already implemented, is prohibiting
747's from running their air conditioners for long
periods while they are on the ground. Another relatively
simple measure would be to install temperature sensors in
the central fuel tank. More expensive preventive actions
include pumping nitrogen into the tank as the fuel is
consumed, and increasing the diameter of the fuel lines,
Marsden says.
Basis for theory
The pertinent facts about
TWA 800 that have led Marsden to propose an electrostatic
cause are: the plane was 25 years old; it refueled at
Kennedy airport and spent about two hours on the ground
with its cabin air conditioners blasting; there was very
little fuel in its central tank, which is located in the
body of the plane; the heat exchangers for the air
conditioners are located under the central fuel tank; and
some of the fuel lines used to pump fuel back and forth
between the wing tanks pass through the central tank.
Jet fuel normally is not
explosive at temperatures below 100 degrees Fahrenheit.
But on TWA 800 the air-conditioner heat exchangers
probably warmed the air/fuel mixture in the tank above
that point. When the aircraft is flying, the energy given
off by the heat exchangers is effectively dissipated to
the outside air. But when the air conditioners are run
while the aircraft is on the ground and the tank is
nearly empty, the heat exchangers put out enough heat to
raise the temperature of the air/fuel mixture into the
danger zone, Marsden says.
Aircraft fuel tanks are
designed with the assumption that the fuel they carry is
explosive. Safety is provided by eliminating possible
ignition sources. According to Marsden, aircraft
designers may have not have fully appreciated the dangers
of flow electrification. Hydrocarbon fuels often carry
small, electrically charged particles. When the fuel
moves through a metal pipe, some of these charges stick
to the inside of the pipe while others continue to move
with the liquid. When combined with the fact that
hydrocarbon fuels are poor conductors of electricity,
this separation causes an electrostatic charge to build
up.
Petroleum industry studies
have shown that it is possible to build up electrical
potentials of thousands of volts within a matter of
minutes by pumping fuel at velocities above one meter per
second, Marsden says. So, if the jet fuel were pumped
from tank to tank at a velocity greater than that to keep
the aircraft in balance, it could have produced the fatal
spark.
The petroleum industry has
also identified other electrostatic conditions that might
have triggered the explosion. One is a spray of liquid
from a small hole or gasket leak. Another is the flow of
hydrocarbon liquid through filters, particularly dirty
filters that contain a build-up of sediments.
"I know that the FAA
has supported some studies on electrostatic effects, but
I don't know how well flow electrification is understood
in the aviation industry," Marsden says.
Not enough information is
publicly available about the design of the 747 fuel tank
or the way that TWA maintained its aircraft to pursue his
theory much further, Marsden says. But he has volunteered
his services to the National Traffic Safety Board (NTSB),
which is conducting the crash investigation, and to the
Boeing Company. "I'd like to do anything that I can
to help make flying safer," he explains.
Last November, he wrote
the NTSB a short letter explaining his theory. Although
he did not receive a reply from the board, about a month
later the New York Times reported for the first
time that the NTSB considered electrostatic discharge
resulting from pumping jet fuel between several fuel
tanks to be a possible cause of the explosion. According
to news accounts, the NTSB now plans to conduct tests on
a fuel tank simulator to check if electrostatic charging
from a stream or spray of fuel could be the ignition
source for the TWA 800 explosion.
"I'd love to get a
look at the reconstruction of the 747 they have back in
Long Island. I think I could get a good idea of what
happened, or at least come up with some simple tests to
figure it out," Marsden says. "After all, I
know what to look for." SR
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