A Leak Monitor for Industry
In space flight operations, hydrogen leaks pose significant
operational and safety problems. In systems that use hydrogen
propellants, such as NASA's Space Shuttle, the location and severity
of a leak must be determined in a timely manner; failure to do
so may breed consequences that range from program delays to loss
of life and equipment.
In 1990, leaks on the launch pad necessitated grounding of
the Space Shuttle fleet until the leak sources could be identified,
a process that was costly both in monetary terms and in disruption
of Shuttle schedules. The incident triggered an intensive government/industry/academic
effort to develop a comprehensive leak monitoring system for
space launch vehicles. And, since industry experienced similar
leak detection difficulties, the effort was expanded to a double-barreled
approach that additionally embraced development of a commercial
automated gas leak system for industrial applications.
A natural gas-powered Ford Crown Victoria is being checked
for hydrogen leaks.
The latter system-the Model HG2000 Automated Gas Leak Detection
System manufactured by GenCorp Aerojet Industrial Products-made
its debut in 1995 at the St. Thomas, Ontario assembly plant of
Ford Motor Company, where Ford is producing autos that operate
on natural gas in response to the air quality requirements of
the 1990 Clean Air Act amendments.
Other objectives of the program, including a system for improving
NASA's leak detection capabilities for operation of current and
future spacelaunch vehicles, were similarly accomplished. The
development of advanced leak sensors and the automated monitoring
system won a 1995 R&D 100 Award, presented annually to the
100 most significant technological advances of the year.
At the time the development effort was launched, no commercial
system existed for detecting hydrogen leaks at low concentrations
in inert (oxygen free) environments with high sensitivity. There
were hydrogen sensors available, but they had serious deficiencies
when used in applications that demanded operation in inert environments.
To address both matters simultaneously-the sensor and the complete
system-the development program progressed along two parallel
fronts.
NASA's Lewis Research Center, in cooperation with Case Western
Reserve University (CWRU), Cleveland, Ohio, undertook development
of advanced point contact hydrogen sensors. An evaluation of
sensor technology led to a decision to use a palladium-silver
solid state sensor that did not require an oxygen atmosphere
to detect hydrogen leaks and could identify concentrations as
low as one part in a million. The sensor was to be fabricated
on a silicon wafer in a structure that would allow high sensitivity
to changes in hydrogen concentrations, and it had to be miniaturized,
or "microfabricated," to minimize power consumption
and allow placement of sensors in a wide variety of locations.
Although the concept of such a sensor had been investigated,
it had never been fully developed as a complete microfabricated
package.
A NASA/industry team developed a hydrogen sensor that fits
on a wafer chip.
The other part of the two-pronged development effort was conducted
by Marshall Space Flight Center and GenCorp Aerojet Industrial
Products, Las Vegas, Nevada. Their job was to develop a complete
microprocessor-based hardware/software system to monitor multiple
microfabricated hydrogen sensors and visually display the source
and magnitude of hydrogen leaks in real time.
Success on both fronts led to accomplishment, in 1994-95,
of multiple objectives: a significant advancement of sensor technology;
a system for NASA use in leak-checking the Space Shuttle propulsion
system; a prototype Aerojet/Marshall system for leak detection
in Reusable Launch Vehicles now being developed; and the first
commercial application of the system, used in Ford Motor Company
production of the 1996 natural gas-powered Crown Victoria auto.
In development tests of the vehicle, which is fueled by compressed
natural gas stored at 3,000 pounds per square inch pressure,
Ford employed conventional pressure decay and soap bubble techniques
that can take hours to determine that leakage is below the acceptable
limit (2.5 cubic centimeters per hour). That clearly wouldn't
do for production line leak checking. Ford wanted a system that
could test all critical joints and provide a leak reading in
less than 15 minutes. Additionally, Ford required that its production
line system work with nonoxygen-containing gas; make leak measurements
10 times lower than the acceptable rate; provide simultaneous
detection at eight locations; and be able to measure leaks during
system pressurization. Tests of the Aerojet system showed that
it was the only system capable of meeting Ford's specifications.
The NASA/CWRU/Aerojet team is continuing to work on improving
the system. CWRU and Lewis Research Center are focusing on increasing
the detectable hydrogen concentration range of the sensor in
order to broaden the range of useful applications. Aerojet and
Marshall are similarly working to improve the complete system
and Aerojet is exploring possibilities for additional commercial
applications, among them safety monitoring of hydrogen facilities,
such as chemical plants and refineries, and monitoring of hydrogen
buildup in nuclear waste depositories.
|
