Senior Design 03-04: Team 9
Alternative
Testing Methods
Hydrostatic Pressure Decay
Hydrostatic leak detection is conducted by pressurizing the test material
from the inside with a liquid. The component to be tested is filled completely
with a liquid, usually water. The liquid is then pressurized to a required
testing pressure. This pressure is then maintained for a set time at which
point the component can be visually inspected to locate leaks. Often times,
a die or tracer is added to the testing liquid to make it easier to visually
locate the leak.
This ability to visually locate the leak is a big advantage over other
leak testing methods, which only test on a “go-nogo” system.
That is, if the purpose of the test is to determine not only if the product
leaks, but also where it leaks, then the hydrostatic method should be
strongly considered as the method of choice.
Other detection methods can also locate the leak, but are often more costly
and prone to error. For instance, helium testing is a very good method
to locate leaks, but the equipment can be very expensive depending on
how small of a leak needs to be detected. Helium leak detectors are also
very prone to seal failure. Often times, the seals need to be soldered
onto the material in order to ensure that no helium molecules escape around
the seal. The hydrostatic method is much more appropriate on leaks that
are not on the molecular order, and therefore do not require such expensive
equipment or molecular tight seals.
Disadvantages to hydrostatic leak testing would include necessary dry
time after testing. Dry time can be a very, very costly thing depending
on part cycle times as well as other testing and shipping requirements.
Often times, eliminating the dry time can lead to much faster cycle times
and accordingly a decrease in overall costs.
Helium Leak Testing
Helium Leak testing is one method for leak testing any closed circuit.
One of the major uses for helium leak tests is for industrial pipelines,
but it is also used for testing heat exchangers. The test for any component
is basically the same. The chamber is sealed at the ends, helium is pumped
in, a helium sensitive meter is then passed over, checking for leaks.
The main use for leak tests is for localizing the leak, although flow
rate can also be measured.
For conducting a pipeline leak test, the ends of the desired pipe section
are sealed off. Helium is then injected into the pipeline and a technician
scans the ground above the pipeline looking for any helium escaping from
a leak. The reason for pipeline leak tests is for localization of leaks.
For scanning the ground above, the technician uses a helium sniffer, which
is a handheld device using a mass spectrometer that is sensitive to helium.
Another use for helium leak testing is for Heat exchangers. For heat exchangers,
a vacuum helium leak test is done. In doing this type of test, the machine
will first carry out a gross leak test using air on the component, so
the helium is not wasted. Then the actual and accurate test will be carried
out using helium in the chamber. The helium wand/sniffer is passed around
the chamber by the technician and any quantity of helium that permeates
through is picked up by the mass spectrometer.
Vacuum leak testing is a faster test than a pressure type based test.
Reducing manufacturer cycle time. Both leak location identification and
leak rate measurements are possible. Flow may be liquid, gaseous or both.
Helium can be measured at a level below that which is present in the atmosphere.
Some major issues concerning helium leak testing include the following:
- The location of the sniffer probe is operator dependent. The reading depends on the duration, angle, and distance of the probe away from the leak.
- Air currents are always present and affect the air to helium ratio that can be detected.
- The type of permeation of the leak will affect the flow rate.
- Helium is present in the atmosphere and will affect the sensitivity of the meter.
