leak
testing - basic principles
What
is a Leak?
A leak is a flow
of gas (or liquid) through the wall of a vessel (via an
imperfection such as a hole, crack or bad seal).
Leaks require a pressure
difference to generate the flow; they always go from higher
pressure to lower pressure.
Leaks are pictured
as going from positive pressure (inside an object) to outside
(at atmospheric pressure).
This is not always
the case (a leak could be from atmosphere to inside an evacuated
object), but it helps to think about it this way because
the units and terminology are based on this model.
Leak
Testing - what units do you use?
For leaks of air
into atmosphere, units are usually expressed as mm3
or cm3 (cc) per second or minute. So 16.6
mm3/sec = 1 cm3/min. A bubble under
water is about 30 - 50 mm3, so 1 bubble per second
is about 30 mm3/sec or 2 cm3/min.
A standard unit of leakage which takes account of air pressure
is the mbarl/sec. (Millibar-litre per second). A leak into
atmosphere of 1 mbarl/sec is equivalent to a volume leak
of 1000 mm3/sec.
Leak
Testing - what technique should be used?
Key
questions at the start of any leak test requirement are:-
| What
size is the component and what is it’s internal
volume? |
Is
there access to inside or is it a sealed unit? |
| What
is the leak limit? |
Is
it rigid or flexible? |
| Does
it have hidden internal volumes that may affect leak
measurements? |
Are
parts at ambient temperature? |
| Are
the parts clean and dry? |
What is the surface finish of any sealing surfaces? |
Based on the acceptable
leak rate limit (shown in ml/sec) alone the following test
method can be used
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Tracer Gas
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Flow
Rate
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Water Submersal
/ Dunk Tank
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High
Vacuum Helium
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Air
Decay
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10-9
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10-8 |
10-7 |
10-6 |
10-5 |
10-4 |
10-3 |
10-2 |
10-1 |
1 |
5 |
Leak
Testing - generic systems
pressure/vacuum
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The
test piece and the reference volume are simultaneously
pressurised (or evacuated) to a preset pressure. The
air in the system is then allowed to stabilise, with
the supply valves all closed. The Differential Pressure
Transducer is automatically zeroed.
After this
stabilisation time, the pressure change in the test
piece is compared to the pressure change in the reference
volume, using the Transducer. If the test piece is
leaking, the difference will increase and be measured,
an alarm limit may be set for a pass/fail decision.
The sequence
is fully automatic, the accuracy and sensitivity of
the system is defined by the method of setting the
preset pressure together with the quality and type
of control valves and Differential Pressure Transducer.
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helium
systems
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A
vacuum pump evacuates the test chamber and test piece
simultaneously to a preset vacuum. At this preset level,
the chamber and the test piece are isolated and the
chamber evacuated further to a very low pressure. A
positive pressure variation is therefore created between
the test piece and the chamber.
Helium gas
is then introduced into the test piece, often in a
10% concentration. A Mass Spectrometer analyses a
sample from the chamber as the vacuum continues to
be drawn. The Mass Spectrometer measures the helium
leakage and sets the pass/fail decision.
The test piece
pressure is often compared to the chamber pressure
before dosing with helium, to avoid saturating the
Mass Spectrometer in the event of a gross leak
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