Leak detection theory- ForTest
LEAK DETECTION THEORY
Unlike most system of units, as for example that measurable or that dimensional, the loss control requires almost necessarily a machinery. The right test method selection depends generally on the following parameters:
- Admissible loss rate value
- Test type: loss location or loss measurement
- Specific list of the tested detail: dimensions, pressure and empty resistance limit, packaging materials, surface finishes etc...
- Utilization and test conditions
- Security and environment parameters
Some of the applicable methods are quoted in the following schedule:
[ Pa m³/Sec]
|Tracer gases and Spectrometer||Helium||Local./ Pass-No Pass||10-11 … 10-6|
|"In bell" compliance interception test||Air||Pass-No Pass||10-6|
|Loss pressure test||Air||Pass-No Pass||10-5|
|Volume flow rate or mass flow test||Air||Pass-No Pass||10-4|
|Visual inspection in water tank and pressure air||Air||Visual||10-4|
The Helium method is not tested, as it is not contemplated in our production. It is considered in fact that the system is placed at the height of the sensitivity and the equipment and management costs make it applicable only where it is really necessary, that is to say on the limits of components for refrigerant gases, microelectronics, pace-maker, etc. The immersion test is not mentioned here too, because it has none of the technical abilities, but for the fact that it can really find and identify the loss point.
The short sensitivity level according to the rule must, in fact, be interpreted as method impossibility to give a measurement, and that means a total uncertainty if applied on product line, and a high management cost due to the impossibility to be automated.
PRESSURE DECAY LOSS DETECTORS
There are essentially two different types for Δp/Δt measure: absolute systems and differential systems. Both of them execute a test cycle that is based on three fundamental phases:
Filling, which allows to pressurize the test cell, Settlement that establishes the volume of the introduced air, and Test where is analyzed the pressure development in order to measure an eventual decay of the same one during the time.
The absolute system, represented in FIG. 1, is the theoretical method most immediate, economical and evident in order to execute that measurement.
The differential system, represented in figure 2, today is employed in those cases where it is necessary to have the same sensibility with very different pressures, or where are executed high pressures tests (>20 Bar), even if we will see later on, that the interception systems are in any case better and safe thanks to the elevated pressures which are brought into play.
The system limits are the following:
- Major pneumatics complexity
- Non positive safety pneumatics
- Double pressure measurement section ( filling and test )
- Lower measurement repeatability
- Longer test times
- Major instrumentations costs
We can analyze the FIG. 2 in order to understand the differences between the two systems and considering the differential application in a symmetrical way, that is to say with a airtight standard piece and a test piece; it is easy to understand that, between the first day test and the following tests, we will see the standard piece with a settlement cumulus both thermal and mechanical equal to "n", while the piece in test equal to "0", because it is substituted test after test. It is for that reason that the system is non comparable with a direct measure in repeatability terms.
Furthermore, what is really important to consider going from a system to another, is that the pressure values measured in Δp/Δt do not often coincide. In fact, in the absolute system, this is the real pressure decay, that is comparable with a certified precision manometer, while the differential measurement is the measure of the difference between two pressures. According to the tested elements, to the symmetrical or not symmetrical usage and to the settlement times, we can consider a proportion from 1:0,8 to 1:0,1 between absolute measure and differential measure: in other terms, the millibar in a second that is measured with an absolute or manometer system, can be considered as 0,8 - 0,1 mb/s on the basis of a differential system. This thing does not mean that the differential system does not work correctly, but it simply means that they are two different types of measure among themselves, and this fact must be considered during the installation phase.
FLOW LEAK DETECTORS
This system allows to measure in a direct way the flow rate or airflow generated by the leak. At the end of the filling and settlement phases the test time is the only factor which is necessary in order to obtain a stable measure about this flow: this time is generally very short (ex.: 0,1 Sec.).
This system is essentially made up of a flow sensor, a pressure sensor and a constant pressure generator in the range of the measurement flow. The pneumatic complexity of the system is in the possibility to give a constant pressure flux but without any bobbing and noises because, unlike Δ, p systems, the measure is in open pressure force.
The flow measure can be done using volumetric temperature-compensated systems or actually, through massive meters.
In the first case it is recommended to value the CNR-UNI 10023 and the ordinary laws of the gases physics for the temperature compensation.
The practical application of these instruments for the leaks measure takes place in four cases:
- when the volume of the detail is not known and variable: for example very flexible packs or bottles;
- when the test times must be reduced at most;
- when it is necessary a continuous measurement of the leak in order to execute analysis and repairs;
- when the leakages which must be measured are so elevated that a Δp system cannot maintain a constant test pressure distorting the leakage calculation of the same: cartridge valves or oil distributors, drawings in general.
Usually Δp systems appear cheaper and more lasting for other industrial applications, thanks to the consumable pneumatic parts.
INTERCEPTION LEAK DETECTORS
Compliance leak test system or interception leak test, means a system that is able to measure the leakages in the environment outside of the cavity to test.
The practical example more popular and explanatory is that of loss measurement of the valves shutter: the air is introduced from a side and the leakage is intercepted from the opposite side.
This concept is extensible to every types of component or piece, considering the possibility to enclose the element in a bell and at the same time to pressurize it from the inside.
The value of this system must be found in the high sensibility (referring to the norm: 10 times > than Δp systems and 100 times than flow detectors) and in the elevated speed of test execution.
The high sensibility is due to the possibility to measure the leakage as pressure rise compared to the “zero” environment, then without offset problems.
The elevated speed is due to the fact that, (except particular applications on elastic components), the whole test is executed during the piece pressurization and the result is almost immediate.
In view of the necessity of a test bell execution, the system is suitable for air/nitrogen high pressure tests, also till 180 Bar, because it can be structured in safety way for people and things.
On the opposite, the instrumentation complexity is to be found in the non positive safety pneumatics: it is for that reason that usually these instruments include a traditional leak-flow section in order to verify the correct leak of the closed bell.
LOSSES RATES ANALYSIS
“Zero” loss does not exist and even if it existed, it should not be possible to measure it.
So it is always good to define prior the permissible loss of the own piece, basing on tables given by the norms.
After this first test it is necessary to study the fluid of exercise ( gas or liquid ) and the operative pressures to which the element to test is subjected.
In case of gas components, where does not exist a “watershed” between the molecular dimensions of the fluids (test/operative), we have to follow the only danger evaluation: for example the same element for city gas can have two allowable loss levels completely different in case it is applied to a domestic environment (kitchen) or through open-wire transmission lines.
Some examples of loss rates established by norms for gas components are:
15 - 60 nCC/hour at 150mBar for kitchen gas Ramps
1 - 5 nCC/minute at 5 Bar for open- wire transmission lines joints
In case of components for liquids (water/blood/fuel/oil, etc. etc….) and also referring to the danger level in loss case, there are normative parameters of leakage which is measured in air where the liquid will not certainly draw, thanks to the molecular link between air and a particular fluid.
Examples of loss rates for liquids are (measured in air, 1 Bar)
0,3 - 0,6 nCC/Minute for fuel containers
2,0 - 3,0 nCC/Minute for water containers
3,0 - 6,0 nCC/Minute for oil containers
In reality, where it is possible, it is better to apply more elevated pressures, in the limits of 1………6 Bar at most. Thanks to this solution, the test costs can be reduced and the test performances can be sensitively improved. Improving the test pressure, we obtain a loss amplification, which generically is not linear to the pressure: if for example we measure 1 nCC/minute at 1 Bar test, the same loss measured at 5 Bar can be more major than 5nCC/minute.
Moreover, a major pressure amplifies the eventual defect, if elastic, chipping the meat us as, for example, in case of welding on plastic or cracks.
In opposition it is necessary to value the negative aspects of major pressures, such as major settlement times in case of plastic elements, “under mask” losses in case for example of brim gaskets where the elevated pressure increases the leak of a defective element and problems connected with the safety for people and surrounding environment.
So the right test pressures must be sought with the collaboration of professional men with an experience which is matured in years and above all with the instrumentation useful to execute the initial tests of the case.