Embed Size (px)
Citation preview
Dr. WiesnerSteuerungstechnik GmbH
Weststraße 4D-73630 RemshaldenPhone +49 7151/9736-0Fax +49 7151/[email protected]
Industrial Leakage Testing
• What does “tight” really mean?• Selecting the suitable test-method • Influencing factors• Overview of leakage-test methods
Tight or untight?
Change in the tire pressure of your car amounts to
• 0.1 bar per minute• 0.1 bar per hour• 0.1 bar per day• 0.1 bar per week• 0.1 bar per month• 0.1 bar per year
Is your tire tight or not?
Pressure drop Leakage rate
0.1 bar/minute rough untight 3880 cm³/min
0.1 bar/hour unacceptable 64.7 cm³/min
0.1 bar/dayacceptable in somesituations
2.7 cm³/min
0.1 bar/week tolerable 0.39 cm³/min
0.1 bar/month acceptable 0.09 cm³/min
0.1 bar/year tight 0.0074 cm³/min
Pressure drop in a car tire
Tire diameter 630 mm; rim-base diameter 350 mm; width 180 mm; filling volume 38.8 l; (215/65 R15)Loss of pressure from 2.5 to 2.4 bar
Holding your tire underwater, you find within oneminute• 100 air bubbles• 10 air bubbles• one air bubble• no air bubble
Is your tire tight or not?
0.1 bar/minute 3880 cm³/min ~924.000 air bubbles/min
0.1 bar/hour 64.7 cm³/min ~15.400 air bubbles/min
0.1 bar/day 2.7 cm³/min ~640 air bubbles/min
0.1 bar/week 0.39 cm³/min 93 air bubbles/min
0.1 bar/month 0.09 cm³/min 21 air bubbles/min
0,1 bar/year 0,0074 cm³/min 1.8 air bubbles/min
Pressure drop in a car tire
Assumed diameter of air bubble d=2 mm; V = 0.0042 cm3
(at d=1 mm: V= 0.00052 cm3 / at d= 3mm: V= 0.014 cm3)
Comparison of admissible leakage rates
Pressure drop in a car tire
Leakage rate Examples appliedto:
0.1 bar/minute 3.880 cm³/min car exhaust muffler(or catalytic converter)
0.1 bar/day 2.7 cm³/min water pump(also applicable to AdBlue®)
0.1 bar/year 0.009 cm³/min fuel tank(gasoline)
0.1 bar/~150 years 0.00006 cm³/min coolant circulationsystem (refrigerator)
• Required tightness dependents on the application and usage of the component
• The admissible leakage rate requires to be defined
“tight” only means
measured leakage rate < admissible leakage rate
Admissible leakage ratesIn the industrial testing sector, leakage is tested against various types of medium applying air or other gases.
The following standard values are used:
CharacteristicAir leakagefrom to
waterproof 0.5 cm³/min 12 cm³/min
oil-proof 0.6 cm³/min 4.5 cm³/min
gasoline-proof0.0006 cm³/min(benzol gas)
3 cm³/min(liquid)
gas-tight derived from application
Definition of leakage rate:
Admissible current of (air)volume per unit of time Units:- cm³/min or ml/min- l/min- mbar l/s
Admissible change in pressure per unit of timeUnits:- mbar/s- mbar/min
Additional information requirements:
• Height of test-pressure or -vacuum
• Where admissible pressure drop is specified: test volume
• Special loads, if any, in the course of testing
Where the admissible leakage rate and test conditions are not specified, these must be derived from the real conditions under which the component will be used.
Conversion: leakage rate / change in pressure
To assess the required test resolution when selecting a pressure-test method or in order to standardize any admissible change in pressure in alignment with a leakage rate, an approximate 1 conversion is carried out based on the Boyle-Mariotte Law.
1 Not taking into account changes in temperature occurring in the course of testing and disregarding real air pressure.
cm³/minml/min l/min mbar l/s Pa m³/s
1 cm³/min1 ml/min 1 0.001 0.0167 0.00167
1 l/min 1 000 1 16.667 1.667
1 mbar l/s* 60 0.060 1 0.1
1 Pa m ³/s* 6 0.006 10 1
Leakage-rate conversion
* 1 mbar l/s (1 Pa m 3/s) corresponds to the amount of leakage causing change in pressureof one mbar (Pa) per second in a volume of one liter (m3).
Test-method identification
This takes place on the basis of:
• leakage rate• properties of test specimen• production conditions• aspects relating to specific field of application• testing costs• required test documentation
Leakage rates and suitable test methods
Properties of test specimen
• Elasticity of test specimen• Hermetically sealed test volume• Highly heat-conductive materials
Production conditions• Special temperature conditions?
• Necessity to carry out leakage detection?• Influencing ambient processes?
Other criteria for selecting suitable test methods
Aspects relating to specific field of application
• Pressure ratios relating to specific application?• Mechanical stress relating to specific application?• Is it necessary to incorporate functional testing?
Testing costs• Unit costs in respect of jigs and fixtures
• Cost of consumption per test• Personnel costs• Unit costs in respect of maintenance, calibration and upkeep
Other criteria for selecting suitable test methods
Other criteria for selecting suitable test methods
Required proof of quality
• Precise quantification of leakage rate required?• Manual rating allowed?• Documentation of test data required?
Influencing variables
Is 0.1 bar change in pressure detectable in this way?
Influencing variables
• Different test methods
• Inadequate measuring accuracy• Non-reproducible adaptation• Operator influences• Physical influences• Behavior of test specimen• Process-specific influences
Clamping and sealing devicesDesign guidelines
No stress on testing area caused by tensional or sealing forces
Implementation:
Fork beds incorporating counter-holding functionSealing head fitted with expanding gasket
Clamping and sealing devicesDesign guidelines
Adaptation corresponding as far as possible to specific field of application
Implementation:
Inflatable gasket sleeve, fitted to a hose nipple from outside
Clamping and sealing devicesDesign guidelines
Minimization of wear on the sealing components of the clamping and sealing deviceto prevent pseudo-rejections
Implementation:Use of a gasket corresponding to the original, but manufactured from material extremely resistant to wear
Clamping and sealing devicesDesign guidelines
Closed flux of force at point of adaptation at high testing pressure(Example: Bursting-pressure test).
Low influence on test specimen as a result of adaptation
Rating criteria for the both principle different leakage-test methods
Air-pressure measuring and testing methods
+ Low investment and running costs, simple operation.- Sensitive to changes in temperature, depending on behavior of test
specimen, sensitivity depends on test pressure and test volume.
Gas verification methods – sniff test / integral test+ Very low leakage rates detectable, insensitive to changes in temperature- High investment and running costs, mostly unsuitable for big leakages
Pressure-testing methodsAir-under-water testing
Method: Test specimen is filled with pressure and moved under water. Bubbles at points of leakage are mostly observed visually or, in seldom cases detected by a ultrasonic sensor equipment.
Pressure-testing methodsAir-under-water testing
Test medium: Compressed air or nitrogenDetectable leakage rates: > 0.1 cm³/minAdvantages: - simple leakage detectionDisadvantages: - test specimens get wet
- often high costs caused by water preraration- operator-dependent- quantitative leakage-rate determination
difficult
Pressure-testing methodsBubbling through method
Method: Test specimen is filled with pressure. Air escaping at points of leakage is refilled and made visible through bubbles appearing in a water inspection glass.
Pressure-testing methodsBubbling through method
Test medium: Compressed air, nitrogen or vacuumDetectable leakage rates: > 0.1 cm³/min (depending on pressure)Advantages: - robust, simple and extremley low-cost
method- no wetting of test specimens
Disadvantages: - operator-dependent rating- quantitative leakage-rate determination
difficult
Pressure-testing methodsBubbling through method
Bubbling throughtest unit
Pressure-testing methodsGeneral features
Method: The test chamber is filled with compressed air (or in more seldom cases with pressurised nitrogen or other gases) or evacuated and shut off. The change occurring in pressure due to leakage is measured and evaluated .
Advantages: - automatable- operator-independent evaluation - low costs - quantifiable leakage-rate determination - automatic documentation capability of test results
Disadvantages: - test result influenced by thermal and elastic changesoccurring to test specimen
Pressure-testing methodsRelative -pressure method
(absolute-pressure or pressure-differential method)
Detectable leakage rates:Volume- and pressure-depending >1 cm³/min
Advantage:Simple, low-cost method
Disadvantage:Test resolution dependent on test pressure
Pressure-testing methods Relative -pressure method
(absolute-pressure or pressure-differential method)
Simple, low-cost relative-pressure test unit
LTScompact
Pressure-testing methodsDifferential-pressure method
Detectable leakage rates:Volume- and pressure-depending>0.1 cm³/min
Advantage:Test resolution independent of test pressure
Disadvantage:Testing technology involves higher complexity
Pressure-testing methods
Pressure-testing methodsDifferential-pressure method
State-of-the-art differential-pressure
test unitINTEGRA DD6
Pressure-testing methodsPressure-rise method
Special feature: pressure-rise is measured inside a hood surrounding the test specimen
Detectable leakage rates: volume-depending >0.1 cm³/min
Advantages: - faster tests possible- suitable for tests with very high testing
pressure
Disadvantages: - high mechanical complexity- evaluation uncertain if test hood is untight
(additional effort for supervision)
Pressure-testing methodsTesting of hermetically sealed components under a test hood
Special feature: The volume inside a hood surrounding the testspecimen is filled with pressure or evacuated. The pressure change, caused by the specimen inthis volume is measured and evaluated.
Detectable leakage rates: volume-depending >0.1 cm³/min
IMPORTANT: Requires an additional pre-test against roughleakage. In addition, the internal volume of the testspecimen must be at least 5 – 10% of the sum total resulting from the dead volume of thetest hood + tube volume + test-circuit volume.
Pressure-testing methodsTesting of hermetically sealed components under a test hood
Basic diagram showing relative-pressure test implementation
Pressure-testing methods Testing of hermetically sealed components under a test hood
Gas verification methods
Gas verification methodsSniff test – general features
Method: Pressurised test gas is filled into a test chamber. The test gas escaping at points of leakage is detected by a probe, primarily operated on manual lines.
Advantages: - independent of temperature and elasticity- leakage point detection is possible
Disadvantages: - operator-dependent evaluation- quantitative leakage-rate determination difficult - risk of working area contamination
Gas verification methodsHydrogen sniff test
Test medium: Forming gas (5% hydrogen / 95% nitrogen)
Detectable leakage rates: >0.0001 cm³/min
Advantages: - simple applicationDisadvantages: - natural constituent of hydrogen in
atmosphere restrictive (0.5 ppm) - sometimes unsuitable for testing plastic
parts due to high hydrogen permeation - risk of cross-sensitivity when testing
anodised aluminum components
Gas verification methodsHydrogen sniff test
Hydrogen sniffer
Gas verification methodsHelium sniff test
- natural constituent of helium in atmosphererestrictive (5 ppm)
- sometimes unsuitable for testing plasticcomponents due to high helium permeation
Disadvantages:
- very low leakage rates verifiableAdvantages:
>0.00001 cm³/minDetectable leakage rates:
Helium, pure or mixed with N2 or airTest medium :
Gas verification methods
Helium sniffer
Gas verification methodsIntegral test – general features
Method: Pressurised test gas is filled into a test chamber. Slightest traces of test gas escaping at points of leakage into a test hook are automatically detected.
Advantages: - operator-independent evaluation - no influence on test result due to thermal and elastic changes occurring to test specimen
- quantifiable leakage-rate determination - automatic documentation capability of test results
Disadvantages: - high mechanical complexity - risk of test-gas contamination due to maloperation orcoarse leakage
Gas verification methodsHydrogen test method (integral test)
Test medium: Forming gas (5% hydrogen / 95% nitrogen)non-combustible
Sensitivity: 1-2 ppmAdvantages: - no vacuum inside test hook
- low effort for sensory and mechanical systems
- test gas easily removableDisadvantages: - 0.5 ppm hydrogen present in natural atmosphere
- sometimes unsuitable for testing plastic components due to high hydrogen permeation
Gas verification methodsHydrogen test method (integral test)
Gas verification methodsLaser leakage-test method (integral test)
- sensory system costlyDisadvantages:
- very low leakage rates verifiable- no high-vacuum necessary - robust, sensory system insensitive to
interference- in most cases low gas consumption costs - no disturbing influences through natural
constituent in atmosphere
Advantages:
>0.0001 cm³/minDetectable leakage rates:
Gas mixture from air or N2 primarily with very low proportion of sulfur hexafluoride (SF6).
Test medium:
Gas verification methodsLaser leakage-test method (integral test)
Gas verification methodsLaser leakage-test method (integral test)
Complete system forlaser leakage-test method
Gas verification methodsHelium mass-spectrometer method (integral test)
- sensory system costly and sensitive- very high outlay for peripherals and test
chambers- 5 ppm helium present in natural
atmosphere- sensitive to moisture and dirt- in most cases unsuitable for leakage rates
above 0.1 cm3/min
Disadvantages:
- extremely low leakage rates verifiable- oldest and most widespread gas
verification method
Advantages:
>10-10 mbar l/sDetectable leakage rates:
Gas mixture from air or N2and heliumTest medium:
Gas verification methodsHelium mass-spectrometer method (integral test)
Diagram illustrating helium integral leakage-test method
Other leakage-test methods
• Flow test method (volume flow)Specially for larger admissible leakage rates
• Flow test method (mass flow)Also for minor leakage rates. Disadvantage: critical applied to rough leakage
• Laser scan methodGas detection verification method for automatically leakage localisation of leakage rates > 0.01 cm3/min. Disadvantage: very costly
• Ultrasonic leakage-test methodManually applied method for detecting sound generated by escaping gas. As an alternative, ultrasonic sources are deployed in the interior of the test specimen. Suitable for localisation of major leakages on very large test objects.
Other leakage-test methods
From practically all the methods previously specified, special applications can be derived catering for better simulation of the conditions to which the test specimens are subjected in actual practice or even for making the test possible at all:
• Pressure or test-gas admittance internally or externally• Internal or external vacuum testing• Special pressure-testing methods for hermetically sealed
components• Cambering for test-gas testing of hermetically sealed components
