Chapter-8 Application of Leak Testing 28-08-2014

7/25/2019 Chapter-8 Application of Leak Testing 28-08-2014

1/13

8APPLICATIONS OF LEAK

TESTINGUntil recent years, leak testing of most objects was performed in a relatively crude

manner. The entire field had been greatly ignored by many groups with the exception of

the American Vacuum ociety. !eakage has now become a serious concern in the

fabrication of nuclear reactor components, unfired pressure vessels, and vessels

containing lethal substances. !eak testing is also used on other components or systemsutili"ed in processing of fluid materials that are affected by the presence of

contaminants that react with the product. !eak test are now made on pyrotechnic

devices, electronic components, automotive, air conditioners, food packages and other

test objects.

8.1 SYSTEM RELIABILITY THROUGH LEAK TESTING

#ne important reason for leak testing is to measure the reliability of the system under

test. !eak testing is not a direct measure of reliability, but it might show a fundamental

fault of the system by a higher than expected leakage rate measurement.

The use of the $leak before break% design concept relies on a detectable leak being the

first indication of a flaw. To the extent this concept is used, the detection of leaks assoon as they occur is critical. !oss of contents can result in many serious conse&uences.

A toxic gas or li&uid may take lives. 'ven a nontoxic contaminant may spoil the

environment.

!eakage might result in loss of lubricant and thus machinery failure. (n the packaging

industry, a leak might result in product spoilage. A high rate of leakage from mechanical

connections might indicate that a gasket is improperly aligned or missing. (n the same

manner, a high leakage valve might show the presence of a misaligned or misthreaded

flange. Therefore, it is possible to detect installation errors by high leakage valves. #f

course, there is always the economic loss associated with a product loss, as well as costs

associated with clean up.

8.2 LEAK TESTING TO DETECT MATERIAL FLAWS

)any leaks are caused by material flaws such as cracks and fissures. ome of these

flaws can be detected by measuring of leakage rates. #ther leaks can be detected by

flaw detection techni&ues that identify leak locations. *owever, neither leak testing

techni&ue will detect all flaws.

!eak testing is therefore complementary to other non+destructive testing techni&ue that

is used to find and evaluate basic material flaws.

Through cracks result in leakage, and through cracks may be precursors of more

extensive cracking or fracture. (n particular, pin holes and tiny internal breaches aredetectable using leak detection techni&ues.

8-1 | P a g e


2/13

8.3 DESIRED DEGREE OF LEAK TIGHTNESS

(n industry, the term $leak tight% has taken on a variety of meanings. A water bucket is

$tight% if it does not allow easily detectable &uantities of water to leak out. A high

vacuum vessel is $tight% if the rate of apparent leakage into the system cannot be

indicated with the e&uipment on hand. #ne might even consider that a gravel truck is

leak tight so long as there are no openings in the truck bed large enough to allow the

smallest muggiest to escape. (n each case, the degree of leak tightness depends on the

individual situations. !eak tightness re&uires that the leakage flow be too small to be

detected. *owever, leak tightness is a relative term. Therefore, it becomes a necessity to

establish a practical level of leak testingsensitivity for any given component under test.

Thus, nothing is leak tight except by comparison to a standard or specification. 'ven

then, the measured degree of leak tightness can be ensured only at the time of leak

testing and under the specific leak testing conditions. !ater operation at higher pressures

might open up leaks.

roduct specifications often recogni"e that perfect leak tightness is impossible, and thus

specify a maximum leak rate. -or many aerospace applications, for example,components and systems are typically re&uired to have a leak rate of less than x /0 +1

standard cubic centimetres of gas at a pressure of one atmosphere 2 x /0 +1 mbar+

!34sec5.

8.4 THE NEED FOR LEAK DETECTION

'ven with today6s complex technology, it is, for all practical purposes, impossible to

manufacture a sealed enclosure or system that can be guaranteed to be leak proof

without first being tested. The fundamental &uestion in leak detection is. 7hat is the

maximum acceptable leak rate consisted with reasonable performance life of the

product8

Anyone who manufactures or uses closed vessels needs leak detection. A partial list of

typical users includes.

+ Any industrial pressure vessel manufacturer.

+ )anufacturers using tubular elements, such as refrigeration e&uipment

manufacturers, petroleum cracking plants, chemical plants, etc.

+ (ndustrial aerosol container manufacturers.

+ Any manufacturer using bellows in his product.

+ Vacuum chamber manufacturers.

+ 9eneral : ; < laboratory users.

+ *ermetically + sealed electronic component manufacturers producing typically,

relays, connectors, &uart" crystals, :ead switches, etc.

7hat type of leakage should these products avoid each manufacturer knows his own

problem for some, it is leakage which will damage the product or impair process, for

others, it is the loss material vital to the product or process.

ome examples are.

Admission of materials through leaks

+ ealed packed foods #xygen, water vapours, bacteria+ emiconductor (= packages. #xygen, water vapour

8-2 | P a g e


3/13

+ harmaceuticals >acteria

+ =ardiac pacemakers, other =orrosive body fluids

implants.

+ =hemical process systems #xygen, water vapour

+ Vacuum tubes all kinds Air 2degradation of vacuum5

+ Vacuum process e&uipment Air, oxygen, other gases

+ 7atches 2conventional or 7ater, vapour or li&uid

digital5

+ ealed temperature and Air 2degradation of vacuum5

pressure sensors

+ =ryogenic storage and Air 2degradation of vacuum

transport units insulations5

'scape of materials through leaks+ :efrigeration and air !oss of refrigerant

conditioning systems

+ >eer and beverage cans !oss of carbonation

+ hock absorbers !oss of oil or compressed gas

+ *ydraulic systems, tor&ue !oss of operating fluid

converters.

+ 'lectrical power transmission !oss of insulating gas

+ Aerosol spray cans !oss of propellant.8.5 APPLICATIONS OF HELIUM LEAK TESTING

The use of helium mass spectrometer leak detectors in commercial, industrial,

scientific, and research organi"ations is expanding at an ever+increasing rate.

These organi"ations have discovered that modern helium leak detection techni&ues

improve operating efficiency, product reliability, and reduce costs. ome example of the

e&uipment that have been leak testing with helium detectors are?

/. *ermetically sealed devices such as relays, connectors, displays, &uart" crystals,

:ead switches, feedthroughs, headers, bellows, and transducers.

3. 'lectronics and aerospace components.

@. uclear reactors.

B. *eat exchangers, containers, pressure vessels.

. Air conditioners and refrigeration systems 2evaporator coils, condenser coils,

compressors, control valves ; etc.5.

1. Automobiles parts.

C. Vacuum system 2furnaces, surfaces analysis, ion implanters, metalli"ers, etc.5

D. Vacuum and pressure systems 2nuclear stations, chemical plants, petro+chemical

towers, underground lines, radio ; TV cables, cryogenic components andsystems etc.5.

8-3 | P a g e


4/13

ome of these are discussed below?

D../ !'AE T'T(9 '!'=T:#(= A:T

D.././ *ermetically+sealed devices

*ermetically+sealed devices, such as small electronic packages, are leak checked using

the inside+out test techni&ue. *elium trace gas is placed inside the package, and leakageis detected by analysis of the tare volume between the part and the fixture in which it is

being testing.

Two fundamental problems in testing sealed packages are?

*ow to put the tracer gas inside the packageF and

*ow to detect Ggross leakers6 2from which the tracer gas has escaped prior to test5.

#ne method of putting tracer gas inside is to make the final assembly in a glove box

filled with the tracer. Although helium can be sealed in at final closure, this is not often

done both because product handling in glove boxes is more difficult and more

expensive, and because some electrical devices perform poorly with helium inside.

'ven if the tracer is put in at final closure, detection of gross leakers remains a problem

since the tracer can dissipate prior to test, especially if the leak is large and the internal

volume is small.

The majority of small sealed products, such as integrated circuits, :eed switches, etc.,

are given their charge of tracer gas after final closure by pressuri"ing them in the tracer

gas for a specified time. This process, known as $bombing% in semiconductor jargon, is

selective, in that it puts tracer gas into leakers only. Typical pressure is atm. 210 psig5

and holding time is / hour, but longer time may be necessary if the leak is small and the

internal volume is large.

D../.3 'lectronic feedthroughs

Testing the electronic packaging for hermetically+sealed devices, such as electronic

feedthroughs, re&uires considerable ingenuity. The parts are small and sensitivity are

relatively high /0+C to /0+D td.==4ec. The glasses to metal seals that bring the

electronic signal from the device sealed in the container to the leads are the points that

must be checked.

The problem involves testing at relatively high speeds and high sensitivities. A test

re&uires evacuation of one side of the part while pressurising with helium on the

opposite side.

D../.@ =athode ray tubes

A conventional automatic leak detector is being used to check all phases of the cathode

ray tube 2=:T5 development process in a oscilloscope manufacturer6s engineering tube

laboratory.

The helium leak detector helps to find leaks &uickly and efficiently, saving on : ; ecause of its low cost and &uick results, the bubble test is also widely

used on consumer products where other tests are not feasible because of their

e&uipment costs or the need for interpretation of test signals whose source and

significance are not immediately obvious.

Telephone utility companies have reported use of solution film bubble emission tests toinspect for damage in telephone cables. -or this test, nitrogen gas is injected within the

cable sheath under suitable pressure.

The bubble testing solution is applied to the exterior surface of the cable. *oles in the

sheath of the telephone cable are detected when the gas leaks out and forms bubble

indications.

roduct and food packaging is tested for leaks using the visual vacuum bubble leak

testing techni&ue for product and food packaging that contains some headspace gas.

This may also be referred to as bubble emission testing when packaging is immersed

under water under atmospheric conditions. The bubble emission test is used todetermine package integrity.

8-6 | P a g e


7/13

(ssues such as packaging material compatibility, sealing machine setup and seal

reliability are crucial for high altitude trucking and airfreight shipments. ackage

integrity is also crucial to consumer safety since heat sealed packages are designed to

provide a contamination free and sterile environment to the product.

!eak testing by bubble emission can be a destructive testing method and just like with

dye penetrant leak testing, manufacturers should expect to waste a certain amount of

package material and products. !eak testing by bubble emission involves submerging

the package underwater and looking for leaks. 7ith the proper e&uipment, vacuum can

also be used to reduce the external pressure on the sealed package, giving sufficient

internal pressure to allow air or gas to leak out from a defect creating a stream of

bubbles.

Vacuum bubble emission testing is performed by filling a test vacuum chamber with

water so that the package is submerged by water. The chamber lid is then closed and

vacuum is applied. Vacuum is slowly increased so the package expands. The package is

observed for a steady progression of bubbles from the flexible container indicating a

leak.

#nce the vacuum is released the package is also inspected for the presence of test fluid

inside the specimen. -lexible packing with little or no head space cannot be reliably

evaluated with this test method. arameters such as the vacuum level during test and the

testing time will vary based on the various types of packaging methods and

re&uirements.

8.8 APPLICATIONS OF ACUUM BO! LEAK TESTING

Vacuum >ox Testing is a practical techni&ue of testing in comparison to the

&uantitative measures used to examine objects. Vacuum box testing can be used to test

objects on which a pressure differential can be created across the area to be examined.The common application areas of vacuum box testing include piping systems,

pressure vessels, and storage tanks.

There are various types of boxes or frames used in Vacuum >ox Testing depending

upon the application area. Vacuum box inspection can be carried out on lap welds,

butt welds and fillet welds. These vacuum boxes are used to examine a small and

specific portion of the welded area to produce accurate results. The vacuum boxes

used in vacuum box testing are fabricated of thick, clear erspex with a closed cell

foam seal along the bottom edge.

>efore proceeding with the vacuum box testing, a vacuum gauge and hose coupling

are fitted in the box. Then a soapy solution is put on the line of welding in the testingobject. After that the vacuum box is placed over the testing object and a vacuity is

created inside the box. ow, the object is observed, if there any drop in the vacuum or

bubble formation on the surface then it shows that there is leak in the weld and the

object is defected or faulty.

Vacuum box testing is a very popular leak testing techni&ue that is widely used to test

various objects against defects, flaws, leaks, or other imperfections. (t is very efficient

and effective way of testing various welded objects for any kind of leaks or defects.

There are basically two main types of vacuum box systems used for performing

vacuum testing?

2a5 *igh ressure )odels that are mainly used for inspection of above ground storagetanks that have been in service.

8-7 | P a g e


8/13

2b5 !ow ressure )odels used for weld integrity checks on new tank builds where

gross defects are the main concern.

8." APPLICATIONS OF FURTHER AND ADANCED TECHNI#UES

D.I./ :A


9/13

F$%. 8.1 R&'$()*&+,* -&)+ /$%*&)$(0 ),+0$,

The radiotracer patch moves along with the fluid and migrates towards the location of

the leak. :ough estimation of the leak flow rate is also possible by measuring the

velocity of the tracer patch movement. The search for the exact location of the leak

can continue only at that side to which the radiotracer is moving. A modification of

this method is to inject a radiotracer from one end and monitor its migration towards

the leak using many detectors installed along the pipe. The pipeline dimensions affect

the velocity of the radiotracer movement along its way from the injection point to the

leak point.

The chance to find small leaks in large diameter pipelines is rather scarce because theradiotracer concentration decreases under the influence of diffusion and dilution with

respect to time. -or larger leaks, this techni&ue works &uite well. The radiotracer

patch migration techni&ue can be employed in some circumstances for both shallow

and deeply buried pipelines. -or shallow buried pipelines, the detection is normally

performed by moving a detector on the ground surface along the pipeline, while for

deeply buried pipelines, the radiotracer patch inside the pipeline is monitored using

radiation detectors lodged into pits dug at regular intervals along the ground surface

projection of the pipeline 2-ig. D.35.


10/13

penetrant leak test should be used prior to other tests, such as hydrostatic and

ultrasonic, to minimise the possibility of flaw passage contamination.

An extended dye penetrant dwell time is usually necessary.


11/13

electrical output recorded on a meter. A tracer gas with a high level of infrared

absorption, such as nitrous oxide, tends to be effective for this method of detection.

REISION E!ERCISE NO. 8

8-11 | P a g e


12/13

APPLICATIONS OF LEAK TESTING

/. A leakage from a mechanical system indicates that LLLLLLLLLLLLLL.

/. !eak testing is complementary to LLLLLLLLLLLLLLLLLLLLLLL.

3. 7hat are the advantages of )odern helium leak detection techni&ue8

@. *ow electronic feed through are tested8

B. 7hat is the leak rate and relative sensitivity level for small electronic feed through8

. ame electronic components leak tested by helium leak testing method.

1. *ow *ermetically sealed devices are leak checked8

C. 7hy the final assembly of the electronic parts in a glove box filled with helium is

difficult8

D. 7hich leak testing techni&ue is most widely used for leak testing8

I. ame the e&uipment tested by halogen leak detectors8

/0. 7here the heated anode halogen vapour detectors are suitable for use8

//. *ow *alogen leak detectors detect the leaks8

/3. 7hat are the advantages the helium leak detector over halogen leak detector (n testing

refrigeration valves8

MULTIPLE CHOICE #UESTIONS

8-12 | P a g e


13/13

APPLICATIONS OF LEAK TESTING

/. !eakage has become a serious concern in the fabrication of?

a. uclear reactor components.

b. Unfired pressure vessels.

c. Vessel containing lethal substances.

d. All of the above.

/. !eak testing is a direct measure of?

a. -abrication process.

b. :eliability.

c. Testing process.

d. one of the above.

3. 7hich of the following gases is injected within the cable sheet under suitable pressureof the telephone cables?

a. *@gas.

b. =#3gas.

c. *e gas.

d. gas.

@. The sensitivity of the helium leak detector is not affected by?

a. The traces of the helium in the plant atmosphere.

b. The tracers of the halogen in the plant atmosphere.

c. >oth a ; b.

d. one of the above.

B. >ubble leak testing techni&ue is preferable due to?

a. (ts low cost.

b. Juick results.

c. *igh sensitivity.

d. >oth a ; b.

8-13 | P a g e

Documents

Chapter-8 Application of Leak Testing 28-08-2014