Helium Leak Testing

An Introduction ToHelium Leak methods and

Techniques

An Introduction ToHelium Leak methods and

Techniques

Mirza Safeer Ahmad

National Centre for NDT

INTRODUCTIONINTRODUCTION

Helium Leak Testing is a sensitive, Helium Leak Testing is a sensitive, reliable and most widely used method reliable and most widely used method of leak detection in manufacturing of leak detection in manufacturing industries.industries.

Typical users are the manufacturers of Typical users are the manufacturers of electronic tubes and other devices electronic tubes and other devices with critical operating conditions.with critical operating conditions.

INTRODUCTIONINTRODUCTION

Mostly the heavy and light fabricated Mostly the heavy and light fabricated tubes, pressure vessels and tubes, pressure vessels and assemblies are tested using Helium assemblies are tested using Helium leak test.leak test.

Helium leak testing is used throughout Helium leak testing is used throughout industry to locate leaks in even the industry to locate leaks in even the most complex pressure, vacuum and most complex pressure, vacuum and hermetic systems and enclosures.hermetic systems and enclosures.

FUNDAMENTALS OF LEAK DETECTION

Leak:A leak may be defined as an unintended A leak may be defined as an unintended crack, hole or porosity in a containing crack, hole or porosity in a containing wall that allows the admission or escape wall that allows the admission or escape of fluid or gas.of fluid or gas.

The basic function of leak detection is The basic function of leak detection is the location and measurements of leaks the location and measurements of leaks in sealed products, which must contain in sealed products, which must contain or exclude fluids.or exclude fluids.




The Need for Leak Detection:It is for all practical purposes, It is for all practical purposes, impossible to manufacture a sealed impossible to manufacture a sealed enclosure or system that can be enclosure or system that can be guaranteed to be completely leaking guaranteed to be completely leaking proof without first being tested.proof without first being tested.

The Need for Leak Detection:The fundamental question in leak The fundamental question in leak detection is; what is the maximum detection is; what is the maximum acceptable leak rate consistent with acceptable leak rate consistent with reasonable performance life of the reasonable performance life of the product?product?

Fundamentals of Leak Detection

Why A Helium Leak Detector?Why A Helium Leak Detector?

SIMS 180TD+

High SensitivityHigh Sensitivity Wide RangeWide Range ReliableReliable QuantitativeQuantitative Dynamic TestingDynamic Testing

Helium Leak DetectionHelium Leak DetectionMass spectrometer is used for helium Mass spectrometer is used for helium leak detection method.leak detection method.Good for leak detection as large as 1 std Good for leak detection as large as 1 std cc/Sec and up to 10 cc/Sec and up to 10 -11 -11 std cc/Sec.std cc/Sec.Extremely sensitive and reliable Extremely sensitive and reliable quantitative measurement capability quantitative measurement capability makes it very important for systems and makes it very important for systems and components which require exact leak components which require exact leak rate specification.rate specification.

Mass Spectrometer Helium Leak Detector

Mass Spectrometer Helium Leak Detector

Why Helium?Why Helium? Low Background (5-20 PPM)Low Background (5-20 PPM) InertInert SafeSafe LightLight Low CostLow Cost

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Operation of Mass Spectrometer Leak Detector


Mass spectrometer is a device for electromagnetic sorting of charged gaseous particles.

Mass spectrometer leak detector (MSLD) is a complete system for locating and measuring the sizes of leaks.



MSLD operating principle consists of ionization of gases in vacuum.

These ions are accelerated with high potential difference and magnetic field.



Factors to be Considered for the Selection of Helium Leak Testing Technique


Size, shape, and location of the equipment to be tested.

Choice between use of pressure or vacuum or both for testing.

Maximum leakage rate specified or that can be tolerated.



Degree of automatic leak testing operation required.

Number of parts or complexity of the system to be tested.



Tracer Probe MethodTracer Probe Method

The mass spectrometer leak detector is connected to the internal volume of an evacuated test object (such as a vessel or piping system) while a helium spray tracer probe is moved over the external surface to detect the specific locations of leaks. The interior of the test piece is


The interior of the test piece is evacuated

A probe is used to directly spray the test gas on the suspected leak sites

Any leaks are evidenced when the tracer gas flows through the evacuated test piece and is detected by the leak detector.



Detector Probe MethodDetector Probe Method

The test object or system is pressurized internally with helium or a gas mixture containing helium.

The mass spectrometer leak detector is connected to the hose of a scanning probe that collects samples of gas leaking from the external surface into the surrounding atmosphere.


To verify probe response before scanning the test object, the probe should be moved past the orifice of a known helium source at the same speed and distance as will be used for the test object.


The detector probe technique can be used to determine leak locations but is inadequate for leakage measurement or for finding leaks smaller than I0-7 Pa-m3/s (I0-6 std cm3/s).



Hood Method (Outside – In Mode)Hood Method (Outside – In Mode)

When vacuum leak testing by the hood technique, the mass spectrometer leak detector is connected to the evacuated interior of the system under test.

The test object or system is then placed under a hood or within a chamber containing helium gas or an air helium mixture usually at atmospheric pressure.


This technique can be used to quantify the total leakage rate of the system.

However, it cannot be used to determine the specific locations of leaks.



Bell Jar Method (Inside – Out Mode)Bell Jar Method (Inside – Out Mode)

Sealed components filled with helium or a gas mixture containing helium are placed in an evacuated testing chamber.

The mass spectrometer connected to this vacuum chamber detects helium leaking from any part of the surfaces of the sealed test objects in the vacuum chamber.


This test does not permit location of leaks on the test object surfaces.

The test part is filled with helium gas or gas mixture containing helium.



Accumulation MethodAccumulation Method

Leaking helium tracer gas is allowed to collect for a period of time before being sampled by the leak detector.

The accumulation method does not usually permit leak location.

However, by sealing off small surface areas and accumulating tracer gas within the sealed volume, areas of leakage can be localized.

Dynamic Testing Dynamic Testing

When Helium leak testing is performed with the high vacuum pump of a helium mass spectrometer continually pumping on the test object.

Static Testing Static Testing

When the vacuum pump is disconnected from the test object.

This method accumulates helium leakage to make a more sensitive measurement and is relatively uncommon

Techniques for Locating LeaksTechniques for Locating Leaks

Spraying Method Sniffer Probe Technique

Spraying MethodSpraying Method

This method is applied to objects and systems that can be evacuated.

MSLD is connected directly to the interior volume of the system under test.

After evacuation of the test object or system the exterior surface or suspected areas are sprayed with a fine jet of helium from a helium probe.


A portion of any helium tracer gas entering the vacuum system through a leak is drawn into the mass spectrometer leak detector.

The presence of helium in the mass spectrometer tube is indicated both audibly and visually by alarms.


The concentration of the helium in the spectrometer tube is indicated by the deflection of the pointer on a panel meter.

Careful scanning with the helium probe permits positive location of leaks.

Sniffer Probe TechniqueSniffer Probe Technique

Internal pressure instead of vacuum is used for conducting a leak test.

This method of testing is referred to as pressure testing.

Such tests require introducing a tracer gas within the test object or use a mixture of a trace gas and some other gas such as nitrogen, under a pressure greater than the atmospheric.



In the sniffing test, the probe used for the detection of leak location is called sniffer probe.

This device attaches to the inlet of the leak detector with a long flexible tube and is used to pinpoint leaks from test parts pressurized with helium.


Vinyl tubing or flexible metal tubing may be used to connect the sniffer to the helium leak detector.

The length of a vinyl probe hose should not exceed 2m (6 ft.) for optimum speed of response and clean up.

The sensitivity of this technique varies with length of the probe hose.

Technique for Accumulationpressure testing


This technique detects the total amount of leakage by enclosing the component under test within the bag, box, shroud or container.

Any gas leaking from the component accumulates in the space (volume) between the component and the enclosure.



A measurement of the accumulation of tracer gas in a measured time period provides a measure of the leakage rate.

The pressure method used to measure this type of leakage with a mass spectrometer leak detector is called accumulation testing.



The vessel to be tested is pressurized with a tracer gas and placed in a sealed room with a leak detector and probe, or the instrument can be outside and the probe connected to the chamber through a port or opening.



The accumulation technique may be applied to vessels of any size or configuration that are capable of being pressurized at greater than atmospheric pressure.

Parts per million testingParts per million testing

In this method, the vessel to be tested is pressurized with helium and placed in a sealed room with a leak detector, or the instrument can be out side and connected to the room by a test line.

A sampling probe is attached with the leak detector and instrument pressure is set and maintained at 25m Pa (0.2 µm of Hg).

Parts per million testingParts per million testing

The percentage of helium in air is 0.000 4%.

To avoid confusion, it may be desirable to “zero” this reading out of the system, after calibration, and before leak testing the vessel. Then any leakage of helium from the pressurised vessel will be picked up by the leak detector.

Global TestGlobal Test

The best way to conduct this type of test is to enclose all or part of the evacuated system under test with an atmosphere containing helium.

The helium will enter the system under test if any leaks are present.

The internal volume of the test object is connected directly to a vacuum pump.

Global TestGlobal Test The leak detector in turn is

connected to the vacuum pump system. Helium leaking from the hood into the test object is detected by the leak detector.

This test permit the determination of a total leakage rate for the entire system.

The porosity of almost any material, including metals and ceramics, can be measured by this method.

Bomb and backfill (bell Jar)Bomb and backfill (bell Jar)

This method is used for sealed objects which can not be attached directly to the detector.

The object is first placed in a vessel containing helium under pressure.

Helium penetrates into the object if it has a leak.

Bomb and backfill (bell Jar)Bomb and backfill (bell Jar)

The object is then removed from the vessel and placed in another vessel which is connected to the detector.

Helium escapes from the object through the leak, and produces a signal.

Vacuum System of MSLDVacuum System of MSLD

Mechanical roughing pump. Mechanical rotary van backing or

fore pump. Oil vapour diffusion pump or turbo

molecular pump. Cryogenic pumping surface. Associated Valve & gauges.

Vacuum System of MSLDVacuum System of MSLD

Capabilities and Limitations of Helium Sniffer Leak testing


The sensitivity of the helium sniffer leak test method in a normal field environment will enable detection of leakage rates in the range of 10-3 to 10-4 pa. m3/sec (10-2 to 10-3 std. cm3/sec), at differential pressure of 100 kPa (1 atm.) using helium mixture of 2 to 5% by volume.



Using the accumulation technique of bagging with polyethylene and rigidly controlling the factors affecting sensitivity, it is possible at a differential pressure of 100 Kpa ( 1 atm.) to detect much smaller leakage rates of 1 x 10-5 pa. m3/sec (10-4 std. cm3/sec) using helium mixture of 2 to 5% by volume.



The increase of sensitivity obtained by bagging and accumulating is directly dependent on time of accumulation

Calibration of Helium Leak Detector


Calibration of the instrument merely correlates the output signal with a known flow of helium

Calibration is usually accomplished by adjusting the gain, or amplifier feedback, until leak rate meter reading agrees with the value stamped on the standard leak.



A standard calibrated helium leak with 100% pure helium at atmospheric pressure is generally used to calibrate the helium leak detector.

The leaking membrane is a silica quartz bulb with diffusion type leakage. The calibration leaks are normally supplied in the 10-6 or 10-10 Std CC/Sec ranges.



The calibration temperature is labelled on each standard leak, and a temperature correction factor is also given.

Concentration of tracer GasConcentration of tracer Gas

The concentration of helium tracer gas shall be approximately 10% by volume at the test pressure, unless otherwise specified by the referencing code section.

The test pressure shall be held a minimum of 30 minute.

MASS SPECTROMETERMASS SPECTROMETER

A mass spectrometer is a device that sorts and filters various types of charged particles.

As gas enters an analyzer, a filament releases a stream of electrons that converts the gas particles into charged ions that are accelerated to a rapid speed.


Under a constant magnetic field, ions that share the same mass move along a specific trajectory toward a collector, which accumulates only the targeted molecules.


Specialized devices, such as helium mass spectrometers, can be configured to remain sensitive only to a particular range of atomic mass, allowing other molecules to pass through the detector without triggering the collector.


Mass spectrometers can only operate within a vacuum, making them effective for leak detection in vacuum systems, but can be adapted for pressurized tracer gas systems through the addition of a probe or an enclosure for collecting leakage.


However, a mass spectrometer’s sensitivity and minimum leak detection rate tend to decrease when the device is applied to a pressure system.


The mass spectrometer produces a beam of positive ions from a sample of tracer gas being investigated, sorts these ions into a spectrum of mass-to-charge ratios, and records or indicates the relative abundance of each species of ion present.


In mass spectrometers, the ion currents of specific ion species are detected electrically.

The signal is usually amplified electronically before being displayed or recorded.


The primary functions of a mass spectrometer instrument are to be sufficiently sensitive to detect all desired ion currents and to be able to resolve or separate completely the ion currents due to different ion species.


Pumping tracer gas samples from leaks in test objects into the vacuum of the instrument.

The ionization of gas sample molecules by electron impact.

The sorting and identification of positive ions according to their mass-to-charge ratios.

MASS SPECTROMETERMASS SPECTROMETER Hot tungsten filaments High Potential Difference and vacuum Analyzing magnetic field Separating slits Target plate Electrometer (charge detector) Amplifier display meter Vacuum Pumps

SNIFFERSNIFFER This simple tracer gas collection

device is connected to a leak detector with a pumping system.

The pump provides the suction necessary to draw tracer gas samples into the sniffer inlet, through the connecting hose, and into the sensing element of the leak detector.

SNIFFERSNIFFER A screw adjustment on the sniffer gun

provides for control of the gas intake and throughout to the leak detector instrumentation.

A sleeve or rubber nozzle on the tip of the probe collects the leaking tracer gas which will travel to the detector.

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Helium Leak Testing