Apart from the vacuum systems themselves and the individual components used in their construction (vacuum chambers, piping,valves, detachable [flange] connections,measurement instruments, etc.), there are large numbers of other systems and products found in industry and research which must meet stringent requirementsin regard toleaks or creating a so-called “hermetic” seal. Among these are many assemblies and processes in the automotive and refrigerationindustries in particular,butalso in many other branches of industry. Working pressure in this case is often above ambient pressure. Here “hermetically sealed” is defined only as a relative “absence of leaks”. Generalized statements often made, such as “no detectable leaks” or “leak rate zero”, do not represent an adequate basis for acceptance testing. Every experienced engineer knows that properly formulated acceptance specifications will indicate a certain leak rate (seebelow)under defined conditions. Which leak rate is acceptable is also determined by the application itself.
Types of leaks
Differentiation is made among the following leaks, depending on thenature of the material or joining fault:
- Leaks in detachable connections:Flanges, ground mating surfaces, covers
- Leaks in permanent connections: Solder and welding seams, glued joints
- Leaks due to porosity: particularly following mechanical deformation (bending!) or thermal processing of polycrystalline materials and cast components
- Thermal leaks (reversible):opening upat extreme temperature loading (heat/cold), above all at solder joints
- Apparent (virtual) leaks: quantities ofgaswill be liberated from hollows and cavities inside cast parts, blind holes and joints (also due to the evaporation of liquids)
- Indirect leaks: leaking supply lines in vacuum systems or furnaces (water, compressed air, brine)
- “Serial leaks”: this is the leak at the end of several “spaces connected in series”,e.g.a leak in the oil-filled section of the oil pan in a rotary vane pump
- “One-way leaks”: these will allow gas to pass in one direction but are tight in the other direction (very seldom) An area which is not gas-tightbut which is not leaky in the sense that a defect is present would be the
- Permeation (naturally permeability) of gas through materials such as rubber hoses, elastomer seals, etc. (unless these parts have become brittle and thus “leaky”).
Calculating leak rate, leak size and mass flow
No vacuum device or system can ever be absolutely vacuum-tightand it does not actually need to be. The simple essential is that the leak rate be low enough that the required operating pressure, gas balance and ultimatepressurein the vacuum container are not influenced. It follows that the requirementsin regard tothe gas-tightness of an apparatus are the more stringent the lower the required pressure level is. In order to be able to register leaks quantitatively, the concept of the “leak rate” with the symbol QLwas introduced; it is measured with mbar · l/s or cm3/s (STP) as the unit of measure. A leak rate of QL= 1 mbar · l/s is present when in an enclosed, evacuated vesselwith a volume of 1 l the pressure rises by 1 mbar per second or, where there is positive pressure in the container, pressure drops by 1 mbar. The leak rate QLdefined as a measure of leakiness is normally specified in the unit of measure mbar · l/s. With the assistance of the statusequation (1.7)one can calculate QLwhen giving the temperature T and the type of gas M, registering this quantitatively as mass flow,e.g.in the g/s unit of measure. The appropriate relationship is then:
(1.7)
(5.1)
where R = 83.14 mbar · l/mol · K, T = temperature in K; M = molar mass in g/mole;Δmfor the mass in g;Δtis the time period in seconds.Equation 5.1is then used
a) to determine the mass flowΔm/Δtat a knownpVgas flow ofΔp· V/Δt(see in this context thepage on thepressure rise test)or
b) to determine thepVleak gas flow where the mass flow is known (see the following example).
Example for case b) above:
A refrigeration system using Freon (R 12) exhibits refrigerant loss of 1 g of Freon per year (at 77°F or 25°C). How large is the leak gas flow QL? According toequation 5.1for M(R12) = 121 g/mole:
Thus, the Freon loss comes to QL = 6.5 · 10–6mbar · l/s. According to the “rule of thumb” for high vacuum systems given below, the refrigeration system mentioned in this example may be deemed to be very tight. Additional conversions for QLare shownin TablesVIIaandVIIbin Chapter 9.
Table VIIa Conversion of throughput (Qpv) units; (leak rate) units
Table VIIb Conversion of throughout (QpV) units: (leak rate) units
Total leak rate < 10-6mbar · l/s:Equipment is very tight
Total leak rate 10-5mbar · l/s:Equipment is sufficiently tight
Total leak rate > 10-4mbar · l/s:Equipment is leaky
A leak can in fact be “overcome” by a pump of sufficient capacity because it is true that (for example at ultimate pressure pendand disregarding the gas liberated from the interior surfaces):
(5.2)
(QLLeak rate,Seffthe effective pumping speed at the pressure vessel)
WhereSeffis sufficiently great it is possible – regardless of the value for the leak rate QL– always to achieve a pre-determined ultimate pressure of pend. In practice, however, an infinite increase ofSeffwill run up against economic and engineering limitations (such as the space required by the system).
Whenever it is not possible to achieve the desired ultimatepressurein an apparatus there are usually two causes which can be cited: The presence of leaks and/orgasbeing liberated from the container walls and sealants.
Partial pressure analysis using amassspectrometeror the pressure rise method may be used to differentiate between these two causes. Since the pressure rise method will only prove the presence of a leak without indicating its location in the apparatus, it is advisable to use a helium leak detector with which leaks can, in general, also be located much more quickly.
In order to achieve an overview of the correlation between the geometric size of the hole and the associated leak rate it is possible to operateon the basis ofthe following, rough estimate: A circular hole 1 cm in diameter in the wall of a vacuum vessel is closed with a gate valve. Atmospheric pressure prevails outside, a vacuum inside. When the valve is suddenly opened all the air molecules in a cylinder 0.39 inches (1cm) in diameter and 1082ft (330m) high would within a 1-secondperiod of time“fall into” the hole at the speed of sound (330 m/s). The quantity flowing into the vessel each second will be 1013mbar times the cylinder volume(see Fig. 5.1).The result is that for a hole 1 cm indiameter QL(air) will be 2.6 · 104mbar · l/s. If all other conditions are kept identical and helium is allowed to flow into the hole at its speed of sound of 970 m/s, then in analogous fashion the QL(helium) will come to 7.7 · 10+4mbar · l/s, or apVleaking gas current which is larger by a factor of 970 / 330 = 2.94. This greater “sensitivity” forheliumis used in leak detection practice and has resulted in the development and mass production of highly sensitive helium-based leak detectors (seepage onleak detectors with mass spectrometers).
Fig 5.1 Correlation between leak rate and hole size
Shown inFigure 5.1is the correlation between the leak rate and hole size for air, with the approximation value of QL(air) of 10+4mbar · l/s for the “1 cm hole”. Thetableshows that when the hole diameter is reduced to 1μm(= 0.001 mm) the leak rate will come to 10-4mbar · l/s, a value which in vacuum technology already represents a major leak (see the rule of thumb above). A leak rate of 10-12mbar · l/s corresponds to hole diameter of 1 Å; this is the lower detection limit for modernhelium leak detectors.Since the grid constants for many solids amount to several Å and the diameter of smaller molecules and atoms (H2, He) are about 1 Å, inherent permeation by solids can be registered metrologically using helium leak detectors. This has led to the development ofcalibratedreference leaks with very small leak rates (seepage oncalibratingleak detectors).This is a measurable “lack of tightness” but not a “leak” in the sense of being a defect in the material or joint. Estimates or measurements of the sizes of atoms, molecules, viruses, bacteria, etc. have often given rise to everyday terms such as “watertight” or “bacteria-tight”; seeTable 5.1.
Compiled inFigure 5.2are the nature and detection limits of frequently used leak detection methods.
Table 5.1 Estimating borderline leak rates. As opposed to vapor, it is necessary to differentiate between hydrophilic and hydrophobic solids. This also applies to bacteria and viruses since they are transported primarily in solutions.
Fig 5.2 Leak rate ranges for various leak detection processes and devices
The standard helium leak rate
Required for unequivocal definition of a leak are, first, specifications for thepressuresprevailing on either side of the partition and, secondly, the nature of the medium passing through that partition (viscosity) or its molar mass. The designation “helium standard leak” (He Std) has become customary to designate a situation frequently found in practice, where testing is carried out using helium at 1 bar differential between (external) atmospheric pressure and the vacuum inside a system (internal, p < 1 mbar), the designation “helium standard leak rate” has become customary. In order to indicate the rejection rate for a test using helium under standard helium conditions it is necessary first to convert the real conditions of use to helium standard conditions (seethe sectionon conversion equations below).Some examples of such conversions are shown inFigure 5.3.
Fig 5.3 Examples for conversion into helium standard leak rates
Conversion equations
When calculating pressure relationships and types ofgas(viscosity) it is necessary to keep in mind that different equations are applicable to laminar and molecular flow; the boundary between these areas is very difficult to ascertain. As a guideline one may assume thatlaminarflowis present at leak rates where QL> 10-5mbar · l/s and molecular flow at leak rates where QL< 10-7mbar · l/s. In the intermediate range the manufacturer (who is liable under theguaranteeterms) must assume values on the safe side. The equations are listed inTable 5.2.
Here indices “I” and “II” refer to the one or the other pressure ratio and indices “1” and “2” reference the inside and outside of the leak point, respectively.
Table 5.2 Conversion formulae for changes of pressure and gas type
Terms and definitions
When searching for leaks one will generally have to distinguish between two tasks:
- Locating leaks and
- Measuring the leak rate.
In addition, we distinguish, based on the direction of flow for the fluid, between the
a.vacuum method(sometimes known as an “outside-in leak”), where the direction of flow is into the test specimen (pressure inside the specimen being less than ambient pressure), and the
b.positive pressure method(often referred to as the “inside-out leak”), where the fluid passes from inside the test specimen outward (pressure inside the specimen being greater than ambient pressure).
The specimens should wherever possible be examined in a configuration corresponding to their later application – components for vacuum applications using the vacuum method and using the positive pressure method for parts which will be pressurized on the inside. When measuring leakrateswe differentiate between registering
a. individual leaks (local measurement) – sketches b and d inFigure 5.4, and registering
b. the total of all leaks in the test specimen (integral measurement) – sketches a and c inFigure 5.4.
Fig 5.4Leak test techniques and terminology.
a: Integral leak detection; vacuum insidespecimen
b: Local leak detection; vacuum insidespecimen
c: Integral leak detection (test gas enrichmentinside the enclosure); pressurized testgas insidespecimen
d: Local leak detection; pressurized test gasinside the specimen
The leak rate which is no longer tolerable in accordance with the acceptance specifications is known as therejection rate. Its calculation is based on the condition that the test specimen may not fail during its planned utilization period due to faults caused by leaks, and this to a certain degree of certainty. Often it is not the leak rate for the test specimen under normal operating conditions which is determined, but rather the throughput rate of atest gas– primarilyhelium– under test conditions. The values thus found will have to be converted to correspond to the actual application situationin regard tothe pressures inside and outside the test specimen and the type of gas (or liquid) being handled.
Where a vacuum is present inside the test specimen (p < 1 mbar), atmospheric pressure outside, and helium is used at the test gas, one refers tostandard helium conditions. Standard helium conditions are always present during helium leak detection for a high vacuum system when the system is connected to a leak detector and is sprayed with helium (spray technique). If the specimen is evacuated solely by the leak detector, then one would say that the leak detector is operating in thedirect-flow mode. If the specimen is itself a complete vacuum system with its own vacuum pump and if the leak detector is operated in parallel to the system’s pumps, then one referstopartial-flow mode. One also refers to partial stream mode when a separate auxiliary pump is used parallel to the leak detector.
Whenusingthepositivepressuremethod,itissometimeseitherimpracticalorinfactimpossibletomeasuretheleakageratedirectlywhileitcouldcertainlybesensedin anenvelopewhichenclosesthetestspecimen. Themeasurementcanbemadebyconnectingthatenvelopetotheleakdetectororbyaccumulation(increasingtheconcentration)ofthetestgasinsidetheenvelope. The “bombingtest”isaspecialversionoftheaccumulationtest(seethepageonIntegral and Industrialtesting).Intheso-calledsniffertechnique,anothervariationoftheofthepositivepressuretechnique,the(test) gasissuingfromleaksiscollected(extracted)byaspecialapparatusandfedtotheleakdetector. ThisprocedurecanbecarriedoutusingeitherheliumorrefrigerantsorSF6asthetestgas.
Leak detection Leak types and rates Vacuum Fundamentals
