HK1084562B

HK1084562B - Leak detector system and method for vacuum pumping leak detector

Info

Publication number: HK1084562B
Application number: HK06103049.9A
Authority: HK
Inventors: A．里帕特
Original assignee: Agilent Technologies, Inc.
Priority date: 2004-05-14
Filing date: 2006-03-09
Publication date: 2009-10-16

Description

Leak detection system and method for vacuum pumping leak detector

Technical Field

The present invention relates to a vacuum pumping system and a method thereof, and more particularly, to a vacuum pumping system having a high compression ratio for light gases such as helium and hydrogen and a method thereof.

Background

Helium mass spectrometer leak detection is a well known leak detection technique. Helium is used as a tracer gas through the smallest leak in a sealed sample. Helium is then drawn into the leak detection apparatus and measured. The amount of helium corresponds to the leak rate. An important component of the instrument is a mass spectrometer tube that detects and measures helium. The mass spectrometer tube ionizes the inhaled gases and mass analyzes them to separate helium components, which are then measured. In one method, the interior of a test piece is connected to a test port of a leak detector. Helium is sprayed onto the exterior of the test piece, drawn into the interior through a leak and measured by a leak detector.

One requirement for mass spectrometry tubes is that the inlet through which helium and other gases pass be maintained at a relatively low pressure, typically below 2 x 10^-4Torr (Torr). Thus, leak detectors typically include a vacuum pumping system, which may include a roughing, diffusion or turbo-roughing pump and an associated forevacuum pump, and a condenser tube. For example, U.S. Pat. No. 3, 4,499,752 issued to Fruzzetti et al, on 19/1985, and U.S. Pat. No. 4,735,084 issued to Fruzzetti, on 5/1988, both describe vacuum pumping systems for helium mass spectrometer leak detection.

One problem with helium mass spectrometer leak detection is that: vacuum pumping systems used to maintain the inlet of the mass analysis tube at the required pressure may have low compression ratios for light gases such as helium. Thus, helium in the ambient environment can enter the vacuum pumping system in the opposite direction and be measured by the mass spectrometer tube. Helium entering the vacuum pumping system does not represent leaks in the sample, thus creating spurious data. This problem is exacerbated when helium is sprayed onto the specimen, thereby increasing the concentration of helium in the ambient environment and increasing the amount of helium that enters the vacuum pumping system in the opposite direction.

Scroll vacuum pumps have been used in helium mass spectrometer leak detection. Scroll vacuum pumps are also used as roughing and/or backing pumps. US5,542,828 issued to Grenci et al on 8/6 1996 describes a scroll pump for supporting a high vacuum pump in a mass spectrometer leak detector.

Conventional scroll vacuum pumps have a relatively low compression ratio for light gases such as helium. The compression ratio can be increased by reducing the clearance and increasing the number of revolutions of the spiral vane of the scroll vacuum pump. However, this approach can add significant cost to the scroll vacuum pump, which is not feasible for low cost and/or portable applications.

Disclosure of Invention

In view of the above-mentioned deficiencies in the prior art, it is an object of the present invention to provide an improved light gas vacuum pumping system and method that overcomes the above-mentioned deficiencies.

In order to achieve the above object of the invention, according to a first aspect of the invention, there is provided a leak detection system including: a helium mass spectrometer leak detector; a primary vacuum pump having an inlet and an exhaust, the inlet in gaseous communication with the helium mass spectrometer leak detector, the primary vacuum pump comprising an oil-free positive displacement vacuum pump, the oil-free positive displacement vacuum pump being a scroll vacuum pump, a multi-stage roots vacuum pump, a multi-stage piston vacuum pump, a screw vacuum pump, or a hook vacuum pump; and a secondary vacuum pump having an inlet connected to the exhaust of the primary vacuum pump, the primary vacuum pump having a primary vacuum pump compression ratio when operating alone, the secondary vacuum pump having a secondary vacuum pump compression ratio when operating alone, the primary vacuum pump and the secondary vacuum pump operating together having a light gas compression ratio greater than the product of the primary vacuum pump compression ratio and the secondary vacuum pump compression ratio.

According to a second aspect of the present invention there is provided a method of vacuum pumping a leak detector, comprising: providing a helium mass spectrometer leak detector; pumping the helium mass spectrometer leak detector using a primary vacuum pump, the primary vacuum pump having an inlet and an exhaust, the inlet in gaseous communication with the helium mass spectrometer leak detector, the primary vacuum pump comprising an oil-free positive displacement vacuum pump that is a scroll vacuum pump, a multi-stage roots vacuum pump, a multi-stage piston vacuum pump, a screw vacuum pump, or a claw vacuum pump; and supporting the primary vacuum pump with a secondary vacuum pump having an inlet connected to the exhaust of the primary vacuum pump, the primary vacuum pump having a primary vacuum pump compression ratio when operating alone and a secondary vacuum pump compression ratio when operating alone, the primary vacuum pump and the secondary vacuum pump operating together having a light gas compression ratio greater than the product of the primary vacuum pump compression ratio and the secondary vacuum pump compression ratio.

According to a third aspect of the present invention, there is provided a leak detection system comprising: a helium mass spectrometer leak detector; a primary vacuum pump having an inlet and an exhaust, the inlet in gaseous communication with the helium mass spectrometer leak detector, the primary vacuum pump comprising an oil-free scroll vacuum pump; and a secondary vacuum pump having an inlet connected to the exhaust of the primary vacuum pump, the primary vacuum pump having a primary vacuum pump compression ratio when operating alone, the secondary vacuum pump having a secondary vacuum pump compression ratio when operating alone, the primary vacuum pump and the secondary vacuum pump operating together having a light gas compression ratio greater than the product of the primary vacuum pump compression ratio and the secondary vacuum pump compression ratio.

Drawings

For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference, and in which:

FIG. 1 is a block diagram of a vacuum pumping system according to a first embodiment of the present invention;

FIG. 2 is a block diagram of a vacuum pumping system according to a second embodiment of the present invention;

fig. 3 is a block diagram of a vacuum pumping system according to a third embodiment of the present invention.

Detailed Description

FIG. 1 shows a block diagram of a vacuum pumping system 10 according to a first embodiment of the present invention. Vacuum pumping system 10 includes a primary vacuum pump 12, a secondary vacuum pump 14, and further includes a valve 16. The primary vacuum pump 12 has an inlet 20 connected to a system 24 to be pumped. Primary vacuum pump 12 further includes an exhaust 30. Secondary vacuum pump 14 has an inlet 40 connected by a conduit 42 to exhaust 30 of primary vacuum pump 12. Secondary vacuum pump 14 further includes an exhaust 50. Optional valve 16 is connected to conduit 42 between exhaust 30 of primary vacuum pump 12 and inlet 40 of secondary vacuum pump 14. When valve 16 is open, exhaust 30 of primary vacuum pump 12 is connected to interpump exhaust 60 and secondary vacuum pump 14 is effectively bypassed.

Primary vacuum pump 12 may be an oil-free or dry positive displacement vacuum pump having a plurality of clearance seals between inlet 20 and exhaust 30. An oil-free vacuum pump is a vacuum pump that does not use lubricating oil in its working volume. It should be understood that: components of the vacuum pump that are independent of the working volume, such as motors, gears or bearings, can use lubricating oil. A scroll vacuum pump is an example of a vacuum pump having a plurality of gap seals between the inlet and exhaust ports. A suitable scroll vacuum pump is the Varian SH 100. Other types of oil-free vacuum pumps having multiple clearance seals between the inlet and exhaust include oil-free multi-stage roots vacuum pumps, oil-free multi-stage piston pumps, oil-free screw pumps, and oil-free claw pumps. All of these first stage pumps are oil-free positive displacement devices. These pumps have tight backlash to form air pockets separated by individual clearance seals between the inlet and exhaust ports. Examples of commercial uses of these pumps include: (1) screw pump-Kashiyama HC-60; (2) roots pump-Alcatel ACP 28; (3) hook pump-edwards qdp 40; (4) piston pump-Pfeiffer xtrady 150-2.

A scroll vacuum pump includes a non-orbiting scroll member, an orbiting scroll member and a drive mechanism. Each of the fixed scroll member and the orbiting scroll member includes a scroll plate and a spiral vane extending from the scroll plate. The helical vanes intermesh together to define air pockets between the vanes. A drive mechanism imparts rotational motion to the orbiting scroll member relative to the non-orbiting scroll member to move the pockets of air between the vanes toward the discharge port of the pump. A tip seal located in a groove at the tip of the spiral vane forms a seal between the scroll members. The pockets between the vanes can be considered a multi-stage scroll pump and the tip seals can be considered to provide tip seals between the pockets between adjacent vanes. So that the scroll vacuum pump has a plurality of clearance seals between its inlet and exhaust ports.

Secondary vacuum pump 14 may be a relatively inexpensive oil-free vacuum pump. One example is an oil-free diaphragm vacuum pump. A suitable diaphragm vacuum pump is KNF N84.3. In other embodiments, secondary vacuum pump 14 may be an oil-free scroll vacuum pump. In embodiments using valve 16, secondary vacuum pump 14 has a lower pumping capacity than primary vacuum pump 12 because secondary vacuum pump 14 is bypassed until a relatively low mass flow rate is required.

In one embodiment, valve 16 is a spring-loaded poppet valve that vents gas to atmosphere through an interpump exhaust 60. Valve 16 may be configured to automatically open when the pressure at exhaust 30 of primary vacuum pump 12 exceeds atmospheric pressure and to automatically close when the pressure at exhaust 30 drops below atmospheric pressure. Thus, the valve 16 is only open during periods of high mass flow. The total mass flow of the two vacuum pumps is dependent only on the capacity of the first stage vacuum pump and not on the capacity of the second stage vacuum pump. When the system 24 is evacuated, most of the gas is pumped by the primary vacuum pump 12 and vented to atmosphere through valve 16. When the mass flow rate drops, secondary vacuum pump 14 evacuates line 42 to a negative pressure, thereby closing valve 16. The pressure differential across valve 16 causes it to close. From this point on, primary vacuum pump 12 and secondary vacuum pump 14 are connected in series to pump system 24. The exhaust region of primary vacuum pump 12 is then pumped to a pressure level that is close to the base pressure of secondary vacuum pump 14. In some cases where gas is not vented to atmosphere, exhaust port 50 and inter-pump exhaust port 60 are connected to a common exhaust pipe (not shown).

Vacuum pumping system 10 is particularly useful for pumping systems for light gases such as helium and hydrogen, which require high compression ratios. Thus, system 24 may be a helium mass spectrometer leak detector. However, the vacuum pumping system 10 is not limited in this regard and may be used in any system for light gases that requires a high compression ratio, as well as other systems.

Because the oil-free vacuum pump 12 and the secondary oil-free vacuum pump 14 operate in series, the light gas compression ratio is greater than that of a single pump alone and is substantially greater than the product of the compression ratios of the pumps. Reducing the discharge pressure of the primary vacuum pump to a low pressure dramatically increases the ability of the pump to compress light gases. This effect can be detected in a helium mass spectrometer leak detector in the event that the background helium value detectable by the leak detector drops to an extremely low value. For example, in a thousand per million helium environment, a 100 liters per minute (lmp) scroll vacuum pump alone may result in a helium background value of about 5 x 10^-8sccs (standard cubic centimeters per second). When a 5lpm diaphragm pump was connected in series with the scroll vacuum pump, the detected helium background dropped by more than a factor of 1000. The stand-alone base pressures for the scroll vacuum pump and the diaphragm pump were 10 millitorr and 4 torr, respectively. If the pumping efficiency of the primary vacuum pump remains constant, the overall compression ratio of the two pumps in series in the above example will only increase by a factor of 190 (760/4). However, because the background value of helium drops by more than a factor of 1000, the helium pumping efficiency of the primary vacuum pump will increase dramatically.

FIG. 2 shows a block diagram of a vacuum pumping system 10 according to a second embodiment of the present invention. Similar components in fig. 1 and 2 have the same reference numerals. In the embodiment shown in FIG. 2, valve 70 has an inlet 72 connected to exhaust 30 of primary vacuum pump 12. A first outlet 74 of valve 70 is connected to inlet 40 of secondary vacuum pump 14, and a second outlet 76 of valve 70 serves as an interpump exhaust 60. The valve 70 may be an electrically or pneumatically controlled two-way valve. The valve 70 may have a first state in which the inlet 72 is connected to the first outlet 74 and a second state in which the inlet 72 is connected to the second outlet 76. The state of the valve 70 is controlled by a control signal on line 80. In the embodiment shown in FIG. 2, valve 70 is controlled by a pressure signal from system 24. Thus, for example, when the pressure in the system 24 is above a selected value, the valve 70 connects the inlet 72 to the second outlet 76, and when the pressure in the system 24 is below a selected value, the valve 70 connects the inlet 72 to the first outlet 74. In other embodiments, the valve 70 is controlled by different conditions, such as the pressure at the exhaust 30 of the first pumping system 12.

In the embodiment shown in FIG. 2, second vacuum pump 14 is operated when a test, such as a leak detection test, is performed, and second vacuum pump 14 is not operated when a test is not performed. The second vacuum pump 14 is stopped when no test is performed to reduce energy consumption. As shown, control switch 82 is connected in series with power supply V of second vacuum pump 14. The test signal causes the switch 82 to close when a test is performed and causes the switch 82 to open when a test is not performed. It will be understood that: switch 82 is closed prior to testing to provide sufficient time to pump excess light gas from system 24. It is further understood that: it is within the scope of the present invention that different techniques may be used to operate and stop secondary vacuum pump 14.

FIG. 3 shows a block diagram of a vacuum pumping system 10 according to a third embodiment of the present invention. Similar components in fig. 1 and 3 have the same reference numerals. In the embodiment shown in FIG. 3, the primary vacuum pump is an oil-free scroll vacuum pump 90 and the secondary vacuum pump is an oil-free diaphragm pump 92. In one embodiment, scroll vacuum pump 90 is a small oil-free vacuum pump having a speed of 50Ipm and a base pressure of 500 mTorr, and diaphragm pump 92 has a speed of 5lpm, KNF N84.3.

In addition, fig. 3 shows the assembly technique used according to the invention. In one embodiment, system 24, scroll vacuum pump 90 or other primary vacuum pump, diaphragm pump 92 or other secondary vacuum pump, and valve 16 or other valve may be enclosed in a single assembly 100, as schematically illustrated in phantom in FIG. 3. Such a single component structure is used in a compact and/or portable system. By way of example, the system 24 may be a helium mass spectrometer leak detector. In other embodiments, scroll vacuum pump 90 or other primary vacuum pump, diaphragm pump 92 or other secondary vacuum pump, and valve 16 or other valve may be enclosed within assembly 110 shown in phantom in FIG. 3.

Having described various illustrative and non-limiting embodiments and aspects thereof, it is therefore evident that various modifications and changes will be apparent to those skilled in the art. Such variations and modifications are included in the description, which is intended to be illustrative and explanatory and not restrictive of the invention. The scope of the invention is to be defined by the proper construction of the following claims and their equivalents.

Claims

1. A leak detection system, comprising:

a helium mass spectrometer leak detector;

a primary vacuum pump having an inlet and an exhaust, the inlet in gaseous communication with the helium mass spectrometer leak detector, the primary vacuum pump comprising an oil-free positive displacement vacuum pump, the oil-free positive displacement vacuum pump being a scroll vacuum pump, a multi-stage roots vacuum pump, a multi-stage piston vacuum pump, a screw vacuum pump, or a hook vacuum pump; and

a secondary vacuum pump having an inlet connected to the exhaust of the primary vacuum pump, the primary vacuum pump having a primary vacuum pump compression ratio when operating alone, the secondary vacuum pump having a secondary vacuum pump compression ratio when operating alone, the primary vacuum pump and the secondary vacuum pump operating together having a light gas compression ratio greater than the product of the primary vacuum pump compression ratio and the secondary vacuum pump compression ratio.

2. The leak detection system as recited in claim 1, wherein the secondary vacuum pump comprises an oilless diaphragm vacuum pump.

3. The leak detection system as recited in claim 1, wherein the secondary vacuum pump comprises an oil-free scroll vacuum pump.

4. The leak detection system as recited in claim 1, further comprising a valve coupled to the exhaust of the primary vacuum pump, the valve configured to provide an interpump exhaust in response to a selected condition.

5. The leak detection system as recited in claim 4, wherein the secondary vacuum pump has a smaller pumping capacity than the primary vacuum pump.

6. The leak detection system as recited in claim 4, wherein the valve comprises a poppet valve configured to automatically open based on a predetermined pressure differential across the valve.

7. The leak detection system as recited in claim 4, wherein the valve comprises a controllable valve configured to connect the exhaust of the primary vacuum pump to an interpump exhaust based on the detected pressure in the system.

8. The leak detection system as recited in claim 1, wherein the primary vacuum pump and the secondary vacuum pump are enclosed in a single housing.

9. The leak detection system as defined in claim 1, wherein the primary vacuum pump and the secondary vacuum pump are configured to provide a high compression ratio for light gases.

10. A method of vacuum pumping a leak detector, comprising:

providing a helium mass spectrometer leak detector;

pumping the helium mass spectrometer leak detector using a primary vacuum pump, the primary vacuum pump having an inlet and an exhaust, the inlet in gaseous communication with the helium mass spectrometer leak detector, the primary vacuum pump comprising an oil-free positive displacement vacuum pump that is a scroll vacuum pump, a multi-stage roots vacuum pump, a multi-stage piston vacuum pump, a screw vacuum pump, or a claw vacuum pump; and

supporting said primary vacuum pump using a secondary vacuum pump having an inlet connected to an exhaust of said primary vacuum pump, said primary vacuum pump having a primary vacuum pump compression ratio when operating alone and a secondary vacuum pump compression ratio when operating alone, the light gas compression ratio of said primary vacuum pump and said secondary vacuum pump operating together being greater than the product of said primary vacuum pump compression ratio and said secondary vacuum pump compression ratio.

11. The method of claim 10, further comprising coupling the exhaust of the primary vacuum pump to an interpump exhaust via a valve based on a selected condition.

12. The method of claim 11, wherein pumping the system using the primary vacuum pump comprises pumping the system using an oil-free scroll vacuum pump, and supporting the primary vacuum pump using the secondary vacuum pump comprises supporting the primary vacuum pump using an oil-free diaphragm pump.

13. A leak detection system, comprising:

a helium mass spectrometer leak detector;

a primary vacuum pump having an inlet and an exhaust, the inlet in gaseous communication with the helium mass spectrometer leak detector, the primary vacuum pump comprising an oil-free scroll vacuum pump; and

14. The leak detection system as recited in claim 13, further comprising a valve coupled to the exhaust of the primary vacuum pump, the valve configured to provide an interpump exhaust in response to a selected condition.

15. The leak detection system as recited in claim 13, wherein the secondary vacuum pump comprises an oilless diaphragm vacuum pump.

16. The leak detection system as recited in claim 14, wherein the valve comprises a poppet valve configured to automatically open based on a predetermined pressure differential across the valve.