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A method for localizing small helium leaks in large cryogenic vessels
NUCLEAR
INSTRUMENTS
AND
METHODS
155
(1978) 573-574 ; O
NORTH-HOLLAND
PUBLISHING
CO.
A M E T H O D F O R L O C A L I Z I N G S M A L L H E L I U M LEAKS IN L A R G E C R Y O G E N I C VESSELS* M. L. MALLORY and H. G. BLOSSER
Michigan State University, Cyclotron Laboratory, East Lansing, Michigan 48824, U.S.A.
Received 19 May 1978 Small leaks in a large cryogenic helium vessel have been localized by rapidly changing the temperature in the leak vicinity. As the helium temperature decreases, the viscosity of helium lowers, thereby causing a greater leak into the vacuum jacket. The relative pressure surge then gives a signature which can be used to evaluate various possible leak locations. Large liquid bath type superconducting coils involve a very difficult leak detection problem in that the helium vessel has typically tens of meters of welded joints, and a very m i n u t e leak through any of these welds into the insulating v a c u u m can seriously reduce the quality of the thermal insulation. Leaks at room t e m p e r a t u r e below the threshold ~) of typical detection e q u i p m e n t ( ~ 10 -9 Torr l/s) can have a significant effect on the v a c u u m insulation quality w h e n c o m b i n e d with the effects of m u c h greater helium flow through the leak at liquid helium t e m p e r a t u r e than at room temperature and the relatively high thermal conductivity of gaseous helium. In the process of constructing * This material is based upon work supported by the National Science Foundation under Grant No. Phy 78-01684. [1~0 Pr'IASE UNDARY
PRESSURE = 1.2 otm
~
a large superconducting h e a v y ion cyclotron magnet at Michigan State University a useful technique has evolved for approximately localizing these very small leaks. T h e key physical p h e n o m e n o n involved in the technique is the rapid variation in the m a s s flow through a given hole or crack as a function of temperature. Fig. 1 shows the viscosity and density of helium 2) from 24 K to 4 K at a pressure of 1.2 a t m (the normal operating pressure of liquid bath cryostats) and indicates that one would expect a large change in flow through a given hole if an appropriate t e m p e r a t u r e change can be induced in the region of the hole. A variety of techniques is available for varying the temperature distribution in the cryostat. T h e cryostat for the cyclotron m a g n e t has for example a total of ten connections provided for various purposes and with appropriate valving, the boil-off gas (measured t e m p e r a t u r e of -~5 K) can be channeled
8 dLIOUID HELIUM
PRESSURE RESPONSE TO PASSINGCOLD HELIUM GAS THROUGH LEAD #1
7'/
"~ _m >
4.0
CLOSED VALVE
i i 2o 1.0
to
N 3.0 a_ 0~ 4
6
8
IO
2 14 6 8 TEMPERATURE (K)
20
22
24
.2
Fig. 1. The viscosity and density of helium from 24 K to 4 K at a pressure of 1.2 atm, the operating pressure of the superconducting cyclotron cryostat, are shown. A temperature change from 24 K to 4 K causes a change in the helium viscosity of ~3 and hence a corresponding change in the leak rate through a crack.
TIME (rain)AFTER OPENING VALVE
Fig. 2. Measurement
of the pressure in the cryostat vacuum
jacket when cold helium gas is passed through the cryostat area containing the crack. Additional leaks have now been found with leak rate five times smaller than the one shown above.
574
M. L. MALLORY
AND H. G. BLOSSER
through any one of these connections. As the flow pattern of the gas is varied, the temperature distribution in the coil shifts, and the effect of the new temperature distribution on the leak rate is observed as a change in pressure in the cryostat insulating vacuum. Noting the differential pressure effect as the flow is directed to one or another of the possible return paths then provides a signature which is used to approximately identify the portion of the cryostat in which the leak is located. Fig. 2 shows an example of the technique, namely a display of the cryostat pressure as flow through a particular coil current lead was first opened and then closed. Three other current leads having the same geometry as current lead H1 and located approximately 8 cm away gave no detectable signal when cycled through a similar flow pattern. The cryostat was then warmed and opened, and the welds tested for leaks at room temperature, where the helium leak detector has a sensitivity of ~ 1 × 10 -9 Torr l/sec. While none were found, the welds in the connection to current
lead #1 were remade and after cooldown we found that the leak was fixed. Two additional leaks of even smaller rates than above have now been found by this technique. Both the location of the repaired welds and the temperatures before flow of boil-off gas indicate that thermal contraction of the metal is not involved in the increased leak rate. With the repairs of each leak, the cryostat pressure has improved and the liquid helium usage has decreased. In summary, the technique herein described can be extremely useful in locating ultra-small helium leaks in large cryogenic helium vessels. Secondly this technique with an appropriate test cryostat can give a leak detection rate sensitivity that is three magnitudes beyond presently achieved leak detection rates at room temperature.
References I) A. Roth, Vacuum technology (North-Holland, AmsterdamNew York-Oxford, 1976). 2) NBS Technical Note 631 (1972).
INSTRUMENTS
AND
METHODS
155
(1978) 573-574 ; O
NORTH-HOLLAND
PUBLISHING
CO.
A M E T H O D F O R L O C A L I Z I N G S M A L L H E L I U M LEAKS IN L A R G E C R Y O G E N I C VESSELS* M. L. MALLORY and H. G. BLOSSER
Michigan State University, Cyclotron Laboratory, East Lansing, Michigan 48824, U.S.A.
Received 19 May 1978 Small leaks in a large cryogenic helium vessel have been localized by rapidly changing the temperature in the leak vicinity. As the helium temperature decreases, the viscosity of helium lowers, thereby causing a greater leak into the vacuum jacket. The relative pressure surge then gives a signature which can be used to evaluate various possible leak locations. Large liquid bath type superconducting coils involve a very difficult leak detection problem in that the helium vessel has typically tens of meters of welded joints, and a very m i n u t e leak through any of these welds into the insulating v a c u u m can seriously reduce the quality of the thermal insulation. Leaks at room t e m p e r a t u r e below the threshold ~) of typical detection e q u i p m e n t ( ~ 10 -9 Torr l/s) can have a significant effect on the v a c u u m insulation quality w h e n c o m b i n e d with the effects of m u c h greater helium flow through the leak at liquid helium t e m p e r a t u r e than at room temperature and the relatively high thermal conductivity of gaseous helium. In the process of constructing * This material is based upon work supported by the National Science Foundation under Grant No. Phy 78-01684. [1~0 Pr'IASE UNDARY
PRESSURE = 1.2 otm
~
a large superconducting h e a v y ion cyclotron magnet at Michigan State University a useful technique has evolved for approximately localizing these very small leaks. T h e key physical p h e n o m e n o n involved in the technique is the rapid variation in the m a s s flow through a given hole or crack as a function of temperature. Fig. 1 shows the viscosity and density of helium 2) from 24 K to 4 K at a pressure of 1.2 a t m (the normal operating pressure of liquid bath cryostats) and indicates that one would expect a large change in flow through a given hole if an appropriate t e m p e r a t u r e change can be induced in the region of the hole. A variety of techniques is available for varying the temperature distribution in the cryostat. T h e cryostat for the cyclotron m a g n e t has for example a total of ten connections provided for various purposes and with appropriate valving, the boil-off gas (measured t e m p e r a t u r e of -~5 K) can be channeled
8 dLIOUID HELIUM
PRESSURE RESPONSE TO PASSINGCOLD HELIUM GAS THROUGH LEAD #1
7'/
"~ _m >
4.0
CLOSED VALVE
i i 2o 1.0
to
N 3.0 a_ 0~ 4
6
8
IO
2 14 6 8 TEMPERATURE (K)
20
22
24
.2
Fig. 1. The viscosity and density of helium from 24 K to 4 K at a pressure of 1.2 atm, the operating pressure of the superconducting cyclotron cryostat, are shown. A temperature change from 24 K to 4 K causes a change in the helium viscosity of ~3 and hence a corresponding change in the leak rate through a crack.
TIME (rain)AFTER OPENING VALVE
Fig. 2. Measurement
of the pressure in the cryostat vacuum
jacket when cold helium gas is passed through the cryostat area containing the crack. Additional leaks have now been found with leak rate five times smaller than the one shown above.
574
M. L. MALLORY
AND H. G. BLOSSER
through any one of these connections. As the flow pattern of the gas is varied, the temperature distribution in the coil shifts, and the effect of the new temperature distribution on the leak rate is observed as a change in pressure in the cryostat insulating vacuum. Noting the differential pressure effect as the flow is directed to one or another of the possible return paths then provides a signature which is used to approximately identify the portion of the cryostat in which the leak is located. Fig. 2 shows an example of the technique, namely a display of the cryostat pressure as flow through a particular coil current lead was first opened and then closed. Three other current leads having the same geometry as current lead H1 and located approximately 8 cm away gave no detectable signal when cycled through a similar flow pattern. The cryostat was then warmed and opened, and the welds tested for leaks at room temperature, where the helium leak detector has a sensitivity of ~ 1 × 10 -9 Torr l/sec. While none were found, the welds in the connection to current
lead #1 were remade and after cooldown we found that the leak was fixed. Two additional leaks of even smaller rates than above have now been found by this technique. Both the location of the repaired welds and the temperatures before flow of boil-off gas indicate that thermal contraction of the metal is not involved in the increased leak rate. With the repairs of each leak, the cryostat pressure has improved and the liquid helium usage has decreased. In summary, the technique herein described can be extremely useful in locating ultra-small helium leaks in large cryogenic helium vessels. Secondly this technique with an appropriate test cryostat can give a leak detection rate sensitivity that is three magnitudes beyond presently achieved leak detection rates at room temperature.
References I) A. Roth, Vacuum technology (North-Holland, AmsterdamNew York-Oxford, 1976). 2) NBS Technical Note 631 (1972).