Control apparatus for liquefied gas container

Abstract: A control apparatus for keeping constant the temperature and pressure of the vapor phase in a very low-temperature controlled liquefied gas container (2) includes pressure sensing means (10a), temperature sensing means (13), two electromagnetic valves (V1)(V2), and a refrigerator (7). When the operate of the refrigerator (7) gives disturbance to the measurement at low temperature, the refrigerator (7) is stopped. The gas vaporizes from the liquid phase (5), so that the gas in the container (2) discharges form the on-off value (V1). In this manner, the temperature and pressure of the liquefied gas in the container (2) can be kept constant. After measuring, the liquid level in the container (2) fall down by evaporation, so that the liquid gas is supplied by the refrigerator (7). In this manner, the liquid level in the container (2) can be kept constant.

Description: The present invention relates to a control apparatus for a liquefied gas container wherein gas within the container in which liquefied gas is stored is condensed by a recondenser of a refrigerator, and the pressure of the gas in the container is controlled to be kept constant, comprising pressure sensing means for sensing the gas pressure in the container and means for discharging the gas from the container. For measuring the extremely weak intensity of magnetic fields arising from organisms, such as human brain, arm, eyeball, and heart, there has been used a superconductive quantum interference device (hereafter referred to as SQUID) comprising in combination a superconductive ring and one or two Josephson junctions, which SQUID is immersed in a liquefied helium gas within a low-temperature controlled vessel. In order to maintain the temperature of the liquefied helium in such a low-temperature controlled vessel to be constant, it is necessary that the pressure in the temperature controlled vessel is kept constant, and that in the case where the temperature rises, the temperature is fallen by the latent heat of vaporization obtained by evaporating the liquefied helium, thereby, excessive gas is discharged outside the temperature controlled vessel. In order to perform the above control, as disclosed in the document JP-A-55-004933, it is necessaey that the discharged helium gas is stored in the low-temperature recovery tank, then the stored helium gas is recovered by the vacuum pump to the recovery tank and returned to the temperature controlled vessel. However, the prior art does not have a function for controlling flow when the helium gas is returned to the temperature controlled vessel. Furthermore the prior art does not disclose a recondenser by a refrigerator in the temperature controlled vessel itself. Therefore, a disturbance of the temperature is caused in the temperature controlled vessel when the helium gas is returned. The object of the invention is to provide a control apparatus for a liquefied gas container which enables the temperature of the liquefied gas in the liquefied gas container to be kept constant within close limits. Said object according to the invention is reached by a control apparatus characterized by an on-off valve for discharging the gas from the container, a gas source for supplying a gas having same composition as the liquefied gas stored in the container, a flow control valve for directing the gas from the gas source to a gas phase of the container, the flow rate of the gas being variable, and control means responsive to an output from the pressure sensing means to open the on-off valve when the gas pressure has become greater than a predetermined positive first value and to open the flow control valve at a predetermined degree of opening for a predetermined period of time when the gas pressure is negative and its absolute value is greater than a predetermined second value. The control apparatus of the invention further comprises temperature sensing means for sensing the gas temperature in the container, the control means being responsive to an output from the temperature sensing means to control the refrigerator so that the detected temperature is equal to a predetermined value. According to the invention, evaporated gas in a container, such as a low temperature controlled vessel, in which liquefied gas is stored is condensed and reliquefied by a recondenser of the refrigerator. When the refrigerator cannot be operated, for measuring the extremely weak intensity of magnetic field. The gas pressure in the container is detected by the pressure sensing means, the gas pressure is greater that the predetermined positive first value, the on-off valve is opened to discharge the gas in the container by, for example, allowing it to be diffused into the atmosphere. When the refrigerator is operated, for supplying the gas into the container so that the gas pressure is negative and the absolute value thereof is greater than the predetermined second value, the temperature of the liquefied gas stored in the container may largely vary, and it is very likely that external air or the like will enter the container, with the result that the moisture in the air will become condensed within the container and that the composition of the container content will become changed. In order to prevent the occurrence of such condition. therefore, when the negative absolute value of the gas pressure is larger than the predetermined second value, a gas having same composition as the liquefied gas stored in the container is supplied from the gas source into the container through the flow control valve, whereby the negative absolute value of the pressure in the container may be reduced to a value of the atmospheric pressure level. The amount of gas to be supplied from the gas source into the container is set to a value at which the liquid level in the liquid phase of the container is equal to the predetermined level and the gas pressure in the gas phase of the container is equal to the predetermined pressure or, for example, atmospheric pressure, and accordingly the degree of opening of the flow control valve and the period of time during which the flow control valve is open are preset so that gas may be supplied to such amount. When gas is supplied at a large flow rate in the case where the temperature of the gas being supplied into the container is relatively high, excessive heat is temporarily introduced so that the temperature of the gas phase may be abruptly changed or sudden boiling may be caused to the liquefied gas stored in the container. The opening of the flow control valve is controlled and the gas flow is cooled so as to prevent the occurrence of such condition. As stated above, according to the invention, gas in the container in which the liquefied gas is stored is condensed and reliquefied by the condenser of the refrigerator. In the case where the condensation capacity of the refrigerator is relatively small or the refrigerator cannot be operated, the gas pressure in the container will rise, and when the gas pressure has become greater than the predetermined positive first value, the on-off valve is opened and the pressure in the container is kept constant. Conversely, when the refrigerator is operated, for supplying the gas into the container. When the gas pressure in the container has dropped to a negative level, and when the negative absolute value of the gas pressure is greater than the predetermined second value, a gas having same composition as the liquefied gas in the container is supplied from the gas source into the container via the flow control valve. The degree of opening of the flow control valve and the time period during which the valve is open are determined so that the liquid level in the liquid phase of the container is equal to the predetermined level and the gas pressure in the gas phase of the container is of a value of the atmospheric pressure level. In this manner, the temperature and pressure of the liquefied gas in the container can be kept constant. Further, according to the invention, the refrigerator is controlled so that the gas temperature in the gas phase of the container is kept at the predetermined value, whereby the temperature of the liquefied gas can be maintained at a constant level within precision limits. BRIEF DESCRIPTION OF THE DRAWINGS Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein: FIG. 1 is a schematic view showing general arrangement of one embodiment of the invention; FIG. 2 is a flow chart for explaining the operation of a processing circuit 27; and FIG. 3 is a schematic view showing a detailed arrangement of a negative governer 18 and its vicinity. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now referring to the drawing, preferred embodiments of the invention are described below. FIG. 1 is a schematic view of general arrangement of one embodiment of the invention. A container 2 in a low temperature controlled vessel 1 is covered with a heat insulating material 3 and is closed by a ceiling plate 4. Liquid helium 5 is stored in the container 2; and immersed in the liquid helium 5 is a superconductive quantum interference device for measuring the extremely weak intensity of a magnetic field arising from, for example, an organism. In order to measure the extremely weak intensity of the magnetic field to high precision limits by means of the SQUID, it is necessary to keep the temperature of the liquid helium 5 constant very precisely within the range of, for example, 4.2 ± 0.1 °K. to this end. moreover the refrigerator, which give disturbance to the SQUID is not operated, the following arrangement is adopted. A gas phase 6 is formed within the container 2 and above the liquid helium 5. A recondenser 8, as a component of a compression type refrigerator 7, is disposed in the gas phase 6. A heat medium such as liquid helium, flows in the recondenser 8 through a transport pipe. Helium gas in the gas phase 6 of the container 2 is condensed and reliquefied in the recondenser 8. Disposed outside the container 2 is a main body 7a of the refrigerator in which the temperature of the heat medium to be supplied to the recondenser 8 is controlled. Piping 10 is provided in an upper part of the gas phase 6 of the container 2, an end 10a of the piping 10 being located above the level 11 of the liquid helium 5 and in the upper part of the gas phase 6. Pressure sensing means 12 is provided on the piping 10 for detecting the gas pressure in the gas phase 6 of the container 2. There is provided temperature sensing means 13 for detecting the temperature of gas in the gas phase 6 of the container 2. The temperature sensing means 13 is disposed adjacent the end 10a of the piping 10 or at other location in the upper part of the gas phase 6 of the container 2. The piping 10 is connected to piping 14, with an on-off valve V1, in the form of an electromagnetic valve, disposed at a mid-point of the piping 14. Gas from the piping 14 via on-off valve V1 may be discharged by being diffused into the atmosphere, but in this embodiment it is arranged that the gas is collected into a buffer tank 16 of, for example. about 100 mm H₂O. In a gas source 17 or pressure vessel is stored compressed helium gas at ordinary temperatures, which gas is supplied to the buffer tank 16. Helium gas from the buffer tank 16 is supplied to a negative pressure governer 18. The negative pressure governer 18 has a function such that it is opened when the pressure from a secondary pipeline 19 drops to a pressure of, for example, less than -3mm H₂O, while it is fully closed when the pressure is higher than that level. The pipeline 19 has a flow control valve V2 interposed therein. Helium gas flowing through the pipeline 19 and flow control valve V2 is passed through a heat transfer tube 24 submerged in liquid nitrogen 23 stored in a cold tank 21 so that it is cooled down to for example, 77 °K, and is then supplied through piping 25 and in turn through piping 10 into the gas phase 6 of the container 2. The cold tank 21 is replenished with liquid nitrogen so that the level of liquid nitrogen 23 is kept constant. A processing circuit 27 which incorporates a computer or the like controls the on-off valve V1 and flow control valve V2 in response to outputs from the pressure sensing means 12 and temperature sensing means 13. For measuring the extremely weak intensity of magnetic fields arising form organisms, the operate of the refrigerator body 7a, which give disturbance to the SQUID, must be stopped. After measuring, the liquid level in the liquid phase of the container fall down by evaporation of the liquid, so that the gas is supplied the refrigerator. The liquid level in the liquid phase of the container can be kept to the predeterminated level. FIG. 2 is a flow chart explanatory of the operation of the processing circuit 27. As earlier stated, the gas phase 6 is provided with a recondenser 8 by which vaporized helium gas is condensed and reliquefied. When the gas pressure P in the gas phase 6 has become higher than a predetermined positive first value P1 which is higher than atmospheric pressure, that is, P1 ≦ P (1) then operation proceeds from step n2 to step n3, at which the processing circuit 27 operates to open the on-off valve V1, while the flow control valve V2 remains shut off. Accordingly, gas in the gas phase 6 is stored into the buffer tank 16, or in another example it is diffused into the atmosphere. The buffer tank 16 may, for example, take the form of an accumulator or the like. When the pressure P in the gas phase 6 of the container 2 is lower than atmospheric pressure, or is negative, and the absolute value of the pressure P is greater than a predetermined positive second value P2, that is, P2 ≦ |P| (2) P ≦ -P2 (2a) the operation proceeds from step n4 to step n5. At step n5, the flow control valve V2 is opened while the on-off valve V1 remains closed. The degree of opening of the flow control valve V2 and the period of time during which it is open are determined such that the amount of gas supplied from the pipeline 10 into the container 2 through the flow control valve V2 coincides with a value at which the pressure in the gas phase 6 is equal to atmospheric pressure. Where the flow rate is excessively large, it is likely that the temperature of the gas phase 6 will fluctuate and, in turn, fluctuations in pressure will result, so that bumping may be caused to the liquid helium 5. It is arranged, therefore, that the temperature of the liquid helium 5 will be kept constant so as not to cause such condition. When pressure P detected by the pressure sensing means 12 is: -P2 ≦ P≦ P1 (3) then operation proceeds from step n4 to step n6, at which the on-off valve V1 is closed and the flow control valve V2 is also closed. When the operate of the refrigerator does not give disturbance the processing circuit 27, in response to an output from the temperature sensing means 13, controls the refrigerator body 7a so that the temperature of the gas phase 6 of the container 2 is kept constant at the predetermined temperature level, whereby the temperature of heat medium supplied to the recondenser 8 is controlled. The refrigerator 7 is a GM (Gifford - McMahon) refrigerator, for example. This GM refrigerator is of such arrangement that a valve disk driven by a valve motor of an expander is switchable from high pressure to low pressure and vice versa, and a displacer is vertically movable through pressure adjustment by surge volume, whereby the heat medium or helium gas is adiabatic and freely expanded for cold a heat station provided on the displacer. The heat station is equipped with an electric heater so that the temperature of the liquid helium supplied to the recondenser 8 can be controlled by electrically energizing the heater. The refrigerator 7 may be of any other suitable arrangement. The arrangement of the negative governer 18 is schematically shown in FIG. 3. In a casing 37 there is provided a diaphragm 28 which is elastically pulled upward by a spring 29 in FIG. 3. A chamber 30 is open to the air. A diaphragm chamber 31 is in communication with a pipeline 32 connected to the buffer tank 16. A valve body 33 is coupled by a valve stem 34 to the diaphragm 28 and is adapted to be seated on a valve seat 35. When the pressure in the pipeline 29 is less than -3 mm H₂O as stated earlier, the diaphragm 28 is displaced downward in FIG. 3 against the spring force of the spring 29, so that the valve body 33 is moved away from the valve seat 35 to open. The invention is applicable not only in connection with liquid helium, but also to a wire range of uses in connection with other liquefied gases. The invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments are therefore to be considered in all aspects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come with in the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.

Claims: A control apparatus for a liquefied gas container (2) wherein gas within the container in which liquefied gas is stored is condensed by a recondenser (8) of a refrigerator (7), and the pressure of the gas in the container (2) is controlled to be kept constant, comprising pressure sensing means (13) for sensing the gas pressure in the container (2) and means for discharging the gas from the container (2), characterized by an on-off valve (V1) for discharging the gas from the container (2), a gas source (17) for supplying a gas having same composition as the liquefied gas stored in the container (2), a flow control valve (V2) for directing the gas from the gas source (17) to a gas phase (6) of the container (1), the flow rate of the gas being variable, and control means responsive to an output from the pressure sensing means (13) to open the on-off (V1) valve when the gas pressure has become greater than a predetermined positive first value and to open the flow control valve (V2) at a predetermined degree of opening for a predetermined period of time when the gas pressure is negative and its absolute value is greater than a predetermined second value. A control apparatus for a liquefied gas container as set forth in claim 1, further comprising temperature sensing means (13) for sensing the gas temperature in the container (2), the control means being responsive to an output from the temperature sensing means (13) to control the refrigerator (7) so that the detected temperature is equal to a predetermined value. A control apparatus for a liquefied gas container (2) as set forth in claim 1, wherein a gas cooler (21) is disposed midway on a pipeline (25) conducting the gas from the gas source (17) to the gas phase (6) of the container (2). A control apparatus for a liquefied gas container (2) as set forth in claim 1, wherein the rate of gas flow through the flow control valve is controlled by a negative pressure governer (18) disposed upstream of the flow control valve (V2). A control apparatus for a liquefied gas container (2) as set forth claim 1, wherein there is provided a buffer tank (16) containing a gas having same composition as the liquefied gas stored in the container (2) and adapted for temporarily storing therein the gas from the on-off valve (V1) and conducting gas therefrom to the gas phase (6) of the container through the flow control valve (V2), the buffer tank (16) being replenished with gas from the gas source (17) when the gas therein falls short of the requirement.

Inventors: KAWANISHI KAZUNORI, KAWASHIMA YOSHIYUKI, TAKAE TSUTOMU, TAMURA ITSURO, KAWANISHI, KAZUNORI, KAWASHIMA, YOSHIYUKI, TAKAE, TSUTOMU, TAMURA, ITSURO
Applicants: OSAKA GAS CO LTD, OSAKA GAS COMPANY, LIMITED
Application year: 1991
Main classification: F17C13
IPC codes: G05D16, F17C13