Byers, Robert E. (New Kensington, PA)
Schmitt, John (Jeannette, PA)

05/266536

03/19/1974

06/27/1972

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Gulf Research & Development Company (Pittsburgh, PA)

137/212

137/392, 137/213

F16K21/18; F17C13/02; F16K21/00; F17C13/00; B67D5/54

141/94,95,96,192,193,197,198,47,48,49,50 340/236 137/386,392,209,212

Bell Jr., Houston S.

We claim

1. Apparatus for flowing liquid from a supply vessel to a use vessel for a predetermined length of time, comprising means to supply pressurized gas to said supply vessel above the liquid therein, means to flow liquid out of said supply vessel under the influence of said pressurized gas, means to sense when said liquid falls below a predetermined level in said use vessel, circuit means operated by said sensing means, said circuit means including means to commence the supply of said pressurized gas to said supply vessel upon operation of said sensing means, and said circuit means including adjustable means to control the length of time pressurized gas is supplied to the upper end of said supply vessel and to thereafter stop the flow of pressurized gas to said supply vessel.

2. The combination of claim 1, wherein said liquid is liquid nitrogen, means to vent boil-off gaseous nitrogen from the upper end of said supply vessel, and means to close said vent means when said pressurized gas supply means are operative.

3. The combination of claim 2, and pin-hole bleeder means on said vent means to reduce the rate and amount of boil-off gaseous nitrogen leaving said supply vessel.

4. The combination of claim 1, said means to flow liquid out of said supply vessel comprising a snorkel tube extending down to a position closely adjacent the bottom of said supply vessel, whereby substantially all of the liquid in said supply vessel will be flowed to said use vessel.

5. The combination of claim 1, said sensing means comprising means to activate and de-activate portions of said circuit means when said device does not sense and does sense said liquid in said use vessel.

6. The combination of claim 1, said means in said circuit means to time and stop the flow of pressurized gas to said supply vessel comprising a unijunction transistor and an RC timing circuit so arranged that the charging time of said RC circuit controls the firing of said transistor, a variable resistance in said RC circuit, a normally closed solenoid valve to control the flow of said pressurized gas to the upper end of said supply vessel, and wherein said unijunction transistor controls said normally closed solenoid valve.

7. The combination of claim 6, wherein said liquid is liquid nitrogen, means to vent boil-off gaseous nitrogen from the upper end of said supply vessel, means to close said vent means when said pressurized gas supply means are operative, a relay in said circuit means, said relay including a first pair of normally closed contacts in series circuit with the coil of said normally closed solenoid valve, said relay including a second pair of normally closed contacts to control the charging of said RC circuit, and wherein the firing of said transistor operates the coil of said relay.

8. The combination of claim 7, said relay including a pair of normally open contacts in series circuit with said relay coil to establish a holding circuit for said relay coil.

This invention relates to apparatus for maintaining a level of a liquid in a vessel, and is particularly adapted for use in instrumentation using liquid nitrogen wherein a predetermined level of liquid nitrogen must be maintained in a relatively small vessel inside the instrument.

The invention was developed specifically for use with the commercial instrument made by American Instrument Company of Silver Spring, Md. and covered by U. S. Pat. Nos. 3,059,478 to Coggeshall et al., and 3,295,720 to T. J. Sloan. The Coggeshall patent covers the basic system, and the Sloan patent relates to an improved control system. In that instrument, liquid nitrogen is transferred from a small internal storage vessel to a still smaller sample or use vessel, and the nitrogen in the sample vessel must be maintained at some predetermined level. The problem is that the size of the internal storage vessel prohibits continuous tests of long duration, on the order of 72 hours for example, because the nitrogen in the internal storage vessel exhausts too quickly. Reference may be had to the above identified Coggeshall patent in which FIG. 1 shows the overall system and reference numeral 32 is the sample or use vessel.

Liquid nitrogen can be purchased in standard large capacity flasks. Broadly, the invention provides means to connect such large standard commercial flasks, which contain sufficient volume of liquid nitrogen for tests of any duration which may be encountered, directly to the sample or use flask inside the instrument. The prior relatively small internal storage flask with its attendant problems and disadvantages is thus completely eliminated. Heretofore, it was necessary to open the instrument and periodically refill the small internal storage flask, and then reassemble the parts and the instrument. Further, this procedure is inherently undesirable, and further has a disadvantage that it does not permit the running of long duration tests. With the present invention, only one external connection must be made between the instrument and the commercial flask, thus eliminating the need to disassemble the instrument, while simultaneously permitting running of long tests.

More broadly, the invention could be used in any environment wherein it is desired to move a liquid from a storage vessel to a use vessel, in which use vessel a predetermined level must be maintained. Another application of the invention could be in handling toxic and/or corrosive liquids wherein it is desired to minimize the possibility of human contact with such liquids. Liquid nitrogen, of course, boils at a temperature well below room temperature, and in any case some nitrogen will be gaseous in order to fill the vessel, and therefore the large storage vessel will have within it a two phase system. The gas in the storage vessel above the liquid is helpful in driving the liquid to the use vessel. However, the invention also provides an auxiliary driving gas, and, if the invention should be used with a liquid which does not boil at about room temperature, then the auxiliary driving gas alone will be sufficient to drive such liquid. Such other liquids might require a change in the level detection means. Depending upon the particular application, any conventional device might be used, such as a float valve, conductivity relay, photo cell, pressure switch, or the like.

Various other schemes have been tried in the prior art to accomplish this goal of providing relatively large capacity liquid storage vessels external to the instrument being serviced or other point of use. One such system includes a heater in the equivalent of the large external supply vessel to thereby cause an increase in gas pressure above the liquid. This scheme suffers from the apparent problems of having to deal with thermodynamic forces which could become tricky and difficult, mechanical difficulties in making a small heater, and moving such a heater into and out of the main supply vessel.

The invention also provides an electrical circuit driven primarily by a level sensing device in the use vessel to control the flow from the supply vessel to the use vessel. The circuit provides means to stop the venting of boil off gas from the supply vessel and to simultaneously commence flow of driving gas from an auxiliary supply. The circuit also includes means to determine the length of time during which liquid will be driven over from the supply vessel to the use vessel, and further provides means to easily change this predetermined time. This portion of the circuit comprises a resistance/capacitance (RC) network to control the firing of a transistor to thereby operate a relay which in turn stops the flow. A variable resistance is included in the RC circuit to control the charging of the capacitor which controls the length of time required for the transistor to fire. The circuit also includes the means to control the timing and sequence of the various steps and operations which occur during a cycle of driving liquid over from the supply vessel to the use vessel.

Another feature of the invention is the inclusion of a control vent on the safety bleed from the supply vessel. This is an easily made pin hole type of device fitted onto the normal opening of the vent controlling solenoid valve.

The above and other advantages of the invention will be pointed out or will become evident in the following detailed description and claims, and in the accompanying drawing also forming a part of the disclosure, in which:

FIG. 1 is an elevational view of an apparatus embodying the invention with some parts shown diagrammatically and other parts broken away and in cross-section;

FIG. 2 is a view of the bleeder vent; and

FIG. 3 is a schematic diagram of the electrical control circuit.

Referring now in detail to the drawing, reference numeral 10 designates a wall of an instrument which contains a vessel 12 within itself. Vessel 12 corresponds to the dewar flask 32 in FIG. 1 of the Coggeshall patent identified above. More generally, vessel 12 represents any use vessel in which it is desired to maintain a predetermined level of a liquid 14. A movable shelf 16 in the instrument supports the vessel 12, and another wall 18 in spaced relation to shelf 16 is provided. The shelf 16 carrying the vessel 12 is movable up and down with respect to a lid member 20. The commercial instrument mentioned above includes means, not shown, to permit this motion. Lid 20 is mounted in a suitably formed opening in the wall 18 by any suitable means, such as bolts. Lid member 20 is formed with a number of openings including openings for passing samples. These sample openings are not shown.

The commercial instrument includes level sensing means 22 mounted in lid 20, and these same means 22, unchanged, are used in the present invention, and thus are not shown or described in detail. Level sensing means 22 includes a thermistor which actually contacts the liquid 14 in the vessel 12, and a transistor circuit, not shown, which functions effectually like a switch in the circuitry of FIG. 3. The thermistor is indicated by the black dot at the end of physical sensing means 22, and the electrical capability of the level sensing means 22 is indicated by the symbol for a normally open switch in FIG. 3. In other environments, other liquid level sensing means could be used.

Means are provided to prevent splashing of the liquid nitrogen as it enters the use vessel. To this end, a length of plexiglas pipe or tube 24 is mounted in another opening in the lid 20 by any suitable means and extends downwardly therefrom into the liquid 14 in the vessel 12. A length of conventional specially constructed long vacuum type insulated glass tubing 26 extends from the wall 10 to the upper end of perforated tube 24.

Means are provided to connect tubing 26 to the inside of wall 10, and to connect an end of a similar transfer tube 28 to the opposite side of the wall, with the liquid carrying openings in the two tubes 26 and 28 in communication with each other. To this end, a pair of plates 30 are provided on opposite sides of the wall 10. These plates are held together by a pair of screws S, which pass through suitable clearance openings in one of the plates 30 and mate with suitably threaded openings in the other plate 30. Conduit 29 is held by the two plates 30, and it is this conduit which serves as the connection between tubing 26 on the inside of wall 10 and tubing 28 on the outside of the wall. A short length of metal tubing 30 is joined to the inside end of conduit 29 by any suitable means such as soldering, and the opposite end of tubing 30 is joined to insulated tubing 26 by a suitable glass-to-metal seal 31. At the opposite outside end of conduit 29, the capability of making a quick connection and disconnection between the transfer tube 28 and this conduit is provided by means of mating flanged ends with an O-ring 33 fitted therebetween, and is finally completed by means of a suitable flange clamp, not shown. This kind of connection is known as an O-ring joint in the art.

The vessel 34 is a staple item of commerce in which liquid nitrogen is sold. Typically, it is a metal vacuum construction vessel, of 50 liters capacity, with outside dimensions of approximately 20 inches diameter and 30 inches high. An adapter 36 is put into the neck of the supply vessel. Gasketing material, not shown, of any suitable type may be provided between adapter 36 and vessel 34.

The opposite end of the insulated liquid nitrogen tube 28 extends through the adapter 36 and has a long snorkel inlet tube 38 which terminates in closely spaced relation to the bottom of the supply vessel 34 in order to be able to deliver substantially all of the liquid in the vessel 34. Adapter 36 further includes the tube 40 which branches off into a pair of tubes 42 and 44. The flow in tube 42 from the auxiliary supply (not shown) into the vessel 34 above the liquid therein is controlled by a normally closed solenoid valve labelled S1. In a similar manner, the flow of gaseous nitrogen in tube 44 out of the vessel 34 for venting purposes is controlled by a normally open solenoid valve labelled S2. The solenoid valves S1 and S2 are themselves conventional. The outer end of the vent tube 44 is provided with a vent bleed device 46, shown in more detail in FIG. 2.

Basically, this device comprises a standard pneumatic fitting having threads 48 at one end, and a nipple 50 at the opposite end. The nipple 50 is modified by the addition of an end plate, as by welding or soldering, which plate is provided with an opening 52 of relatively small diameter. In the successfully constructed embodiment, the hole 52 was on the order of one thirty-second of an inch in diameter. The purpose of the bleeder 46 with its pin hole 52 is to slow down the rate at which the nitrogen escapes from the supply vessel 34, to thereby even further lengthen the time between required refills of the main nitrogen supply vessel 34. In other applications, a needle valve or the like could be provided in place of bleeder 46 to further control venting.

In the successfully constructed embodiment vessel 12 was at about the same height as the top of vessel 3 It is thought that no siphoning occurred because 34. little of the nitrogen would gradually gasify in tubes 28 and 26 when the driving pressure is relieved thus blocking such a gravity flow. In other environments, when dealing with substances which are liquid at room temperature, this potential problem can be easily avoided by locating vessel 12 higher than the supply vessel.

The circuit 54 is built on a "hot" or supply line 56 and a ground line 58. Direct current from a suitable not shown was used to operate the various coils. A main on/off switch 55 is provided in line 56. The coil of the solenoid valve S1 is located in a line 60 which is connected to the ground line 58 at one end and terminates at a junction point 62. A line 64 containing the first set of contacts 66-1 on a double manual switch connects point 62 to the hot line 56. The second set of contacts 66-2 of this switch is in a line 68 in series circuit with the coil of the normally open solenoid valve S2. Operation of switch 66 will open the normally closed solenoid valve S1 and will close normally open solenoid valve S2 to override the automatic circuitry described below in the event it should be desired to supply liquid from supply vessel 34 to use vessel 12 at a time other than when the circuitry indicates that liquid supply is required. The double switch 66 is primarily used for start-up.

A line 70 extends from supply line 56 to a point 72 and contains the switch or switch-like part of level sensing device 22 which functions as an open switch in normal operation. That is, "switch" 22 closes and supplies power to point 72 only when liquid is required in vessel 12, and otherwise keeps the remaining circuitry inactive so long as the liquid in vessel 12 is at or above the predetermined level.

Three lines 74, 76, and 78 branching from point 72 contain the remaining circuitry. Line 74 contains a pair of normally closed contacts 82 on a relay R1 and a diode 84 biased as shown. The diode 84 serves to protect transistor 94 for a direct connection to line current when manual switch contacts 66-1 are operated. The line 74 thereafter terminates at point 62 described above. Line 76 contains a pair of normally closed contacts 86 on the relay R1 and terminates at a point 88. A variable resistance device or potentiometer 90 is connected to point 88 and to a line 92 which contains a capacitor C1, and which finally terminates at the ground line 58. A unijunction transistor 94 has one terminal connected by a line 96 to the line 92 between potentiometer 90 and capacitor C1, and a second terminal connected by a line 98 to the point 88. Line 98 contains a resistor 100 which normally provides a reverse bias on transistor 94. The last line 78 contains, in series circuit from left to right, a diode 102, a junction point to jumper line 80, a pair of normally open contacts 104 on relay R1, a resistor 106, a connecting point to a line 108 to the third terminal on transistor 94, and the coil of relay R1. The jumper line 80 connects to line 68 between the two elements therein. The diode 102 serves the same function as diode 84 but with respect to switch contacts 66-2. The resistor 106 is current limiting because the R1 relay coil must be low resistance in order to be in line 78 in the same circuit with transistor 94, which, of course, is a low current device. The three terminals of the transistor 94 are connected to the lines 96, 100 and 108 and are called, respectively, emitter, base 2, and base 1.

OPERATION

Looking at FIGS. 1 and 3 of the drawing simultaneously, the operation of the invention can best be understood by setting forth one cycle of the fill or liquid transfer operation. Various different things occur at various different times, the sequence being controlled by the circuit 54. FIG. 3 shows the apparatus at a stable condition, i.e., the liquid in vessel 12 is at or above device 22 and thus the contacts of device 22 is open thereby rendering the subsequent circuitry inactive.

A cycle begins when the level in vessel 12 falls and the device 22 effectively conducts. The first occurences are that the two solenoid valves S1 and S2 operate to thereby stop venting the supply vessel 34 via line 44 and to supply driving force in the form of gas under pressure from the auxiliary supply via line 42. Solenoid S1 opens because normally closed contacts 82 in line 74 on relay R1 remain in their normally closed state thus connecting now activated point 72 to ground via the coil S1. The coil of solenoid valve S2 is activated at once via lines 78, 80 and 68, to close this normally open valve to stop venting.

Nitrogen was used and is preferred over air for the auxiliary gas drive in the successfully constructed embodiment because service air often contains moisture. Water in the main supply vessel 34 will change the boiling point of the nitrogen therein, and this change in boiling point has a deleterious effect on the operation of the instrument of the Coggeshall patent. However, more generally, in other environments, any gas may be used as the auxiliary drive, including air, so long as the nature of the particular environment is such as to permit the use of that particular gas.

The next occurrence in the sequence is the firing of the transistor 94 to activate the coil of the relay R1 via the lines 108, 96 and 76 and back to the activated point 72. The length of time required for the transistor to become conducting across the lines 96 and 108 is the length of time during which liquid will be driven over from vessel 34 to vessel 12 because of the operation of the two solenoid valves, as described above. Variable resistance 90 and capacitor C1 determine this length of time in the usual manner of an RC circuit. Current leakage through the resistor 90, the rate of which is controlled by the setting thereon, slowly charges capacitor C1 until the voltage increases to the point at which the transistor via the line 96 fires. The emitter voltage builds up at 96 until the reverse bias on line 98 is overcome, at which moment the device 94 becomes forwardly biased and current flows via lines 96 and 108 (basel) and to coil R1.

The next three occurrences happen simultaneously upon firing of transistor 94. They are, first the establishment of a holding circuit for relay R1 via closing of contact 104 in line 78; secondly, cessation of the supply of driving gas because of the opening of normally closed contacts 82 on the relay R1 in the line 74 to thereby deactivate coil S1; and finally, deactivation of the Rc circuit via opening of the normally closed contacts 86 in the line 76. Note that the solenoid valve S2 remains closed thus holding the main supply vessel 34 sealed in the interim between the end of the third group of occurrences and the beginning of the fourth and last group of occurrences thus to prevent gaseous nitrogen resulting from natural boiling in tube 28 from interrupting the flow of the liquid nitrogen later.

The final set of occurrences are caused by the arrival of the liquid up to the predetermined level in the vessel 12 to thereby cause contacts 22 to return to their open condition. When contacts 22 open again, the coils of relay R1 and solenoid valve S2 in the lines 78 and 68 become deactivated. The system and circuit have now completed a fill cycle and the circuit has returned to the FIG. 3 configuration.

An advantage of the circuit of FIG. 3 is that it automatically provides for shut-off in the event the level in use vessel 12 should reach the predetermined height prior to the expiration of the RC determined time and accompanying firing of transistor 94. In this event the last set of occurrences happens at once, S1 returns to its normally closed condition, and no further positive drive occurs. In either case, a certain amount of "coasting" will occur after S1 closes and before S2 re-opens because of the trapped gas pressure above the liquid in supply vessel 34. This phenomenon is an advantage because, together with the bleeder, it consumes the auxiliary gas supply, and it reduces the number of fill cycles per unit time thereby conserving the liquid nitrogen. The "coasting" causes a tailing off in the supply rather than a sharp cut-off, thus enhancing the above advantages and providing smoother operation.

While the invention has been described in detail above, it is to be understood that this detailed description is by way of example only, and the protection granted is to be limited only within the spirit of the invention and the scope of the following claims.