US 4,887,857 AGrant
Method and System for Filling Cryogenic Liquid Containers
Issue Date:1989-12-19
•9 Claims
•9 Drawing Sheets
Abstract
Substance loss is minimized in a station for loading a container with cryogenic substance stored in a tank. A throttle vent valve is provided at the outlet vent of a container being loaded for controlling the differential pressure between the storage tank and the container. The throttle vent valve is adjusted to maintain the differential pressure at a value equal to the optimum differential pressure for minimizing substance loss. The optimum differential pressure is selected by determining the filling loss for a plurality of values of differential pressure and selecting the differential pressure which produces the minimum filling loss.
Metadata
Assignee
- Air Products and Chemicals, Inc.
Inventor
- James VanOmmeren
Application Information
Application Number:US 2937328
Filing Date:1989-01-05
Priority Date:1986-07-22
Art Unit:243
Classifications
IPC:
F17C 1300
Field of Search:
6214149-551;4-7;11;69;82;39;47-51;98;83
Patent Drawings (9 sheets)
Description
A portion of this patent document contains material which is subject to copyright protection. The owner has no objection to the facsimile reproduction by anyone of the patent document or patent disclosure as it appears in the Patent and trademark Office patent file or records, but otherwise reserves all rights whatsoever.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of loading liquefied gases into containers.
2. Description of Related Art
Typically a filling station has a large storage tank in which a cryogenic substance is stored in liquid form. Portable cylinders, which are superinsulated to maintain the cryogenic substance in its liquid form, must be periodically refilled from these filling stations and transported to a place of use.
During the transfer of liquefied gases from the storage tank to the portable cylinder, a portion of the product gas is wasted. These filling losses, depending on the circumstances, may be a significant percentage of the product gas.
It is known to transfer cryogenic substances from a storage tank to a liquid cylinder using pressurized transfer filling and centrifugal pump filling. In pressurized transfer filling the pressure head within the storage tank is used to force substance through pipes into a cylinder. In centrifugal pump filling, a centrifugal pump is disposed in line between the storage tank and the liquid cylinder for transferring substance.
The cylinder being filled includes two connections associated with filling, an inlet port and an outlet vent. Substance is loaded into the cylinder through the inlet port while the outlet vent is left open, allowing any liquefied gas which returns to a gaseous form to vent to the atmosphere. As substance flows through a filling station the substance absorbs heat causing the substance to change state into gas and causing high venting losses due to excessive flashing from the pressure letdown between storage tank and cylinder pressure as a substance enters the cylinder.
The conventional liquid cylinder filling technique fills the cylinder with the vent valve completely open, incurring high venting losses due to excessive flashing of the cryogenic entering the cylinder. Fill line length, diameter, restrictions, and insulation potential contribute to the overall filling loss. Many liquid cylinders are overfilled due to operator error. This can present a serious safety hazard in those fill plants where the cylinder vents into the enclosed fill building. This liquid fill venting mode also causes the filling loss to drastically increase and results in a cylinder which is filled beyond the DOT fill density limit, requiring further venting to achieve the shipping limit.
A number of prior systems have attempted to deal with these large filling losses. These systems include recirculating systems to prevent loss of flashed vapor, top filling the cylinder with pumps and pump aided transfer systems. None of these have been entirely satisfactory.
The recirculating systems have recirculated the flashed vapor generated when the liquid from the tank has entered the cylinder. Recirculating the flashed vapor back to the tank can result in a no loss system. However, there is a serious risk of contamination of the tank if a contaminated liquid cylinder has been filled. Also the heat absorbed by the recirculated vapor is added to the storage tank, an undesirable event. Further, a sophisticated operator is required to run this system.
Top filling with a pump generally has operated only under ideal conditions in which the plumbing between tank and cylinder is precooled and the liquid cylinder is cold. Under typical conditions the cylinder must be blown down periodically to avoid losing pump prime or damaging the seals. Further, the operation takes 10 to 12 minutes on average and requires a sophisticated operator to deal with pump problems and maintenance.
U.S. Pat. No. 4,475,348 discloses use of an automatic throttling valve to vent the cylinder being filled whenever the pressure in the cylinder reaches approximately 10 PSI less than the pressure in the storage tank regardless of filling station configuration or substance. This method decreases filling losses to some degree but its effectiveness varies with station configuration and substance.
It is known that during centrifugal pump transfer of substance from a storage tank to a cylinder, centrifugal pumps are subject to cavitation. Cavitation is caused when the cryogenic substance absorbs thermal energy causing the substance to vaporize in the pump inlet and bubbles of the vapor to be carried to the impeller of the pump. The pump rotor then spins more rapidly in the gas bubble since the gas offers much less resistance than the liquid. This rapid spinning causes friction and heat which warms the gas further causing further vaporization. Unless the motor is stopped when this occurs, the pump motor could burn out or the casing or rotor of the motor could break due to internal friction. If the substance being loaded is liquid oxygen, there is a high potential for a safety hazard.
Rattan in "Cryogenic Liquid Service", Chemical Engineering, Apr. 1, 1985, page 95 discloses bleeding a small liquid stream through a hole in a pump to keep the pump cool to deal with this problem. However, in very hot areas, a large amount of substance must be wasted by this method. Another method disclosed in this same article, is bringing the pressure within a system up to a level that prevents flashing.
Another danger present when liquid cylinders are loaded with a cryogenic substance is that if the cylinder is overfilled, liquefied gas product is discharged from the outlet vent of the cylinder. It was a common practice to continue filling a cylinder until liquefied product was discharged from the outlet vent as a way of determining when the cylinder was full. In addition to wasting product this can be dangerous since the liquefied gas may injure an operator by cryogenic burns or asphyxiation or cause an explosion or a fire.
SUMMARY OF THE INVENTION
The present invention is an optimized, integral, and automated liquid filling system capable of performing both pressure transfers and pumped transfers with minimal filling losses.
The system includes a programmable controller which calculates the optimum filling differential pressure from the system input parameters, recalculates the optimum differential pressure during fill and maintains that varying optimum differential pressure throughout the filling cycle by actuating a control valve in the vent line.
The pumped transfer system will determine the point of pump cooldown, automatically start the pump motor, and sense for pump cavitation throughout the fill. A cavitation signal will automatically stop the pump motor, provide for more cooldown time, and restart the motor.
The present invention also provides a completely automated filling system with backup shutoff devices, equipment monitoring instruments, optimized control logic, and output light and alarm indications. This automation performance eliminates operator error and liquid full venting, improves process safety, and yields DOT regulation filled liquid cylinders.
A major advantage of the present invention is improved operating and shipping safety due to
(a) elimination of liquid full venting, reducing the asphyxiation hazard for N.sub.2 /Ar and the fire hazard for O.sub.2 if venting into the fill building;
(b) automatic shutdown of the pump under cavitation conditions, reducing oxygen pump fire concerns; and
(c) significant reduction in the number of overfull or liquid full cylinders, decreasing the potential for venting liquid through the pressure safety valve during shipping and storage.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a diagram of the system of the present invention.
FIG. 2 shows a more detailed diagram of the system of FIG. 1.
FIG. 3 shows a flow chart representation of a routine for controlling the operations of the system of FIG. 2.
FIGS. 4 to 6 show continuations of the routine of FIG. 3.
FIG. 7 shows a block diagram representation of a model for calculating cylinder filling losses.
FIGS. 8 and 9 show graphs of filling loss as a function of cylinder pressure.
DETAILED DESCRIPTION OF THE INVENTION simplified diagram of automated pressure/pump transfer liquid cylinder fill station 10 under control of a controller 12 of the present invention. Fill station 10 loads cryogenic substance 16 such as liquid oxygen, liquid nitrogen, liquid argon or other liquefied gases from storage tank 14 through pipe 24 and fail/close solenoid controlled valve 28 into liquid cylinder or container 18 under the control of a controller 12. The pressure of tank 14 is transmitted to controller 12 by pressure transducer 20 and the pressure of cylinder 18 is transmitted to controller 12 by pressure transducer 66 permitting controller 12 to determine the differential pressure between tank 14 and cylinder 18. Substance 16 may be transferred from storage tank 14 to cylinder 18 either by pressure transfer using the pressure head within tank 14 to move substance 16 ("pressure transfer") or by centrifugal pump transfer using pump 34 ("pump transfer").
Variable throttle vent valve 68, controlled by actuator 70, is provided in system 10 to control the back pressure within cylinder 18 and thereby to optimize the differential pressure between tank 14 and cylinder 18 for station 10 during pressure transfer of substance 16. The differential pressure is optimized for a fill station 10 to minimize the filling loss of substance 16 during the loading operation.
The optimum differential pressure for different fill stations 10 varies depending on the type of substance 16 and parameters such as the pipe length between tank 14 and cylinder 18, the diameter and the thermoconductivity of the material of construction of the pipes between tank 16 and cylinder 18, and the insulation on the pipes. This optimum differential pressure may change during the filling operation in response to changes in the system variables. A method for calculating the optimum differential pressure for a selected fill station will later be described.
When the initial optimum differential pressure for system 10 is calculated, it is stored as a set value in controller 12. Controller 12 then controls the pressure within cylinder 18 during the fill operation by reading pressure transducers 20,66 and adjusting variable throttle vent valve 68 in accordance with the tank pressure to cause the differential pressure of system 10 between tank 14 and cylinder 18 to be substantially equal to the stored set value of optimum differential pressure. Although the differential pressure between tank 14 and cylinder 18 is chosen as the value to be optimized and monitored in station 10, differential pressure between substance 16 being loaded and cylinder 18 may be optimized and monitored for points upstream of cylinder 18 other than tank 14. System variables are monitored and the optimum differential pressure iteratively recalculated during the filling process.
In addition to the optimum differential pressure, controller 12 also controls the flow of substance 16 from tank 14 to terminate the flow in response to an overfill error condition and controls actuation of pump 34 to prevent cavitation. In an error condition, either during pump transfer or pressure transfer of substance 16, cylinder 18 may be overfilled causing liquefied substance 16 to exit cylinder 18 through outlet vent 54 and vent pipes 64,92. The presence of liquefied substance 16 in pipe 64 is detected by thermocouple 56 which is disposed in pipe 64 substantially close to outlet vent 54.
Thermocouple 56 produces a signal at its output proportional to temperature. The output of thermocouple 56 is applied by way of line 100 to controller 12. When controller 12 determines that liquefied substance 16 is present within pipe 64 causing the temperature of pipe 64 to fall below a predetermined low level, controller 12 terminates the supply of substance 16 to cylinder 18. The predetermined low level of temperature which causes controller 12 to terminate the supply of substance 16 is substantially equal to the temperature of liquefied substance 16 within tank 14 calculated at cylinder 18 fill pressure.
Controller 12 terminates the supply of substance 16 by applying a signal by way of line 82 to solenoid 30 which causes solenoid controlled valve 28 to close. When solenoid control valve 28 closes, substance 16 is prevented from passing through pipe 24 to cylinder 18. Thus system 10 controls the supply of substance 16 to cylinder 18 in accordance with the temperature detected in vent pipe 64 substantially near outlet vent 54 of cylinder 18.
During pump transfer of substance 16 to cylinder 18, controller 12 of station 10 controls pump 34 to prevent cavitation of pump 34. When a pump transfer fill operation using pump transfer begins, valve 28 is opened without activating pump 34 permitting substance 16 to flow through pipe 24 to pump 34 thereby cooling pump 34. Valve 38 is closed during pump transfer to prevent substance 16 from travelling through pipe 37 and bypassing pump 34.
Thermocouple 40, disposed in pipe 39 substantially near pump 34, detects the presence of liquefied substance 16 within pipe 39 and thereby the temperature of pipe 39 and of pump 34 and produces a signal related to the temperature of pump 34. Pipe 39 is preferably provided with a fitting (not shown) having a thermal well disposed within one foot of pump 34. Thermocouple 40 may thus be positioned within the well to detect the presence of substance 16 at the outlet of pump 34 while not being subjected to the force of liquefied substance 16 being impelled from pump 34.
Pump 34 is a small (approximately five horsepower) pump. Because the mass of pump 34 is small, the presence of liquefied substance 16 in pipe 39 indicates that pipe 24 and pump 34 are sufficiently cool to prevent cavitation since substance 16 must travel through pipe 24 and pump 34 to reach pipe 39. The signal produced by thermocouple 40 is applied to controller 12 by way of line 96.
When controller 12 determines that pump 34 is sufficiently cool to prevent cavitation, controller 12 activates pump motor 36 by way of line 84. Pump motor 36 is coupled to pump 34 by coupling 35 and drives pump 34 causing liquefied substance 16 to be pumped from tank 14 to cylinder 18. Thus the transfer of liquefied substance 16 by pump 34 begins after pump 34 is cooled to approximately the temperature of substance 16 thereby preventing the formation of gas bubbles within pump 34 during the pumping operation which may cause cavitation of pump 34.
Referring now to FIG. 2, a more detailed representation of fill station 10 is shown. In fill station 10 cylinder 18 is positioned on scale 94 during the liquid loading operation. Scale 94 produces an output signal representative of the weight of substance 16 within cylinder 18. The output of scale 94 is monitored by controller 12 by way of input line 98. Controller 12 may be a conventional microprocessor or programmable controller. The control code of Appendix C is used to monitor and adjust the parameters of fill station 10 during filling.
Controller 12 is programmed to determine when the desired weight of liquefied substance 16 has been transferred to cylinder 18 from tank 14. In response to a determination by controller 12 that cylinder 18 contains the desired weight of substance 16, controller 12 terminates the supply of substance from storage tank 14 by controlling solenoid 30 and thereby valve 28 by way of output line 82 as previously described.
Thus fail/close solenoid controlled valve 28 may be closed by controller 12 in response to the occurrence of either of two events. First, when cylinder 18 contains a predetermined amount of substance 16 as indicated by scale 94, controller 12 closes valve 28. Secondly, as a backup method, if cylinder 18 overfills, thus causing the presence of liquefied substance 16 in pipe 64, thermocouple 56 detects a drop in process temperature at output pipe 64 causing controller 12 to close valve 28.
Station 10 also includes two shutdowns: remote shutdown 78 and hardware shutdown 60. An operator may use remote shutdown 78 to indicate to controller 12 that a filling operation on station 10 should be terminated at any time regardless of the internal substance temperature of pump 34 or vent pipe 64. Additionally, as a further safety precaution, hardware shutdown 60 may terminate operation of station 10 automatically in response to the temperature of pipe 64 and independently of controller 12.
Hardware shutdown 60 monitors thermocouple 56 through temperature switch 62 and closes valve 28 and stops pump 34 in response to a backup set point independently of controller 12. Hardware shutdown 60 thus serves as a backup for controller 12 during an overfill error if controller 12 is out of order permitting station 10 to terminate the supply of substance 16 during controller 12 failure.
Referring now to FIG. 3, flow chart 110 is shown. Flow chart 110 is a representation of the operations programmed and stored within controller 12 for controlling the operation of fill station 10. The first step in the filling operation is attaching liquid cylinder 18 as shown in block 112. Cylinder 18 includes inlet port 52 and outlet vent 54. Inlet port 52 is coupled to line 51 for receiving substance 16 from storage tank 14. Outlet vent 54 of cylinder 18 is coupled to pipe 64 for venting of substance 16 gasified during filling of cylinder 18.
During the filling process, as liquid substance 16 enters cylinder 18, some liquefied substance 16 vaporizes due to heat input and pressure letdown. The gaseous substance must be vented. In conventional filling operations outlet vent 54 was left open to the atmosphere to permit this flashed vapor to escape. Additionally, if cylinder 18 is overfilled liquefied substance 16 overflows through vent 54. In station 10, however, temperature measurement, pressure measurement and variable back pressure are provided by coupling vent 54 to thermocouple 56, pressure transducer 66 and variable throttle valve 68 respectively on output line 64.
Several manual valves are then opened as shown in block 114. These valves include optional manual valve 26 in line 24 which must be opened if provided within system 10 to allow substance 16 to flow from tank 14. If centrifugal pump 34 is to be used to transfer substance 16 to cylinder 18 ball valves 32,50 must be opened and valve 38 must be closed to permit substance to flow through pump 34 and not bypass pump 34 through pipe 37. If the pressure transfer method is used to fill cylinder 18 then valve 38 must be opened and ball valves 32,50 closed to allow substance 16 to flow around pump 34 by passing through pipe 37.
When cylinder 18 is connected and the required manual valves are open, scale 94 is zeroed and the fill weight is set as described in block 116. The TARE, or zeroing, operation is performed to cause the weight of the cylinder to be ignored by scale 94. For example, if 280 pounds of liquid nitrogen are to be loaded into cylinder 18, then after empty cylinder 18 is on the scale and the scale is zeroed when the scale reads 280 pounds it can be determined that there are 280 pounds of nitrogen in the cylinder.
To cause controller 12 to terminate the supply of substance 16 when 280 pounds of nitrogen have been loaded into cylinder 18, a fill weight of 280 pounds would be entered on dial 95 of scale 94. A relay (not shown) within scale 94 is closed when the weight of substance 16 within cylinder 18 reaches the set point of dial 95. The closing of the relay within scale 94 is detected by controller 12 by way of input line 98.
Cylinder valves 52,54 are then opened as shown in block 118 and a determination is made in decision 120 whether substance loading is to be performed by pressure transfer or pump transfer. If substance loading is to be performed by pressure transfer, two techniques may be followed: a fast technique (path 124) and a cooldown technique (path 126). During pressure transfer, the pressure within tank 14 is used to force substance 16 into cylinder 18. Typical values for the pressure in tank 14 are 50 psi to 150 psi.
If the fast technique of pressure transfer is used, path 124 is followed and variable throttle vent valve 68 and solenoid controlled fill valve 28 are fully opened as described in block 128. This permits substance 16 to flow through pipes 24,39,51 and inlet port 52 to cylinder 18 and to cool cylinder 18 with substantially little back pressure causing the coldest substance 16 to contact the internal surface of cylinder 18. A determination is made at decision 130 whether the temperature of thermocouple 56 is approximately -150.degree. F. which indicates that cylinder 18 is sufficiently cold to further minimize product loss. The temperature of -150.degree. F. is empirically determined and may vary for other product gases.
Thermocouple 56 produces a signal proportional to the temperature in pipe 64 substantially close to outlet valve 54 of cylinder 18. The signal produced by thermocouple 56 is amplified by operational amplifier 58 and applied to controller 12 by way of input line 100 of controller 12. If the temperature of thermocouple 56 is not substantially equal to the temperature of liquefied substance 16, as calculated at cylinder 18 filling pressure, a determination is made at decision 132 whether the temperature of thermocouple 56 is less than the initial temperature before the loading process began. If the temperature of cylinder 18 does not drop below the initial value within a period of time after valve 28 is open an error condition is indicated because if substance 16 is flowing into cylinder 18 as it should cylinder 18 must cool down.
If the temperature of thermocouple 56 is not less than the initial temperature, a timeout routine is executed as shown at decision 134. The timeout decision of 134 is intended to indicate that the execution of the program of controller 112 loops through decisions 130,132,134 for a predetermined period of time waiting for thermocouple 56 to indicate a drop in temperature below the initial value. If the drop in temperature does not occur before this timeout period is over, solenoid valve 28 is closed and an alarm on scan panel 86 is sounded as indicated in block 138 and execution ends at terminal 140.
Once the temperature of thermocouple 56 has fallen below the initial value as determined by decision 132, execution loops through decisions 130,132 until the temperature of thermocouple 56 has reached -150.degree. F. indicating that cylinder 18 has cooled down sufficiently. As shown in block 136, controller 12 applies a signal by way of output line 90 to voltage-to-pneumatic transducer 74 to adjust the pressure of cylinder 18 and optimize the differential pressure of system 10. Voltage to pneumatic transducer 74 receives input instrument air or nitrogen of a predetermined pressure from line 76 and applies a controlled pressure by line 72 to actuator 70. Controller 12 may include digital to analog converters for producing analog signals such as the signal applied to actuator 70.
Actuator 70 causes variable throttle vent valve 68 to close in block 136 until the required pressure in cylinder 18 is produced in accordance with pressure readings of pipe 64 by pressure transducer 66 to achieve optimum differential pressure. Valve 68 may be a conventional throttle valve such as the cryogenic 316SS Globe control valve of the V1S series, manufactured by Jamesbury, with one R2A pneumatic actuator set for fail open on instrument air loss. A typical valve body size is three-quarters of an inch but the valve body size may range from approximately one-half inch to one and one-quarter inch, depending on the type of fill station.
Controller 12 monitors the pressure within tank 14 by reading the output of pressure transducer 20. Pressure transducer 20 is coupled to tank 14 by pipe 22 which opens onto the interior of tank 14. Thus controller 12 may determine the differential pressure between tank 14, including liquefied substance 16 head pressure within tank 14 and cylinder 18 by comparing the outputs of pressure transducers 20,66. The determined value of differential pressure is compared with the stored optimum set value of differential pressure and the pressure of cylinder 18 is adjusted accordingly by adjusting throttle valve 68. The optimum differential pressure is calculated and set in block 136 as set forth in Appendices A,B.
If the cooldown technique of pressure transfer is used rather than the fast technique as previously described, execution follows path 126 to block 144 in which the optimum differential pressure is set immediately rather than after cylinder 18 cools down as described for the fast technique of path 124. The technique of path 126 may be used if cylinder 18 is initially in a precooled condition, allowing filling of substance 16 to occur immediately at the optimum differential pressure. Solenoid controlled fill valve 28 is opened by way of output line 82 as shown in block 146 and thermocouple 56 is compared with the initial temperature in block decision 148 to determine whether cylinder 18 is beginning to cool down indicating that substance 16 is flowing into cylinder 18 as previously described.
If cylinder 18 does not begin cooling within a period of time determined by the timeout of decision 142, as previously described for the timeout of decision 134, then there may be a leak of substance somewhere between tank 14 and cylinder 18 and solenoid valve 28 is closed and an alarm of scan panel 86 is sounded as shown in block 138 as previously described. Whether pressure transfer proceeds by the cooldown technique or the fast technique, execution proceeds to off page connector 150 with the optimum differential pressure already set by adjusting valve 68.
Referring now to FIG. 4, execution proceeds from off page connector 150 of routine 110 to on page connector 152 of routine 190 and a determination is made at decision 154 whether the temperature of thermocouple 56 has reached the temperature of liquid substance 16 being transferred indicating an overfill error. If substance 16 being transferred is liquid nitrogen, the liquid temperature detected by thermocouple 56 is -310.degree. F.; if substance 16 is liquid oxygen, the liquid temperature is -285.degree. F.; for liquid argon, the temperature is -290.degree. F.
In an alternate embodiment, a single low temperature set point of approximately -250.degree. F. may be used for any of the above substances 16. In another alternate embodiment, substance 16 may be liquid hydrogen or helium and a suitable temperature set point is selected for these product gases. In another alternate embodiment, the low temperature set point is determined by controller 12 and is a function of the type of cryogenic substance 16 being transferred and cylinder 18 fill pressure as sensed by pressure transducer 66.
If the temperature of thermocouple 56 has not reached the temperature of liquefied substance 16 as determined by decision 154, liquefied substance 16 has not reached pipe 64 indicating that an overfill condition does not exist. Therefore, a determination is made at decision 156 whether the cutoff weight entered on dial 95 of scale 94 has been reached. To make this decision, controller 12 reads a single output bit of scale 94 by way of input line 98 in which the output bit of scale 95 indicates whether the weight of substance 16 in cylinder 18 has reached the weight set on dial 95. If the cutoff weight has not been reached, execution loops back to block 136 by way of off-page connector 157 and on-page connector 159. At block 136, the optimum differential pressure is again calculated and set as set forth in Appendix A pages 6,7 and Appendix B pages 7,9. If the pressure within tank 14 has changed since the last time block 36 was executed, the new value of pressure is used in the calculations of block 136 and a new value of optimum differential pressure is determined.
Thus, during the filling operation execution loops through decisions 154,156 waiting for the cutoff weight to be reached or, in the event of a failure of digital scale 94, for an overfill. When the cutoff weight has been reached as determined by decision 156, variable throttle vent valve 68 an solenoid controlled fill valve 28 are closed as shown in block 158 and a fill alarm and a fill light on scan panel 86 are activated by controller 12 by way of output line 88 as shown in block 160.
The operator of fill station 10 then closes cylinder valves 52,54 as indicated in block 162 and a blowdown is performed as shown in block 164. In the blowdown the lines which carry substance 16 are emptied to prevent vaporization of substance 16 within the lines from causing a pressure build up due to continued heat input from ambient temperature. Such a pressure build up could rupture a line. Cylinder 18 is then disconnected as shown in block 178 and execution is terminated at end 180.
If the temperature of thermocouple 56 is substantially equal to the liquid temperature as determined by decision 154, indicating an overfill, solenoid controlled fill valve 28 is closed by controller 12 as shown in block 166. Vent control valve 68 is fully opened to permit venting of the overflow of liquefied substance 16 through vent line 92 as indicated in block 168 and an alarm and an overfill light on scan panel 86 are activated as indicated in block 170.
Cylinder inlet valve 52 is then manually closed as indicated in block 172 and a blow down of the fill line and cylinder 18 is performed as shown in block 174. Cylinder outlet or vent valve 54 is then closed as indicated in block 174 and cylinder 18 is disconnected as shown in block 178.
Referring now to FIG. 5, a flow chart representation of pump transfer routine 200 is shown. Execution proceeds to on page connector 202 of pump transfer routine 200 from off page connector 122 of routine 110 when a determination is made at decision 120 that pump transfer is to be performed. Pump transfer is started at block 204. The optimum back pressure, as determined from the optimum differential pressure set value stored in controller 12 and the pressure in tank 14, is set at block 206 by a signal by way of output line 90 from controller 12 to voltage to pneumatic transducer 74 which controls variable throttle vent valve 68 as previously described. Additionally, solenoid controlled valve 28 is opened to permit substance 16 to begin to flow through pipe 24 to cylinder 18.
Controller 12 then waits a predetermined period of time to determine whether substance 16 has actually begun to flow once solenoid controlled valve 28 is opened. This determination is made in the manner previously described at decision 210 in which the temperature in vent pipe 64, as monitored by thermocouple 56, is compared with the initial temperature when the transfer operation began. If the temperature of cylinder 18 has not fallen below the initial temperature as determined by decision 210, a determination is made by decision 208 whether the time out period has elapsed. If the time out period has not elapsed, execution loops between decisions 208,210 until either the time out period does elapse or the vent temperature decreases below the initial temperature.
If the vent temperature does not drop below the initial temperature before the end of the timeout period, indicating a possible failure condition such as improper cylinder 18 connection, solenoid controlled valve 28 is closed as shown in block 212, the alarm and error light of scan panel actuated in block 216, and routine 200 is terminated at end 220.
If the vent temperature does fall below the initial temperature before period, as determined by decisions 208,210, a determination is made at decision 214 whether pump 34 temperature has substantially reached the liquid temperature as calculated by controller 12 according to the tank 14 pressure received from pressure transducer 20. This indicates that pump 34 is sufficiently cool to prevent cavitation. Controller 12 determines the temperature of pump 34 by monitoring thermocouple 40 which produces a signal representative of the temperature within pipe 39 preferably within one foot of pump 34. This temperature drops when substance 16 reaches pipe 39 indicating that pump 34 is sufficiently cool to prevent cavitation.
The signal produced by thermocouple 40 is amplified by operational amplifier 42 and applied to controller 12 by way of input line 96. When pump 34 is sufficiently cool to prevent cavitation, pump motor 36 is activated by controller 12 by way of output line 84 as indicated in block 218 and the optimum pressure is calculated and set as shown in block 221 as set forth in Appendices A, B.
In an alternate embodiment, controller 12 may wait for a predetermined period of time after detecting the presence of liquefied substance 16 at the outlet of pump 34. This allows an additional cooling period to be certain that pump 34 is cool enough to prevent cavitation. However, if pump 34 is small enough, this is not necessary.
When pump motor 36 is actuated, determinations are made whether the temperature within pipe 64 has substantially reached liquid temperature to detect an overfill error and whether the weight within cylinder 18 has reached the cutoff weight as previously described in the description of pressure transfer. Thus, a determination is made at decision 222 whether the temperature of pipe 64, as indicated by the output of thermocouple 56, has substantially reached liquid temperature. If the temperature at pipe 64 has not reached liquid temperature, a determination is made in decision 226 whether the cutoff weight has been reached.
If the cutoff weight of substance 16 within cylinder 18 has not been reached as determined by decision 226, a determination is made at decision 232 whether the differential pressure between the input and the outlet of pump 34 is low indicating cavitation of pump 34. If the differential pressure as determined by differential pressure switch 48 is low as determined at decision 232, pump motor 36 is stopped as indicated in block 238, and a determination is made how many times this condition has arisen.
If the pump motor 36 shutdown condition has arisen less than four times, pump cooldown is permitted to continue as shown in block 224 and a determination is again made whether the pump temperature has substantially reached liquid temperature at decision 214. Valves 44,46 are provided at the inputs to differential pressure switch 48 to selectively prevent passage of substance 16 to differential pressure switch 48 and equalization valve 50 is provided to permit bypassing of differential pressure switch 48 for isolating differential pressure switch 48 from the rest of station 10, for example during maintenance.
If the differential pressure between the inlet and the outlet of pump 34 is not low, as indicated by differential pressure switch 48 in decision 232, a determination is made at decision 240 whether the temperature of pump 34, as determined from thermocouple 40, is greater than the liquid temperature +5.degree. F. indicating an error condition in which substance 16 is not passing through pump 34 as expected. If the temperature of pump 34 is greater than the liquid temperature +5.degree. F., pump motor 36 is stopped as shown in block 238.
If pump motor 36 has been stopped fewer than four times as determined in decision 234, cooldown is continued as previously described. If the pump temperature is not substantially greater than the liquid temperature +5.degree. F., execution returns to decision 221 and station 10 continues filling cylinder 18 and waiting for the cutoff weight to be reached.
Thus, during the loading of substance 16 into cylinder 18 by the pump transfer method, fill station 10 monitors the cutoff weight at decision 226 and also monitors vent temperature at pipe 64, the differential pressure across pump 34 and the temperature of pump 34 to detect error conditions. It will be understood by one skilled in the art that these determinations, made at decisions 222,226,232 and 240, are shown as being performed sequentially by controller 12 but may be performed in parallel by a plurality of controllers or independent circuits. For example, a dedicated circuit for monitoring the temperature at vent pipe 64, independently of the programming of controller 12, may interrupt the loading operation when the temperature of vent pipe 64 reaches a predetermined low level.
Referring now to FIG. 6, there is shown flow chart 250 which is a continuation of the operations of pump transfer routine 200. When pump motor 36 has been stopped at block 238 four times, either because the differential pressure of differential pressure switch 48 is low or the temperature of thermocouple 40 is high, execution proceeds from off page connector 236 of pump transfer routine 200 to on page connector 252 of routine 250. The choice of four as the number of passes through the routine stopping and restarting pump 34 is empirically chosen. Pumps such as pump 34 often require two startup attempts before catching prime.
After four startup attempts, solenoid controlled valve 28 is closed to terminate the flow of substance 16 as shown in block 254 and variable throttle vent valve 68 is completely opened to vent cylinder 18. Additionally, the alarm on scan panel 86 and a cavitation alarm on scan panel 86 are activated as shown in block 258 and execution is terminated at end 260.
When the cutoff weight of cylinder 18 has been reached as determined by decision 226 of routine 200, execution proceeds through off page connector 230 to on page connector 262 of routine 250. Because cylinder 18 has reached the required weight at this point, solenoid controlled fill valve 28 is closed as shown in block 264 and variable throttle vent control valve 68 is closed as shown in block 266. The horn and fill light of scan panel 86 are activated as shown in block 268. The operator then closes cylinder valves 52,54 as shown in block 270 and blow down is performed as indicated in block 272. The loading operation is then complete. Cylinder 18 is therefore disconnected as indicated in block 274 and execution is terminated at end 299.
During the loading of cylinder 18, if the temperature of pipe 64 reaches the temperature of the liquid being loaded, as determined by decision 222, indicating an overfill condition, execution proceeds through off page connector 228 of pump fill routine 200 to on page connector 276 of routine 250. During an overfill condition, the first operation performed is closing of solenoid controlled fill valve 28 to terminate the supply of substance 16 as indicated in block 278.
Vent control valve 68 is opened completely at block 280 to permit venting of liquefied substance 16 which has reached pipe 64. The alarm and overfill light of scan panel 86 are activated at block 290. The operator then closes cylinder inlet port 52 as shown in block 292 and a blowdown of the fill line is performed at block 294. Additionally, a blowdown of cylinder 18 must be performed at block 296 followed by closing cylinder vent valve 54 at block 298. Cylinder 18 may then be disconnected as shown at block 274.
Controller 12 is programmed to provide a separately identifiable error message for each error condition which may arise within station 10, for example the errors determined at decisions 134, 142, 154, 208, 232, 234, and 240. This permits an operator to easily determine which error condition has arisen. Additionally, the duration of each timeout period, such as those at decisions 134, 142, and 208, may be individually selected and optimized by adjusting corresponding time parameters within the program of controller 12.
Controller 12 also accumulates total pump running hours to indicate when scheduled maintenance is needed. In addition, total filling losses may be accumulated over time to assist in inventory control.
Referring now to FIG. 7 there is shown a flow chart of model routine 300 for modelling filling losses during loading of cylinder 18 with a cryogenic substance 16. This model may be used to determine the optimum differential pressure for fill stations such as fill station 10 for minimizing filling losses. The optimum differential pressure for an individual fill station depends on many parameters such as the length, diameter, construction material and insulation material of the pipes through which substance 16 must pass to reach cylinder 18. The optimum differential pressure also depends on the type of cryogenic substance 16 which is transferred and may vary during the filling process.
The routines modelled by model 300 are run prior to the loading of cylinder 18 and accept as their inputs parameters relating to a specific fill station such as fill station 10. This model may be run repeatedly for a fill station with all parameters remaining constant except for the pressure of cylinder 18 and thereby the differential pressure between storage tank 14 and cylinder 18. The filling loss for each value of pressure within cylinder 18 is calculated by model 300 and an optimum differential pressure is selected by reference to these results and determining which value of differential pressure produces minimum loss of substance 16.
This optimum differential pressure is stored as a set point within controller 12 and compared with values of differential pressure determined during a pressure transfer. The values of differential pressure during a pressure transfer are determined by monitoring the pressure of tank 14 and the pressure of cylinder 18 using pressure transducers 20,66 respectively. The differential pressure of fill station 10 during pressure transfer is adjusted by adjusting the pressure in cylinder 18 with throttle valve 68 to a pressure set point determined from the optimum differential pressure set point and the pressure within tank 14.
By repeatedly running model 300 as described, there may be produced graphs of filling loss versus cylinder pressure as shown in FIGS. 8 and 9 in which each line on graphs 340,360 represents a plurality of runs of model routine 300 for a single fill station in which the pressure within cylinder 18 is varied while the remaining parameters are kept constant. For example, the curves of graph 340 are all plotted for a fill station in which the tank pressure was constant at fifty psig, the outer diameter of the fill line was seven-eighths inch, and no insulation was present on the fill lines. The pressure within cylinder 18 was varied from zero to fifty psig. Curve 342 was plotted for a seven-eighths inch outer diameter fill line, a fill line length of one hundred feet and pressure within cylinder 18 varying from zero to fifty psig.
Curve 344 was plotted by holding the fill line length constant at seventy-five feet while varying the pressure within cylinder 18 from zero to fifty psig. Similarly, curves 346,348 were produced by holding the fill line length at fifty feet and at twenty-five feet respectively while varying the pressure within cylinder 18 over the same range. By reference to curves 342-348, it can be seen that when the pressure of cylinder 18 is varied while the remaining parameters are held constant, there is a cylinder pressure, and therefore a station differential pressure, which produces a minimum filling loss. This optimum differential pressure can vary greatly with fill line length, from eight psig at twenty-five feet to twenty-five psig at one hundred feet.
The curves of graph 360 are plotted with tank pressure held constant at fifty psig, a fill line outer diameter of seven-eighths inch and a one inch foam insulation on the fill line. Curves 362,364,366,368 were produced by inputting fill line lengths of one hundred feet, seventy-five feet, fifty feet and twenty-five feet, respectively, while varying the pressure of cylinder 18 between zero and fifty psig. As previously described, a minimum product loss may be determined for each curve 362-368.
Similar graphs may be prepared using model 300 for fill stations in which the tank pressure may be any desired value other than fifty psig, for example, seventy-five or one hundred psig. Additionally, runs of model 300 may be performed using any outer diameter fill line, such as one-half inch or five-eighth inch outer diameter. Such graphs may also be prepared for different thermal conductivity of materials, cylinder 18 fill volumes, substances 16, etc.
Thus, it may be seen that when a fill station is specified according to its fill line length, fill line outer diameter, insulation, etc., model 300 may be used to vary the pressure within cylinder 18 to determine the minimum fill loss as a function of differential pressure for that station.
At block 306 of model 300, pipe line heatleak due to convection (Q.sub.c) and pipe line heatleak due to radiation (Q.sub.r) are calculated. The convection heat loss (Q.sub.c) is calculated according to: ##EQU1## in which T.sub.A is the ambient temperature, T.sub.L is the liquid temperature, h.sub.i is the heat transfer coefficient of the wetted surface between the pipes carrying substance 16 and substance 16 itself, A.sub.i is the total wetted area between the pipes and substance 16,.DELTA.r is the thickness of the pipe and of the insulation, respectively, A.sub.lm is the log mean of the pipe area or insulation area, h.sub.o is the heat transfer coefficient between the outer layer of insulation and ambient, and A.sub.o is the outer area of the insulation.
The pipeline heatleak due to radiation (Q.sub.R), also calculated at block 306, is calculated as:
in which .sigma. is the Stephan-Boltzmann constant, .epsilon. is the emissivity constant of the outer surface of the insulation, A.sub.o is the outer pipe area, T.sub.A is the ambient temperature and T.sub.surf is the surface temperature of the insulation when the surface is assumed to have no ice.
At block 308 a determination is made of the amount of loss due to pipeline cool down (Q.sub.PCD). This loss includes both the heat absorbed from the pipe and the heat absorbed from the insulation around the pipe. This determination is given as:
in which m.sub.p is the mass of the entire pipeline which carries substance 16, m.sub.i is the mass of all the insulation respectively on the pipes which carry substance 16, C.sub.p is the specific heat for the pipes and for the insulation and .DELTA.T is the difference between the initial pipe and insulation temperature and the temperature of substance 16. K is a percentage less than 100% which indicates the amount of insulation which is cooled, providing a temperature gradient across the insulation thickness between substance 16 temperature and ambient temperature.
Cylinder heatleak (Q.sub.CH) is determined at block 312 from the normal evaporation rate (NER) of the substance being loaded assuming that an average of one-half of the final volume of cylinder 18 is exposed during the filling operation. Therefore cylinder heatleak is given as: ##EQU2## in which NER is the normal evaporation rate which may be, for example, 1.5% per day for liquid oxygen at 1 atmosphere, w is the total cylinder liquid mass, and .DELTA.H.sup.v is the latent heat of vaporization for the liquid substance 16.
Cylinder cool down (Q.sub.CCD) is calculated at block 316 assuming that there is no thermal resistance in the inner vessel within cylinder 18 and that 37% of the super insulation mass of cylinder 18 is cooled to liquid temperature during cylinder cool down. The heat loss due to cylinder cool down using these assumptions is:
in which M.sub.v is the mass of the inner vessel and Mi is the mass of the super insulation of cylinder 18.
At block 320 vapor displacement is calculated. When substance 16 first enters cylinder 18, some of substance 16 vaporizes filling cylinder 18 with vapor. This vapor is displaced by liquefied substance 16 as cylinder 18 is filled. The displaced vapor is vented through outlet vent 54. The displaced vapor is lost product gas and is calculated in block 320 in order to determine overall product loss. It is approximately equal to the volume of cylinder 18.
In order to build pressure within tank 14 for transfer of substance 16, substance 16 may be subcooled by passing substance 16 through external coils to cause a controlled amount of vaporization. The vapor generated is returned to the vapor space of tank 14. The vapor may be periodically vented to control the pressure within tank 14. This subcooling of substance 16 also helps prevent cavitation because substance 16 is transferred before it reaches liquid saturation at the higher pressure and substance 16 is thus less likely to vaporize when it reaches pump 34. The amount of product gas lost due to subcooling is determined in block 322. Losses due to overfills are determined in block 324.
The amount of work performed by pump 34 and pump motor 36 may also be included, and they are estimated in block 326 as the electrical power supplied to pump motor 36. The loss due to cool down of pump 34 is equal to the mass which is in contact with substance 16 multiplied by the specific heat of the material of construction and temperature differential between substance 16 temperature and initial pump temperatures, and this loss is calculated in block 328.
The Joule-Thompson flashing loss is calculated in block 329. This loss occurs when cryogenic substance 16 passes from a higher pressure region, such as a region substantially near tank 14, to a lower pressure region, such as a region substantially near cylinder 18. The transition from higher pressure to lower pressure causes some of substance 16 to boil off. Assuming isenthalpic conditions and using the "Lever Rule" on a pressure, temperature, enthalpy diagram, the flashing losses are calculated as:
in which H.sub.1.sup.L is the higher pressure enthalpy, H.sub.2.sup.L is the lower pressure enthalpy, and H.sub.2.sup.v is the latent heat. The percent loss calculated in equation (6) is multiplied by the total amount of product gas or substance 16 transferred from tank 14 to obtain the amount of substance 16 lost due to flashing.
In block 330 all of the losses calculated in blocks 306-329 are summed to determine the total filling loss and execution ends at terminal 332. The pipeline and cylinder heatleak losses are time dependent, therefore an iterative procedure must be used to obtain the total filling losses. The process represented by model 300 is then rerun for a plurality of different values of differential pressure between tank 14 and cylinder 18 while the remaining parameters specifying station 10 and substance 16 are held constant. A value of differential pressure is selected which produces a minimum amount of total filling loss at block 330.
This optimum differential pressure for station 10 is stored in controller 12 and used to adjust throttle vent valve 68 during filling. The entire process of performing a plurality of runs of model 300 and selecting an optimum differential pressure must be performed for each different configuration of a fill station and for each different product gas.
When model 300 is used to simulate filling losses due to pressure transfer, certain losses, such as the losses calculated in blocks 322, 326, 328 which are associated with pump 34, need not be calculated. A FORTRAN program, written in a structured form understandable to those of ordinary skill in the art, which performs the calculations required for calculating product loss during such a pressure transfer appears at the end of this specification as Appendix A.
Additionally, a FORTRAN program for calculating filling losses during pump transfer appears at the end of this specification as Appendix B. Since many of the losses simulated by model 300 occur during both pressure transfer and pump transfer the programs of Appendices A, B overlap. The program of Appendix B may be used to optimize the pressure of cylinder 18 with respect to the amount of venting loss due to subcooling.
The program of Appendix B may also be used to model the losses for sequential filling of a plurality of cylinders 18 by pump transfer. During the first filling of a cylinder 18 the losses due to building feed pressure calculated in block 322 and pump cooldown calculated in block 328 are higher than the losses due to these considerations during subsequent fillings because during subsequent fillings the pressure is already built up in tank 14 and pump 34 is already cooled down.
Thus, if model 300 as implemented in Appendix B is run a plurality of times in view of the changing values of temperature and pressure in tank 14 and temperature of pump 34, the total filling loss for a plurality of cylinders 18 may be determined. This information may be used to determine the minimum number of cylinders 18 which must be filled sequentially to make pump transfer economically desirable.
The first cylinder filled by pump transfer causes losses which are higher than the losses required to fill by pressure transfer because pressure transfer does not require subcooling of tank 14 or cooling of pump 34. However, subsequent fillings cause less filling loss than pressure transfer because substance 16 passes through station 10 more quickly causing less heatleak loss and less operator time. There is thus a crossover point after which filling by pump transfer is more econonomicaly desirable than filling by pressure transfer. By running model 300 repeatedly and summing the losses incurred for a plurality of cylinders 18 for both types of transfer, this crossover point may be determined.
The control code of Appendix C is written in MACBASIC, a controller version of BASIC, and is designed to run on an Analog Devices Micro Mac 6000 Controller Computer. The code is exemplified for a 50 foot, uninsulated 3/4 inch copper fill line using liquid nitrogen. In general use, the appropriate curve generated by the FORTRAN simulations of Appendices A, B for a particular plant configuration and cryogenic substance is loaded into the control code, which then controls the operation of the filling system to maintain the optimum differential pressure. ##SPC1##
Claims
I claim:
1. A method for minimizing cryogenic substance loss in a filling station having a storage tank storing cryogenic substance for loading a container having an outlet vent with a throttle vent valve for adjusting the differential pressure between the substance being loaded and the container, comprising the steps of: (a) first determining a value of filling loss for each of a plurality of values of differential pressure; (b) selecting and storing prior to loading an optimum value of differential pressure from the plurality of values to produce the minimum filling loss; (c) loading substance into a container; (d) continuously monitoring the differential pressure during loading; (e) comparing the monitored differential pressure to the optimum differential pressure; (f) adjusting the throttle vent valve to maintain the monitored differential pressure at a value substantially equal to the optimum differential pressure value; (g) selecting at least one new optimum value of differential pressure during loading of the container; (h) adjusting the throttle vent valve to bring the differential pressure to a value substantially equal to the new optimum differential pressure value during loading of the container; and (i) repeating steps (g) and (h) until the filling process is terminated.
2. The method of claim 1 in which the station is provided with a fill valve for controlling the flow of substance from the storage tank to the container and step (c) is preceded by the steps of: opening the fill valve for permitting substance to flow from the storage tank to the container thereby cooling the container; sensing the temperature substantially near the outlet vent of the container; determining whether the temperature has reached a predetermined level; and adjusting the throttle vent valve in response to the temperature determination for providing container cool down prior to adjusting the throttle vent valve.
3. The method of claim 1 further comprising the steps of: sensing the weight of the substance loaded into the container; determining when a predetermined weight of substance is loaded into the container; and controlling the fill valve for terminating the supply of substance to the container in response to the weight determination.
4. The method of claim 1 further comprising the steps of: sensing the temperature substantially near the outlet vent of the cylinder; determining whether the temperature has reached a predetermined level; and controlling the fill valve in response to the outlet vent temperature to terminate the supply of substance to the container for preventing substance from overflowing from the container.
5. The method of claim 1 in which the station is provided with a pump for transferring substance from the storage tank to the container further comprising: (a) supplying substance to the pump; (b) sensing the pump temperature; (c) determining when the sensed temperature has reached a predetermined level; and {(d) controlling the pump motor in response to the determination.
6. The method of claim 1 in which the station is provided with a pump for transferring substance from the storage tank to the container, further comprising: sensing the differential pressure across the inlet and outlet of the pump during motor operation; controlling the pump motor in accordance with the sensed differential pressure.
7. A method for minimizing substance loss in a filling station having a storage tank storing cryogenic substance for loading a container having an outlet vent with a throttle vent valve for adjusting the differential pressure between the substance being loaded and the container, the station having a fill valve for controlling the flow of substance from the storage tank to the container, comprising the steps of: (a) selecting an optimum differential pressure; (b) loading substance into the container, thereby cooling the container; (c) monitoring the differential pressure during loading; (d) sensing the temperature substantially near the outlet vent of the container; (e) determining whether the temperature has reached a predetermined level; and (f) adjusting the throttle vent valve to bring the monitored differential pressure to a value substantially equal to the optimum differential pressure in response to the temperature determination for providing container cool down prior to adjusting the throttle vent valve; (g) selecting at least one new optimum value of differential pressure during loading of the container; (h) adjusting the throttle vent valve to bring the differential pressure to a value substantially equal to the new optimum differential pressure value during loading of the container; and (i) repeating steps (g) and (h) until the filling process is terminated.
8. The method of claim 7 further comprising the steps of: sensing the weight of the substance loaded into the container; determining when a predetermined weight of substance is loaded into the container; and controlling the fill valve for terminating the supply of substance to the container in response to the weight determination.
9. The method of claim 7 further comprising the step of: controlling the fill valve in response to the outlet vent temperature to terminate the supply of substance to the container for preventing substance from overflowing from the container.
Patent Citations (7)
Non-Patent Literature (7)
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