Abstract:
A freeze and thaw system using knowledge-based hardware and methodology is provided for repair of oil-filled pipes. Liquid nitrogen is boiled off inside freeze jackets installed over the pipe, exiting in a superheated vapor state. Liquid nitrogen flow rates, controlled by the temperature controllers, are based on the requirements of the freeze process to maintain a desired level of superheat. The freeze operation is divided into distinct phases each with a different nitrogen superheated exhaust temperature. At the end of the repair, electric heaters wrapped over the pipe proceed to thaw the frozen oil. The heater controller thaws the frozen plugs based on an embedded time versus temperature protocol. The freeze prevention heaters are wrapped over the freeze jackets to prevent ice formation over the cold freeze jackets while the freeze plug is formed and maintained. This technology results in less thaw and freeze time at reduced liquid nitrogen usage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to a system for freezing and thawing stationary oil in a section of a pipe to create a frozen plug. As these underground pipes do not incorporate any isolation valves, the plugs serve that function. This allows repair in the oil-filled pipes that also contain high voltage electric cables. At the end of the repairs, the frozen plug is thawed to initiate oil flow through the pipe. 
         [0002]    Fluid filled pipes are used in a variety of applications such as equipment used in food processing systems, water delivery systems, cooling of High Voltage electric cables, electrical distribution systems as well as the exploration, delivery and processing of oil, natural gas and their end products. The fluid may be in the pipes for a variety of purposes, such as delivery of the fluid to an end point, to transfer or deliver thermal energy to a system, or to isolate a component within the pipe from the external environment. 
         [0003]    One exemplary application is a system having long distances of uninterrupted dielectric oil filled pipe containing electric cables to transmit high voltage electric power. These systems are used to transfer electrical power from electrical production facilities to areas where the electrical power will be consumed. In some areas, many miles may separate electrical production and consumption areas. It is not uncommon for transmission cables to extend over 8 miles (13 km) to 15 miles (24 km). Several different types of conductors or cables are used in the transmission of high voltage electricity. For underground cables, the conductors are housed inside a steel pipe containing dielectric fluid. The pipe is typically carbon steel having a quarter-inch (0.64 cm) wall. The pipe protects the power cables and acts as a vessel for containing the dielectric fluid. One issue that arises in underground installations is the heat and electrical fields generated by the high voltage cables. The high-pressure dielectric fluid surrounding the cables provides both cooling and electrical insulating functions. 
         [0004]    During normal use over an extended period of time, leaks of dielectric fluid through the steel pipe can sometimes occur. These leaks may be due to a number of reasons, such as shooting fluid jet from an adjacent water or steam pipe, mechanical fatigue, or corrosion for example. It should be appreciated that the leakage of the dielectric fluid may be undesirable environmentally and result in degradation in the performance of the cable. Since there are few or no valves installed in transmission cables, one method of repair involves freezing the dielectric fluid on either side of the area needing repair. The freezing creates a highly viscous plug of frozen dielectric fluid that allows the repair area to be isolated for draining and maintenance functions. 
         [0005]    Since dielectric fluid has a low freezing point, a cooling liquid, such as liquid nitrogen for example, is used to form the freeze plugs. Typically, the liquid nitrogen is introduced into the freeze jackets that are installed to encircle the pipe. While the method is effective, it does have several drawbacks. There is poor contact between the freeze jackets and the outside wall of the pipe. Since the heat transfer mechanism is by conduction, poor contact between the jackets and the pipe does not help the heat transfer. Thus large amounts of liquid nitrogen are required over an extended period of time. Typically, it takes 10 to 12 hours for a proper freeze plug to form. Also, since these transmission cables may be located in a metropolitan area, any delays in the delivery of adequate quantities of liquid nitrogen may cause issues in forming and maintaining the freeze plugs. 
         [0006]    This process of forming the freeze plug also results in a large block of ice forming around the outside surfaces of the freeze jackets. While the block of ice may assist in holding the cable and freeze plug temperatures, it also lengthens the time it takes to thaw the freeze plug. Typically, using this method, it takes 8 to 24 hours to thaw the oil around the transmission cables once the repairs have been completed. To facilitate the thawing, it is common to use propane or kerosene heaters that blow hot air across the accumulated ice. It should be appreciated that since the transfer of electrical power is terminated once the repairs are initiated, and it cannot be restored until the oil around the transmission cables has been thawed, therefore it is desirable to minimize the amount of time needed to form and thaw the freeze plugs. 
         [0007]    It should also be appreciated that it would be desirable to reduce the freeze and thaw time periods in other applications as well. For example, in a food processing facility, production could be halted during the repair, or in a water distribution system, water would not be delivered to end customers during the repair. 
         [0008]    While existing systems and methods for thawing pipes are suitable for their intended purposes, there still remains a need for improvements, particularly regarding the amount of time needed to thaw the fluid filled pipes. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with one embodiment of the invention, a device for warming a frozen pipe is provided. The device includes a first heater disposed about the frozen pipe, the first heater having a heating element. An insulation layer is disposed about the first heater. A second heater is disposed about the heater. A temperature sensor is arranged in thermal contact with the heating element. 
         [0010]    In accordance with another embodiment of the invention, a device for generating a freeze plug is provided. The device includes a jacket and a first tube fluidly coupled to the jacket adjacent a first end. A second tube is fluidly coupled to the jacket adjacent a second end, wherein the second end is opposite the first end. A first valve is coupled to the first tube. A first temperature sensor is thermally coupled to the second tube. A controller is electrically coupled to the first temperature sensor and the first valve, the controller includes a processor that is responsive to executable computer instructions for modulating the first valve in response to a signal from the first temperature sensor such that a superheated level of a gas flowing from the jacket through the second tube is maintained. 
         [0011]    In accordance with another embodiment of the invention, a method of operating a device for warming a frozen pipe is provided. The method includes the step of operating a first heater, wherein the first heater has a heater element and is disposed about the frozen pipe. A first temperature of the heater element is monitored. The first heater is disabled when the first temperature exceeds a first threshold. A second heater is operated at a substantially constant second temperature, wherein the second heater is disposed about the first heater. 
         [0012]    In accordance with another embodiment of the invention, a method of operating a device for forming a freeze plug in a pipe is provided. The method includes the steps of flowing a cryogenic liquid into a first chamber, wherein the first chamber is in thermal contact with the pipe. The cryogenic liquid is boiled in the first chamber to form a cryogenic superheated gas. A first temperature of the cryogenic gas exiting the first chamber is monitored. A first valve is modulated to adjust the flow of the cryogenic liquid in response to the first temperature. The first temperature of the cryogenic gas is maintained at a desired level of superheat by modulating the first valve. A first heater is operated at a substantially constant second temperature and a freeze plug is formed in the pipe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Referring now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike: 
           [0014]      FIG. 1  is an illustration of a prior art high voltage conductor arrangement; 
           [0015]      FIG. 2  is a perspective view illustration of a device for forming and thawing a freeze plug in a pipe in accordance with an exemplary embodiment of the invention; 
           [0016]      FIG. 3  is a sectional view illustration of the device of  FIG. 2  installed on high voltage conductor of  FIG. 1 ; 
           [0017]      FIG. 4  is a side plan view illustration of the device of  FIG. 2 ; 
           [0018]      FIG. 5  is a schematic illustration of a controller for the device of  FIG. 2 ; 
           [0019]      FIG. 6  is a schematic illustration of a temperature sensor arrangement for the device of  FIG. 2 ; 
           [0020]      FIG. 7  is a block diagram illustration of an exemplary method of operating the device of  FIG. 2 ; 
           [0021]      FIG. 8  is a temperature profile illustration for the exhaust temperature for the device of  FIG. 2 ; 
           [0022]      FIG. 9  is a block diagram illustration of another embodiment method of operating the device of  FIG. 2 ; 
           [0023]      FIG. 10  is a block diagram illustration of another embodiment method of operating the device of  FIG. 2 ; 
           [0024]      FIG. 11  is a block diagram illustration of an embodiment for thawing a frozen pipe; and, 
           [0025]      FIG. 12  is a temperature profile illustration for the thaw heater temperature setting for the device of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Fluid filled pipes are used in a wide variety of applications. In some applications the pipes do not include valves that allow the conduit to be segmented for maintenance or repair. Valves may not be included for a number of reasons. For example in the transmission of high voltage electrical power, the electrical power conductors  20  are housed within a pipe  22  that contains a dielectric fluid  24 . The dielectric fluid  24  isolates the power conductors  20  and also helps heat transfer that is generated by the power conductors  20  during operation. Since a valve would interfere with the power conductors  20 , fluid filled high voltage transmission lines operate contiguously over long distances with no mechanism for segmenting the sections of pipe. 
         [0027]    It should be appreciated that high voltage transmission lines may operate contiguously for many miles. To drain the entire line would be a lengthy and costly procedure. Further, service to customers along the entire length of the transmission line would be interrupted. Therefore, in the event the high voltage transmission line needs repair, maintenance or upgrading, the pipe  22  needs to be segmented to minimize the amount of dielectric fluid  24  that needs to be drained. While the embodiments herein discuss the application of a device for forming, maintaining and thawing a freeze plug with respect to high voltage transmission lines, this is for exemplary purposes and not intended to be limiting. It should be appreciated that the device discussed herein may be used in any application requiring a removable plug in a fluid filled pipe or conduit. 
         [0028]    Referring now to  FIGS. 2-6 , an exemplary device  26  for forming a plug in a fluid filled pipe will be described. In general, the device  26  consists of systems for freezing, thawing, providing a cryogenic supply, display/logger and various controllers, such as the freeze and thaw controller  54 X for example. In the case of high voltage electrical lines, the pipe  22  will be located below grade, such as under a street for example. The pipe  22  is accessed in a chamber or pit  27  is formed around the pipe  22 . It should be appreciated that other pipes  29  or utilities, such as electrical power lines, telecommunications line or water lines may also be located in the pit  27 . It should be appreciated that the operation of the freeze/thaw device  26  should not interfere with the operations of the other utilities. As will be made clearer herein, one of the advantages of the device  26  is the flexibility it provides in operating in these types of environments. Since repairs may be performed over an extended period of time, in most embodiments, a temporary shelter  31  is constructed over the pit  27  to protect the device  26  from the environment. 
         [0029]    The device  26  receives a cooling fluid from a source  28 . The source  28  may be a tank trailer for example. In the exemplary embodiment, the cooling fluid is a cryogenic fluid, such as liquid nitrogen for example. The cooling fluid leaves the source  28  through a tube  30 . The tube  30  may include a number of components to regulate the flow of the cooling fluid, such as a pressure regulator  32  and relief valve  34  for example. 
         [0030]    The tube  30  terminates at a manifold  36 . In the exemplary embodiment, the manifold  36  includes four segments  38 ,  40 ,  42 ,  44 . In the exemplary embodiment, the four segments  38 ,  40 ,  42 ,  44  are substantially identical. As will be discussed in more detail below, the segments  38 ,  40 ,  42 ,  44  deliver the cooling fluid to a portion of a freeze jacket that is coupled to the pipe  22 . Each segment bifurcates into a pair of tubes  46 ,  48 . The first tube  46  includes a bypass valve  50 . The second tube  48  includes a solenoid valve  52 . As will be discussed in more detail below, each of the solenoid valves  52  is electrically coupled to a freeze controller  54 . In the exemplary embodiment, the bypass valve  50  is a manually operated normally closed valve that allows the flow of cooling fluid through to the jacket in the event that the solenoid valve  52  does not operate properly. 
         [0031]    The pair of tubes  46 ,  48  connect back together and couple to a tube  56  that connects the pair of tubes  46 ,  48  to the freeze jackets  58 ,  60 . In the exemplary embodiment, the freeze jackets  58 ,  60  each include a first half  62  and a second half  64  as illustrated in  FIG. 3 . A hinge  66  rotatably couples the first half  62  and second half  64  to each other. The hinge  66  allows the freeze jackets  58 ,  60  to be easily installed and removed from the fluid filled power conductor pipe  22  without disturbing the pipe  22 . In one embodiment, one or more temperature sensors  61  are positioned between the inner diameter of the freeze jackets  58 ,  60  and the pipe  22 . Further, it should be appreciated that the freeze jackets  58 ,  60  may also be arranged in another form or shape that allows the cooling fluid to be introduced to substantially the entire circumference of the pipe  22 , such as in four segments for example, without deviating from the intended scope of the claimed invention. 
         [0032]    Each of the tubes  56  are fluidly coupled to one of the halves  62 ,  64  to allow the cooling fluid to flow into a chamber  68 ,  70  shown in  FIG. 3 . The chambers  68 ,  70  extend substantially along the length of the jackets  58 ,  60 . An exhaust tube  72  is coupled to the end of each chamber  68 ,  70 . In the exemplary embodiment, the jackets  58 ,  60  are made from a suitable material, such as steel, copper or aluminum, that has high thermal conductivity properties and is able to withstand thermal cycling between ambient and cryogenic temperatures. 
         [0033]    Disposed about the jackets  58 ,  60  are thaw heaters  74  as shown in  FIG. 3 . In the exemplary embodiment, the thaw heater  74  is a resistance heater capable of operating over a cryogenic temperature range of −200° C. (the temperature of liquid nitrogen in the jackets  58 ,  60 ) to 100° C. (maximum heater surface temperature). The thaw heater  74  includes an element  76  that generates thermal energy in response to the application of an electrical current flowing through the element  76 . In the exemplary embodiment, the thaw heater  74  has a power rating of 1.1 kW to 1.8 kW and has a surface area of 8.75 ft 2  for a 10-inch (0.254 meters) diameter pipe  22 . The thaw heater  74  may also include a temperature sensor or a thermal cutout switch  78  arranged to monitor the temperature of the element  76 . As will be discussed in more detail below, the cutout switch  78  is coupled to the thaw controller  54 T to allow a modulated operation of the thaw heater  74  under a wide variety of temperatures. In the exemplary embodiment, the thaw heater  74  maintains a temperature of the outer wall of the pipe  22  in the range of 0° C. to 100° C. during the thaw mode of operation. In another embodiment, the thaw heater  74  is an induction type heater. In one embodiment, the device  26  includes a single controller  54 T that combine the functions of the freeze controller  54  and the thaw controller  54 T. In one embodiment, the thaw controller  54 T change modes of operation from a freeze mode to a thaw mode through a manual actuation by the operator. 
         [0034]    In some embodiments, such as the one illustrated in  FIG. 5  for example, there may be a freeze controller  54  associated with each exhaust conduit  72 , for a total of four freeze controllers  54 . Further, there may be two thaw heaters  54 T each with an associated thaw controller  54 T for a total of two thaw controllers  54 T in device  26 . 
         [0035]    Disposed about the thaw heater  74  is an insulation layer  80 . In the exemplary embodiment, the insulation layer  80  is a flexible closed-cell foam, such as Armaflex™ manufactured by Armacell LLC for example. The insulation layer  80  may be made from any suitable material having an operating range on the order of −185° C. to +177° C. A second, or freeze prevention heater  82  is disposed about the insulation layer  80 . The freeze prevention heater  82  is a resistance heater having a heater element (not shown) that generates thermal energy in response to an electrical current. As will be discussed in more detail below, the freeze prevention heater  82  minimizes the buildup of ice due to the freezing of condensed water vapor. In one embodiment, the freeze prevention heater  82  is coupled to the thaw controller  54 T. In another embodiment, the freeze prevention heater  82  includes an integrated control circuit that maintains the freeze prevention heater  82  at a constant temperature. In the exemplary embodiment, the freeze prevention heater  82  has a power rating of 0.5 kW to 0.66 kW and a surface area of 9.5 ft 2  for a 10-inch (0.254 meter) diameter pipe  22 . 
         [0036]    In one embodiment, a copper mesh  86  is wrapped around the pipe  22  between the jacket  58 ,  60  and the pipe  22 . The copper mesh  86  assists in improving the conductive heat transfer between the jacket  58 ,  60  and the pipe  22 . 
         [0037]    In one embodiment, a set of temperature sensors  61 , such as a thermocouple for example, are coupled to the pipe  22  as illustrated in  FIG. 4 . In the exemplary embodiment, these temperature sensors  61  are disposed on the outer surface of the pipe  22 , with the temperature sensors  61 A,  61 C being located adjacent the ends of the freeze jackets  58 ,  60  and the temperature sensor  61 B being located between the freeze jackets  58 ,  60 . As will be discussed in more detail below, temperature sensors  61  are coupled to a display  59  to provide repair personnel with an indicator on the freeze condition of the pipe  22 . 
         [0038]    An additional set of temperature sensors  88  is thermally coupled to measure the temperature of the fluid in the exhaust tubes  72 . In the exemplary embodiment, the temperature sensors  88 A,  88 B,  88 C,  88 D are mounted to the outer surface of each of the exhaust tubes  72 . The fluid may be a liquid, however, in the exemplary embodiment, the fluid in the exhaust tubes  72  is a superheated cryogenic gas. In other embodiments, the temperature sensor  88  is mounted within a thermal well (not shown) arranged in the exhaust tubes  72 . In yet other embodiments, the exhaust tubes  72  include an opening and the temperature sensor  88  is positioned within the opening to directly measure the fluid temperature. Each of the temperature sensors  88  are electrically coupled to a data transmission media  89  as shown in  FIG. 6 . 
         [0039]    The freeze controller  54  is electrically coupled to the temperature sensors  88 , and the solenoid valves  52 . The thaw controller  54 T is coupled with the thaw heater  74  and temperatures sensors  61  and  78 . The device  26  operation is controlled by controllers  54 ,  54 T. Controllers  54 ,  54 T are suitable electronic devices capable of accepting data and instructions, executing the instructions to process the data, and presenting the results. Controllers  54 ,  54 T may accept instructions through user interface, or through other means such as but not limited to electronic data card, voice activation means, manually operable selection and control means (e.g. buttons and switches), radiated wavelength and electronic or electrical transfer. Therefore, controllers  54 ,  54 T can be, but is not limited to, a microprocessor, microcomputer, a minicomputer, an optical computer, a board computer, a complex instruction set computer, an ASIC (application specific integrated circuit), a reduced instruction set computer, a programmable logic controller (PLC), an analog computer, a digital computer, a solid-state computer, a single-board computer, a computer network, a desktop computer, a laptop computer, or a hybrid of any of the foregoing. 
         [0040]    Controllers  54 ,  54 T are capable of converting the analog voltage or current level provided by sensors, such as temperature sensor  61  for example, into a digital signal indicative of the temperature of the outer pipe surface pipe  22 . Alternatively, the sensors  61 ,  88  and  78  may be configured to provide a digital signal to controllers  54 ,  54 T, or an analog-to-digital (A/D) converter (not shown) may be coupled between sensors  61 ,  88  and  78  and controllers  54 ,  54 T to convert the analog signal provided by sensors  61 ,  88 , and  78  into a digital signal for processing by controller  54 ,  54 T. Controllers  54 ,  54 T use the digital signals act as input to various processes for controlling the device  26 . The digital signals represent one or more system data including but not limited to pipe surface temperature (T pipe ), heater element temperature (T element ), gas exhaust temperature (T exhaust ), solenoid valve  52  operating state and the like. It should be appreciated that the freeze controller  54  and the thaw controller  54 T do not operate simultaneously. In one embodiment, the thaw controller  54 T is activated by the operator once the flow of the cryogenic fluid is stopped. 
         [0041]    Controllers  54 ,  54 T are operably coupled with one or more components of device  26  by data transmission media  89 . Data transmission media  89  includes, but is not limited to, twisted pair wiring, coaxial cable, and fiber optic cable. Data transmission media  89  also includes, but is not limited to, wireless, radio and infrared signal transmission systems. In the embodiment shown in  FIGS. 2 ,  3  and  5 , transmission media  89  couples freeze controller  54  to solenoid valve  52  and temperature sensor  88  and the display unit  59 . In the embodiment shown in  FIGS. 2-5 , the transmission media  89  couples the thaw controller  54 T to the thaw heater  74 , and the temperature sensor  78 . Controllers  54 ,  54 T are configured to provide operating signals to these components and to receive data from these components via data transmission media  89 . 
         [0042]    In general, freeze controller  54  accepts data from sensors  88  and is given certain instructions for the purpose of comparing the data from sensors  88  to predetermined operational parameters. Controller  54 T accepts the data from sensors  78  ( FIG. 3 ). Controller  54  provides operating signals to solenoid valve  52 . Controller  54  may also accept data from other components, indicating, for example, the pressure of the cooling fluid, or the volume of cooling fluid remaining. The freeze controller  54  compares the operational parameters to predetermined variances (e.g. low flow rate, low pressure, supply level) and if the predetermined variance is exceeded, generates a signal that may be used to indicate an alarm to an operator or a connected computer network (not shown). Additionally, the signal may initiate other control methods that adapt the operation of the device  26 , for example the thaw heater  54 T may change the operational state of thaw heater  74  to compensate for the out of variance operating parameter. For example, if sensor  78  detects a heater element  76  temperature above a predetermined threshold, the thaw controller  54 T may modulate or turn off the thaw heater  74  to maintain a desired operating temperature. As will be described in more detail below, to prevent damage to the thaw heater  74  over a temperature range from −110 C to + 100  C. The thaw controller  54 T may initiate a change in operation state signal to thaw heater  74  resulting in the thaw heater  74  being cycled on and off during the process of thawing the pipe  22 . Another example includes a level of superheat in the exhaust gas that is below a desired level for maximizing the absorption of heat from the pipe  22  while minimizing cooling fluid usage. The controller  54  may initiate a change in state to the valve  52 , changing the flow rate of cooling fluid into the chambers  68 . 
         [0043]    The data received from sensors  61 ,  78 ,  88  may be displayed on a user interface coupled to controllers  54 ,  54 T. The user interface may be an LED (light-emitting diode) display, an LCD (liquid-crystal diode) display, a CRT (cathode ray tube) display, or the like. A keypad may also be coupled to the user interface for providing data input to controllers  54 ,  54 T. 
         [0044]    In addition to being coupled to one or more components within device  26 , freeze controller  54  may also be coupled to external computer networks such as a local area network (LAN) and the Internet. The LAN interconnects one or more remote computers, which are configured to communicate with controller  54  using a well-known computer communications protocol such as TCP/IP (Transmission Control Protocol/Internet(̂) Protocol), RS-232, ModBus, and the like. 
         [0045]    Controllers  54 ,  54 T may include a processor coupled to a random access memory (RAM) device, a non-volatile memory (NVM) device, a read-only memory (ROM) device, one or more input/output (I/O) controllers, and a LAN interface device. I/O controllers are coupled to sensors  61 ,  78 ,  88 , valves  52 , and alternatively to a user interface for providing digital data between these devices and bus. I/O controllers may also be coupled to analog-to-digital (A/D) converters, which receive analog data signals from sensors  61 ,  78 ,  88 . 
         [0046]    A NVM device is any form of non-volatile memory such as an EPROM (Erasable Programmable Read Only Memory) chip, a disk drive, or the like. Stored in NVM device are various operational parameters for the application code, such as the methods illustrated in  FIGS. 5-8  for example. The various operational parameters can be input to the NVM device either locally, using a keypad or remote computer, or remotely via the Internet using a remote computer. It will be recognized that application code can be stored in a NVM device rather than a ROM device. 
         [0047]    Controllers  54  and  54 T include operation control methods, such as the methods shown in  FIGS. 5-8  for example, embodied in application code. These methods are embodied in computer instructions written to be executed by processor, typically in the form of software. The software can be encoded in any language, including, but not limited to, assembly language, VHDL (Verilog Hardware Description Language), VHSIC HDL (Very High Speed IC Hardware Description Language), Fortran (formula translation), C, C++, Visual C++, Java, ALGOL (algorithmic language), BASIC (beginners all-purpose symbolic instruction code), Perl, ladder logic, visual BASIC, ActiveX, HTML (HyperText Markup Language), and any combination or derivative of at least one of the foregoing. Additionally, an operator can use an existing software application such as a spreadsheet or database and correlate various cells with the variables enumerated in the algorithms. Furthermore, the software can be independent of other software or dependent upon other software, such as in the form of integrated software. 
         [0048]    An exemplary Display/Logger  59  is illustrated in  FIG. 5 . In this embodiment, the device has a display  59  for displaying temperature data from the temperature sensors  61 ,  78  or  88 . The controller  54 T also includes a first thaw controller  63 , a second thaw controller  65 , and four freeze controllers  67 A,  67 B,  67 C,  67 D. The first thaw controller  63  and the freeze controllers  67 A,  67 B are electrically coupled to control the operation of freeze jacket  58 . Similarly, the second thaw controller  65  and freeze controllers  67 C,  67 D are electrically coupled to control the operation of freeze jacket  60 . 
         [0049]    A pair of data terminals  55 ,  57  are provided to interface the controllers  63 ,  65 ,  67 A,  67 B,  67 C,  67 D with the temperature sensors  61 ,  88 . In one embodiment, first data terminal  55  receives data from the temperature sensors  61 A,  61 B,  61 C. In one embodiment, the data terminals  55 ,  57  are incorporated into and integral with the data logger/display  59 . Similarly, the second data terminal  57  receives data from temperature sensors  88 A,  88 B,  88 C and  88 D. As discussed above, the temperature data from each of the temperature sensors  61 ,  88  is provided to the repair personnel through display/logger  59 . In one embodiment, the thermocouple wire from the temperature sensors  88  is divided at terminal  57  as shown in  FIG. 6 . In this embodiment, the conductor  71  from the temperature sensor  88  is split into two conductors  73 A,  73 B after the terminal  57 . One of the conductors  73 A couples the temperature sensor  88  to the display  59 , while the other conductor  73 B provides a connection to a freeze controller, such as freeze controller  67  for example. In the exemplary embodiment, the dividing of the conductor  71  is accomplished by soldering a second wire onto a thermocouple wire. 
         [0050]    During operation, the device  26  may be considered to operate in four modes, a freeze mode  90 , a maintenance mode  92 , warm up mode  93 , and a thaw mode  94  as shown in  FIG. 7 . These modes of operation provide for different temperatures during operation as shown in  FIG. 8 . As will be discussed in more detail below, by varying the temperature profile, the device  26  provides advantages in shortening the time to form and thaw a freeze plug at reduced liquid nitrogen flow. 
         [0051]      FIG. 9  illustrates an exemplary embodiment of a method  95  of operating the device  26 , when the device  26  is operating in modes  90 ,  92 ,  93  and  94 . The method  95  starts in freeze mode  90 . In this mode, the freeze prevention heater  82  is activated in block  96  in parallel with the flow of cooling fluid, such as liquid nitrogen for example, in block  98  into the freeze jackets  58 ,  60 . It should be appreciated that when utilizing cold cooling fluids, such as a cryogenic cooling fluid for example, water will tend to condense on and about the jackets  58 ,  60 . This condensed water also then freezes forming a mass of ice on the large outer surfaces of the jackets. In prior art systems, the mass of ice was very large resulting in extended thaw times as the operators should not chip away at the ice using tools but use thaw requires propane type heaters. It should be appreciated that this was undesirable since it resulted in extended periods (up to 24 hours) before the pipe  22  could be returned to service and there was a potential for damage from the tools. In this embodiment, by operating the freeze prevention heater  82  at a constant temperature, such as 12.8° C. for example, only a minimal amount of ice forms having little or no impact of the thaw time. 
         [0052]    The cooling fluid flows within each of the chambers  68 ,  70  absorbing heat from the fluid  24  through the pipe  22 . In the exemplary embodiment, the cooling fluid boils in the chambers  68 ,  70 . This allows the cooling fluid to exhaust through the exhaust tube  72  as a superheated vapor. The solenoid valves  52  are modulated by freeze controller  54  in block  100  to maintain a desired level of superheat by controlling the exhaust temperature T exhaust  measured by the sensor  88 . In the embodiment using liquid nitrogen as a cooling fluid, the boiling temperature of the liquid nitrogen is −196.3° C. and the method  95  maintains T exhaust  at −175° C. This allows the freeze jackets  58 ,  60  to remove heat from the pipe  22  through conduction, radiation, convection and the significant boiling enthalpy of approximately 85 Btu/lb. The method  95  loops between the modulating of valve  52  in block  100  and the flowing of cooling fluid in block  98  until the freeze plug is formed. 
         [0053]    This method of using superheated fluid provides advantages over the prior art systems that simply filled the chambers  68 ,  70  with liquid nitrogen and allowed the liquid nitrogen to flow out the exhaust tube  72 . The generation of a superheated gas allowed the generation of a frozen oil plug, within a high voltage transmission pipe  22 , in 6-10 hours for an 8-inch conduit and 8-12 hours for a 10-inch pipe. Further, the freeze plug was created and maintained while simultaneously reducing the amount of liquid nitrogen by 30%-45%. This provides the advantage of reducing costs in both material and labor. The variance in the freeze time depends on a number of factors including environmental conditions. An exemplary example of parameters for freeze times in an arbitrary example included in Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Dielectric 
                   
                 N2 
                   
                   
               
               
                   
                 Fluid 
                 Pipe O.D. 
                 exhaust 
                   
                 Total Freeze 
               
               
                 Pipe Size 
                 Type 
                 Temp 
                 temp. 
                 Soak Time 
                 Time 
               
               
                   
               
             
             
               
                 10 inch 
                 DCL100 
                 −160 C. 
                 −175 C. 
                 4 to 8 
                 8 to 12 
               
               
                   
                   
                   
                   
                 hours 
                 hours 
               
               
                   
               
             
          
         
       
     
         [0054]    The method  95  remains in freeze mode  90  until a freeze plug is formed from the fluid  24  within the pipe  22  that is sufficiently solid enough and long enough to withstand the pressure of the fluid  24  upstream from the area that maintenance or other tasks need to be performed. In one embodiment, the upstream pressure ranges from 80-150 pounds per square inch (551.6 kPa-1723.8 kPa). In this embodiment, it is desirable to have a freeze plug with a length of 72 inches (1.8 meters) to 91 inches (2.31 meters) to provide a freeze plug with a desired factor of safety. Once a freeze plug of the desired size is formed, the method  95  transfers to maintenance mode  92 . 
         [0055]    Once the freeze plug is formed and pressure tested by the operators, the flow rate of the cooling fluid may be reduced to maintain the pipe  22  at a desired temperature above the freezing point of the fluid  24 . In the exemplary embodiment, the fluid  24  is a dielectric fluid having a freezing point of between −50° C. (DCL-500 type fluid) to −75° C. (DCL-100 type fluid). In this embodiment, the set point for T exhaust  is increased in block  102  such that T exhaust  may be set to vary, between −120° C. to −145° C. To achieve this, the method  95  monitors T pipe  and T exhaust  in block  104  and modulates the valve  52  in block  106  to maintain the desired temperatures. The method  95  continues to loop between block  104  and block  106  until a predetermined amount of time before the freeze plug is to be thawed. In the embodiment of a high voltage transmission line, the freeze plug may be in place for several weeks while repairs, maintenance or upgrade tasks are performed. A predetermined amount of time prior to the completion of the tasks, the method  95  enters a warm up mode  93  operations. In block  108 , the set point for T exhaust  is changed, to gradually increase T pipe , to between −85° C. to −115° C. In the exemplary embodiment, the predetermined amount of time is 24 hours. An exemplary example of parameters for maintaining the freeze plug in an arbitrary example included in Table 2. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Min. N2 
                   
                   
               
               
                 Pipe 
                 Dielectric 
                 Exhaust 
                 Est. Centerline 
                 N2 Exhaust 
               
               
                 Size 
                 Fluid Type 
                 Temp. 
                 Temperature 
                 Temperatures 
               
               
                   
               
             
             
               
                 10 inch 
                 DCL100 
                 −145 C. 
                 −100 C. 
                 Variable &lt; −125 C. 
               
               
                   
                   
                   
                 to −120 C. 
               
               
                  8 inch 
                 DCL150 
                 −120 C. 
                 −80 C. 
                 Variable &lt; −100 C. 
               
               
                   
                   
                   
                 to −95 C. 
               
               
                   
               
             
          
         
       
     
         [0056]    Once the tasks associated with pipe  22  have been completed, the method  95  shifts from warm up mode  93  to thaw mode  94 . In thaw mode  94 , the method  95  first stops the flow of the cooling fluid in block  110 . The thaw heater controller  54 T is activated to turn on the thaw heater  74  in block  112 . The method  95  monitors the temperature of the heater element  76  in the thaw heater  74  to maintain a temperature below a desired maximum temperature, such as 80° C. for example. The method  95  modulates the thaw heater  74  in block  114  and loops back to block  112  until the freeze plug is thawed. It should be appreciated thaw heater  74  operates over a wide operating environment from −210° C. to 100° C. By maintaining the heater element  76  temperature and modulating the operation of the thaw heater through the thaw heater controller  54 T advantages may be gained, in that the reliability and useful life of the thaw heater  74  are extended without significantly impacting the time needed to thaw the freeze plug. In one embodiment, the pressure in the pipe  22  on either side of the device  26  is monitored, such as by pressure transducers  91  (FIG.  4 ) for example. It should be appreciated that the pressure transducers  91  may be positioned at a distance upstream and downstream from the device  96  Once the pressure on either side of the device  26  has equalized, the freeze plug is considered “thawed” for operational purposes and the method  95  exits the thaw mode  94 . An exemplary example of parameters for thawing the freeze plug in an arbitrary example included in Table 3. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Dielectric 
                 N2 Exhaust Temp 
                 Pipe O.D. 
                   
                 Thaw time if only 
               
               
                 Pipe 
                 Fluid 
                 24 hours before 
                 Temp at end 
                 Thaw time 
                 natural convention 
               
               
                 Size 
                 Type 
                 thaw 
                 of Thaw 
                 with heaters 
                 is used 
               
               
                   
               
             
             
               
                 10 
                 DCL100 
                 −100 C. 
                 −50 C. 
                 2.5 hours 
                 8 to 10 
               
               
                   
                   
                   
                   
                   
                 hours 
               
               
                  8 
                 DCL150 
                  −95 C. 
                 −20 C. 
                 1.5 hours 
                 5 to 8 
               
               
                   
                   
                   
                   
                   
                 hours 
               
               
                   
               
             
          
         
       
     
         [0057]    Referring now to  FIG. 10 , another method  116  of operating the device  26  is illustrated. The method  116  starts in block  118  and initiates freeze mode  90 . The freeze prevention heater is turned on in block  120  and its temperature is set for 12.8° C. As discussed above, this prevents or minimizes the build up of water condensate forming ice on the jackets  58 ,  60 . In parallel with the activation of the freeze prevention heater, the method  116  initiates the flow of cooling fluid in block  122 . The exhaust temperature T exhaust  is measured in block  124 . It should be appreciated that the exhaust temperature T exhaust  may be continuously monitored during freeze mode  90 . 
         [0058]    The method  116  then proceeds to query block  126  where it is determined if the freeze plug has been formed. This query may be determined in a number of ways, such as by the temperature measured by sensor  61 B or the exhaust temperature being at a desired level for a particular period of time, sometimes referred to as a “soak time”. In the exemplary embodiment, the determination of whether the freeze plug has been formed includes the steps of determining whether the predetermined soak time has been reached in block  127 . In the exemplary embodiment, the soak time for a 10″ pipe is approximately 6 hours. If block  127  returns a positive, the operator performs a pressure test in block  129  where a predetermined pressure differential is placed across the freeze plug. If the pressure differential is maintained for a predetermined amount of time, the query block  129  returns a positive. If either query block  127  or query block  129  return a negative, the method  116  proceeds to block  128 . In another embodiment, the presence of the freeze plug is determined by using at least two temperature sensors  61 A,  61 C at each end of the freeze jackets  58 ,  60 . In this embodiment, when the temperature at the ends of the freeze jackets  58 ,  60  reaches a desired level, the freeze plug has been formed. 
         [0059]    If the query block  126  (or query blocks  127 ,  129 ) returns a negative, the method  116  proceeds to query block  128  where it is determined whether T exhaust  is substantially equal to −175° C. It should be appreciated that the temperature T exhaust  is set depending on the size of the pipe  22 . Where the pipe  22  is 10-inches (0.254 meters) in diameter, T exhaust  is set to −175° C., in the case of an 8-inch (0.2032 meters) diameter, T exhaust  is set to −135° C. If query block  128  returns a positive, the method  116  loops back to block  124 . If query block  128  returns a negative, the method  116  proceeds to block  130  where the flow of the cooling fluid is adjusted, such as with solenoid valve  52  for example, to achieve the desired T exhaust . It should be appreciated that the modulation of the flow of cooling fluid in block  130  may be an on-off modulation, or a continuous variation of the flow rate over the operational range of the valve. It should further be appreciated that the controlling of the exhaust temperature may be arranged with hysteresis to avoid continuously cycling the solenoid valve  52 . 
         [0060]    If the query block  126  returns a positive, the method  116  proceeds to block  132  and the maintenance mode  92  is initiated. Block  132  adjusts the flow of the cooling fluid allowing T exhaust  to vary at a temperature at less than −145° C. The temperature T exhaust  will depend on the diameter of the pipe  22 . In one embodiment, the pipe  22  has a 10-inch (0.25 meter) diameter and the desired T exhaust  is −145° C. In another embodiment, the pipe  22  has an 8-inch (0.2 meter) diameter and the desired T exhaust  is −120° C. Block  132  allows the device  26  to keep the freeze plug in place with a sufficient safety factor while also minimizing the amount of cooling fluid consumed. It should be appreciated that the temperature T exhaust  is continuously monitored during maintenance mode  92 . 
         [0061]    With the freeze plug in place, the operator may then drain the fluid  24  from the segmented section of the pipe  22  downstream from the freeze plug and perform maintenance, repair or upgrade tasks as shown in block  134 . In one embodiment, two devices  26  are installed on the pipe  22  on either side of the section where tasks need to be performed. In this embodiment, two freeze plugs are formed creating an isolated segment. In this embodiment, only the fluid  24  in the isolated segment needs to be drained to allow tasks to be performed. This is advantageous in applications, such as high voltage transmission lines, where the pipe  22  may extend for many miles and only a small section needs attention. 
         [0062]    A predetermined amount of time before the freeze plug is to be thawed, the method  116  proceeds to block  136  where the cooling fluid flow rate is modulated to maintain the freeze plug in pipe  22 . The method  116  then proceeds to query block  137  where it is determined if the repairs are almost complete. In the exemplary embodiment, the query block  137  returns a positive when there is 24 hours prior to the end of repairs. If query block  137  returns a negative, the method  116  loops back to block  134  where repairs continue and the cooling fluid is modulated. 
         [0063]    Once query block  137  returns a positive, the method  116  exits maintenance mode  92  and enters warm up mode  93 . In block  139 , the temperature of the pipe  22  is gradually increased over the predetermined time period by modulating the cooling fluid flow rate such that the temperature T exhaust  is at the desired temperature, such as −110° C. The warm up temperature that T exhaust  is set will depend on the application. In the exemplary embodiment, the warm up temperature of −110° C. is sufficient to allow a refreezing if repair operations take longer than expected. 
         [0064]    When the operator is ready to thaw the freeze plug, the method  116  proceeds to thaw mode  94  in block  138 . In block  138 , the flow of the cooling fluid to the jackets  58 ,  60  is stopped. The method  116  then proceeds to block  140  where the thaw heater  74  is turned on. The method  116  then proceeds to query block  141  where it is determined if the thermal cutout, such as switch  78  for example has been activated. If query block  141  returns a positive, meaning that the thermal cutout was activated and the thaw heater  74  is operating at or above a desired temperature. The method  116  then proceeds to block  143  where the thaw heater  74  is deactivated until the temperature of the thaw heater  74  is less than the desired maximum temperature. If the query block  141  returns a negative, or once the thaw heater  74  has cooled, the method  116  proceeds to block  142  to determine if the pressure in pipe  22  has equalized on either side of the device  26 . 
         [0065]    If query block  142  returns a negative, the method  116  loops back to block  140  and continues to thaw the pipe  22  with the thaw heater  74 . If the query block  142  returns a positive, then the fluid pressure is the same on either side of the device  26  which indicates that the freeze plug is thawed for operational purposes. It should be appreciated that some fluids, such as dielectric fluid under go a phase change and some portions of the fluid may remain in the solid state or frozen. However, once there is sufficient phase change into a liquid state to allow the pressure to equalize, the method  116  proceeds to block  144  and terminates. 
         [0066]    The method  116  provides advantages in reducing the amount of time it takes to thaw the freeze plug. Typically in prior art solutions, the time to thaw a freeze plug was between 8 to 24 hours. Using the method  116 , this time to thaw was reduced to 2 hours, which reduces costs in labor and returns the pipe  22  to service in less time. 
         [0067]    A method  146  of thawing a frozen pipe  22  is illustrated in  FIG. 11 . It should be appreciated that the method  146  may be used either alone, or as part of thaw mode  94  with methods  95 ,  116 . Method  146  starts by initiating a timer in block  150 . The method  146  then proceeds to activate or turning on the thaw heater  74  in block  148 . The method then proceeds to block  149  where the thaw heater temperature setting T setting  is set to a desired initial set point. In the exemplary embodiment, the initial set point is T setting =20° C. The method  146  then proceeds to block  151  where the pipe  22  temperature T pipe  is measured and then to block  152  where the temperature T element  of the thaw heater element  76  is measured. 
         [0068]    The method  146  then proceeds to query block  154  where the thaw heater temperature is compared to a threshold temperature. In the exemplary embodiment, the threshold temperature is 80° C. If query block  154  returns a positive, the method  146  proceeds to block  156  where the thaw heater  74  is either modulated (e.g. electrical current is reduced), or cycled off. By maintaining the temperature T element  below the threshold temperature, the reliability and useful life of the thaw heater  74  is increased. Further, it was found that the cycling of the thaw heater  74  to maintain the temperature T element  below the threshold temperature did not have a significant impact on the amount of time it took to melt the freeze plug. 
         [0069]    If the query block  154  returns a negative, the method  146  proceeds to query block  158  where it is determined if the timer (set in block  150 ) has expired. In the exemplary embodiment, the initial timer setting is for 30 minutes. If query block  158  returns a positive, the method  146  proceeds to block  160  where the temperature T setting  is incremented. In the exemplary embodiment, the temperature T setting  is set to 20° C. for 30 minutes, then incremented by an additional +10° C. for two consecutive a thirty minute time periods. After the initial three time periods, the timer is set to a 15 minute time period and the +10° C. incremental increases continue until the T setting  reaches 80° C. or the freeze plug is thawed as illustrated in  FIG. 12 . The timer is reset in block  162 . 
         [0070]    If the query block  158  returns a negative, or once the timer has been reset in block  162 , the method  146  proceeds to query block  164  where it is determined if the pressure has equalized on either side of the device  26 . Query block  164  determines if the freeze plug has been effectively thawed or melted to allow operations to continue. If the query block  164  returns a negative, the method  146  loops back to block  151 . If the query block  164  returns a positive, indicating that the freeze plug has been melted a sufficient amount to continue operations, the method  146  proceeds to block  166 . In block  166 , the method  146  turns off the thaw heater  74 . 
         [0071]    The device  26  and the methods of operation provide a number of advantages over the prior art. The device  26  and methods of operation generate the freeze plug in a shorter period of time while using 30% to 50% less cooling fluid. The device  26  and methods of operation provide further advantages in the thawing of the freeze plug reducing the time for thawing from 8 to 24 hours to 2 hours. The use of two freeze jackets in the device  26  also allows for redundancy in the event one jacket is damaged or malfunctions during operation. 
         [0072]    It should be appreciated that while the embodiments have been described herein with respect to the forming of a freeze plug in high voltage electrical power pipes, the claimed invention should not be so limited. Other applications in which these embodiments may be used include, but are not limited to: boric acid piping use in nuclear power, oil pipelines, hydraulic lines, fruit juices pipes and in the fuel lines and deicing of airplane wings for example. 
         [0073]    An embodiment of the invention may be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. The present invention may also be embodied in the form of a computer program product having computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, USB (universal serial bus) drives, or any other computer readable storage medium, such as random access memory (RAM), read only memory (ROM), or erasable programmable read only memory (EPROM), for example, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. The present invention may also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. A technical effect of the executable instructions is to manage the formation and thawing of a freeze plug in a conduit. 
         [0074]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.