Patent Application
20180372258
The field of the invention is high voltage heating systems. More particularly, the invention relates to designs for high voltage skin effect heat tracing systems.
In the oil and gas industry, pipelines are generally heated over distances of many miles. Skin effect electric heat tracing systems may be suited for long transfer pipelines up to 12 miles (20 km) or 15 miles (25 km) per circuit. These skin effect heat tracing systems are generally engineered for a specific application, such as for material transfer lines, snow melting and de-icing, tank foundation heating, subsea transfer lines and prefabricated, pre-insulated lines. In a skin effect heat tracing system, heat is generated on the inner surface of a ferromagnetic heat tube that is thermally coupled to the pipe to be heat traced. An electrically insulated, temperature-resistant conductor is installed inside the heat tube and connected to the tube at a far end thereof. An alternating current (AC) is passed through the insulated conductor and returns through the heat tube.
At voltages above about 5 kV, the ferromagnetic heat tube of a skin effect heat tracing system is prone to the corona effect: a localized discharge resulting from transient gaseous ionization that occurs as a charge difference builds up between the surface of the tube and the surface of the insulated conductor inside the tube. Specifically, this localized discharge occurs when the charge difference exceeds the breakdown electric field for the gas disposed between the heat tube and the insulated conductor (about 3 V/μm for air). The corona effect can become a significant issue for longer pipelines having heat tracing systems that require a higher voltage potential to drive current because the higher voltage potential results in greater charge build-up between the heat tube of the skin effect tracing system and the insulated conductor surrounded by the heat tube. The accumulated static electricity can damage or prematurely age the insulation and other components of the heat tracing system and can sometimes result in electric arcing as accumulated static electricity discharges between the conductor and the heat tube.
It may be desirable to heat long pipelines (for example, on the order of 36 miles) and to handle voltages larger than 5 kV and up to 10 kV or higher. Thus, it may be desirable to use a system or method to reduce or eliminate the risk of partial discharge in heating systems.
The present invention overcomes the aforementioned drawbacks by providing a pressurized skin effect heat tracing system capable of withstanding continuous applied voltages above at least 5 kV with reduced corona effect.
In one aspect, the present invention provides a skin effect heat tracing system including a heat tube and a heater cable including a core conductor and an electrical insulation layer surrounding the core conductor. The heater cable may be sized to lie within the heat tube so that gas-filled space is defined between an outer surface of the electrical insulation layer and an inner surface of the heat tube. The gas-filled space may be pressurized above an external gas pressure outside the heat tube.
In some embodiments, the skin effect heat tracing system may include a sealed heater circuit that is substantially airtight and that comprises the heat tube and the heater cable. The sealed heater circuit may include a pull box, a front power connection box, an end termination box, and/or a splice box.
In some embodiments, the heater cable may be a round, unshielded heater cable, a round, shielded heater cable, a ribbed, unshielded heater cable, or a ribbed, shielded heater cable.
In some embodiments, the gas-filled space may be pressurized to between 20 and 30 pounds per square inch absolute.
In another aspect, the present invention provides a system that includes a carrier pipe, a skin effect heat tracing system disposed at a surface of the carrier pipe, and a gas source coupled to at least one component of the skin effect heat tracing system. The gas source may be configured to supply pressurized gas to the at least one component.
In some embodiments, the skin effect heat tracing system of the system may include a heater circuit in contact with the surface of the carrier pipe. The at least one component may include the heater circuit. The heater circuit may include a heat tube and a heater cable. The heater cable may include a core conductor and an electrical insulation layer surrounding the core conductor. The heater cable may be sized to lie within the heat tube so that gas-filled space is defined between an outer surface of the electrical insulation layer and an inner surface of the heat tube, the gas-filled space may be pressurized by the gas source above an external gas pressure outside the heat tube.
In some embodiments, the skin effect heat tracing system may further include an end termination box at an end of the carrier pipe and a pull box coupled to the end termination box via the heater circuit. The at least one component may further include at least one of the pull box and the end termination box. The heater circuit, the end termination box, and the pull box may be sealed so as to be substantially airtight.
In some embodiments, the system may further include a leak detection device coupled to the skin effect heat tracing system. The leak detection device may be configured to determine a leak rate of the at least one component.
In some embodiments, the system may further include a controller coupled to the skin effect heat tracing system. The controller may be configured to monitor an internal pressure of the at least one component and to instruct the gas source to supply the at least one component with additional pressurized gas in response to detecting that the internal pressure is less than a predefined pressure threshold.
In some embodiments, the pressurized gas may be air, nitrogen, argon, or sulfur hexafluoride.
In another aspect, the present invention provides a method of operating a skin effect heat tracing system. The method includes supplying, by a gas source, pressurized gas to one or more components of the skin effect heat tracing system to increase an internal gas pressure of the one or more components to be higher than an external gas pressure outside the one or more components.
In some embodiments, the method may further include, before supplying the pressurized gas, performing, with a leak detection device, a leak check on the one or more components to determine a leak rate.
In some embodiments, the method may further include determining, by the leak detection device, that the leak rate exceeds a predefined threshold, and sending, with the leak detection device, an alert to a controller coupled to the skin effect heat tracing system, the alert requesting maintenance of the skin effect heat tracing system in order to repair leaks.
In some embodiments, the method may further include asserting, by the leak detection device and in response to determining that the leak rate exceeds the predefined threshold, a flag indicative of an end condition, the flag instructing the controller to cease operation of the skin effect heat tracing system.
In some embodiments, the pressurized gas may be supplied by the gas source until the internal pressure of the one or more components is at a defined pressure level of between 20 to 30 pounds per square inch absolute.
In some embodiments, the method may further include determining, by a controller, that the internal gas pressure of the one or more components is less than a predefined pressure threshold, and instructing, by the controller, the gas source to supply additional pressurized gas to the one or more components.
In some embodiments, the one or more components may include a heater circuit having a heater cable disposed within a heat tube, a pull box, a front power connection box, an end termination box, and/or a splice box.
In some embodiments, the pressurized gas may include air, nitrogen, argon, or sulfur hexafluoride.
The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.
Generally, skin effect heat tracing systems include a heat tube with an internal heater cable.
The heater cable 10 can be surrounded by gas 18 (sometimes referred to herein as “gas-filled space” 18) except at a point 20, at which the heater cable 10 is in contact with an inner surface 22 of the heat tube 12. For example, the gas 18 may be air, nitrogen, argon, sulfur hexafluoride (SF6), or any other applicable gas that may be introduced to the space between the heater cable 10 and the heat tube 12. The positioning of the heater cable 10 within the heat tube 12 produces non-uniform electrical fields when electrical power is applied to the heater cable 10, with the highest electric field being located where the heater cable 10 contacts the heat tube 12 (i.e., at point 20). The insulation layer 16 may be, for example, made from polyethylene, perfluoroalkoxy resin (PFA), ethylene propylene rubber (EPR), ethylene propylene diene monomer (EPDM) rubber, or silicon rubber. Electric charge accumulates on the outer surface of the insulation layer 16 during operation of the skin effect heat tracing system. If the voltage between the heater cable 10 and the heat tube 12 exceeds the breakdown voltage for the gas in the gas-filled space 18, the accumulated electric charge discharges as corona (that is, partial discharge, or PD). More specifically, PD occurs primarily at the point 20 due to the charge differential between the outer surface of the insulation 16 and the inner surface 22 of the heat tube 12 (which may be electrically grounded), and further due to the relatively close proximity of these two surfaces. Protracted PD can erode the solid insulation 16 and eventually break down the insulation 16 this point 20. Protracted PD also tends to initiate defects (voids, imperfections, contaminants, etc.) in the heat tube 12. Thus, it may be advantageous to reduce PD in order to reduce the risk of insulation erosion and tube defects. Furthermore, reducing PD decreases the risk of igniting flammable vapors or dusts that may be present near the heat tube 12. Also, reducing PD decreases the effects of PD-generated ozone. In particular, ozone generated by PD can react with gas (e.g., nitrogen in the air) and moisture to create nitric acid, which increases corrosion of steel and other metal alloys, and, furthermore, ozone may have a deleterious effects on wire insulation (e.g., the insulation 16).
To limit the risk of this corona effect and potential electrical arcing events, conventional skin effect heat tracing systems generally limit heat circuit lengths to about 25 Kilometers (15 miles) from a single source using supply voltages approaching 5 kV. Embodiments of the present invention, in contrast, provides a skin effect heat tracing system that can operate at over 5 kV such as at 7.5 kV, 10 kV, or higher, and at circuit lengths over 25 Kilometers with comparatively less PD, which is enabled via the pressurization of gasses within one or more components the skin effect heat tracing system. As will be described in greater detail below, increasing system pressure reduces PD, which allows the system to operate at higher voltages and further allows for increased circuit lengths to be realized.
Accordingly,
As shown in
More specifically, as shown in
The pull boxes 26 can allow the individual heat circuits to be modified, replaced, or serviced without disturbing the insulation 38. Heat circuit lengths may be determined by a combination of cable size, cable voltage, temperature rating, heat tube size, and attachment method. Additionally, one or more splice boxes may be dispersed between adjacent pull boxes 26 to protect a splice between two heater cables 10 in a single heat circuit segment. However, in some embodiments, heater cables 10 may be long enough to travel between pull boxes 26 such that the splice boxes are unnecessary.
Generally, the internal gas pressure in components of the system 24 may initially be at atmospheric pressure, nominally 14.7 pounds per square inch absolute (psia). According to some embodiments, during heating operations, one or more components of the system 24 may be pressurized above atmospheric pressure, such as to approximately 20-30 psia. In other words, one or more components of the system 24 may have an internal pressure of about 5-15 pounds per square inch gauge (psig) compared to an external pressure of the system 24.
In some embodiments, the components may be subjected to internal gas pressurization via a gas source 41, which may include a compressor, a pressurized gas tank, or other suitable pressurizing tools. While in the present example, the gas source 41 is shown to be coupled to the system 24 through the end termination box 28, it should be understood that, in other embodiments, the gas source 41 may be coupled to the system 24 at other locations. For example, the gas source 41 may instead be coupled to one of the splice boxes, the front power connection box, directly to the heat tube 12, one of the pull boxes 26, or any other applicable component of the system 24 to supply pressurized gas to the system.
As another illustrative, non-limiting example, in some embodiments of the system 24, voltages observed at individual heat circuits located near the end termination box 28 may be lower than voltages observed at individual heat circuits located elsewhere along the carrier pipe 36, and PD may be less likely to occur at these lower voltage locations. Thus, in some embodiments, only the front power connection box and selected heat circuits, pull boxes 26, and/or splice boxes located more than a predefined distance from the end termination box 28 may be sealed and pressurized, while those located less than this predefined distance from the end termination box 28 (e.g., components at less risk of experiencing PD) may remain unsealed and unpressurized. By only pressurizing components at comparatively higher risk of experiencing PD, unnecessary pressurization of system components with low or zero PD risk may be avoided, thus reducing the likelihood of losing gas due to leaks.
Additionally, in some embodiments, the gas source 41 may include a leak detection device 39, which may check the portion of the system 24 to be pressurized in order to determine whether leaks are present in that portion. In alternate embodiments, this leak detection may be performed using a leak detection device separate from the gas source 41. The leak detection device 39 may also determine whether the leak rate of the system 24 is significant enough to impede or prevent the effective pressurization of the system 24 (e.g., whether the detected leak rate exceeds a predetermined threshold). If significant leaking is detected, the leak detection device 39 may provide an alert to the control panel 30 (e.g., to a computer controller or processor in the control panel 30) indicating that maintenance should be performed on the system 24 in order to repair leaks before performing heating operations with the system 24.
Generally, at least the heat tube 12 of the system 24 can be pressurized—that is, each end of the heat tube 12 may be sealed so that the space 18 defined between the heater cable 10 and the heat tube 12, as shown in
To accomplish this pressurization, the components of at least a portion of the system 24 (e.g., the portion that is to be pressurized) may be effectively sealed so that these components are substantially airtight (i.e., airtight or of such a low gas leak rate that pressurizing 5-15 psi is technically and economically feasible). For example, all connection joints of the heat tube 12 may be made with airtight welds to ensure the heat tube 12 is substantially airtight. The pull boxes 26 may include covers that are sealed, for example with silicone, such as room-temperature-vulcanization (RTV) silicone, or other types of sealants or via other sealing methods. The end termination box 28 and the front power connection box may be sealed, for example, with gaskets or other sealing methods. In addition, or alternatively, one or more of the heat tubes 12 may be sealed by cable glands (not shown) at locations where the heater cables 10 enter and exit the heat tubes 12 (e.g., at the end termination box 28, the front power connection box, and/or the splice boxes).
As described above, reducing PD is advantageous in skin effect heat tracing systems, and pressurizing the system 24 decreases the tendency for PD) to occur. More specifically, as a gas is pressurized in an electric field, the mean free path of an electron moving in the electric field before it collides with a gas molecule is shortened due to the higher number of gas molecules present. In other words, increasing the pressure increases the number of atoms/molecules in a static (e.g., not dynamic), confined volume, which increases the chance of an electron colliding with those atoms/molecules as it moves across that static volume (thereby decreasing the free mean path of the electron). Decreasing the mean free path in this way reduces the collision energy of the electron and molecule. As a given gas molecule must receive a defined amount of energy before ionizing, this reduction in collision energy decreases the likelihood that the gas molecule will ionize and generate a partial discharge.
For example, it has been shown that PD is proportional to absolute temperature such that, at a constant pressure, increasing temperatures correspond to a reduction of the number of gram moles of gas present. As such, increasing temperatures correlate to increasing PD current. In a general experiment, PD increased by about three to five times as temperature increased from 20 degrees Celsius (° C.) to 150° C. In another heat tube experiment, PD increased with increased heat tube temperature, as shown in the graph 42 of
As shown in
n
2
/n
1
∝P
2
/P
1
according to the Ideal Gas Law, PV=nRT, where P=absolute pressure. Thus, in applications where a conventional, unpressurized skin effect heat tracing system operates around 150° C., according to
Thus, at a pressure of about 6.5 psig, and 150° C., PD could be reduced by a factor of four compared to ambient pressure levels. Returning to
Accordingly, the pressurization of skin effect heat tracing system 24 can reduce the risk of PD and, thus, improve the scope of the applications for which the system 24 can be used. More specifically, embodiments of the present invention reduce electrical fields (and partial discharge thereby) in gas around the heater cable 10 located within the electrically conductive tube 12 (e.g., which may be grounded) in a quantifiable fashion by pressurizing the tube 12 and/or other components of the system 24. Since electron collision energy is decreased via this pressurization, higher voltages can be applied to the heater cable 10 with comparatively reduced risk of PD. Consequently, the skin effect heat tracing system 24 of the present invention can include a heat circuit deployed with longer distances between line lead connections compared to conventional, unpressurized systems, such as greater than 15 miles, up to 36 miles, or greater than 36 miles, and at higher voltages, such as above 5 kV, up to 10 kV, or higher than 10 kV.
In light of the above, some embodiments of the invention further include a method of providing a pressurized skin effect heat tracing system 24 to reduce the risk of PD during operation.
At step 66, power connections may be made between a subset of the components that are electrically powered and a power source (e.g., connecting the heat tube 12 and the heater cable 10 to the power source 40 via the pull boxes 26, the front power connection box, the end termination box 28, and the control panel 30 and the transformer 32 of
At step 68, one or more components of the system, such as heat tubes, pull boxes, splice boxes, the front power connection box, and/or the end termination box, may be made substantially airtight via one or more sealing methods, as described above.
Finally, at step 70, the one or more components, now sealed, may be pressurized (e.g., via a gas source such as gas source 41 of
Beginning with the first process, at step 72, a leak check may optionally be performed using a leak detection device (e.g., leak detection device 39 of
At step 74, a gas source (e.g., the gas source 41 of
At step 76, a power source (e.g., power source 40 of
At step 78, if the controller determines that the internal pressure of the components of the system has fallen below Pth, the method returns to step 74 so that the components may be re-pressurized by the gas source. In other words, when the sensed pressure falls below Pth, the controller instructs the gas source to re-pressurize the components. Otherwise, if the internal pressure of the components of the system remains above Pth, the method returns to step 76 and power continues to be applied to the conductors, enabling the continued normal operation of the skin effect heat tracing system. In some embodiments, the controller can further analyze pressure data over time and generate an alert if the analyzed data indicates a leak (e.g., if the system drops below Pth a certain number of times over a time period or if the system drops below Pth within a pre-determined time period after being re-pressurized).
Turning now to the second process, at step 80, the controller may periodically check for one or more end conditions while steps 72-78 are being performed. For example, end conditions may include the detection that the leak rate of the system exceeds a predefined threshold, the detection of a user- or system-provided instruction to cease operation of the skin effect heat tracing system, the detection that power is no longer being applied to the skin effect heat tracing system. As an example, if it is detected that the operating voltage of a given heat circuit is reduced below approximately 5 kV, the skin effect heat tracing system may automatically stop applying pressurized gas to that heat circuit.
At step 82, if the controller has not detected an end condition the method returns to step 80. Otherwise, if the controller has detected an end condition, the method ends. In some embodiments, ending the method may include ceasing the application of power and pressurized gas to the system. In some embodiments, ending the method may also further include depressurizing the components of the system. Alternatively, in some embodiments, the method of
It should be noted that, for embodiments having multiple, separately sealed heat circuits, some heat circuits may remain off (e.g., not receiving power) while other heat circuits receive power, such that heat may be applied only to selected sections of the carrier pipe. In such embodiments, the gas source may be switchably connected (e.g., through one or more valves, which may be controlled by a controller of the control panel) to each of the sealed heat circuits, and may supply pressurized gas to only those heat circuits that are active (e.g., that are receiving power). In this way, gas is not wasted on inactive heat circuits that may not require pressurization
It will be appreciated by those skilled in the art that while the invention has been described above in connection with particular embodiments and examples, the invention is not necessarily so limited, and that numerous other embodiments, examples, uses, modifications and departures from the embodiments, examples and uses are intended to be encompassed by the claims attached hereto. The entire disclosure of each patent and publication cited herein is incorporated by reference, as if each such patent or publication were individually incorporated by reference herein. Various features and advantages of the invention are set forth in the following claims.
This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 62/522,928 filed on Jun. 21, 2017, the entire contents of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62522928 | Jun 2017 | US |
