The present invention pertains in general to methods and apparatus for heat-tracing and in particular to methods and apparatus for supplying electrical power to proximity effect heat-tracing systems.
Viscous fluids, particularly heavy oil and molten sulfur, are often most conveniently transported by pipelines. Movement of viscous fluids through such pipelines is facilitated by maintaining the fluids at an elevated temperature.
Steam- or electrically-heated tracer pipes are commonly welded to pipelines in order to maintain the pipelines at temperatures above the ambient temperature. However, it is generally costly to provide steam or conventional electrical resistance heating in tracing pipes for the length of a long-distance pipeline. This cost is due, in part, to the need for energy input points at intervals of a few hundred to a few thousand feet.
One type of electrical heating used in long-distance pipelines is proximity effect heating, proximity effect heating is a form of impedance heating in which an insulated conductor passes through the interior of a ferromagnetic pipe called a heat tube. An alternating current power source is connected between a first end of the heat tube and a first end of the conductor, while a second end of the heat tube is connected to a second end of the conductor. Current flows from the power source through the conductor to the second end of the conductor and returns through the heat tube. Heat is generated by the I.sup.2 R loss of the return current flow, by hysteresis and eddy currents induced within the heat tube wall by the alternating magnetic field around the insulated conductor, and by the I.sup.2 R loss in the insulated conductor.
By a process of electromagnetic induction, the current in the insulated conductor causes the return current to concentrate at the inner surface of the heat tube which is the surface of the tube nearest the conductor. This phenomenon of current concentration is properly referred to as the proximity effect, although the term "skin effect" is often applied.
One advantage of proximity effect heat-tracing systems is that energy inputs may be spaced up to several miles apart. However, the voltage required to energize a proximity effect circuit is directly proportional to the length of the circuit so that conventional proximity effect circuits having energy inputs spaced at intervals of several miles require input voltages in the order of several kilovolts to cause a sufficiently large flow of current in the insulated conductor. Insulated cable for use at these voltage levels has been cited, in U.S. Pat. No. 3,524,966, as costing up to 50% of the installation cost of a heat generating pipe.
An approach to supplying additional lengths of proximity effect circuit is shown in U.S. Pat. No. 3,523,177, wherein a secondary side of a first transformer serving as a power source to a first length of circuit feeds electricity to a primary side of a second transformer in series. The second transformer serves as a power source to a second length of circuit. Additional lengths of circuit are supplied by adding further transformer in series. A major disadvantage of this approach is that all lengths of the heating circuit are in series so that the energy input to each cannot be made and broken independently in order to compensate for a broken wire in a section or to regulate heating on a section-by-section basis.
An approach to reducing electrical insulation costs is disclosed in U.S. Pat. No. 3,524,966, wherein advantage is taken of the fact that the difference in electrical potential between the insulated conductor and the heat tube declines gradually to zero from the first end to the second end. By lowering the grade of insulation in a stepwise fashion from the first to the second end, a major reduction in cost is obtained. Although use of this approach reduces the insulation cost of operating under a given applied voltage, it does not allow the flexibility to choose to lower the voltage to be applied or to extend the length of circuit supported by a given applied voltage.
As a practical matter in existing circuits, the length of a proximity effect circuit is limited by the voltage that can be applied and the voltage that can be applied is limited by the voltage rating of the insulation around the cable used as the conductor within the heat tube. Consequently, it is highly desirable to substantially reduce the voltage required for a given length of proximity effect circuit or to substantially increase in the length of proximity effect circuit which can be energized by a given voltage.