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Home / Issue Archive / 2007 / June #6 / Using Electric Heat Tracing Systems for Long Pipelines

№ 6 (June 2007)

Using Electric Heat Tracing Systems for Long Pipelines

By N. Khrenkov

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Electric heat tracing is the most suitable and functional tool to ensure stable transmission of liquids by pipeline. Advantages of electric heat tracing systems are quite extensive: they are featured by low material-output ratio and easy installation; they are corrosion-proof and defrosting-proof and can be powered from the plant's power supply network. These systems are equipped with automated control systems that accurately and, in accordance with the selected algorithm, support the preset operation mode; they are easily integrated with upper level automated control systems and may be used for complex and branched pipelines.

Ensuring a stable process flow requires full or partial compensation of heat losses; in case of the process interruption, it is necessary to maintain the minimum permissible temperature of liquid; pipes are to be heated to a predetermined temperature for restarting the process after interruption (cold start-up).

The problems mentioned are successfully solved by applying cable heat tracing systems for various pipelines.

The most important criterion while selecting a design option for a heat tracing system is the system's total cost (Fig. 1). In order to select an optimum low-cost and efficient system for a specific application, it is necessary to consider the pipeline's length, a temperature level to be maintained, and a need to install a power supply network.

The longer the pipeline, the more complicated parallel power supply network is required for the heat tracing system, which means supply of additional control cabinets, temperature detectors, distribution boxes, etc. So, a system with a minimum-cost parallel power supply network will be an optimum solution for pipelines 15-30 km long.

Heating elements of electric heat tracing systems are various cables and tapes (resistive, self-regulating, or Longline resistive elements) subject to application.

A parallel power supply network is required to supply power to resistive and self-regulating cables and to apply voltage to heating sections. If a pipeline is relatively short (up to 150 m), the parallel network dimensions are minimal, likewise expenditures connected with this network. Pipelines 200-500 m long can also be heated by means of resistive and self-regulating cables. In this case, the parallel network cost will be comparable to expenditures connected with heat tracing cables.

For pipelines 500-3,000 m long, it is economically feasible to apply special three-phase Longline resistive cables of series resistance in a star-connected circuit, which are powered from one end only. Such cable functions as a heating element and a feeder at the same time. A cable makeup includes a conductor of three copper tapes enclosed in organic-silicon rubber, which makes it flexible. Copper braiding and bandage improve mechanical protection.

A Skin Electric Current Tracing (SECT) system functions without a parallel power supply network, and thus requires minimum costs for heating pipelines 3 to 30 km long. This system is powered from one end for pipelines up to 15 km long, and from two ends or from the central pipeline section for pipelines 25 to 30 km long.

Special heating elements are used in the SECT system. Their operation is based on a skin-effect and proximity effect in ferromagnetic conductive materials under alternative current of commercial frequency.

The heating element is a specially treated tube of low-carbon steel with an outer diameter of 20 to 60 mm and wall thickness of at least 3mm. A special conductor of non-magnetic material (copper or aluminum) with a cross section of 10 to 50 sq. mm is placed inside the tube.

The conductor is reliably connected to the steel tube at the far end of a heated pipe run; at the near end, alternative voltage (50 Hz) is applied to a section between the conductor and the tube. Similarly to the Longline resistive cable system, the SECT system also functions as both a heating element and a feeder.

The alternative current flows through the whole cross-section of the inner conductor, since under AC of commercial frequency, no noticeable skin-effect occur in non-magnetic material with low electrical resistance. In contrast, in ferromagnetic outer conductor (steel tube), skin-effect is distinct, i.e. current flows only in a thin inside surface layer about 1 mm thick, and is virtually zero in the outer tube surface. Due to insignificant skin-layer, up to 80 percent of heat is generated in the steel tube (Fig.2).

The nature of dependance of the heat tracing system cost on the pipeline's length has been confirmed by practical knowledge based on designing and operation of Teplomag systems manufactured by the Special Systems and Technologies (SST) Company

Special Systems and Technologies has extensive experience in engineering, manufacturing, installation, start-up and maintenance of heat tracing systems for pipelines of various lengths at oil and gas facilities. Over 2,000 Teplomag systems have been installed at main pipeline facilities.

SST Design Center and Assembly Subdivision provide for a full package of services and development of "turnkey" projects. Based on the data of preliminary heat calculations, they identify the cost of the most efficient heat tracing system and develop the design, according to which all components of the system are manufactured. All products undergo up to 14 quality control levels.
SST products are certified for application in explosion hazard zones in Russia, Ukraine, Kazakhstan and Belarus.

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