SES – 200

Skin effect heating system can provide a cost-effective alternative to Series resistance heat tracing on long pipelines. A pipeline up to 25 kilometres plus long can be traced from a single power point.

Skin effect heating system generates heat in the “Heat Tube” by the return electrical current flowing through the impedance of the inner skin of the heat tube.

Maximum Maintenance Temperature: 200°C

Specifications:

Typical Characteristics of product range

Available for hazardous area applications – Zone 1 & 2

Maximum Application Temperature to 200°C

Maximum Circuit length – 25000m

SKIN-Effect Trace Heating Systems

Long Pipeline Trace Heating

1. Destination of the skin-system

SKIN-SYSTEM also referred as Induction-Resistance Heating System (IRHS) is designed for heating, temperature maintenance, and frost protection of pipelines of various purposes having length 2 to 200km and more. The purpose of heated pipeline can be any, but among the basic types the following should be outlined:

– Water conduits (at gas field development and operation);
– discharge pipelines (transporting crude oil);
– sulphur lines (transporting liquid sulphur);
– pipelines transporting viscous chemical substances
   (in-plant interdepartmental pipelines of chemical and petrochemical enterprises).

2. Design styles of the skin-system

Depending on operating temperature and maximum allowable pipeline temperature three design styles of the skin-system can be outlined:

1. low-temperature;
   maintaining temperature 3° to 5°C;
   basic destination – frost protection of water conduits;
2. medium-temperature;
   maintaining temperature up to 60°C;
   basic destination – crude oil transporting pipelines;
3. High-temperature;
   maintaining temperature up to 200°C;
   basic destination –pipelines transporting viscous oil products, sulfur, chemical substances.

Design styles depending on type of pipelining are as follows:

– above-ground;
– buried (subsurface, underground);
– subsea (submarine, underwater).

One system can include combination of various design styles, e.g. in the case of passing above roads and passages (above-ground) or under rivers (underwater). Each of this style requires special decisions and materials used. Method of pipeline insulation is an important factor. It can be:

– Pre-insulation (factory insulation) of pipeline fragments;
– sectional insulation at site.

Depending on required heating power skin-systems consisting of 1, 2, or 3 heating elements (tubes) are used.

3. Description of the skin-system and principle of its operation

Special heating elements are used in the SKIN-SYSTEM that operation is based on skin-effect (proximity effect) in ferromagnetic conductive materials on alternative current of commercial frequency (50Hz).

The heating element is specially treated tube of low-carbon steel having 20 to 60mm outer diameter and wall thickness not less than 3mm with special conductor of non-magnetic material (copper or aluminium) having 10 to 50mm2 cross section placed inside. The conductor is reliably connected to the steel tube at far end of heated pipe run; at near end of the run alternative voltage is supplied between the conductor and the tube that value is calculated basing on power output needed and the length of the heated pipe run.

When the voltage is supplied in this manner, the current in copper conductor flows in one direction and returns through the steel tube in the opposite direction.

The alternative current flows through the whole cross-section of the inner conductor because on commercial frequency there is no noticeable skin-effect in non-magnetic material having low electrical resistance. In contrast, in ferromagnetic outer conductor (steel tube) skin-effect is pronounced, i.e. the whole current flows not in the whole depth of the tube’s wall but in thin surface layer about 1 mm thick. At that this current conducting layer is situated at the inner surface of the steel tube.

As described above, the current flows in inner surface of the skin heater’s tube, in its outer surface the current is virtually absent and there are no electrical potentials that makes the system safe for maintenance staff. This effect allows to earth the tube at any place. The tube is obligatory earthed at the near and at the far end of the heating circuit.

When the system is powered by high voltage (3 to 5kV) heat output occurs in both conductors. If the system is properly designed, 60 to 80% of heat output takes place in the steel tube, and only 20 to 40% in the inner (copper) conductor.

Electrically the skin-system is arranged to ensure continuity of both the inner conductor (skin conductor) and the tube of the skin heater being the return conductor of the system.

4. Advantages of the skin-system

Virtually the only economically sound method of heating pipelines having length 5 to 6km and more.

Lower investment cost as against other heating systems for pipelines. Even for pipelines 3 to 5km long the cost of the SKIN-system is still comparable with that of other applicable heating methods (serial-resistance or self-regulating heating cables).

Long lifetime – because heavy-gauge skin conductor in the SKIN-system is released from heat generation function and is practically used as built-in parallel power supply line. Metal heating tube is also long lived in case of proper treatment since it is installed under a layer of thermal insulation and protective casing of the pipeline. Solid heat-producing elements in the form of steel tubes provide mechanical stability and protect current-carrying conductors from damage. Operating lifetime of the SKIN-system is not less than 25 years.

Electrical and explosion safety. Outer surface of heat-producing element (skin tube) has zero potential to ground, the tube is earthed and completely screens and protects the current carrying skin conductor. Properly designed connections, pull boxes and connection boxes also provide system safety.

Good thermal contact provided. Metal heat-producing element (skintube) is directly welded to the product transporting pipeline or is fixed to it by means of special element. Special heat conducting paste is used to improve heat transfer from the skin tube to the heated pipeline.

Balancing of currents. Supply unit patented by SST LLC is used in power supply system that provides balance in primary three-phase network when feeding single- and two-phase induction-resistance load. Using this unit solves phase unbalance problem uneven current distribution to phases). Phase unbalance is principal defect of all single- and two-phase systems installed up to now.

Simple installation. Heat producing elements have no outer electrical isolation that can be damaged at installation. To ensure supplementary protection of the conductor corrugated polymer tube is preliminarily pulled into the SKIN-tube. At that force required for conductor pulling into the tube is also reduced (the method is patented).

3. The equipment being delivered

The set of equipment being delivered for electrical heating system installation includes all materials and equipment required for system installation and connection.

Skin-conductor (induction-resistance conductor, IRC) is highly reliable stranded copper conductor manufacturing specially for skin-systems. The conductor has double isolation made of thermal-resistant materials. Its operating lifetime is at least 25 years.

Skin-heater (induction-resistance heater, IRH) or SKIN tube is metal ferromagnetic tube having normalized value of magnetic permeability. The tube is delivering ready to install. Its operating lifetime is at least 25 years.

Fixing of the skin tube to the main pipe is performed with the help of point welding (if this welding is permitted) or with collar clamps by means of special tool. The method of fixing is determined after consultation with the customer.

Connection boxes are used for facilitation of pulling of the skin-conductor into the skin tube and for installation of connection couplings between sections of the skin-conductor.

Connection couplings of the skin-conductor are multiplayer highly reliable high voltage connecting elements used for connection of sections of the skin-conductor.

Step down transformer – single-, two-phase step down transformers are used that are specially manufactured for the specific project.

Control cabinets are designed for giving out signals controlling heating system turn ON and OFF and for system parameters monitoring. Control of temperature parameters is performed by means of air temperature sensors and heated pipeline temperature sensors using PSTAB multifunction controller. The remote control panel provides the possibility to get information regarding skin-system condition and to send it to dispatching point. Control system permits to optimize electric energy consumption depending on the ambient air temperature.

Packaged Transformation Substation (PTS) for system power supply (delivering by additional order) is designed for installation of the step-down transformer and also protection devices and system start up devices from both high voltage and low voltage sides. The PTS has modular closed design, the equipment inside is protected against atmospheric impact. Spare parts for electrical heating system and special installation tools can be included in the delivery by additional agreement.

Balancing unit is additional high voltage device using as a part of single-tube skin systems that allows connecting them to industrial three-phase networks without overloading of some individual phases.

SPECIFICATION