(SKIN-EFFECT BASED HEATING SYSTEM FOR LONG PIPELINES)
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 purpose 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);
- sulfur lines (transporting liquid sulfur);
- pipelines transporting viscous chemical substances
(in-plant interdepartment 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:
maintaining temperature 3° to 5°C;
basic destination – frost protection of water conduits;
maintaining temperature up to 60°C;
basic destination – crude oil transporting pipelines;
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:
- 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 these style
requires special decisions and materials used. Method of pipeline insulation is an important factor.
It can be:
- preinsulation (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 aluminum) 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)
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 (skin tube) 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 pats 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 to connect them to
industrial three-phase networks without overloading of some individual phases.
|Operating temperature range||- 50 °C to +200 °C|
|Electrical power supply||up to 4.0 KV, 50Hz|
|Linear heat output||up to 100 W/m for one heating element|
||Tube of low-carbon steel,having diameter 20 to 60mm, wall thickness 2.5 to 4mm, its electrophysical parameters being tasted and stabilized|
||Special conductor with high voltage(up to 4 kV) endurance resistant to thermal load (up 200 °Ñ) and to mechanical loadings during installation|
|Anticorrosion protection of the skin –heater||Painting, epoxy or other coating (at the request of the customer)|
|Pulling and connection boxes|
|High voltage connectors|
|Completed Packaged Transformation Substations(PTS)
||Protected rooms for special transformers and devices for regulation,control and communication|