SKIN-SYSTEM
(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:
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 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)
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 (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 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 to connect them to industrial three-phase
networks without overloading of some individual phases.
SPECIFICATION
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 |
Construction: |
|
Heat-producing element |
Tube of low-carbon steel,having diameter 20 to 60mm, wall thickness 2.5 to 4mm, its electrophysical parameters being tasted and stabilized |
Сurrent-carrying conductor |
Special conductor with high voltage(up to 4 kV) endurance resistant to thermal load (up 200 °С) 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 |
Balancing equipment |
|
Thermoconductive paste |