Tube Skin Thermocouples
Skin Type Thermocouple are designed to measure the temperature of the body.
It is bonded to the surface of the body to which temperature is to be measured
through an insulating pad, so that it collects temperature. This model of
thermocouple is typically used within high temperature industry and/or corrosive
environments where fast and accurate temperature measurement is critical.
The most important consideration for effective Skin Type Thermocouple is its
design which can provide repeatability, durability and accuracy. To achieve
these functions thermocouple must incorporate with its design which should
provide excellent insulation properties, material compatibility and good physical
contact with the tube and it should also have the ability to withstand high
radiant temperatures and stresses and in some cases harsh combustion
chemistry.
Tube skin thermocouples accurately measure surface temperature in fired
process furnaces and boilers. The sensor comes with a protection pad (weld
pad) at the tip of the thermocouple. The protection pad is designed to minimize
disruption and damage to the device, caused by flame impingement and
radiant heat.
Figure 1. tube skin thermocouples
There are a number of applications for tube skin thermocouples. Each process
requires sensors that are purposely designed and matched with the furnace or
boiler.
OPTIMUM EFFECT TUBE SKIN
For a tube skin temperature sensor to work at its optimum range, the
thermocouple sheath must remain in close contact with the tube. The weld pad
should be positioned at the critical point and the remaining cable should be
routed away from the direct heat – along the coolest side of the vessel. This
installation technique is important as it allows the thermocouple to utilize the
process tube as a heat sink.
In addition to the weld pad expansion loops as part of design considerations. A
furnace can experience a movement of between 50 mm and 350 mm, from
start up, to reach its process temperature.
If such incremental expansions are not considered through the integration of
expansion loops, the Tube Skin Thermocouple will be subjected to strain and
even breakage. Expansion loops are therefore recommended to ensure the
optimum lifetime of the Thermocouples helping to reduce commissioning and
maintenance problems.
There are a variety of designs such as: V-pad, Weld-pad or constructions with or
without a protection-pad.
Types of Skin Type Thermocouple V-Pad:
Figure 1. V-Pad Skin Type Thermocouple
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V-Pad design includes a machined v-shaped block that is welded to the
mineral insulated cable.
Compaction of the mineral insulation inside of the v-pad protects the
measuring junction from radiant temperature influences.
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Thermocouple junction welded into the base of the v-pad.
V-pad block is designed to minimize the conducted heat influence from
the mi-cable.
Designed to limit the influence of high radiant temperature at the
measuring point.
Material compatibility for various service conditions.
No special machining of the contact block is required to match the tube
profile.
The narrow v-shaped block allows a full penetration weld between the
junction and tube surface. This eliminates potential air gaps and
substantial measurement inaccuracies.
Fast temperature response behavior combined with reliability.
Higher accuracy can be achieved with the addition of an optimized
shield.
Figure 2. Fan Tip tube skin thermocouple
Due to its 8mm diameter sheath made from Hastelloy-X, the ‘Fan Tip’ tube skin
thermocouple, developed by Okazaki Manufacturing Company, can be bent
and coiled to enable movement in both the horizontal and vertical axes. The
40mm end tip of the thermocouple is designed to be welded to the heater
tubes on both sides, which significantly reduces the chances of the
thermocouple breaking off whilst in service.
Currently used by BP, Esso, Exxon Mobil and STATOIL, the Fan Tip thermocouple is
ideal for any process burning or heating applications, including hazardous area
oil and gas, chemical and petrochemical installations. The Fan Tip is suitable for
gas-fired, light fuel oil and heavy fuel oil heaters. The sheath can be supplied in
either 316 stainless steel for gas-fired heaters, Hastelloy-X for light fuel oil, or dual
sheath AR-161 version for arduous, heavy fuel oil applications.
The design of the Fan Tip thermocouple is such that it can be used in all process
heater applications. The length of the sheath can be increased by up to 30
meters. The sheath can be bent in a cold state to meet any fixture and exit point
on the heater tube.
Figure 3. Different fixing types of Skin Thermocouples
SHEATH MATERIALS
Mineral Insulated cable used in the manufacture of tube skin temperature
sensors can be supplied with SS310, SS446, Hastelloy or HR160 sheath.
SPECIAL PROTECTION MINERAL INSULATED CABLE
Tube skin temperature sensors are manufactured from mineral insulated cable.
Different types of sheath materials depending on the application. The sheath
can be further protected with a second outer sheath, for example: fiber frax
sleeve.
Diameters available from 1,5 mm (1/16”) up to 12,7 mm (1/2”) in all metric and
imperial sizes. All constructions longer than 1.000 mm will be supplied with weld
clamps.
References:
https://www.thermo-electric.nl/wp-content/uploads/2014/05/TEtube.skin-FINAL-RGB.pdf
 https://www.engineerlive.com/content/22053
 www.okazaki-mfg.co.uk
 https://rueger.com/application/files/6914/9207/6920/S_70_E.pdf
 http://www.tempsens.com/pdf/articles/Skin-Type-Thermocouple-WriteUp.pdf
 https://www.wika.us/upload/BR_Surfacetemperature_Productreview_e
n_us_83304.pdf
 https://www.thermo-electric.nl/tube-skin-temperature-sensors/
 https://www.thermo-electric.nl/wp-content/uploads/2014/05/TEtube.skin-FINAL-RGB.pdf
 https://www.engineerlive.com/content/22053
 https://rueger.com/application/files/6914/9207/6920/S_70_E.pdf
 https://www.thermo-electra.com/getpdf.php?groep=129&taal=en
 http://www.te-direct.com/products/thermocouples/tube-skin/
http://www.dailyinst.com/assets/6.-for-email----daily-thermetrics-tstcproduct-brochure.pdf
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