Skin Thermocouple : A Reliable Solution For Industrial Tube Surface Temperature Measurement

Introduction to Skin Thermocouples

Skin thermocouples are specially designed temperature elements (constructed without thermowell and with installation accessories) which are used for the measurement of temperature of surface of any fire tube, heater tube, reactor tube, boiler or any other industrial tube assembly where it is essential to install the sensor only at the surface and not to be immersed inside the tube. These type of sensors are installed at the surface or skin of tube so named as ‘skin thermocouple’.  Tip portion of skin thermocouple is to be in contact with the tube surface (whose temperature is to be measure) through a weld pad and guide tube assembly. For senor’s mounting, weld clamps are welded to the tube inside which thermocouple is fix in such a manner that during the periodic maintenance part, it can be easily pull-out and separated from the tube.

Constructional Features

Any skin thermocouple is manufactured simply by connecting a mineral insulated cable with head enclosure. Along with it, for the proper installation and precise temperature measurement, some accessories are to be supplied. The material of construction of these additional parts depends on the client’s recommendation and process technical requirement. Main parts of any skin thermocouple includes:

• Ex-proof head enclosure with or without transmitter
• MgO insulated MI cable (also termed as ‘sheath’)
• Weld-pad (to be weld on tube and covered by shield)
• Guide-Tube (to be attach with weld-pad and covered by shield)
• Heat-shield (for neutralize the ambient impact on junction)
• Coil / Expansion loops (Right or Left depends on direction of heat flow)
• Weld clip / Mounting clamps (for additional support to sheath)
• An adjustable fitting / Stub near head assembly
  

Figure-1: Tube skin thermocouple assembly welded to surface of heater / reactor tube

Technical Purpose of Parts and Accessories

• Ex-proof head enclosure is connected at the termination end for covering of terminals and ceramic block. It can be provided with head-mountable temperature transmitter or if transmitter is not used then the temperature signal can be observed in control room through compensating cable. Head enclosures are provided with dual cable entries and IP-67 protection.
• Magnesium oxide is used as an insulation medium for separation of two elements of thermocouple. MOC of these two elements may different depends on the type of thermocouple (i.e. For ‘K’ type, chromel and alumel elements are used and insulated by MgO power (About 99% pure). The element and insulation medium is covered by metallic tube and termed as ‘sheath’.
• Weld-pad is used to weld on the tube directly at the last end so that it can make a direct contact between thermocouple sheath and tube and accurate temperature measurement can be obtained.
• Guide-tube is used as a cover for the termination end of thermocouple sheath and welded at the middle part of weld-pad so that sheath can be inserted in it and can form a direct contact.
• Heat-shield is used with the weld-pad in order to prevent the impact of ambient atmospheric conditions and flame temperature on the measureable hot junction of the thermocouple. The use of a guide tube allows more accurate temperature measurement of tube surface.
• Coil in sheath or expansion loops are designed in order to avoid the impact of expansion of the tube because of thermal growth. If expansion loops are not considered while manufacturing the thermocouple, it may damage at the time of tube expansion so it is always recommended to consider loop. These loops may be of ‘S’ shaped, Coil form, Spiral form, ‘V’ shaped etc. ‘Right’ or ‘Left’ side loop can be formed based on the tube expansion direction.
• Weld clips or Mounting clamps are welded with tube surface at a regular distance from each other in order to provide the additional support to the sheath. MI cable or sheath can be easily removable from these clamps.
• On the sheath, an optional adjustable compression fitting or stub may be used as per the requirement for the connection with the heater-wall. MOC of this fitting/stub may be SS or Inconel grade depends on the operating temperature.

Technical Specifications

If you want to know about how to select the best Skin Thermocouple you can refer to our blog “How to select Skin Type Thermocouple?”

Classification Based on Construction

Depending on the type of construction of pad, there are following 4 types of skin thermocouples:

1. Knife edge type skin thermocouple
2. Welded pad type skin thermocouple
3. Interchangeable curved type skin thermocouple
4. Strap on or clamp-on type skin thermocouple

Response time of a knife edge thermocouple is better (about 5-6 seconds) than other 3 types (about 10 seconds). Strap on construction is lesser used in petrochemical industries. Welded-pad type is lesser accurate than a knife edge construction while both can maintain similar repeatability. Pictorial representation of all 4 types can be shown in figure 2-5 as follows.

Figure-2: Knife edge pad layout

Figure-3: Welded pad layout

Figure-4: Curved type layout

Figure-5: Strap or clam-on type layout

Step-by-Step Procedure for Installation of Tube Skin Thermocouple

Step-1: The 1^st step of skin thermocouple commissioning is the cleanliness of the surface of heater tube, where skin thermocouple is to be installed. This surface cleaning involve removal of dust and rust which may present on the surface. Dust removal involves an ordinary cleaning by a waste cloth and rust removal involves grinding. During the grinding procedure, we have to make sure that grinding is not as much as it reduce the wall thickness of tube. Grinding is to be done mainly where the junction of thermocouple is located. The grinding area depends on the length of heat shield and the number of mounting clamps and their size. Before starting the installation, head-enclosure should be removed from sheath by its opening.

Step-2: The 2^nd step involve welding of weld-pad (attached with sheath and guide tube) on the tube surface. Before this welding, we have to make sure about the routing of thermocouple and its clamps on the surface. Weld pad should be weld from 3 sides in order to maintain the proper contact of sheath/junction with tube surface.

 Step-3: The 3^rd step involve welding of heat-shield on the tube surface over the weld-pad in such a manner that there will be minimum impact of ambient conditions on the thermocouple junction and the accurate temperature measurement can be obtained. While welding the heat shield on the tube- surface, make sure that proper fillet rod is selected and welding is done on 3 sides.

 Step-4: The 4^th step involve welding of mounting clamps on the tube-surface at a regular distance. The number of clamps depends on total length of thermocouple and recommended maximum distance between two consecutive clamps is 500mm. These clamps should not be direct weld with sheath. Dye penetrant test may be performed for all weld joints in order to ensure weld-strength.

 Step-5: The 5^th step involves re-connection of head-enclosure with thermocouple sheath and lead wires to the terminal block inside the head. Surface temperature readings can be obtained through compensating/extension cables in the control room

Conclusion

Installation of skin thermocouple provides a precise way to measure tube surface temperature in various applications where its not recommended to immerse temperature element inside the tube or pipe. Tempsens provide skin thermocouples with a variety of sheath MOC and sizes as per the client’s technical requirement. The use of applicable accessories like heat shield, weld pad, guide tube, mounting clamps allows skin thermocouple to provide accurate temperature data with faster response time. Its flexible construction with expansion loops provide an additional feature so that its performance will not much get affect by tube expansion due to thermal growth. Instead of an ordinary thermocouple or RTD, skin thermocouples are comparatively a better solution for temperature measurement and now a days it is globally acceptable in various process industries.

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FAQs

• What is a skin thermocouple? Mention its various Accessories.
  Skin thermocouple is a mineral insulated based temperature element which is specially used for the measurement of temperature of surface of any tube without immersed in it. It is installed on the skin of tube so named as skin thermocouple. The accessories of any skin thermocouple include weld-pad, guide-tube, heat-shield, mounting-clamp, stud etc.

• What is the purpose of expansion loops provided with a skin thermocouple?
  Expansion loops provide flexibility to the installed thermocouple on the tube so that there will not be any effect of tube expansion due to the thermal growth. Expansion loop is an essential requirement which ensures that there will not be any damage of insulation or element due to the thermal growth of tube.

• What is the purpose of heat shield used with a skin thermocouple?
  Heat-shield will act as an outer covering to the weld-pad and guide tube so that there will not be any impact of ambient conditions during the temperature measurement by sheath tip. Use of heat-shield with weld-pad allows accurate temperature measurement with good response time. It is filled with an insulation medium known as ‘kowool’.

• What is the purpose of guide tube provided with a skin thermocouple?
  Guide tube will act as an outer covering to the thermocouple sheath. It is welded with weld-pad and weld-pad is then weld with tube. Shape of guide tube is quarter / half curved depends on the tube diameter. Thermocouple sheath is inserted in the guide-tube.

• What are the design criteria to select sheath material of a skin thermocouple?
  Selection of sheath material depends mainly on the operating temperature to be measure, Some other factors include material of tube whose temperature to be measure, hardness level of sheath material and compatibility with thermocouple element / MgO insulation.

• What are the application areas where skin thermocouples used?
  Skin thermocouples are widely used for tube surface temperature measurements in various industrial segments like petrochemicals, steel, power plants, cement, boiler, furnaces, chemical applications etc.

• What are the factors affecting the response time of a skin thermocouple?
  Response time is mainly affected by the contact level between thermocouple sheath and tube and type of junction used in between thermocouple elements. Grounded junction has better response time than ungrounded type. Similarly knife edge type skin thermocouple has lesser response time than other three types due to the excellent contact between sheath tip and tube surface. Sometime response time can also be increase by the use of thermal silicon paste on the sheath body.