Measurement of surface temperature of refinery fired heater tubes is difficult because of the high temperature gases and fumes which surround these tubes. These gases are 300 to 400 deg C hotter than the heater tube at the point of measurement. This aspect and the heat transfer along the length of Thermocouple sheath to its point of attachment
on the heater tube tends to cause the Thermocouple to show relatively higher temperature than the desired wall temperature. This problem can get aggravated if there is direct flame impingement at the point of attachment of Thermocouple to the heater tube.

Best results can be obtained by grounding the Thermocouple hot junction in "Wedge shaped" tip and attach the same by TIG welding it to the tube surface.

"Wedge shaped" Skin Thermocouples are perhaps, most ideal for the reliable monitoring of tube surface temperatures in fired heaters, for the prolonged tube life and ensuring safe and efficient operation.

Skin Thermocouples offer
• many advantages for temperature   measurement
• Thick wall sheath and compact mass of   MgO isolate the thermocouple conductors   from combustible products, ensuring a   long and drift free usable life.
• The Thermocouples are self supporting   within the fired heater box or in the   furnace because of the inherent high   temperature 8 strength of thick wall   sheaths.
• Terminal heads can be mounted      directly to the Thermocouple cold end   with the help of a compression fitting.
• Thermocouple sheaths are pliable
  enabling easy routing and installation.

"Wedge shaped" skin Thermocouple is l0mm or l2.7mm diameter with a sheath wall thickness of nominal 3mm. The insulating material is highly compacted magnesium oxide (MgO).

The Thermocouple junction is grounded into a "Wedge shaped" tip enabling a direct and totally metallic heat transfer path from the tube wall to the hot junction of Thermocouple. Firm attachment of "Wedge shaped" tip to the heater tube by TIG weld joint, almost eliminates effect of hot combustion gases on the measurement by Thermocouple.

"Wedge shaped" skin Thermocouple offers most accurate measurements, improved longevity, better reliability and ease of installation for tube skin temperature measurement.

Twelve standard installation configurations of the "Wedge shaped" skin Thermocouples are available to fit a particular application. These are factory formed as per customers requirement.

• Clean tube surface area 75mm long in the direction of Thermocouple axis x 50mm wide by grinding or by scrubbing.
• Locate and clamp Thermocouple firmly in desired location. The full length of "Wedge shaped" tip must make firm contact with tube.
• Select filler wire from table Welding method: Tungsten Inert gas (TIG).
  

• Perform root weld pass on both sides of "Wedge shaped" tip using 1.6mm" filler wire. Welds must   overlap each other at joining point of "Wedge shaped" tip and tube and run full length of "Wedge   shaped" tip.
• Perform secondary weld pass on both sides of "Wedge shaped" tip using 2.5mm" filler wire. Welds to   run full length of "Wedge shaped" tip.
• Allow the weld joint to be cooled to a temperature of 300°C between every pass.
• Perform final weld pass on both sides of "Wedge shaped" tip using 2.5mm" filler wire.
• Thermocouples mounted perpendicular to tube axis: Locate Retaining clip at tangency point of   Thermocouple and tube and weld at both ends only using 2.5mm " filler rod.
• Axial mounted Thermocouples: Locate retaining clip as desired and weld same as step 8.
• Thermocouples requiring configuring or reforming at site should best be done under the supervision of   our technician. This involves use of special tools and experienced hand and we will be pleased to   depute our technician to solve the site problems.