Heater Tube Skin Thermocouples

Heater tube skin thermocouples (TSTC) are installed on a very challenging service, to measure tube surface temperature in hydrocarbon processing plants. These thermocouples are exposed to harsh process environment which involve more than one of the following:

⦁ high surrounding temperature,

⦁ oxidizing and reducing atmospheres,

⦁ sulphurous gases,

⦁ burner flame impingement

⦁ expansion & movement of tubes

These are typical conditions. As a thermocouple manufacturer, the right solution for the design, metallurgy & routing / installation has to be in a joint discussion involving the end user to understand:

⦁ nature of the likely failure of thermocouple,

⦁ Thermocouple installation & routing arrangement,

⦁ Location of burner

⦁ Expansion & movement of tubes and its effect on the thermocouple,

⦁ Process conditions

A long-lasting and reliable thermocouple recipe can be one which can address all the concerns listed above. There is no ‘ONE SIZE FITS ALL’… for a skin thermocouple.

As an end user, it is important to keep an eye on the following specifications and check points:

i)Thermocouple sheath size — its outer diameter, and wall thickness play a vital role in improving the life of thermocouple. Harsh environments inside furnaces, demand users to select sizes of 12.7mm sheath diameter & 3mm wall thickness.

Lower diameter and wall thickness are easier for installation, removal and maintenance, especially for replacement purpose, but this should be determined by the user depending on the furnace environment conditions.

ii)) Thermocouple sheath material — there are few metallurgies which are popularly used for TSTCs:

⦁ SS310

⦁ SS446

⦁ Inconel 600*

⦁ Inconel 625*

⦁ Hastelloy X**

⦁ HR160 ***

⦁ Alloy TD#

Typically, in heater furnaces, it is a combination of oxidizing and reducing atmospheres, and sulphurous gases, with high temperature. Each of the above metallurgies has its own strengths. However, studies and real time data, along-with our experience on TSTCs have led to conclusions that major and minor alloying elements play a crucial part in imparting mechanical strength at elevated temperature, resistance to corrosion, and withstanding the atmosphere inside the furnace. Generally, if it is a Nickel Chromium alloy, then it is recommended to have minimum 20–25 % Chromium. Additional alloying elements such as Molybdenum, Cobalt, Silicon, Niobium, provide the requirement properties such as:

⦁ Better pitting and crevice corrosion ( Moly )

⦁ Stabilization during welding to avoid IGC ( Niobium )

⦁ Resistance to SCC ( Nickel)

⦁ Good for oxidizing environment (Chromium)

Ferritic Steels such as SS446 are very popular for use as sheath material in such applications, as they meet all the required properties, except for creep strength at elevated temperature. Due to this, there is deformation of SS446, during such furnace operations, which can lead to reduced life and performance of the thermocouples. Considerable focus has to be given to the installation and proper routing support for such thermocouples, to mitigate the concerns due to high temperature deterioration of mechanical properties.

iii)Conductor diameter — Typically maintaining a proper proportion of conductor diameter and insulation is the capability of the manufacturer. Standards such as BS EN 601515 define these dimensions. However, for TSTC with higher wall thickness, the conductor diameter has to be suitably achieved to meet the strength requirements. An acceptable value would be 16 AWG for duplex (sheath dia of 9.5mm) and has to be certified by the supplier.

iv) Routing: This is an important aspect during installation to ensure that the thermocouple follows the correct route to the measuring point. Care has to be taken to align an expansion loops, and the thermocouple sheath needs to be routed & laid along the heater tube as much as possible to maintain proper support & avoid burn out due to flame impingement. Too much unsupported length should be avoided.

v)) Additional requirements to explore would be an extra heat resistant insulating sleeve over the thermocouple sheath, which can be wrapped to improve the life of thermocouple.

At PYRO ELECTRIC, we have an experience of over 25 years in handling design and manufacture of TSTC, as well as site installations. Besides this, our specialists have visited extensive installations during shutdowns to assess the thermocouple conditions, studied the failures, carried out analysis, and improvised on designs which have suited the particular plant process dynamics.