The most valid method for temperature control and measurement during heat treatments performed in an oven, is based on the use of thermocouples. Other systems, surely valid for other processes, can easily introduce detection errors with enough entity to ruin the result of the treatment itself.
For example, temperature measurement through infrared measurement devices is less indicated due to the problems connected with the reflectivity of the treated materials, depending on the conditions of the item’s surface, changeable during the thermal cycle (formation of oxides).
Various kinds of thermocouples can be used, different for geometry, temperature of use and method of fixing to the items.
Sheathed thermocouples are used in high temperature treatments and/or high turbulence of hot gases. They are obtained through drawing machining and have two isolated conductors in mineral oxide and an external protection sheath in Inconel or AISI 310 stainless steel.
Thermocouples with fiber insulation have a very practical use and are manufactured, on the other hand, with conductors, isolated in a temperature-resistant fiber sheath.
In accordance with their kind of insulation they can be used up to the temperature of 1200°C and their peculiarity is that they are normally single use.
They are supplied in reels, easily traceable through the manufacturing batch number from which the single thermocouples are obtained and they can have variable length in order to reach all the parts of the item on which temperature shall be measured. After each treatment, the wire is thrown away (single use).
The two types of mentioned thermocouples are also distinguished (IDENTIFIED) according to the way they are fixed:
Sheathed thermocouples need a mechanical fixing through nut and stud, slotted and welded to the item to be treated;
Thermocouples with fiber insulation are fixed to the item through capacitive discharge in compliance with the Code ASME VIII, Div. 1 – 2.
TRATER is provided with a very large number of sheathed thermocouples (about 300), periodically calibrated, with a length up to twenty meters and always ready for use in the plant.
In addition, TRATER buys thousands of meters of single-use wire for thermocouples of different kind and for different temperatures.
Location of thermocouples on the items
Only a correct location of thermocouples, in contact with the most critical points of the items, can ensure an efficient temperature control and a good result of the treatment. The measurement of air temperature alone is not enough and generates inevitable bad results.
Thermocouples are generally positioned onto the items according to the following criteria:
on the maximum and minimum thickness;
on the intermediate thicknesses;
on the superior and inferior part of the item;
on the sides of the item;
inside the accessible closed chambers, when thicknesses are higher than 50 mm;
on zones, sections, appendixes and other particulars that can be of specific interest or geometrically differ from the rest of the item.
Other factors must be carefully evaluated in order to position an adequate number of thermocouples for control of the thermal cycle and each single case is studied and deeply examined by our Thecnical Office, in relation to:
the type of treatment to be performed;
the geometrical complexity, dimensions and weight of the item to be treated;
the characteristics of the used oven (the kind and number of burners and the automatically controlled zones)
the positioning of the item into the same oven.
In case of a simultaneous treatment of more items with the same geometry and mass, it is possible to divide the thermocouples on more items, applying to the oven’s load the previously indicated parameters.
For the treatment of most complex items, such as heat exchangers with fixed sheets or in case of particular materials, TRATER provides double thermocouples in the most critical locations with a back-up function in case of malfunctioning.
In TRATER, temperature recording is continuous and automatic during the entire thermal cycle.
It is performed through recorders with multiple reading points with written trace or through acquisition devices, able to record the entire thermal cycle and reproduce it on graphical or numeric print.
The detected data are used by the operator, who controls the heat treatment, to verify that no anomalie occur during the cycle, that can ruin the result of the treatment. In the end, they are used as documentation of the performed job (time/temperature diagram).
In order to monitor particularly complex thermal cycles on very delicate items, with a very high number of thermocouples (up to 72 recording points), our computer-science technicians have designed a particular software to simultaneously manage all the readings, able to indicate the real-time trend of the thermal cycle to the operator that controls the treatment, to calculate the maximum deviations of temperature in order to compare them with the admissible ones and to calculate the distributions of the temperature averages on parts of the item (for example on the exchangers: the average temperature of the shell, the average temperature of the tube bundle, the average temperature of each tubesheet, etc.).
Every diagram indicates the identified number of the treatment, the date of execution and the identification data of the treated item together with the number and the kind of thermocouples, their accuracy class, the time and temperature scale, with the signature of the Technical Responsible Person of the treatment.
Calibration of equipment
As previously indicated, an adequate positioning of the thermocouples on the pieces to be treated and a valid recording of thermal cycle will ensure a reliable control of the treatment temperature; in any case, we cannot have trustworthy results if the thermometric equipment is not able to provide a sufficient accuracy or if they vary their measurement in time.
For this reason, in TRATER all thermometric equipment is calibrated before its use in the plant and periodically, in relation to their conditions of use.
Once manufactured (or bought), a thermocouple is subject in time to the loss of its accuracy due to different factors, such as:
Alternating of thermal cycles;
These factors, changing the characteristics of the materials, vary their thermal-electrical properties and, consequently, the measurement.
It is very difficult to estimate the life of a thermocouple, since it depends on different factors, some of which are not predictable, such as in the case of mechanical stresses.
In any case, it is advisable that no long period of time passes between a calibration and the next one.
In TRATER a calibration is performed every six months for sheathed K-type thermocouples and a quarterly for the recorders.