Trater has invested in software and systems capable of managing hundreds of thermocouples positioned simultaneously in each oven, in order to immediately provide the operator with the distribution of temperatures on the items and the relative expansion of the surfaces, to avoid upsettings and plastic deformations of the components , which lead to shortenings of the life of the items due to stress corrosion cracking. TRATER uses hundreds of kilometers of thermocouples every year to guarantee correctly treated items.
It has staff 24 hours a day in charge of monitoring thermal cycles. Unattended thermal cycles are not carried out. TRATER also makes this service available to its valued customers for the monitoring of thermal cycles conducted in other companies.
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MEASUREMENT TECHNIQUE
The most valid method for controlling and measuring temperature during heat treatments carried out in the oven is based on the use of thermocouples. Sheathed thermocouples are used for treatments at high temperature and/or with strong turbulence of hot gases. They are made through drawing, with the two conductors insulated in mineral oxide and the external protection sheath in inconel or AISI 310 stainless steel.
Thermocouples with fibre insulation are very practical to use and are instead made with the two conductors insulated in a temperature-resistant fibre braid. In relation to their type of insulation they can be used up to a temperature of 1200°C and have, as a peculiar characteristic, the fact that they are normally used in a disposable way. They are supplied in skeins, distinguished by manufacturing batch and easy to trace, from which the individual thermocouples are obtained, of variable length and suitable for reaching all parts of the item on which the temperature is to be measured. At the end of each treatment, the used thread is scrapped (disposable).
THE ARRANGEMENT OF THE THERMOCOUPLES ON THE ITEMS
Only a correct position of the thermocouples, in contact with the items, in the most critical points, will guarantee effective temperature control and the success of the treatment. Measuring the oven air alone is not sufficient and leads to inevitable failures. In general, thermocouples must be positioned on the items according to the following criteria:
Other factors must be carefully evaluated to position an adequate number of thermocouples to be used to control a thermal cycle and each individual case is studied and examined in depth by our Technical Office, in relation to:
In case of the simultaneous treatment of more items with the same geometry and mass, it is possible to divide the thermocouples onto several items by applying the parameters previously exposed to the oven load. For the treatment of more complex products, such as, for example, in the treatment of fixed tubesheet exchangers or in the case of particular materials, Trater applies double thermocouples in the most critical points, with a back-up function in the event of failure.
TEMPERATURE RECORDING
For the control of particularly complex thermal cycles to be adopted on very delicate items, with a very high number of thermocouples (up to 72 recording points), our IT technicians have developed a particular software for the simultaneous management of all the readings, able to indicate in real time to the operator responsible for controlling the treatment, the progress of the thermal cycle, calculate the maximum temperature deviations to compare them with the admissible ones and calculate the average temperature distributions on parts of the item (for example in the exchangers : the average temperature of the shell, the average temperature of the tube bundle, the average temperature of each tube sheet, etc…).
THE TIME TEMPERATURE DIAGRAM
Each diagram indicates the identified number of the treatment, the date of performance and the identification data of the treated items. The number and type of thermocouples used, their precision class, the time and temperature scale are also specified, as well as the signature of the Technical Manager of the treatment.
THE CALIBRATION OF THE EQUIPMENT
As seen, a suitable arrangement of the thermocouples on the items to be treated and a valid recording of the thermal cycle will guarantee reliable control of the treatment temperature; However, we will not be able to have reliable results if the thermometric equipment used is not able to provide a sufficient accuracy or if it is subject to vary its measurements over time. For this reason, at Trater, all thermometric equipment is calibrated before its use in the plant and periodically, in relation to the conditions of use.
Once a thermocouple has been manufactured (or purchased), it is subject, over time, to losing its accuracy due to various factors such as:
- the alternation of thermal cycles;
- high temperatures;
environmental contamination; - mechanical stresses.
These factors, by changing the characteristics of the materials, vary the thermoelectric power and, consequently, the measurement. It is very difficult to estimate the life of a thermocouple in general, as it depends on various factors, even unpredictable ones, as in the case of mechanical stresses.
It is better, however, not to leave too long a period between one calibration and the next, so as not to incur measurement errors in a large number of treatments. In Trater we carry out a six-monthly calibration for K-type thermocouples in reusable sheaths and a quarterly calibration of the recorders.
