HEAT TREATMENT OF HEAT EXCHANGERS
The heat treatment of heat exchangers is definitely the activity in which TRATER has developed more experience (THE MOST), and is able today to support (SUPPORTING) the Customer in this complex process.
The treatment of heat exchangers is A complex OPERATION due to the combination of metallurgical and thermodynamical problems and in general for the geometrical complexity of the items.
Furthermore, the use of more and more performing materials, difficult to be treated and with specific corrosion resistance, mechanical and creep characteristics, makes it necessary to face the heating of exchangers paying the highest attention and commitment, handling the work just after (?) an in-depth engineering and metallurgical study.
A badly executed (PERFORMED) treatment, OBTAINED applying wrong methods and without an efficient temperature control on the whole item, can bring to macroscopic geometrical deformations (SUCH AS bending of the tubes, deformation of the sheets and of the screwing and flexion of the tube), cracking of the weld of the tube-sheet/s, and even though not immediately detectable, it can generate a tensile stress state in correspondence of specific geometries of the items (tubesheets, welds between tubes and tubesheets, tubes) with a sufficient high value to expose the item to phenomena of stress corrosion cracking and hydrogen embrittlement, in particular operative conditions.
Insufficient temperatures can bring to a missing tempering of the welds and the insufficient relieving of the residual stress state during the welding processes, calandering or mechanical preliminar workmachining, making the joints weak, reducing the resistance to creep of the material and exposing the item to previously mentioned phenomena of stress corrosion cracking.
Furthermore, the consequences of oxidative processes on both internal and external surface of the tubes are not to be underestimated due to the influence in (DURING THE) service of the exchanger (pollution of the process products and reduction of the heat exchange) and due to the generation of exothermal heat, proper of oxidation phenomena.
The treatment of the fixed tube-sheet heat exchangers
During the treatment, the shell of the exchangers heats and cools up easily because it is touched by the turbulent gases generated by the burners, while the tube bundle, closed in the shell (in a firm environment, without direct heat supply, if not per natural convection which determinates a stratification of the air), heats and cools up much more slowly, generating big differences in the temperature of the item.
In particular, the elongation of the shell during the heating phase, since it is not together with (MANCA UN VERBO - WELDED?) the tubes, can cause the deformation of the tube-sheets, the collapse of the welds between tube and sheet and/or the stretching (saldature tubo-piastra e/o lo stiramento) of the tubes; the local upsets (ricalcature locali) of the material will surely correspond to tensile stressed zones (zone sollecitate a trazione) after the heat treatment.
Invece (ON THE CONTRARY)(perchè? non è "invece", si comporta come durante il riscaldamento), in relation (ACCORDING) to the mass and the geometry of the tubes, DURING THE COOLING PROCESS the tube bundle of the item cools up with lower rates than the shell and compressive efforts beyond the elastic limit of the material (in temperature) can occur with (CAUSING) upsettings, deformations and generation of residual tensile stress states, that can expose the item in service, as already stated, to the known problems of stress-corrosion, fatigue and hydrogen embrittlement.
Furhtermore, in the field of the complex geometry of a fixed tube-sheet exchanger, the lacking achievement (?) of the correct soaking temperature on the entire item will generate zones in which there is no stress relieving and tempering of welds and the high hardnesses, acting as points of concentration of the efforts, that can generate cracks in service (? tutto il paragrafo).
Various studies have demonstrated that hardness values of the weld and the heat-affected zone of the welds (?) under 200HB (for carbon steels) and under 240HB (for low-alloyed or alloyed steels) are optimal for the service of these items even when subject to corrosive attack, as indicated by the NACE directives.
Treatment of the tube bundles
In many cases, it is necessary to treat the entire tube bundle of an exchanger with "U" tubes or a floating tubesheet.
The problems are the same of the ones described for the fixed-tube sheet heat exchangers.
In relation to the dimensions of the bundle, the number of tubes, their diameter, thickness, geometry and number of diaphragm present, the most external tubes of the item heat up and cool up with higher speed than the internal ones causing deformations with (OF) remarkable entity, due to temperature difference and consequent dilatation difference, and new residual stress states, due to generated constraints with the baffles and tubesheets.
The geometrical distortions can make impossible the insertion of the bundle into the shell and reduce the thermal efficiency of the heat exchanger, while the residual stresses can expose the item to serious damages during operation.
Treatment method of heat exchangers
In the nineties TRATER has developed technologies for the treatment of heat exchangers previously described (TECHNOLOGIES o HEAT EXCHANGERS?) able to reduce and, in most cases, cancel all of the indicated (MENTIONED) problems.
Thanks to the initial intuition of forcing hot gases with controlled volume and temperature inside the tubes with the aim to uniform the temperature of the items, TRATER has developed more sophisticated techniques for heating and cooling up the items to treat exchangers and tube bundles geometrically (more) complex, in carbon steel, low-alloyed and alloyed steel, alloys with high nickel, titanium, copper content and with particular oxidation problems.
TRATER has furthermore developed really valid and efficient methods to control temperature in all the most critical parts of the items, using in some cases up to 140 thermocouples, applied into direct contact with the item; through an especially generated software, able to calculate the real time average temperatures and the deviations between the various parts of the heat exchanger, and a 24-hours a day monitoring through our highly specialized personnel, TRATER is able to ensure the best treatjment result.
Over the years, (in Italy and abroad), TRATER earned the title of strategic company, supporting the most important industrial realities in Italy and in Europe, specialized in the construction of heat exchangers, boosting the acquisition of important jobs in an industry sector that does not know crisis and has, therefore, always been a real driver of the economy, evolving at the same level of technologies for the energy recovery.