Investigation of the influence of flexible operating modes of industrial furnaces on the service life of metallic high-temperature components

The pro­duc­ti­vi­ty and relia­bi­li­ty of ther­mopro­ces­sing plants are of decisi­ve importance for their ope­ra­tors and manu­fac­tu­r­ers. In the con­text of the ener­gy tran­si­ti­on in Ger­ma­ny, the goal of sus­taina­bi­li­ty for socie­ty as a who­le is beco­ming incre­asing­ly important. Stra­te­gies for plant main­ten­an­ce are evol­ving from reac­ti­ve and pre­ven­ti­ve mea­su­res to pre­dic­ti­ve, con­di­ti­on-ori­en­ted func­tion­al main­ten­an­ce. This appli­es in par­ti­cu­lar to ener­gy-inten­si­ve plants, for exam­p­le for the heat tre­at­ment of metals.

High-tem­pe­ra­tu­re loa­ded com­pon­ents in indus­tri­al fur­naces expe­ri­ence acce­le­ra­ted creep defor­ma­ti­on under ther­mal cycling com­pared to iso­ther­mal loa­ding, which in com­bi­na­ti­on with cor­ro­si­ve dama­ge limits the ser­vice life of the­se com­pon­ents. One exam­p­le of such com­pon­ents are radi­ant tubes, which are used for indi­rect hea­ting in indus­tri­al fur­naces in which inert gas atmo­sphe­res are used. At the end of their ser­vice life, the radi­ant tubes are no lon­ger func­tion­al due to cracks or seve­re defor­ma­ti­on (see Figu­re 1) and the tubes have to be shut down. The redu­ced power input decrea­ses the pro­duc­ti­vi­ty of the fur­nace until the tubes are even­tual­ly repla­ced during a plant shut­down. The repla­ce­ment results in direct cos­ts for new radi­ant tubes, as well as indi­rect cos­ts due to the lost pro­duc­tion capa­ci­ties. The increase in com­po­nent ser­vice life thus repres­ents a signi­fi­cant opti­miza­ti­on poten­ti­al for plant operators.

Figu­re 1: Dou­ble-P-type radi­ant tubes at the end of their ser­vice life

The aim of the rese­arch pro­ject is to inves­ti­ga­te the influence of fle­xi­ble fur­nace ope­ra­ti­on on the ser­vice life of metal­lic high-tem­pe­ra­tu­re com­pon­ents. In the cour­se of the pro­ject, a mathe­ma­ti­cal model will be crea­ted to descri­be the creep defor­ma­ti­on and mate­ri­al dama­ge of sel­ec­ted mate­ri­als, which will sub­se­quent­ly be used in nume­ri­cal models to cal­cu­la­te the defor­ma­ti­on of various radi­ant hea­ting tubes under the action of dif­fe­rent tem­pe­ra­tu­re cycles. A novel, mecha­nism-based creep model is being deve­lo­ped with expe­ri­ments on uniaxi­al­ly loa­ded creep spe­ci­mens at the Insti­tu­te for Mate­ri­als Sci­ence (IfW) at TU Darm­stadt. The rese­arch insti­tu­te OWI Sci­ence for Fuels gGmbH (OWI) is inves­ti­ga­ting the chro­mi­um eva­po­ra­ti­on rate of mate­ri­al samples under com­bus­ti­on atmo­sphe­res in cor­ro­si­on tests, with which the mate­ri­al model is being exten­ded to include a term for mate­ri­al dama­ge. Also, the expe­ri­ments are trans­fer­red here to a more com­plex stress situa­ti­on in the form of beam spe­ci­mens sup­port­ed on one side in long-term tests.

At the Depart­ment for Indus­tri­al Fur­naces and Heat Engi­nee­ring, the expe­ri­ments are being exten­ded to sin­gle-ended radi­ant tubes (SER) as an exam­p­le of high-tem­pe­ra­tu­re com­pon­ents. For this pur­po­se, a new test rig (Figu­re 2) is being set up for the pro­ject, in which radi­ant tubes are loa­ded with dif­fe­rent tem­pe­ra­tu­re chan­ges in long-term tests. The set­up is equip­ped with pro­cess con­trol tech­no­lo­gy based on indus­tri­al stan­dards. A modu­lar design allows the instal­la­ti­on of dif­fe­rent radi­ant tube geo­me­tries and thus enables long-term use of the test rig. After the tests, the tubes are mea­su­red three-dimen­sio­nal­ly to eva­lua­te the defor­ma­ti­on of the tubes.

Figu­re 2: Modu­lar test rig with two sin­gle-ended radi­ant tubes

Nume­ri­cal models deve­lo­ped during the pro­ject and vali­da­ted with the test results of all pro­ject part­ners, enable the fin­dings from the expe­ri­ments to be trans­fer­red to more advan­ced load cases. A two-stage simu­la­ti­on pro­cess is fol­lo­wed for the cal­cu­la­ti­on of the radi­ant tube life­time: Nume­ri­cal flow simu­la­ti­ons are used to cal­cu­la­te the tran­si­ent tem­pe­ra­tu­re dis­tri­bu­ti­on of a radi­ant hea­ting tube (Figu­re 3). The results are then trans­fer­red to struc­tu­ral simu­la­ti­ons in which the tran­si­ent defor­ma­ti­on of the radi­ant hea­ting tube is cal­cu­la­ted using the deve­lo­ped mate­ri­al model.

Figu­re 3: Exem­pla­ry results from a tran­si­ent nume­ri­cal simu­la­ti­on: SER tem­pe­ra­tu­re pro­files during bur­ner on-off-firing

Final­ly, the nume­ri­cal models are used to set up inves­ti­ga­ti­ons for various indus­tri­al appli­ca­ti­on sce­na­ri­os. The resul­ting com­pre­hen­si­ve data­ba­se on com­mon high-tem­pe­ra­tu­re mate­ri­als and radi­ant hea­ting tubes enables plant ope­ra­tors to opti­mi­ze the ope­ra­ti­on of their plants with regard to the ser­vice life of instal­led components.