Heat exchanger life extension via in-situ reconditioning
A method of in-situ reconditioning a heat exchanger includes the steps of: providing an in-service heat exchanger comprising a precipitate-strengthened alloy wherein at least one mechanical property of the heat exchanger is degraded by coarsening of the precipitate, the in-service heat exchanger containing a molten salt working heat exchange fluid; deactivating the heat exchanger from service in-situ; in a solution-annealing step, in-situ heating the heat exchanger and molten salt working heat exchange fluid contained therein to a temperature and for a time period sufficient to dissolve the coarsened precipitate; in a quenching step, flowing the molten salt working heat-exchange fluid through the heat exchanger in-situ to cool the alloy and retain a supersaturated solid solution while preventing formation of large precipitates; and in an aging step, further varying the temperature of the flowing molten salt working heat-exchange fluid to re-precipitate the dissolved precipitate.
1. A method of in-situ reconditioning a heat exchanger comprising the steps of:
a. providing an in-service heat exchanger comprising a precipitate-strengthened alloy wherein at least one mechanical property of said heat exchanger is degraded by coarsening of said precipitate, said in-service heat exchanger containing a molten salt working heat exchange fluid;
b. deactivating said heat exchanger from service in-situ;
c. in a solution-annealing step, in-situ heating said heat exchanger and molten salt working heat exchange fluid contained therein to a temperature and for a time period sufficient to dissolve said coarsened precipitate;
d. in a quenching step, flowing said molten salt working heat-exchange fluid through said heat exchanger in-situ to cool said alloy and retain a supersaturated solid solution; and
e. in an aging step, further varying the temperature of said flowing molten salt working heat-exchange fluid to re-precipitate said dissolved precipitate.
2. A method in accordance with claim 1 wherein said precipitate is a gamma-prime (γ′) precipitate.
3. A method in accordance with claim 1 wherein said solution-annealing step is carried out at a temperature in the range of 870° C. to 1150° C.
4. A method in accordance with claim 1 wherein said solution-annealing step is carried out by energizing a heating jacket.
5. A method in accordance with claim 1 wherein said quenching step is carried out at a temperature in the range of no lower than the lowest temperature at which said working fluid will remain sufficiently fluid to flow to a temperature below a working temperature of said heat exchanger.
6. A method in accordance with claim 5 wherein said temperature range is 550° C. to 650° C.
7. A method in accordance with claim 1 wherein said aging step is carried out at a temperature in the range of 600° C. to 850° C.
8. A method in accordance with claim 1 wherein said aging step is carried out at a maximum temperature no greater than a normal operating temperature of said heat-exchanger.
9. A method in accordance with claim 1 wherein said aging step further comprises flowing a power cycle fluid through said heat exchanger.
10. A method in accordance with claim 1 wherein said aging step further comprises energizing a heating jacket.
11. A method in accordance with claim 1 further comprising an additional, subsequent step of:
f. reactivating said heat exchanger to service.
12. A method in accordance with claim 1 further comprising an additional step of, after said deactivating step and prior to said solution-annealing step, draining a power cycle heat-exchange fluid from said heat exchanger.