Thermal Fatigue | Materials And Corrosion Control

Thermal fatigue cracks usually initiate on the surface of the component and are generally wide and often filled with oxides due to elevated temperature exposure. Cracks may occur as single or multiple cracks. They propagate transverse to the stress and they are usually dagger-shaped, transgranular, and oxide filled. However, cracking may be axial or circumferential, or both, at the same location.

·         Key factors affecting thermal fatigue are the magnitude of the temperature swing and the frequency (number of cycles).

·         Time to failure is a function of the magnitude of the stress and the number of cycles and decreases with increasing stress and increasing cycles.

·         Startup and shutdown of equipment increase the susceptibility to thermal fatigue. There is no set limit on temperature swings; however, as a practical rule, cracking may be suspected if the temperature swing exceeds about 200°F (93°C).

·         Damage is also promoted by rapid changes in surface temperature that result in a thermal gradient through the thickness or along the length of a component. For example: cold water on a hot tube (thermal shock); rigid attachments and a smaller temperature differential; inflexibility to accommodate differential expansion.

All materials of construction

Equipment:

o   Mix points of hot and cold streams such as locations where condensate comes in contact with steam systems, such as
de-superheating or attemporating equipment.

o   Coke drum shells and coke drum skirts where stresses are promoted by a variation in temperature between the drum and skirt.

o   Notches (such as the toe of a weld) and sharp corners (such as the intersection of a nozzle with a vessel shell) and other stress concentrations may serve as initiation sites.

o   In steam generating equipment, cracking occurs:

§  At the toe of the fillet weld, as the change in section thickness creates a stress raiser.

§  At the ends of an attachment lug and if there is a bending moment as a result of the constraint (circumferential cracks)

§  At rigid attachments between neighboring tubes in the superheater and reheater. Slip spacers designed to accommodate relative movement may become frozen and act as a rigid attachment when plugged with fly ash.

§  In tubes in the high temperature superheater or reheater that penetrate through the cooler water wall tubes at the header connection if the tube is not sufficiently flexible. These cracks are most common at the end where the expansion of the header relative to the waterwall will be greatest.

§  At steam actuated soot blowers due to rapid cooling of the tube if the first steam exiting the soot blower nozzle contains condensate. Similarly, water lancing or water cannon use on waterwall tubes may have the same effect.

o   Designs that incorporate reduction of stress concentrators, blend grinding of weld profiles, and smooth transitions Controlled rates of heating and cooling during startup and shutdown of equipment to lower stresses.

o   Considering differential thermal expansion between adjoining components of dissimilar materials

·         Designs should incorporate sufficient flexibility to accommodate differential expansion.

o   In steam generating equipment, slip spacers should slip and rigid attachments should be avoided.

o   Drain lines should be provided on soot-blowers to prevent condensate in the first portion of the soot blowing cycle.

·         In some cases, a liner or sleeve may be installed to prevent a colder liquid from contacting the hotter pressure boundary wall

·         External SWUT inspection can be used for non-intrusive inspection for internal cracking and where reinforcing pads prevent nozzle examination.

·         Heavy wall reactor internal attachment welds can be inspected using specialized ultrasonic techniques.