CFD was used to confirm that poor design of the steam inlet nozzle was the main contributor to deaerator head to shell weld cracking, and confirmed proposed design improvements. This recent Becht project illustrates one of the causes of deaerator cracking.
A through-wall crack had been found at the head-to-shell junction at the steam inlet end of the drum. This crack was attributed to corrosion fatigue, a common occurrence in deaerators. The crack was most likely initiated at a weld surface defect on the I.D. of the drum and grew with time. The daily operating cycles of the drum during periods of reduced steam demand and thermal stresses which we attributed to a poorly designed steam inlet nozzle were the main contributors to the crack growth.
A large diameter superheated steam inlet nozzle extended through the head of the drum terminating 18” into the vessel. The steam exited through a rectangular shaped slot opening on the underside of the pipe which directed the flow of superheated steam directly into the condensate on the bottom of the drum near the shell-to-head weld.
Our computational fluid dynamics (CFD) analysis showed that there was very little dispersion of the steam exiting the nozzle and that the velocity of the steam mixing with the condensate was relatively high. The superheated steam contact with the cooler condensate resulted in a violent reaction with localized heating and cooling of the vessel shell. This cycling can cause thermal stresses which can result fatigue cracking. Generally, fatigue cracking occurs at welds and heat affected zones adjacent to the weld. Notably no such weld cracking occurred at the opposite end drum where there is no steam inlet nozzle. Figures 1a and 1b demonstrate that the flow trajectory of the incoming steam acts more like a high velocity jet of steam directed at the condensate in the bottom of the drum.
To provide better dispersion, we recommended that the inlet pipe be modified with multiple and wider distributed holes. In this configuration, the nozzle acts as a sparger which better disperses the steam in all directions and at lower velocities as can be seen in Figures 2a and 2b.