Andy Mountford, Senior Technical Lead, Training

A TEMA AES exchanger with carbon steel tubes produces 7.5 bar steam on the shell side with a weight fraction vapor of 0.3 at the shell exit (Figure 1). The operator has seen no evidence of tube damage in the operating unit, but the Xist vibration analysis flags fluidelastic instability (FEI) at the bundle and shell exit, specifically with an FEI ratio of 2.34.

Figure 1. Geometry of steam generator and region of vibration concern
Figure 1. Geometry of steam generator and region of vibration concern

HTRI recommends limiting this ratio to 0.8 in Xist to avoid possible FEI tube vibration damage. Values above unity indicate that tube damage is probable. Accordingly, the Xist Vibration Analysis report flags the FEI ratio and bundle exit crossflow velocity as excessive (with asterisks), and the shell exit velocity is also flagged (Figure 2).

Figure 2. Bundle and shell entrance/exit sections of the Xist Vibration Analysis report
Figure 2. Bundle and shell entrance/exit sections of the Xist Vibration Analysis report

A common “fix” to eliminate shell and bundle exit vibration is to add a support at the outlet region and/or remove tuberows to increase the flow area. However, these measures would prove costly for an existing unit and may ultimately be unnecessary if the operator can use Xvib to examine and discount the vibration concerns

The evaluation starts with the creation of an Xvib case from the Xist 2D Tube Layout drawing for a tube located in the shell and bundle exit region (Figure 3).

Figure 3. Selected tube for Xvib analysis
Figure 3. Selected tube for Xvib analysis

The Xist shell exit velocity of 13.58 m/s is manually specified in the outlet span of the Xvib model. The Xvib results show FEI ratios in the range 0.19 – 0.29 (Figure 4), which are well below unity, indicative of a low vibration potential for the selected tube, thereby confirming that the Xist vibration analysis of the exit region is excessively conservative in this case.

Figure 4. Results of Xvib analysis for selected tube
Figure 4. Results of Xvib analysis for selected tube

The conservatism in the Xist analysis arises because the shell and bundle exit velocities are applied to the longest span in the outlet region, a span in the baffle window, yet tubes in the baffle window are far from the shell and bundle exit areas in this case, as shown in Figure 5, and are therefore not subjected to high exit velocities.

Figure 5. The Xist analysis is based on the longest span in the outlet region (indicated)
Figure 5. The Xist analysis is based on the longest span in the outlet region (indicated)

Use of the longest outlet span in the Xist analysis produces a critical velocity one order of magnitude lower than the critical velocity associated with the tubes immediately under the outlet nozzle, which are more rigidly supported. Xvib accurately calculates the natural frequencies, and critical velocities, of selected tubes from anywhere in the bundle, applying a rigorous modal weighting that ensures a more precise vibration assessment. In this case, the Xvib analysis confirms that costly remedial measures are not warranted.

Read the corresponding article, Goodbye to bad vibes: A comparison of HTRI's tube vibration analysis tools, for some key differences between Xist and Xvib.