Apr-2024

Solving a tube failure problem due to flow stratification using the Xtend tube-insert

A vacuum heater located at a major oil refinery experienced tube failures at the top of the radiant section, and inspection showed a substantial layer of coke in the top half of the tubes at the point of failure.

Kshitij Deshmukh
XRG Technologies

Article Summary

It was conjectured  by site subject matter experts that there is a high probability of flow stratification in these tubes, leading to uneven heating and coking. Other hypotheses failed to account for overheating at the top of the tubes rather than the flame-facing bottom of the tubes. Figure 1 shows a typical crack in the top of one of the tubes.

Flow stratification in the tube (where liquid remains on the bottom of the tube and vapour on top) would account for significantly different heat transfer coefficients between the lower and upper surface of the tube and potential overheating on the top surface. Additionally, the vaporized and overheated fraction of the flow would account for tube degradation and coke formation on the top surface of the tube.

XRG Technologies used multiphase CFD simulations to analyze the problem. The first step was to reproduce the field observations. The CFD model consisted of flue gas flow around the tubes, tube metal thickness, and multiphase process flow inside the tubes. The CFD results showed flow stratification in the tubes and the resulting change in heat transfer resulted in the overheated top surface of the tube, consistent with the field observations.

The second part of the CFD study demonstrated the effect of using Xtend inserts in the same tubes to reduce the flow stratification and the resulting temperature increase. XRG Technologies developed the Xtend tube-insert specifically to counter flow stratification and increase the heat transfer coefficient inside tubes. Figure 2 shows several sections of inserts linked together.

Figure 3 shows the liquid volume fraction at the tube surface. The current field installation on the left shows fluid stratification with mostly vapour on the top while modeling of the Xtend inserts on the right shows the inside tube surface is wetted by the liquid phase as the flow is swirled by the Xtend insert.

Figure 4 shows tube metal temperature profile in the field installation on the left and with Xtend inserts on the right. The overheating of the top surface of the tube, as seen in the field, due to flow stratification is alleviated by the Xtend inserts creation of a  swirling flow that  prevents stratification.

The swirling of the flow by the Xtend inserts prevents the overheating of the top surface of the tube as it has a more uniformly liquid wetted surface compared to a tube without the insert. This is shown in Figure 5 at the exit of the tube.

The Xtend inserts can increase pressure drop for the process flow, but the magnitude was lower than expected. In this instance, only 50% of the allowable  pressure drop increase was used to achieve an acceptable result.

Based on these modeling results, the site is now planning installation of the Xceed inserts during an upcoming turnaround on this heater. 

The Xtend inserts can improve operating economics by preventing tube ruptures from flow stratification, extending tube life, reducing coking, lowering  TMTs and reducing downtime.