Revamps 2026 Issue

specific zones where cross-flow velocities are exceed- ing the thresholds at which vibration becomes a concern. These zones do not always align with where people expect to find them. Understanding what is actually happening inside each shell from a two-phase flow perspective is essential and often skipped in favour of bulk thermal calculations. Slug, stratified, and annular flows all behave very differently from a pressure pulsation standpoint, and a shell that is running in slug flow regime is fundamentally more prone to inducing vibration than one running in annular or dispersed flow. Assessing the flow pattern in each individual shell, using recognised flow regime maps adapted for the specific fluid properties and geometry, helps identify which shells in a series arrangement are most at risk and provides a rational basis for deciding where interventions will have the most impact. The piping configuration is an area where problems are created during design and discovered during operation. It deserves more attention during front-end engineering than it typically receives. The straight-length requirement after bypass mixing points exists because flow entering a shell nozzle immediately after a junction is inherently non-uni- form, including swirl, asymmetric velocity distribution, and, in two-phase flow, severe phase separation at the junction itself. The switch from horizontal cut to vertical cut baffles is one of the more impactful modifications available for two-phase shell-side services Checking actual installed piping configurations against licensor requirements and, where deficiencies are found, evaluating whether any modifications are feasible within the existing layout should be part of any systematic vibra- tion investigation. CFD simulations For cases where the flow regime analysis and piping review do not conclusively identify the root cause, or where mul- tiple shells need to be evaluated and it is not clear which to prioritise, two-phase CFD modelling provides a level of insight that simplified analytical methods cannot match. Visualising where slug formation initiates, how it evolves through the shell, and where the resulting pressure pulsa- tions are most intense gives the engineering team a much more defensible basis for deciding what to change and where. CFD in this context is not a theoretical exercise; it directly informs practical decisions about baffle modifica- tions, nozzle repositioning, or inlet device additions. Baffle modifications The switch from horizontal cut to vertical cut baffles is one of the more impactful modifications available for two-phase

shell-side services and is worth discussing in some detail. Horizontal cut baffles, with cuts at the top and bottom of the shell, create a flow path where liquid and vapour nat- urally separate. Liquid drains to the low side and vapour accumulates at the top. In a two-phase service, this segre- gation is not just inefficient from a heat transfer standpoint; it is mechanically problematic because the alternating liquid and vapour slugs passing over the tube bundle create cycli- cal loading on the tubes and baffles. Vertical cut baffles, with the cut on the left and right sides of the shell, force the flow across the bundle in a direction that tends to keep the two phases better mixed. The result is more uniform tube loading, reduced pulsation amplitude, and meaningfully lower vibration risk. The trade-off has some impact on LMTD and thermal efficiency. However, in high-pressure hydrogen service, that is a trade most oper- ators are willing to make. Mechanical anti-vibration measures For exchangers that are being retubed or replaced, there are several design features worth incorporating specifically to improve vibration resistance. Removing a small num- ber of tube rows near the baffle tips, where unsupported tube spans are longest and vibration amplitudes highest, reduces the risk of those tubes becoming the weak point. Adding de-resonating baffles, which are intermediate supports placed specifically to change the natural fre- quency of the tube bundle and avoid resonance with the excitation frequency of the flow, is another measure that has proven effective in practice. Strengthening tube-to- tubesheet joints and ensuring that baffle-to-shell fits are tight enough to prevent baffle rattle without being so tight as to prevent thermal expansion are details that matter more in this service than in lower-pressure, lower-temper- ature applications. Case 3: Fouling and plugging in crude preheat train exchangers If there is one problem that every refinery engineer has dealt with at some point, it is fouling in the crude preheat train. It does not announce itself dramatically. There is no sudden trip, no visible leak, no alarm in the middle of the night. It creeps in slowly, and by the time the numbers are telling a clear story, the damage to heat recovery is already significant. The crude preheat train exists to recover heat from hot product and reflux streams leaving the atmospheric dis- tillation column, using that energy to preheat incoming crude before it enters the fired heater. A well-performing preheat train reduces heater duty and cuts fuel consump- tion directly. When fouling takes hold, that heat recovery deteriorates, the heater compensates by burning more fuel, and operating costs climb steadily every day until the next turnaround. Case background The fouling problem presented in a way entirely typical of crude preheat fouling: a gradual but persistent decline in crude outlet temperature from the preheat train over the course of an operating run. At the start of the run (SOR),

Revamps 2026