Engineering task and calculation objective
The GVLV module performs the flow-induced vibration analysis for tube bundles of shell-and-tube heat exchangers. It is driven in the background by the design module WTS and cannot be called separately; the results are passed to WTS and documented there. The analysis examines whether the shell-side cross flow over the tubes can excite flow-induced vibrations.
Flow-induced tube vibrations are one of the most frequent causes of damage in shell-and-tube equipment: fluidelastic instability, vortex shedding and turbulent excitation lead to tube failures at the baffle penetrations, to wear in the baffle holes and to leaks at the tube-to-tubesheet expansion. The analysis therefore compares the natural frequencies of the tubes – depending on span lengths, tube geometry and added mass from the fluid – with the excitation mechanisms of the cross flow, checking in particular the critical velocity for fluidelastic instability.
Excluded from the scope of calculation are exchangers with two shell-side passes and a longitudinal baffle (TEMA type F), bundles with disc and doughnut baffles, and tubes with low external fins.
Calculation workflow
- Data transfer from WTS: Geometry and operating data – tube dimensions, tube pitch, baffle spacings, shell-side flow velocities and fluid properties – are taken over from the thermal-hydraulic design in WTS; no separate input is required.
- Determine the natural frequencies of the tubes: The tubes are treated as multi-span beams with the baffles as supports. The natural frequency depends on the bending stiffness, the tube mass including the fluid contents and the vibrating hydrodynamic mass of the outside medium; the largest span lengths – often in the window zones – are governing.
- Evaluate the excitation mechanisms: For the governing cross-flow velocities, the excitation types are checked: fluidelastic instability (critical velocity according to the stability criterion), vortex shedding with resonance check against the Strouhal frequency, and turbulent buffeting excitation; for gases additionally the acoustic resonance of the shell-side space.
- Perform the verification: The actual velocities are compared with the critical values and the frequency margins are assessed. The results are returned to WTS and appear there in the documentation.
- Derive design consequences: If limits are exceeded, typical remedies are: smaller baffle spacings or intermediate supports, a modified tube pitch, impingement protection at the inlet nozzle, larger nozzles or a modified baffle geometry – followed by a re-evaluation.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Outside diameter of shell | Da | m |
| Shell outside diameter lengthwise tubes | Da,y | m |
| Inside diameter of shell | Di | m |
| Shell inside diameter lengthwise tubes | Di,y | m |
| Diameter of expansion joint | DK | m |
| Outside diameter of tubes | da | m |
| Inside diameter of tubes | di | m |
| Tube length between tubesheets | L | m |
| Thickness of front tube sheet | s | m |
| Thickness of rear tube sheet | s | m |
| Type of construction | Wärmeübertragers | - |
| Diameter | DU | m |
| Diameter lengthwise tubes | DU,y | m |
| Number | nU | - |
| Plate thickness | sU | m |
| Central baffle spacing | Umlenkbleche | m |
| Inlet baffle spacing | Umlenkblech | m |
| Height of baffle cut | H | m |
| Diameter of bores | dB | m |
| Inside diameter of inlet nozzle | Ds,e | m |
| Inside diameter inlet nozzle lengthwise tubes | Ds,e,y | m |
| Inside diameter of outlet nozzle | Ds,a | m |
| Inside diameter outlet nozzle lengthwise tubes | Ds,a,y | m |
| Clear distance tubesheet - nozzle center inlet | vorne | m |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Number of analysed resonant frequencies | i | - |
| Instabilität | Instabilität | - |
| Instabilität | Instabilität | - |
| Instabilität | Instabilität | - |
| Resonant frequency of tube row with the highest degree of danger K* | Instabilität | 1/s |
| Instabilität | Instabilität | % |
| Instabilität | Instabilität | % |
| Instabilität | Instabilität | % |
| Maximum critical gap velocity | Instabilität | m/s |
| Critical inlet volume flow | Instabilität | m³/h |
| Instabilität | Instabilität | - |
| Anregung | Anregung | - |
| Instabilität | Instabilität | - |
| Anregung | Anregung | - |
| Anregung | Anregung | - |
| Anregung | Anregung | - |
| Anregung | Anregung | % |
| Anregung | Anregung | % |
| Anregung | Anregung | % |
| Anregung | Anregung | % |
| Anregung | Anregung | % |
| Anregung | Anregung | % |
| Strouhal Number 1 | 1 | - |
| Strouhal Number 2 | 2 | - |
Calculation options
Type of construction
Without expansion joint · With expansion joint · Floating head · Gland flange · U-Tubes
Position of outlet nozzle
at the bottom · at the top
State of matter
liquid · 1
Heat exchanger type
Tube bundle heat exchanger (cylindrical shell) · Cross-flow heat exchanger (rectangular shell)
Frequently asked questions
Why is fluidelastic instability the most critical excitation mechanism?
Above the critical cross-flow velocity, the tube motions couple through the flow field in a self-amplifying way: the vibration amplitudes no longer stay bounded but build up until the tubes strike each other. Unlike vortex resonance, there is no "passing through" the critical range – the design must ensure that the critical velocity is not exceeded anywhere in the bundle.
Why are TEMA F exchangers and disc and doughnut baffles excluded?
In TEMA type F, a longitudinal baffle divides the shell side, creating leakage flows across the baffle and asymmetric velocity fields that are not represented by the standard flow models. Disc and doughnut baffles produce a radially directed flow pattern for which the usual cross-flow correlations of the vibration analysis do not apply. These configurations require a separate assessment.
Which locations in the bundle are most vibration-prone?
The tube spans with the largest span lengths and the highest cross-flow velocities: the window zones of the baffles, the U-bends of U-tube bundles (large free length, low stiffness) and the region below the inlet nozzle, where the approach velocity is highest. This is typically also where damage starts.
What does acoustic resonance mean in gas-flowed shell sides?
In gas flows, standing sound waves can form transverse to the shell. If their natural frequency coincides with the vortex shedding frequency at the tubes, an intense howling tone with high sound pressure levels develops, additionally loading baffles and tubes. The remedy is detuning baffles installed parallel to the flow direction.