Vortex Shedding and Tube Frequency Assumptions
Vortex Shedding and Tube Frequency Assumptions
(OP)
A flow induced vibration analysis of a steam surface condenser uses the following methods to determine the propensity to vibrate:
1) how close the tube structural frequencies lie to the vortex shedding frequency
2) the maximum tube vibration amplitude at resonance
3) Connor's fluid elastic instability gap assessment
My question is, how are these assessments affected by the presence of single phase water droplets in the steam vapour? Could the presence of single phase drops change vortex shedding or structural frequencies?
1) how close the tube structural frequencies lie to the vortex shedding frequency
2) the maximum tube vibration amplitude at resonance
3) Connor's fluid elastic instability gap assessment
My question is, how are these assessments affected by the presence of single phase water droplets in the steam vapour? Could the presence of single phase drops change vortex shedding or structural frequencies?





RE: Vortex Shedding and Tube Frequency Assumptions
The general rule is that the forcing frequency must be outside +/- 20% of the tube natural frequency, for single tubes.
If you are dealing with tube bundles you have to deal with self-excited, fluid-elastic motion of the bundle. This is a non-linear effect and depends on the deflection amplitudes. ASME N1300 is a good source and is covered in texts that deal with flow-induced vibration.
RE: Vortex Shedding and Tube Frequency Assumptions
RE: Vortex Shedding and Tube Frequency Assumptions
there is a considerble body of work dealing with the problem, what standard is used for design: Heat Exchanger Inst, ASME, proprietary, etc.?
RE: Vortex Shedding and Tube Frequency Assumptions
Among six similar condensers, only one tube bundle contains core tubes that are fretting at supports and, coincidentally, the same one has been running in both spray and tube condensing mode, while the others only in tube condensing mode.
I think there must be a correlation. Do you know of any work dealing with condensing vapour quality and FIV?
RE: Vortex Shedding and Tube Frequency Assumptions
direct spray on the tube bundle can mean several things.
if the spray was introduced upstream, and well mixed before entering the hex, ok you have "two phase", but if liquid is being directed against the tubes directly, the governing condition involves liquid phase at high velocity
most heat exchangers I've worked with did not use spray injection and in fact required premixing and baffle plates to prevent direct impingement
look at the spray nozzles, estimate the liquid phase exit velocity, identify if the spray nozzles are functioning and properly placed
simply jetting high velocity spray against the tubes without proper mixing is going to create all sorts of issues
vortex shedding in a liquid phase is a bit more complex than gas or vapor phase, not only because of the increased desity, but also because there is a tendency for the sheeding process to occur at a fraction of velocity normally associated with the strouhal condition
RE: Vortex Shedding and Tube Frequency Assumptions
The fundamental single tube frequency is 60 hz and the highest vortex shedding frequency (according to the Manufacturer) is 1/2 that (30hz). So either the liquid phase is lowering structural response or raising shedding frequency - or both.
Could this happen and why would it affect only the coldest, first pass coils deep inside the bundle?
RE: Vortex Shedding and Tube Frequency Assumptions
The damage described suggest that flow induced resonance is occurring at a lower velocity than predicted by the vendors calculation. Could be a number of reasons including the actual construction
RE: Vortex Shedding and Tube Frequency Assumptions
RE: Vortex Shedding and Tube Frequency Assumptions
RE: Vortex Shedding and Tube Frequency Assumptions
1) flow is in vapour phase, droplets are gravimetrically separated from the vapour
2) peripheral tubes are under the most severe conditions because as the vapour penetrates into the tube bundle, a portion of it condenses, reducing its velocity, the remainder moves upward
3) the weight of the film condensate on tube surface per inch length is not included in equivalent tube weight
The effective weight of the tube per unit length is the sum of contained fluid, the metal (4.6 x more) and displaced fluid (6 x less).
RE: Vortex Shedding and Tube Frequency Assumptions
Rather than droplet entrainment, could the result be an extremely dry, high quality vapour that arrives, with high velocity at the inner cold coils, driving the vortex shedding frequency up to the first fundamental for the tube spans just in that area of the coils?
RE: Vortex Shedding and Tube Frequency Assumptions
RE: Vortex Shedding and Tube Frequency Assumptions
Perhaps having too much condensing capacity and really coarse controls could lead to transient coil flooding? Could this lead to a 'steam cracking' phenomenon where plumes of vapour rise between core tubes that are coated with a thick descendiing film of condensate and rapid collapse sends shock waves through the tube bundle?
RE: Vortex Shedding and Tube Frequency Assumptions
good luck
RE: Vortex Shedding and Tube Frequency Assumptions
If I understood the mechanism, then I could set limits on acceptable leakage past spray and coil control valves in all the other condensers. Running on sprays with leaky coil flow or running on coils with leaky spray flow could cause the same fretting degradation.