Why do outside tubes of an exchanger suffer fretting the most
Why do outside tubes of an exchanger suffer fretting the most
(OP)
Well I am doing another anlysis on why some Type 304 stainless steel tubes failed by fretting in one of our exchangers. One of the things I noticed with the tubes located on the outrside perimeter of the tube bundle is that they tend to fail mainly by fretting. I know fretting in exhcnager tubes is usually a result of vibrations by high velocities, but can't understand why the outside tubes are the most vulnerable.





RE: Why do outside tubes of an exchanger suffer fretting the most
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RE: Why do outside tubes of an exchanger suffer fretting the most
Are the outer tubes located physically closer to one another? Does the shell-side flow path cause high-velocity streams to impinge in crossflow or swirling flow on the tube bundle (outer tubes may bear the brunt of such if it is so)? Is the fretting occuring at baffles, and are the baffles secured against motion?
RE: Why do outside tubes of an exchanger suffer fretting the most
Second comment - I have seen on occasion where support plate holes are not sized properly in one part of the tube support plate(s), and it is possible this could be the case.
RE: Why do outside tubes of an exchanger suffer fretting the most
If you ever see this the check list is short.
1. is there any possibility of corrosion of the support plates? Make them out of the correct material.
2. are the flow diffusers adequate?
3. Were the vibration calculations rigorous enough?
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Plymouth Tube
RE: Why do outside tubes of an exchanger suffer fretting the most
If it is a non-condensing application, then the velocity is ever decreasing as heat is transferred from the gas stream as it penetrates the bundle, result being that the velocity is highest in the outer rows.
rmw
RE: Why do outside tubes of an exchanger suffer fretting the most
If your heat exchanger tube has liquid flows, then the surface corrosion you are seeing is more likely due to cavitation and not fretting. The cavitation pitting occurs when the local surface fluid's temperature and pressure are such that boiling occurs. As the vapor bubbles rapidly expand and collapse on the tube surface, very high pressures can result. If the pressure are high enough and/or occur enough times, the tube surface will eventually fail in compressive fatigue. The result of which is the surface pitting (or more correctly "spalling") that you see.
This same cavitation erosion is commonly seen on the liquid cooled surfaces of diesel engine cylinder liners. It is understood that the fluid cavitation is mainly due to the cylinder liner wall vibrating from internal pressure spikes each time that cylinder fires.
Hope that helps. I know a bit about engines, but not much about heat exchangers.
Terry
RE: Why do outside tubes of an exchanger suffer fretting the most
RE: Why do outside tubes of an exchanger suffer fretting the most
"Scientists dream about doing great things. Engineers do them." -James Michener
RE: Why do outside tubes of an exchanger suffer fretting the most
The shells side is a naptha/steam vapor mix, and the tube side is water. The purpose of the exchanger is to warm up water. There is an impingiment plate at the vapor entrance but it only covers a certain area. I've been told by one of the other engineers that the vapor enters at a rate of 300 ft/s and so given that the failure occurs mainly on the peripheral tubes, I believe that this high velocity is playing a role on the fretting wear observed (not corrosion just wear)
RE: Why do outside tubes of an exchanger suffer fretting the most
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Plymouth Tube
RE: Why do outside tubes of an exchanger suffer fretting the most
Very few (if any) impingement plates that I have seen on the original design were adequate for the service.
rmw