Hi,
We have a 6 inches line connected to a heatexchanger. The line vibrates visibly. Natural frequency of the system was calculated and it is 3.34 Hz but the system fails in thermal loading, since temperature is 150 degree C. Nozzle loading also increases than designed for heatexchanger, i.e 1800 N.m.
Any help would be appreciated for arresting vibrations.

Overloaded nozzle, failed thermal stress limit, vibration .... Seems like a rubbish design to me.

If it's overloaded and over-stressed, why bother trying to control vibration ? That will most risk make the loads and stresses worse.

Surely you should be correcting the poor engineering by re-design ?

A more refined analysis for this type of situation maybe suitable...Say for the nozzle loading you can request the Mechanical group to do a detailed FEA analysis by providing them the actual loadings..But with regards to vibration problems you can request from a consultant who specialize in dynamic problems of this type. Depends if you can convince your project manager for the cost of the study...They can make a full package for this type of problems..They have tools now like the FEA tools in Caesar where the flexibility in fittings might help reduce the thermal loads.

That is the most common problem in the EPC industry..We only do static analysis during the design stage complying to the codes. But we have limited knowledge on how to tackle vibration problems (just the 4Hz criteria).. And then during start up and operation that is where vibration problems starts and then we visually check it site checking where is the direction of vibration and restraint it on that direction..But I've read in one of the consultant for this type of dynamic problems that sometimes just putting a restraint in the direction without doing a detailed dynamic analysis might worsen the problem..

Anyway, good luck!! We have also faced this type of problem which is a steady state vibration type.. As Moverz said poor design, in our case it's really poor design because they only provide one hanger support for that two phase flow system without friction holding the pipe in place. So my colleague stress guy recommended to put friction supports and guide & stopper and check the thermal loads and stress and now the vibration is minimized to an acceptable level in the eye of the operator.

So good design practice is very important. Although we cannot really predict that 100% we can solve vibration problems during design stage.

I'm open for correction in my statements above. Just sharing my thoughts..


Cheers to all!!!!!!