4 replies to this topic

[Raj Mehta]

When should one use 30 degree tube layout and when 60 deg tube layout ? We gave 60 deg tube layout to one of our client and during commissioning encountered a lot of vibration in the surface condenser. The reason highlighted later was the orientation of tube layout. 

 

Can someone please help to distinguish both the tube layouts with their pros and cons which one should consider while designing. Also, How would tube pitch contribute to vibrations in either of the tube layout (30 deg & 60 deg)

 

Thanking you, 

Edited by Raj Mehta, 17 April 2013 - 09:25 PM.

[srfish]

TEMA has a section on flow induced vibration. In section V-10 they have an  equation for an estimate for the critical velocity. This velocity if exceeded would cause too large of tubing flutter. The result of the calculation will give a higher critical velocity for the 30 degree tube layout. 

[Raj Mehta]

So which one is better & safer ? 30 degree or 60 degree ? Also i guess the inlet pattern / orientation is crucial here. am i right ? 

 

Thanks

[Padmakar Katre]

So which one is better & safer ? 30 degree or 60 degree ? Also i guess the inlet pattern / orientation is crucial here. am i right ? 

 

Thanks

 

Hi,

The answer is there in Mr. srfish's reply. when the calculated critical velocity is on higher side, means the ratio of actual to critical velocity is lower and your equipments is out of that vibrational issue. So 30 deg layout is better.

I will advice you to visit the below website,

 

http://www.gulleyassociates.com/

 

Very helpful and lot of good information available.

[chemsac2]

Raj,

 

Fluid elastic instability is seen when FEI ratio is more than 0.8. FEI ratio which is ratio of critical and cross-flow velocities is higher for 60 deg layout compared to 30 deg layout for a given pitch ratio. This is due to lower fluid instability constant for 60 deg layout. Refer attachment for formula and graph showing value of beta.

 

Apart from fluid elastic instability, tube pitch also affects vortex shedding frequency. Exact effect depends on tube natural frequency. Refer attachment.

 

I have not seen 60-deg layout being used. Have read somewhere that 60 deg layout is helpful in avoiding acoustic vibration when everything else fails.

 

Pitch ratio is something that is not used for getting rid of vibration as it leads to increased cost. Baffle spacing and baffle type are preferred methods.

 

Regards,

 

Sachin

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