Posted by: Jay_Stress
Natural Frequency basis - 09/20/18 12:36 PM
Hi
As per our Design basis, piping system carrying two phase fluid is to be designed to have a natural frequency of 4 Hz or more. can anyone tell me the basis of this 4Hz figure?
Regards
JP
Posted by: Dave Diehl
Re: Natural Frequency basis - 09/20/18 02:48 PM
You will see such limits in the Energy Institute's Piping Vibrations Guidelines and DNV-RP-D101. Two phase flow may excite harmonic response of lower modes of piping vibration. By staying above 4 Hz you will reduce the likelihood of such piping failures.
Posted by: anubis512
Re: Natural Frequency basis - 09/21/18 08:29 AM
The 4 Hz value is also seen in Chapter 8 of Kellogg's Design of Piping Systems.
A few observations:
The EIG calls anything 4 Hz and lower as "Flexible," and thus prone to the highest rate of failure due to the EIG.
The EIG's estimates for natural frequencies is higher than that of CAESAR's, I find. This may be due to lack of mass (fluid, insulation) in their estimations.
You should be careful when using 4 Hz as your final arbiter. It's important to understand the kind of problem you have.
By specifying 4 Hz as a minimum natural frequency, you're assuming that small slugs in your system will never exceed 4 events per second and cause a runaway condition.
However, you could have large slugs at 0.1 Hz that can still cause your system to fail or fatigue, despite not having a runaway condition.