Dear Expert,

I am working with vibration issue in existing vertical degassing boot vessel, crude inlet piping and vapour gas outlet piping. Please note that the existing crude inlet piping supports (Guide, limit stop and U-clamp support) are damaged due to vibration caused by (Two phase flow) slug. Since these areas are weak points.

Kindly refer the attached schematic drawing for the existing degassing boot crude inlet line and vapour outlet piping arrangement connecting on top of tank.

Here i have observed that the entire vertical degassing boot vessel, crude inlet piping (Three raisers with one single nozzle) and vapour outlet piping is under critical vibration. The degassing boot/tank bottom ends are fixed independently with heavy anchor bolts and foundations,
The boot height is around 18 meters from the ground level. I have noted the vibration at site, whenever slug passing to degassing boot, the entire setup is vibrating. Hence the vessel is flexible and allowing to move here and there (No support restrain for boot vessel, except bottom anchor of vessel), also the vibration is transfer to vapour outlet piping.

Kindly advise how to stop the entire vibration with below options,

1. Degassing boot vessel shall be supported properly with
intermittent guide arrangement or.

2. Crude inlet piping and vapour outlet piping shall be supported properly with additional guide,
limit stops and hold-down supports.

Please share your experience and explore more passible solution to move forward.

By
Rajivgandhi

Process conditions: Unknown.
Slug velocities: Unknown.
Slug frequency: Unknown.
Slug reaction: Unknown.
Thicknesses: Unknown.
Materials: Unknown.
Lengths: Unknown.

There will be a limit to the veracity of any specific advice to be offered, here.

General advice, however...

A low natural frequency of the pipe in the direction of pipe and turns will have deleterious effects due to slug reactions, and vibrations due to slug reactions will be aligned with pipes and turns.

This is different from flow induced vibrations, which are aligned with perpendicular directions of the pipe.

Dear Expert,

Process conditions: 93 Degree Celsius design temp. and 1960 Kpa design pressure.

Slug velocities: 8 m/s

Slug frequency: 3 Hz

Slug reaction: 11027 N

Thicknesses: 9.52 mm

Materials: API 5L Gr. B

Lengths: approximately 75 meters

Please note that the issue is 18 m height of boot is vibrating with the influence of slug is passing through 20 inch small crude inlet pipe and it is vibrating the entire big boot vessel, is it correct.

Kindly advise supporting the pipe is good or support the vessel is good.

By
Rajivgandhi

My expectation is that an improper process design has resulted in a gas line carrying slug loads, and the expectation is that the stress analyst can offer a solution that permits the process designer's mistake to go uncorrected, which is likely a tall order - maybe impossible.

Please be aware that we cannot offer actionable advice or solution - you have to perform the analysis or pay someone to do so on your behalf. It is not a straightforward yes or no answer.

Michael's observation is true. Whenever, a vibration happens at the site, a stress engineer is the first one who is always called upon even without proper root cause analysis.

I believe, it's always a good start to find the root cause before anything is done on the piping side.

Solving the vibration problem is a complex task. Giving it to a consultant who has vast experience in solving problems of this kind would be advisable.

Sometimes the solution is a trial and error, and it's hard to guarantee to a client that after giving the recommendation the vibration will reach its acceptable level.

As a start, you can request the vibration levels at critical points (displacement/velocity vs. frequency) and compare them to industry guidelines. Normally, it will show above the limits if severe vibration is observed.

Secondly, you can model the vibration characteristics of the piping system by performing the modal analysis (making sure that the boundary condition is as accurate as possible). In the end, you will want to avoid resonance by keeping the natural frequency of the piping system away from the excitation frequency (+/- 20%).

Another tool is performing harmonic analysis to check the stress levels at critical points. But normally, harmonic analysis will have a dramatic effect if the two frequencies are very near each other (in which amplification of vibration occurs).

If you are preparing a report, make sure that you cover all your basis. In the end, it is difficult to define the excitation frequency as it can be broadband, in which even if you increase the frequency of the piping, it may happen that it will coincide with the excitation frequency which does not occur currently (but it is just there in the system waiting to be excited).

So yes, there is no straightforward answer to a vibration problem as you need to establish firmly the source of excitation.

As a saying goes, prevention is always better than cure.

But in this case, since the problem is already there, something needs to be done anyway.

Cheers!!