Caesar II makes snubbers active for dynamic analysis. Modal analysis suggests that addition of snubbers result in higher natural frequency of the piping system.

In reality snubbers will lock up when the force acts quickly on piping, such as slug loads.

My question is whether use of snubbers is a legitimate way to stiffen up piping system subject to vibrations, for example, due to a two phase flow. Will snubbers be effectively locked up in such cases? Caesar II seems to suggest so.

In cases of runaway vibrations, theoretically, it could. But I'm not sure what would be a good metric for sizing such an animal if you were going to design for such a case. There might be a rule of thumb somewhere out there.

Hello fellow stressers,

I'm also interested to know from our fellow stressers if someone have specified a snubber for a two phase flow system and worked successfully in the field. For my 20 years as stress engineer I haven't specified one. I have done one vibration problem in the field related to two phase flow and fortunately a conventional hold down guide does suppress the low frequency vibration of around 3 Hz (a typical resonance problem). I just put a gap on the lateral direction since it is a high temperature piping and have a big thermal movement in the lateral direction. Fortunately the mode of vibration is on the vertical direction of the elbow (Caesar II mode shape matches the one observed in the field at approximately 3 Hz) and the thermal load is going down so a hold down guide does the trick since there is no upward thermal movement to absorb.


Warm Regards,

With two phase flow, you have a spectrum of flow conditions, based on relative velocities.

1. Mist flow = "high" gas rate, "low" liquid rate
2. Annular flow = "high" gas rate, "high" liquid rate
3. Stratified flow = "low" gas rate, "low" liquid rate, no vertical sections in the line
4. Bubbly flow = "high" liquid rate, "low" gas rate
5. Slug/plug flow = Stratified flow with vertical sections, or at "medium" combinations.

For Mist and Bubbly flow, there are generally no "two phase flow" concerns. They can largely be treated as Gas and Liquid systems accordingly, with the understanding that you can still end up with erosion or cavitation issues. However, I do not classify this as a "two phase flow" problem - just a linespeed problem.

For (steady) stratified flow, there generally should not be any perturbations. This is analogous to the pipes and culverts under the streets carrying rain water.

For annular flow, my gut feel are that vibrations are going to be proportional to the dynamic pressure of the system - i.e. 1/2 density times velocity squared. In a perfectly annular system, you would have a perfectly hollow cylinder of liquid along the walls of the pipe, and a gas cylinder down the annulus of the pipe. The less perfect this flow, the more turbulent waves exist along the radius of the interface. But ultimately, annular flow results in random noise, which I've seen typically dealt with by anchoring the pipe down, everywhere.

I've never seen an empirical method for estimating slug/plug flow size and velocity, though my understanding is that there exist software packages that purport to do so. It would be this impulse load and only this impulse load I would consider designing against, and therefore adding snubbers for.

Thanks Michael. Your approach make sense to me. Maybe if I experience a high temperature system with large slug/plug flow, I may use this snubber in case there is a constraint in re-routing the line. Also, If I remember right my senior stress engineer before have a rule of thumb regarding the percent liquid present in the mixture to consider as vibrating line where the 4 Hz can be used to check if the system has enough restraints as a countermeasure. But of course, it's hard to predict the actual frequency of the two phase system in design stage. Maybe this 4 Hz value was established through years of experience in the industry where most of the vibration frequency occurs. In my case I have experienced the 3 Hz frequency of vibration of two phase system.


Warm Regards,

I have little opinion aside from speculation as to the history of 4 Hz, but I know it's a mandated requirement for specific lines, which are generally understood to be capable of 2 phase flow.