Dynamic Loads in Piping Design/ Bracing Design

[Muckerj]

Hello all,

I have been an anonymous user for some time and I have always found this website full of good information. So here I am tonight joining as a registered member. Hope to have many interesting discussions with you all in the years to come.

Actually, the reason I signed up tonight was because I have some problem at work that I can't solve. It's not actually my responsibility but somehow it has landed on my lap (Lucky me). Anyway, it's related to piping design dynamic loads. Perhaps a bit of history would help......

We recently issued a Non Compliance Report against our EPC contractor concerning bracing design and installation. Our contractual reference is B31.3, NORSOK as well as Company's standard. Basically, they have not installed bracing following the principles below:
1. 2- plane bracing
2. Bracing not close to significant mass (Bending stress concern)

Anyway, while we are waiting for a response to this NCR, I have been told to identify the work scope and issue it for our EPC contractor to fix (We don't have a lot of time left on the project so every problem needs an immediate resolution). So my idea was to include all systems subject to dynamic loads because those systems could be subject to vibration. However, as I have digged deeper, the answer is not so straight forward. For the sake of clarity, I will split my questions into bracing and dynamic loads determination. I think it's important to separate the two even though they are closely related subjects

Bracing
Originally I was only looking at bracing of small bore connection- local vent and drain (cantilever). However, I have come across the Energy Institutes Guideline, and they consider pipe away connections too in addition to cantilever type. So my question is
-'Do pipe away small bore branch connections require bracings even if there are adequate supports for those pipe away lines?'
-' Should pipe size larger than small bore be braced too?'
I know in the EI guideline it outlines a quantitative approach but because of the time that I have and the fact that we are not the EPC contractor, we aren't looking to perform that kind of analysis. Instead, I am doing a qualitative analysis like I said before.

Dynamic Loads
While I am preparing a technical note for the bracing issue, I have to also assess each possible dynamic load case as outlined in the standards mentioned earlier. Basically, if certain cases are not valid, I will overlook them. Below is a list of dynamic loads that I have identified:
- PSV reaction/ flow transient
- Slug
- Water hammer/ Surge due to fast opening/ closing valves
- Acoustic fatigue
- Flow transients
- Mechanical excitation/ Pulsation due to positive displacement equipment (Group them into one because they are both related to PD type equipment)

Ok, here are my questions:
- Some of the loads such as PSV and slug load has been considered in the static stress analysis (I am aware of the limitation of stress analysis not being able to address dynamic load fully). And conclusion is that stress is within acceptable range. Does that mean vibration is not possible (My instinct is no)? If no, I guess the static analysis would not have covered vibration induced fatigue assessment either (i.e. fatigue failure due to any of the dynamic loads mentioned above)

- My gut feeling tells me that vibration assessment for any of the above dynamic loads would be along the line of determining the natural frequency of the pipework. How about for forced vibration (i.e. exciting frequency not near natural frequency), say PSV pop off which exerts a force on the pipe work, does that need to be determined too?

I know the most sensible approach to vibration would be to following EI's guideline but it doesn't appear an option so I am just after some fast and hard rules.

I hope I explain well enough. Thanks for your time and would welcome any feedback.

 

[goodguy1405]

Hello,
I would like to know if you are using any finite element analysis software? Solidworks has a great finite element analysis tool that can address these questions.

 

[saplanti]

I think you need to read the Energy Institute Guideline in detail. It has everything you need with appropriate logical diagrams.
It is a simplistic process that you can use before consulting the experts. Ones you determine the cause of the dynamic load by going through the process information, the guideline gives very simplistic approach to solve the problem. This part requires data with some experience.

I have been witnessed many times that some do not want to read anything but immediately get the answers from others. I do not know this might be the case, but I would still suggest to read the guideline.

Whatever we say about bracing it will not be as good as detailed in the guideline.

Since you do not give any information about the process and piping and your question is very general, it is difficult for others to give particular information.

In case there is a pressure reducing valve with large turn-down ratio and a large gas velocity you may suspect the valve is the cause of vibration. The guideline has an answer for this as well.
When you mention about the slug, so this might be wellhead piping or following gas gathering line before the separator. Unfortunately, the operators do not give proper information about slug occurrence, their volume, frequency and velocity. So the pipe stress engineer makes assumptions that do not meet the reality.

I guess you need to go through the entire process design information, get validation of them with the operator and use the guideline to see if you can solve it.

In case the pulsation (gas or liquid?) is an issue you need to support the piping adequately and may need to use pulsation dampener in case the content is liquid. If this is a compressor piping and you are asking all these question please consult an expert.

Hope it helps.

Ibrahim Demir