Hello Stressers,

I have come across a spec where there is a requirement of 4Hz as a minimum natural frequency for "Minimum Flow Line" and support design to be 2 times the static load.

I'm familiar with the "Two Phase" line which is basically identified by process group in Line Index which shows the Fluid phase as "M" which means mixed phase flow.

But how to identify that the line is considered as "Minimum Flow Line". What are the hints, keypoints to know that the line is a "Minimum Flow Line". Usually I don't see this from a usual EPC Specs for Piping Stress Analysis. Can I request Process group if they can identify this in the Line Index?

Your help is highly appreciated, as this might help in avoiding vibration problem to be taken into account in the "Design Stage".

Many Thanks & Cheers,

That's an odd phrasing. Without the whole statement, it's somewhat nebulous in my mind what's intended.

In the offshore world in the US, "flowlines" specifically refer to lines that come from the wells, as well as between facilities. I.E. random fluctuations are expected.

4 Hz is a catchall for "flowlines," but I've also read 4 Hz is a catchall for environmental effects.

So I can imagine that the statement in question could indicate that flowlines are to be analyzed at a minimum, but more piping could be subject to the same analysis based on engineering judgement.

Thanks Michael for your opinion. Actually in my previous experience our senior Stress Engineer requires us to do a Modal Analysis where the 4 Hz is used as the minimum fundamental frequency to almost all critical lines in order to determine if the piping system is well restrained and well guided. But it's not an absolute requirement especially for high temperature lines where flexibility is required rather than a rigid piping system. Just a quick tool to determine the stability of the system against environmental effects.

Anyway, vibration at the "Design Stage" cannot be guaranteed 100% can be solved but good design practices that were developed through the years can be used as a countermeasure.

Cheers...

The term “minimum flow” generally means the lowest continuous flow the pump is permitted to operate, without reference to duration, vibration level or other criteria.
It is the flow below which the pump should not be operated continuously. The piping which supplies minimum flow to pump is known as “minimum flow line”. Normally a piping from pump discharge connected to suction supplies “minimum flow” to pump. It supplies required fluid to pump suction when there is now supply in sump/source (equipment, tank etc.).
Few years back I was assigned trouble shooting job for vibration in minimum flow line. The problem was solved by adding supports in the system (increasing natural frequency of the system).
May be these lines are designed for flow turbulence, that why there is specific requirement of 4Hz.
Point to keep in mind for future projects.
Khalid

Thanks Khalid for sharing your experience. From your description of how the system works there is a possibility of "Flow Induced Vibration" because of the mixing of flow from discharge line to suction line. Also, this system can then be identified from P&ID if there's a connection between a suction and discharge piping. And maybe the "Energy Institute Guideline" can also be used to evaluate "LOF's" based from the given process parameters. Actually I have remembered one of my Senior Stress Engineer commented on my Stress package when I was still new to Stress Analysis regarding a pump suction. He commented that it's a vibrating system but for other pump suction system he didn't comment the same. So I suspect, that is a minimum flow line. He doesn't want me to use the base support type but rather a shoe type support and also request the structural guys to provide foundation considering vibration. Maybe he experience it before also for this type of line. Sometimes experience is really the "best teacher".

Cheers to all!!!