Full Flow in Pipes 1

[more22]

I have the following scenario:
A 6" line in which different pumps can discharge into. The minimum flow that can flow through the line is 25 gpm. Is there a way of calculating/verifying whether the line is going to be full of liquid at this minimum flowrate? The reason why I am asking is because I will be installing an ultrasonic flowmeter and two phase flow would affect the speed of sound through the air & liquid media (if the line is not full of liquid).

 

[Montemayor]

more22:

You are touching on the very subject that piping designers are reknowned for: liquid fluid flow in a system that is liquid full. Most young engineers forget that the main underlying basis for the Darcy-Weisbach equation to hold and be applicable, the flow in the pipe must be liquid-full. If this prerequisite is not met, then you have 2-phase flow and the Darcy-Weisbach relationship falls apart.

The way "pipers" design their piping runs is dependent on their fulfilling that requisite. They inherently know this and a real pro piper knows exactly what he is doing. One way to accomplish this to to use vertical offsets and to include the liquid metering instrumentation downstream of the vertical offset - by building in a positive liquid static head downstream of the measuring device. This is called know-how or just plain expertise. An engineer never calculates his piping under the basis you are inferring: calculating for a pseudo-minimum flow that ensures that the pipe will be liquid-full. What is done in the major engineering contractor offices of the world is that expert pipers see to it that the piping is laid out and installed in such a manner that it is kept flooded of "liquid-full 100% of the time.

This is a good, practical question on real-life fluid flow and the way it is designed in the actual, process plants of the world. Theory is great; and the experienced designer knows the under-pinnings of his theory and fulfills the prerequisites necessary for the actual instrument to work in accordance with that theory and its basis.

I hope you have an experienced and veteran piper on your project team. You will recognize that such a team member is worth his weight in gold.

Good luck.