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[Chris Haslego]

----------Originally posted by Shaheryar Qureshi----------------
I am trying to clear this up:

Although we look at recommended pressure drops and velocities during line sizing for either gas or liquid.

But for Liquids pressure drop is limiting and for gases velocities are limiting.

How does this tie in to Gravity flow i.e. liquid drain lines?

Need comments /critiscim

Thanks
Shaheryar Qureshi

------------Response from Mr. Art Montemayor------------------
Shaheryar:

I am not confused by your questions, but I have difficulty in deciphering what you are seeking. Nevertheless, I'll comment on your points as follows and perhaps this will help to clear up some things:

1. You make the general statement that pressure drop is limiting for liquids, while velocity is the limiting factor for gases. This is not only untrue in the general sense, it basically is reversed in most applications. You have much more freedom of design with supercooled liquids than you have with compressible fluids - such as vapors and gases. The critical pressure drop is the absolute controlling factor for gases and vapors: you cannot flow any more mass than that flowing at sonic conditions (that's why the guys in the plant call it "Choked flow").

You do not have this phenomena with liquids unless you are flashing (like a near-saturated liquid) and getting a resultant vapor - which can lead to the "choked" condition. This also introduces the subsequent problem of cavitation and its destructive effects on control valves and equipment.

This is why the Darcy-Weisbach equation works for liquids and can only be applied to gases/vapors when the total pressure drop is less than 10% of the initial driving force. Make sure that you fully understand the basis for the Darcy-Weisbach equation and the limitations that sonic flow imposes on a gas/vapor. A lot of young engineers out in industry still have not been exposed to these very basic principles and, as a result, are formulating some erroneous solutions in fluid mechanics. You cannot force a compressible fluid to flow (mass rate-wise) any faster than the critical, sonic velocity regardless of a higher pressure drop caused by reducing the downstream pressure. For air, this occurs when the absolute pressure ratio is 0.528; i.e., when the downstream absolute pressure (P2) is 52.8% of the upstream absolute pressure (P1). This is such a very important basic principle that I often recommend engineers go to Milton Beychok's website where he has an excellent description of how this principle is employed and used in everyday practical applications. I recommend you go there. Milton has done a great job of bringing this subject out of the academic closet and putting it right where it belongs: on the engineer's dinner plate. Milton's Web Site is:

www.air-dispersion.com

2. Liquid gravity flow (in a pipe) is subject to having one of two practical conditions imposed on it:
a. A self-venting feature;
b. An equalization line or connection.

A liquid cannot drain on it's own (limited by gravity) without one of the two above features. The conventional kitchen sink is an example of this. The diameter of the drain line has to furnish an ability to self-vent; otherwise, the draining will not take place under steady conditions. It will be sporadic or slug-like. If you connect the liquid source with the liquid target using an equalization line, the liquid will drain under a steady flow. If you inspect your sanitary drain system in your home you will see that there is a mandatory vent line on the drain line that does this: it equalizes the drain system to the atmosphere - which is the same pressure the kitchen sink is at.

I hope this helps. This is a great subject that is often treated as mediocre or too simple by young engineers and winds up causing a lot of grief for them in their applications and design.
Art Montemayor