5 replies to this topic

[gatty]

Yes of course, and the arrow indicates the direction of flow

i want to know y is it so ......y not both d directions ???

[Art Montemayor]

Gautam:

I’m going to assume that you made a typographical mistake in writing your query and that what you meant to write is the following (without your horrible abuse of the correct English capitalization of the first letter in a sentence and the letter "y"):

“I want to know why a globe valve works preferably in one direction only - i.e., is the direction of flow specified in a globe valve?”

The correct response is to give no importance to the presumed arrow cast on the body in relief. The important item to be concerned about is the precision and the integrity of the valve’s stem seal. If this stem seal is correctly machined and sealed, it should withstand the higher pressure of the two pressures existing – one on each side of the valve – when the valve is completely in the closed position. Those are the conditions under which and for which the globe (and gate) valve is made to operate.

Think about it and you will agree that it is only practical, common sense that the globe and gate valve withstand their rated design pressure regardless of which way the fluid is flowing. And it is the pressure rating that is the critical criteria – never the “flow rating”, since there is no such thing. There are various situations that bear this ability out:

1) There are two sub-types of globe valve: the conventional, straight-through flow globe valve and the 90^o (“angle”) globe valve. I have personally used the angle globe with the flow going in both directions. “under-and-up” or “over-and-under” when describing the fluid’s path around the valve’s plug.

2) My personal preference is to use the globe valve with the flow being under-and-over. My reason for this is that the valve can control pressure and flow with more positive action since it doesn’t have to overcome the counter force created when the fluid is traveling in the opposite direction. With the over-and-under type of flow, the fluid imposes a counter force on the plug because the upstream pressure is bearing against the back side of the plug and trying to force it closed.

3) However, there are occasions when the valve is better employed with the flow being over-and-under. Take, for example, the case where I have had to employ an angle control valve directly on the flanged nozzle at the top of an Amine Stripper tower. I have had to do this because the resultant outlet of the control valve is a hot mixture of Amine solution and corrosive CO₂. It is impossible to install this type of valve with the flow under-and-over because this forces the pneumatic diaphragm to be in the horizontal position and the diameter of this diaphragm forces the valve to be located further away from the Stripper’s nozzle and exposes a longer pipe nipple to corrosive action before the fluid gets into the Stripper. The solution is to orient the valve with the diaphragm in the vertical position and this allows the valve to be flanged directly up against the Stripper. As a result, the flow winds up being over-and-under. I will illustrate these orientations in an Excel sketch if readers are unable to visualize the important difference this makes.

4) Another similar application where I have had no recourse is a patented process that I designed and developed for the automatic production of Dry Ice Pellets. In this process, I expand liquid CO₂ through an expansion valve with the result being a 50% mixture of gaseous CO₂ and solid Dry Ice snow. I cannot afford to allow the expanded products to remain static because the Dry Ice snow will precipitate and form a solid plug in the process. I must introduce the gas + snow mixture directly and immediately into a chamber where the snow is compressed and extruded into pellets. In order to accomplish this efficiently and without plugging, I employ a globe control valve that is oriented exactly like the one I employ in the Amine Stripper previously mentioned. by machining the valve’s outlet internal port in a “Vee” shape (similar to a megaphone or cone) the liquid CO₂ flows over-and-under, expanding immediately and entering the compression chamber without any obstacles. This is the only mechanical orientation that allows for successful and continuous service.

5) Another illustration of globe valves used in both ways is a 2-tower adsorption installation where both towers are inter-connected with a manifold made up of globe valves. One tower operates under pressure while the other tower is being re-generated and is devoid of pressure. The towers are interchanged from process service to regeneration on a sequenced cycle. The valves continuously see pressure on one side for a half-cycle and then no pressure for the other cycle half.

I apologize to those readers who are unable to visualize what I’ve described. It’s easy for me because I’ve successfully done it in the field many times and I know of yet many other examples. If need be I can sketch out the installations on a spreadsheet and attach it to this thread.

What Profe writes is true. The arrow can indicate the direction of flow. However, he/she leave out an important detail: the flow can be in any direction – regardless of arrows or any other indication. It simply doesn’t matter or make a difference in the practical application. In all fairness, if pressure drop makes a difference, you will find that the over-and-under flow direction yields less pressure drop than the under-and-over type.

I hope this answers your query and leaves no doubts in your mind and you will start to write in a mature manner and not abuse the English language.

[sachin arora]

Hi all

I egree with Art, and mi afirmation was about pressure drop, this is greater in the opposite direction that in the correct one. I think.

hi all ,
this is specificaly for PROFE
I JUST WANT TO SEE A LINE DIAGRAM FOR WHAT U EXPLAINED IN YOUR REPLY
IT WILL BE OF GREAT HELP TO THIS YOUNG ENGINEER


SACHIN