Static Pressure Question

[FSB01]

There is something that I would like to ask: Is there a Hydrostatic pressure present in closed system (filled with liquid) if pressure pumped into system is higher than static head? My answer would be that as long as we are within gravity field there must be static head present. Some of my coworkers disagree and say that is true only for an “open” system, while in a “closed” system static head is not present.
I am confused as those guys are more knowledgeable than I.
Thanks

 

[TGS4]

FSB01 - you are correct in your answer. And to prove it, go back to first principles. Draw a control volume around a unit of the liquid, and have the control volume surface cross the pipe (so that pressure is a component for global equilibrium.

You're on the right track, and don't let some "old hands" confuse you - you can't argue with first principles!

 

[Ashereng]

Could your coworkers be refering to atmospheric contribution to overall head?

In an open tank, total pressure is = atmospheric press + hydrostatic head

In a closed tank, total pressure is = vapor space press + hydro static head

The effect of outside atmospheric pressure is eliminated since there is usually a blanket gas providing the pressure.

 

[FSB01]

Well, it looks that after a discussion they come to conclusion that they might be wrong (still just "might be"). I think that they were mixing overcoming static pressure when pumping fluid and removing it.

 

[davefitz]

ASME Section I recognizes hydrostatic pressure must be additive to a closed system vapor pressure when determining design pressure for downcomers and headers . In some cases, a large coal fired boiler ( tower type) can be over 300 ft tall, and the desing pressure of teh bottom of the lowest downcomer may be more than 100 psi greater than steam drum pressure or "master stamping pressure".

 

[krb]

Could it be that they are thinking in a closed system, the static pressure on the suction and discharge side of the pume is in general equal, and therefore cancels out? I know most of the HVAC water systems I deal with, that is the case. That is not to say it does not exist, only that it is equal.

KRB

 

[rconner]

Just a comment with hope it's basically right and doesn't confuse the situation further! From say a practical standpoint I think most common pressure gauges are calibrated in "gauge pressure" or "psig" etc. I think what this means is that in effect absolute atmospheric pressure at sea level is in effect the "zero" gauge position. In other words, if the gauge is at the bottom of a water column that has an open water surface at the top, the gauge will read/show pressure due only to the height/unit area weight of that water column, not a larger reading say of that plus the air weight pressing on the upper water surface.

 

[Ashereng]

If I may be a bit nik picky.

"Most gauges in use are "Gauge Pressure" gauges."

"Gauge Pressure" gauges are referenced to atmospheric pressure.

"Absolute Pressure" gauges are referenced to vacuum.

Both types of gauges are in common use.

You usually can not "calibrate" a "Gauge Pressure" gauge to be an "Absolute Pressure" gauge, and vice versa. I haven't seen one that can do that yet.

 

[rconner]

I take no offense -- thanks!