Anyone heard this rule of thumb?

Rich Kontny_3

Larry simplified things as a column of water open at the top, no matter what diameter exerts .434 lbs. of pressure per one foot of heigth with the column (or cylinder) open on top. This is gravity at work and varies somewhat at extreme high elevations. For the sake of conversation regarding pressure needed at the top or pressure exerted at the bottom of a column we use .434/ft pressure.

This .434 number is used a great deal in plumbing also as we need to design around low and excessive pressures. Excessive pressures are often found in high rise applications and therefore require prv,s and reliefs to protect against excessive pressure concerns. In low water conditions sometimes street pressure is substantially reduced by elevation and must be overcome by booster pumps etc.

Heating/cooling concerns are similar yet we for the most part are only trying to overcome heigth to provide adequate flow of the liquid we are handling (usually h2o)

A bit wordy perhaps but it made sense to me and continues to do so.

So it takes 2.31 feet of head to create 1# of pressure.


Rich K.

Radman

Can anyone explain the science behind this rule of thumb?

My general rule of thumb is always NO RULES OF THUMB, DO THE MATH! However while attending a solar class the instructor suggested a rule of thumb for static fill pressure on a CLOSED LOOP glycol system to be 1psi per 2.31 feet of vertical distance between the bottom of the tank and the top of the array. When questioned about this he stood on it and did not give an explaination. Can someone elucidate the source of this figure?

Tony_23

No thumbs there : )

It takes 1 psi to lift water 2.31 ft vertically. Don't have that pressure you don't fill to the top. Simple as that. No rule of thumb, just physics

RJ_4

foot of head

If a pipe is filled with water 2.31 ft. the pressure will be 1psig. It is recommended to have at least 5 psig. at the highest point of the system , to help with air removal. Example: A system with a pump in the basement and a 20' height to the top of a solar panel would figure as 20'divided by 2.31 = 8.6l lbs. add 5 lbs. total static press. = 13.61 lbs.

Radman

Pressure irrelevant to \"filling\"

I understand the concept, but it still does not explain its relationship to static pressure. Filling a solar system in a closed loop is done with a pump in an open loop process, using a pump with sufficient head to deliver the fluid to the collectors and through the piping and its related pressure drops. Once the system is filled however, the "fill pressure" is related to the value that the system is pressurized to. At this point, whether there is 10psi on the guage or 50 psi on the guage the system is full, and there is most certainly fluid in the collectors. Therefore, how can the weight of a foot of water be anywhere related to static fill pressure in a closed loop system?

Radman

Don't you mean 2.31 x the 20 ft of head + 5psi???

Unknown

No, he means

1)- Water will always seek its own level.
2)-It could be spread-out like an octopus, but the fact remains.
3)-A vertical column filled to 2.31ft in height equals 1 PSI guage pressure at the bottom of the column.

Dave

Radman

Another way to look at it

Ok, forget solar for a second and let's say we have a hydronic heating system in a 10 story building. Based upon this rule of thumb, we would need (120ftx2.31)+5= 282.2psig of fill pressure?

Unknown

Radman

120Ft divided by 2.31= 51.94 PSI + 5= 56.94 PSI is needed if the water feed is introduced at the systems lowest point(basement).
If the feeder is introduced to the system at a higher level, the pressure is calculated from that point to the highest altitude(120Ft), but is still 56.94PSI in the basement.
Once the system is filled statically, the pump choice you use overcomes the friction loss to move it.

Dave

Radman

Ok, right, BUT...

Ok, sorry that's right 120/2.31=51.94+5 assures me that if my city pressure is at least 60 psi I will get water up to the 10th floor. This still does not answer my question as to how that relates to static fill pressure, which at this point in my example could be anywhere between 12-20psig. If there is water in the entire system, and the system is closed, then why is this relevant to my static pressure on the system. If you set the expansion tank pressure to the same value as the fill pressure, and the system is static, why would I need my fill pressure to be set at 56psi? If I set the system pressure to 30psig, then does that mean that the water in the system only made it to the 6th floor? No, that can't be because where did the water go? It's not in the expansion tank because that pressure is also at 30psig, and it surely didn't compress. What am I missing here?

Unknown

Radman

OK, static fill is like standing there doing nothing.
When water is heated it expands right?, so your exp. tank could be tied-in to the system at a higher altitude(than the basement), to reduce its pre-charge.
BTW-30 PSI(from basement) will get you to 69.30Ft vert. hght. no matter how spread-out the entire system is.

Dave

Larry Weingarten

Let's assume...

... the instructor was trying to keep air out of your systems. You need enough psi, (measured at the bottom) to fill the system to the top. Closed or open doesn't matter here; if it's 23.1 feet tall, you'll need 10 psi at the bottom. Then you get to add some pressure and use an expansion tank to maintain that pressure. Air can't leak in :~) I'm prolly repeating what others have said...

Yours, Larry

Radman

Ok Guys, Thanks!

Ok, I get it, and now I understand where I am getting confused. I appreciate everyone drilling it in, and also appreciate the patience. Thanks all.

Mike T., Swampeast MO

It still appears in a closed, pressurized system. Check pressure at the basement at the boiler and also check on the highest radiator (2 or 3 floor house easier to notice). The pressure will be lower at the top by 1 psi for every 2.31' above where you checked the pressure in the basement.

Radman

Thanks Rich!

That explaination also helps, thanks for taking the time. I guess in all of the time I have been doing this I never considered this equation as it relates to fill on closed loop. It really makes sense now, and it also makes me go "OH, so that's why we were having that problem!" Don't you love that? It proves a more important point, we are always learning, and as soon as you stop you are pushing up daisies!

Arthur

Rule of the thumb

Do not confuse static pressure with circulating pressure, Because the water is filled in a closed loop the static pressure will be whats required to fill the system up to the very top. Then the pressure on the both sides of the pump when off will be the same. However the pressure the pump generates to make the water circulate will be whats required to make the water circulate,
The circulating pressure will be the difference in pressure on both sides of the pump ignoring the static head pressure.
I've run into consultant engineers who can't seem to grasp this concept.

Unknown

Rule of thumb

the static head head pressure in any open or closed loop water system comes into play when filling an empty system.

If you don't have the pressure to reach the to of the system it wont be full.

Some times people no matter how well versed confuse the term static head with head pressure, both are convertable to feet in head. Feet in head or feet of head is the term most closely attributed to pumps.

Jake