Author Topic: Water pressure question  (Read 3689 times)

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Offline madlabs

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Water pressure question
« on: January 19, 2012, 10:13:50 AM »
Hi all,

So, at my neighbors place (which will be off grid) the have their well about 20-30' above their building site. For the sake of this discussion, let's say 20'. That would make 8.6 psi pressure at the build site. So, if we took a large diameter pipe, say 4", ran it down the hill to the build site and then choked it off to a smaller diameter, say 3/4", there would be an increase in pressure, no?

I have been looking around for a calculator to figger this out, but no luck. I'm guessing that it's out there and I am not searching for the right term.

Thanks!

Jonathan
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Offline halfcrazy

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Re: Water pressure question
« Reply #1 on: January 19, 2012, 11:22:07 AM »
If it was gravity flow than I would say no but I may be thinking incorrect.

Ryan

Offline tomw

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Re: Water pressure question
« Reply #2 on: January 19, 2012, 12:11:08 PM »
Bigger tube means more flow.

Pressure is essentially the "column" of water above the outlet and effectively the same, regardless of the outlet size. Oversimplified but that is how it works.

Sorry to blow your idea out of the water [pun intended], but that is my understanding of it.

Tom
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Offline oztules

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Re: Water pressure question
« Reply #3 on: January 19, 2012, 05:36:18 PM »
It's the weight of the water that creates the pressure you feel.

The weight of the water in  20' of 4"  vertical pipe is more than the weight of water in a 20' @ 1 inch pipe.......

But we measure pressure as a per unit area amount..... so even for the 4" example, we take only 1 square inch of surface area at the bottom and look at that column of water only.... all the other square inches of cross section are the same........

So the diameter of the pipe counts for nil.... it is the height of the water that determines the pressure in ....lbs PER square inch...... more square inches gives you more lbs, but the same lbs/square inch..... which is your pressure.


Clear as mud..............oztules
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Offline madlabs

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Re: Water pressure question
« Reply #4 on: January 20, 2012, 10:58:17 AM »
Drats! But part of me still thinks why isn't it like when you put your thumb over the end of a hose, you trade some volume for some pressure. Don't folks decrease pipe size in micro hydro to increase velocity?

Thanks, and forgive my ignorance!

Jonathan
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Offline tomw

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Re: Water pressure question
« Reply #5 on: January 20, 2012, 11:15:10 AM »
Drats! But part of me still thinks why isn't it like when you put your thumb over the end of a hose, you trade some volume for some pressure. Don't folks decrease pipe size in micro hydro to increase velocity?

Thanks, and forgive my ignorance!

Jonathan

Don't confuse velocity with pressure.

It is like amps and volts. Lots of volts [pressure] is not much without some pressure [amps]

I usually explain electriocal power using water but it works the other way, too, I guess.

Tom
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Offline Dale S

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Re: Water pressure question
« Reply #6 on: January 20, 2012, 02:29:43 PM »
Tom, shouldn't you first "pressure" read volume instead.
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Offline rossw

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Re: Water pressure question
« Reply #7 on: January 20, 2012, 02:45:06 PM »
Tom, shouldn't you first "pressure" read volume instead.

Actually, I think the second one should be "volume".

Voltage is like "pressure", Amps are like "volume" or "flow".

Offline oztules

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Re: Water pressure question
« Reply #8 on: January 20, 2012, 03:02:23 PM »
"Drats! But part of me still thinks why isn't it like when you put your thumb over the end of a hose, you trade some volume for some pressure. "

You need to look at what is really happening. Without the thumb over the output orifice, there is no back pressure in the hose. All the water pouring out the actual end is no longer under any pressure... as there is no restriction here (there is in the hose itself from wall friction etc).... so the water falls out, pushed only by the water behind it trying to get out too.

When you restrict it, you build up back pressure. If you block it off you will build up full pressure.

So if we had a pressure gauge tapped into the hose say 1 foot in from the outlet, with no thumb, we get very low pressure, but max volume, but when we clamp the end of the hose, we get the static high pressure, and no volume..... so we can use anywhere in between those two states to do things.

If we partly open the outlet (spray a bit about) the pressure now in the hose tries to push whatever volume of water it can, out through the smaller orifice. It cannot push with more pressure than the static pressure (blocked hose).... and will always be less once we release water out.

Friction losses in the hose (result of velocity of the water and hose diameter ) work against us too.

So the smaller the outlet hole, the greater the pressure can build towards static max..... and the more pressure available to push the max water out of the hole.... so it moves faster.

But the pressure can never be higher than the static pressure (except for inertial change... ie the instant you block it..... you must decelerate the water that was moving. This can be used for ram pumping to very high pressure.).


Nothing is for nothing in this universe.... so for turbines we use jets to get the best combination of water velocity driven by back pressure. Restrict too much and we get high back pressure..... but we lose the mass.

If we  don't restrict enough, we lose the velocity needed to turn the turbine fast enough.... but you cant beat the turned off  (static) pressure. You can balance the pressure:velocity part to get what you need, but not pressure max.... the more you open the jet, the less pressure built up in the pipe to push the water out with.

just more mud....................oztules
Flinders Island...... Australia

Offline Dale S

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Re: Water pressure question
« Reply #9 on: January 20, 2012, 03:20:12 PM »
Tom, shouldn't you first "pressure" read volume instead.

Actually, I think the second one should be "volume".

Voltage is like "pressure", Amps are like "volume" or "flow".

Damn, I had a 50/50 chance, I knew one of em should be volume.
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Offline Wolvenar

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Re: Water pressure question
« Reply #10 on: January 20, 2012, 03:20:43 PM »
Wow, I always thought of it as voltage = flow speed, amperage= pressure, volume= storage capacity ( in watt hours)
Correct me in how I am wrong

My reasoning,
Voltage/ Flow Either can be very fast but if there is no pressure there can be very little work done.
Amperage/ Pressure If you have to much amperage, it will burn/damage its vessel. as such pressure will do the same
Volume /KWh =  Think of it as storage capacity.
Watts =  I cannot think of the term for a water equivalent without referring back to a measure of energy such as joules..                                                            However it directly  relates as water speed times pressure= work capacity at any given time of measurement.

Ok so thats just how I have always understood it
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Offline rossw

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Re: Water pressure question
« Reply #11 on: January 20, 2012, 03:24:34 PM »
Wow, I always thought of it as voltage = flow speed, amperage= pressure, volume= storage capacity ( in watt hours)
Correct me in how I am wrong

My reasoning,
Voltage/ Flow Either can be very fast but if there is no pressure there can be very little work done.
Amperage/ Pressure If you have to much amperage, it will burn/damage its vessel. as such pressure will do the same
Volume /KWh =  Think of it as storage capacity.
Watts =  I cannot think of the term for a water equivalent without referring back to a measure of energy such as joules..                                                            However it directly  relates as water speed times pressure= work capacity at any given time of measurement.

Ok so thats just how I have always understood it

I think of "amperage" as being like "flow", because you need a large wire to carry a lot of amps, just like you need a large pipe to carry a lot of water (flow). Wire insulation needs to be thicker for higher voltage (pressure) just like pipe wall thickness has to be thicker for higher pressures.

A high pressure (voltage) CAN cause a high flow (current) as long as your pipes/wires are big enough

Offline oztules

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Re: Water pressure question
« Reply #12 on: January 20, 2012, 08:24:30 PM »
Lemmeesee....


If we take voltage, we can see that it pushes the electrons through a circuit.
If we take pressure, we use it to push the water molecules through the water circuit (hose, pipe etc)

So pressure is volts..... psi

Flow rate is the current .... number of electrons or water molecules flowing at the moment.

So Amps is flow rate .... gallon per minute, cubic feet/second. etc


If we get the flow rate and divide by time we get volume..... If a coulomb /sec is an amp, then  a gallon per second/second is a volume.

So a coulomb is Volume .... gallons or cubic feet etc.

Now :D

So watts are pressure X flow rate..or psi X gpm

Watt hours are psi X gpm X hours


even more mud................oztules

anyone want to try capacitance?? perhaps area x head  :-\

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Offline wdyasq

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Re: Water pressure question
« Reply #13 on: January 21, 2012, 06:59:46 AM »
"even more mud................oztules

anyone want to try capacitance?? perhaps area x head "

Well, in the hose and pressure discussion, the hose as it expands when the big digit is over the end, act sort of like a capacitor storing energy. Partially removing  the big digit must be similar to a variable resistor.

Just stirring the mud .....

Ron

BTW, this has been a good discussion. It clarified how I can describe sparky stuff to the ignorant.

Offline Rover

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Re: Water pressure question
« Reply #14 on: January 21, 2012, 07:23:01 AM »
"BTW, this has been a good discussion. It clarified how I can describe sparky stuff to the ignorant"

Just use a tazer and take questions afterwards. :)

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