Bernoulli Question & Static Head-Irrigation System
Bernoulli Question & Static Head-Irrigation System
(OP)
Want to know everyone's thoughts on this application.
Pumping from a vented to atmosphere wastewater effluent tank downhill through an irrigation network.
The irrigation pipe is small diameter PE pipe (about 1/2") with pressure compensating discharge orifices. These orifices emit a maximum of .62GPH over a range of 15-75 PSI. In each zone there would be about 3,100 of these orifices and about 6100' of this pipe. A zone would have a header/return manifold of 3" PVC and the irrigation tube would be connected between the two headers.
We are trying to determine the actual static head of the system when we "flush" a zone. During flushing we open a valve back at the pump tank at the top of the hill and create a circular network.
When flushing the last zone we pump downhill (-125' head) through 700' of 4" PVC, through the irrigation piping, and then back uphill (+125' head) through 1000' of 4" PVC to the very same tank that the pump is located in.
The orifices are the point in question. The downhill leg will stay full (or nearly) and the uphill leg will be full and the irrigation tubes will be full except the orifices. We are pumping about 100 GPM. The only open part of the system is those very tiny orifices and these are providing uniform backpressure so that all orifices discharge at the same rate against a 15-75 psi influent pressure. We are using 35 psi.
Thoughts on the actual static head?
Pumping from a vented to atmosphere wastewater effluent tank downhill through an irrigation network.
The irrigation pipe is small diameter PE pipe (about 1/2") with pressure compensating discharge orifices. These orifices emit a maximum of .62GPH over a range of 15-75 PSI. In each zone there would be about 3,100 of these orifices and about 6100' of this pipe. A zone would have a header/return manifold of 3" PVC and the irrigation tube would be connected between the two headers.
We are trying to determine the actual static head of the system when we "flush" a zone. During flushing we open a valve back at the pump tank at the top of the hill and create a circular network.
When flushing the last zone we pump downhill (-125' head) through 700' of 4" PVC, through the irrigation piping, and then back uphill (+125' head) through 1000' of 4" PVC to the very same tank that the pump is located in.
The orifices are the point in question. The downhill leg will stay full (or nearly) and the uphill leg will be full and the irrigation tubes will be full except the orifices. We are pumping about 100 GPM. The only open part of the system is those very tiny orifices and these are providing uniform backpressure so that all orifices discharge at the same rate against a 15-75 psi influent pressure. We are using 35 psi.
Thoughts on the actual static head?





RE: Bernoulli Question & Static Head-Irrigation System
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RE: Bernoulli Question & Static Head-Irrigation System
However, the dissenters from this position are hedging that the orifices create an open point in the system and that basically refutes what is conventionally thought.
RE: Bernoulli Question & Static Head-Irrigation System
your total discharge through 3,100 emitters is about 30 gpm, so not sure where the 100 came from.
RE: Bernoulli Question & Static Head-Irrigation System
In this case the 1/2" tubing is spread across a supply header manifold that is about 115' wide with a irrigation tube every 5' along that length. The irrigation tube runs are about 235' each in length. These dimensional numbers are not exact.
Again, all of these calculations for flow and dose volume are from the manufacturer supplied design spreadsheet and we've utilized it many, many, times for design purposes.
The only issue in question here is the determination of static head and if it has any sizable effect on pump TDH.
RE: Bernoulli Question & Static Head-Irrigation System
The pressure head at the orifices is due to the static pressure that exerts a force on its container. As the elevation of the orifices drops, the static pressure increases.
You cannot measure the static head at the orifices since the orifices are open.
During flushing when you open a valve back at the pump tank, you are reducing the pressure head.
RE: Bernoulli Question & Static Head-Irrigation System
In other words, if you pump more than the orifices expell, static head will be found at the fluid level somewhere on the other side of the orifices. If you pump less than what all the orifices can expell, you will find the static head fluid surface somewhere within the group of orifices, close to where the combined total of the first-in-line orifices equal the pump rate. If you pump more than what can be expelled by the orifices, the static head fluid surface line will build to the top of tank overflow, or to the static head capacity of your pump at that given flowrate.
There will be substantial resistance to flow from your 1/2" pipe, so the total head your pump delivers to move the fluid and raise or lower the static surface will be a function of the flowrate, as only what is left over, after friction loss at the given flowrate is subtracted from pump discharge head, will be available to lift the static surface higher at any given time.
Each orifice reduces the remaining static head, after flow losses are subtracted from pump head, mostly because each flowing orifice increases the total flow in the pump, which in turn makes the pump deliver less of an initial discharge head... see the pump curve discharge head vs flowrate.
From "BigInch's Extremely simple theory of everything."
RE: Bernoulli Question & Static Head-Irrigation System
Is there a static head in this nearly closed loop system since we pump down the hill and back up it to the same location because the orifices are in the system. Some in our office say it has a static head of 125'.
Or would it be?
In this closed loop system and using waterlevel to waterlevel differential as the determinant, which would be zero here, resulting in a static head of zero on the pump.
I am in the camp that the design point of the pump is 90-100 GPM at approximately 95-105' TDH.
Others in the office are in the camp that the design point of the pump is 90-100 GPM at 225' TDH.
RE: Bernoulli Question & Static Head-Irrigation System
RE: Bernoulli Question & Static Head-Irrigation System
total head = static head + friction head.
design head can be set at anything.
Neither does it matter if the required head is all static, all friction, or a combination of the two.
If the liquid surface is inside the tank in a closed loop system, then yes, you have no static head and +up -dn = 0 All pressure drop around the circuit is from friction of flow alone. Flowrate will vary as water is removed from the orifices, also doesn't matter. The orifices do not matter as long as they are flowing water out and only water is running past them, no air is getting in and setting a different fluid level than what you have in the tank. Some water must be getting to the tank... THAT'S ALL that's required to have closed circuit.
From "BigInch's Extremely simple theory of everything."
RE: Bernoulli Question & Static Head-Irrigation System
From "BigInch's Extremely simple theory of everything."
RE: Bernoulli Question & Static Head-Irrigation System
The pump's head reqired to so will be the head in the tank + friction losses in the upflow line, - head in the tank + friction losses in the downflow line) = only friction losses for the total combined flow of orifice flows + tank fill flowrate.
From "BigInch's Extremely simple theory of everything."
RE: Bernoulli Question & Static Head-Irrigation System
RE: Bernoulli Question & Static Head-Irrigation System
However, in your application where the elevation changes markedly (125-feet), this method of pressure control may not be provide satisfactory results. The water pressure will be higher at the low points and force more water into your irrigation tubes.
For the best accuracy in flow distribution, you should consider adding additional pressure regulators at the low elevation discharge points of the supply header pipe.