Bernoulli Question & Static Head-Irrigation System

[jefftb]

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?

 

[katmar]

The static head is the difference in height between your pump centre line and the liquid surface in your tank. The rest of the head that your pump "sees" is due to friction.

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"An undefined problem has an infinite number of solutions"

 

[jefftb]

I feel that the static head is basically waterlevel to waterlevel in this case and it would vary from pump centerline to discharge point or return point since they would almost be the same.

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.

 

[cvg]

suggest you set this up in epanet or some other software to do your sizing. Based on the limited information given, you cannot get 100 gpm through 1/2 inch pipe and maintain a pressure less than 75 psi. I would say you would be lucky to get more than 5 gpm.

your total discharge through 3,100 emitters is about 30 gpm, so not sure where the 100 came from.

 

[jefftb]

The 100 GPM (approx) is derived from a Excel spreadsheet the irrigation tubing manufacturer provides for calculation of proper dose flowrate.

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.

 

[bimr]

The static head of a pump is the maximum height (pressure) it can deliver.

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.

 

[BigInch]

The orifices do not affect the actual static head, unless you actually have a water surface level on the other side of one of the orifices. Static head is always the difference between water level and the pump centerline.

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."

 

[jefftb]

The question would be this at the very basic:

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.

 

[cvg]

assuming that your pump is designed with sufficient pressure and capacity to keep the pipe flowing full all the way back to the header line at the tank, than static head is equal to pump elevation minus discharge elevation.

 

[BigInch]

design point has nothing to do with static head alone.
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."

 

[BigInch]

Thinking about this just one little more step further, pumping to a tank, even at a low flowrate through the pump going to irrigation orifices, in fact even if the pumped flowrate is less than the sum of the outflow from all the orifices doesn't matter either. Water in the tank will run "backwards" or downwards to the orifices to meet all the orifices' combined flowrates if necessary, as long as the pipe will handle that flowrate from tank to orifices without excessive pressure loss. You will actually feed the orifices from two directions.

From "BigInch's Extremely simple theory of everything."

 

[BigInch]

In which case the head of the pump required for that scenario might actually be pretty low, if you need any at all. But then why would you pump, so we have to assume you are pumping to fill the tank too. Then you will have to reach the head elevation in the tank at the tank fill rate plus the orifice outflow rate.

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."

 

[jefftb]

We have to provide positive inlet pressure to the irrigation tube between 25 & 35 psi. There is a direct relationship between inlet pressure and maximum length of irrigation tube lineal runs. We also have to get the flush water back to the irrigation tank.

 

[bimr]

It is common in a recirculating loop to control the pressure in the loop at the discharge end of the supply header pipe through the use of a back pressure regulator. This method will maintain a constant pressure throughout the loop.

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.