How to increase psi in my gravity irrigation system?

[plowhog]

At the center of my irrigation system I have a 2 1/2 inch pvc pipe with 35 psi. Max flow is about 45 gpm. I want to increase the psi as some sprinklers require 40 psi. My target is 45 psi. I don't want to use an electric pump. Gravity only.

The psi is generated because the pipe runs up a moderately steep grade maybe 400 feet. The inlet to that 2 1/2 inch pipe is roughly 90 feet above where I measure psi. (.43 psi per one foot of height /head.)

The inlet is in a 'water box.' That box is fed by a long section of 6 inch pvc pipe that travels on a very shallow grade then dumps water into that box. The shallowness of the grade is why I only get 45 gpm through it. The water box also has another exit pipe which is the overflow. If I use less than 45 gpm, the excess goes out the overflow.

I believe i could increase psi by joining the 6 inch and 2 1/2 inch sections. But, I am concerned about restricting my flow in the 6 inch line even more by doing this. I don't want to lose volume. The 2 1/2 inch pipe stays full unless I overtax it by exceeding 45 gpm of irrigation. And the 6 inch pipe is never full. Not even close, less than 1/4 full I would estimate.

The intake of the 6 inch line is about 1/3 mile away. The pipe is nearly full there even though only 45 gpm is being fed into it there.

I am concerned about connecting a 'shallow gradient' pipe onto one with more slope. I can't find anything on google about this.

Anyone have ideas?

 

[RedNeckRacin]

The only ways to increase pressure in a gravity system is to reduce the frictional losses or increase the head. You only have 90x.43= 38.7psi so you can't get any better than that.

Can you move the inlet box farther up slope to get more head pressure?

 

[4570Man]

The only ways to increase pressure in a gravity system is to reduce the frictional losses or increase the head. You only have 90x.43= 38.7psi so you can't get any better than that.

Can you move the inlet box farther up slope to get more head pressure?

I’m assuming that he measured the pressure with no flow so just static pressure. Bigger pipes would flow more but the static pressure will remain the same without gaining more head.

 

[MossRoad]

At the center of my irrigation system I have a 2 1/2 inch pvc pipe with 35 psi. Max flow is about 45 gpm. I want to increase the psi as some sprinklers require 40 psi. My target is 45 psi. I don't want to use an electric pump. Gravity only.

The psi is generated because the pipe runs up a moderately steep grade maybe 400 feet. The inlet to that 2 1/2 inch pipe is roughly 90 feet above where I measure psi. (.43 psi per one foot of height /head.)

The inlet is in a 'water box.' That box is fed by a long section of 6 inch pvc pipe that travels on a very shallow grade then dumps water into that box. The shallowness of the grade is why I only get 45 gpm through it. The water box also has another exit pipe which is the overflow. If I use less than 45 gpm, the excess goes out the overflow.

I believe i could increase psi by joining the 6 inch and 2 1/2 inch sections. But, I am concerned about restricting my flow in the 6 inch line even more by doing this. I don't want to lose volume. The 2 1/2 inch pipe stays full unless I overtax it by exceeding 45 gpm of irrigation. And the 6 inch pipe is never full. Not even close, less than 1/4 full I would estimate.

The intake of the 6 inch line is about 1/3 mile away. The pipe is nearly full there even though only 45 gpm is being fed into it there.

I am concerned about connecting a 'shallow gradient' pipe onto one with more slope. I can't find anything on google about this.

Anyone have ideas?

Are you saying:
- 6" pipe feeds a water box.
- water box feeds a 2.5" pipe.
??

If so, then:
If you join the top of the 2.5" pipe to the bottom of the 6" pipe (skipping the waterbox), then, yes, your pressure at the end of the 2.5" pipe will increase by whatever pressure is added by the pressure in the 6" pipe.

Measure the pressure at the bottom of the 6" pipe and see if it's 10psi.

It doesn't matter what grade angle the pipe is, or how big the diameter of the pipe is. It's the total height of the fall that determines the PSI in a gravity system.

 

[jmc]

Could you elaborate on why the 6" pipe is only "1/4 full", yet it's "almost full" at its intake?

And by "full", for example, "1/4 full", do you mean the water stream's cross-section is 1/4 of the pipe's cross-section or do you mean 1/4 the length of the pipe is solid water?

 

[RedNeckRacin]

....

It doesn't matter what grade angle the pipe is, or how big the diameter of the pipe is. It's the total height of the fall that determines the PSI in a gravity system.

Static pressure you are correct, but that is 100% not true for a flowing condition. The pipe will have increasing frictional losses with increasing velocity and decreasing pipe size. That's fluid mechanics 101.

 

[RedNeckRacin]

I’m assuming that he measured the pressure with no flow so just static pressure. Bigger pipes would flow more but the static pressure will remain the same without gaining more head.

Agreed. Except the frictional losses would be less in a larger pipe of the same material.

 

[MossRoad]

Static pressure you are correct, but that is 100% not true for a flowing condition. The pipe will have increasing frictional losses with increasing velocity and decreasing pipe size. That's fluid mechanics 101.

Yes, I was referring to static pressure. Do you think he's gonna do the equations necessary to figure all of that out for frictional losses in a gravity system? He already has 35PSI where he wants 45 PSI. He wants a 10PSI boost. If he ads the 6" pipe at the top of the column, if it doesn't have at least 10PSI to begin with, he'll never get to his desired 45PSI. Check there first before doing anything else. If he doesn't have it, the only way he's gonna boost it is with a pump, or a higher-in-elevation source.

From here:
Pipe Pressure Drop Calculations Formula, Theory and Equations

"Calculating the Pressure Drop in a Pipe
To calculate the pressure loss in a pipe it is necessary to compute a pressure drop, usually in fluid head, for each of the items that cause a change in pressure. However to calculate the friction loss in a pipe for example, it is necessary to calculate the friction factor to use in the Darcy-Weisbach equation which determines the overall friction loss.

Friction Factor The friction factor itself is dependent on internal pipe diameter, the internal pipe roughness and the Reynold's number which is in turn calculated from the fluid viscosity, fluid density, fluid velocity and the internal pipe diameter.

There are therefore a number of sub-calculations that must take place to calculate the overall friction loss. Working backwards we must know the fluid density and viscosity properties, know the pipe diameter and roughness properties, calculate the Reynold's number, use this to calculate the friction factor using the Colebrook-White equation, and finally plug in the friction factor to the Darcy-Weisbach equation to calculate the friction loss in the pipe.

After calculating the pipe friction loss we then need to consider possible fitting losses, change in elevation and any pump head added. Summing these losses/gains will give us the overall pressure drop in the pipe.'

 

[plowhog]

Could you elaborate on why the 6" pipe is only "1/4 full", yet it's "almost full" at its intake?

And by "full", for example, "1/4 full", do you mean the water stream's cross-section is 1/4 of the pipe's cross-section or do you mean 1/4 the length of the pipe is solid water?

That is a significant question, which is still a mystery to me. The pipeline is closed with no mid-span access points. So I can only "see" the intake of the pipe and the outflow of the pipe. The intake is a very shallow grade and it is close to 100% to the top of the 6" pipe. The outflow is only about 1/4 full which is the cross section of the 6" pipe.

Although there conceivably could be a restriction, I can measure at the intake location and find about 45 gpm. It's the same at the outflow location- 45 gpm. So I am guessing the gradiant at the intake is more shallow than the gradient at the outflow.

I did examine for any high spots in the pipeline that might form air pockets. I know those can significantly reduce flow. Two spots had the pipe running slightly uphill-- raised up by a tree root that has grown over time. I added air release valves in these location but it didn't seem to do anything.

Since the water seems "backed up" in the 6 inch line to cause the intake to be nearly 100% of the pipe, I am concerned about connecting the (shallow grade) 6" line directly to the (steeper gradient) 2 1/2 inch line.

Maybe I need to cap the 6" line and see what kind of pressure would build in it. That poses a few issues (i.e. overflow at the source) but I could do it long enough to do a test.

Maybe I need to plumb the 6" line to the 2 1/2 inch line and see what I get? But I would probably tee off with a gate valve also so I could bleed off pressure if it became too much. I don't think I will raise pressure enough to cause pipe or joint problems but it is very old PVC and I don't want to go above 45 psi. Circa 1970.

 

[MossRoad]

You could put a pressure regulator on the system down at the outflow end not to exceed 45psi.