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1

Sunday, October 10th 2010, 2:07pm

Static Pressure vs. Working Pressure

So I bought a water pressure gauge and attached it to the outlet on the PVB and got a static pressure of about 50-52 psi depending on when I measure it. From there I plan on using 1-1/4 inch piping throughout, 1" valves, and pgp heads and a max of 15 gpm for any zone. You might recall from another thread that my 5 gal bucket test gave 25 gpm but I will only use 15 gpm. So to calculate my working pressure do I deduct pressure loss only from everything that is downstream from where I measured the static pressure (the outlet on the PVB)? or Do I need to also include the pressure loss from the water meter, 50 ft. of mainline, and the PVB that are upstream of where I measured the static pressure?

Central Irrigation

Supreme Member

Posts: 305

Location: Central Minnesota

2

Sunday, October 10th 2010, 2:45pm

You must deduct pressure loss of everything the water has to flow through.



Static pressure is defined as water pressure that is not flowing. Working pressure is the pressure you have with a certain flow. I would only concern yourself with deducting pressure losses in the meter, backflow, valve. Since you have stated that you'll be using 1 1/4" pipe, your losses (of the pipe) will be very minimal (1psi/100'). If you want to error on the side of caution, deduct 3-5psi from your worst zone (largest and/or farthest from the water source).



You should have no problem ending up with 35-40psi at the heads.



If you want to get even more technical, what type of soil are you in. Clay soil turf seems to do better with a 2GPM nozzle where as sandy soils prefer 3GPM nozzles. It may be worth thinking about. You could actually decrease the number of zones by nozzling down.

pass1

Active Member

Posts: 33

Location: east coast

3

Sunday, October 10th 2010, 5:18pm

The length of service line, in your case, 1" copper from the city main to the meter must be included in your pressure loss calculations. You never really said how long that pipe is in your previous posts. Don't forget any elevation changes.

4

Sunday, October 10th 2010, 5:25pm

So If I take 50 static and deduct the meter, mainline, PVB, valves, laterals, etc... I'm down about 15 to 35? Darn, that sucks. Can anyone explain how I have a 5/8 inch meter, 50 psi static, and I can get 25 gpm out of the PVB? Doesn't seem right. Having a hard time accepting that. Seems that with a 5/8 inch meter and 50 static I should be only getting maybe 12 gpm. I wonder if the pressure gauge is just wrong and the pressure is actually higher. Will the PGP work properly at 35 psi?

pass1

Active Member

Posts: 33

Location: east coast

5

Sunday, October 10th 2010, 8:26pm

A bucket test is a guide at best in determining the amount of water you can run. Your flow design (gpm) should be based on how much pressure you need to operate the heads for the worst case zone. Your bucket test is water flow in a open flow condition. You were measuring an approximate flow not taking into account the pressure. If you were to attach a pressure guage somewhere in the piping as you were doing the bucket test you would probably see a significant drop in the operating pressure for the flow you say you are getting. (25gpm). Pipe friction loss charts don't lie. You should figure all your losses through pipe, valves, backflow, meter, elevation etc. ( plus 10% for misc. ) at the flow you want and see if you have the desired pressure left to operate the furthest head in the worst zone. If not, lower you flow (gpm) and do the calc's again until you have the pressure you need. You can also make adjustments to your pipe size to decrease friction loss.
Pipe sizing is done backwards from the last head on a zone back towards the point of connection.

Central Irrigation

Supreme Member

Posts: 305

Location: Central Minnesota

6

Sunday, October 10th 2010, 8:36pm

35 psi at the head is just fine.

When I design systems I try to shoot for 40psi, but only a long time professional would be able to tell the difference between 35psi and 40psi.



Also, like Pass1 stated, for every foot below where you tested your pressure from, you will gain 0.433 psi. So, if you have a head 10' below your pvb, you will have 4.33psi more at the head.



35 psi will perform just fine, just make sure and watch your head spacing. A PGP guy will comment and tell you the appropriate spacing based on your pressure.

7

Monday, October 11th 2010, 6:18pm

I think I remember reading in irrigation tutorials that head spacing should not exceed working pressure.

Central Irrigation

Supreme Member

Posts: 305

Location: Central Minnesota

8

Monday, October 11th 2010, 6:41pm

You're absolutely correct. However different manufacturers perform differently. I would space a Toro 800 at 33' with your pressure, but a PGP could be different.

ReddHead

Advanced Member

Posts: 67

Location: South Jersey

9

Tuesday, October 12th 2010, 7:00am

So If I take 50 static and deduct the meter, mainline, PVB, valves, laterals, etc... I'm down about 15 to 35? Darn, that sucks. Can anyone explain how I have a 5/8 inch meter, 50 psi static, and I can get 25 gpm out of the PVB? Doesn't seem right. Having a hard time accepting that. Seems that with a 5/8 inch meter and 50 static I should be only getting maybe 12 gpm. I wonder if the pressure gauge is just wrong and the pressure is actually higher. Will the PGP work properly at 35 psi?

Yeah, it sucks. I didn't want to accept it when I did my tests either, but it's true. You're getting 25GPM but you probably only have 5-10 PSI as the water exits the PVB, if that. If you hooked up a pressure gauge right next to the outlet of the PVB @ 25GPM you'd see very low pressure. If you had a valve on the outlet of the PVB and gradually closed it to reduce the amount of flow the pressure on the gauge would start to rise.

Oh and for what it's worth I have a cheaper hose bib gauge like you. If I open my hose bib fast enough (it fully opens with 1/4 turn) it will spike to 82PSI and stay there. If I slowly open the valve it clocks in at 51PSI. The gauge is cheap so it just sits at the highest reading it sees until all of the pressure is relieved.

HooKooDooKu

Supreme Member

10

Tuesday, October 12th 2010, 1:06pm

RE: Static Pressure vs. Working Pressure

So I bought a water pressure gauge and attached it to the outlet on the PVB and got a static pressure of about 50-52 psi depending on when I measure it. From there I plan on using 1-1/4 inch piping throughout, 1" valves, and pgp heads and a max of 15 gpm for any zone. You might recall from another thread that my 5 gal bucket test gave 25 gpm but I will only use 15 gpm. So to calculate my working pressure do I deduct pressure loss only from everything that is downstream from where I measured the static pressure (the outlet on the PVB)? or Do I need to also include the pressure loss from the water meter, 50 ft. of mainline, and the PVB that are upstream of where I measured the static pressure?


Your "bucket test" gave you the gpm with a "working pressure" of ZERO. That's why everyone has been saying you can't go by the bucket test.

As for pressure losses, you have to account for ALL pressure losses. To be perfectly accurate, you would even have to account for pressure losses in the pipes that run from the water tower to the water main running down your street. But that's a dynamic system where your pressure messurement (50-52 psi) varies as other water customers turn water on/off in there homes as well (so you can never get a TRUE static pressure).

So your best starting point is to make multiple static pressure reading at various times of the day and begin with your lowest reading. Even then you have to account for possible variables such as future pressure losses when more houses are built that use the same water mains, or if others have been running irrigation systems at the times you've been making your measurements.

So beginning with your lowest static pressure, you have to account for pressure losses in all the pipes (water main to meter, meter to backflow preventer, backflow preventer to valves, valves to sprinkler heads). Then you have to account for pressure losses through the devices (water meter, backflow preventer, valves). For pipes, you'll need pressure loss charts. For devices, you will have to look up manufacturers specs for your planned gpm (www.irrigationtutorials.com will give you some rough guestimates at what some of these might be, but if you try to use the guestimates, you have to add more fudge factors).

It's not unreasonable for pressure losses to be something like 3psi for the meter, 12 psi for the backflow preventer (if its an RPZ), 5 psi for the valves, and 5 psi for the pipes. If those numbers were accurate, that would leave you a working pressure of only 25 psi. With a PVB, that 12 psi is likely replaced with something on the order of 2-3 psi... and that would put your working pressure at about 35 psi. So it's not unreasonable that starting with a static pressure of 50 psi you only get about 35 psi to work with.

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