You are not logged in.

TimCA2

Unregistered

1

Sunday, September 14th 2008, 11:29am

Static vs dinamic presure

Good morning all,



I am trying to design my system . My well cut in is 44 psi and cut out is 58 psi. These are at the presure tank. When I run a hose to the to an isolated head on a stake (MP3000 head) and check the dinamic presure it is about 28 psi. My question is should I be using static or dinamic presure when designing my system?

Thanks

Tim

HooKooDooKu

Supreme Member

2

Sunday, September 14th 2008, 3:37pm

It's a little more complex than that.

To simplify for a moment, let's pretend like you're on city water with a static pressure of 50psi. That means that while no water is flowing through the system, the pressure is 50psi, and it will be 50 psi everywhere in the system. What static pressure represents is the amount of pressure you are starting with.

Dynamic pressure, on the other hand, is the pressure when water is flowing, and the pressure is litterally different everywhere in the system. How do you lose water pressure? As it flows through anything. Objects like water filters, backflow preventers, and irrigation valves act like single point pressure losses. The amount of the pressure loss depends upon flow rate. So a valve might lose 4.7 psi at 5gpm (gallons per minute) and it might lose 5.4 at 10 gpm. The other primary source of pressure loss is the pipes. The pressure loss is usually measured in psi per 100 feet of pipe, and because it depends both on pipe size and flow rate, this type of information has to come from pressure loss charts. And because the inside diameter is different for different types of pipes, you need a chart for the exact type pipe you are using. Changes in elevation will also affect pressure. An elevation gain of 10' will loose about 4psi while an elevation loss of 10' will gain about 4psi.

Finally, for a sprinkler to work, it has to have a certain amount of pressure when the water gets to the sprinkler. The amount of pressure at the head determines the performance of the head.

So the way you have to work it out is layout your system. Then assume a working pressure at each head. Assume something like 30psi. Then for your selected pressure, determine what is the flow rate of the head at that pressure. Then look at all the pipes. Determine what is going to be the flow rate through each pipe. If you are assuming 30psi at every head, that means that the flow through any segment of pipe equals the flow of one head times the number of heads the pipe feeds. Then based on the pressure loss charts for the pipe, you have to figure out how much pressure you expect to lose through each segment. So if you have a 25' section that the charts (for your pipe size and flow rate) indicates a pressure loss of 4psi (per 100'), that segment will lose 1 psi.

So once you know all of your pressure losses, you start with the static pressure, work out all the pressure losses through the system, and see if you don't still have 30psi or more of pressure at the head. If the pressure losses leave less than 30spi at the head, you have to change the design. Try again with a lower working pressure, or increase the size of pipes for less pressure loss. From there, it's pretty much trial and error.

It's a lot of reading, but these principles are better spelled out at www.irrigationtutorials.com.

Lowvolumejeff

Advanced Member

Posts: 88

Location: Seattle Area

3

Sunday, September 14th 2008, 5:58pm

Excellent summary

Nice answer :thumbsup:

Small addition. Aside from picking up pressure from upsizing the pipes, is look at the type of valves you are using. Many of the commonly used ones are pressure killers. I use Weathermaitic silver bullets (AKA 12000 series) which is good for drip zones, and only cost you 3.4 PSI at 12 GPM. It's bigger brother, the 21000 series (black bullet) at 12 GPM losses less than 2 PSI. Many of the common Rainbird valves cost you 4.7 to 5.5 PSI at 12 GPM.

I buy my valves from this sites sponser, www.sprinklerwarehouse.com

So, if you are tight on pressure, upsizing the valve may be more practical and less expensive than upsizing pipe.

If your STATIC pressure is below 50, you may need to do both, or look at adding a booster pump. I know very little about pump systems, but do know that if your static is the 50's, you need to avoid all pressure hogs. What type backflow are you using? Remeber to calculate its pressure loss also.

You stated the dynamic pressure at the MP 3000 on the stake was 28 PSI. I think they do best at pressures above 30. Was this the last head on a zone? Where you running more than this head? If you are designing the system, how did you test this? Your answers will help us help you.

Thanks again to HooKooDooKu for the excellent answer before mine.

Jeff

4

Monday, September 15th 2008, 8:08am

You should not check you pressure using a hose. Why? because your not using a hose in you sprinkler system.

here is my advise and I work only with well water. Didnt I read that your pump can produce 14 gpm? If so then your fine. tap into the mainline and get the pipe out side of the house use a 1" febco PVB and 1" every where. Design your system with 5 heads per zone. Use the # 2 nozzles or maybe # 3 test one zone first before you do the rest of the heads. While running that zone watch your pressure gauge and it should stay in the 40's. If so then you wont cycle your pump and thats what you want. If you would like I can tell you all of the heads and valves etc that I would use. Let me know.

Similar threads

Rate this thread