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New Member


Thursday, August 14th 2008, 3:27pm

Question about pressure loss on a hill

I am trying to install a sytem on a house that is about 60 feet higher than the meter. The service line to the house is 1 1/4 Sch 40 with a distance of 600 feet from the meter to the house. The static pressure at the house is about 44-52 psi(it varies). I tapped the service line just before it enters the house and did a flow test that showed I am getting about 10 gpm at 30psi. I figured pressure losses to be about 9 psi from the farthest head to the point I tapped the service line. That includes the BF device but not the service line or meter. Do I need to figure in the 26 psi lost coming up the hill or do I disregard it since I am measuring the static pressure at the house?


Advanced Member

Posts: 88

Location: Seattle Area


Thursday, August 14th 2008, 7:57pm

Great question, but need more information, OR you already have all you need to know.

You say you get a dynamic (flowing) pressure of 30 psi at 10 gpm, at the POC. All you need to do is calculate the pressure loss for the back flow assembly, the friction loss of the pipes, and the loss of the valves you are using. Calculate that to the furthest head, and you can design your system. You may have already done this, when you say the PSI loss to the furthes head is 9 PSI. Sounds like a lot, and can be reduced by upsizing the PVC diameter, and chnging the type and/or size of valve. If you have 21 PSI at the furthest head, you might consider MP rotors, or maxipaw impacts. Both work at lower pressures, and the MP rotors work on lower volumes, meaning you can put more per zone, but have longer run times.To get good coverage patterns, most pop up sprayers need about 30 psi. Rotors even more.

Friction loss in schedule 40 PVC is of course dependent on pipe diameter. For each 100 feet, my charts tell me:

3/4 inch loses 7.74 psi/ 100 feet. (also, this size should not handle more than 8 GPM due to water velocity) 1 inch = 2.39 pSI losss/100 ft 1 1/4 loss is 0.63, 11 1/2 loss is 0.31, and 2 inch is 0.09 loss.

So, upsizing long pipe runs will increase pressure at the head. Valves also eat pressure. I use Weathermatic silver bullets (loss 3 psi at 10 GPM) Many of common Rainbird valves are twice that.

Now, a very important distinction. Dynamic vrs static pressure. When you say you measured 30 psi at a flow of 10 GPM, I am assuming you took the pressure while the water was flowing at 10 gpm. I know you probably did, but other reading this might not realize that if you take the pressure with no water flowing, that is static pressure. If one were to measure the pressure, then turn on the water and measure the flow, that would be different. Dynamic pressure is meassured at a certain flow rate. Usually a special device is needed to accomplich this, and few home owners have one. ( I don't have one, but want one....hint to wife for Christmas :D )

If the 10 GPM @ 30 psi is not your dynamic flow, write back for more explaination. Also, how did you get the 9 PSI loss at the furthest head? Sounds to me you know what you are doing.

Hope I helped, at least some. Jeff


New Member


Thursday, August 14th 2008, 10:37pm

Thanks for the response Jeff. The 10gpm@30psi was just a simulation. I rigged up a device to connect to the POC with a pressure gauge and a manual ball valve. I choke down the valve to the expected dyanamic pressure and check the flow. I don't have a flow/pressure meter to find the actual dynamic pressure.

I figured the pressure losses with loss charts. 294' of 1"CL200=3.35psi,1"valve=1.5psi,double check=4psi. If I include the service line, 600' of 11/4"=3.8psi and 5/8" meter=3.7psi it adds up to about 16 psi loss. That would leave me about 28psi to work with based on 44psi static pressure. That might work ok, but what about the loss from the 60' of elevation(60x.433=26psi). That would leave me with nothing to work with. Do I need to count the elevation loss since it's probably already affecting my static pressure reading at the house?


Advanced Member

Posts: 88

Location: Seattle Area


Friday, August 15th 2008, 1:11am

I'm impressed

Great job with th dynamic pressure determinatuion. That is how I am doing it, but I probably get a lot wetter than you, since many of my static pressures are in the 85 - 110 PSI range, and I get flows of 30 GPM. I know, you wish you had that problem. I want onje of those Toro dynamic pressure determiners.

Unless I am misunderstanding, I think you have your problem licked. If you got your 30 PSI and 10 GPM reading at the house, it is from that point on you calculate your pressure loss. Since he elevation gain is behind your 30/10 reading, it is moot. Since the 30 PSI/10GPM readings were taken after the street line and meter, they do not have to be recalculated. So,that adds 7 PSI. or your loss is 9 psi. Upsize your pipe to 1 1/4" Class 200 drops your loss per 100 foot from 1.3 to 0.42. So, over 300 feet, that adds 2.6 psi. If you go to 1 1/2, the friction loss drops to 0.22/100 ft, saving you another 0.5 psi or so.

What valve are you using. 1.5 psi loss is good. You could go to a Rainbird EFB-CP 1 inch, cost you only 0.7 PSI.

Well, upsizing a pipe is costly, especially with the recent price increases (look for a drop in fall when demand decreases), it is your best option. Last time I checked, the price increase form 1.25 to 1.5 inch pipe was not that much, fittings were also no that much more. The Rainbird valve gives you some more. So that is 3.8 psi gain. OR, DCA = -4 PSI, 300 ft 1.5 inch - .66 psi, low friction losss valve -0.7 PSI total - 5.3 psi Net 30 PSI - 5.3= 24.7 PSI.

Not great, but certainly doable with pop ups (15 psi minimum Rainbird 1800 series). Maxipaws need 25 psi, and mp rotors like 30 psi. I was taught to move my heads in closer when pressure is lower. Manufacturers inflate their numbers. Not sure if that is true, but you might use 12 foot nozzls spaced at 10 feet. Most pop ups (sprays) deliver about 2 GPM.

So, what is your pressure at 8 GPM. May be the solution. Then using a low flow head like the MP rotor may be possible, allowing wider coverage per zone, but with less flow. watering longer per zone. Your friction loss numbers will improve with less flow.

Thats enough to ponder for one night, check my math, it has been a long day in the trenches.

Good luck, let me know, Jeff


Supreme Member


Friday, August 15th 2008, 8:17am

RE: Question about pressure loss on a hill

...Do I need to figure in the 26 psi lost coming up the hill or do I disregard it since I am measuring the static pressure at the house?

Ignoring friction losses for the moment, where ever the pressure gauge was located (elevation wise), then that is the water pressure at that elevation regardless of where the water originated from.

So if you measured the pressure at the house, and the house is at the top of the hill, you will have at least that must pressure to start with. For every irrigation head that is located below the elevation of that pressure guage, it will actually gain some water pressure. If the house is not at the highest point on your property, and you're going to have spray heads at an elevation higher than where the pressure guage was located, they are going to lose pressure.

Here's an interesting though that just occured to me. If you wanted to install a drip irrigation system and wanted to water some plants with it located back down at the bottom of the hill, if your valves are located at the top of the hill where the 30psi was taken, you would have to run the lateral line back down the hill and install a pressure regulator AT THE BOTTOM OF THE HILL in the middle of the latteral line. If the starting pressure was 30psi at the top of the hill, then you run the lateral down to an elevation point 60' lower, the water pressure in the lateral line at the bottom of the hill would be over 50psi... too much for a drip irrigation.


Supreme Member

Posts: 5,292

Location: Metro NYC


Friday, August 15th 2008, 11:11am

A lot will depend on how much area is being irrigated, and at what elevations relative to where the measurements are taken, but if it's a large enough area, this would be a situation for a booster pump. A half-horse jet pump might be just about perfect for the job.

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