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ReddhEad

Advanced Member

Posts: 67

Location: South Jersey

1

Friday, August 13th 2010, 1:34pm

Accounting for water hammer

I've been researching what size and type of piping I should be using for my main line. In this area of NJ 80% of the installs are using pulled poly. I have about a 100ft mainline that will only be delivering at max 10GPM at (if I'm lucky) 35psi. When I do my calculations using 1.25" poly pipe at 100ft I see surge pressures approaching 400psi toward the end of the main line. A longer pipe resulting in greater surge pressure as you reach the end according to the formulas I read. Even if I installed poly pipe with a 200psi working pressure rating, would it be able to withstand that huge pressure surge when a valve snaps shut at the end of the line?

Maybe I'm just calculating the actual pressure surge incorrectly? I've used quite a few online calculators and even did the math by hand. The local irrigation supply I contacted doesn't even stock 160psi 1.25" poly. They said if I wanted it they would have to special order it. The highest they carry is 100psi. I was planning on a 1.25" mainline to keep as much pressure getting to the heads as possible.

I'd like to use poly because I can just make nice smooth turns with less fittings. I guess I could use Sched 80 PVC if that's what it is going to come down to. I just worry about that huge pressure surge coming back into the house plumbing and popping my ice maker or under sink water filter line.

HooKooDooKu

Supreme Member

2

Friday, August 13th 2010, 11:43pm

As I understand it (from what I learned at www.irrigationtutorials.com) is that the way to size pipe to avoid water hammer is to make sure the water speed in the pipes never exceeds 5 feet per second.

The best guess I can make (without knowing the exact size of the pipe you are using at a pressure loss chart to match it) based on info at http://www.hunterindustries.com/Resources/pdfs/Technical/Domestic/LIT194w.pdf is that your flow is 10GPM, the water speed in 1" poly pipe will be just under 4 feet per second.

Wet_Boots

Supreme Member

Posts: 4,064

Location: Metro NYC

3

Saturday, August 14th 2010, 3:24am

Step one - describe the water supply. What is the static pressure? What is the size of the supply line from the street to the house? what size of water meter do you have?

ReddhEad

Advanced Member

Posts: 67

Location: South Jersey

4

Saturday, August 14th 2010, 6:41am

Static pressure is 51psi at the hose bib. Supply line from the street is 1" copper and about 40ft long before it hits the house and goes right into a 5/8" meter. After the meter I was planning on running 1 1/4" Sched 40 to the backflow preventer on the outside of the house. Total length of that run would be about 25ft. Then it would change over to 1 1/4" poly pipe. The last valve in the line would be about 100ft out. At 10GPM my flow rate should be very low, under 2 ft/sec.

Thanks

Wet_Boots

Supreme Member

Posts: 4,064

Location: Metro NYC

5

Sunday, August 15th 2010, 4:29pm

You can stop worrying about water hammer. Your mainline could probably run to the next zip code before water hammer became a problem. The problem with trying to read a table to get a number, is that you don't have valves that snap closed, and in the seconds during their closure, the flow is slowing down as well.

ReddhEad

Advanced Member

Posts: 67

Location: South Jersey

6

Tuesday, August 17th 2010, 7:31pm

You can stop worrying about water hammer. Your mainline could probably run to the next zip code before water hammer became a problem. The problem with trying to read a table to get a number, is that you don't have valves that snap closed, and in the seconds during their closure, the flow is slowing down as well.
Thanks Boots.

Yeah I kind of went overkill on the 1.25" mainline. The consensus always seemed to be that the lower your keep the GPM the more reliable the system would be. I think I'll recalculate what happens to price and pressure when I drop to a 1" mainline. For my GPM it should be negligible. The valve closure time was definitely my struggle point. Some charts say to use .40 seconds and others say to use .10 seconds. I guess keeping it under 5ft/sec is all I really need to worry about.

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