Sprinkler caused Water Hammer...help ! ! !

[timfogl]

Having problems with my sprinkler system and water hammer. Have discovered that the hammer occurs not when the sprinkler valve shuts-off but when they TURN ON. Was thinking hammer arrestors would solve my problem but now I am wondering if that would work.

My system.
Main line 3/4"...high water pressure 95PSI.
T off main line before it goes into house..line to sprinkler system steps up to 1" before going thru antisiphon valve and on to 20 valves. All 1" line PVC to either 3/4 risers to lawn heads/trees or 3/4 risers to pressure regulated 25psi drip lines. Main line into house has pressure regulator with pressure at about 65psi +-. Every sprinkler valve causes some hammer and it is primarily heard near the area of the water heater. Water heater has an expansion tank.

Eight valves in my system(I am on an acre of land)are about 90 feet away from where the main line enters the house(into a basment)and the line to the water heater runs the length of the house to get to the water heater. Probably another 60 feet. With water hitting a sprinkler head or a dripper line more than 150 feet away from the water heater, it's hard to believe it could run all the way back to cause water hammer??

Installed inline PVC drains on all line runs so that when pressure is non existent they are supposed to drain the line...mainly to prevent winter freeze in pipes. This of course causes the lines to be probably pretty much empty when a sprinkler valve turns on and it takes a big slug of water(1" line) away from the smaller main line feed. I am trying to get my head around how this causes my water hammer and what I can do to resolve the issue. I am beginning to get worried about water pipe leaks and with a full finished basement that would castastrophic.

ANY REPEAT ANY help or suggestion certainly appreciated....
Thanks.
Tim

[LazyPup]

To explain your problem we must begin with a simple rule of physics. The venturi principal postulates that when velocity of a fluid is increased, the pressure is proportionally decreased. When all valves on a system are closed the water is at rest and the internal pressure will be equal to the applied pressure,whether from a municipal water source or a well pump. When a facuett is opened flow begins. As the velocity of flow increases the internal pressure in the pipe decreases proportionally. When a valve is closed suddenly, the velocity stops and the pressure immediately rises to equal the supply pressure. The sudden increase in pressure, and to a minor degree the inertia of the moving water causes a physical shock in the line which produces one loud banging sound which we call "water hammer". To prevent water hammer inverted air chambers are built into the piping system, called water hammer arrestors or in a worst case scenario Expansion tanks such as you have on your water heater. The idea is to trap air in the vertical tube or expansion tank. As the water pressure rises rapidly the trapped air can compress to absorb the shock.

In your case the problem is slightly opposite. The irrigation lines are empty when the system is off. When the system is turned on, the water rushes in to fill the system, thus a high velocity of flow and an associated instantaneous drop in line pressure. The noise you are hearing in the vicinity of the water heater is the air rapidly expanding in the expansion tank to compensate for the pressure change.
There may also be an associate water hammer as the irrigation lines are filled and the internal pressure begins to rise.

One possible solution would be to install a backflow preventer valve on the house line immediately after the pressure regulating valve to prevent backflow from the house lines to the irrigation lines when the irrigation is first turned on.

Also, the code now requires water hammer arrestors to be installed in the near proximity of any fast acting valves. The main valve on your irrigation system would be a fast acting valve and may require a water hammer protector on the irrigation system.

[timfogl]

LazyPup,
Thanks for all the information. Few questions remain.
1: When you say "install a backflow preventer valve on the house line immediately after the pressure regulating valve to prevent backflow from the house lines to the irrigation lines...
[:0]Do you mean install backflow preventer inside basement after PRV has decrease pressure to +-65psi? So basically it is another antisipon valve like on the sprinkler system only reversed to stop backflow from the house...this of course is a major undertaking...tearing out walls insulation etc etc etc.
2. Is there a combo PRV one direction backflow preventor other direction? Would be wayyyyyyyyy easier to sweat out current PRV and install a combo in it's place...no walls come down, etc etc...
any such luck???

Thanks again.
Tim

[LazyPup]

the backflow preventer is a short check valve .about 2.5 inches overall lenght, could be attached directly to the PRV with a close nipple. Would only alter the present PRV by about another 3 inches overall.

[Troy]

LazyPup, is there anything you don't know? [] I am impressed!

[timfogl]

LazyPup,
Thanks for more info. Do they come in sizes, etc or does the pipe size determine that. Source? Home Depot, Lowes, will also check online source PlumbingSupply.com.
This just may be doable if I can get a midget plumber...thanks.
Tim

[timfogl]

Most everything I looked at was huge...just like I have in my current sprinkler set-up. Only thing I found that may be what you are talking about is at this link

http://www.sprinkler.com/go-item-backflow_preventers-1117.html

Let me know if this is what you are talking about or give me a link to a product....thanks.
Tim

[LazyPup]

all you need is a check valve. They are sized to fit the size of pipe you have. For a 3/4 inch supply line the check valve would look like a 3/4in brass coupling about 2.5 to 3 inches long. They have a female pipe thread on each end.

You could attach a 3/4 inch brass close nipple to the threaded output of your PRV then screw the check valve onto the nipple and connect your copper line to the threads on the other end of the check valve.

I have located one on a google search to give you a visual of what it looks like, complete with internal cutaway view.

http://www.checkall.com/UN/un3.htm

[timfogl]

LazyPup,
Wanted to let you all know that I FINALLY.....got the problem fixed. It turned out to be the reverse water hammer you suggested an inline check valve worked SUPER....
Thanks again to all that offerred so long ago....
Take Care.
Tim