Sprinkler valve issue - water hammer
 

I recently replaced all of my sprinkler valves. The old ones were leaking/not working, and the whole thing was a mess. I ripped everything out and started over with all new stuff. I even installed a cool new digital timer.

Everything works great except for the pipes "slam" badly when the valves turn on or off. I've mitigated the problem somewhat by turning down the supply valve. It still rattles the pipes, but not as bad. I'm pretty sure the noise is the pipes banging against the joists where the straps are loose.

The system is divided into two sets of 3 valves. Interestingly, after I completed the first set of 3 valves and ran it for a week, I didn't notice any hammering. It wasn't until I set up the valves in the back yard that the problem started.

I'm wondering if there was some technical detail I got wrong or left out. I'm using lower-end RainBird valves, 3/4" Sch 40 PVC pipe from the house all the way through to where it connects to the existing 3/4" Sch 20 pipes in the ground.

Any other ideas or possible solutions?

you need an air chamber in front of the valve. Shut off the water supply, put a T-connection (I'm assuming you have PVC), with the open T-connection facing up, put a length of PVC, say a foot, and a cap. Turn water on, the air in the vertical 1 foot pipe with the cap will compress the air in it and act like a cushion.

Check your water pressure. I had the problem until I checked my pressure. I had a bad pressure regulator. I replaced it the problem went away.

I was also going to suggest checking your water pressure, as too high pressure can cause banging and/or whistling pipes. If your pressure is over about 70 lbs. then replace your pressure regulator.

THat is exactly where I would look first. These valves have an optimum pressure range of about 40 -100 psi, 80 being more than you will ever need.

IIRC when I checked mine it was like 90. New regulator, set to 50 I think..

uh oh, another "while you're in there"...

Anti-siphon/backflow valves are supposed to be used on hard-lined irrigation systems. They keep ground water from getting back into the house system. It's probably code but the city might not check.

There are water-hammer arresters(basically a vertical pipe full of air) which provide a cushion effect near the source of the slamming. I think most of these require some overhead room between the joists(6-8in). There may be sealed ones with internal springs available which can be mounted horizontally.
You might want to check the main line pressure as well. Rent the gauge.

If stuff is getting taken apart, an individual shut off valve for each run is handy for maintinence.

Is the height of the T, w/r/t the rest of the system, important?

I don't think I have a pressure regulator, or if I do, it's at the curb.

When was your house built? I've not seen a house without a pressure valve in over 20 years.

If it's missing, you can install one easily enough.

1961

Do the thing with the "T". It's quick, cheap and easy. Old time plumbers used to do this inside houses. If not, then go to Plan B.

FWIW, my 1949 CA bungalow had these "T"'s in several places. It took an old plumber to explain their function.

Hugh is correct, any quick closing valve needs a water hammer arrestor. The linear movement of the water comes to a complete halt and hammers the pipe. We install them for washers, diswashers and even ice makers. In lieu of an arrestor an air chamber could be installed, a pice of pipe tee'd in and capped off just past the valve.

What I don't understand is why I get the hammer when the valve opens.

The bulbs are coming on now. When I re-did the shower, I noticed the old fixture had an extra 1' or so of pipe on each side pointing straight up on both the hot and cold side. Now, when we turn off the hot water in the other bathroom (downstream from this one) we get a small 'knock' from the pipes.

The valve opening rate coupled with the system pressure is what determines water hammer, as I recall. If you can change the valve closure rate it will eliminate the hammer effect. This is why you see "no-slam" check valves used as pump control valves and other control valve modulated on closure rate so as to prevent system transients like this.

Another question, do you have vacuum breakers installed in the line to prevent back flow?

Those are incorporated into the sprinkler valves themselves.

This afternoon I installed a 'mini arrestor' on the line, which reduced the knock by about 80% or so. I think I'll also put a 'riser' thing in next to the valves.

That's what I thought. These usually vent to atmosphere and therefore should give relief within a specified pressure range.

Are the vacuum valve heads at least a foot to a foot and a half higher than the highest sprinkler head?

I'm not sure the backflow preventers built into the heads us the same as a whole system BFP. Between the heads and the water source can be dozens of potential leaks. A good BFP near the water supply would be more effective.