Help with BAD water hammer from a long downhill run!

[tobinator]

I have a long downhill run from my utility room to a lawn hydrant. Buried run is about 150' long (3/4" black poly)and there is an elevation drop of at least 32'. As any plumber would guess, there is a severe water hammer in the house when there is any action on the hydrant below.

There's a shut-off valve before 3/4" Copper exits the stem wall below grade, then a 90° bend and a transition to poly. Mostly closing the valve reduces the hammer, but at the expense of flow.
Pipes in the utility room are well secured
Downhill run is pretty straight (and would be tough to change)
Water pressure in house is 50/70 PSI (pump) I shudder (get it?) to think what it is at the hydrant.
Single family well

I was thinking about a big 18" home-made hammer arrestor right after the 90° in 3/4" galvanized with a schraeder valve on the cap so I can recharge it every summer. Only about 6" of pipe would be above grade and it could be well secured to the stem wall and a straight line to the hydrant.

Any thoughts about a remedy?

How about the best way and some easier ways?


BTW, I did consult my plumber as he was retro-ing the house and before the ditch was finished. He made no mention of water hammer. Of course he also forgot to sweat a bunch of fittings, cut 3/4 through three consecutive floor joists and bailed out before his work was done, so I guess I'm not too surprised... I had to finish the project because the other contractors wanted to start over and I was almost out of money. So much for taking the lowest bid.

Thanks in advance!
Matt

 

[Jadnashua]

Some hydrants close slower which may solve the issue. A air column quickly gets its air charge dissolved into the water stream and becomes useless. Put a real hammer arrestor on it. These typically have a sealed piston isolating the air from the water, and the air stays in them for a very long time. Because of the elevation drop, your pressure at the hydrant is significantly higher than in the house - think water tower effect (gravity is a real factor here). You may need an oversized hammer arrestor of several in parallel to absorb the energy when you shut the valve.

 

[Sponsor]

 

[tobinator]

I just thought they might not be robust enough. Do you believe otherwise? There is a big force at work. Not just a dishwasher solenoid on level 1/2" pipe.

Matt

 

[Jadnashua]

http://siouxchief.com/Supply/Arresters-And-Trap-Primers/Water-Hammer-Arresters2 they do come in various sizes...you may need one like the mega or industrial sized ones. Other companies make them, too. A mini one is not likely sufficient...

 

[tobinator]

Thanks! Definitely worth a try. I just bought one of the larger ones on **** for $18.00. Let's see if this works.

If not, I might try a pressure reducing valve at the stem wall. Anybody know how to calculate the pressure gain for a ~35 foot drop? 40PSI at the hydrant would be plenty...

 

[Jadnashua]

0.433#/ft, with no fiction losses. So, 35'x0.433=15# or so. That isn't excessive - it would be a problem if it exceeded 80psi or so.

 

[Valveman]

Anytime you hear a "thump" from water hammer, the pressure spikes from 5 to 10 times the pressure you see on the gauge. There is a "big force at work here". The best answer would be to "slowly" close the hydrant, giving the water time to slow down. Quickly closing the hydrant is like dropping a huge boulder in front of a moving train. There is going to be a crash. I have stopped water hammer before by simply replacing a ball valve with a gate valve. The gate valve can only be closed so fast, where a 1/4 turn of the ball valve is an instantaneous closing which causes water hammer. Once I trained my wife to close them slowly, I haven't had any more problems with water hammer from my frost free hydrants.