Siphon problem with above ground water tank/pump

[saxman2u]

Several heads on my irrigation system are leaking when my irrigation pump cuts off. The irrigation pump is 30 ft lower in elevation than the bottom of the 1200 gallon water tank that supplies water to the pump. I keep about 4 feet of water in the tank, so the pump is essentially 34 ft below the water surface. The pump is about 120 feet away in linear distance with a 2" PVC line feeding the pump. The pump has a 1.5" input on it and a 1.25" output on it.

After several years of correct operation, several sprinkler heads are leaking and it is not front low head drainage. Some of lower heads are not even leaking. I can hear water passing through the valve solenoid. It is making a whirring sound, faint but noticeable.

Am I better off to install small check valves under all the sprinkler heads? This is a pain, messy, and time consuming. Or, do I have to install an anti-siphon or vacuum break device up by the water tank?

My pressure to the sprinkler heads is right at 55 PSI with the PCV and the valve solenoid diaphragm is not damaged. I am also unsure as to why the problem just surfaced since I installed the system in 2010?

This question popped up years ago but I would like a few more details with correct equipment and install locations/height/material.

Thanks for your help,
Scott

 

[Tom.G]

Based on the mostly complete description, I'll have to assume that the solenoid valve is supposed to stop outflow from the water tank.

So the condition you have is:
1) Water is not to flow when valve is closed. This was proven true in past operation.
2) Water is now flowing when valve is supposed to be closed.

Possible Causes:
A) Water is flowing somewhere other than thru valve.
B) Valve is not closed when it is supposed to be.
C) You are imagining things, there is no water flowing.

If I had to choose the most likely cause it would be B).

Solution most likely to be effective:
Replace the valve.

P.S. For less stringent requirements on the valve, you could mount it at the tank outlet and install a vacuum-breaker just below it. That way the valve has only four feet of head on it rather than 34 feet. But that means the pump will startup dry unless you delay its' start after the valve opens.

 

[CWatters]

Can you confirm that the sprinkler heads are lower than the water level in the tank? In which case it sounds like the problem is due to wear in the pump or the valve causing it to let water by (eg nothing to do with a syphon effect). If that's the situation then fixing the valve should work.

 

[Averagesupernova]

Couple of things here. The lowest sprinkler heads do not leak but higher ones do. You can hear water flowing through the solenoid valve that is supposed to be closed. Sounds like a two fold problem. It should be a given that a closed solenoid valve should not allow fluid to pass through. Points to a defective solenoid valve. But, if that were the case then shouldn't all sprinkler heads leak especially the lower ones? I would bet there is a diaphragm in each sprinkler head that shuts off flow once pressure gets below a certain point. Points to this being defective also. Maybe the components just need a good cleaning.

 

[CWatters]

Perhaps the lower ones don't leak because all the water has escaped (from the higher ones) before it can get to them?