What am I missing when blowing out water from my sprinkler system?

Question

I am trying to blow out my sprinkler lines and I put together an adapter to connect the air compressor to the faucet pictured below.  The air compressor was charged to 140PSI, and then I turned on a zone and opened the air compressor valve to 40PSI (into the faucet).  But as far as I could tell, nothing was happening as far as water getting discharged from the system.  It appeared that all of the air was flowing out of the pressure vacuum breaker.

The past couple of years I have hired someone to blow out the sprinkler lines and I know they attach to that faucet.  I also know that when they are clearing the water from the system the sprinkler heads popup and the water discharges from them.   So what am I doing wrong?

EDIT (2010-12-28):
I am not sure why I was not able to blow out the lines through the pressure vacuum breaker (I imagine it is not sealing correctly as said by @gregmac is his answer) so I ended up installing a new valve beyond the PVB which is where I attached the air compressor.

gregmac · Accepted Answer

My guess would be that there is something wrong with the backflow preventer.

Most likely, the air inlet valve is not sealing properly. When pressurized from the side marked "normal flow" (which is where your tap is), it should open the check valve, and also force the air inlet valve shut.

It's probably a rubber gasket, and might be worn out and need to be replaced, or maybe something is physically holding it open. Try pushing on it to see if you can get it to close all the way. Some silicon grease around the gasket will help it form a good seal and prolong the life of the gasket.

Mike Powell · Answer

I haven't done this work before so I don't have much to add, but did find this tutorial that mentions removing that backflow preventer before blowing out. Of course, then you'd need to make a new adapter since your faucet won't be connected to the system anymore. And this also doesn't explain why others have been able to do it in the past with the backflow preventer in place. Overall this is a pretty crap answer, sorry!

Answered by Mike Powell on December 11, 2020

John Blue · Answer

Your pressure is probably too low to shut the backflow, so all of the air is coming out the PVB.  You've got to get sufficient pressure to seal the PVB.  Mine takes 50 - 60 psi using a compressor that delivers 6-7 CFM in that PSI range.

Also consider the professionals use compressors delivering much higher CFM...their compressor may be quite capable of closing the valve at 40psi.

cog · Answer

Sometimes DIY sized compressors have an easier time if you can find the spring that is preventing the blowout preventer from closing (under air pressure) and temporarily take it out.  This  would be the spring that is mounted to the cap, in the case of 800M4QT watts units, but mechanically the same idea applies to others.

I used to be able to do mine, and it wasn't a CFM thing that made it impossible, after a blowout preventer replacement. The new spring was too strong for ~90PSI, and my lower-than-commercial CFM.

Jason · Answer

Mine had a spider web inside preventing the valve from closing.

What am I missing when blowing out water from my sprinkler system?

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