Replacement backflow valve piston

[WobblyHand]

Been having a problem with a back flow valve for my irrigation system. The back flow valve weeps, and recently has gotten a lot worse. Took it apart mostly to see what the insides looked like. Cleaned the plastic piston and found a crack in the face. This piston takes 45 psi. If I hadn't found the crack, I would have just replaced the o-ring and reassembled it. Think I will make a replacement piston.

Removed the o-ring, is seems to be 3/32" and 1.25" ID, which makes it a 124. With the use of calipers and a micrometer, measured up the piston and made a CAD model. I made the ears wider, as they are just locating a spring. The cross in the bottom was replaced by a raised disc. There is also a second spring in the valve which seats against the cross. The bottom of the valve has some metal spring guides which touch the region between the end of the cross and the ID. Here is the CAD rendering so far.

I need to work on the angles of the ears, as they are not quite correct. I think this first attempt will be in aluminum even though this isn't the best material for longevity as this will be immersed in water with steel springs on both sides. I have some 304, but I had some problems with machining it.

Not quite sure how to machine this piece just yet. Making the raised button inside the bore seems like that would be interesting. Thought I would machine the ears last and hold the workpiece in some soft jaws for the mill work. Not sure about the o-ring groove, do they make 3/64" radius grooving tools?

 

[Cadillac]

Brass it’ll take the elements.

 

[WobblyHand]

Brass it’ll take the elements.

Very obvious choice. $16 for 1.5" OD 2" long, if I buy 2 of them. May remake it in brass. First go at it, to learn, hope to make most of my mistakes in aluminum first!

I do have some 1.5" OD 304, but I find it difficult hard to machine.

 

[Provincial]

You can grind a radius tool pretty quickly from a HSS tool bit. A set of radius gauges is a nice addition to your tool box, but you can use this part for checking the tool you grind for this job.

A test run in aluminum makes sense. The part won't last long, but it will prove the concept.

You should also be able to make a tool to cut that raised button if you are careful to make the relief right.

Unless that button acts as a stop, you might be able to redesign the part to center the spring on the bore, rather than the button.

 

[WobblyHand]

You can grind a radius tool pretty quickly from a HSS tool bit. A set of radius gauges is a nice addition to your tool box, but you can use this part for checking the tool you grind for this job.

A test run in aluminum makes sense. The part won't last long, but it will prove the concept.

You should also be able to make a tool to cut that raised button if you are careful to make the relief right.

Unless that button acts as a stop, you might be able to redesign the part to center the spring on the bore, rather than the button.

Thanks for your help.

I have ordered some brass, but will make a prototype out of aluminum. It helps me refine the steps needed to make a good piece.

Making a nice radius that small seems like it might be hard. That's my primary concern. Roughing it out won't be difficult. Haven't attempted grinding under magnification before! Won't take much movement to erase the whole feature!

I think the button is necessary as a spacer to load the piston spring correctly. It might be nice to have a groove to locate the spring, but it's tight in there and the visibility will not be good. Will machine that feature blind as it is. I will measure the piston spring to confirm what I just said. Designs often fail because of poor assumptions, so it's worth checking!

Since this is a backflow valve, I do want to maintain the correct spacing and features on the piston. I don't know how finely tuned this valve is, or how sensitive it is to tolerance. So I will try to be as faithful to the original design as I can execute.

 

[b4autodark]

Orbit Sprinkler Systems Brass 3/4" atmospheric vacuum breaker $13.28, Amazon.

 

[WobblyHand]

Orbit Sprinkler Systems Brass 3/4" atmospheric vacuum breaker $13.28, Amazon.

Wow. Probably couldn't buy the brass for that. I'll keep that in mind. I have a 1" system, though. Not sure the water pressure is sufficient to make up for the restriction. My system was designed 10 or so years ago at a 45psi water pressure. Lately the city has been reducing the wp. Some of my zones have too many heads for the reduced pressure now. So I'm going with a full sized valve.

I did buy some brass, it should arrive Saturday. So in a way, I'm committed. So next I need to cut a HSS blank and to grind a couple tools. As nice as a micro 100 radius tool is, I can't justify the cost for this single part, $85 is too much for me. If I was making a bunch, or if I was selling them, I'd buy it without hesitation.

 

[b4autodark]

$21.95 for a 1" brass one at Sprinkler Warehouse.

 

[WobblyHand]

$21.95 for a 1" brass one at Sprinkler Warehouse.

Wow, you keep coming up with some interesting stuff. Still going to rebuild what I have. Really don't want to have to remake all the auxiliary plumbing if I don't have to. The valve is probably not the same overall dimensions as my Watt. My city is pretty picky about backflow valves. I know on my boiler they specified the brand and model number. Never seen that before. This is probably due to quality issues they have experienced city wide with other backflow valves in that application. Is the backflow valve freeze proof? Mine is a freeze proof model.

Have you ever ordered from Sprinkler Warehouse? Is that the place out of Florida? From looking them up, I don't get a good impression, although it's hard to separate the true information from the rest.

Supposedly, my brass is coming in later in the afternoon, so I plan to make that piston. If it works, great. If not, there's apparently multiple fallbacks.

 

[b4autodark]

Wobblyhand, I have never ordered anything from Sprinkler Warehouse so I can't vouch for them.

I applaud you for building the part, I to have been known to stand at my lathe for a couple days making something I could have bought, do it just out of spite or curiosity I guess.

Your boiler backflow preventer is a whole different animal because there is pressure in your boiler unlike your lawn sprinkler system. I was a master plumber for more years than I care to remember and had a backflow certification that required a separate license from the plumbing license and related training. The backflow laws in my state, Minnesota, are very tough, as they should be. The state law requires that reduced pressure zone backflow preventers be tested and inspected annually and rebuilt every 5 years. It's big business and they take it seriously. The largest one I ever dealt with was a tandem 12" set up at an airport.

The one on your boiler is likely a Watts 9D.