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Just my opinion, I think I'd ask the supply house you got the flanges from for the Vendor area manager's phone number/email and tell him you have multiple failures all of one size and ask if he'd be willing to look at what you have and provide some information. The sales manager for the flange manufacturer should be willing to offer some help.

All flanges with both companies were exactly the same 6" sch 80

Forgot, Pipe size it too can be a factor

With the information provided it's not possible to pin point the problem. But I'd ask you to take a look at the velocity (flow rate) of the chilled water. Look at the hangers/supports. Is the gap between the flanges the same all the way around? Does crack develop at part or full flow? Where do the crack develop at: top, bottom, sides, all the way around? It could be resonance. If you are still working on it do you have a vib guy you can have take a look at the pipe resonance at various flow rates

Just thoughts, Good Luck

That is what Lasco is saying. I'm quite sure my fitter did everything properly. I'm convinced that its a defect in the flanges. I was just trying to find other that are having the same issue. I have found one other company that has had 4 failures just like the ones I have had with the exact same flanges with the same pressure. I know they have qualified pipe fitters too.

That’s within in its limits.

I also wonder if any lateral stress or improper tightening is a factor?

SCH 80 about 43F

what Temperature Chilled Water?
SCH 40 or 80 PVC?

I found out another company in town has had 4 failures in exactly the same way at about 80 PSI. Same size and type of flange. FYI mine were on chilled water and the pumps were on VFD's for soft start.

I might think an engineer could see the problem. Cracks due to what would be the start point. Vibration, expansion/contraction, manufacturer defect, and all the stuff Answer-Man wrote.

Something is not correct to have 3 failures within the same pipe system/circuit. Just to name a few I have run across: design paramters vs actual parameters, pressure, temperature, chemical compatability, type gasket, pipe stress, bolting tension, supports, type of pumps, pulsations in the fluid.....

Since it's been a couple days any update from the original posting

Lasco PVC flange failure

I have been doing this for 38 years and never saw a Vanstone flange fail. We have had 3 failures on the same job and Lasco clamed its not their fault. The flange cracks at the rim under the retaining ring that the bolts go in.

Has anyone else had a similar failure? I know of one other I recently found out about.

Thanks