4" 150# Lug Butterfly Valve on a chilled water return line that is leaking past the seat. I asked the owner and only the CHW with a rust inhibitor and am anti-bacterial agent are present. The original valve leaked and started to blow open even though the handle was locked in place. We put two other valves (1 brand spankin' new) on and both leaked past the seat, and by leak I mean about 1 gal/min.
The owner swears that the pressure in the main that it ties in to is only 70 psi, but this makes me think it is a lot higher than they think. But there are no pressure taps on the line.
If this is a chilled water return line it will be at a lower pressure that the supply side but most likely only by what ever the pressure drop of the chilled water system is. I don't think I've ever seen a system that ran in a true vacuum on the return side unless there was a problem at the strainer or that nature.
I would try to get an idea of the true system pressure and if that appears normal, get the valve manufacture involved they may have some valuable imput.

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